-
Synthesis of a New LigandN,N’-trans-bis-(tert
-butyl)-9,10-dihydro-9,10-ethano-
anthracene-11,12-dimethanamine
Yap Woon Teck
under the direction ofProf. Christopher C. Cummins and John-Paul
F. Cherry
Department of ChemistryMassachusetts Institute of Technology
Research Science InstituteAugust 7, 1999
-
Abstract
The main aim of the project is to synthesize a new ligand with
the distinct proper-
ties of being sufficiently bulky to form stable amido complexes
that do not readily react
with the metal alkyldiene catalyst. Despite its bulkiness, it
still has to be applicable to
a variety of metals by routes analogous to those usually
employed for synthesizing metal
dialkylamides of both the transition and main-group metals[1].
We attempted to synthe-
size the new ligand,
N,N’-bis-trans-(tert-butyl)-9,10-dihydro-9,10-ethano-anthracene-11,12-
dimethanamine (bTAA), was synthesized from the reaction of
tert-butylamine with trans-11-
,12-bis-(toluene-4-sulfonyloxymethyl)-9,10-dihydro-9,10-ethano-anthracene
(Refer to Figure
3). The product was characterized by means of spectroscopy,
including: 1H and 13C nuclear
magnetic resonance (NMR) as well as Fourier
Transformation-Infrared (FT-IR).
-
1 Introduction
There was much research in the synthesis, study and
characterization of metallic and semi-
conducting nanoclusters at the beginning of this decade[2]-[30].
The ultimate aim was to
synthesize semiconductor clusters of a predictable and desired
size such that the structure
and properties from the electronic perspective are intermediate
between those of an atom
or molecule and those of the bulk material[1]. These clusters
are interesting from both the
theoretical and experimental standpoints[20]-[33].
Theoretically, the clusters would have
unprecedented optical and electronic properties which could be
greatly utilized in the opti-
coelectronic industry[34], [35]. Experimentally, the synthesis
of the clusters is challenging,
since a sample would have to be nearly monodisperse before the
phenomenal characteristics
of a given size cluster can be observed[1].
Various methods of synthesizing the nanoclusters such as using
zeolites, colloids and
inverse micelles, have been developed [7]-[13]. In 1991, Cummins
et al. synthesized the lig-
ands, trans-2,3-bis(tert-butylamidomethyl)norborn-5-ene (bTAN)
and trans-2,3-bis[(trimet-
hylsilyl)amidomethyl]norborn-5-ene (bSAN) with the purpose of
preparing metal-containing
monomers that can be polymerized by living ring-opening
metathesis polymerization to form
clusters (Refer to Figure 1) [1], [29], [36]. Ring-opening
metathesis polymerizaton provides a
convenient and elegant route to produce the nanoclusters that
had been greatly researched
upon.
In follow-up studies by Cummins et al., metals such as Sn, Pb
and Zn had been attached
to the new ligands. The clusters had remarkable potential to be
polymerized to form the
nanoclusters that had initially provided the impetus for the
research. However, the reac-
tivity and electron-richness of the double bond in the
norborn-5-ene cyclopentadienyl (Cp)
ring makes the new ligands highly susceptible to electrophilic
attack. Moreover, both the
Sn(bTAN) and Pb(bTAN) complexes (Refer to Figure 1) are
relatively unstable, believed by
Cummins et al. to be due to the amides in bTAN being strong
electron donors, thus placing
1
-
too much electron density on Sn(II) and Pb(II) and leading to
the ultimate reduction of the
metals[36].
N,N’-trans-bis-(tert-butyl)-9,10-dihydro-9,10-ethano-anthracene-11,12-dimethanamine
(bTAA) was conceived of with the intention of synthesizing a new
ligand that retains the
desirable properties of bTAN. Comparing bTAA with bTAN, the
former retains the desirable
properties of being bulky enough to form stable amido complexes
that do not readily attack
the metal alkyldiene catalyst due to steric hindrance. In fact,
bTAA is even bulkier than
bTAN, due to the replacement of the norborn-5-ene Cp ring with
the bulkier anthracene ring.
