4752 J. Am. Chem. Soc., Vol. 121, No. 20, 1999
Leigh et al.
Laser flash photolysis of 5a and 6a in hexane-pyridine
mixtures results in the formation of “stable” absorptions centered
at 370 nm, which are assigned to the pyridine ylide of singlet
((trimethylsilyl)methyl)methoxysilane (7cMe) was available from a
previous study.8
1
-Ethyl-1-methoxy-1,2,2,2-tetramethyldisilane (8Me) was prepared
from 1-ethyl-1-phenyl-1,2,2,2-tetramethyldisilane by the method of
Auner and co-workers.58 1-Ethyl-1-phenyl-1,2,2,2-tetramethyldisilane
was prepared by reaction of (ethylmethylphenylsilyl)lithium and
trimethylchlorosilane in anhydrous tetrahydrofuran, using the general
(
trimethylsilyl)carbene (11). From the variation in the yield of
the ylide as a function of pyridine concentration, a value of
.07-0.4 ns is estimated for the lifetime of the carbene in
0
deoxygenated hexane solution at room temperature, assuming
59
procedure of Gilman and co-workers. The compound was isolated as
9
-1 -1
a value of k ) (1-5) × 10 M
s
for the rate constant for
a colorless liquid (bp 50-54 °C, 10.2 mmHg) and identified on the
pyridine trapping of the carbene. This affords an estimate of
1
basis of the following spectroscopic data: H NMR δ 0.07 (s, 9H),
9
-1
k1,2-Me ∼ (8 ( 6) × 10 s for the rate constant for formation
0.31 (s, 3H), 0.89 (m, 2H), 1.00 (m, 3H), 7.32 (m, 3,H), 7.43 (m, 2H);
13
of 1,1,2-trimethylsilene by [1,2]-methyl migration in the carbene
C NMR δ -6.74, -1.78, 5.41, 8.18, 127.68, 128.26, 134.06, 138.56;
intermediate, a value comparable to those for [1,2]-hydrogen
IR (neat) 3068.0 (w), 2952.6 (s), 2893.4 (m), 1427.3 (m), 1245.8 (s),
1104.5 (s), 1010.3 (m), 859.3 (s), 834.2 (s), 782.8 (s) 720.4 (m); MS
m/z(I) ) 222 (18), 193 (15), 179 (11), 149 (34), 135 (44), 121 (100),
migration in simple alkyl- and dialkylcarbenes.56 In the case of
the nitrogenous precursors at least, silene formation probably
also occurs by a competing, direct excited-state pathway. The
product distribution obtained from photolysis of pentamethyl-
disilanylketene (6c) in the presence of alcohols suggests that
1
2
05 (25), 73 (22); HRMS calcd for C12H22Si 222.1260, found 222.1261.
The disilane (7.1 g, 0.032 mol) was dissolved in pentane (30 mL) in a
flame-dried 100-mL two-neck round-bottom flask equipped with a
reflux condenser, nitrogen inlet, rubber septum, and magnetic stirrer.
The solution was cooled with an ice bath, and trifluoromethanesulfonic
acid (2.83 mL, 0.032 mol) was added via syringe. The mixture was
stirred for 1 h at 0 °C and then allowed to warm to room temperature,
after which it was stirred for another 30 min. The mixture was then
recooled to 0 °C, and a solution of sodium methoxide in methanol
[1,2]-trimethylsilyl migration in disilanylcarbenes is almost 10
times faster than [1,2]-methyl migration.
Further studies of the photochemistry of R-silylcarbene
precursors and the reactivity of the corresponding carbenes and
silenes are in progress.
(prepared from sodium (1.3 g) and 10 mL of anhydrous methanol) was
added slowly, resulting in the formation of a white precipitate. The
mixture was stirred for an additional 1 h and then filtered. After
evaporation of solvent, the product was distilled under vacuum to yield
a colorless oil (2.2 g, 0.0125 mol, 39%; bp 58 °C, 70 mmHg) which
Me
Experimental Section
Ultraviolet absorption spectra were recorded on a Perkin-Elmer
Lambda 9 spectrometer interfaced to a Pentium microcomputer. Gas
chromatographic (GC) analyses were carried out using a Hewlett-
Packard 5890II+ gas chromatograph equipped with a conventional
heated injector, a flame ionization detector, a Hewlett-Packard 3396A
integrator, and DB1 or DB1701 megabore capillary columns (15m ×
was identified as 8 on the basis of the following spectroscopic data:
1
H NMR δ 0.093 (s, 9H), 0.147 (s, 3H), 0.678 (q, 2H), 0.961 (t, 3H),
3.411 (s, 3H); 13C NMR δ -3.613, -1.546, 6.832, 7.863, 51.415; IR
(neat) 2953.9 (s), 2879.1 (s), 2827.3 (s) 1460.5 (m), 1246.5 (s), 1186.6
(w), 1087.1 (s), 1008.7 (m), 864.9 (m), 834.5 (m), 776.8 (m), 734.5
(m); MS m/z(I) ) 176 (1.2), 161 (19), 147 (11), 133 (100), 117 (16),
103 (14), 75 (30), 73 (27), 59 (34).
