The cyclopropylcarbinyl route to γ-silyl carbocations

Article abstract of DOI:10.3762/bjoc.15.170

The mesylate derivative of cis-1-hydroxymethyl-2-trimethylsilylcyclopropane has been prepared, along with a number of related mesylates and triflates with substituents on the 1-position. These substrates all solvolyze in CD₃CO₂D to give products derived from cyclopropylcarbinyl cations that undergo further rearrangement to give 3-trimethylsilylcyclobutyl cations. These 3-trimethylsilylcyclobutyl cations are stabilized by a long-range rear lobe interaction with the γ-trimethylsilyl group. When the substituent is electron-withdrawing (CF₃, CN, or CO₂CH₃), significant amounts of bicyclobutane products are formed. The bicyclobutanes are a result of γ-trimethylsilyl elimination from the cationic intermediate that has an unusually long calculated Si-C bond. The solvolysis chemistry of mesylate and triflate derivatives of trans-1-hydroxymethyl-2-trimethylsilylcyclopropane and 1-substituted analogs can be quite different since these substrates do not generally lead to 3-trimethylsilylcyclobutyl cations.

Full text of DOI:10.3762/bjoc.15.170

The cyclopropylcarbinyl route to γ-silyl carbocations
Xavier Creary
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 1769–1780.
Department of Chemistry and Biochemistry, University of Notre
Dame, Notre Dame, IN 45556, USA
doi:10.3762/bjoc.15.170
Received: 25 March 2019
Accepted: 03 July 2019
Published: 24 July 2019
Email:
Xavier Creary - creary.1@nd.edu
This article is part of the thematic issue "Reactive intermediates –
carbocations".
Keywords:
bicyclobutane; carbocation; cyclopropylcarbinyl; rearrangement;
silicon
Guest Editor: S. R. Hare
© 2019 Creary; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The mesylate derivative of cis-1-hydroxymethyl-2-trimethylsilylcyclopropane has been prepared, along with a number of related
mesylates and triflates with substituents on the 1-position. These substrates all solvolyze in CD3CO2D to give products derived
from cyclopropylcarbinyl cations that undergo further rearrangement to give 3-trimethylsilylcyclobutyl cations. These
3-trimethylsilylcyclobutyl cations are stabilized by a long-range rear lobe interaction with the γ-trimethylsilyl group. When the sub-
stituent is electron-withdrawing (CF3, CN, or CO2CH3), significant amounts of bicyclobutane products are formed. The bicyclobu-
tanes are a result of γ-trimethylsilyl elimination from the cationic intermediate that has an unusually long calculated Si–C bond. The
solvolysis chemistry of mesylate and triflate derivatives of trans-1-hydroxymethyl-2-trimethylsilylcyclopropane and 1-substituted
analogs can be quite different since these substrates do not generally lead to 3-trimethylsilylcyclobutyl cations.
Introduction
Carbocations, positively charged trivalent carbon compounds Cyclopropylcarbinyl cations are an extensively studied system
and reactive intermediates, have continued to fascinate chemists [8,9]. Initial interest was derived from the fact that both cyclo-
since the early discoveries of tropylium [1,2] and trityl [3-7] propylcarbinyl and cyclobutyl substrates 1 and 2, where X
salts. Many of the giants of organic chemistry during the last represents diazonium ion [10,11], chloride [10], or naphthalene-
century contributed heavily to the development of carbocation sulfonate [12] leaving groups, reacted in aqueous solvents to
chemistry. This article will deal with three types of carboca- give an identical mixture of products 3, 4, and 5 (Scheme 1).
tions that have been of intense and fundamental interest over Additionally, solvolysis rates were far greater than expected for
the years, i.e., cyclopropylcarbinyl cations, electron-deficient primary and strained secondary systems. To account for these
cations, and silyl substituted carbocations. A brief overview of facts, it has been suggested that there are common cationic
these types of carbocations is warranted.
