Silicon migration from oxygen to carbon and decarbonylation in methoxytriphenylsiloxycarbene

Article abstract of DOI:10.1021/ol006117+

Thermolysis of 2-methoxy-2-triphenylsiloxy-5,5-dimethyl-Δ³-1,3,4-oxadiazoline affords methyl triphenylsilylformate and methyl triphenylsilyl ether via methoxytriphenylsiloxycarbene. Kinetics show that the carbene undergoes reversible 1,2-triphenylsilyl migration (Brook rearrangement) as well as irreversible decarbonylation. Computed transition states and activation energies (B3LYP/6-31+G*) suggest that the migration of the silyl group from oxygen to carbon occurs through an "in plane" transition state with the carbene lone pair forming a new bond to silicon. Decarbonylation involves a four-membered ring, achieved by nucleophilic attack of the oxygen atom of the methoxy group at silicon.

Full text of DOI:10.1021/ol006117+

ORGANIC
LETTERS
2000
Vol. 2, No. 18
2733-2736
Silicon Migration from Oxygen to
Carbon and Decarbonylation in
Methoxytriphenylsiloxycarbene
,†
John Paul Pezacki,* Paul G. Loncke,‡ Joseph P. Ross,† John Warkentin,*,†,§ and
,‡
Timothy A. Gadosy*
Department of Chemistry, McMaster UniVersity, 1280 Main Street West,
Hamilton, Ontario, Canada L8S 4M1, and Department of Chemistry and Biochemistry,
Concordia UniVersity, 1455 DeMaisonneuVe BlVd. Ouest,
Montre´al, Que´bec, Canada H3G 1M8
gadosy@alcor.concordia.ca
Received May 26, 2000
ABSTRACT
Thermolysis of 2-methoxy-2-triphenylsiloxy-5,5-dimethyl-∆3-1,3,4-oxadiazoline affords methyl triphenylsilylformate and methyl triphenylsilyl
ether via methoxytriphenylsiloxycarbene. Kinetics show that the carbene undergoes reversible 1,2-triphenylsilyl migration (Brook rearrangement)
as well as irreversible decarbonylation. Computed transition states and activation energies (B3LYP/6-31+G*) suggest that the migration of the
silyl group from oxygen to carbon occurs through an “in plane” transition state with the carbene lone pair forming a new bond to silicon.
Decarbonylation involves a four-membered ring, achieved by nucleophilic attack of the oxygen atom of the methoxy group at silicon.
∆3-1,3,4-Oxadiazolines (1) are now established thermal
sources of dialkoxy-, alkoxyamino-, and alkoxythioalkoxy-
carbene intermediates1,2 that can also function as photo-
chemical precursors of dialkylcarbenes or cycloalkylidenes
for kinetic studies by LFP (Scheme 1).3 The special properties
of silyl moieties in carbene chemistry that have emerged
recently4,5 including the apparent migration of trimethylsilyl
to the carbene (or carbenoid) site of a cyclopropylidene, to
† McMaster University.
‡ Concordia University.
§ E-mail: warkent@McMaster.ca.
(1) Warkentin, J. In AdVances in Carbene Chemistry, Vol. 2; Brinker,
U., Ed.; JAI Press Inc.: Greenwich, 1998; pp 245-295.
Scheme 1
(2) (a) Couture, P.; Warkentin, J. Can. J. Chem. 1997, 75, 1264. (b)
Couture, P.; Warkentin, J. Can. J. Chem. 1997, 75, 1281. (c) Kassam, K.;
Venneri, P.; Warkentin, J. Can. J. Chem. 1997, 75, 1256. (d) Kassam, K.;
Warkentin, J. Can. J. Chem. 1997, 75, 120. (e) Kassam, K.; Warkentin, J.
J. Org. Chem. 1994, 59, 5071. (f) Er, H.-T.; Pole, D. L.; Warkentin, J.
Can. J. Chem. 1996, 74, 1480.
(3) (a) Pezacki, J. P.; Pole, D. L.; Warkentin, J.; Chen, T.; Ford, F.;
Toscano, J.; Fell, J.; Platz, M. S. J. Am. Chem. Soc. 1997, 119, 3191. (b)
Pezacki, J. P.; Wood, P. D.; Gadosy, T. A.; Lusztyk, J.; Warkentin, J. J.