Moreover, the replacement of the norborn-5-ene Cp ring (which
contains the carbon-carbon
double bond) with the inert benzene rings of the anthracene
makes the new complex less
susceptible to electrophilic attack by electrophiles. bTAA
should also be able to retain the
characteristic of being a versatile ligand, applicable to a
variety of metals by routes analogous
to those usually employed for synthesizing metal dialkylamides
of both transition and main-
group metals. As in bTAN, the tert-butyl groups in bTAA would
not only have an important
steric effect but would also give the metal complexes that form
later, the desirable property
of solubility in hydrocarbon solvents, allowing solution-cast
films to be prepared[36]. This
also aids in the crystallization of the complexes, allowing
X-ray diffraction to be utilized
for determining the structure of the complexes. One of the most
desirable properties that
bTAA has over bTAN is that bTAA has a bulky anthracene base
which is extremely rich
in electrons. Hence, upon coordination to the metal atoms, bTAA
may produce a new
organometallic complex which would have unprecedented,
interesting properties which can
be further explored. This is especially true for the metal
atoms, as there may be a shift in
the electron density towards the metal atoms, thus imparting new
chemical and electronic
properties to the metal concerned (Refer to Figure 2 for
structures of bTAN, bSAN and
bTAA).
Here we attempt to develop a better ligand, bTAA, via the
reaction of anthracene with
dimethyl fumarate (Refer to Figure 3).
2
-
NN
Pb(bTAN)
Pb
NN
Sn
Sn(bTAN)
NN
Sn
ClCl
Sn(bTAN)Cl2
NN
ZnPh
PhZn
(ZnPh)2(bTAN)
Figure 1: Diagram showing Metal Complexes of bTAN; Pb(bTAN) and
Sn(bTAN) are un-stable due to too much electron density on the Pb
and Sn atoms; (ZnPh)2(bTAN) andSn(bTAN)Cl2 are stabilized by the
presence of the phenyl groups and chlorine atoms
3
-
NH
NHNH
SiMe3
HN
Me3Si
HN
bTAN bSAN
bTAA
NH
Figure 2: Diagram showing the structures of
trans-2,3-bis(tert-butylamidomethyl)norborn-5-ene (bTAN),
trans-2,3-bis[(trimethylsilyl)amidomethyl]norborn-5-ene (bSAN), and
N,N’-tr-ans-bis-(tert-butyl)-9,10-dihydro-9,10-ethano-anthracene-11,12-dimethanamine
(bTAA); InbSAN, the tert-butyl groups in bTAN are replaced by the
trimethylsilane groups; In bTAA,the norborn-5-ene base is replaced
by the anthracene base
4
-
O
O
O
O
C
H3CO
O
COCH3
O
TsO
+ CH3
HN
dioxane
NH
OTs
2x TsCl/Pyridine
2x tBuNH2/DMF110oC
2x LiAlH4
OH
OH
H3C
110oC
anthracene
dimethyl fumarate
trans-9,10-dihydro-9,10-ethano-anth
racene-dicarboxylic
acid-(11.12)-dimethyl ester (1)
trans-11,12-bis-hydroxymethyl-9,10-dihydro-9,10-ethano-anthracene
(2)
trans-11,12-bis-(toluene-4-sulfonyl
oxymethyl)-9,10-dihydro-9,10-ethan
o-anthracene (3)
N,N'-trans-bis-(tert-butyl)-9,10-dihydro-9,10-ethanoanthracene-11,12-dimethanamine
(4)
Figure 3: Reaction mechanism for the synthesis of
N,N’-trans-bis-(tert-butyl)-9,10-dihydro-9,10-ethanoanthracene-11,12-dimethanamine,
illustrating how 4 is obtained from the initialreaction of
anthracene with dimethyl fumarate
5
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2 Materials and Methods
All experiments were done under a nitrogen atmosphere in a
Vacuum Atmospheres drybox
or by using standard Schlënk techniques, unless otherwise
stated. Reagent grade diethyl
ether and tetrahydrofuran (THF) were distilled from purple
sodium benzophenone ketyl
under nitrogen. All dioxane, pyridine and dimethyl formamide
were used as bought. 13C
and 1H NMR data are referenced with C6D6 at 128.39 parts per
million (ppm) and 7.16
ppm, respectively; pyridine-d6 was referenced at 8.74 ppm. All
NMR spectra were recorded
in benzene-d6, on the Varian Mercury-300 spectrometer unless
otherwise noted. Infrared
spectra were recorded on a Bio-Rad 135 Series FTIR
spectrometer.