intermediates in these solvolysis reactions of 1 and 2. Labelling
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[13-15], stable ion [16-19], and computational studies [19] generated and that they can derive stabilization by a variety of
implicate the involvement of three degenerate cyclopropyl- mechanisms. Chief among these cations are the α-trifluoro-
carbinyl cations, 6a, 6b, and 6c, in equilibrium with cyclobutyl methyl [22-24], α-cyano [22,25-29], α-carbonyl [30-33], and
cation 7, as well as the homoallylic cation 8 (Scheme 2). α-phosphoryl [34,35] analogs of 9. Carbocations of type 9
Cations 6 are stabilized by the cyclopropyl ring and are there- will be examined in conjunction with the cyclopropyl-
fore much more stable than simple primary carbocations. The carbinyl–cyclobutyl manifold.
cyclobutyl cation 7 is also quite stabilized relative to simple
secondary carbocations. This cation has been called a “bicy-
clobutonium” cation, 7a, which is a nonclassical cation (a
cation containing hypercoordinated carbon) that could be
derived from protonation of bicyclobutane [20]. Another poten-
tial mode of stabilization is by an interaction of the cationic
center with the adjacent strained cyclobutyl bonds as in 7b.
Figure 1: Electron-deficient carbocations.
The third type of carbocation that will be incorporated into this
paper is the trimethylsilyl-substituted carbocation [36-44]. We
have been interested in long-range interactions of silicon with
both carbene [45-48] and carbocation centers [49,50]. Along
these lines, γ-trimethylsilyl cations of general type 11 have been
generated under stable-ion [51] as well as solvolytic conditions
[52-54]. They are greatly stabilized by the “rear lobe” type of
interaction shown involving the γ-trimethylsilyl group. A num-
ber of related cations are also stabilized by analogous γ-silyl
Scheme 1: Solvolyses of cyclopropylcarbinyl and cyclobutyl sub-
strates.
interactions [55-59], which have also been termed ”percaudal”
interactions [56]. Certain carbenes can also be stabilized in a
similar fashion [60,61]. Thus substrates of type 10 solvolyze in
protic solvents with large rate enhancements (anchimeric assis-
tance) to generate carbocations 11 as reactive intermediates
(Scheme 3). These cations 11 capture solvent molecules to give
exclusively products 12 with net retention of configuration, a
characteristic of carbocations that are stabilized by this type of
rear lobe interaction.
Scheme 3: Solvolyses of γ-trimethylsilylcyclobutyl substrates.
Scheme 2: The cyclopropylcarbinyl–cyclobutyl–homoallyl cation mani-
fold.
A series of cyclopropylcarbinyl substrates 13 and 14 (Figure 2),
A second class of carbocations that this article will deal with is where X is a leaving group and R is an electron-donating group
the so-called “electron-deficient” carbocation, i.e., carbocations and E is an electron-withdrawing group, have now been exam-
9 (Figure 1) substituted with electron-withdrawing groups E ined. The goal was to evaluate the cyclopropylcarbinyl to
[21]. Many studies have shown that such cations can indeed be cyclobutyl cation rearrangement. Can these substrates lead to
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γ-trimethylsilyl-substituted cyclobutyl cations 11 and what are
the fates of such carbocations? Answers to these questions were
sought.
Figure 2: Substrates of interest.
Results and Discussion
Phenyl-substituted systems
The first compounds to be examined were the mesylates 19 and
20. These substrates were prepared as shown in Scheme 4. Irra-
diation of ethyl 2-diazo-2-phenylacetate (15) in vinyltrimethyl-
silane as solvent gave an isomeric mixture of esters 16. Subse-
quent reduction with lithium aluminum hydride gave a mixture
of alcohols 17 and 18, which could be readily separated by
silica gel chromatography. The assignment of stereochemistry
Scheme 4: Synthesis of mesylates 19 and 20.
of these isomers was based on shielding effects in both 1H and product along with 8% of the alkene 22. It is proposed
13C NMR spectra. For example, the trimethylsilyl singlet in 18 (Scheme 5) that these products arise from stepwise formation of
appears at δ −0.30 (shielded by the cis-phenyl group), while the the cyclopropylcarbinyl cation 23. This cation can rearrange via
trimethylsilyl singlet in 17 appears at δ 0.14 (deshielded by the migration of bond a to give the cyclobutyl cation 24. The cis-
trans-phenyl group). Such effects are in complete agreement nature of the phenyl group and the hydrogen in cation 23 neces-
with calculated shifts based on B3LYP/6-31G* calculated struc- sarily results in the formation of the γ-silyl-stabilized cation 24.
