β-Trimethylsilyl cyclopropylcarbenes

Article abstract of DOI:10.1021/jo001112b

Thermal decomposition of the in situ generated lithium salt of the tosylhydrazone derivative of cyclopropyl trimethylsilylmethyl ketone gave 1-cyclopropyl-1-trimethylsilylethylene, a product of exclusive silyl migration. Thermal decomposition of the sodium salts of tosylhydrazone derivatives of 1-trimethylsilylcyclopropyl alkyl ketones also gave methylenecyclopropane products derived from trimethylsilyl migration. These reactions were interpreted in terms of rapid trimethylsilyl migration to carbene-like centers that compete effectively with ring expansion processes of cyclopropylcarbenes. Computational studies (B3LYP/6-31G*) suggest that cyclopropyl stabilization of carbenes is more effective than β-trimethylsilyl stabilization. However, β-trimethylsilyl stabilized conformations are easily attained, and these conformations can lead to silyl migrations. There are two minimum energy conformations of methyl-1-trimethylsilylcyclopropylcarbene, 27, and the rotational barrier to interconversion of these conformations (5.4 kcal/mol) is substantially lower than in the parent cyclopropylcarbene (15 kcal/mol). The onset of a stabilizing interaction in the transition state between the carbene vacant orbital with the adjacent Si-C σ-orbital is proposed. Computational studies also show a very small (2.0 kcal/mol) barrier for trimethylsilyl migration in trimethylsilylmethyl cyclopropylcarbene, 11.

Full text of DOI:10.1021/jo001112b

J . Org. Chem. 2001, 66, 1115-1121
1115
â-Tr im eth ylsilyl Cyclop r op ylca r ben es
Xavier Creary* and Mark A. Butchko
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556
creary.1@nd.edu
Received J uly 24, 2000
Thermal decomposition of the in situ generated lithium salt of the tosylhydrazone derivative of
cyclopropyl trimethylsilylmethyl ketone gave 1-cyclopropyl-1-trimethylsilylethylene, a product of
exclusive silyl migration. Thermal decomposition of the sodium salts of tosylhydrazone derivatives
of 1-trimethylsilylcyclopropyl alkyl ketones also gave methylenecyclopropane products derived from
trimethylsilyl migration. These reactions were interpreted in terms of rapid trimethylsilyl migration
to carbene-like centers that compete effectively with ring expansion processes of cyclopropylcarbenes.
Computational studies (B3LYP/6-31G*) suggest that cyclopropyl stabilization of carbenes is more
effective than â-trimethylsilyl stabilization. However, â-trimethylsilyl stabilized conformations are
easily attained, and these conformations can lead to silyl migrations. There are two minimum energy
conformations of methyl-1-trimethylsilylcyclopropylcarbene, 27, and the rotational barrier to
interconversion of these conformations (5.4 kcal/mol) is substantially lower than in the parent
cyclopropylcarbene (15 kcal/mol). The onset of a stabilizing interaction in the transition state
between the carbene vacant orbital with the adjacent Si-C σ-orbital is proposed. Computational
studies also show a very small (2.0 kcal/mol) barrier for trimethylsilyl migration in trimethylsilyl-
methyl cyclopropylcarbene, 11.
In tr od u ction
diacy of carbene 3 and analogous cyclopropylcarbenes has
recently been questioned in the formation of cyclobutene
derivatives. J ones and Shevlin3 have shown that cyclo-
propylmethylcarbene generated by alternative methods
gives vinylcyclopropane (and not 1-methylcyclobutene) as
the major product. Platz4 has shown that a substituted
cyclopropylcarbene is effectively trapped by alkene or
amines (in preference to ring expansion). These studies
led to the suggestion that cyclobutene products may not
arise from free carbenes such as 3. Instead some ener-
getic nitrogen-containing intermediate may be a direct
source of the ring expanded product.
Singlet carbenes 1 are prone to intramolecular rear-
rangement via 1,2-shifts to the carbenic center.1 The
chemistry of cyclopropylcarbene 3 is no exception and is
characterized by a remarkably facile ring expansion to
give cyclobutene, 4. This chemistry of 3 was first de-
scribed by Shechter and Friedman2 and was based on the
pyrolysis of the tosylhydrazone salt 5. Thermal decom-
position of 5 gave cyclobutene as the major product,
presumably via the intermediacy of the diazocompound
6 and the cyclopropylcarbene 3. However, the interme-
We5 and others6 have been interested in the chemistry
of carbenes containing silicon in adjacent and more
remote positions relative to the carbenic center. Our
previous studies have shown that the trimethylsilyl group
undergoes very facile migration to carbenic centers. The
alkene 8 derived from silyl migration is the major product
formed from carbene 7,5a while the silyl migration
product 10 is formed exclusively from 9.5b With these
results in mind, we wanted to determine the fate of
cyclopropyl carbenes with silicon in the â-position as in
11 and 12. Will silyl migration compete successfully with
the rapid ring expansion process usually seen in cyclo-
propyl carbenes? Reported here are the results of these
studies.
(3) Thamattoor, D. M.; J ones, M., J r.; Pan, W.; Shevlin, P. B.
Tetrahedron Lett. 1996, 37, 8333.
(4) Huang, H.; Platz, M. S. Tetrahedron Lett. 1996, 37, 8337.
