0 Copyr@ht 1983 American Chemical Society
Volume 2 , Number 1 , January 1983
Intramolecular Reactions of Alkenylsilylenes Thomas J. Barton" and Gary T. Burns Department of Chemistry, Iowa State University, Ames, Iowa 500 11 Received July 14, 1982
Thermally generated 4-butenylmethylsilylene was found to cyclize in 56% yield to 4-methyl-4-silacyclopentene. This isomerization is proposed to occur through P-CH insertion to form the often postulated Methyl-4-(2-pentenyl)silylenealso vinylsilacyclopropane which then rearranges via 1 , 3 4 1 ~ migration. 1 undergoes intramolecular allylic C-H insertion to directly form 3,4-dimethyl-4-silacyclopentene (30%). Methyl-5-(1,3-pentadienyl)silyleneundergoes intramolecular .n addition to give predominantly (52%) 5-methyl-5-silacyclohexa-l,3-diene.A silabicyclo[3.1.0]hexene intermediate is also proposed for the transformation of methyl-l-(2,4-hexadienyl)silyleneto 1-methyl-1-vinyl-1-silacyclopent-2-ene (68%), but C-C rather than Si-C bond homolysis is invoked as the penultimate step.
Introduction Recently we reported' that flash vacuum pyrolysis
(FVP) generated allylmethylsilylene (1) isomerized to 1-methylsiletene (3) and suggested that this occurred via initial intramolecular .n addition to form silabicyclo[l.l.O]butane 2 followed by homolysis of the internal Si-C bond and 172-hydrogenmigration. We now wish to report the extension of this work to selected, longer chain alkenyl and dienyl silylenes.
Me-
Si
1
/
4
/*
Me
Me
silicon analogue of 4 would exclusively undergo P addition if produced under conditions favoring intramolecular reaction. To this end the desired silylene precursor, disilane 6, was synthesized by the addition of l-aziranyl-lchlorotetramethyldisilane to a solution of 4-butenyl bromide followed by quaternarization with methyl iodide and displacement of the amine by methanol to afford 1-(4-butenyl)-l-methoxytetramethyldisilane(6) in 64 % yield. The FVP of 6 was conducted a t 690 "C (1 X
Me H
3
2
Me
6
Results and Discussion 4-Butenylmethylsilylene (8). In contrast to the behavior of allylcarbenes, generation of 4-butenylcarbene (4) by catalytic decomposition of the corresponding diazo precursor does not lead t o the product of intramolecular P addition, bicyclo[2.1.O]pentane (5), but rather to 1,4pentadiene by hydrogen shift.2 Since the corresponding
n-n *D :CH
4
5
shift by a 4-butenylsilylene would involve the unknown process of thermal rearrangement of a silylene t o a silene (&Si=C&) via hydrogen migration, it was hoped that the (1)Burns, G. T.; Barton, T. J., J. Am. Chem. SOC.,in press. (2) Kirmse, W.; Grassman, D. Chem. Be?. 1966,99, 1746.
Me
0
t Me3SiOMe
H
Me
7 (56%)
torr) with an 81% mass recovery. In addition t o the expected product of reductive elimination, MeaSiOMe (79%), only a single volatile product was obtained: 4-methyl-4silacyclopentene (7) in 56% yield (absolute, GC). In consideration of the origin of 7, it should be first noted that direct insertion of silylene 8 into a vinyl P-CH bond would produce the unobserved isomeric silacyclopentene 9. Also, a mechanistic route of P addition and central Si-C bond homolysis affords diradical 10, for which 1,Zhydrogen migration to form 9 would be expected to be a t least competitive with a 1,3-hydrogen shift to produce 7. Thus, we favor initial silylene insertion into an allylic
0276-7333/83/2302-0001$01.50/0 0 1983 American Chemical Society
2 Organometallics, Vol. 2, No. 1, 1983
r
.D
-
addition
-9,
Si-Me
Barton and Burns
Me
Me
H
7
Me
M
e
m
Me3si'
H CH3
680 'C
-4 10 t o r r
'Me
\Me
16
20 (13%)
Me
13
Met hyl-5-( 1,3-pentadienyl)silylene ( 16). Extension of this study to dienylsilylenes was accomplished by the FVP of 5-( l-methoxytetramethyldisilany1)-1,3-pentadiene (15) a t 700 "C torr) with 87% mass recovery. In addition to MesSiOMe ( 8 4 % ) ,two volatile products were isolated and found to be isomeric with silylene 16. The major product was determined to be 5-methyl-hilacyclohexa-1,3-diene (18)and was formed in 52% yield. The minor product (13%) was found to be 3-methylene4-methyl-4-silacyclopentene (20). Both of these products presumably arise from addition of the divalent silicon in 16 t o the terminal double bond. The intermediate silabicyclo[3.l'.0]hexene 17 apparently undergoes preferential opening of the internal Si-C bond followed by 1,2-hydrogen shift to produce 18. Competition in the form of external Si-C bond homolysis affords a silacyclopentenyl diradical 19 which rearranges to 20. Methyl-l-(2,4-hexadienyl)silylene(22). Increasing the length of the alkadienyl chain of 16 by a single methyl (3) Gaspar, P. P.; Hwang, R.-J. J. Am. Chem. Soc. 1974, 96,6198. Ishikawa, M.; Ohi, F.; Kumada, M. J. Organomet. Chem. 1975,86, C23; and references therein.