tures of 17 and 18. Additionally, nOe studies on 17 confirm the This cation is the source of the acetate 21. Alternatively, cation
stereochemical assignment. Conversion to mesylates 19 and 20 23 can rearrange by migration of the b bond of the cyclo-
using mesyl chloride and triethylamine was straightforward.
propane. This leads to the β-silylcyclobutyl cation 25, which
can subsequently desilylate to give the minor product, the
Mesylate 19 reacts readily in CD3CO2D at 20 °C (Table 1) to alkene 22. Interestingly, formation of the γ-silyl cation 24 is
give the substituted cyclobutyl acetate 21 (92%) as the major preferred over the β-silyl cation 25.
Scheme 5: Reaction of mesylate 19 in CD3CO2D.
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Reaction of the isomeric mesylate 20 in CD3CO2D gives the the cis-nature of the phenyl and TMS groups in 26 requires that
same rearranged products 21 and 22. These products are these groups are closer to each other in 27. Shown in Figure 3
accounted for mechanistically in Scheme 6. The initially formed are M062X/6-311+G** calculated structures and energies of
cyclopropylcarbinyl cation 26 rearranges by migration of the a cations 27 and 24, which are distinct energy minima, along with
bond of the cyclopropane to give the cyclobutyl cation 27. This the transition state 28 which connects these two cations. Cation
cation 27 is different from the γ-silyl-stabilized cation 24 in that 27 derives most of its stabilization from the phenyl group, while
Scheme 6: Reaction of mesylate 20 in CD3CO2D.
Figure 3: M062X/6-311+G** calculated structures and relative energies of cations 24, 27, and transition state 28.
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the TMS group in the 3-position provides no cross-ring stabi- Mesylate 31 reacted readily in CD3CO2D to give the cis-
lization. The calculated barrier for ring inversion of 27 to give cyclobutyl acetate 34 as the major product (Scheme 8), along
the lower energy rear lobe stabilized γ-trimethylsilyl cation 24 with a small amount of cyclobutene (35). The rate of 31
is only 2.4 kcal/mol. Calculations at the B3LYP/6-31G*, (Table 1) is not substantially enhanced relative to the unsubsti-
B3LYP/6-311+G**, MP2/6-31G*, and the MP2/6-311+G** tuted cyclopropylcarbinyl mesylate (33). The small rate en-
levels lead to the same conclusions, i.e., cations 24 and 27 are hancement factor of 3.56 is consistent with a small inductive
distinct energy minima with a very low barrier for conversion stabilization of the initially formed cationic intermediate. This
of 27 to 24. Therefore, formation of 27 under solvolytic condi- behavior is completely analogous to that of the phenyl analog
tions should readily yield 24, and subsequently the substitution 19 and a similar mechanistic pathway is proposed. The initially
product 21. The small amount (4%) of elimination product 22 is formed cyclopropylcarbinyl cation 36 rearranges to the γ-silyl-
a result of rearrangement of 26 to the β-trimethylsilyl cation 25 cyclobutyl cation 37, the source of the major product 34. The
as described in Scheme 5.
desilylated product 35 arises from the alternative β-trimethylsi-
lylcyclobutyl cation.
Unsubstituted and methyl-substituted
systems
The behavior of mesylate 32 is in contrast to that of 31 and the
Attention was next turned to potential γ-trimethylsilylcy- phenyl analog 20. Five products, 35, 38, 39, 40, and 41, are ob-
clobutyl cation systems lacking phenyl stabilization. Thus pure tained and these products are formed in essentially the identical
Z- and E-alcohols 29 and 30 were each cyclopropanated under ratio as seen in our previous study of the trans-mesylate 42
Simmons–Smith conditions, and the resultant stereochemically [52]. The similarity of products formed from acetolysis of 32
pure alcohols were converted to mesylates 31 and 32, respec- and 42 implies that the same cation rearrangement manifold is
tively (Scheme 7). For rate comparisons, cyclopropylcarbinyl involved. Scheme 9 gives a mechanistic rationale for these
mesylate 33 [62,63] was also prepared.