Am. Chem. Soc. 1998, 120, 8681. (c) Pezacki, J. P.; Warkentin, J.; Wood,
P. D.; Lusztyk, J.; Yuzawa, T.; Gudmundsdottir, A. D.; Morgan, S.; Platz,
M. S. J. Photochem. Photobiol. A: Chem. 1998, 116, 1. (d) Pezacki, J. P.;
Couture, P.; Dunn, J. A.; Warkentin, J.; Wood, P. D.; Lusztyk, J.; Ford, F.;
Platz, M. S. J. Org. Chem. 1999, 64, 4456. (e) Tae, E. L.; Zhu, Z.; Platz,
M. S.; Pezacki, J. P.; Warkentin, J. J. Phys. Chem. A 1999, 103, 5336.
10.1021/ol006117+ CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/11/2000

the exclusion of allene formation (eq 1),6 led us to try to
generate (MeO(Ph3SiO)C:) by the oxadiazoline route.
°C was only 6.5 × 10-6 s-1, too small to account for the
formation of 8 from oxadiazoline 6 via 7 (Table 1).
1
Table 1. Thermolysis Rate Constants Determined by H NMR
Spectroscopy
Treatment of acetoxyoxadiazoline 57 with triphenysilanol
and catalytic trifluoroacetic acid afforded 6 in ca. 33% yield
(Scheme 2). After chromatography on silica, 6 was heated
precursor
solvent
thermolysis rate constant (110 °C)
6
benzene
methanol
benzene
methanol
kobs ) 8.1 × 10-5 s-1
kobs ) 6.1 × 10-5 s-1
kobs ) 6.5 × 10-6 s-1
kobs ) 8.2 × 10-4 s-1
7
Scheme 2
Thermolysis of 6 in toluene did not afford bibenzyl, and
added TEMPO did not lead to any detectable products of
radical trapping.9,10 It is therefore likely that carbene 9
(Scheme 3) is responsible for the formation of both 7 and 8.
Scheme 3
in degassed benzene for 24 h at 110 °C, in a sealed tube.
Major products were acetone, methyl triphenylsilylformate
(7), and methyl triphenylsilyl ether (8), with 7:8 ) 1:3
(Scheme 2).
Isolation of 7 and heating it in benzene (150 °C, 24 h,
sealed tube) converted it cleanly to 8, as reported by Brook
and co-workers.8 The rate constant for this process at 110
Thermolysis of 6 in methanol-d4 afforded orthoformate 10,
the H NMR spectrum of which showed both the expected
2
OCD3 and the orthoformyl CD signals. However, the yield
was low (∼8%) and both ester 7 and ether 8 were coproducts.
The majority of the product was ether 8 (unlabeled). The
rate constant for conversion of 7 to 8 is much larger in
methanol than in benzene (Table 1), suggesting that this
process occurs through a polar transition state. Thermolysis
of 7 in CD3OD at 110 °C also afforded 10, again in low
yield, Scheme 3. Thus, it appears that rearrangement of the
carbene intermediate is fast relative to trapping with metha-
nol, which we assume occurs with a bimolecular rate constant
(ignoring an isotope effect) of ca. 1 × 105 M-1 s-1, the
measured rate constant for trapping dimethoxycarbene.11
The most straightforward interpretation of these results
involves thermal cycloreversion of 6, to generate nitrogen
gas and a carbonyl ylide12 that fragments rapidly to acetone
and 9. Carbene 9 rearranges by migration of the triphenylsilyl
group from oxygen to carbon by a Brook rearrangement.
Several cases of gas-phase migrations of an alkyl group from
oxygen to carbon in dialkoxycarbenes are known. From the
preferential migration of 2,2,2-trifluoroethyl relative to
methyl or ethyl,13 it appears that negative charge develops
in the migrating group. Those dialkoxycarbenes do not
rearrange appreciably in solution, however, suggesting that
triphenylsilyl is a superior migrating group. This discovery
has implications for the thermal generation of other carbenes
by rearrangements of silyl analogues. It may be possible to
design systems with other double bonds, such as CdN, that
will rearrange similarly (i.e., diaminocarbenes, R2NC(dNR)-
SiR3 f R2N(NRSiR3)C: and alkoxyaminocarbenes, ROC-
(dNR)SiR3 f RO(NRSiR3)C:, or R2NC(dO)SiR3 f R2N-
(OSiR3)C:).