17
1
0-0. /S!
Me/ \H
Me
18 (52%)
group had a profound and unexpected effect on the observed chemistry. Thus, when methyl-l-(2,4-hexadieny1)silylene (22)was generated by FVP of hexadienyldisilane 21 (680 "C torr)), a very clean formation of 1-methyl-1-vinyl-1-silacyclopent-2-ene (25)was obtained. The absolute yield of 25 was 68%, but on the basis of the yield of Me3SiOMe (73%), hence the amount of silylene 22 formed, the yield of 25 was a remarkable 93%. Formation of 25 is most economically rationalized by initial P addition to form silabicyclo[3.1.0]hexene 23. Apparently the methyl substitution on 23 is sufficient to alter the course of decomposition from that taken by 17. Although the carbon-carbon bond rupture required for the conversion of 23 to diradical24 is not the normal expectation for a monocyclic silacyclopropane, this process has ample precedent in the ring-expansion reactions between silylenes and and cyclic dienes such as furan^,^ ~yclopentadiene,~ 1,3-~yclooctadiene.~
f=)l /\
H
14 (29%)
19
I
Mea Si\
Me
7':
Me
'H
9 (not observed)
(-MeaSiOMe)
12
Me3SiOMe t
15
C-H bond to form an intermediate vinylsilacyclopropane 11 which undergoes a formal 1 , 3 4 1 ~ migration 1 to afford 7. Invocation of 11 is of additional interest in that vinylsilacyclopropanes have long been thought to be the initial intermediates in the bimolecular reactions of silylenes and 1,3-dienes to produce 4-sila~yclopentenes.~ Thus, it would appear that we have achieved the same intermediate through an intramolecular silylene reaction. Methyl-4-(2-pentenyl)silylene(13). Intramolecular allylic C-H insertion by silylene appears also to be involved in the thermochemistry of methyl-4-(2-pentenyl)silylene (13). Thus, when 4-(l-methoxytetramethyldisilanyl)-2pentene (12) (synthesized in a similar fashion as for 6 in only an 8% yield) was subjected to FVP (680 "C torr)) only two volatile products were observed and isolated by gas chromatography. T h e products were MeaSiOMe (67%) and 3,4-dimethyl-4-silacyclopentene(14)(29%, 43% based on Me3SiOMe). Formation of 14 is again most easily rationalized as arising from cyclization of silylene 13 through insertion into an allylic C-H bond.
Ma-SI
torr)
NeO'
10
Me
-
Me3Si -SI
* \
8
11
(7
700 ' C
Me$
SI
I-
Me0
-
t o r r ) (-Me3SiOMe ( 7 3 % ) )
680 ' C
Me
Me
21
I
M e - -
SI: -Me
Me
I
22
Me
23
-
\ Me
24
25 (68%)
In summary one can conclude that intramolecular T additions of appropriately substituted silylenes can be (4) Chernyshev, E. A.; Krasnova, T. L.; Stepanov, V. V.; Labartkava, M. 0. Zh. Obshch. Khim. 1978.48.,~2798. .~ (5) Hwang, R.-J.; Conlin, R.'T.; Gaspar, P. P. J. Organomet. Chem. 1975, 94, C38.
(6) Childs, M. E.; Weber, W. P. Tetrahedron Lett. 1974, 4033.