(5) (a) Creary, X.; Wang, Y.-X. Tetrahedron Lett. 1989, 19, 2493. (b)
Creary, X.; Wang, Y.-X. J . Org. Chem. 1994, 59, 1604. (c) Creary, X.;
Wang, Y.-X. Res. Chem. Intermed. 1994, 20, 201. (d) Creary, X.; J iang,
Z.; Butchko, M.; McLean, K. Tetrahedron Lett. 1995, 37, 579.
(6) Baird, M. S.; Dale, C. M.; Dulayymi, J . R. A. J . Chem. Soc., Perkin
Trans. 1 1993, 1373. (b) Kirmse, W.; Konrad, W.; Schnitzler, D. J . Org.
Chem. 1994, 59, 3821. (c) Pezacki, J . P.; Loncke, P. G.; Ross, J . P.;
Warkentin, J .; Gadosy, T. A. Org. Lett. 2000, 2, 2733.
(1) For reviews of carbene chemistry, see: (a) Kirmse, W. Carbene
Chemistry, 2nd ed.; Academic Press: New York, 1971. (b) Carbenes;
J ones, M., J r.; Moss, R. A., Eds.; Wiley: New York, 1973 and 1975;
Vols. I and II.
(2) Friedman, L.; Shechter, H. J . Am. Chem. Soc. 1960, 82, 1002.
10.1021/jo001112b CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/24/2001

1116 J . Org. Chem., Vol. 66, No. 4, 2001
Creary and Butchko
extracted with hexane prior to pyrolysis to remove any
trace of 14 (hexane soluble) that might have arisen from
the Shapiro reaction. These transformations are consis-
tent with the intermediacy of the carbene 11, which
undergoes exclusive trimethylsilyl migration to the car-
bene center. This trimethylsilyl migration is substantially
faster than either ring expansion or hydrogen atom
migration.
Attention was next focused on the carbene 12. 1-Tri-
methylsilylcyclopropanecarboxaldehyde9 was converted
to the corresponding tosylhydrazone 18. Deprotonation
with NaOCH3 followed by vacuum pyrolysis of the dry
sodium salt led to a product mixture (56% yield) contain-
ing 95% of the cyclobutene 19 as well as 5% of the
methylenecyclopropane 20. Again, no fragmentation
product, trimethylsilylacetylene, was observed. The chem-
istry of the diazocompound 21 is therefore beginning to
deviate from that of the parent diazocompound 6. Forma-
tion of the small amount of methylenecyclopropane 20
suggests that silicon migration can begin to compete with
ring expansion in a carbene-like transition state10 or in
the carbene 22.
Resu lts a n d Discu ssion
Tosylh yd r a zon e Sa lt P yr olyses. The approach used
to generate carbenes in this study was the thermal
decomposition of tosylhydrazone salts.7 Tosylhydrazones
are usually derived from the corresponding aldehydes or
ketones. However, since cyclopropyl trimethylsilylmethyl
ketone is not readily available, an alternative approach
was used to generate the tosylhydrazone salt precursor
to carbene 11. The tosylhydrazone derivative of cyclo-
propyl methyl ketone, 13, was doubly deprotonated with
2 equiv of n-BuLi at -78 to -30 °C. Silylation with 1
equiv of ClSiMe3 led to the lithium salt 16, which was
dried and then pyrolyzed under vacuum at up to 180 °C.
The sole product formed from this series of reactions was
the alkene 14. No propargyltrimethylsilane, a product
of carbene fragmentation, was observed. The product 14
does not arise via the Shapiro reaction8 since the dilithio
derivative 15 does not eliminate LiTs and molecular
nitrogen at -30 °C. Additionally, the dry salt 16 was
1-Trimethylsilylcyclopropylmethyl ketone11 was con-
verted in fashion similar to the tosylhydrazone 23a , and
the dry sodium salt was pyrolyzed under vacuum. The
(7) For a discussion of the tosylhydrazone salt pyrolysis method of
diazocompound and carbene generation, see: (a) Bamford, W. R.;
Stevens, T. S. J . Chem. Soc. 1952, 4735. (b) Kaufman, G. M.; Smith,
J . A.; Vander Stouw, G. G.; Shechter, H. J . Am. Chem. Soc. 1965, 87,
935.
(9) Wells, G. J .; Yan, T.-H.; Paquette, L. A. J . Org. Chem. 1984, 49,
3604.
(10) As in refs 3 and 4, concerted loss of molecular nitrogen and
rearrangement remains a possibility.
(8) Shapiro, R. H. Org. React. 1976, 23, 405.

â-Trimethylsilyl Cyclopropylcarbenes
J . Org. Chem., Vol. 66, No. 4, 2001 1117
F igu r e 1. B3LYP/6-31G* structures of carbenes 27a , 27b, the 90° constrained form 27c, and the transition state 27-TS for
interconversion of 27a and 27b.
behavior was significantly different from that of the salt
5 in that the methylenecyclopropane 25a was the major
product (65%) and only 35% of the ring expanded cy-
clobutene 24a was formed. Analogous behavior is seen
in the tosylhydrazones 23b and 23c derived from 1-tri-
methylsilylcyclopropyl ethyl ketone and 1-trimethylsi-
lylcyclopropyl n-butyl ketone. Silyl migration very suc-
cessfully competes with the ring expansion process in all
of these systems.