0
.53 mm; Chromatographic Specialties, Inc.). Mass spectra and GC/
MS analyses were recorded on a Hewlett-Packard 5890II gas chro-
matograph equipped with an HP-5971A mass selective detector and a
DB5 fused silica capillary column (30m × 0.25 mm; Chromatographic
Specialties, Inc.). Semipreparative gas chromatographic separations were
carried out using a Varian 3300 gas chromatograph equipped with a
thermal conductivity detector and a 3.8% UCW982 on 80/100 Supel-
coport (24 ft × 0.25 in.; Supelco, Inc.) stainless steel column.
Preparative-scale photolysis of 6b in the presence of MeOH was
carried out on a deoxygenated 15 mL sample of the ketene (0.032 M)
and the alcohol (0.8 M) in hexane. The solution was placed in two 1
cm × 15 cm quartz tubes, which were then sealed with rubber septa,
deoxygenated with a stream of argon, and placed in a merry-go-round
surrounding a 16 W zinc resonance lamp (214 nm; Philips 93106E).
Irradiation of the solutions for 18 h resulted in the formation of a very
clean mixture of 6b and a single product in a ∼1:2 ratio by capillary
GC. The solvent was removed on the rotary evaporator, and the
components were then separated by semipreparative GC. The product,
which was isolated in 99.3% purity (by capillary GC) after this
2
,2,4-Trimethylpentane (isooctane) and hexane (BDH Omnisolv)
both exhibited an absorbance of <0.2 at 193 nm in a 3 mm cell and
were used without further purification. Methanol, methanol-O-d, and
tert-butyl alcohol were of the highest purity available and were used
as received from Aldrich Chemical Co. Pyridine (BDH Reagent) was
distilled from anhydrous barium oxide. Ethyldimethylchlorosilane was
used as received from Aldrich Chemical Co. (Trimethylsilyl)diazo-
methane (3a) was obtained from Aldrich as a 2 M solution in hexane
and was purified by semipreparative gas chromatography. (Trimeth-
ylsilyl)diazirine (5a) was prepared by photolysis of an argon-saturated
procedure, was identified as 1-(methoxydimethylsilyl)-1-(trimethylsilyl)-
1
ethane (7bMe) on the basis of the following spectroscopic data:
H
NMR δ -0.130 (q, 2H), 0.084 (d, 6H), 0.090 (s, 9H), 1.018 (d, 3H),
13
3.224 (s, 3H); C NMR δ -2.096, -1.950, -0.896, 8.026, 8.897,
49.963; 29Si NMR δ 2.49, 18.82; MS m/z ) 176 (20), 175 (100), 147
(7), 131 (5), 117 (6), 89 (46), 73 (20), 59 (35), 45 (11).
0
.2 M solution of 3a in pentane, in a Rayonet reactor fitted with 10
RPR-419X lamps (419 nm). The progress of the photolysis was
followed by UV spectroscopy and GC and continued to >97%
conversion of the diazo compound. The resulting solution was
concentrated by careful distillation and then re-diluted with hexane or
hexane/pyridine mixtures to an appropriate concentration for laser flash
photolysis experiments (0.04-0.1 M).
Analytical-scale photolyses were carried out using the 16 W zinc
lamp or in a Rayonet reactor containing five RPR-3000 lamps. Aliquots
of hexane or isooctane solutions containing 6 (0.01 M) and methanol
or tert-butyl alcohol (0.5 M) were placed in Suprasil quartz cuvettes
(3-mm × 10-mm), sealed with rubber septums, and saturated with dry
argon. One tube was left in the dark to monitor ground-state esterifi-
cation over a time period of 5-10 h; this was found to be significant
only for 6a in the presence of methanol. The others were irradiated in
a merry-go-round for periods of up to 1 h, with periodic monitoring of
the photolyzates by capillary GC. Products were identified by capillary
GC co-injection with authentic samples of the corresponding alkoxy-
silanes.
The silylketenes 6a-c were prepared as described previously4
and were purified by semipreparative GC, affording samples of >99%
purity as estimated by capillary GC. Ethyldimethyl-tert-butoxysilane
3,57
(7aBu) was prepared by reaction of ethyldimethylchlorosilane with tert-
butyl alcohol and triethylamine in ether, purified by preparative GC,
1
and identified on the basis of spectroscopic data: H NMR δ 0.064 (s,
H), 0.507 (q, 2H), 0.905 (t, 3H), 1.220 (s, 9H); 1 C NMR δ 0.35,
.97, 10.39, 32.03, 71.88; MS m/z(I) ) 160 (0.05), 145 (15), 131 (19),
9 (20), 87 (22), 75 (100), 61 (22), 59 (21), 45 (13). Dimethyl-
3
6
6
8
Nanosecond laser flash photolysis experiments employed a micro-
computer-controlled detection system6 and the pulses from a Lu-
,60
(
56) Platz, M. S.; Modarelli, D. A.; Morgan, S.; White, W. R.; Mullins,
M.; Celebi, S.; Toscano, J. P. Prog. React. Kinet. 1994, 19, 93.
57) Pommier, A.; Kocienski, P.; Pons, J.-M. J. Chem. Soc., Perkin Trans.
1998, 2105.
(58) Boetze, B.; Herrschaft, B.; Auner, N. Chem. Eur. J. 1997, 3, 948.
(59) Gilman, H.; Peterson, D. J.; Wittenberg, D. Chem. Ind. 1958, 1479.
(60) Leigh, W. J.; Workentin, M. S.; Andrew, D. J. Photochem.
Photobiol. A: Chem. 1991, 57, 97.
(
1