-
Ahmad, S., Lockett, A., Shuttleworth, T. A., Miles-Hobbs, A.
M.,Pringle, P. G., & Bühl, M. (2019). Palladium-catalysed
alkynealkoxycarbonylation with P,N-chelating ligands revisited: a
densityfunctional theory study. Physical Chemistry Chemical
Physics, 21(16),8543-8552. https://doi.org/10.1039/c9cp01471c
Peer reviewed version
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Received00thJanuary20xx,Accepted00thJanuary20xx
DOI:10.1039/x0xx00000x
Palladium–catalysedAlkyneAlkoxycarbonylationwithP,NChelatingLigandsRevisited:ADensityFunctionalTheoryStudyShahbaz
Ahmad,a Ashley Lockett,a Timothy A. Shuttleworthb Alexandra
Miles-Hobbs,b Paul G.PringlebandMichaelBühl,*a
ArevisedinsitubasemechanismofalkynealkoxycarbonylationviaaPdcatalystwithhemilabileP,N-ligands(PyPPh2,Py=2-pyridyl)hasbeen
fully characterisedat theB3PW91-D3/PCM levelofdensity functional
theory.Key intermediatesonthis route are acryloyl (η3-propen-1-oyl)
complexes that readilyundergomethanolysis.With
twohemilabileP,N-ligandsand one of themprotonated, the overall
computed barrier is 24.5 kcalmol-1,which decreases to 20.3
kcalmol-1 uponprotonation of the second P,N-ligand. This
newmechanism is consistent with all of the experimental data
relating tosubstituent effects on relative reaction rates and
branched/linear selectivities, including new results on
themethoxycarbonylationofphenylacetyleneusing(4-NMe2Py)PPh2and(6-Cl-Py)PPh2ligand.Thisligandisfoundtodecreasecatalytic
activity over PyPPh2, thus invalidating a formerly characterised in
situ base mechanism.
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Introduction
The regioselective, direct synthesis of fine chemicals
fromsustainable and abundant resources is highly desirable
forindustrial chemical manufacture. The key challenges within
thisarea include the availability of commercially viable
catalysts,efficient reaction time and conditions, realistic
isolationprocedures,broadsubstratescopeandhighatom-economy.Usingtransition
metal catalysts, homogeneously catalysedcarbonylations are
important industrial processes1-5 that can beconducted with high
chemo- and regioselectivities, efficientlyextending carbon
chains.3,6 Transition metal
catalysedalkoxycarbonylation(hydroesterification)ofalkynes(Scheme1)isa
direct route to the corresponding acrylate esters with
100%atomeconomy.7-14
Scheme1
Methoxycarbonylation of propyne yields methyl
methacrylate(MMA),7, 8, 15-21 the precursor to
poly(methylmethacrylate), alsoknown as Perspex.22 There is a
growing demand in the use
ofPerspexforliquid-crystaldisplay(LCD)screens,especiallyintouchscreenelectronics.23
Currently,an important route toMMAonan industrial scale
isatwo-step process from ethene. The first step is thehomogeneously
catalysed methoxycarbonylation of ethene toyield methyl propionate
(MeP) followed by a
heterogeneouslycatalysedconversionofMePtoMMA.22,24-27
Due to their hemilabile coordination modes, P,N-ligands are
ofconsiderable interest in homogeneous catalysis. Directhomogeneous
methoxycarbonylation of propyne under
mildconditionsisanattractiverouteforthesynthesisofMMAusingahemilabile
Pd(P,N-chelate) catalyst.7-10, 28-30 Drent’s initial
worksuggestedacarbomethoxymechanismwithterminationinvolvingintramolecular
proton transfer from a protonated 2-pyridyldiphenylphosphine
(PyPPh2) ligand.
7Subsequent labelling
studiesbyScrivantietal.suggestedthatthecyclemightbeinitiatedbyaprotontransferfromPyPPh2ontocoordinatedalkyne.
7,11
The Bühl group has previously applied state-of-the-art
densityfunctionaltheory(DFT)studiestounravelthemechanisticdetailsof
homogeneous methoxycarbonylation of propyne using ahemilabile
Pd(P,N-chelate) catalyst. A number of possiblepathways (previously
labelledA - D) were considered, of whichonly one appeared to be
consistentwith observed activities
andselectivities(pathwayD,Scheme2).29,30Thismechanisminvolvesproton
shuttling by the pyridyl groups in the initiation
andterminationsteps.Thependantpyridylmoiety(whenprotonated)can act
as an in situ acid, protonating coordinated propynefollowed by
thermodynamically favoured CO insertion and thendeprotonating
coordinated methanol to promote rapid esterformation. The
unfavourable steric interaction between thearomatic ringof the
ligandandthemethylgroupof
thepropynepromotestheobservedhighregioselectivity(Scheme2).