Com p u ta tion a l Stu d ies. Ab initio molecular orbital
computational studies can offer some insights into the
behavior of the diazocompound 26 as well as the carbene
27 derived from 26. Recently, density functional compu-
tational methods have been applied to carbene inter-
mediates,12-17 and this method has been used in the
present studies. Geometries were optimized at the B3LYP/
6-31G* level, and single point energies were also calcu-
lated at the B3LYP/6-311+G**//B3LYP/6-31G* level. The
B3LYP/6-31G* calculated structure of 26 shows a con-
formation where the Si-C-C-N dihedral angle is ap-
proximately 90° (87°). Analogous to the parent cyclopro-
pylcarbene 3,18 there are two minimum energy conforma-
tions of the carbene 27 as shown in Figure 1. Conforma-
tion 27a has the trimethylsilyl group anti to the methyl
group and the vacant orbital on the carbene aligned
essentially in conjugation with the cyclopropane ring. The
second minimum energy conformation (27b) has the
trimethylsilyl and methyl groups in a syn-orientation.
This syn-conformation 27b is 0.53 kcal/mol lower energy
than 27a at the B3LYP/6-311+G**//B3LYP/6-31G* level.
Conjugation of the vacant carbene orbital with the
cyclopropane ring therefore appears to be more important
than conjugation with the filled C-Si σ-orbital.
To evaluate the importance of the interaction of the
carbene vacant orbital with the C-Si σ-orbital, the
energy of conformation 27c was calculated. Conformation
27c is not an energy minimum, and this structure was
arrived at by constraining the Si-C-C-CH3 dihedral
angle to 90°.19 This conformation 27c is only 3.6 kcal/
mol higher in energy than 27a and corresponds to a loss
of cyclopropyl stabilization of the carbene. By way of
contrast, the analogous constrained conformation (H-
C-C-H dihedral angle ) 90°) of the parent cyclopropyl-
carbene 3 is 15.1 kcal/mol higher energy than the anti-
conformation of 3.20 These findings are completely in
line with a stabilizing interaction of the carbene center
(11) Blankenship, C.; Wells, G. J .; Paquette, L. A. Tetrahedron 1988,
44, 4023.
(12) Wong, M. W.; Wentrup, C. J . Org. Chem. 1996, 61, 7022.
(13) Xie, Y.; Schreiner, P. R.; Schleyer, P. v. R.; Schaefer, H. F., III.
J . Am. Chem. Soc. 1997, 119, 1370.
(18) (a) Schoeller, W. W. J . Org. Chem. 1980, 45, 2161. (b) Wang,
B.; Deng, C. Tetrahedron, 1988, 44, 7355. (c) Shevlin, P. B.; McKee,
M. L. J . Am. Chem. Soc. 1989, 111, 519. (d) Chou, J .-H.; Mckee, M. L.
DeFelippis, J .; Squillacote, M.; Shevlin, P. B. J . Org. Chem. 1990, 55,
3291.
(14) Sulzbach, H. M.; Platz, M. S.; Schaefer, H. F., III; Hadad, C.
M. J . Am. Chem. Soc. 1997, 119, 5682.
(15) Zhu, Z.; Bally, T.; Stracener, L. L.; McMahon, R. J . J . Am. Chem.
Soc. 1999, 121, 2863.
(16) Geise, C. M.; Hadad, C. M. J . Am. Chem Soc. 2000, 122, 5861.
(17) Creary, X.; Butchko, M. J . Am. Chem. Soc., submitted for
publication.
(19) The optimal Si-C-C-CH3 alignment of 90° was chosen on the
basis of carbocation studies by Lambert et al.; see: (a) Lambert, J . B.
Tetrahedron 1990, 46, 2677. (b) Lambert, J . B.; Zhao, Y.; Emblidge,
R. W.; Salvador, L. A.; Liu, X.; So, J .-H.; Chelius, E. C. Acc. Chem.
Res. 1999, 32, 183. (c) Lambert, J . B.; Wang, G.-t.; Finzel, R. B.;
Teramura, D. H. J . Am. Chem. Soc. 1987, 109, 7838. (d) Lambert, J .
B.; Liu, X. J . Organomet. Chem. 1996, 521, 203.

1118 J . Org. Chem., Vol. 66, No. 4, 2001
Creary and Butchko
The question arises as to which carbene conformation
is generated in pyrolysis of diazocompound 26. In prin-
ciple, loss of molecular nitrogen from 26, without exten-
sive reorganization, would give a conformation 27c with
the carbene vacant orbital aligned with the C-Si σ-bond.
While this carbene conformation is not an energy mini-
mum, it is a favorable conformation for SiMe3 migration.
The possibility therefore exists that loss of N2 from 26 is
concerted with silicon migration, i.e., a carbene as a
discrete intermediate may not be involved in the forma-
tion of methylenecyclopropane 25.
Consider next the possibility of formation of carbene
27 as a discrete intermediate. Carbene 27a cannot ring
expand since the TMS and methyl groups are anti.
Carbene 27a is also in an unfavorable conformation for
silicon migration. However, upon bond rotation, 27a
passes through a conformation close to 27c, where SiMe3
migration should be facile. Conformation 27a can there-
fore lead to the silyl migration product 25. On the other
hand, conformation 27b is the conformation that can ring
expand to give 24. However, analogous to the proposal
of J ones, Shevlin,3 and Platz,4 the discrete intermediacy
of carbene 27b in formation of 24 remains open to
question. These suggestions are summarized in the
following scheme.