products. Capture of an unrearranged discrete cyclopropyl-
carbinyl cation 43 gives the major product 38, while migration
of bond c to the cationic center gives rearranged cation 44, the
source of the rearranged acetate 39. Ring expansion via migra-
tion of bond b in 43 gives the β-trimethylsilyl-stabilized
cyclobutyl cation 45, and subsequent desilylation provides
cyclobutene (35). Alternatively, cyclobutyl to homoallylic
cation rearrangement leads to the homoallylic products 40 and
41 via internal mesylate return or solvent capture. Of interest is
the fact that no product 34 (derived from γ-trimethylsilyl-stabi-
lized cation 37) is formed. Our previous computational study
[52] provided insight into the lack of involvement of cation 37.
This study at the B3LYP/6-31G* level suggested that migra-
tion of bond a in 43 is not viable since the resultant cation 47 is
not an energy minimum at this level, but a transition state.
However, a current study at the M062X/6-311+G** level finds
Scheme 7: Synthesis of mesylates 31 and 32.
Scheme 8: Reaction of mesylate 31 in CD3CO2D.
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Scheme 9: Reaction of mesylate 32 in CD3CO2D.
that both conformations 47a and 47b are energy minima. While mechanistic scheme involving the γ-trimethylsilyl-stabilized
47a lies 10.8 kcal/mol above 37, the barrier for inversion of 47a cation 52 is likely.
to 37 is quite large (24.9 kcal/mol). Hence there is no viable
route to 37.
The isomeric trifluoroacetate 49 (shown in Table 1) gives
methylcyclobutene (51) (68%) as the major acetolysis product,
In order to complete the study of substrates 13 with electron-do- along with minor products that are identical to those previously
nating groups, the methyl analog 48 was prepared from the cor- reported [52] in solvolysis of the trifluoroacetate derivative of
responding cyclopropylcarbinyl alcohol, which was available (1r,3r)-1-methyl-3-(trimethylsilyl)cyclobutanol. As in the case
from methyl 2-diazopropanoate by a process completely analo- of mesylate 32, the γ-trimethylsilyl-stabilized cation 52 is
gous to the synthesis of the phenyl analog 17. The mesylate de- apparently not formed from trifluoroacetate 49 due to stereo-
rivative was too reactive for rates to be measured and hence the chemical constraints.
trifluoroacetate derivative 48 was studied. Acetolysis gave the
acetate 50 along with a smaller amount of methylcyclobutene Systems with electron-withdrawing groups
(51, Scheme 10). This reactivity is completely analogous to that Attention was next turned to cyclopropylcarbinyl systems
seen in the phenyl and hydrogen analogs 19 and 31, i.e., a substituted with electron-withdrawing groups. Previously Tilley
Scheme 10: Reaction of trifluoroacetate 48 in CD3CO2D.
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Table 1: Solvolysis rates for substrates in CD3CO2D at 20.0 °C.
Compound
k (s−1)
krel
(for ROMs)
1.71 × 10−4
1.0
3.8
33
19
20
31
32
48
49
6.50 × 10−4
1.26 × 10−3
6.09 × 10−4
6.89 × 10−4
Scheme 11: Bicyclobutane formation from a γ-trimethylsilyl cation.
Scheme 12. Reduction of the ester mixture 57 with lithium alu-
minum hydride gave a chromatographically separable mixture
of alcohols 58 and 59. Stereochemistry of the alcohol 58 was
established by long-range 19F coupling to the cis-trimethylsilyl
group hydrogens (JH-F = 0.9 Hz). Long-range 19F coupling to
the TMS methyl groups of 58 was also observed in the
13C NMR spectrum (JC-F = 2.1 Hz) [64,65]. This long-range
19F coupling is not observed when the CF3 group is trans to the
TMS group in the isomer 59.