(4) For a review of R-siloxycarbenes generated from acylsilanes, see:
Brook, A. G. In The Chemistry of Organic Silicon Compounds; Patai, S.,
Rappoport, Z., Eds.; John Wiley and Sons: New York, 1989; Chapter 15,
pp 984-988. Also see: (a) Brook, A. G.; Kucera, H. W.; Pearce, R. Can.
J. Chem. 1971, 49, 1618. (b) Duff, J. M.; Brook, A. G. Can J. Chem. 1973,
51, 2869. (c) Bourque, R. A.; Davis, P. D.; Dalton, J. C. J. Am. Chem. Soc.
1981, 103, 697.
(5) (a) Pole, D. L.; Sharma, P. K.; Warkentin, J. Can. J. Chem. 1996,
74, 1335. (b) Kirmse, W.; Guth, M.; Steenken, S. J. Am. Chem. Soc. 1996,
118, 10838. (c) Gonza´lez, R.; Wudl, F.; Pole, D. L.; Sharma, P. K.;
Warkentin, J. J. Org. Chem. 1996, 61, 5837. (d) Kirmse, W.; Konrad, W.;
Schnitzler, D. J. Org. Chem. 1994, 59, 3821. (e) Walsh, R.; Wolf, C.;
Untiedt, S.; de Meijere, A. J. Chem. Soc., Chem. Commun. 1992, 421. (f)
Walsh, R.; Untiedt, S.; de Meijere, A. Chem. Ber. 1994, 127, 237. (g)
Shimizu, H.; Gordon, M. S.; Organometallics 1994, 13, 186. (h) Barton,
T. J.; Lin, J.; Ijadi-Maghsoodi, S.; Power, M. D.; Zhang, X.; Ma, Z.;
Shimuzu, H.; Gordon, M. S. J. Am. Chem. Soc. 1995, 117, 11695. (i) Creary,
X.; Wang, Y.-X. J. Org. Chem. 1994, 59, 1604.
(6) Baird, M. S.; Dale, C. M.; Al Dulayymi, J. R. J. Chem. Soc., Perkin
Trans. 1 1993, 1373.
(7) Kassam, K.; Pole, D. L.; El-Saidi, M.; Warkentin, J. J. Am. Chem.
Soc. 1994, 116, 1161.
(8) (a) Brook, A. G. J. Am. Chem. Soc. 1955, 77, 4827. (b) Brook, A.
G.; Mauris, R. J. J. Am. Chem. Soc. 1957, 79, 971.
(9) It is well established that other metallo formates containing Sn, Se,
or Te can undergo thermal or photochemical M-C bond homolysis to give
acyl radical/metal radical pairs. Acyl radicals then can undergo either
decarboxylation or decarbonylation. See: Lucas, M. A.; Schiesser, C. H.
J. Org. Chem. 1996, 61, 5754 and references therein.
(10) Some dialkoxycarbenes fragment in solution to radical pairs. See:
(a) Venneri, P. C.; Warkentin, J. J. Am. Chem. Soc. 1998, 120, 11182. (b)
Merkley, N.; El-Saidi, M.; Warkentin, J. Can. J. Chem. 2000, 78, 356. (c)
Reid, D. L.; Herna´ndez-Trujillo, J.; Warkentin, J. J. Phys. Chem. A 2000,
104, 3398.
(11) (a) Du, X.-M.; Fan, H.; Goodman, J. L.; Kesselmayer, M. A.; Krogh-
Jespersen, K.; LaVilla, J. A.; Moss, R. A.; Shen, S.; Sheridan, R. S. J. Am.
Chem. Soc. 1990, 112, 1920. (b) Moss, R. A.; Wlostowski, M.; Shen, S.;
Krogh-Jespersen, K.; Matro, A. J. Am. Chem. Soc. 1988, 110, 4443.