Scheme2.Bühl’spathwayD.29,30
Basedonthismechanism,an increase inbasicityof the2-pyridylmoiety
should facilitate the critical proton transfer steps, andindeed it
was predicted computationally that the
4-dimethylamino-2-pyridylligand(4-Me2N-Py)PPh2shouldlowertheoverallbarrierofthewholecyclesignificantly,thusincreasingtheoverall
catalytic activity.29 In the Pringle groupwe have put
thisprediction to the test for the methoxycarbonylation
ofphenylacetylene. Rather disappointingly, it transpires that
themorebasic ligand(4-Me2N-Py)PPh2doesnot increasetheactivityover
the parent PyPPh2 and indeed, a decrease in activity isobserved. We
have thus revisited the original mechanismcomputationally and now
present a new pathway that
isconsistentwithallavailableexperimentalinformation.
ResultsandDiscussion
1.MethoxycarbonylationofPhenylacetyleneTo test the predicted
effect of the (4-Me2N-Py)PPh2 ligand, wesynthesised it and used it
in the
Pd-catalysedmethoxycarbonylationofphenylacetylene.Weusedthissubstraterather
than propyne, because it is a liquid at room temperatureand thus
more easily handled. We have confirmedcomputationally that our
hypothesised mechanism is notdependent on this particular choice of
substrate. We
haverecomputedpathwayDatthesamelevel,replacingtheMeC≡CHwith PhC≡CH.
As documented in the Supporting Information (SI,see Table S1 and
Figures S1 and S2) the general shape of thereactionprofile, aswell
as keybarriers are very similarongoingfrom propyne to
phenylacetylene. Importantly, for
bothsubstrates,essentiallythesamelowering
intheoverallbarrierofthewholecycleiscomputedongoingfromthePyPPh2tothe(4-Me2N-Py)PPh2
ligand (see SI). Predictions made for propyne
assubstrateshouldthusbeentirelytransferabletophenylacetylene.
We had anticipated that the dimethylamino group in
(4-Me2N-Py)PPh2wouldbe sufficiently basic to beprotonated in the
veryacidic conditions under which the catalysis is carried out
andtherefore may not produce the desired electron-rich
pyridylgroup.For this reason, inaddition to thepreviously reported
(4-
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Me2N-Py)PPh2 we also prepared the p-anisyl ligand
(4-MeO-Py)PPh2byasimilarroute(Scheme3)inthebeliefthatthiswouldhavethepropertiesofanelectron-richpyridylsubstituentwithoutasignificantriskofprotonationofthemethoxysubstituent.
Scheme3.Preparationof(4-Me2N-Py)PPh2and(4-MeO-Py)PPh2
The phenylacetylene methoxycarbonylation (Scheme 1, R =
Ph)resultsobtainedunderthestandardconditions(MethodA
intheExperimental)withthe2-pyridylphosphinesaregiveninTable1.Itis
clear that the catalysts derived from (4-Me2N-Py)PPh2 and
(4-MeO-Py)PPh2arenotasactiveastheparentPyPPh2.
Table1Catalyticmethoxycarbonylationofphenylacetylene.
Entry Liganda Mb Convat
15minbConvat1hc
Convat4.5hc
Selectivityd
1 PyPPh2 A 99 100 992 (4-Me2N-
Py)PPh2A 51 78 95
3 4-MeO-Py)PPh2 A 86 98 994 PyPPh2 B 88 >995 (6-Cl-Py)PPh2 B
99
>99aPy=2-pyridyl.bReactionconditionsaregivenintheExperimentalSection.
Method A was used for Entries 1-3 and Method B forEntries 4-5.
cConversion and selectivity determined by 1H NMR.Each result is an
average of 2 or more runs. cdThe rest of
theproductwasthelinearisomer.
Since these new results do not support our previously
proposedmechanism, pathway D, we have revisited the
mechanismcomputationally and have uncovered a new pathway which
isconsistentwiththeexperimentalresults.ForconsistencywithourpreviousresultswewilllabelthenewpathwayE.
2.RevisedInSituBaseMechanism(E)2.1GeneralMechanism.AmoredetailedconformationalanalysisofthePyPPh2ligandsinintermediates1-9hasnowrevealedthatsomerotamerswithdifferentorientationofthePysubstituentareslightly
lower in energy than the ones reported previously. Inaddition, we
have located the transition states of all steps
thatinvolveadditionofreactantsordissociationofproduct,whichhadbeenneglectedbefore.Noneoftheserefinementsresultedinanyqualitative
changesof themechanism.Suchaqualitative
changewasfound,however,whentheacylintermediate(4ainScheme2)was
scrutinised further. What has been revealed is that anisomeric
acryloyl complex is accessible (complex 4
discussedbelow),whichisslightlyhigherinenergythan4aby1.1kcalmol-1.OnpathwayD,migratoryCO
insertion into4aaffordeda
looselyboundMMAproductthatwaseasytodissociatefromthemetal.In
contrast, migratory insertion in the new complex 4 affords
averystableproductcomplexwithstronglyboundMMA(complex7discussedbelow).ThisnewthermodynamicsinkonpathwayD,whichhadbeenoverlookedbefore,would
raise theoverall free-energyspanof thewholecycle from22.9kcalmol-1
29, 30
to41.5kcalmol-1(addingthefree-energydifferencebetweenthepresent
isomer 7 and the previous TS5–6), which would seem to
bedifficult to overcome even at the elevated
experimentaltemperature.However,anewmechanismformethanolysisoftheacryloylcomplexwasfoundwithasignificantlylowerbarrierthanthatoftheacylcomplex(7ainScheme2),whichmakesthewholeprocessviableagain.Basedonthesefindingswehavenowtracedacompletecycle(termedpathwayE),whichisdiscussedindetailbelow.