7.4
3.6
4.0
1.31 × 10−7a 76b,c
8.91 × 10−8a 52b,c
aExtrapolated from data at higher temperatures. k for 48 at 60.0 °C =
2.58 × 10−5 s−1; k for 48 at 80.0 °C = 2.33 × 10−4 s−1; k for 49 at
60.0 °C = 1.62 × 10−5 s−1; k for 49 at 80.0 °C = 1.42 × 10−4 s−1.
bMesylate is too reactive for rate to be measured. cAssuming mesy-
late reacts 105 faster than trifluoroacetate.
and co-workers [55] have examined the triflate 53 and found
that this system solvolyzes with rear lobe TMS participation
(Scheme 11). The unusual feature in solvolysis of 53 is the for-
mation of the highly strained bicyclobutane 55 as the sole prod-
uct. It was therefore of interest to see if the cyclopropylcarbinyl
to cyclobutyl rearrangement could be used to access the carbo-
cation 54, and subsequently, bicyclobutane 55. It was also of
interest to see if other bicyclobutanes could be formed if the
CF3 group were replaced by other electron-withdrawing groups
that we have previously examined in carbocation forming reac-
tions.
Scheme 12: Formation of triflates 60 and 61.
Additional cyclopropylcarbinyl systems containing the electron-
withdrawing cyano and carbomethoxy groups were prepared in
an analogous fashion as shown in Scheme 13. Carbomethoxy-
cyano carbene addition to vinyltrimethylsilane followed by lith-
ium borohydride reduction of the ester functionality of 63 gave
a separable mixture of alcohols 64 and 65. The stereochemistry
of the product 65 was established using nOe studies. Cyano to
carbomethoxy conversion in 65 to give alcohol 66 was straight-
The requisite trifluoromethyl-substituted cyclopropylcarbinyl forward. Triflate derivatives 67 and 68 were prepared since
systems were prepared by addition of the carbene derived from analogous mesylate derivatives were relatively unreactive.
the diazoester 56 to vinyltrimethylsilane as shown in Triflate 69 was a highly reactive substrate that could only be
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Scheme 13: Formation of triflates 67, 68, and 69.
prepared in about 80% purity. The less reactive mesylate deriv- unrearranged substitution products 70. In the cases of 68 and
ative 75 was therefore prepared and used for kinetic studies.
69, some rearranged substitution products 71 were also formed.
The mesylate 75 gave the same initial products as the triflate
The triflates 61, 68, and 69 (with electron-withdrawing groups 69. However, the bicyclobutane 72c formed from mesylate 75
trans to trimethylsilyl) were all solvolyzed in CD3CO2D and was not completely stable at 40–60 °C, but degraded slowly to a
results are shown in Scheme 14. Since the triflate 69 was highly mixture of other products. The bicyclobutanes 55, 72b, and 72c
reactive and could not be isolated in pure form, the mesylate de- were quite stable in CD3CO2D at 20 °C, where triflate studies
rivative 75 was used in kinetic studies that were carried out in were carried out.
the 40–60 °C range. Rates of reaction of mesylate derivatives
(Table 2) were all substantially slower than the parent mesylate The bicyclobutane products 55 and 72 are a result of desilyl-
33 or the phenyl, methyl, or H analogs. This is attributed to a ation of the γ-silyl cations 54 and 74. Why are bicylobutanes
significant inductive destabilizing β-effect of the group E on the formed from cations 54 and 74 and not from cations 24, 37, and
initially formed cation 73. The triflates all produced significant 52, which do not have electron-withdrawing groups? Previous
amounts of bicyclobutane products 55 and 72 along with some studies have shown that “electron-deficient” cations 9, where
Scheme 14: Reactions of substrates with electron-withdrawing groups in CD3CO2D.
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E = COR [66], CN [25], CF3 [67], and PO(OEt)2 [34], readily
eliminate β-hydrogens to form alkenes as major products. They
do not readily capture solvent at the cationic center. It is there-
fore expected that nucleophilic attack at the cationic centers of
54 and 74 will be slowed. Table 3 shows results of calculations
on the γ-trimethylsilylcyclobutyl cations shown in Figure 4 at
different levels of theory. The presence of the electron-with-
drawing group results in an increase in the Si–C3 bond length
relative to the cations 24 and 52. Also, the cross-ring C1–C3
distance is decreased. In the language of resonance theory, these
features are in line with increased contributions of form 74a to
the overall structure of the cation. These features suggest more
facile nucleophilic attack should occur at silicon, favoring bicy-
clobutane formation. Also included in Table 3 are calculated
bond lengths in the phosphoryl-substituted cation 74d, which
also shows a very long Si–C bond. Preferred trimethylsilyl
elimination from this intermediate is in line with the behavior of
mesylate 76, which gives exclusively the bicyclobutane 77 on
solvolysis in CH3CO2H (Scheme 15).