2.1 Synthesis of
trans-9,10-dihydro-9,10-ethano-anthracene-dica-
rboxylic acid-(11.12)-dimethyl ester (1):
Anthracene (20.00g, 112 mmol) and dimethyl fumarate (24.26g, 168
mmol) were dissolved in
anhydrous dioxane (100mL) in a 250mL round-bottom flask, and
refluxed at a temperature
of 110◦C for 72 hours, whereby the solution turned dark brown.
The solution was dried
in vacuo, yielding a pale brown, powdery solid which was
subsequently dissolved in hot
methanol (100mL, 65◦C), forming a dark brown solution which was
filtered to remove any
insoluble impurities. The dark brown filtrate was cooled, giving
cream-coloured crystals of
1 which were filtered and dried in vacuo. Yield: 22.08g
(61.0%)
O
O
O
O
C
H3CO
O
C OCH3
O
110oC
trans-9,10-dihydro-9,10-ethano-anthracene-dicarboxylic
acid-(11.12)-dimethyl ester (1)
CH3dioxaneH3C
anthracene
+dimethyl fumarate
Figure 4: Diels-Alder1 reaction of anthracene with dimethyl
fumarate to yield 1
6
-
O
O
O
O
CH3H3C
Figure 5: Diagram showing electron pushing in the Diels-Alder
reaction of anthracene withdimethyl fumarate to yield 1
2.2 Synthesis of
trans-11,12-bis-hydroxymethyl-9,10-dihydro-9,1-
0-ethano-anthracene (2):
A 1L-3 neck flask was equipped with a 250mL dropping funnel
(right neck), a reflux condenser
with N2-inlet atop (centre neck), a stopper on the left neck and
a magnetic stirbar. Under
N2 atmosphere in the drybox, dry diethyl ether (300mL) was added
to the flask, followed by
lithium tetrahydridoaluminate [LAH] (5.34g, 141 mmol). The
reaction mixture was taken
out of the box and cooled to 0◦C and flushed with N2. The
resultant dark yellow solution
from the dissolution (under N2 pressure) of 1 (21.68g, 67.3
mmol) in a mixture of diethyl
ether and THF (200mL; 2:3) was added to the dropping funnel. A
dropwise addition, which
required 20 minutes, was commenced. After stirring for 18 hours,
the resultant greenish-grey
reaction mixture, which had been allowed to warm up to room
temperature, was quenched by
chilling to 0◦C. The reaction mixture was then worked up by
carefully adding, sequentially,
H2O (5.34g), 15% NaOH (5.34g) and H2O (16.02g), producing a
white, powdery precipitate
which was filtered off and washed with extra ether (2 x 100mL)
and THF (2 x 100mL). The
combined ether and THF filtrates were dried (MgSO4) and excess
solvent was removed in
1Refer to Appendix A for further explanation on Diels-Alder
reactions
7
-
vacuo to yield 2 as a white, powdery solid. Yield: 13.40g
(71.8%)
C
H3CO
O
C OCH3
O
H
AlH
H
HLi +[ ]
-+
OH
OH
lithium tetrahydridoaluminate
2trans-11,12-bis-hydroxymethyl-9,10-dihydro-9,10-ethano-anthracene
(2)
trans-9,10-dihydro-9,10-ethano-anthracene-dicarboxylic
acid-(11.12)-dimethyl ester (1)
Figure 6: Reduction of 1 with LiAlH4 to yield 2
2.3 Synthesis of
trans-11,12-bis-(toluene-4-sulfonyloxymethyl)-9-
,10-dihydro-9,10-ethano-anthracene (3):
2 (13.40g, 50.3 mmol) was dissolved in pyridine (100mL) in a 1L
round-bottom flask and
chilled to 0◦C while stirring. p-Toluenesulfonyl chloride [tosyl
chloride] (23.02g, 121 mmol)
was added in portions over 6 minutes. After 30 minutes, the
pyridine•HCl precipitate beganto appear. After 24 hours, during
which the reaction mixture was allowed to warm up to
room temperature (25◦C), water (500mL) was added while stirring.