F igu r e 2. B3LYP/6-31G* energy levels of carbene 27 and the
rearrangement products 24 and 25. B3LYP/6-3111+G**//
B3LYP/6-31G* energy levels are shown in parentheses.
in 27c with the conjugating adjacent silicon-carbon
bond. In support of this suggestion, the bond from silicon
to the cyclopropyl carbon increases from 1.902 Å in 27a
to 1.957 Å in 27c. The Si-C-C bond angle also decreases
to 102° in 27c. When the Si-C-C-CH3 dihedral angle
in 27c is not constrained, attempted geometry optimiza-
tion leads to SiMe3 migration and formation of methyl-
enecyclopropane 25 without barrier.
The transition state for interconversion of 27a and 27b
was located on the energy surface and corresponds to 27-
TS. This transition state has one imaginary frequency
and hence corresponds to an energy maximum, which lies
5.4 kcal/mol above 27a . The Si-C-C-CH3 dihedral angle
of 65° indicates minimal carbene stabilization from the
cyclopropyl ring but the beginnings of stabilization by the
adjacent silicon. In line with this interpretation is the
lengthening of the Si-C bond to 1.925 Å. However,
maximal interaction with the adjacent Si-C bond, as in
27c, has not been achieved. The energy levels of these
carbene conformations are shown in Figure 2.
The computational profile of carbene 22(R-H) (Figure
3) is analogous to that of carbene 27(R-CH3). There are
two minima (22a and 22b) with 22b being lower in
energy by 1.0 kcal/mol at the B3LYP/6-311+G**//B3LYP/
6-31G* level. The transition state (22-TS) for intercon-
version of these conformations lies 6.5 kcal/mol above
22a . The Si-C bond length of 1.938 Å is again indicative
of some stabilization of this transition state by the
beginning of an interaction of the carbene vacant orbital
with the Si-C bond.
An explanation for the smaller amount of silyl migra-
tion (5%) in the products derived from diazocompound
21 would be desirable. The large amount of ring expanded
product 19 might well arise from a concerted loss of N2/
ring expansion process, as suggested for the parent
diazocompound 6.3,4 This possibility cannot be ruled out
with current information, but there is no obvious reason
(such as steric effects) why R-CH3 substitution in diazo-
compound 26 should change the behavior. Another pos-
sibility involves a subtle conformational difference be-
tween diazocompounds 21 and 26. The calculated
minimum energy structure of 21 has a Si-C-C-H
dihedral angle of 77°. Loss of molecular nitrogen from
21, along with a 77° bond rotation, would lead preferen-
tially to the syn-carbene 22b. A larger rotation would be
required to generate the anti-conformation of 22a . The
syn-conformation 22b is the conformation that leads to
the ring expanded product 19 (95%). Diazocompound 26
(20) Our calculated rotational barrier for conversion of anti-3 to
syn-3 is 15.1 kcal/mol at the B3LYP/6-31G* level. This transition state
is very close in structure (H-C-C-H dihedral ) 96°) and energy to
the 90° constrained structure. A previous barrier to rotation of 13.1
kcal/mol for conversion of anti-3 to syn-3 (single point MP4SDQ/6-31G*
approximated value) has been reported.18c,d

â-Trimethylsilyl Cyclopropylcarbenes
J . Org. Chem., Vol. 66, No. 4, 2001 1119
F igu r e 3. B3LYP/6-31G* structures of carbenes 22a , 22b, and the transition state 27-TS for interconversion of 22a and 22b.
F igu r e 4. B3LYP/6-31G* structures of carbenes 11 and the transition state for silyl migration 11-TS.
differs in that the 87° Si-C-C-CH3 dihedral angle could
lead to either carbene 27a or 27b with comparable
facility.
â-silyl stabilization is the Si-C bond length of 1.957 Å,
as well as the Si-C-C-C dihedral angle of 106°.
Although this angle is not exactly 90°, it still permits a
substantial interaction of the Si-C bond with the carbene
vacant orbital. The total energy of 11 is also in line with
this suggestion. Carbene 11 is 11.1 kcal/mol more stable
than the isomeric carbene 27b, where only cyclopropyl
stabilization is important. Finally, the transition state
for migration of the trimethylsilyl group (11-TS) has been
located, and it lies only 2.0 kcal/mol above 11. This is
significantly smaller than previously calculated barriers
for hydrogen migration to typical carbene centers.14,21
This remarkably small calculated barrier to silyl rear-
rangement is consistent with the experimentally ob-
served exclusive formation of alkene 14 from 11. The
â-trimethylsilyl group not only stabilizes carbene 11, but
this interaction leads to facile trimethylsilyl migration.
The B3LYP/6-31G* calculated structure of carbene 11
is shown in Figure 4. This carbene appears to be
stabilized simultaneously by the cyclopropyl group as
well as the â-trimethylsilyl group. A manifestation of this
(21) Ford, F.; Yazawa, T,; Platz, M. S.; Matzinger, S.; Fu¨lscher, M.
J . Am. Chem. Soc. 1998, 120, 4430.

1120 J . Org. Chem., Vol. 66, No. 4, 2001
Creary and Butchko
1
116-119 °C, was collected. H NMR of 23a (CDCl3) δ 7.83 (d,
J ) 8.4 Hz, 2 H), 7.29 (d, J ) 8.1 Hz, 2 H), 2.42 (s, 3 H), 1.60
(s, 3 H), 0.73-0.69 (m, 2 H), 0.62-0.59 (m, 2 H), -0.14 (s, 9
H). Anal. Calcd for C15H24N2O2SSi: C, 55.52; H, 7.45. Found:
C, 55.56; H, 7.46.
