Silicon-Based Blocking Agents

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Silicon-Based Blocking Agents

Gerald L. Larson Vice President, Research Products

Materials for protection of:AlcoholsPhenolsDiols

AminesCarboxylic Acids

Thiols

Materials for:Derivatization for gas chromatographic analyses

Synthetic transformations

Supplement to the Gelest Catalog, “Silicon Compounds: Silanes & Silicones”which is available upon request.

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2

SILICON-BASED BLOCKING AGENTS

General ConsiderationsThe ideal protecting group for an active-hydrogen moiety such as an alcohol or amine would be one that wouldmimic the hydrogen atom itself, but be much more flexible in its reactivity. It would readily go on in high yield,be stable over as wide a variety of reaction conditions as possible and, at the same time, be selectively removable inthe presence of other functional groups including other protecting groups. While no single silyl group can fulfillall of these conditions in all cases, the available range of silicon-based blocking agents can offer the syntheticchemist viable answers to nearly every protection-deprotection challenge. The ability to vary the organic groupson silicon introduces the potential to alter the R3Si group in terms of both its steric and electronic characteristicsand thereby influence the stability of the silylated species to a wide variety of reaction and deprotection condi-tions. This allows the synthetic chemist to select a silyl protecting group that can, for example, be simultaneouslyremoved during the work-up of the reaction step(s) that required the protection, or that can be selectivelyremoved in the presence of another silyl or other protecting group.

In addition to having the ability to be fine-tuned to fit a particular need, the ease and high-yield introductionand deprotection of the silyl protecting groups contribute significantly to their popularity and utility. The commonly utilized trisubstituted silyl protecting groups and their acronyms are shown below.

Si

Me

Ph

Me Si

Ph

Ph

Me Si

Et

Et

Et

Si

Me

Me

Si

Me

Me

Si

Ph

Ph

Si

Me

Me

Si

Me

Me

Me

TMS DMPS MDPS TES

DMIPS TBS TBDPS TDS

Si

TIPS

The use of silicon-based blocking agents has been reviewed with regard to reaction/deprotection,1-7 oxidation ofsilyl ethers,8 and selective deprotection9.

Introduction of the Silyl GroupsIn general smaller groups on silicon make the trisubstituted silyl group more reactive and easier to introduce. Thus,a general relative reactivity scale is: Me3Si > Ph2MeSi > Et3Si > iPrMe2Si > tBuMe2Si > tBuPh2Si > ThMe2Si (whereTh = thexyl) > iPr3Si. The leaving group on silicon also plays a role in both reactivity and byproducts. The generalrelative reactivity of R3Si-X as a function of X is: X = CN > OTf > I > Br > Cl > trifluoroacetamido > acetamido >R2N > RO. Others, such as perchlorates, and sulfates, though very reactive, are not very practical. These generalreactivity trends will not apply to all sets of reaction conditions, but present a practical working guide.

The introduction of the silyl group depends on the nature of the silyl group itself and the substrate. Silyl groupsare most often used for the protection of alcohols in the form of their silyl ethers. Generally, the less stericallyhindered the silyl group the easier it is to introduce (and, indeed, to remove). The introduction of the stericallyunimpeded trimethylsilyl group to a primary, secondary or tertiary alcohol is a straightforward process takingplace with a variety of reagents under several mild, high-yield reaction conditions. On the other hand, the intro-duction of the more sterically demanding tert-butyldimethylsilyl group requires reaction of the alcohol with tert-butyldimethylchlorosilane in the presence of imidazole as a catalyst10 and the formation of the tert-butyldimethylsi-lyl ether of tertiary alcohols is very difficult. Alternatively, a more reactive form such as tert-butyldimethylsilyl trifluoromethanesulfonate can be used.

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Consideration of ByproductsThe introduction of a silyl group onto an alcohol, amine or other active hydrogen system is the result of a substitution reaction at silicon releasing the protonated form of the leaving group from silicon. This leavinggroup can be acidic, e.g. HCl, HBr, HI, HO3SCF3; basic, e.g. NH3, Me2NH, Et2NH; or neutral, e.g.CH3CONH2, CH3CONHMe, CF3CONH2, CF3CONHMe. The effect of the nature and physical propertiesof these leaving groups on the reaction product and purification must be considered when selecting a silicon-based protecting group.

Stability of Silyl-Protected Functional GroupsThe relative stabilities of the silyl-protected functional groups, for example, alcohols as silyl ethers, is roughly thesame as the relative rates for their introduction. It must be remembered, however, that the stability of the system isdependent upon the specific reaction conditions and, in particular, the pH. For example, phenyl-substituted silylethers are equal or more reactive than their trimethylsilyl counterparts under alkaline conditions, but less reactiveunder acidic conditions. In general terms, however, the relative stabilities of the silyl-protected functional groups willfollow the order of: iPr3Si > ThMe2Si > tBuPh2Si > tBuMe2Si > iPrMe2Si > Et3Si > Ph2MeSi > Me3Si. Again, thisorder is meant to be a general guide as the actual stabilities will depend on the pH of the medium as well as otherreaction conditions. The results of a study of the stability of various silyl ethers as a function of the groups on sili-con and the nature of the alcohol towards various hydrolysis conditions and common organic reagents are shown inTable 1. The reader is referred to the excellent review by Crouch and Nelson on the selective deprotection of silylgroups in synthesis.9

The Trimethylsilyl, TMS, GroupCommon reagents for this reaction are trimethylchlorosilane, TMCS, and hexamethyldisilazane, HMDS,although others are used depending on the leaving group from silicon. The reaction of trimethylchlorosilanewith an alcohol liberates HCl, which must be trapped. This is normally achieved with the presence of a tertiaryamine. Hexamethyldisilazane reacts with alcohols to give ammonia as a byproduct. This can be simply led awayfrom the reaction or trapped. The reactions of HMDS in the absence of a catalyst are oftentimes quite slow.Common catalysts are TMCS, lithium salts, ammonium chloride among others. Trimethylsilylation reagentswhich liberate neutral byproducts are bis(trimethylsilyl)trifluoroacetamide, bis(trimethylsilyl)acetamide and othersas listed in Table 2.

If greater reactivity is necessary one can turn to the more reactive species such as the (dimethylamino)trimethylsi-lane, cyanotrimethylsilane, and trimethylsilyl triflate.

Deprotection of the TMS group is usually accomplished by mild hydrolysis in aqueous or alcohol medium.Trimethylsilyl ethers can be deprotected in the presence of TES ethers.11,12

The Triethylsilyl, TES, GroupThe triethylsilyl protecting group is primarily used for the protection of alcohols, although other functional groupscan be and have been protected as their TES derivatives. The stability of the triethylsilyl ethers falls between thoseof the corresponding TMS and TBS ethers. The most common reagent for the introduction of the TES group istriethylchlorosilane and the methods used for the reaction are those that are employed for TMCS. TES ethers canbe selectively removed in the presence of TBS ethers.13

The tert-Butyldimethylsilyl, TBS, GroupThe TBS group is used for the protection of alcohols10, amines14, thiols14, lactams14, and carboxylic acids14. TheTBS group is commonly introduced via tert-butyldimethylchlorosilane, although other, more-reactive derivatives,such as the triflate, can be used. Because of its steric bulk the reaction of an alcohol with tert-butyldimethyl-chlorosilane requires the presence of imidazole as a catalyst.10 The high stability of TBS-protected groups to avariety of reaction conditions, its clean NMR characteristics, and its ease of removal with fluoride ion makes it apopular choice among the silicon-based blocking agents. TBS ethers can be removed in the presence of TIPS,15

and TBDPS,16 ethers.

The Triisopropylsilyl, TIPS, GroupThe TIPS group is one of the most sterically hindered silyl protecting groups, being removed only slowly understandard acid- or base-catalyzed hydrolysis conditions. TIPS ethers show a stability greater than TBS ethers.Reaction with tetrabutylammonium fluoride is a common method for TIPS desilylations.17 The TIPS group isusually introduced from triisopropylchlorosilane, but triisopropylsilane15 or the triflate16 can also be used.