Diethyl ether (300mL)
was added and the white precipitate, which would not go into the
diethyl ether, was filtered
off. The organic layer of the reaction mixture (i.e., the
ether/pyridine mixture) was removed
and dried in vacuo, producing a yellow powder, which was washed
with diethyl ether until
it yielded a white powder of 3. Yield: 21.86g (75.6%)
TsOSH3C
O
O
Cl
p-toluenesulfonyl chloride
OH
OHOTs[[+2
trans-11,12-bis-hydroxymethyl-9,10-dihydro-9,10-ethano-anthracene
(2)
trans-11,12-bis-(toluene-4-sulfonyloxymethyl)-9,10-dihydro-9,10-ethano-anthracene
(3)
Figure 7: Reaction of 2 with p-toluenesulfonyl chloride to yield
3
8
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2.4 Synthesis of
N,N’-trans-bis(tert-butyl)-9,10-dihydro-9,10-et-
hanoanthracene-11,12-dimethanamine (4):
3 (11.54g, 20.1 mmol) was dissolved in dimethyl formamide [DMF]
(88.07g) in a 250mL thick-
walled glass vessel, producing a colourless solution. Upon
addition of excess tert-butylamine
(36.72g, 502 mmol), the reaction mixture turned pale yellow. The
glass vessel was tightly
closed with a Teflon stopcock and was then immersed in a 110◦C
bath. The reaction mixture
was stirred magnetically and maintained at 110◦ for 22.5 hours.
A white sublimate appeared
at the top of the vessel. The sublimate was shaken back into the
reaction mixture and
redissolved. The resultant, homogeneous, intense bright yellow
solution was cooled to 25◦C
and poured into a 1L separatory funnel containing 1.33 M NaOH
(250mL) and the resulting
mixture was extracted with 30-65 petroleum ether (3 x 200mL).
The combined petroleum
ether extracts were dried with MgSO4 and all the solvent was
removed in vacuo, forming a
yellowish-white powder. The powder was redissolved in diethyl
ether and 4 was recrystallized
from the resultant bright yellow solution, to yield a white,
microcrystalline powder. Yield:
2.13g (28.2%)
TsOCH3
NCH3
H3C
H
H
HN[tert-butylamine
+OTs [
xsNH
110oC
DMF
trans-11,12-bis-(toluene-4-sulfonyloxymethyl)-9,10-dihydro-9,10-ethano-anthracene
(3)
N,N'-trans-bis-(tert-butyl)-9,10-dihydro-9,10-ethanoanthracene-11,12-dimethanamine
(4)
Figure 8: Reaction of 3 with tert-butylamine to yield 4
9
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3 Results and Data
The product obtained had only a single melting point of 193◦C.
The Fourier-Transformation
Infrared (FT-IR) spectrum of the product that was obtained from
the reaction of 3 and
tert-butylamine showed that there was no detectable N-H
stretching around the region of
3300-3500 cm−1 (Refer to Appendix D). From this, it was evident
that the product did not
contain any hydrogen atoms which were directly bonded to the
nitrogen atoms.
Moreover, the product, 4, showed no apparent reaction with
butyllithium, an extremely
strong deprotonating reagent. This is corroborated by the fact
that the 1H NMR spectra
taken before and after the reaction of 4 with butyllithium were
exactly the same and that
there was no detectable difference in the intensity of any
peaks. This showed that 4 had not
been deprotonated by the butyllithium (See Appendix E and F).
There was no reason why
4 should not be deprotonated, unless there was no hydrogen to
begin with.