Inaddition,anotherisomerofcomplex1hasbeenlocated,which
isstabilisedthroughanintramolecularNH...Nhydrogenbond(1a,Figure
1), and which is now taken as the zero point of all
ourenergies.Before initiating thewhole reactionbyprotonating
thecoordinated alkyne, 1a first must rearrange to Pd(II) complex
1(ΔG1a!1 = 7.8 kcal mol
-1 and ΔG‡1a!1 = 14.4 kcal mol-1). The
protonation of propyne in complex 1 gives rise to an
agosticintermediate2i(ΔG1!2i=-5.4kcalmol
-1andΔG‡1!2i=5.7kcalmol-
1).AlowkineticbarrierviaTS2i–2suggestsafastconversionof2iinto2
(ΔG2i!2 = -9.5 kcalmol
-1 andΔG‡2i!2 = 0.8 kcalmol-1). CO
displaces the chelating nitrogen of the pyridylmoiety via
TS2–3forming intermediate3 (ΔG2!3=-3.4kcalmol
-1andΔG‡2!3=8.6kcalmol-1) through a large enthalpic gain (Figure
1). This part isessentially identical to thepreviouspathwayD,
except for someminor modifications of the energetics from the
differences inconformations.
Figure 1. Free energy profile using methanol as the model
solvent for initial proton transfer and CO uptake
(B3PW91-D3/ECP2/PCM level). Energies (ΔHandΔG) are in kcal mol-1
relative to 1a.
Thekeystepisnowthedirectformationoftheacyloylcomplex4throughmigratoryCOinsertion(Figure2,theelectronicstructureofthiscomplexisdiscussedbelow).Thefinalstepofmethanolysishas
now been divided into two sub-steps, the one that leads
totheproductionofMMAandtheotherthatdissociatesMMAfromthe catalytic
system (Figure 2 and Figure 3, respectively). In thefirst sub-step,
MeOH associates to the complex via a hydrogenbondingMeO–H.....N
interactiontothenitrogenofoneof
the2-PyPPh2moietiesgivingrisetoanintermediate5(ΔG4!5=1.5kcal
N
Z
Li N
Z
PPh2
ClPPh2
Z = NMe2 or OMe
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mol-1).Then,theacryloylgrouprearrangesviaTS5–6(ΔG5!6=2.9kcal
mol-1 and ΔG‡5!6 = 3.6 kcal mol
-1) to form a very reactiveketene-like intermediate. The
associated MeOH can easilyperform a nucleophilic attack at the
activated acyl group of
theketeneviaTS6–7,assistedbythenearbyPygroupwhichactsasan in situ
baseaccepting theproton fromMeOH (Figure4). Thisstep leads to the
formation of a low-lying MMA coordinatedproduct7 (ΔG6!7 = -24.1
kcalmol
-1 andΔG‡6!7 =2.7 kcalmol-1).
TheeaseofthisstepwithitsverylowbarrierisremarkableifonerecallsthatonpathwayD,methanolysisoftheacylcomplex7aisthemostdifficultstep,withthehighestactivationenergy.
Figure 2. Carbonylation,MeOHuptake, and formationof
keteneandMMA.Energies(ΔHandΔG) areinkcalmol-1relativeto1a.
Figure3.MMAdissociationandregenerationof1a.Energiesareinkcalmol-1relativeto1a.Thesolidlineindicatesfreeenergiescorrectedforbasis-setsuperpositionerror(BSSE),withtheBSSE-correctedΔHandΔGencircled.
In 7, MMA is strongly bound to the metal atom and
fulldissociationishighlyendergonic,withacomputedΔGof11.0kcalmol-1
(for the reaction 7→ [Pd(PyPPh2)(HPyPPh2)]
+ (8)
+MMA).WelocatedanumberofassociativeinterchangepathwayswhereMMAisreplacedwithfreshreactant,propyne,throughtransitionstateswithfourligandscoordinatedtoPd.However,thefullMMAdissociation
indicated to be more favourable than all the
otherpathways(withakineticbarrierofΔG‡7!8=16.8kcalmol
-1).Thepropyneuptaketo8 regeneratesthecomplex1a (ΔG8!1a=
-16.5kcalmol-1).
Scheme4.CatalyticcycleofpathwaysD(right)andE(left)involvingP,NligandasaninSituBase,accordingtoDFT.