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The Thexyldimethylsilyl, TDS, GroupThe TDS group was proposed as a silyl blocking agent that would have more stability than the TBS group andwhose reagent, TDS-Cl, would be less expensive to produce commercially.18 TDS-Cl is a liquid making it some-what easier to handle this air-sensitive material than the corresponding solid TBS-Cl. The TDS group providesprotection for alcohols, amines, carboxylic acids, and thiols. In the even of low reactivity the more reactive triflate derivative, TDS-OTf, can be used. Reaction of TDS-Cl with alcohols is carried out under conditions likethose for the TBS-Cl silylations.

The tert-Butyldiphenylsilyl, TBDPS, GroupThe TBDPS group has greater steric demands than the TBS group and, therefore, tends to form more stable silylated species than their corresponding TBS analogs.19 Alcohols, amines,20 amides,21 hydroxamic acids,22 andcarboxylic acids23 are among the groups that have been blocked by the TBDPS group. Deprotection is usuallythrough the reaction with tetrabutylammonium fluoride, TBAF.

Phenyl-Substituted Silyl GroupsPhenyldimethylsilyl, diphenylmethylsilyl, and triphenylsilyl groups are also used to protect alcohols. While thesereagents bring no special steric advantages to the blocking agent picture they do bring an electronic one.Phenylsilyl ethers are more stable towards acid with an increase in stability following an increase in phenyl substitu-tion.24 A similar stability towards alkaline hydrolysis is not seen. The stability of the diphenylmethylsilyl ethers hasbeen carefully studied.25 Introduction of the phenyl-substituted silyl groups is through their respective chlorides.

Other Trialkylsilyl GroupsIn principle any trialkylsilyl group can be considered a silyl blocking agent, though not all are, of course, used inthat manner. The more notable ones can be found in the Tables along with pertinent information concerningreactivity and stability.

Bridging Blocking AgentsThe commonly employed silicon-based agents for the protection of diols and related species such as diamines andhydroxyacids are shown below.

The use of various silylene units such as the dimethyl-(DMS)26, diethyl-(DES)27, diisopropyl-(DIPS)28, di-tert-butyl-(DTBS)28,29 and diphenylsilylene(DPS)30 groups have been employed for the protection of diols, hydroxyacids,diamines, and similar difunctional systems. Here again, as expected, the more hindered the silicon center themore stable the silylated species becomes. Thus, the DTBS ethers of diols are hydrolytically stable between pH 4and pH 10. The 6- and 7-membered ring systems from 1,3- and 1,4-diols, respectively, are more stable than the

Si

Me Me

Si

Et Et

Si

Ph Ph

Si

Si

SiSi

O

Si Si

Me

Me

Me

Me

Me

Me

Me

Me

DMS DES DPSDIPS

DTBS STABASE TMDS

Si

Si

BSBTIPDS

i Pr

Si

O

Si

i Pr

i Pri Pr

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5-membered rings that result from the reaction of 1,2-diols. The disilylethane derivative, tetramethyldisi-lylethane (STABASE) is used for the protection of primary amines, including those of the esters of aminoacids.31

The tetraisopropyldisiloxanyl (from TIPDS) unit is highly useful for the protection of the 3’,5’-dihydroxy unitsof nucleosides.32 The benzostabase group can be used to protect primary aliphatic and aromatic amines.33,34

Special Silicon-Based Blocking AgentsDiisopropylchlorosilane has been shown to provide both protective and stereoselective reactive properties in the sily-lation/reduction of β-hydroxyketones.35 The chloromethyldimethylchlorosilane has been very effectively usedin the analytical arena taking advantage of the greater electron capture detection of the chlorine atom.36,37

Suitably substituted alcohols can be reacted with bromomethyldimethylchlorosilane to silylate the hydroxyl,followed by a radical cyclization and functionalization of the silicon-carbon bond to provide diols, or α-methyl alcohols.38, 39 The (pentafluorophenyl)dimethylsilyl group has been successfully used in the derivatiza-tion of hydroxysteroids, which can be detected at femtogram levels.40 Carboxylic acids are protected as theirtrimethylsilylethyl esters with the advantage of being readily cleaved with the elimination of ethylene.41

DeprotectionThe clean deprotection or desilylation of a silyl-protected functional group is essential to its utility. The relative sta-bility correlation of trisubstituted silyl ethers towards hydrolysis under acid catalysis is TMS ≅ DMPS ≅ MDPS <TES ≅ DMIPS < TPS < TBS < TDS < TIPS < TBDPS < DTBMS.42 The relative stabilities towards hydrolysis inbase is TMS ≅ DMPS ≅ MDPS < DMIPS ≅ TES < TBDPS ≅ TBS < TDS < TIPS < DTBMS.42 Denmark andcoworkers25 have looked at the steric and electronic effects on the desilylation of silyl ethers in acid and base. In addition the long-range structural effects on the desilylation of silyl ethers has been investigated.43

reagentsHCl - KF - CH3MgBr n-Butyl LAH - Pyridinium

Blocking group Substrate THF methanol in ether lithium THF Chlorochromate

n-butanol <15 min 2 min 48 hr 2 hr 30 min <30 min(CH3)3Si- cyclohexanol <15 min 2 min >48 hr 3 hr 1 hr <30 min

t-butanol <15 min 24 hr >48 hr 50 hr 24 hr <30 min

n-butanol <15 min 2 hr no reaction 24 hr 1 hr <30 min(C2H5)3Si- cyclohexanol <15 min 20 hr no reaction >48 hr 2 hr <30 min

t-butanol <15 min no reaction no reaction no reaction no reaction 1 hr

cyclohexylMe2Si- cyclohexanol < 15 min 10 hr no reaction 36 hr 2 hr <30 min

iPr(CH3)2Si- cyclohexanol 10-30 min 24-30 hr no reaction >60 hr 3 hr <30 min

n-butanol >3 hr no reaction no reaction no reaction 25 hr 10 hrtBuMe2Si- cyclohexanol >3 hr no reaction no reaction no reaction >50 hr >20 hr

t-butanol no reaction no reaction no reaction no reaction no reaction >20 hr

n-butanol 16 hr no reaction no reaction no reaction >30 hr 22 hrtHexylMe2Si- cyclohexanol 30 hr no reaction no reaction no reaction no reaction 50 hr

t-butanol no reaction no reaction no reaction no reaction no reaction no reaction

iPr3Si- cyclohexanol no reaction no reaction no reaction no reaction no reaction >72 hr

n-butanol no reaction 100 hr no reaction no reaction no reaction no reactiontBuPh2Si- cyclohexanol no reaction no reaction no reaction no reaction no reaction no reaction

t-butanol no reaction no reaction no reaction no reaction no reaction no reaction

Table 1

RESISTANCE OF SILYLATED ALCOHOLS TO CHEMICAL TRANSFORMATIONSt1/2 for Si-OR bond scission at room temperature

101



CF3C

O

NSi(CH3)3

CH3

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TRIMETHYLSILYL BLOCKING AGENTSTable 2

COMMENTS

SIT8510.0 Reacts in presence of HCl acceptor.6 WillTrimethylchorosilane, TMCS silylate strong acids with expulsion of HCl.44

[75-77-4] High purity grade available, SIT8510.1Protects hindered alcohols w/ Mg/DMF.45

SIT8430.0 More reactive than SIT8510.0 Less reactive,Trimethylbromosilane, TMBS but more Less reactive, photolytically stable than [2857-97-8] SIT8564.0.46

SIT8564.0 Extremely reactive silylating agent.46

Trimethyliodosilane, TMIS Used with HMDS for hindered alcohols.47

[16029-98-4] Forms enol silyl ethers with ketones and SIT8620.0.48

SIT8620.0 Strong silylating agent for C- or O-silylations.48,49

Trimethylsilyltrifluoro- Reacts w/ nitroalkanes to give N, N-bismethanesulfonate TMSOTf (trimethylsiloxy)enamines.50,51

[27607-77-8] These are useful reagents.52

SIT8585.0 Releases toxic HCN upon reaction. Extremely Trimethysilyl Cyanide reactive silylating agent for acids and alcohols.TMSCN Amines and thiols react more slowly than acids [7677-24-9] and alcohols. Does not react with amides, ureas

or carbonates.27 Silylates amino acids.53

SIH6110.0 Releases ammonia upon reaction. Both trimethylsilylHexamethyldisilazane, HMDS groups used. Silylations catalyzed by SIT8510.0[999-97-3] and other reagents.6