Hence it was deduced and inferred that the desired ligand,
N,N’-trans-bis-(tert-butyl)-
9,10-dihydro-9,10-ethanoanthracene-11,12-dimethanamine had not
been produced. There-
fore, a new structure of the product, 5, that the resulted from
the reaction of 3 with tert-
butylamine was proposed (Refer to Figure 9) :
N
Figure 9: Proposed Structure of the Product, 5 Obtained from the
Reaction of 3 withtert-butylamine
10
-
N
A
B
CD
F
GH
I
E
Figure 10: Structure for 1H NMR analysis
With reference to the structure in Figure 10 and Appendix B, it
can be deduced that,
1H NMR (C6D6) δ 7.182, 7.030, 7.010, 6.891, 6.961, 6.941 are due
to the hydrogens at
positions A, B, C and D; 4.072 is due to the hydrogens at E;
2.303, 2.280, 2.273 are due to
the hydrogens at F; 1.616, 1.599, 1.584 are due to the hydrogens
at G; 2.773 is due to the
hydrogens at H and 0.859 are due to the hydrogens at I.
N
1
2
3 4
5
6
11
11117
9
8
10
Figure 11: Structure for 13C NMR analysis
With reference to the structure in Figure 11 and Appendix C, it
can be deduced that, 13C
NMR (C6D6) δ 152.141, 140.358, 126.971, 126.067, 125.991,
121.697 are due to the carbons
at positions 1, 2, 3, 4, 5 and 6; 53.506 is due to the carbon at
7; 52.607 is due to the carbons
at 8; 46.713 is due to the carbons at 9; 46.317 is due to the
carbons at 46.317 and 26.958 is
due to the carbons at 11.
11
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4 Discussion
4.1 Reasons For The Failure to Produce bTAA via the Reaction
of 3 with tert-butylamine
There are several reasons why the attempt to form
N,N’-trans-bis(tert-butyl)-9,10-dihydr-
o-9,10-ethanoanthracene-11,12-dimethanamine from the reaction of
3 with tert-butylamine
failed and resulted in the formation of the new organic molecule
5 (Refer to Figure 9).
Firstly, comparing the desired ligand, bTAA, with bTAN, we
observe that bTAA has an
anthracene base compared to the norborn-5-ene Cp ring of bTAN.
Sterically, the 2 benzene
rings of the anthracene base surrounding the bridge may actually
“push” the tert-butyl group
upwards when one molecule of tert-butylamine reacts with 3. The
nitrogen is now closer to
the tosylate group and 5 is more likely to be formed.
In addition, intramolecular electron transfer may take place,
resulting in the formation of
a covalent bond between the nitrogen atom and the carbon atom,
thus producing a closed-
ring cyclic compound, 5.
Furthermore, the anthracene base may actually cause a strain in
the C-N bond of 4,
as the N-C-N bond angle in 4 is rather small, as the bond length
of N-C and C-N in 4
are relatively short. Thus, the tert-butyl group is again forced
upwards, resulting in the
formation of 5.
Finally, looking at the proposed reaction mechanism for the
formation of 5 from the
reaction of 3 with tert-butylamine (Refer to Figure 12), we
notice that initially the tosylate
group detaches from 3, to be replaced by the tert-butylamine
group. Simultaneously, the
tert-butylamine group loses a hydrogen atom, which combines with
the tosylate group to
form the alcohol. As the second tosylate group detaches from 3,
one of two reactions can
occur:
1. The nitrogen atom that is already attached to 3 could lose
another hydrogen atom
12
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and form a pentagonal, closed ring with the carbon atom on the
opposite side. At the
same time, another equivalent of the alcohol is formed.
2. A new tert-butylamine molecule could collide with the carbon
atom (which is on the
opposite side from the nitrogen atom), with the correct
collision geometry to form the
desired ligand, 4.
Comparing the two scenarios, it would be much easier for the
attached nitrogen atom
to rotate about the carbon atom, and form a closed ring with the
other carbon atom. The
intramolecular transfer in this case would also be much more
rapid than the collision and
reaction of another tert-butylamine with the molecule. Moreover,
the strain in the N-C bond
(as mentioned above), may actually facilitate the forming of the
closed ring.
Therefore, 5 is formed.