ThekeyintermediatesinpathwayEaretheacryloylcomplexes4-6. Such
complexes are known (for selected examples seereferences 31-36),
but their involvement in alkoxycarbonylationhas, to our knowledge,
not been suggested before. From
theoptimisedstructuresinFigure4andthebonddistancesinTable2itcanbeseenhow,uponadditionandreorientationofMeOH,themetal
atom isdisplaced fromaposition closer to the
carbonylCatomtotheterminalmethylenegroup.Inisomer6theligandhassignificant
ketene character, apparent from the short
C2-C3distanceandanincreasedC2-C3-Oabondangleapproaching180°.The
trends in bond distances are reflected in the
computedWibergbondindices(WBIs),ameasureforthecovalentcharacterof a
bond37 (which tends to adopt values close to 1 and 2 forcovalent
single and double bonds, respectively, affording
lowervaluesforbondswithhighioniccharactersuchasthePd-CbondsinTable2).
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Figure4.Optimisedstructuresofintermediates4-6
Scheme5.Labellingschemeforcomplexes4-6.
Table2:Selectedbonddistances(inÅ)withWibergbondindices[insquarebrackets]ofcomplexes4-6,aswellasselectedbondangles
(in degrees, for atom labelling see Scheme 5),
B3PW91-D3/ECP2/PCMlevel.Parameter 4 5 6
d(Pd-C1) 2.213[0.38] 2.295[0.31] 2.063[0.53]d(Pd-C2) 2.236[0.20]
2.263[0.18] 2.265[0.20]d(Pd-C3) 2.104[0.48] 2.054[0.54]
2.757[0.15]d(C1-C2) 1.398[1.46] 1.382[1.56] 1.446[1.20]d(C2-C3)
1.435[1.18] 1.457[1.11] 1.361[1.40]d(C3-Oa) 1.183[1.95] 1.187[1.93]
1.159[2.08]d(C3-Ob) n.a. 3.481[0.00] 2.966[0.02]a(C1-C2-C3) 114.8
114.8 120.3a(C2-C3-Oa) 146.1 140.8 170.8
Inthepathwaysdiscussedsofar,wehaveassumedthatonlyoneof the
PyPPh2 ligands is protonated. Under the strongly acidicconditions,
however, it may well be possible that a significantfraction of the
catalyst has both ligands protonated. We havetherefore recalculated
the crucial steps on pathways E fordiprotonated, dicationic
intermediates (see Figure S3 in the SI).Introduction of the second
proton decreases the MMAdissociation barrier to 13.2 kcal mol-1,
but the overall
barrierremainscomparabletothatofmonocationicpathway,asnowtheoverallbarrierhasbeenshiftedtotheprotontransferstepratherthan
the product dissociation step (7+ + Propyne → TS1–2i+ +
MMA, ΔG‡ = 16.8 kcal mol-1). The concomitant quantitativechanges
to the energetics along with the selectivity
andsubstituenteffectsarediscussedbelow.
Figure 5 Intermediates for product release on the dicationic
version of
pathwayE.ThesolidlineindicatesBSSE-correctedfreeenergies,withBSSE-correctedΔHandΔGencircled.
2.2Selectivityandsubstituenteffects.Thepathwaysdiscussedsofar
produce branched MMA, the main product observedexperimentally. The
linear/branched selectivity is
determinedearlyonthepath,uponintramolecularprotonationofcoordinatedpropynein1.ThisstepinournewpathwayEisthesameasintheoriginal
pathway D (where observed selectivities are wellreproduced). The
minor changes in conformational preferencesfound in thepresentwork
lead tonegligible changes in the finalenergetics: Intermediate 1
(leading to a branched product) ismore stable by ΔG = 2.5 kcal
mol-1 (ΔH = 3.4 kcal mol-1)
thanintermediate1L(leadingtothelinearproduct).Theappearanceofthe
new intermediate 1a and its equivalent 1aL (leading to
thelinearproduct)slightlyaffectsthecomputedselectivities,becausethe
highest point on the branched pathway is TS1a-1 (i.e.formation
of1),whereas on the linear pathway it isTS1-2L (i.e.the protonation
of the alkyne, see Figure 6). The two
kineticbarriersleadingtotheisomericproductsdifferbyΔΔG‡=3.7kcalmol-1,
corresponding to a high selectivity towards the branchedproduct at
45 °C (Table 3), which is consistent with theexperimental results.
On the dicationic pathway, no isomercorresponding to 1a exists, and
the selectivity is determined bythe difference between TS1-2i+ and
TS1L-2L+ (see Figure
S4),affordingaslightlyreducedΔΔG‡=2.9kcalmol-1.
SubstitutingthePymoietywitha6-Me-Pygroupfurtherenhancesthe
selectivity toward the branched product. This is fully borneout
inourcalculations,where thegreater stericeffectsongoingfrom PyPPh2
to by (6-Me-Py)PPh2 increases ΔΔG
‡ from 3.7
kcalmol-1to6.3kcalmol-1.Thelattervaluecorrespondstoaselectivityof99.99%,ingoodagreementwiththeexperimentalobservationstowardsselectivity.
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Figure6.Pathwaysforformationofbranched(right)andlinear(left)products;energies(ΔHandΔG)areinkcalmol-1relativeto1a.SelectivityisgovernedbythedifferencebetweenTS1a-1andTS1-2L.