SID3605.0 Similar to SID6110.0 and SID3398.0 LiberatesDimethylaminotrimethylsilane, Me2NH upon reaction. Liberates Me2NHTMSDMA upon reaction. Silylates urea-formaldehyde[2083-91-2] polycondensates.54 Silylates phosphorous acids.55

SID3398.0 Releases diethylamine upon reaction. ModeratelyDiethylaminotrimethylsilane, strong silylating agent. Selectively silylates TMSDMEA equatorial over axial hydroxyls.56

[996-50-9]

SIB1846.0 More reactive than SIH6110.0. Releases neutralBis(Trimethylsilyl)acetamide, acetamide upon re-action. Both silyl groups used.BSA Used for silylation in analytical applications.57

[10416-59-8] Reactions catalyzed by acid. Forms enol silyl ethers in ionic liquids.58

SIB1876.0 More reactive than BSA (SIB1846.0).Bis(Trimethylsilyl)- Commonly used for analytical purposes.trifluoroacetamide, Reacts very well in DMF or acetonitrile.57

BSTFA[25561-30-2]

SIB1878.0 By-product is urea.Bis(Trimethylsilyl)urea, Used for alcohols and acids.59

BSU Used in synthesis of penicillins and [18297-63-7] cephalosporins.60

SIM6576.0 Silylation reagent similar to SIB1846.0,N-METHYL-N-TRIMETHYLSILYL- but with liquid, volatile byproductTRIFLUOROACETAMIDE MSTFA[24589-78-4]

CF3

O

N

SiMe3

SiMe3

O

HN

HN

SiMe3

SiMe3

Si

Me

Me

Me Cl

Si

Me

Me

Me Br

Si

Me

Me

Me I

Si

Me

Me

Me O3SCF3

Si

Me

Me

Me CN

NH

Me3Si SiMe3

Si

Me

Me

Me NMe2

Si

Me

Me

Me NEt2

Me

O

N

SiMe3

SiMe3

Table 2TRIMETHYLSILYL BLOCKING AGENTS

102



TRIMETHYLSILYL BLOCKING AGENTSTable 2 (continued)

COMMENTS

SIT8590.0 Powerful silylating agent for alcohols. Trimethylsilylimidazole, TMSIM Does not react with aliphatic amines.6

[7449-74-3]

SIT8572.0 Reacts with 1°, 2° and 3° alcohols.63

2-Trimethylsiloxypent- 2-en-4-one[13257-81-3]

SII6460.0 By-product is acetone.Good for alcohols Isopropenoxytrimethylsilane, and acids.64 Provides acetonides with diols.65

IPOTMS[1833-53-0]

SIM6496.0 Used for silylation of acids, alcohols, thiols, 1-Methoxy-1-trimethysiloxy- amides and ketones.66,67

2-methyl-1-propene[31469-15-5]

SIM6571.5 Used for 3° alcohols and enolizable ketones.68,69

Methyl Trimethylsilylacetate[2916-76-9]

SIE4901.6 Silylation of ketones, alcohols, acetylenes, Ethyl Trimethylsilylacetate thiols under the influence of fluoride ion.70-73

[4071-88-9]

SIA0555.0 Neutral propylene by-product. Acid-catalyzedAllytrimethylsilane silylations. Used for acids,74 and thiols75.[762-72-1] Employed in the synthesis of

N-trimethylsilylpyridinium triflate an activetrimethylsilylating agent.76

SIT8588.52-(Trimethylsilyl)ethoxymethyl chloride, 95%[76513-69-4]

SIT8580.0Trimethylsilyl Azide[4648-54-8]

TRIALKYLSILYL BLOCKING AGENTS

COMMENTS

SIT8250.0 Stability of ethers intermediate between TMS Triethylchlorosilane and TBS ethers.77 Good for 1°, 2°, 3° alcohols.[994-30-9] Can be cleaved in presence of TBS,

TIPS and TBDPS ethers.9

SID3603.0 Very reactive triethylsilyl protecting group.N,N-Dimethylaminotriethylsilane Dimethylamine by-product produced.[3550-35-4]

SIT8335.0 More reactive than SIT8250.0 Useful for more Triethylsilyl- hindered alcohols.78

trifluoromethanesulfonate[79271-56-0]

SII6462.0 Ethers comparable in stability to those of TES-Isopropyldimethyl- protected ethers.79

chlorosilane[3634-56-8]

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7

Si

Et

Et

Et Cl

Si

Et

Et

Et O3SCF3

Si

Me

Me

i-Pr Cl

N

N SiMe3

OEt

Me3Si

O

Me3Si

O OSiMe3

OMe3Si

Me

Me OSiMe3

OMe

OMe

Me3Si

O

Table 2 (continued)

TRIMETHYLSILYL BLOCKING AGENTS

Table 3TRIALKYLSILYL BLOCKING AGENTS

Si

Et

Et

Et NMe2

103




Table 3 (continued) COMMENTS

COMMENTS

SIB1935.0 and Excellent for 1° and 2° alcohols. Silylation catalyzedSIB1935.2 by imidazole. Stable to many reagents.10

tert-Butyldimethylchlorosilane Can be selectively cleaved in presence of acetate,[18162-48-6] THP and benzyl ethers80 among others.81

SIB1935.4tert-Butyldimethylchlorosilane,1M in THF[18162-48-6]

SIB1935.5tert-Butyldimethylchlorosilane,2.85M in toluene[18162-48-6]

SIB1967.0 More reactive than SIB1935.082 Converts acetatestert-Butyldimethylsilyltri- to TBS ethers.83

fluoromethanesulfonate[69739-34-0]

SIB1966.0 Employed in silylations for analytical purposes.84,85

N-(tert-Butyldimethylsilyl)-N- purposes.84,85

trifluoroacetamide[77377-52-7]

SIT8384.0 Ethers more stable than TBS ethers.15 ProtectsTriisopropylchlorosilane carboxylic acids.86 Used in synthesis of[13154-24-0] free-4-hydroxylhexopyranoses.87

SIT8387.0 More reactive than SIT8384.017 Used to make Tsoc, Triisopropylsilyl- (triisopropylsilyloxycarbonyl) protecting groups fortrifluoromethanesulfonate amines.88

[80522-42-5]

SIT8385.0 Silylates strong acids with loss of hydrogen.17

Triisopropylsilane Silylates 1° alcohols selectively.89

[6485-79-6]

SIA0535.0 Reaction w/ triflic acid and then pyridine givesAllyltriisopropylsilane the active triisopropylsilylating agent, [24400-84-8] triisopropylsilylpyridinium triflate.76

SIT7906.0 Ethers show stability similar to or greater than the Thexyldimethylchlorosilane TBS ethers.Used for 1° and 2° amines.18,90

[67373-56-2] Selective for 1° alcohols.90

SIB1933.5. Blocking agent w/ potential stabilitytert-Butyldiethylchlorosilane approaching that of TIPS.[107149-55-3]

PHENYL-CONTAINING BLOCKING AGENTSTable 4

COMMENTS

SIB1968.0 Forms more stable ethers than TBS ethers.91

tert-Butyldiphenylchloro- Used to protect phenols,92 carboxylic acids,20

silane amines,93 and amides.94,95

[58479-61-1]

SIP6728.0 Used in analytical procedures.96

Phenyldimethylchlorosilane[768-33-2]

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Si

Ph

Ph

Me3C Cl

Si

Me

Me

Ph Cl

Si

Me

Me

Me C Cl3

Si

Me

Me

Me3C O3SCF3

Si

Me

Me

Me3C

NH

CF3

O

Si

i-Pr

i-Pr

i-Pr Cl

Si

i-Pr

i-Pr

i-Pr O3SCF3

Si

i-Pr

i-Pr

i-Pr H

Si

Me

Me

Cl

Me

Me

Me

Me

Si

Me

Me

Me C Cl3

Si

i-Pr

i-Pr

i-Pr

Si

Et

Et

Me C Cl3

Si

Me

Me

Me3C Cl

Table 3 (continued)


Table 4PHENYL-CONTAINING BLOCKING AGENTS

104



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9

PHENYL-CONTAINING BLOCKING AGENTSTable 4 (continued)

COMMENTS

SID4586.0 Similar to SIP6728.0. Emits ammonia upon reaction.Diphenyltetramethyl- Used for silylation of capillary columns.97

disilazane[3449-26-1]