(12) Couture, P.; El-Saidi, M.; Warkentin, J. Can. J. Chem. 1997, 75,
326.
(13) Suh, D.; Pole, D. L.; Warkentin, J.; Terlouw, J. K. Can. J. Chem.
1996, 74, 544.
2734
Org. Lett., Vol. 2, No. 18, 2000

To probe for the mechanism of silyl migration, B3LYP/
6-31+G* calculations with the Gaussian 98 suite of pro-
grams14 were carried out on the 1,2-silyl migration in a model
compound, methoxysiloxycarbene (MeOCOSiH3).
The transition state structures, TSAD and TSBE, for 1,2-
silyl migration in the trans-trans conformer (A) and the cis-
trans conformer (B) of methoxysiloxycarbene are shown in
Figure 1. Also shown are the corresponding products, anti
analysis (NPA),15 is consistent with nucleophilic attack by
the carbene lone pair at silicon. There is a buildup of 0.197e
and 0.178e on the silyl group, along with 0.003e and 0.025e
on C6, and a loss of 0.164e and 0.157e from O7 in TSAD
and TSBE, respectively. The electron buildup on C6 is likely
due to the developing C6-O7 double bond.
Frontier molecular orbital coefficients obtained for meth-
oxysiloxycarbene also support the notion of nucleophilic
attack by the carbene lone pair at silicon. The magnitudes
of these coefficients suggest that the HOMO is primarily
made up of nonbonding σ orbitals on O2, O7, and C6 while
the LUMO is in essence the O7-Si σ* orbital. Thus, since
the geometries of A and B allow maximum interaction
between the C6 σ nonbonding orbital (HOMO) and the O7-
Si σ* orbital (LUMO), they favor nucleophilic attack by the
carbene σ lone pair at silicon. This mechanism resembles
one postulated for acyl migrations in acyloxycarbenes that
involves nucleophilic attack of the carbene carbon onto the
carbonyl carbon during acyl migration.16
The mechanism by which the carbene and the ester are
decarbonylated is more complex. The overall result in the
case of 9 is also reminiscent of the rearrangement of
alkoxyhalocarbenes, which do so by fragmentation to carbon
monoxide, carbocation, and halide ion (eq 2). The latter then
Figure 1.
methyl silylformate (D) and syn methyl silylformate (E). The
computed activation barriers for 1,2-silyl migration in A and
B are 10.3 and 7.62 kcal/mol, respectively. These barriers
were calculated as the differences between the zero-point
corrected electronic energies of the transition states and that
of A, the lowest energy ground-state carbene conformer.
Both transition state structures are in keeping with nu-
cleophilic attack by the carbene lone pair at silicon. The
dissociating O7-Si bond lengthens from 1.71 Å in A to 1.92
Å in TSAD, while the C6-O7-Si angle shrinks from 115.4°
to 73.4°. Likewise, the O7-Si bond lengthens from 1.72 Å
in B to 1.91 Å in TSBE, while the C6-O7-Si angle shrinks
from 114.8° to 73.7°. The O2-C6-O7-Si dihedral angles,
175.6° in TSAD and 180.0° in TSBE, indicate essentially
planar transition state structures (Figure 1). This means that
in these structures, the breaking O7-Si bond eclipses the
carbene σ nonbonding orbital. The charge development in
the two transition states, obtained from natural population
combine, in the absence of carbocation scavengers, to afford
alkyl halide in high yield.17,18 An analogous process in the
case of 9 would involve the triphenylsilyl cation/methoxide
anion pair. However, methoxide is sufficiently more basic
than halide ions to make such an ion pair mechanism unlikely
in benzene. We propose that the ester is decarbonylated via
the carbene which cyclizes to afford the four-membered
intermediate 14 by attack of the silophilic methoxy oxygen
at silicon (Scheme 4).
Scheme 4
(14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
Cammi, M.; Mennucci, R.; B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; 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.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.;
Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.7,
Gaussian, Inc.: Pittsburgh, PA, 1998.
This mechanism is supported by the fact that the conver-
sion of ester 7 to ether 8 is more than 10-fold slower than
thermolysis of 6 in benzene, but 6 gives rise to 7 and 8 in
(16) Moss, R. A.; Xue, S.; Liu, W.; Krogh-Jespersen, K. J. Am. Chem.