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Inthecontextoftheoverallactivitiesdiscussedinthenextsection,wealsoevaluatedtheselectivity-determiningdifferenceinthefreeenergybarrierfortwootherligandswithNMe2andClsubstituentsin
4- and 6-positions of the 2-pyridyl moiety, respectively. In
allthese cases, the high branched selectivity is predicted to
bemaintainedorslightlyenhanced(ΔΔG‡valuesinupperhalfofTable3). Only
for the dicationic mechanism with PyPPh2 and
(4-Me2N-Py)PPh2ligands,aslightreductioninselectivitywouldbepredicted(lowerhalfofTable3).
Table3:Effectsofdifferent2-pyridylphosphineligandsystemson
branched to linear products selectivitiesa and the
overallenergybarriers (energyspan,ΔG‡MARI!HETS).All
thevaluesaregiveninkcalmol-1.
Ligand ΔΔG‡ %Branched(at45°C)EnergySpan
MechanismEPyPPh2 3.7 99.71 16.8(6-Me-Py)PPh2 6.3 99.99
16.8(4-Me2N)PyPPh2 4.4 99.91 18.2(6-Cl-Py)PPh2 5.9 99.99
15.9(4-Cl-Py)PPh2 4.8 99.95 17.9MechanismE(Dicationic)PyPPh2 2.9
98.99 16.8(6-Me-Py)PPh2 4.7 99.94 18.2(4-Me2N-Py)PPh2 2.5 98.12
19.6(6-Cl-Py)PPh2 5.7 99.99 16.4(4-Cl-Py)PPh2 3.8 99.78 16.7
aFor mechanism E, ΔΔG‡ = ΔG‡1L!2L – ΔG‡1!1a (cf. Table S6), for
mechanism E
(dicationic),ΔΔG‡=ΔG‡1L!2L+–ΔG‡1!2i
+(cf.FigureS4)2.3 Catalytic activity and substituent effects.
The overall kineticefficiency and, in particular, substituent
effects on it, can
beevaluatedusingKozuchandShaik’s38-40energeticspanmodel.Thismodel
identifies the rate-limiting states (as opposed to a
singlerate-limiting step) as those that maximise the energy
differencebetween the lowest intermediate and the highest
transition stateon a continuous (free-)energy profile of a
catalytic cycle. On themonocationicreactionpathwayE,wehave
identified
intermediate7asthemostabundantreactionintermediate(MARI)andTS7–8ashighestenergytransitionstate(HETS).TheresultingoverallbarrierbetweenMARI
andHETS is 16.8 kcalmol-1, consistentwithahighTOF at 45 °C. For
pathway D, the overall barrier was originallyreported to be 22.9
kcal mol-1, however this value is erroneousbecause one of the
important intermediates, namely 7 in thecurrent investigation
(which is the MARI for pathway E)
wasoverlookedandmissingfromthereactionprofile.TheMARIonthereaction
profile of pathway E ismore stable by 19.3 kcalmol-1 infree energy
(21.6 kcal mol-1 in enthalpy) than the MARI on
thereactionprofileofpathwayD.Thus,theoriginalpathwayDwithits
HETS for methanolyis of the acyl complex should have an
actualoverall free energy barrier of more than 40 kcal mol-1, much
toohightobeovercomeundertheexperimentalconditions.
For the original pathwayD, introduction of an NMe2 group in
4-position of the 2-Py moiety was predicted to lower the
overallbarrier notably (by 3.6 kcal mol-1).29, 30 In contrast, in
our
newmechanismE,thesamesubstitutionraisestheoverallbarrierfrom16.8kcalmol-1
to18.2kcalmol-1 (seeenergyspanvalues
inupperhalfofTable3).ThisincreaseintheoverallbarriershoulddecreasetheTOFrelative
to thatof theoriginalsystem,
ingoodagreementwiththeexperimentalresultsdiscussedinSection1.Thereasonforthisdifferent
substituenteffect inpathwaysD andE is that in
theformer,thehighesttransitionstate(HETS)isthatformethanolysis,which
is significantly reduced as the basicity of the Py group
isincreased, whereas in the latter (pathway E), the HETS is
forproduct release,which isonly littleaffectedby thebasicityof
theligand.
For thedicationic route,whichwewouldexpect tobecomemorerelevant
with decreasing pH, an overall barrier of 16.8 kcal mol-1between
MARI and HETS is computed, i.e. same as that on themonocationic
pathway E. The MARI on both
pathways(monocationicanddicationic)are7and7+,thoughdifferentspecieswere
identified controlling the overall turnover frequency
(XTOF).Details of XTOF for the dicationic pathway are given in
Table S4.Going from the PyPPh2 to the (4-Me2N-Py)PPh2 ligand on
thedicationicpathway slightly increases theoverall barrier,
from18.2kcalmol-1 to19.6kcalmol-1 (which shoulddecrease
thepredictedTOF).41
Since an increase of ligand basicity causes the overall activity
todecrease slightly, a reduction of basicity might actually have
theoppositeeffect,i.e.increasetheoverallactivity.Inordertotestthispossibility
computationally, we considered two chlorinatedderivatives. When a
Cl atom is placed on the 4-position of thepyridyl moiety, an
increase in the overall free energy span ispredictedon
themonocationicpathway;however theoverall
freeenergyspanslightlydecreasedforthedicationicpathway(Table3).When
theCl atom is placedon the6-position (thereby
combiningelectronicandstericeffects),aslightdecreaseintheoverallbarrierispredictedforbothmonocationicanddicationicpathways(by0.9kcalmol-1and0.5kcalmol-1,compareentriesforPyPPh2and(6-Cl-Py)PPh2
inTable3).