SIP6729.0 Reacts with alcohols in presence of Wilkinson’sPhenyldimethylsilane catalyst.98

[766-77-8]

SID4552.0 Stability versus other silyl ethers studied.25

Diphenylmethylchloro- studied.25

silane[144-79-6]

SIT8645.0 Ethers hydrolyze comparably to TMS ethers in Triphenylchlorosilane base and 4 times slower in acid.99 Can lead to[76-86-8] solid products.100

SIA0575.0 Reaction w/triflic acid and then pyridine givesAllyltriphenylsilane triphenylsilylpyridinium triflate, an active[18752-21-1] triphenylsilylating agent.76

SID4552.5 More reactive than SID4552.0. LiberatesDiphenylmethyl dimethylamine upon reaction. (dimethylamino)silane[68733-63-1]

SIB1026.4 Reactivity and stability similar to that of SID4586.01,3-Bis(4-biphenyl)-1,1,3,3-tetramethyldisilazane

SPECIALTY SILICON-BASED BLOCKING AGENTSTable 5

COMMENTS

SIP6716.1 Named Flophemesyl group. Ethers detectable at Pentafluorophenyldi- femtogram level by ECD. Forms excellent derivativesmethylchlorosilane for mass spectral analysis.102,103

[20082-71-7]

SIT8589.2 Used for protection of acids.104,105

Trimethylsilylethanol[2916-68-9]

SIC2285.0 Can form cyclic products with appropriate Chloromethyldimethyl- 1,2-difunctional substrates.36 Used in analytical chlorosilane applications for greater ECD detectability.37

[1719-57-9]

SIB1890.0 Has been applied to the synthesis of diols and Bromomethyldimethyl- β-methyl alcohols.38,39

chlorosilane[16532-02-8]

SID3535.0 Silylates and reduces β-hydroxyDiisopropylchlorosilane ketones selectively.35

[2227-29-4] Photochemically removable365

Si

Me

Ph

Me H

Si

Me

Ph

Ph Cl

Si

Ph

Ph

Ph Cl

Si

Ph

Ph

Ph

Si

Me

Ph

Ph NMe 2

p-Ph-Ph

Si

HN

Si

Ph-Ph-p

Me Me

MeMe

F F

Si

FF

F

Me

Me

Cl

Me3Si

OH

Si

Me

Me

ClCH2 Cl

Si

Me

Me

BrCH2 Cl

Si

i-Pr

i-Pr

H

Cl

Ph

Si

HN

Si

Ph

Me Me

MeMe

Table 4 (continued)

PHENYL-CONTAINING BLOCKING AGENTS

Table 5SPECIALTY SILICON-BASED BLOCKING AGENTS

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SPECIALTY SILICON-BASED BLOCKING AGENTSTable 5 (continued)

COMMENTS

SIH5840.4 Potential blocking agent for fluorous phase(Heptadecafluoro-1,1,2-2- synthesis.106

tetrahydradecyl)-dimethylchlorosilane[74612-30-9]

SIB1815.5 Potential blocking agent for fluorous phaseBis(tridecafluoro-1,1,2,2- synthesis.106

tetrahydrooctyldimethylsiloxy)-methylchlorosilane

SIB1837.0 Sterically-hindered for theBis(trimethylsiloxy) protection of diolsdichlorosilane[2750-44-9]

BRIDGING SILICON-BASED BLOCKING AGENTSTable 6

COMMENTS

SID4120.0 and Reacts with alcohols,107 diols,108 andSID4120.1 hydroxy carboxylic acids109.Dimethyldichlorosilane Employed as a protecting group/template[75-78-5] in C-glycoside synthesis.110

SIB1072.0 More reactive than SIB4120.0.Bis(Dimethylamino)- Reacted with diols,111 diamines,112 anddimethylsilane treatment for glass113.[3768-58-9]

SIB1068.0 Similar to SIB1072.0.Bis(Diethylamino)-dimethylsilane[4669-59-4]

SIH6102.0 Silylates diols with loss of ammonia. Similar inHexamethylcyclo- reactivity to HMDS.114,115

trisilazane[1009-93-4]

SID4510.1 Reacts with alcohols,116 diols,111 2- hydroxybenzoicDiphenyldichloro- acids117.silane[80-10-4]

SID3402.0 Similar to, but more stable derivatives than Diethyldichlorosilane dimethylsilylenes.[996-50-9]

SID3537.0 Forms tethered silyl ethers from diols.118-120

Diisopropyldichloro- Protects 3’,5’ hydroxyls of nucleosides, but less silane effectively than SIT7273.0121

[7751-38-4]

Si

Me

Me

Cl

Cl

Si

Me

Me

NMe2

NMe2

Si

Me

Me

NEt2

NEt2

HN

Me2Si

NH

SiMe2

NH

Me2Si

Si

Ph

Ph

Cl

Cl

Si

Et

Et

Cl

Cl

Si

i-Pr

i-Pr

Cl

Cl

CF3(CF ) CH CH SiMe Cl 2 7 2 2 2

O

Si Si

Me

ClMeMe

C6 13Fn

2

Table 5 (continued)

SPECIALTY SILICON-BASED BLOCKING AGENTS

Table 6BRIDGING SILICON-BASED BLOCKING AGENTS

106



��


11

BRIDGING SILICON-BASED BLOCKING AGENTSTable 6 (continued)

COMMENTS

SID3205.0 Used to protect 1,2-diols,29 and 1,3- diols122.Di-tert-butyldichlorosilane Forms 4,6-cyclic di-tert-butylsilylenediyl ethers [18395-90-9] w/glycopyranosides.123

SID3345.0 More reactive than SID3205.0.28 ConvertsDi-tert-butylsilylbis(tri- 1,3-diols to cyclic protected 1,3-diols.124

fluoromethanesulfonate) Reacts w/1,3-diols in preference to 1,2-diols.125

[85272-31-7]

SID3534.0 More reactive than SID3345.0.Di-isopropylbis(tri- Protects diols366

fluoromethanesulfonyl) silane[85272-30-6]

SIB1042.0 Protection for 1° amines,126,127 including aminoBis(dimethylchlorosilyl)- acid esters128.ethane[13528-93-3]

SIT7273.0 Highly useful for protection of 3’,5’-Tetraisopropyldichloro- dihydroxynucleosides.32

disiloxane Protects 1,2-diequatorial diols.129

[69304-37-6]

SIB1084.0 Used to protect anilines,34 amines,35 and1,2-Bis(dimethylsilyl)- amino acids130.benzene[17985-72-7]

SID1048.2 Potentially useful silylating agent for diols and 1,3-Bis(chlorodimethylsilyl)- related systems.propane[2295-06-9]

SIT7087.0 Used in the protection of 3’,5’-1,1,3,3-Tetracyclopentyl- dihydroxynucleosides.dichlorodisiloxane

www.gelest.com

Si

Me3C

Me3C

O3SCF3

O3SCF3

SiSi

Cl Cl

Me

Me

Me

Me

i-Pr

Si

O

Si

i-Pr

Cl Cl

i-Pri-Pr

Si

Si

H

MeMe

H

MeMe

Me2Si SiMe2

Cl Cl

Si

O

Si

Cl Cl

Si

Me3C

Me3C

Cl

Cl

Table 6 (continued)

BRIDGING SILICON-BASED BLOCKING AGENTS

107



�� Gelest, Inc.