4.2 Alternative Pathways to Obtain the Desired Ligand,
N,N’-tra-
ns-bis-(tert-butyl)-9,10-dihydro-9,10-ethanoanthracene-11,12
-dimethanamine
From Figure 13 and 14, the pathway might be successful in
producing the desired ligand
bTAA, as the formation of the dicarbonyl from the reaction with
tert-butylamine is actually
analogous to another reaction that is found to be successful in
literature. In literature,
the tert-butylamine was replaced by ortho-tert-butylaniline.
Moreover, the reduction of the
dicarbonyl with lithium tetrahydridoaluminate is a very standard
method of removing the
carbonyl groups. An additional advantage is that the nitrogen
and carbon atoms would have
absolutely no opportunity to form a closed ring as the oxygen
atom in the carbonyl group
would sterically hinder the formation of the closed ring. Thus,
5 is less likely to be formed.
13
-
OTsCH3
NCH3
CH3
H
H
NH CH3S
O
O
OH
NH
N
CH3S
O
O
-OH+
CH3S
O
O
OH
N
OTs
+
+
+OTs
+
+ +
+
+-
CH3S
O
O
-O
Figure 12: Proposed Reaction Mechanism for the Formation of 5
from 3 and tert-butylamine,whereby the pentagonal closed-ring
forms, instead of the desired 4
14
-
+ OO
OH
O
H
C
HO
O
COH
O
C
Cl
O
CCl
O
C
HN
O
CNH
O
HN
NH
dioxane
110oC
SOCl2
xs tBuNH2-HCl
LiAlH4
Figure 13: Alternative Reaction Pathway 1 to Obtain the Desired
Ligand, via the reactionof anthracene with fumaric acid
15
-
Cl
O
Cl
O
C
Cl
O
CCl
O
C
HN
O
CNH
O
HNLiAlH4
RNH2
NH
+
dioxane
110oC
Figure 14: Alternative Reaction Pathway 2 to Obtain the Desired
Ligand, via the reactionof anthracene with fumaryl chloride
16
-
Cl
O
Cl
O
CH3
NCH3
CH3
H
H
NH
O
HN
O
NHHN
HN
+
LiAlH4
NH
Figure 15: Alternative Reaction Pathway 3 to Obtain the Desired
Ligand, via the reactionof fumaryl chloride with
tert-butylamine
17
-
Cl
O
Cl
O
CH3
NCH3
CH3
H
H
NH
O
HN
O
HN
C
HN
O
CNH
O
+
LiAlH4 NH
Figure 16: Alternative Reaction Pathway 4 to Obtain the Desired
Ligand, via the reactionof fumaryl chloride with
tert-butylamine
18
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4.3 Yields
Comparing our yields of between 60% to 76% to those found in
literature, and especially
analogous reactions by Cummins et al., they were significantly
lower by about 5% to 10%.
This may be due to the fact that the analogous reactions done by
Cummins et. al. in-
volved molecules containing norborn-5-ene rings as the base
whereas ours have an anthracene
base in place of the Cp ring. Sterically, the anthracene base is
bulkier and may actually cause
our molecule to be less reactive, kinetically, and hence the
yields are lower.
The second reason is that we did not optimize the conditions for
the reactions, whereas
those found in literature were optimized results. Hence, our
yields are comparatively lower.
4.4 Applications of the Ligand
Potential applications for the ligand are few, mainly to
synthesize new metal clusters or
complexes that may be of interest. The metal complexes
themselves, may hold the key
to obtaining catalysts which can be used for polymerization
reactions or the synthesis of
complex organic molecules, just to name a few. In addition, they
may prove to be more
efficient and economical than those catalysts currently in use
in many industrial processes.
This area of research is still awaiting further
investigation.
4.5 Future Studies
Upon successful synthesis of the ligand, bTAA, several
experiments can be done to produce
the desired versatile metal complexes which would then form the
basis of further experimen-
tation and investigation. Some of these future studies are
illustrated in Figure 17, where M
= Cr, Mo and MCl3 = CrCl3(THF)3, CrCl3, MoCl3(THF)3.