Organometallics, Vol. 2, No. I , 1983 3
Alkenylsily lenes extremely clean and synthetically useful even under the rather severe thermal conditions of this work. However, it appears from these limited data (and related work from our laboratory) that when allylic C-H bonds are available and suitably located t h a t C-H insertion by the divalent silicon will predominate. Indeed, allylic activation of the C-H bond is not required as Gusel'nikov' has recently demonstrated that methyl(trimethylsilylalky1)silylenes such as 26 react predominantly through p-CH insertion followed by methylsilylene ejection from the transient silacyclopropane to afford the corresponding alkenylsilanes. This is consistent with our finding that n-butylsilylene 28 cleanly affords 1-butene.
..
Me3 SICH2CH2CH2 -SiMe
-
-HQ;M~
Me3Si C H 2 -CH-CH2 \ /
26
'Si'
// 'Me Me3S iCHzCH=CH2
Me
-> I (CH
Me 3 SI
I
2 ) 3C H 3
680 0C(10-4 t o r r )
/
-
MeaSiOMe t
Me0
27
..
Me\#
Me SICHzC H z C H z C H 3
-
Hzd-!
\
- H 5.1 M e
CH-CHzCH3
28 HzC=C
HC H 2 C H 3
In addition to extensions of this work, we are currently attempting to generate these and other alkenylsilylenes under photochemical conditions in the hopes of observing the bicyclic intermediates. E x p e r i m e n t a l Section General Data. Pfoton NMR spectra were recorded on a Varian Model EM-360 spectrometer. GC-mass spectral (GC-MS) data were collected at 70 eV on a Finnegan Model 4023 mass spectrometer, and exact mass measurements were obtained on an AEI MS-902 mass spectrometer. Gas chromatographic separations were performed on a Varian-Aerograph Series 1700 instrument. Unless otherwise specified, all yields are calculated from NMR measurements with internal standards and are absolute. Flash vacuum pyrolyses (FVP) were carried out by evaporating the silylene precursor through a horizontal 30-cm quartz tube packed with quartz chips and heated in a tube furnace. Products are collected in a liquid Nz cooled trap. 1-(4-Butenyl)-l-methoxytetramethyldisilane(6). A solution of 9.85 mmol of 4-butenylmagnesium bromide was prepared by the slow addition of 1.33 g (9.85 mmol) of 4-butenyl bromide to a stirred suspension of excess Mg turnings in anhydrous EtzO. To this solution was added 1.554 g (8.03 mmol) of l-aziranyl-lchlorotetramethyldisilane' over a 10-min interval. After the solution was stirred for 8 h at room temperature, 1.0 mL of Me1 was added, and this was followed immediately by the addition of 0.8 mL of MeOH. After being stirred for 15 min, the solution was poured into twice its volume of hexane, fiitered through celite, and concentrated by rotary evaporation. The residue was GC separated on a 20-ft, 20% SE-30 column programmed from 130 "C at 2 "C/min to afford 0.334 g (21%) of pure 6. On a larger scale reaction (33 mmol) the product was isolated by distillation (60 "C (1torr)) in 64% yield: NMR (DCCl,) 6 0.22 (s,9 H), 0.27 (s, 3 H), 0.67-1.0 (m, 2 H), 1.91-2.49 (m, 2 H), 3.50 (s, 3 H), 4.78-5.21 (m, 2 H), 5.58-6.28 (m, 1 H); mass spectrum, m / e (% (7) Gusel'nikov, L. E.; Lopatnikova, E.; Polyakov, Yu. P.; Nametkin, N. S. Dokl. Akad. Nauk SSSR 1980,253 (6), 1387. (8) Burns, G. T.; Barton, T. J., submitted for publication.