Con clu sion s
Thermal decomposition of the tosylhydrazone salt 16,
a potential precursor to carbene 11, gives the product
from 1,2-silyl migration exclusively. The potential ring
expansion, fragmentation, or 1,2-hydrogen migration
processes do not compete with the facile 1,2-silyl migra-
tion. 1-Trimethylsilylcyclopropylcarbenes (or transition
states having carbenic character) rearrange to form
cyclobutenes along with varying amounts of methyl-
enecyclopropanes derived from silyl migration. The meth-
ylenecyclopropane products are significant in that meth-
ylenecyclopropanes are not observed in the rearrangement
of the parent systems, which do not contain trimethylsilyl
groups. 1,2-Silyl migration to carbene like centers again
successfully competes with the ring expansion process.
Computational studies (B3LYP/6-31G*) show two mini-
mum energy conformations for 1-trimethylsilylcyclopro-
pylcarbenes 22 and 26 with the cyclopropyl ring conju-
gated with the carbene vacant orbital. The 90° rotated
conformation, which is presumably the optimal orienta-
tion for silyl interaction with the carbene vacant orbital,
was not an energy minimum. Rotational barriers of 6.5
and 5.4 kcal/mol for interconversion of conformations of
22 and 26 have been calculated. These barriers are
significantly lower than the 15.1 kcal/mol barrier for
interconversion of the parent cyclopropylcarbene con-
formers. It is suggested that one carbene conformer leads
to cyclobutene (ring expansion), while the other con-
former, upon rotation, can lead to methylenecyclopro-
panes (silyl migration). When in competition, cyclopropyl
stabilization wins out over â-trimethylsilyl stabilization
of carbenes. However the ready availability of silyl
stabilized carbene conformations allows for facile silyl
migration to cyclopropyl carbenic centers.
P r ep a r a tion of 1-Tr im eth ylsilylcyclop r op yl Eth yl Ke-
ton e. Ethyllithium was prepared by dropwise addition of a
solution of 2.18 g of ethyl bromide (20.0 mmol) in 5 mL of ether
to a mixture of 367 mg of lithium wire (containing 1% sodium)
(52.8 mmol) in 15 mL of ether at 0 °C. The mixture was stirred
at 0 °C for 1 h and at room temperature for an additional 15
min. The ethyllithium solution was then added dropwise to a
solution of 1.181 g of 1-trimethylsilylcyclopropyl carboxylic
acid22 (7.459 mmol) in 15 mL of ether at 0 °C. The reaction
mixture was then stirred at room temperature for 8 h,
quenched with 25 mL of saturated NH4Cl solution, and
extracted with ether. The ether extract was washed with
saturated NaCl solution and dried over a mixture of Na2SO4
and MgSO4. After filtration, the ether solvent was removed
using a rotary evaporator. The residue was chromatographed
on 11 g of silica gel. Elution with 10% ether in hexanes gave
436 mg (34% yield) of 1-trimethylsilylcyclopropyl ethyl ketone.
1H NMR (CDCl3) δ 2.24 (q, J ) 7.2 Hz, 2 H), 1.09 (d of d, J )
4.2, 6.4 Hz, 2 H), 1.00 (t, J ) 7.2 Hz, 3 H), 0.78 (d of d, J )
3.7, 5.9 Hz, 2 H), 0.049 (s, 9 H). 13C NMR (CDCl3) δ 212.7
(quat), 31.4 (t, J ) 125 Hz), 20.8 (quat), 11.2 (t, J ) 163 Hz),
8.3 (q, J ) 127 Hz), -2.2 (q, J ) 119 Hz). HRMS (EI) calcd for
C9H18OSi 170.1127, found 170.1091.
P r ep a r a tion of Tosylh yd r a zon e 23b. A suspension of 446
mg of p-toluenesulfonyl hydrazide in 0.5 mL of methanol was
stirred as a solution of 400 mg of 1-trimethylsilylcyclopropyl
ethyl ketone prepared above in 2.5 mL of methanol was added
at room temperature. The mixture became homogeneous, and
after 18 h the mixture was cooled in a freezer. Crystallization
of the tosylhydrazone 23b occurred, and the excess methanol
solvent was decanted. The remaining solid was washed with
a minimal amount of cold hexanes. The solid was dried for
several hours on the rotary evaporator to give 454 mg of
tosylhydrazone 23b (57% yield), mp 68-70 °C. 1H NMR
(CDCl3) δ 7.82 (d, J ) 8.4 Hz, 2 H), 7.30 (d, J ) 8.1 Hz, 2 H),
2.42 (s, 3 H), 1.93 (q, J ) 7.8 Hz, 2 H), 0.97 (t, J ) 7.8 Hz, 3
H), 0.74-0.68 (m, 2 H), 0.64-0.58 (m, 2 H), -0.15 (s, 9 H).
Anal. Calcd for C16H26N2O2SSi: C, 56.76; H, 7.74. Found: C,
57.19; H, 7.60.