Table 7

DEPROTECTION OF 1° SILYL ETHERS IN THE PRESENCE OF OTHER 1° SILYL ETHERS9

Deprotection of: In the Presence of:

1° TMS 1° TES 1° TIPS 1° TBS 1° TBDPS

1° TMS NaHCO3367 NaOH/EtOH132

MCM-41368

NaOH/EtOH132

Cu(NO3)2133

Ce(NO3)3133

[Bu2(NCS)Sn]2O134

BiCl3369 Bi(O2CCF3)3

369

K2CO3370, MCM-41368

NaOH/EtOH132

Cu(NO3)2133

Ce(NO3)3133

HCl132,135

1° TES TFA371

H2/Pd-C372

MCM-41368

HF/py136

TCNQ/MeCN/H2O137

DDQ/MeCN/H2O137,138

CSA373

IBX/DMSO374

MCM-41368

H2/Pd-C372

SiF4/CH2Cl2139

DDQ/MeCN/H2O137,138

MCM-41/MeOH140

CSA375,376, H2/Pd-C372,377

MSOTf/HCO2DPM/SiO2378

ZnBr2/H2O379

1° TIPS TMSOTf/HCO2DPM, SiO2378

1° TBS HCl/EtOH132

H2SiF6/tBuOH141,142

NaOH/EtOH15

Cyclohexene/PdO143

Alumina144

H2SO4380

CSA381,382

PPTS383

H2SiF6384

H2/Pd-C372,377

TMSOTf/Et3N/MeOH385

CeCl3•7H2O/NaI386

Decaborane387

TsOH/THF/H2O145

Cl2CHCO2H146,147

TBAF148

TBSOTf149

HOAC388

PPTS389

TBAF390

DDQ391

MnO2/AlCl3392

DMSO/H2O393

H2/Pd-C372

HCl132,150,394,395

HOAc/THF/H2O19,151-154

CSA/MeOH155,156,164

PPTS157,158,399-403, TsOH159

MeOH/CCl4160

Cu(NO3)2133

Ce(NO3)3133

Cyclohexene/PdO143

Alumina145

SiF4/CH2Cl2139

DDQ/MeCN/H2O137,138

AcBr/CH2Cl2161, TMSOTf162

HF-py/THF163

PPTS/EtOH164

H2SO4396, TFA397.398

TsOH404, 405, CSA406-408

LL-ALPS-SO3H409

AcCl/MeOH410, Zn(BF4)2411

Cu(OTf)2/Ac2O412, InCl3

413

CeCl3•7H2O/NaI386

Ce(OTf)4/THF/H2O414

PdCl2(MeCN)2415, I2/MeOH416

Br2/MeOH417, IBr418

LiCl/DMF419, CCl4/MeOH420

CeCl3•7H2O421, ZnBr2/H2O

379

ZrCl4/Ac2O422, HF-pyr423,424

I2/KOH400, H2/Pd-C372,377

Bu4NOH/MeOH425, TBAF426

TMSOTf/HCO2DPM/SiO2378

Decaborane387

1° TBDPS KOH427 TBAF/HOAc166,428,429

TBATB/MeOH165

NaOH429-431

Bu4NOH428

LiAlH4167

HF-py432, TBAF390

108



Table 8

DEPROTECTION OF 1° SILYL ETHERS IN THE PRESENCE OF 2° SILYL ETHERS9



1° TMS Rexyn 101168

K2CO3/MeOH169

Alumina170

HOAc433

Swern conditions434

NaHCO3367

Swern conditions434

BF3OEt2171

1° TES Swern conditions434 HOAc172

TMSOTf/i-Pr2NEt452

HF-pyr367,453

LiOH367

HOAc/H2O/THF172,174

RMgX175, HCl454, HBr/PPh3436

CSA384,451,455, PPTS456, TFA457,458

HF/pyr459,460, HF447, TMSOTf/i-Pr2EtN461, TBAF384, TBAF/HOAc439, KF447, Swern cond.462

SiF4/CH2Cl2139

DDQ/MeCN/H2O137,138

TsOH448

citric acid463

TMSOTf/i-Pr2NEt461

TMSOTf/Et3N/MeOH385

1° TIPS TBAF176 , CSA464

TBAF453, SiF4465

POCl3-DMF466

(TfO)2O-DMF466

CBr4/MeOH467

CAN/SiO2468

TFA/H2O/THF177 TFA/H2O/THF178

CSA469

1° TBS HF-pyr373 HCl15,179,470,471

HOAc 180,181,473-477

TsOH/PPTS182

HF/py183,184

NaOH15

Cyclohexene/PdO143

TsOH185,186

H2SO4472

HOAc87-91

CSA381-383,470,478-481

PPTS482

TsOH483

NH4Cl/MeOH484

HF-pyr479,480,485-487

TBAF488 polymeric DCKA617

TBAF•3H2O384

HCl187,489, HOAc172,490-492

TFA/H2O/THF188,189

CSA/MeOH156, PPTS190

Acid Amberlite191

NH4F192

HF/MeCN193,194

H2SiF6/tBuOH142, HF/py195

H2/wet Pd196, AgOAc197

NBS/DMSO198, TBAF199,200

MeOH/CCl4160, NaOH15

Cyclohexene/PdO201

Alumina141,202

CAN/iPrOH203

POCl3/DMP204

(CF3SO2)2)/DMF204

CSA381-383,401,479,480,493-502

PPTS389,401,496,497,519-544

TsOH404,405,510

TsOH/Bu4NHSO4511

TFA398,512-516, acidic CHCl3517

Cu(OTf)2/Ac2O412, BCl3

518

HF-py479,480,485-545,, HF546,547

H2SiF6548, NH4F

549

TBAF/HOAc452,550,564, TBAF551-555

TAS-F556, CBr4-hν557,558

Jones rgt.523, LiBr/18-C-6559

CAN/i-PrOH560, POCl3-DMF466,561

(TfO)2O-DMF466

H2/Pd-C372, QFC562

Bu4NBr3/MeOH425

MnO2/AlCl3392, Oxone563

CAN/SiO2468

HOAc/H2O/THF205,206

TsOH207-209

PPTS158,210,211

TBAF212

HF/py113

HF/Et3N214

IBr/DCM81

HCl179

HOAc758

CSA407,408,759,760

PPTS761-764

TsOH764-766

HF-pyr696

HF469,767-769

H2SiF6713

TMSOTf/Et3N/MeOH385

Cu(OTf)2/Ac2O412

Zn(OTf)2411

K2CO3466

NaOH752

TBTU770, QFC polymeric DCKA618

InCl3413

1° TBDPS TBAF/HOAc565,566

NaOH/DMPU567

NaH/HMPA215

HF-pyr568

KOH/18-C-6569

NaH/HMPA218

NaOH217

KOH/MeOH218

TBAF199,219,220,524,529,576

NH4F193,570,571

NaOH/DMPU567

KOH/18-C-6569

TBAF/HOAc428,519,526,565, 568,566,572-575

TAS-F577

NaH/propargyl alcohol395

HF/py223

alumina144

CSA578

HF-py432,579

NH4F570

TBAF469,580

POCl3-DMF466

(TfO)2O-DMF466

�� Gelest, Inc.

109



Table 9




1° TMS PPTS224

1° TES HF/py235,479,525,533,587

CSA376,586

PPTS435

Amberlyst-15583

TBAF/HOAc440

HOAc771

1° TIPS TBAF235 SiF4224

1° TBS HF584,585 CSA376,581,586

HF-pyr479,525,533,587

TBAF227,543,772

TBAF/HOAc588

CSA219,228 407,408,773

NH4F41

H2SiF6142

SiF4/CH2Cl2139

MeOH/CCl4160HF-py544

oxone563

1° TBDPS TBAF/HOAc566 TAS-F774

Table 10



1° TMS 1° TES 1° TIPS 1° TBS 1° TBDPS2° TMS Amberlyst 15168 TBAF464 HCl229

K2CO3/MeOH230,231

TsOH589

citric acid548

H2SiF6548

NaOH471

HCl232

TsOH233

Acetone/Me2C(OMe)2/CSA234

PhSeCl/K2CO3235

TBAF236

PPTS237,238

Alumina170

K2CO3/MeOH239

HOAc590, CSA591, TsOH589,592,594

BF3-OEt2590, TMS-OTf531

2° TES HF/py225 TFA177

H2SiF6/HF/H2O240

HCl595

HOAc469

CSA380,451

PPTS596

H2SO4380

Ph3P-HBr380

HF-pyr367,384,453

HF-Et3N597

DDQ/MeCN/H2O138

HCl/py241

NH4Cl242,243

TBAF/THF/NH4Cl(s)242

HF-py370

I2/Ag2CO3598,599

Pd/C, MeOH600

H2, Pd/C372

HCl135,234,520,529,603,604

HOAc245,395

Cl3CCO2H246

TsOH247,382,602-604

HF/MeCN248,249

PhSeCl/K2CO3235

TBAF156,599,607

DDQ/MeCN/H2O138

SiF4/CH2Cl2139

CSA/CHCl3/MeOH250

CSA601, PPTS164,602

H2SO4607, BF3-OEt2

382,602

Et3N-HF608

HF-pyr609,2,4,4,6-tetrabromo-2,5-cyclohexadienone/PPh3

611

H2, Pd/C612

ZnBr2/H2O379

2° TIPS

2° TBS H2SiF6/HF/H2O208

CSA596

DIBAL-H251

MnO2/AlCl3392

HCl15,252

HOAc/H2O/TH253,254

TsOH255,256

PPTS158,257,432,613

BF3OEt2258

TMSOTf162

HF/MeCN188

TBAF155

SiF4/CH2Cl2139

CSA/MeOH164

H2SiF6569, TMS-OTf/Et3N

385

TMS-OTf432,614, Cu(OTf)2/Ac2O412

InCl3413, LiAlH4

615,616, IBr775

P2O5/(MeO)2CH2405, LiCl/DMF419

polymeric DCKA618

ZnBr2/H2O379, Zn(BF4)2

411

2° TBDPS NaH/HMPA259

TBATB/MeOH165

110



�� Gelest, Inc.