relative intensity) 202 (lo), 187 (381, 147 (411, 133 (75), 98 (go), 97 (81), 89 (70), 75 (85), 73 (100); calcd for CgHz2Siz0m / e 202.120 93, measured mle 202.120 85. FVP of 6. Disilane 6 (0.3340 g, 1.65 mmol) was distilled (45 "C (1 X lo4 torr)) through a quartz tube heated to 690 "C. The pyrolysate was collected in a N2-cooledtrap and represented an 81% mass recovery (0.2718 9). The pyrolysate was separated by GC on a 20-ft, 20% SE-BO/Chromosorb W column using a temat 2 "C/min. Two products were perature program of 100 "-, obtained. The first was identified as Me3SiOMe (79%) on the basis of spectral comparison with an authentic sample. The second product was identified as 4-methyl-4-silacyclopentene(7):9 56%; NMR (C6D6) 6 -0.11 (d, 3 H, J = 4 Hz, collapses to s with hv at 6 4.22), 1.35 (center of doubled AB, collapses to broadened AB with hv at 6 4.38, 4 H), 4.38 (SiH, m), 5.88 (br s, 2 H); mass spectrum, mle (% relative intensity) 98 (65), 97 (loo), 83 (77), 81 (23), 70 (54),67 (ll),57 (ll),55 (55), 54 (14), 53 (27); the GC retention time and spectral properties exactly matched those of an authentic sample prepared by LiA1H4reduction of 4-chloro4-methyl-4-silacyclopentene. 4-(l-Methoxytetramethyldisilanyl)-2-pentene (12). To a stirred suspension of excess Mg turnings in 30 mL of Et20 was added a solution of 2.361 g (15.8 mmol) 4-bromo-2-pentene and 2.462 g (12.9 mmol) of 1-chloro-1-aziranyltetramethyldisilane'in 20 mL of EtzO at a rate sufficient to maintain a gentle reflux. After the reaction had subsided, the liquid was decanted from the Mg and excess methyl iodide and methanol were added sequentially. The solution was stirred for 2 h and then diluted with hexane, filtered through celite, and concentrated by rotary evaporation. The residue was separated by GC on a 20-ft, 20% SE-30 on Chromosorb W column by using a temperature program of 150-220 "C at 2 "C/min to afford 0.218 g pure 12 (8%): NMR (DCClJ 6 0.06 (9, 12 H), 1.01 (d, J = 7 Hz, 3 H), 1.59 (d, J = 5 Hz, 3 H, collapses to s with hv at 6 5.26), 1.84 (br q, J = 7 Hz, collapses to sharp q with hv at d 5.26, 1 H), 3.36 (s, 3 H, OMe) 5.26 (m, 2 H, collapses to brd s with hv at 6 1.59);m / e (% relative intensity) 216 (15), 201 (lo), 147 (loo),132 (15), 107 (30), 88 (20), 73 (70); calcd for CloHz40Si2m / e 216.13658, measured m / e 216.13645. FVP of 12. Disilane 12 (0.2184 g, 1.01 mmol) was distilled (25 "C torr)) through a quartz tube heated to 680 "C with 68% mass recovery. Two products were isolated by GC using a 20-ft, 20% SE-30 on Chromosorb W column with a temperature program of 100-220 "C at 2 "C/min. The first product was Me3SiOMe in 67% yield. The other product was identified as 3,4-dimethyl-4-silacyclopentene (14): 29% yield, 43% based on Me3SiOMe;NMR (C6D6)6 0.07 (d, J = 4 Hz, collapses to s with hv at 6 4.12,3 H), 1.12 (d, J = 7 Hz, 3 H), 0.89-1.82 (m, 3 H), 4.12 (SiH, m, 1H), 5.75 (br s, 2 H); mass spectrum, m / e (% relative intensity) 112 (35), 111 (ll),97 (loo), 95 (34), 84 (32), 83 (15), 71 (30), 70 (21), 69 (27), 67 (12), 59 (16), 58 (30), 55 (23), 53 (15). 54l-Methoxytetramethyldisilanyl)-1,3-pentadiene (15). To a stirred solution of 8.92 mmol of pentadienyllithium" in 40 mL of Et20 at -78 "C were added 1.688 g (8.72 mmol) of l-aziranyl-1-chlorotetramethyldisilane'in one portion. The solution was allowed to warm to room temperature, and the consumption of the chlorodisilane was followed by GC. After 40 min, 10 mL of hexane was added followed by the sequential addition of 1.0 mL of Me1 and 0.50 mL of MeOH. Within 30 min (GC monitor) the reaction was complete. The reaction mixture was diluted with 5 times its volume of hexane and filtered through celite, and the filtrate was concentrated by rotary evaporation. The residue was GC separated on a 5-ft, 12% SE-30 column at 160 "C to afford 1.070 g of 15 (57%): NMR (DCCl3) 6 0.22 (5, 9 H), 0.27 (9, 3 H), 1.80 (d, J = 8 Hz, 2 H), 3.49 (s, 3 H, SiOMe), 4.74-6.71 (m, 5 H); mass spectrum, m / e ( % relative intensity) 214 (5), 199 (6), 147 (loo), 117 (33), 89 (37), 73 (54); calcd for C10H22Si20m / e 214.12093, measured m / e 214.12041. (9) Silacyclopentene 7 has been previously reported,1° but the structural assignment was solely based on the mass spectrum (which was not reported other than the apparent molecular ion). (10) Jenkins, R. L.; Kedrowski, R. A.; Elliott, I. E.; Tappen, D. C . ; Schlyer, D. J.; Ring, M. A. J. Organomet. Chem. 1975, 86, 347. (11) Bates, R. B.; Gosselink, D. W.; Kaczynski, J. A. Tetrahedron Lett. 1967, 199, 205.