Exp er im en ta l Section
P r ep a r a tion of Lith iu m Sa lt 16. A solution of 1.272 g of
tosylhydrazone 132 (5.04 mmol) in 8 mL of THF was cooled to
-78 °C, and 6.3 mL of 1.6 M n-butyllithium (10.1 mmol) was
added. The mixture was then warmed to -20 °C over 30 min
and then recooled to -50 °C. Trimethylsilyl chloride (561 mg;
5.16 mmol) was added at -50 °C, and the mixture was then
gradually warmed to room temperature. After 3 h the solvent
was removed using a rotary evaporator to give 1.87 g of the
crude lithium salt 16. The salt 16 was washed with about 10
mL of hexanes, and the hexanes were decanted using a pipet.
After evacuation using a rotary evaporator, the last traces of
hexanes were removed at a pressure of 0.1 mm.
P r ep a r a tion of Tosylh yd r a zon e 18. A suspension of 271
mg of p-toluenesulfonyl hydrazide (1.46 mmol) in 2 mL of
methanol was stirred as a solution of 208 mg of 1-trimethyl-
silylcyclopropane carboxaldehyde9 (1.46 mmol) in 0.5 mL of
methanol was added at room temperature. The reaction
mixture became homogeneous and was kept at room temper-
ature for 4.5 h. 1H NMR analysis of an aliquot showed that
the reaction had gone to completion. The tosylhydrazone 18
did not crystallize, even upon standing in methanol in the
freezer for 5 days. The solution of 18 in methanol was
converted directly to the sodium salt by reaction with sodium
methoxide. H NMR of 18 (CDCl3) δ 7.80 (d, J ) 8.4 Hz, 2 H),
7.31 (d, J ) 7.8 Hz, 2 H), 6.67 (s, 1 H), 2.43 (s, 3 H), 0.78-0.74
(m, 2 H), 0.71-0.67 (m, 2 H), -0.070 (s, 9 H).
P r ep a r a tion of Tosylh yd r a zon e 23a . A suspension of 167
mg of p-toluenesulfonyl hydrazide (0.897 mmol) in 1 mL of
methanol was stirred as a solution of 135 mg of 1-trimethyl-
silylcyclopropyl methyl ketone11 (0.866 mmol) in 0.5 mL of
methanol was added at room temperature. The mixture
became homogeneous, and after 20 h the mixture was cooled
in a freezer. The tosylhydrazone 23a (271 mg, 97% yield), mp
P r ep a r a t ion of 1-Tr im et h ylsilylcyclop r op yl n -Bu t yl
Keton e. A solution containing 510 mg of 1-trimethylsilylcy-
clopropyl carboxylic acid (3.228 mmol) in 12 mL of ether was
stirred at 0 °C, and 5.0 mL of 1.6 M n-butyllithium (8.0 mmol)
was added under nitrogen. The mixture was then warmed to
room temperature for 20 h, recooled to 0 °C, and then quenched
with 15 mL of saturated NH4Cl solution. The ether phase was
separated, washed with saturated NaCl solution, and dried
over a mixture of Na2SO4 and MgSO4. After filtration, the ether
was removed using a rotary evaporator and the residue was
chromatographed on 11 g of silica gel. Elution with 5% ether
in hexanes gave 346 mg (54% yield) of 1-trimethylsilylcyclo-
1
propyl n-butyl ketone. H NMR (CDCl3) δ 2.19 (t, J ) 7.3 Hz,
2 H), 1.58-1.42 (m, 2 H), 1.34-1.20 (m, 2 H), 1.08 (d of d, J )
3.6, 5.7 Hz, 2 H), 0.88 (t, J ) 7.3 Hz, 3 H), 0.78 (d of d, J )
4.0, 6.1 Hz, 2 H), 0.048 (s, 9 H). 13C NMR (CDCl3) δ 212.4
(quat), 37.8 (t, J ) 124 Hz), 26.5 (t, J ) 125 Hz), 22.4 (t, J )
120 Hz), 21.0 (quat), 14.0 (q. J ) 125 Hz), 11.1 (t, J ) 162
Hz), -2.2 (q, J ) 120 Hz). HRMS (EI) calcd for C11H22OSi
198.1440, found 198.1450.
P r ep a r a tion of Tosylh yd r a zon e 23c. A suspension of 249
mg of p-toluenesulfonyl hydrazide in 0.5 mL of methanol was
stirred at room temperature as a solution of 262 mg of
1-trimethylsilylcyclopropyl n-butyl ketone in 2.5 mL of metha-
nol was added. The mixture was stirred at room temperature
for 17 h, and some of the methanol was then removed using a
rotary evaporator. The mixture was cooled in a freezer, and
the excess methanol was decanted from the solid that formed.
1
(22) Ainsworth, C.; Kuo, Y.-N. J . Organomet. Chem. 1972, 46, 73.

â-Trimethylsilyl Cyclopropylcarbenes
J . Org. Chem., Vol. 66, No. 4, 2001 1121
The last traces of methanol were removed using a rotary
evaporator to give 470 mg (97% yield) of tosylhydrazone 23c,
mp 79-80 °C. 1H NMR (CDCl3) δ 7.82 (d, J ) 8.4 Hz, 2 H),
7.78 (br s, 1 H), 7.30 (d, J ) 8.1 Hz, 2 H), 2.42 (s, 3 H), 1.87
(m, 2 H), 1.34-1.18 (m, 4 H), 0.82 (t, J ) 7.1 Hz, 3 H), 0.68
(m, 2 H), 0.57 (m, 2 H), -0.15 (s, 9 H). 13C NMR (CDCl3) δ
164.6 (s), 143.8 (s), 135.6 (s), 129.5 (d, J ) 162 Hz), 128.2 (d,
J ) 165 Hz), 27.7 (t, J ) 129 Hz), 27.4 (t, J ) 127 Hz), 23.1 (t,
J ) 126 Hz), 21.6 (q, J ) 128 Hz), 14.4 (s), 13.7 (q, J ) 125
Hz), 8.5 (t, J ) 163 Hz), -2.4 (q, J ) 119 Hz).