Soc. 1996, 118, 12588.
(17) Moss, R. A. Acc. Chem. Res. 1999, 32, 9691 and references therein.
(18) Yan, S.; Sauers, R. R.; Moss, R. A. Org. Lett. 1999, 1, 1603.
(15) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,
899.
Org. Lett., Vol. 2, No. 18, 2000
2735

1:3 ratio. Moreover, both 6 and 7 afforded the same product
of apparent trapping of 9 with methanol.
indicates that there is a buildup of 0.130e on C1 and a
depletion of 0.048e at O7 as well as a buildup of 0.017e on
O2 and a loss of 0.119e from C6. These trends are in accord
with nucleophilic attack by O7 at C1.
Two pathways for the decarbonylation of methoxysiloxy-
carbene were computed by means of B3LYP/6-31+G*. The
first was for nucleophilic attack by O7 on C1 of B, while
the second was for nucleophilic attack by O2 on the silicon
atom of C. The ground-state structures, B and C, as well as
the corresponding transition state structures, TSBF and TSCF,
are shown in Figure 2. The decarbonylation product, methyl
For the second pathway, the breaking O7-Si and O2-
C6 bonds become elongated from 1.337 and 1.729 Å in C
to 1.471 and 1.968 Å in TSCF, respectively. The Si-O7-
C6 and O2-C6-O7 angles decrease from 107.5° and 128.0°
in C to 98.4° and 104.0° in TSCF. The O2-C6-O7-Si
dihedral angle in TSCF is 1.9°, indicating that the breaking
O7-Si bond is aligned with the O2 σ nonbonding orbital.
Moreover, there is a buildup of 0.116e on O2 and 0.060e on
the silicon atom along with a loss of 0.021e from C6 and
0.124e from O7. These features are in accord with nucleo-
philic attack by the methoxy oxygen on the silicon atom with
concomitant formation of carbon monoxide. Intrinsic coor-
dinate (IRC) calculations confirmed that TSCF is the transi-
tion state for decarbonylation of C and suggested that the
mechanism may be concerted.
The calculated activation barrier for decarbonylation via
the cis-trans conformer (B) is 39.8 kcal/mol, and that for
decarbonylation via the trans-cis conformer (C) is 16.9 kcal/
mol. On the basis of these activation barriers, decarbonylation
should occur exclusively by nucleophilic attack of the
methoxy oxygen σ lone pair at the silyl moiety of the trans-
cis conformer (C). This conclusion is also consistent with
the frontier molecular orbital coefficients. As described
earlier, the HOMO is made up primarily of nonbonding σ
orbitals on O2, O7, and C6, while the LUMO consists
primarily of the O7-Si σ* orbital. Thus, the geometry of
the trans-cis conformer (C) is set up for maximum interac-
tion between the O2 σ orbital (HOMO) and the O7-Si σ*
orbital (LUMO). For the alternative pathway (i.e., decarbo-
nylation via B), although the HOMO has a sizable coefficient
on O7, the coefficient of the LUMO on C1 is negligible,
making nucleophilic attack by O7 at C1 an unlikely process.
Figure 2.
silyl ether (F), is also shown in Figure 2.
In the first pathway, the dissociating C1-O2 and C6-
O7 bonds lengthen from 1.463 and 1.345 Å in B to 2.454
and 1.370 Å in TSBF while the developing O2-C6 bond
shortens from 1.312 to 1.207 Å. The C1-O2-C6 angle
shrinks from 122.5° in B to 93.4° in TSBF, while the O2-
C6-O7 angle increases slightly from 110.4° to 113.2°.
Moreover, the C1O2C6O7 dihedral angle in TSBF is 0.0°,
meaning that the breaking C1-O2 bond is aligned with the
O7 σ nonbonding orbital. These features are consistent with
nucleophilic attack by O7 on C1 in concert with the
formation of carbon monoxide, C6-O2. NPA analysis
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for continued
support through an operating grant and a scholarship for
J.P.P.
OL006117+
2736
Org. Lett., Vol. 2, No. 18, 2000