Organometallics 1983,2, 4-8
4
FVP of 15. Disilane 15 (0.8097 g, 3.78 mmol) was distilled (70 torr) through a horizontal quartz tube heated at 700 "C (1x "C. The pyrolysate collected in a N,-cooled trap weighed 0.7032 g for an 87% mass recovery. The three products were isolated by preparative GC on a 20-ft, 20% SE-30 on Chromosorb W column with a temperature program of 70 "C to 200 "C at 2 OC/min. The first product was Me3SiOMe (84%). The second product was identified as 5-methyl-5-silacyclohexa-1,3-diene (18): 52%; NMR (CC14)6 0.20 (d, J = 4 Hz, 3 H, collapses to s with hv at 6 4.16), 1.39-1.76 (m, 2 H), 4.16 (q, 1 H), 5.52-5.98 (m, 3 H), 6.40-6.81 (m, 1 H); mass spectrum, m/e (% relative intensity) 110 (72), 109 (36), 95 (loo), 93 (21), 84 (15), 83 (ll),82 (12); calcd for C6HloSim/e 110.0552, measured m/e 110.0545; the spectral properties of 18 were identical with those of an authentic sample prepared by LiAlH4reduction of 5-methyl-5-chloro-5-silacyclohexa-1,3-diene. The third product was identified as 3methylene-4-methyl-4-silacyclopentene(20): 13%;NMR (C&) 6 0.05 (d, J = 4 Hz, 3 H), 1.30 (center of m, 2 H), 4.42 (m, Si-H, 1 H), 5.28 (br s, 1 H), 5.65 (br s, 1 H), 5.90 (m, 1 H), 6.42 (m, 1 H); mass spectrum, m/e (% relative intensity) 110 (70), 109 (47), 95 (loo), 69 (36), 67 (42), 58 (16), 55 (27), 53 (33), 43 (73). 1- ( 1-Methoxytetramethyldisilanyl)-2,4-hexadiene(21 ) To a stirred solution of 1.065g (13 mmol) of 1,4-hexadiene(Aldrich) in 3.0 mL of THF at -78 OC was added 6.0 mL of 1.44 M (8.6 "01) n-BuLi (Ventron)in one portion. The solution was warmed to room temperature during which time an exothermic reaction occurred and two layers were formed. After addition of enough THF to rehomogenize the orange solution, 1.815 g (8.12 mmol) of 1-chloro-1-(diethylamino)-tetramethyldisilane*was added in one portion. The solution was stirred at room temperature for 4 h, and then 4 mL of CH31and 3 mL of MeOH were sequentially added. After being stirred an additional 1.5 h, the solution was diluted with hexane, filtered through celite, and concentrated by rotary evaporation. The residue was distilled at 63-68 "C (0.1 torr) to afford 1.253g of pure 21: 68%; NMR (DCC13)6 0.21 (s, 9 H), 0.27 (s,3 H), 1.77 (apparent d, overlapped CHzC=C and C=CMe, collapses to br s with hu at ca. 6 5.80,5 H), 3.47 (5, OMe, 3 H), 5.20-6.33 (vinyl m, 4 H); mass spectrum, m/e ( % relative intensity) 228 (8), 213 (5), 147 (loo), 117 (32), 89 (25), 73 (55), 59 (30); calcd for CllHZ40Si2m/e 228.1366, measured m/e 228.1360. FVP of 21. Disilane 21 (0.5010 g, 2.20 mmol) was distilled (40 "C (W4torr)) through a quartz tube heated at 680 "C with 95% mass recovery (0.4742 9). Two products were isolated by prep-
.