41At lowpH,our calculations suggest that
thisligandwouldthusimpartanincreaseinactivity.Encouragingly,thisprediction
is supported by the results obtained at Shell42
whichshowasignificantincreaseinactivitywiththe(6-Cl-Py)PPh2ligand,in
the methoxycarbonylation of propyne albeit under slightlydifferent
reaction conditions [reaction
temperature30°Cand45°CforPyPPh2and(6-Cl-Py)PPh2,respectively].Wehavealsotestedtheactivity
of the catalyst derived from (6-Cl-Py)PPh2 in
themethoxycarbonylationofphenylacetyleneandfoundthatthe(6-Cl-
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Py)PPh2 ligand does indeed produce amore active catalyst
underthesameconditions(Table1,Entries4and5)aspredicted.
ConclusionsInsummary,wehavetestedpredictionsbasedonDFTabouthowtoincrease
the activity of palladium catalysts with P,N hemilabileligands in
alkyne alkoxycarbonylation. The simple
ligandsubstitutionfrominsilicodesign,namelygoingfromPyPPh2tothe(4-Me2-N)PyPPh2
ligand,was realised experimentally, but failed toproduce the
predicted rate enhancement.We have thus
revisitedandrevisedtheoriginallyproposedmechanism(D)computationallyat
the B3PW91-D3/PCM level of density functional theory. Onreaction
profiles of the revised mechanism (E), highly reactiveacryloyl and
ketene-type intermediates are identified,which havevery low
barriers for the alcoholysis step and an overall
kineticbarrierofΔG‡=16.8kcalmol-1.
Barriers controlling branched/linear selectivity are comparable
inboth pathways D and E, which are improved on going from
2-PyPPh2to(6-Me-Py)PPh2and(6-Cl-Py)PPh2ligandsystemstowardsthebranched-formingroute.Both(6-Me-Py)PPh2and(6-Cl-Py)PPh2ligand
systems are analogous to each other on controlling
theselectivities,andthelatteralsodecreasestheoverallbarrierto15.9kcalmol-1.
Underhigheracidconcentrations,modeledbytwoprotonatedP,N-ligands,
alkyne alkoxycarbonylation may follow the dicationicversionof
insitubasemechanism,againwithanoverallbarrierof16.8kcalmol-1
UnliketheresultsobtainedfortheoriginalpathwayD,onthenewpathwayE
the (4-Me2N-Py)PPh2 ligand system isnow indicated
todecreasethecatalyticactivity.Ontheotherhand,aslightdecreaseintheoverallbarrierispredictedforthe(6-Cl-Py)PPh2ligandsystemat
higher acid concentrations. This prediction was
testedexperimentally and the results show that the (6-Cl-Py)PPh2
doesindeed produce a more active catalyst for the carbonylation
ofphenylacetylene.
Wehope that our detailed computational insightswill help in
thedesignoffurtherimprovedcatalystsforcarbonylationbytuningthestereoelectronicpropertiesoftheligand.
ExperimentalSection
LigandSynthesisandCatalysisAllreactionswerecarriedoutunderanatmosphereofdrynitrogen,unlessotherwisestated,usingstandardSchlenklinetechniquesandoven
dried (200 °C) glassware. CH2Cl2, Et2O, THF, toluene andhexane were
collected from a Grubbs type solvent purificationsystem,43 and
deoxygenated by bubbling with N2 for 30
minutes.CD2Cl2wasdriedoveractivated4Åmolecular sieves for72hoursand
deoxygenated by successive freeze-pump-thaw
cycles.MeOHwaspurchasedasanhydrous,storedover3ÅmolecularsievesanddeoxygenatedbybubblingwithN2
for30minutes.
1H, 13Cand 31Pand NMR spectra were recorded at ambient
temperature unlessotherwise statedon Jeol ECP (Eclipse) 300, Jeol
ECS 300, Jeol ECS400, Varian 400-MR, Varian VNMRS 500 spectrometers
and aBrukerAvanceIIIHD500spectrometerequippedwitha13C-observe
(DCH) cryogenic probe. Chemical shifts δ are given in parts
permillion (ppm) and coupling constants J are in Hz. 1H and
13Cchemical shifts were referenced to residual solvent peaks.
31Pchemicalshiftswerereferencedto85%H3PO4.MassSpectrawererecordedbytheUniversityofBristolMassSpectrometryServiceona
VG Analytical Autospec (EI) or VG Analytical Quattro
(ESI)spectrometer. Elemental Analysis was carried out by
theMicroanalyticalLaboratoryoftheSchoolofChemistry,UniversityofBristol.