An attempt to isolate a pure sample of cyclobutene 24a by
preparative gas chromatography led to complete thermal
rearrangement to 2-methyl-3-trimethylsilyl-1,3-butadiene. 1H
NMR of 2-methyl-3-trimethylsilyl-1,3-butadiene (CDCl3) δ 5.74
(d, J ) 2.6 Hz, 1 H), 5.43 (d, J ) 2.6 Hz, 1 H), 4.95 (d, J ) 1.3
Hz, 1 H), 4.90 (d, J ) 1.1 Hz, 1 H), 1.88 (bs, 3 H), 0.17 (s, 9 H).
13C NMR (CDCl3) δ 152.71, 146.37, 125.10, 113.75, 22.14,
-0.41.
P yr olysis of th e Sod iu m Sa lt of 23b. Vacuum pyrolysis
of the dry sodium salt of 23b (R ) Et) gave an 80% yield of a
mixture, containing 60% of 25b and 40% of 24b as determined
by 1H NMR spectroscopy. The cyclobutene 24b completely
rearranged to 2-ethyl-3-trimethylsilyl-1,3-butadiene during
preparative gas chromatography. 1H NMR of 2-ethyl-3-tri-
methylsilyl-1,3-butadiene (CDCl3) δ 5.69 (d, J ) 3.0 Hz, 1 H),
5.40 (d, J ) 3.0 Hz, 1 H), 4.84 (d, J ) 0.9 Hz, 1 H), 4.78 (d, J
) 0.8 Hz, 1 H), 2.19 (q, J ) 7.5 Hz, 2 H), 1.02 (t, J ) 7.5 Hz,
3 H), 0.14 (s, 9 H). 13C NMR of 2-ethyl-3-trimethylsilyl-1,3-
butadiene (CDCl3) δ 153.43 (quat), 153.40 (quat), 124.86 (d of
d, J ) 155, 158 Hz), 109.84 (t, J ) 156 Hz), 28.47 (t, J ) 126
P yr olyses of Tosylh yd r a zon e Sa lts. Gen er a l P r oce-
d u r e.7 The tosylhydrazones 18 or 23 (1.00 equiv) were placed
in a flask, and 1.06 equiv of NaOCH3 (approx 0.6 M in
methanol) was added with stirring. After the tosylhydrazone
dissolved, the methanol solvent was removed using a rotary
evaporator. The solid salt was further dried by evacuation at
15 mm and at 0.05 mm. The flask containing the dry salt was
then fitted with a short path distillation head and a receiver
flask and placed in an oil bath. The temperature of the oil bath
was gradually raised to 80 °C, and then the receiver flask was
cooled in a dry ice/acetone bath. The temperature of the oil
was then raised gradually to 180 °C while maintaining the
system under vacuum. During this time the pressure rose to
approximately 2 mm and then decreased back to 0.05 mm. The
products of these pyrolyses collected in the cold receiver flask.
Structures were determined by standard spectroscopic tech-
1
Hz), 12.68 (q, J ) 127 Hz), -0.65 (q, J ) 119 Hz). H NMR of
25b (CDCl3) δ 2.26 (q of quintets, J ) 1.3, 7.5 Hz, 2 H), 1.08-
1.05 (m, 2 H), 1.05 (t, J ) 7.5 Hz, 3 H), 0.99-0.96 (m, 2 H),
0.12 (s, 9 H). 13C NMR of 25b (CDCl3) δ 130.97 (quat), 129.38
(quat), 27.65 (t, J ) 123 Hz), 14.37 (q, J ) 126 Hz), 2.76 (t, J
) 160 Hz), 1.00 (t, J ) 161 Hz), -0.88 (q, J ) 119 Hz). HRMS
(EI) calcd for C9H18Si 154.1178, found 154.1159.
1
niques. Product ratios were determined by H NMR spectros-
P yr olysis of th e Sod iu m Sa lt of 23c. Vacuum pyrolysis
of the dry sodium salt of 23c (R ) n-Bu) gave a 91% yield of
a mixture containing 57% of 25c and 43% of 24c as determined
by 1H NMR spectroscopy. The cyclobutene 24c completely
rearranged to 2-n-butyl-3-trimethylsilyl-1,3-butadiene during
preparative gas chromatography. 1H NMR of 2-n-butyl-3-
trimethylsilyl-1,3-butadiene (CDCl3) δ 5.68 (d, J ) 3.0 Hz, 1
H), 5.40 (d, J ) 3.1 Hz, 1 H), 4.82 (d, J ) 1.2 Hz, 1 H), 4.78 (d,
J ) 2.0 Hz, 1 H), 2.17 (t, J ) 7.3, 2 H), 1.42-1.24 (m, 4 H),
copy. Pure samples of products were isolated by preparative
gas chromatography. The following procedures are representa-
tive.
