Organometallics 1983, 2, 199-200
of Canada. We thank Mr. B. McDonald and Dr. R. Lenkinski for the Bruker 400-MHz 'H and 2H NMR measurements. Registry No. Deuterium, 7782-39-0; allyl alcohol, 107-18-6; P-hydroxypropanal, 2134-29-4; a-hydroxyacetone, 116-09-6; acrolein, 107-02-8;hydride, 12184-88-2;PdC1,2-, 14349-67-8.
199
proceed not through direct C-H insertion but by rearrangement of a transient silirene intermediate. However, silirenes have never been observed in these reactions with terminal acetylenes.
t Sllacyclopropene to Vlnylsllylene Isomerization In Reactions of Sllyl- and Hydrldosllylenes with Acetylenes Thomas J. Barton,' Stephanie A. Burns, and Gary 1. Burns Department of Chemistry, Iowa State University Ames, Iowa 5001 1 Received Juk 6, 1982
Summary: Evidence is presented that silacyclopropenes with silyl or hydride substitution on silicon thermally rearrange to vinylsilylenes.
The thermochemistry of silirenes 1 (silacyclopropenes) is a key feature in the longstanding question of the mechanism for disilin 2 formation in the reaction of silylenes (R,Si:) and acetylenes. Among the several suggested mechanisms' for this reaction is the dimerization of initially formed silirenes across the silicon-carbon ring bond., Until 1976 silirenes were unknown, and this
4
gve -
/
\
R
2
+
Me3SiCMe
-
Me3Si-*;id
HCECH
7
6 Me3S1
D
H
H
V H
8 R
J
We report here several reactions8 that demonstrate yet another thermal rearrangement pathway taken by these versatile silirenes. When 2-cyclopropyl-2-methoxyhexamethyltri~ilane~ (6) was pyrolyzed at 450 OC in a flow apparatus employing acetylene as the carrier gas, the pyrolysate contained in addition to the expected Me,SiOMe (from a eliminationlo) only a single product, silane 12 in 52% yield [NMR (CS,) 6 -0.40-0.90 (m, 5 H), 0.16 (s, 9 H), 2.26 (d, 1 H, J = 1Hz, collapses to s with hv at 6 4.23), 4.23 (m, 1H), 6.68 (d of d, 1H, J = 20 and 6 Hz, collapses to d, J = 20 Hz with hv at 4.23), 6.98 (d, 1 H, J = 20 Hz); calcd for C9HI5Si2(M - CHJ m / e 179.07123, measured m / e 179.071191. The formation of 12 corresponds to the addition of expected silylene 7 to two molecules of acetylene. We propose that an initially formed silirene 8 rearranges to vinylsilylene 10, either through a diradical intermediate or via prior isomerization to l-silacyclopropene 9.'' Silylene 10 then can react with a second molecule of acetylene to form silirene 11 that, not being possessed with a good migrating group on silicon, rearranges to 12 by hydrogen migration to silicon. (Me3SiliSid
R-CEC-SIR~R
?
H
9
/
Me3Si
10
1
Si h', e
P I
i. polymer
mechanism could not be tested. However, in the intervening 6 years that silirenes have been synthetically available, the reports of their thermal behavior has produced a chaotic picture. Thus, variably substituted silirenes are said to dimerize to disilins 2,3 polymerize: extrude silylenes to form acetylenes 3,5 and isomerize to silylacetylenes 46-usually as remarkably clean processes. It has also been suggested7that the reactions of silylenes with terminal acetylenes to produce ethynylsilanes 5 (1) For a review of the early mechanistic work on this reaction see: Barton, T. J.; Kilgour, J. A. J. Am. Chem. SOC.1976, 98, 7746. (2) Atwell, W. H.; Weyenberg, D. R. J. Am. Chem. SOC.1968,90,3438. (3) Isikawa, M.; Fuchikami, T.; Kumada, M. J. Organomet. Chem. 1977, 142, C45. (4) Conlin, R. T.; Gaspar, P. P. J. Am. Chem. SOC.1976, 98, 3715. (5) Seyferth, D.; Annerelli, D. C.; Vick, S. C. J. Am. Chem. SOC. 1976, 98, 6382: and reference 6a. (6) (a) Ishikawa, M.; Nishimura, K.; Sugisawa, H.; Kumada, M. J. Organomet. Chem. 1980, 194, 147. (b) Ishikawa, M.; Nakagawa, K.-I.; Kumada, M. J. Organomet. Chem. 1980,190, 117. (7) Haas, C. H.; Ring, M. A. Inorg. Chem. 1975, 14, 2253.
H
H
11
/
SlI\'E3
12 (52%)
If this mechanistic pathway is correct, it should be operative for other silylsilylene/acetylene reactions, and we fiid this to be so. Copyrolysis of trisilane 13 and acetylene cleanly afforded the expected 3,6,6-trimethyl-3,5-disilahept-4-en-1-yne (14) in 48% yield [NMR (CSJ 6 0.15 (s, 9 H), 0.29 (d, 3 H, J = 4 Hz, collapses to s with hu at 6 4.37), 2.29 (d, 1 H, J = 1Hz, s with hv at 6 4.37), 4.37 (m, 1H), 6.51 (d of d, 1 H, J = 20 and 6 Hz, collapses to d, J = 20 (8) The pyrolyses reported here were run on a 0.5-1.0-g scale, and yields are absolute by GC. (9) The synthesis and spectral properties of 6 are being reported elsewhere: Burns, S. A.; Burns, G. T.; Barton, T. J. J.Am. Chem. SOC., in press. (10)Atwell, W. H.; Weyenberg, D. R. J. Am. Chem. SOC.1968, 90, 3438. (11) Barton, T. J.; Burns, G. T.; Goure, W. F.; Wulff, W. D. J. Am. Chem. SOC.1982, 104, 1149.