Thin Layer Chromatography (TLC) was performed usingMerck Kieselgel
60 F254 (Merck) aluminium backed plates (0.25mm layer of silica).
Flash column chromatographywas performedusing a Biotage Isolera
Spektra One Chromatographic Isolationsystem and the solvent system
stated. DMAE (dried over 4 Åmolecular sieves) was purchased from
commercial suppliers andpurified before use. Other commercial
reagents were used assuppliedunlessotherwisestated.PyPPh2,
44(4-Me2N-Py)PPh245were
madebyliteratureprocedures.Synthesisof(4-OMe-Py)PPh22-Bromo-4-methoxypyridine(0.500g,2.66mmol)wasdissolvedinEt2O(10cm3).Thiswascooledto-78°Candn-BuLi(1.70cm3,2.72mmol,1.6Minhexanes)wasaddeddropwise,givingabrightorangesolution.Afterstirringfor30minutesatthistemperature,ClPPh2(0.40cm3,2.22mmol)wasaddeddropwiseandthereactionallowedtowarmtoambienttemperatureandwasstirredfor2hours,afterwhichdeoxygenatedH2O(10cm
3)wasaddedtoquenchthereaction.TheEt2OlayerwasextractedandtheaqueouslayerwashedwithEt2O(2x10cm
3).Theorganicportionswerecollected,driedoverNa2SO4,filteredandthesolventremovedinvacuotoyieldthecrudemixtureasapinkoil.Recrystallisationfromhexane(ca.5cm3)at-20°Cgavetheproductasanoffwhitesolid(0.527g,81%).1HNMR(400MHz,CD2Cl2):δH8.50-8.49(m,1H,pyH(H-6)),7.42-7.33(m,10H,phH),6.73-6.71(m,1H,pyH(H-5)),6.66-6.65(m,1H,pyH(H-3)),3.72(s,3H,OCH3).
13C{1H}NMR(126MHz,CD2Cl2):δC166.1(d,
1JC,P=4.5Hz,pyC(C-2)),165.6(d,3JC,P=4.2Hz,
pyC(C-4)),152.0(d,3JC,P=13.5Hz,pyC(C-6)),137.0(d,1JC,P=10.8
Hz,phC(C-1)),134.7(d,2JC,P=19.9Hz,phC(C-2andC-6)),129.6(s,phC(C-4)),129.1(d,3JC,P=7.2Hz,phC(C-3andC-5)),115.3(d,
2JC,P=20.1Hz,pyC(C-3)),108.4(s,pyC(C-5)),55.6(s,OCH3).
31P{1H}NMR(162MHz,CD2Cl2):δP-2.8(s,Ph2PR).HR-MS(EI):m/zcalc.forC18H16NOP[M]
+=293.0970;obs.=293.0980.Elem.Anal.found(calc.forC18H16NOP):C,73.26(73.61);H,5.30(5.50);N,4.76(4.73).Synthesisof(6-Cl-Py)PPh2Thisligandwasmadebyamodificationofaliteraturemethod.46Ph2PH(2.52g,13.5mmol)inTHF(15cm
3)wascooledto-78°Candn-BuLi(8.45cm3,13.5mmol,1.6Minhexanes)wasaddeddropwise,givingabrightorangesolution.Afterstirringthereactionmixturefor30minat-78°C,thereactionallowedtowarmtoambienttemperatureandwasstirredfor1hour.Thereactionmixturewasthenaddeddropwisetoasolutionof2,6-Dichloropyridine(2.00g,13.5mmol)inTHF(20cm3)at-78°C.Thereactionallowedtowarmtoambienttemperatureandwasstirredfor18hours.Volatileswereremovedinvacuoandthereactionwasdissolvedintoluene(20cm3).DeoxygenatedH2O(20cm
3)wasthenadded.Thetoluenelayerwasextractedandtheaqueouslayerwashedwithtoluene(3x10cm3).Theorganicportionswerecollected,driedoverMgSO4,filteredandthesolventremovedinvacuotoyieldthecrudemixtureasapaleorangesolid.RecrystallisationfromMeOHgavetheproductasawhitesolid(2.44g).TheMeOHsupernatantwasthenplacedina-20°Cfreezerwhereprecipitationoccurred.Thesupernatantwasremovedand
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anyremainingsolventremovedinvacuotoyieldadditionalproductasawhitesolid(combinedyield=2.97g,74%).31P,13Cand1HNMRdataallagreedwiththeliteraturevalues.47CatalyticmethoxycarbonylationofphenylacetyleneAdapted
from previously reported procedure.48 Catalysis
wasperformedusingaBaskerville“Multi-Cell”autoclave.Method A: The
ligand (0.110 mmol) was added to the autoclaveand the system put
under an atmosphere of N2. Solutions ofPd(OAc)2 (5.50 x 10
-3 mmol) in MeOH (0.5 cm3) and TsOH.H2O(0.220 mmol) in MeOH (0.5
cm3) were added, followed
byphenylacetylene(5.50mmol).ThiswasthenwashedinusingMeOH(0.5cm3)andtheautoclaveflushedwiththreecyclesofCO(ca.10bar).Theautoclavewasthenpressuredto45barandheatedto60°C.Aftereither1houror4.5hours,theautoclavetransferredtoanicebathandoncecooled,thesystemwasvented.Method
B: The ligand (0.55 x 10-3 mmol) was added to theautoclave and the
system was put under an atmosphere of N2.Solutions of Pd(OAc)2
(2.75 x 10
-3 mmol) in MeOH (0.5 cm3) andTsOH.H2O(0.10mmol)inMeOH(0.5cm