P yr olysis of th e Lith iu m Sa lt 16. The dry salt 16 was
prepared as described previously. Vacuum pyrolysis of 760 mg
of 16, as described above, gave 206 mg (64% yield) of the
vinylcyclopropane 14 as the exclusive product. 1H NMR
(CDCl3) δ 5.34 (d of d, J ) 1.3, 2.6 Hz, 1 H), 5.17 (d of d, J )
0.7, 2.6 Hz, 1 H), 1.41 (m, 1 H), 0.64 (d of d of d, J ) 4.0, 6.0,
8.2 Hz, 2 H), 0.44-0.41 (m, 2 H), 0.13 (s, 9 H). 13C NMR
(CDCl3) δ 153.77 (quat), 119.67 (t, J ) 151 Hz), 15.26 (d, J )
156 Hz), 6.48 (t, J ) 161 Hz), -1.54 (q, J ) 119 Hz). HRMS
(EI) calcd for C8H16Si 140.1021, found 140.1005.
0.89 (t, J ) 7.3 Hz, 3 H), 0.14 (s, 9 H). HRMS (EI) calcd for
1
C
11H22Si 182.1491, found 182.1462. H NMR of 25c (CDCl3) δ
2.24 (t of quintets, J ) 1.3, 7.5 Hz, 2 H), 1.42 (quintet, J ) 7.5
Hz, 2 H), 1.29 (sextet, J ) 7.5 Hz, 2 H), 1.10-1.07 (m, 2 H),
0.96-0.93 (m, 2 H), 0.90 (t, J ) 7.5 Hz, 3 H), 0.11 (s, 9 H). 13
C
P yr olysis of th e Sod iu m Sa lt of 18. The tosylhydrazone
18 prepared above (1.46 mmol) was reacted with 2.6 mL of
0.58 M NaOCH3 in methanol (1.50 mmol). After removal of
the solvent and pyrolysis as described above, 103 mg (56%
yield) of a mixture containing 95% 1923 and 5% 2024 was
collected. Structural assignments were made by comparison
with reported spectral data. A sample of 20 was isolated by
preparative gas chromatography. 1H NMR of 20 (CDCl3) δ 5.97
(quintet, J ) 2.0 Hz, 1 H), 1.16-0.96 (m, 4 H), 0.11 (s, 9 H).
These observed signals are in accord with the literature
NMR of 25c (CDCl3) δ 131.67 (quat), 128.00 (quat), 34.46 (t, J
) 125 Hz), 31.94 (t, J ) 125 Hz), 22.74 (t, J ) 124 Hz), 14.09
(q, J ) 124 Hz), 3.25 (t, J ) 158 Hz), 1.13 (t, J ) 159 Hz),
-0.82 (q, J ) 119 Hz). HRMS (EI) calcd for C11H22Si 182.1491,
found 182.1476.
Com p u ta tion a l Stu d ies. Ab initio molecular orbital cal-
culations were performed using the Gaussian 94 and Gaussian
98 series of programs.25 Energy minima were characterized
via frequency calculations which showed no imaginary fre-
quencies. Electronic energies are presented without zero point
vibrational energies. Transition states showed one imaginary
frequency.
1
spectral data for 20. H NMR of 19 (CDCl3) δ 6.47 (t, J ) 0.9
Hz, 1 H), 2.66 (t of d, J ) 1.2, 3.6 Hz, 2 H), 2.54-2.52 (m, 2
H), 0.047 (s, 9 H). 13C NMR of 19 (CDCl3) δ 156.10 (quat),
147.63 (d, J ) 166 Hz), 32.15 (t, J ) 136 Hz), 31.83 (t, J )
137 Hz), -2.23 (q, J ) 119 Hz).
Ack n ow led gm en t is made to the National Science
Foundation for support of this research.
P yr olysis of th e Sod iu m Sa lt of 23a . Vacuum pyrolysis
of the dry sodium salt of 23a (R ) CH3) gave a 63% yield of a
mixture containing 65% of 25a and 35% of 24a . The cy-
clobutene 24a 23 is a known compound with reported spectral
data. Thus, spectral data for 25a and 24a were extracted from
Su p p or t in g In for m a t ion Ava ila b le: Structures and
energies of diazocompounds 21 and 26, carbenes 11, 22, and
27, carbene rearrangement transition states 11-TS, 22-TS,
and 27-TS, and spectral data for 23c. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
the spectrum of the product mixture. H NMR of 25a (CDCl3)
δ 1.86 (quintet, J ) 1.8 Hz, 3 H), 1.14-1.10 (m, 2 H), 0.93-
0.89 (m, 2 H), 0.11 (s, 9 H). 13C NMR of 25a (CDCl3) δ 131.64
(quat), 123.35 (quat), 19.66 (q, J ) 126 Hz), 3.98 (t, J ) 160
J O001112B
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Liashenko, A.; Piskorz, P.; Komaromi, I. Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen, W.;
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1
Hz), 0.40 (t, J ) 160 Hz), -1.56 (q, J ) 119 Hz). H NMR of
24a (CDCl3) δ 2.54 (m, 2 H), 2.27 (m, 2 H), 1.74 (m, 3 H), 0.067
(s, 9 H). 13C NMR of 24a (CDCl3) δ 157.76 (quat), 144.24 (quat),
34.10 (t, J ) 134 Hz), 27.46 (t, J ) 137 Hz), 18.14 (q, J ) 125
Hz), -1.30 (q, J ) 118 Hz).
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Y.; Lukovskii, B. A.; Polonskii, A. V.; Beletskaya, I. P. Russ. J . Org.
Chem. 1993, 29, 100.