0276-733318312302-0199$01.50/0 0 1983 American Chemical Society
200 Organometallics, Vol. 2, No. 1, 1983
Communications
Hz with hu at 6 4.37), 6.79 (d, 1 H, J = 20 Hz); calcd for C7HI3Si2 (M - CH3) m / e 153.0556, measured m / e 153.05591. 3h'e
I
' M e 3 S )ikiMe
HC=CH
(m, 1 H, SiH), 5.49 (m, 1 H, d of J = 4 Hz with hu at 6 1.87), 5.68 (m, 1 H, d of J = 4 Hz with hv at 6 1.87); mass spectrum virtually identical with that of 191.
9
.e-ii-i.=-
w,-
i
\
1
13
\
SiMe,
1 4 (48%)
This novel rearrangement of a silirene to a vinylsilylene is not restricted to systems having silicon available as a migrating group. Thus, generation of methylsilylene by the pyrolysis of 1,1,1,2-tetramethyldisilane in an acetylene stream afforded, in addition to Me,SiH, 3-methyl-3-silapent-4-en-1-yne (17) in 26% yield [NMR (D6C6)6 0.22 (d, 3 H, J = 4 Hz, s with hu a t 6 4.44), 2.12 (d, 1 H,J = 1.2 Hz, s with hu at 6 4.44), 4.44 (m, 1 H, SiH), 5.92 (s, 3 H); m / e 96 (M+, 25%), calcd for C&Si (M - H) m / e 95.0317, measured m / e 95.03141. Thus, it appears that hydrogen migration from silicon to carbon occurs to transform silirene 15 to vinylsilylene 16 followed by the same addition-rearrangement sequence as before (vide supra). Independent generation of vinylsilylene 16 in an acetylene stream by the pyrolysis of vinyldisilane 18 was conducted to determine the viability of the intermediacy of 16 in this reaction. Me3SiOMe and 17 were the sole volatile products. ve I
ve,5iAih2
-
tle,si+
+
..
Me-s--Y
Me
-4 -C E C V e
Me
i
A
20
It is of interest to speculate that vinylsilylenes and appropriately substituted silirenes are in equilibrium at our thermal conditions. Methylvinylsilylene (16) is known to isomerize to 3-sila-1-butyne (22), and it has been suggested that this occurs through the intermediacy of silirene 15.'l
16
Me
15
22
,
H
Me
Me
OMe
4
I
18
I
17
In order to demonstrate that these rearrangements were not unique to reactions of parent acetylene, copyrolysis of tetramethyldisilane and propyne was undertaken. Thus, generation of methylsilylene in a stream of propyne produced a remarkably clean reaction affording two products, 19 and 20, in a combined yield of 73% (ca. 3:l). Structural assignments were made on the basis of spectral data [ 19: NMR (DCClJ 6 0.29 (d, 3 H, J = 4 Hz, s with hu at 6 4.21), 1.83 (d of d, 3 H,J = 6 and 1 Hz), 1.91 ( 8 , 3 H), 4.21 (m, 1 H, SM), 5.59 (d of d of q, 1 H, J = 18,3 and 1 Hz, d of d with hu a t 6 1.83), 6.29 (d of q, 1 H, J = 18 and 6 Hz, d of J = 18 Hz with hv a t 6 1.83); calcd for C7HI2Si(M+, 39%) m / e 124.07083,measured m / e 124.07105. 20: NMR (DCC1,) 6 0.34 (d, 3 H, J = 4 Hz, s with hu at 6 4.21), 1.87 (m, 6 H, overlapped allylic and acetylenic methyls), 4.21
21
This work establishes a link between 15 and 16, and, thus, lends strength to that proposal as it seems inevitable that 15 would in the absence of traps be bled from the equilibrium as 22. The interesting question of whether l-silacyclopropene 21 is an intermediate in an equilibrium between 15 and 16 has not been experimentally attacked in this work. However, the recent calculations of Gordon12 suggest that 21 may be a thermodynamically unreasonable expectation.
Acknowledgment. The support of this research by the National Science Foundation is gratefully acknowledged. Registry No. 6,83268-89-7; 7,83802-32-8; 12,83802-27-1; 13, 69545-89-7; 14, 83802-28-2; 17, 83802-29-3; 19, 83802-30-6; 20, 83802-31-7; HCECH, 74-86-2; Me,SiOMe, 1825-61-2; Me,SiSi(Me)H2,81633-92-3; Me,SiH, 993-077; HCECMe, 74-99-7; Methylsilylene, 55544-30-4. __
(12) Gordon, M. S.; Koob, R.D.J. Am. Chem. SOC.1981,103, 2939.