3)wereadded,followedbyphenylacetylene(5.50mmol).ThiswasthenwashedinusingMeOH(0.5cm3)andtheautoclavewasflushedwiththreecyclesofCO(ca.10bar).Theautoclavewasthenpressurisedto45barandheatedto60
°C. After 15 minutes, the autoclave was transferred to an
icebathandoncecooled,thesystemwasvented.For both methods A and B, a
small sample of the product wasdissolved in CDCl3 and analysed
by
1H NMR spectroscopy.Conversion and selectivity was determined by
integration of thephenylacetylene alkynyl proton (δH 3.10 ppm) and
the
methylatropate(δH6.38and5.90ppm)andmethylcinnamate(δH7.71and6.42ppm)alkenylprotons(seeSIforthespectra).DFTComputationsWe
have used B3PW9149-51 hybrid functional, which has beensuccessfully
validated to study related (2-pyridyl)thiourea
Pd(II)complexes52andforarangeofreactionsthatrelyuponmetals.53-56WhencoupledwithGrimme’sDFT-D3,57-59includingBecke-Johnsondamping,60,
61 this functional benchmarks well against explicitlycorrelated
CCSD(T).62 DFT-D3BJ correction has been computed
fortheminimisedgeometries.Geometries of all complexes were fully
optimized at theB3PW91/ECP1 level, where ECP1 corresponds with the
6-31G**basissetonallnonmetalatoms, inconjunctionwiththeSDDbasison
Pd, denoting the small-core Stuttgart-Dresden relativisticeffective
core potential (ECP) together with its valence basis set.The nature
of all the possible minima and transition states wasverified by
frequency calculations within the harmonicapproximation. Harmonic
frequencies were computed analyticallyand were used to obtain
enthalpic corrections from standardthermodynamic expressions at
298.15 K.
ThermochemicalcorrectiontermsδEGwerecarriedoutasdifferenceofthereactionfree
energy of a given step (ΔEB3PW91/ECP1 ) and the
correspondingfreeenergy(ΔGB3PW91/ECP1):
𝛿𝐸! = ∆𝐺!!!"!"/!"#! − ∆𝐸!!!"!"/!"#! (1)To obtain starting
structures for the transition states, connectingthe intermediates,
potential energy profile calculations wereperformed at the same
level, B3PW91/ECP1. All potential
energyprofilecalculationswerecomputedby increasing
themetal−liganddistance by 0.1 Å and optimizing the remaining
geometricparameters using loose convergence criteria. Taking the
highestpoints of these paths, full transition state optimisations
wereperformed using QST3 algorithm63 and were confirmed to link
to
the respective reactants and products using intrinsic
reactioncoordinate(IRC)calculations.64,65
The energies of the pre-optimised complexes were refined
usingthesamefunctionalandanECP2level.AtthislevelPdwastreatedwith
the same SDD pseudopotential and valence basis as in
ECP1whereas6-311+G**basissetwasusedforallotheratoms.Solventeffectswereincludedbyapolarisablecontinuum(PCM)66-68modelwith
methanol as a solvent. DFT-D3BJ corrections were added
toaccuratelyaccountforthemissingdispersion.Thefinal∆Gand∆Hvaluesarecalculatedas:
∆G=∆E+δESolv+δEDFTD3BJ+δEG (2)
∆H=∆E+δESolv+δEDFTD3BJ+δEH (3)
where∆E,andδESolvarecomputedattheB3PW91/ECP2level,δEGand δEH are
computed at the RI- B3PW91/ECP1 level.WBIs werecomputed during
natural population analysis.69 The energy spansandfreeenergies
fortheproductdissociationwereobtainedaftercounterpoise corrections,
which were calculated by
performingsingle-pointcalculationsattheB3PW91/ECP2level(seeTablesS5a–
S5p on the SI). All calculations were performed using
Gaussian09.70
ConflictsofinterestTherearenoconflictstodeclare.
AcknowledgementsWe thank EaStCHEM and the School of Chemistry
for support.Computations were carried out on a local Opteron PC
clustermaintainedbyDr.H.Früchtl.WealsothankMrLukeCrawford
fortechnical assistance with some calculations. The Bristol
ChemicalSynthesis Centre for Doctoral Training (BCS CDT) funded by
theEngineering and Physical Sciences Research Council
(EPSRC)(EP/G036764/1)andtheUniversityofBristolarethankedforaPhDstudentship(toT.A.S.).
Keywords:alkoxycarbonylation•alkynes•carbonylation•densityfunctionalcalculations•homogeneouscatalysis•methylmethacrylate•palladium•reactionmechanisms
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