15

Table 11

DEPROTECTION OF 2° SILYL ETHERS IN THE PRESENCE OF OTHER 2° SILYL ETHERS9



2° TMS TBAF260

SiO2-Cl/ NaI776

HF-pyr533,582

TBAF446

KF567

Citric Acid/MeOH261

TBAF/HOAc262,263

FeCl3264

TBAF464

KF447

NaOH471

H2SiF6548

HCl265

TsOH266,267

HOAc/H2O/THF268

PPTS269,622

HF/Et3N270

Citric Acid/MeOH271,272

Cu(NO3)2133

Ce(NO3)3133

DDQ/wet EtOAc273

K2CO3/MeOH239

HOAc620,621

TsOH594

CSA623

HF-pyr533,582 HF/Et3N624

BF3-OEt2625,626

K2CO3627-629

TBAF630, KF447,567

TBAF274

Cu(NO3)2133

Ce(NO3)3133

K2CO3628,629

NaIO4631

2° TES HOAc13,275

TBAF276,436,498

TsOH632

TFA633,634

HF-py533,579

TBAF/ HOAc439

(NH4)HF2635

KF447

NaOH/DMPU567

TFA177,277,278

HOAc279-281

PPTS11,479,636,

MoO5/HMPA272,283

WO5/HMPA282,283

HF-py/py-THF284

H2SO4380 TFA637

HF-py367,638,639

Zn(OTf)2/EtSH656

NH4F637

Amberlyst-15641

HOAc/H2O/THF278

TFA177,277,278,633,634,652,

PPTS11,285,286,613,623,646,651,683

HF/MeCN287

HF/py287,288,384,437,454,533,609,582,606,653,654

DDQ/MeCN/H2O138

MoO5/HMPA282,283

WO5/HMPA282,283

PTSA/MeOH289

TfOH/H2O-THF290

TBAF/THF291

PPTS/MePH292

MCM-41/MeOH140

HCl642,643, HOAc569,644,645

CSA373,596

TsOH460

HF-Et3N435,655

HF781

Zn(OTf)2/EtSH656, TiCl3(O-iPr)654

TBAF/HOAc439,658

TBAF395,436,498,659,660, KF447

NaOH/DMPU567MCM-41368

PdCl2/CuCl/H2O661

HOAc/H2O/THF293

CSA294

TBAF155,295,665,666

SiF4/CH2Cl2139

DDQ/MeCN/H2O138

HF/MeCN287

HF/py286

HCl662

HOAc469,663

HF664

K2CO3667

NaIO4631

2° TIPS HF536,674 TBAF297,668,669

PTSA/MeOH298

TsOH480

HF/Et3N639, LiAlH4

670

NaIO4631

2° TBS HCl299 TBAF654 HCl15,671,672

H2SiF6(aq)142

HOAc280,281

HF/MeCN188

TBAF263,300,383

CSA479,673

HF536,674

Et3N-3HF675

H2SiF6/Et3N676

HCl15

CSA302,439,677

TsOH303

HF/MeCN188,304-306

LiAlH4167

KF/H2O307

TBAF274,308-311,499,619,534,535,616,651,683-

688,777

H2SO4678, HF-py679,680

HF681

BF3-OEt2682

P2O5/(MeO)2CH2405

MnO2/AlCl3392

TsOH312-314

HCO2H/THF/H2O315

CSA316

PPTS317,318,694,695

HF/MeCN319

TBAF200,320,321

SiF4/CH2Cl2139

Cu(NO3)2133

Ce(NO3)3133

DDQ/MeCN/H2O138

HCl689,690

HOAc691-693

HF-pyr696,697

TMS-OTf432,614

BF3-OEt2698

Sc(OTf)3699

NaIO4631

2° TBDPS TBAF/THF179

111



Table 12




2° TMS TsOH267,322 HOAc268

CSA700

KF447,567

HF-py533,582

2° TES TsOH323

HF-pyr531,579

HCl642,643, PPTS701

HF-py384,533,582

TBAF436,702

HOAc324

CSA199

HCl440

TfOH703

2° TIPS

2° TBS H2SiF6/tBuOH/H2O

142 HOAc325

CSA326,327

TBAF328,329

TfOH703

HF704

2° TBDPS

Table 13




3° TMS PPTS/MeOH330

PPTS778

HCl595

ClCH2CO2H/MeOH146

HCl705

BH3-SMe2706

HCl398

HOAc771

BF3-OEt2707,708, K2CO3

398

3° TES HF-Et3N597

TBAF709

SiO2331

TBAF/ NH4Cl709

TBAF709

3° TIPS

3° TBS LiAlH4326,332

3° TBDPS

Table 14




3° TMS TBAF/HOAc263,711 HCl333,398,705

LiAlH4334

TBAF335

FeCl3336

HF681 , BH3-THF712

TBAF/HOAc711

K2CO3398

HCl224

H2SiF6713,714

K2CO3715

3° TES TBAF/HOAc711 Et3NHF337,338

HF-Et3N435,655

TBAF/HOAc711

3° TIPS

3° TBS TBAF/HOAc711 TBAF324

TBAF/HOAc711

3° TBDPS

�� Gelest, Inc.

112



�� Gelest, Inc.


17

Table 15




3° TMS HCl/THF339

3° TES

3° TIPS

3° TBS CSA340

3° TBDPS

Table 16

DEPROTECTION OF PHENOLIC SILYL ETHERS IN THE PRESENCE OF ALKYL SILYL ETHERS9


1° TMS 1° TBS 2° TBS 3° TBS 1° TBDPS 2° TBDPS

ArOTMS Dowex 1-X8 (HO- form)341

TBAF342,343

BiCl3369

Bi(O2CCF3)3369

Bi(OTf)3369

PIFA-MK10717

TBAF344

SiO2718

ArOTES NaOH132

ArOTBS NaOH132

K2CO3/Kryptofi x 222/MeCN345

TBAF346,347,429,717,719-721

KF/18-crown-6347

KF/Al2O3/MeCN348

KF-Al2O3722

CsF/RX/DMF728

K2CO3724, CsCO3

725

Et3NO726, LiOH727

NaOH/ TBAH728

KOH729

LiOH/RX/ DMF730

TMG731

PIFA-MK10717

DMSO/H2O393

K2CO3/Kryptofi x 222/MeCN345

TBAF344,346

CSA732

TBAF719,733,734

K2CO3724

CsF/CH3CN735

Et3NO726

NaOH/TBAH728

KOH736

LiOH/RX/DMF730

KF-Al2O3737

10% HCl349 TBATB/MeOH165

Zn(BH4)2411

TBAF750,751

NaOH/TBAH728

ArOTIPS PIFA-MK10717 NaOH754 TBAF753

NaOH754

ArOTBDPS NaOH132

PIFA-MK10717

TMG731 TMG731

113



�� Gelest, Inc.