19
-
LiHN
NH N
NLi
Li
N
N
M
Cl
N
Li
N
N
MN
N
N
MNa+
NN
M
+ -2
MCl3
---
+
Na
Na ++
Figure 17: Future Reactions To Be Studied Upon Successful
Synthesis of 4, where M =Cr, Mo; The addition of Na could reduce
the metal complex to form a univalent anion orremove the chloride
from the metal; Reaction with Li[N(tBuAr)] could form a
metal-nitrogencomplex
20
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5 Conclusion
From our unsuccessful attempts to synthesize
N,N’-trans-bis(tert-butyl)-9,10-dihydro-9,10-
ethanoanthracene-11,12-dimethanamine, it can be seen that the
reaction of 3 with tert-
butylamine is not a viable method of synthesizing the desired
ligand.
The product which we have obtained (Refer to Figure 9) still
remains to be further puri-
fied and characterized by elemental analysis. Its exact
structure has yet to be characterized
by X-ray crystallography, which we are still investigating. Its
chemical and physical proper-
ties e.g. crystal packing are yet largely unknown and thus
remains to be further explored.
The proposed pathways for synthesizing
N,N’-trans-bis(tert-butyl)-9,10-dihydro-9,10-et-
hanoanthracene-11,12-dimethanamine (Refer to Figure 13 and 14)
are still under investi-
gation. Further experimentation has to be carried out to
determine the versatility of the
desired ligand to be applied to various transition and
main-group metals, its exact chemi-
cal properties, whether its theoretical advantages over bTAN are
existent, and its practical
flaws.
6 Acknowledgments
The author would like to thank Mr. John-Paul F. Cherry for his
kind guidance and assistance
during the entire course of the research internship and the
writing of the research paper.
He wishes also to thank Professor Christopher C. Cummins of the
Chemistry Department,
Massachusetts Institute of Technology, for his invaluable advice
and inception of the research
topic.
He also desires to extend his thanks to the Cummins Research
Group and anyone who
has helped out in one way of another.
The author thanks Mr. Eric Ford, Miss Deborah C. Yeh, Mr. Daniel
Kaganovich and
Miss Aimee Crago for their advice and constructive criticisms
during the writing of the paper
and for looking through the manuscripts.
21
-
Finally, he wishes to express his gratitude to the Center for
Excellence in Education
and the Research Science Institute for providing this priceless
opportunity for the scientific
challenges and the social experiences.
22
-
References
[1] Cummins C. C.; Beachy M. D.; Schrock R. R.; Vale M. G.;
Sankaran V.; Cohen R. E.J. Am. Chem. Soc. 1991, 113, 1153-1163.
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24
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A Brief Explanation of Diels-Alder Reactions
Diels-Alder reactions are basically 1,4-cycloaddition reactions
of dienes, which were devel-
oped by two German chemists, Otto Diels and Kurt Alder, in 1928.
Diels-Alder reactions
have great versatility and much synthetic utility and are
especially useful in organic synthesis
of compounds, as they can often form larger molecules that are
cyclic and hence can be used
precursor reactions that lead to more interesting ones.
Essentially, a Diels-Alder reaction is one that takes place
between a conjugated diene
(which is a 4π-electron system) and a dienophile, a compound
containing a double bond
(which is a 2π-electron system). The product of a Diels-Alder
reaction is known as an
adduct.
The simplest case of a Diels-Alder reaction is that, which takes
placed between 1,3-
butadiene and ethene, where the former is the conjugated diene
and the latter is the
dienophile. The reaction is carried out at 200◦C under high
pressures to yield hexene as
the adduct. Typical yields are about 20%.
200oC+
Figure 18: Diels-Alder reaction of 1,3-butadiene (diene) and
ethene (dienophile) to formhexene
With respect to the reaction of dimethyl fumarate and
anthracene, the conjugated diene
is the anthracene whereas the dienophile is the dimethyl
fumarate. Trans-9,10-dihydro-9,10-
ethano-anthracene-dicarboxylic acid-(11.12)-dimethyl ester is
produced as the adduct, with
yields typical ranging from 60% to 75%.