Table 2: Solvolysis rates for substrates in CD3CO2D at 20.0 °C.
Compound
k (s−1)
krel
(for ROMs)
1.71 × 10−4
1.00
33
75
61
60
68
67
1.26 × 10−7a 7.3 × 10−4
2.25 × 10−4
1.25 × 10−3
2.14 × 10−4
1.61 × 10−3
1.3 × 10−5b
7.3 × 10−5b
1.3 × 10−5b
9.4 × 10−5b
Scheme 15: Bicyclobutane formation from mesylate 76 in CH3CO2H.
aExtrapolated from data at higher temperatures. k at 40.0 °C =
2.24 × 10−6 s−1; k at 50.0 °C = 8.40 × 10−6 s−1; k at 60.0 °C =
2.85 × 10−5 s−1. bAssuming triflate reacts 105 faster than mesylate.
The final item to be addressed is the behavior of triflates 60 and
67 with electron-withdrawing CF3 and CN groups cis to the tri-
Figure 4: γ-Trimethylsilyl cations.
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Table 3: Calculated bond lengths (Å) of γ-trimethylsilyl cations.
Cation
Bond
B3LYP/
6-31G*
B3LYP/
6-311+G**
MP2/
6-31G*
MP2/
6-311+G**
M062X/
6-311+G**
Si–C3
C1–C3
1.962
1.916
1.959
1.914
1.975
1.760
1.970
1.759
1.970
1.736
24
52
Si–C3
C1–C3
1.999
1.717
1.994
1.719
1.990
1.665
1.983
1.675
1.984
1.652
Si–C3
C1–C3
2.016
1.662
2.013
1.659
2.004
1.636
1.998
1.645
2.000
1.616
37
Si–C3
C1–C3
2.018
1.658
2.018
1.655
2.009
1.625
2.002
1.632
2.007
1.601
74c
74d
74b
54
Si–C3
2.013
1.663
2.012
1.659
2.008
1.624
2.003
1.630
2.004
1.602
C1–C3
Si–C3
C1–C3
2.046
1.694
2.045
1.688
2.037
1.652
2.028
1.663
2.031
1.623
Si–C3
C1–C3
2.034
1.646
2.037
1.642
2.024
1.616
2.019
1.623
2.024
1.595
methylsilyl group. These substrates gave exclusively unre- further rearrangement of cations 80 untenable. Indeed, M062X/
arranged substitution products 78 and 79 when reacted in 6-311+G** calculations show that the potential rearranged
CD3CO2D (Scheme 16). The lack of rearrangement products cation 81 (E = CN) is not even an energy minimum, but a tran-
suggests that these potent electron-withdrawing groups make sition state.
Conclusion
1-Substituted-cis-2-trimethylsilylyclopropylcarbinyl mesylates
and triflates 13 solvolyze in CD3CO2D to give products derived
from 3-trimethylsilylcyclobutyl cations. These cationic interme-
diates are stabilized by a long-range rear lobe interaction with
the γ-trimethylsilyl group. When the substituent is electron-
withdrawing (CF3, CN, or CO2CH3), significant amounts of
bicyclobutane products are formed. The bicyclobutanes are a
result of γ-trimethylsilyl elimination from the cationic interme-
diate. Computational studies support a carbocation intermedi-
ate with an unusually long Si–C bond, indicative of increased
demand for Si–C hyperconjugation due to the electron-with-
Scheme 16: Reactions of triflates 60 and 67 in CD3CO2D.
drawing group. With the exception of the phenyl substitution,
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Supporting Information File 2
M062X/6-611+G** calculated structures, energies, and
Cartesian coordinates for carbocations and transition states.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-170-S2.pdf]
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ORCID® iDs
Xavier Creary - https://orcid.org/0000-0002-1274-5769
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