Table 17

DEPROTECTION OF ALKYL SILYL ETHERS IN THE PRESENCE OF PHENOLIC SILYL ETHERS9


ArOTMS ArOTES ArOTBS ArOTIPS ArOTBDPS

1° TMS Dowex CCR-2 (H+

form)341

HCl132 HCl132

Amberlite IR-120 (H+

form)351

HCl132

2° TMS Amberlite IR-120 (H+

form)351

1° TES Pd/C/MeOH600 ZnBr2/H2O379 ZnBr2/H2O

379 ZnBr2/H2O379

2° TES PPTS738

BiBr3/Et3SiH779

ZnBr2/H2O379

ZnBr2/H2O379 ZnBr2/H2O

379

1° TBS HCl132,352

TFA353, PPTS345,735

HF/MeCN345,354,355

BF3OEt2345,356

SiF4/CH2Cl2139

MeOH/CCl4160

TMSCl/H2O/CH3CN357

Oxone/MeOHaq358,563

Nafi on-H/NaI739

LL-ALPS-SO3H409

AcCl/MeOH410

TMS-Cl/H2O740

TMS-Cl/NaI/H2O740

Me2SBr2739

TBSOTf/THPOAc741

BiBr3/H2O/MeCN742

BiCl3/NaI743

CeCl3•7H2O727

CuOTf, Ac2O412

Sc(OTf)3/H2O699

ZnBr2/H2O379, InCl3

413

ZrCl4/Ac2O422

decaborane387

Ce(OTf)4/THF/H2O414

CBr4/MeOH467

I2/MeOH744

CAN/SiO2468

Oxone/MeOH563

H2/Pd-C780

HCl132

Amberlite IR-120 (H+

form)351

HF-pyr746

BF3-OEt2747

ZnBr2/H2O379

ZrCl4/Ac2O422

HCl132,476

ZnBr2/H2O379

InCl3413

BiOClO4745

2° TBS TFA359,749

BF3OEt2345,356

HF/MeCN346

PPTS345

SiF4/CH2Cl2139

Nafi on-H/NaI739

BiBr3/H2O/ MeCN742

BiCl3/NaI743

CeCl3•7H2O727

ZnBr2/H2O379

InCl3413

Me2SBr2739

TFA749

ZnBr2/H2O379

ZnBr2/H2O379

1° TIPS HCl132

CBr4/MeOH467

I2/MeOH744

HCl132

Sc(OTf)3/H2O696

CBr4/MeOH467

I2/MeOH744

1°TBDPS HCl132 HCl132

Sc(OTf)3/H2O699

CBr4/MeOH467

I2/MeOH744

114



�� Gelest, Inc.


19

Table 18

DEPROTECTION OF SILYLENE- AND DISILOXANE-PROTECTED DIOLS IN THE PRESENCE OF 2° ALKYL SILYL ETHERS9


2°TMS 2°TES 2°TIPS 2°TBS 2°TBDPS

DTBS silylene HF/py29,122,360,361

KF/MeOH307,362,363

TIPDS silioxane HF/MeCN364

Table 19

DEPROTECTION OF ARYL SILYL ETHERS IN THE PRESENCE OF ANOTHER ARYL SILYL ETHER9


ArOTMS ArOTES AROTBS ArOTIPS ArOTBDPS

ArOTMS

ArOTES

ArOTBS SbCl3754

KF-Al2O3737

CCl4/MeOH755

NaClO2/NaH2PO4/py756

HCl754

ArOTIPS

ArOTBDPS

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GENERIC SILYLATION PROCEDURES

1. Hexamethyldisilazane, SIH6110.0, in the trimethylsilylation of alcohols.

One equivalent of the alcohol to be silylated is mixed with 0.5 equivalents of hexamethyldi-silazane,SIH6110.0, in an inert solvent or without solvent. Warming the reaction to 40 - 50 °C or adding a drop oftrimethylchlorosilane can significantly accelerate the rate of reaction. The reaction is allowed to continue untilno further evidence of the evolution of ammonia is observed. For primary and secondary alcohols the reactionis quite rapid and nearly quantitative. For tertiary alcohols the reaction may be slower.

2. Trimethylchlorosilane, SIT8510.0 and SIT8510.1, in the trimethylsilylation of alcohols.

One equivalent of the alcohol, 1.1 equivalent of pyridine or triethylamine are mixed in an inert solvent andone equivalent of trimethylchlorosilane, SIT8510.0 or SIT8510.1, is added. The amine can also be used asthe solvent of the reaction. The reaction can be followed by any of the standard techniques including thinlayer and gas chromatography. The reaction is quite fast with primary and secondary alcohols and slower withtertiary alcohols. The trimethylsilylation of amides and amines can be accomplished by a modification of thisprocedure wherein the reaction mixture is heated to reflux for 16 h.

3. Trimethylbromosilane, SIT8430.0, or Trimethyliodosilane, SIT8564.0, in the trimethylsilyla-tion of alcohols.

One equivalent of the alcohol, 1.1 equivalent of a suitable amine base are mixed in an inert solvent and thetrimethylbromosilane, SIT8430.0, or trimethyliodosilane, SIT8564.0, is added. These reagents are more reactive towards hindered alcohols than the trimethylchlorosilane.

4. Trimethyliodosilane, SIT8564.0, Hexamethyldisilazane, SIH6110.0 combination in thetrimethylsilylation of hindered alcohols.

The alcohol (1 equiv.), trimethyliodosilane, SIT8564.0, (2.2 equiv.) and hexamethyldisilazane (1.1 equiv.) aremixed in pyridine as solvent and the reaction mixture is stirred at room temperature.

5. Trimethylsilyltrifluoromethanesulfonate, SIT8620.0, or Trimethylsilyl Cyanide, SIT8585.1, inthe trimethylsilylation of hindered alcohols.

Trimethylsilylation with these very reactive silanes is carried out by the simple mixing of the alcohol and thesilane in an inert solvent and allowing the reaction to occur, usually at room temperature. CAUTION:Hydrogen cyanide is generated in the reaction with trimethylsilyl cyanide.

6. Allyltrimethylsilane, SIA0555.0, in the silylation of carboxylic acids.

The acid (1 equiv.) is dissolved in carbon tetrachloride (other solvents such as dichloromethane can probablybe used as well) and allyltrimethylsilane, SIA0555.0, is added. To this reaction mixture is added about 3 drops(for 10 mmol reaction) of trifluoromethanesulfonic acid. The reaction is very fast and is complete when theevolution of propylene ceases.

7. Triethylchlorosilane, SIT8250.0, tert-Butyldimethylchlorosilane, SIB1935.0, tert-Butyldiphenylchlorosilane, SIB1968.0, Triisopropylchlorosilane, SIT8384.0, orThexyldimethylchlorosilane, SIT7906.0, in the silylation of alcohols.

The alcohol (1 equiv.) is dissolved in DMF along with the chlorosilane (1.1 equiv.) and imidazole (2.2 equiv.)or 2,6-lutidine (2.2 equiv.). The reaction is usually heated to about 40 °C for 10 - 20 h for completion.

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18. tert-Butyldimethylsilylation of an alcohol in dichloromethane.

The tert-butyldimethylsilylation of an alcohol has been carried out by treating 0.89 equiv. of tert-butyldimethylchlorosilane in dichloromethane with 0.91 equiv. of the alcohol, 1.19 equiv. of triethylamineand 0.036 equiv. of 4-dimethylaminopyridine at room temperature for several hours.

19. Triisopropylsilyltrifluoromethanesulfonate, SIT8387.0, in the silylation of alcohols.

The alcohol (1 equiv.) is reacted with triisopropylsilyltrifluoromethanesulfonate, SIT8387.0, indichloromethane with 2,6-lutidine (2.2 equiv.) as catalyst. The reaction can be carried out as low as -78 °C in less than 5 h.

10. tert-Butyldimethylsilyltrifluoromethanesulfonate, SIB1967.0, in the silylation of alcohols.

The alcohol (1 equiv.) is treated with tert-butyldimethylsilyltrifluoromethanesulfonate as a 1 M solution in a50:50 by volume mixture of pyridine and dichloromethane. Additional SIB1967.0 may have to be added tofinish the reaction for less reactive alcohols.

11. Di-tert-Butylsilylbis(trifluoromethanesulfonate), SID3345.0, in the silylation of diols.

The diol (1 equiv.) is reacted with di-tert-butylsilylbis(trifluoromethanesulfonate), SID3345.0, (1.2 equiv.)and 2,6-lutidine (3.0 equiv.) in chloroform and allowed to react at 0 to 25 °C. 1,3-Diols and 1,4-diols are,in general, more reactive than 1,2-diols.

12. tert-Butyldiphenylchlorosilane, SIB1968.0, in the silylation of primary amines.