In Diels-Alder reactions, two new σ bonds are formed at the
expense of two π bonds of
the diene and the dienophile. Since σ bonds are generally
stronger than π bonds, formation
25
-
O
O
O
O
C
H3CO
O
C OCH3
O
110oC
trans-9,10-dihydro-9,10-ethano-anthracene-dicarboxylic
acid-(11.12)-dimethyl ester (1)
CH3dioxaneH3C
anthracene
+dimethyl fumarate
Figure 19: Diels-Alder reaction of anthracene (diene) with
dimethyl fumarate (dienophile)to yield 1 (adduct)
of the adduct is usually energetically favoured. However, most
Diels-Alder reactions are
reversible.
Diels-Alder reactions are highly stereospecific. This is shown
by the following observed
phenomena.
1. The reaction is a syn2 addition of the dienophile to the
diene and the configuration of
the dienophile is retained in the product.
2. Out of necessity, the diene must react in the s-cis
conformation rather than the s-trans.
3. Diels-Alder reactions primarily occur in an endo3 rather than
exo4 fashion when the
reaction is kinetically controlled.
As a result, the following phenomena are observed:
1. A trans dienophile gives a trans adduct; a cis dienophile
gives a cis adduct. Two
examples that illustrate this fact is the reaction between
dimethyl maleate (a cis-
dienophile) and 1,3-butadiene (a diene) and that between
dimethyl fumarate (a trans-
dienophile) and 1,3-butadiene (a diene).
2A syn addition reaction is one that places parts of the adding
reagent on the same side/face of thereactant
3An organic group that is on the same side as the longest bridge
of bridged rings is said to be endo4A group that is anti to the
longest bridge is said to be exo
26
-
H
H COCH3
COCH3
O
OH
COCH3
HCOCH3
O
O
+
Dimethyl maleate(a cis-dienophile)
1,3-butadiene(a diene)
Dimethyl 4-cyclohexene-cis-1,2-dicarboxylate
Figure 20: Reaction of a cis dienophile giving a cis adduct
H
H3COC H
COCH3
O
HCOCH3
O
1,3-butadiene(a diene)
Dimethyl 4-cyclohexene-trans-1,2-dicarboxylate
+
Dimethyl fumarate(a trans-dienophile)
O
COCH3
OH
Figure 21: Reaction of a trans dienophile giving a trans
adduct
27
-
2. Reactions in the s-trans conformation would, if it occurred,
produce a six-membered
ring with a highly-strained trans double bond. Hence, this
course of the Diels-Alder
reaction has never been observed.
s-cis Conformation s-trans ConformationFigure 22: Diagram show
cis and trans conformations of 1,3 butadiene
O
RX
C
O
R
+
Figure 23: Diagram showing why Diels-Alder reactions does not
take place for the transform of the diene
28
-
B 1H NMR Spectrogram of 4 in deuterated benzene
(C6D6)
ppm
12
34
56
7
6.961
6.981
7.010
7.030
7.160
7.182
6.941
4.072
2.773
2.280
2.303
2.273
1.616
1.599
1.584
0.859
7.19
1.86
2.00
2.25
2.40
12.28
29
-
C 13C NMR Spectrogram of 4 in deuterated benzene
(C6D6)
ppm
20
40
60
80
100
120
140
160
180
200
220
240239.071
152.141
128.708
140.358
128.390
128.067
126.971
126.067
125.991
121.697
46.713
52.607
53.504
46.317
26.958
23.241
14.818
30
-
D Fourier-Transformation Infrared Spectrum of 4 in
deuterated benzene (C6D6)
1000
2000
3000
4000
-25
-20
-15
-10-505
Tranmittance
Wav
enum
ber
(cm
-1)
31
-
E 1H NMR Spectrogram of 4 before Reaction with
Butyllithium in deuterated benzene (C6D6)
ppm
12
34
56
7
6.961
6.981
7.010
7.030
7.160
7.182
6.941
4.072
2.773
2.280
2.303
2.273
1.616
1.599
1.584
0.859
7.19
1.86
2.00
2.25
2.40
12.28
32
-
F 1H NMR Spectrogram of 4 After Reaction with B-
utyllithium in deuterated benzene (C6D6)
ppm
12
34
56
7
6.961
6.981
7.010
7.030
7.160
7.182
6.941
4.072
2.773
2.280
2.303
2.273
1.616
1.599
1.584
0.859
7.19
1.86
2.00
2.25
2.40
12.28
33