The amine (1 equiv.) is reacted with tert-butyldiphenylchlorosilane, SIB1968.0, and triethylamine (1.5equiv.) in acetonitrile at room temperature for 1-3 h. Secondary amines do not react.

13. 1,2-Bis(chlorodimethylsilyl)ethane, SIB1042.0, in the silylation of primary amines.

The amine (1 equiv.) is reacted with the 1,2-bis(chlorodimethylsilyl)ethane (1 equiv.) and triethylamine (2 equiv.) in dichloromethane at room temperature for 2-3 h. A convenient way to isolate the product is to filter, concentrate, add pentane, filter again and concentrate to give the product in high purity withoutdistillation.

14. 1,1,3,3-tetraisopropyl-1,3-dichlorodisiloxane, SIT7273.0, in the silylation of nucleosides.

The nucleoside (1 equiv.) is reacted with the silane, SIT7273.0, and imidazole (4.4 equiv.) in DMF at roomtemperature. The yields are about 80 percent.

15. 1,2-Bis(dimethylsilyl)benzene, SIB1084.0.

The amine (1 equiv.) is reacted with the silane, SIB1084.0, with a catalytic amount of Wilkinson’s catalyst,tris(triphenylphosphine)rhodium (I) chloride, in toluene solution.

16. Trimethylsilylethanol, SIT8589.2, in the protection of a carboxylic acid.

The acid (1 equiv.) is reacted with 2-trimethylsilylethanol, SIT8589.2, (1 equiv.), dicyclohexylcarbodiimide(1 equiv.) in ethyl acetate with a catalytic amount of DMAP added. A typical reaction is about 12 h at room temperature.

17. Triisopropylchlorosilane, SIT8384.0, in protection of a sulfonic acid.

The acid (1 equiv.) is reacted with triisopropylchlorosilane, SIT8384.0, (1.4 equiv.) and triethylamine (1 equiv.) in dichloromethane for 1 h at -35 °C and 15 h at room temperature.

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18. Triisopropylsilyltrifluoromethansulfonate, SIT8387.0, as precursor to Triisopropylcarbamate,Tsoc, protecting group.

The amine to be protected in the presence of triethylamine is reacted with carbon dioxide (introduced as agas or as dry ice) at -78 °C for 30-60 min. This mixture is then treated with triisopropylsilyltrifluo-romethanesulfonate, SIT8387.0, at room temperature. The reaction mixture can be washed with waterwithout hydrolysis of the protecting group.

19. Formation of acetonides of diols with Isopropenoxytrimethylsilane, SII6460.0.

The diol (1 equiv.) is mixed with isopropenoxytrimethylsilane, SII6460.0, in an inert solvent (THF, ether,toluene) and 1 or 2 drops of concentrated HCl or trimethylchlorosilane, is added. The reaction is completein less than 30 min.

20. Dimethylaminotrimethylsilane, TMSDMA, SID3605.0 and Diethylaminotrimethylsilane,TMSDEA, SID3398.0.

The alcohol or amine (1 equiv.) is reacted with the aminotrimethylsilane and dimethylamine or diethylamineis distilled off as the reaction proceeds. These reagents are particularly useful in the silylation of amines.HMDS, SIH6110.0, is preferred for the trimethylsilylation of alcohols.

An alternative procedure for the silylation with the aminosilanes is with acid catalysis. This is best achievedwith ammonium sulfate, trimethylchlorosilane as well as trichloroacetic acid.

21. Trimethylsilylimidazole, TMSI, SIT8590.0.

Trimethylsilylimidazole, SIT8590.0, is a very reactive trimethylsilylating agent, especially for alcohols. It is typically reacted with an equivalent amount of the hydroxyl group(s) in the presence or absence of an acid catalyst.

22. N,O-Bis(trimethylsilyl)acetamide, BSA, SIB1846.0.

The alcohol (1 equiv.) is reacted with N,O-bistrimethylsilylacetamide, SIB1846.0, (0.5 equiv.) in an inertsolvent. The reaction proceeds faster with a small amount of trimethylchlorosilane as catalyst.

23. N,O-Bis(trimethylsilyl)trifluoroacetamide, BSTFA, SIB1876.0.

The alcohol (1 equiv.) is reacted with N, O-bis(trimethylsilyl)trifluoroacetamide, SIB1876.0, (0.5 equiv.) in an inert solvent with or without trimethylchlorosilane catalysis. This has the advantage of producing the liquid by-product, trifluoroacetamide, which is oftentimes more easily removed than the solid acetamidefrom SIB1846.0 or diphenylurea from SIB1878.0.

24. N, N-Bis(trimethylsilyl)urea, BSU, SIB1878.0.

Two equivalents of this solid trimethylsilylating agent is reacted with the alcohol (1 equiv.) in an inert solvent. The solid, insoluble diphenylurea produced is readily removed by filtration and the product is purified.

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DEPROTECTION OF SILYL ETHERS

1. Acid-Catalyzed cleavage of trimethylsilyl ethers.

The silylated alcohol (0.4 mmol) in dichloromethane (4 mL) is treated with a drop of 1N HCl and the reac-tion mixture is stirred for 30 min.

In a transeterification approach a 0.5 M solution of the trimethylsilylated alcohol in methanol is treated withpyridinium p-toluenesulfonate (PPTS) at room temperature for 30 min. The lower boiling trimethyl-methoxysilane is removed by distillation.

2. Base-Catalyzed cleavage of trimethylsilyl ethers.

The mildest conditions for the base-catalyzed cleavage of trimethylsilyl ethers is the treatment of a methanolsolution of the silylated alcohol with an excess of potassium carbonate for 1-2 h.

3. Selective cleavage of a triethylsilyl ether with hydrogen fluoride-pyridine - representative proce-dure for the cleavage of silyl ethers with HF•pyridine.

Treatment of 180 mmol of the silylated alcohol with 4 mL of the stock solution of hydrogen fluoride-pyridinesolution (2 mL of HF•pyridine, 4 mL pyridine and 16 mL THF) for 2-3 h results in the cleavage of the tri-ethylsilyl ether.

4. Acid-catalyzed cleavage of triethylsilyl ethers.

A cold (0 °C) methanol solution of the silyl ether is treated with p-toluenesulfonic acid (0.33 equiv.) for 1-2 h.

A solution of the silyl ether in THF is treated with an aqueous solution of trifluoromethane-sulfonic acid.

5. Cleavage of a tert-butyldimethylsilyl ether with tetra-n-butylammonium fluoride - representa-tive procedure for the deprotection of silyl ethers with TBAF.

A solution of the silyl ether in THF (approximately 4 M) is treated with 3 equiv. of 1 M tetra-n-butyl-ammo-nium fluoride in THF at ambient temperature until the silyl ether is converted. This usually requires from 2 - 16 h.

6. Cleavage of a tert-butyldimethylsilyl ether with tris(dimethylamino)sulfur-(trimethylsilyl)diflu-oride, SIT8715.0 - representative procedure for the deprotection of silyl ethers with TAS-F.

A dilute (0.4 M) solution of the alcohol in THF is added to the SIT8715.0 at ambient temperature and theresulting solution is stirred for 1 - 2 h.

7. Cleavage of a tert-butyldimethylsilyl ether with hydrofluoric acid - representative procedurefor the deprotection of silyl ethers with HF.

Hydrofluoric acid (49% aqueous solution, excess) is added to the silyl ether in acetonitrile at 0 °C. After stir-ring for a short time (10 - 30 min usually) the reaction is carefully quenched by the addition of saturatedaqueous sodium hydrogen carbonate (CAUTION: STRONG EVOLUTION OF CARBON DIOXIDE).

8. Cleavage of a tert-butyldiphenylsilyl ether with tetra-n-butylammonium fluoride in acetic acid.

A stock solution of TBAF in acetic acid is prepared (0.15 mL of HOAc per 1.0 mL of 1 M TBAF in THF).The silyl ether is dissolved in THF and reacted with an excess of the stock solution for several h.

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Acknowledgements:Sincere thanks are given to Georg Thieme Verlag, Stuttgart, Germany,http://www.thieme.de/chemistry/journals.html, publishers to Synthesis, for permission to use data fromthe article Selective Deprotection of Silyl Ethers, T.D. Nelson and R.D. Crouch, Synthesis, 1996, 1031.

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