arative GC. The first product was Me,SiOMe in 73% yield. The second product was identified as l-methyl-l-vinyl-l-silacyclopent-2-ene (25): 68% yield, 93% based on Me3SiOMe; NMR (DCCl,) 6 0.32 (s, 3 H), 0.81 (m, 2 H, collapses to br s with hv at 6 2.56), 2.56 (m, 2 H, collapses to br s with hv at 0.81, collapses to br t with hv at ca 6 6), 5.44-6.57 (m, 4 H, C2H3+ 1 vinyl H on ring), 6.87 (d oft, J = 10 Hz, 2 Hz, collapses to d with J = 10 Hz with hv at 6 2.56, 1 H); mass spectrum, m/e (% relative intensity) 124 (2), 109 (55), 97 (30), 96 (loo), 83 (20), 81 (15), 55 (20); calcd for C7H12Sim/e 124.0708, measured m/e 124.0712. 1-Methoxy-1-n-butyltetramethyldisilane (27). To a stirred solution of 3.250 g (14.5 mmol) of 1-chloro-1-(diethylamino)tetramethyldisilanes in 50 mL of EhO were added 11.0 mL (0.158 mol) of 1.44 M n-BuLi (Alfa) over a 10-min period. After the solution was stirred for 1h, 3 mL of Me1 was added, followed by excess NaOMe. The solution was stirred for ca. 6 h and then diluted with 100 mL of pentane, filtered through celite, and then concentrated by rotary evaporation. Distillation of the residue at 50-55 "C (0.1 torr) afforded 1.559 g pure 27 53% yield; NMR (DCCl,) 6 0.18 (s, 9 H), 0.20 (s, 3 H), 0.51-1.58 (m, 9 H, butyl), 3.44 (s, 3 H, OMe); mass spectrum, m/e ( % relative intensity) 204 (3), 189 (4), 131 (20), 117 (loo), 75 (67), 73 (28); calcd for C9H2,Si20m/e 204.136 58, measured m/e 204.13661. FVP of 27. Disilane 27 (0.5626 g) was distilled through a horizontal quartz tube heated to 680 "C with 42% mass recovery (0.2387 9). The volatile products were isolated by preparative GC (20-ft, 20% OV-101 on Chromosorb W at 110 "C) and identified as Me,SiOMe and 1-buteneby spectral comparison with authentic samples. Due to difficulties in quantitative manipulation, no attempt was made to establish yields in this reaction.
Acknowledgment. The support of this research by the National Science Foundation is gratefully acknowledged. We are also thankful for partial support by Dow Corning Corp. Registry No. 6,83134-61-6; 7,55544-25-7;8,83134-62-7; 12, 83134-63-8; 13,83134-64-9; 14, 83134-65-0; 15,83134-66-1; 16, 83134-67-2; 18, 78698-04-1; 20, 83134-68-3; 21,83134-69-4; 22, 83134-70-7; 25, 83134-71-8; 27, 83151-99-9; 28, 83134-72-9; 1aziranyl-1-chlorotetramethyldisilane,83134-73-0; 4-butenyl bromide, 5162-44-7; 4-bromo-2-pentene, 1809-26-3; pentadienyllithium, 54962-98-0; 1,4-hexadiene, 592-45-0; l-chloro-l(diethylamino)tetramethyldisilane,83134-74-1.
1,5-Silyl Migrations as a Route to Acyclic I-Sila-I ,3-butadienes Thomas J. Barton,' William D. Wulff, and Stephanie A. Burns Department of Chemistry, Iowa State Univers/ty, Ames, Iowa 500 1 1 Received July 15, 1982
cis-l-(Pentamethyldisilanyl)-3-methyl-l,3-butadiene was synthesized in three steps and subjected to flash vacuum pyrolysis a t 635 " C . Three of the four products were isomeric silyl hydrides which arose from 1,5 migration of trimethylsilyl from silicon to carbon to produce an intermediate 1-sila-l,&butadiene which underwent 1,5-hydrogen migration back to doubly bonded silicon. (2)-7,7,8,8-Tetramethyl-7,8disilanon-5-en-4-one was synthesized in an attempt to extend this rearrangement to 1,5-silicon migration to oxygen. A remarkably clean pyrolysis afforded a 2-sila-2,3-dihydrofuran (86%), most probably resulting from initial oxygen attack on internal silicon followed by 1 , 2 4 1 ~migration. 1 I n 1979 we reported that 1-methyl-1-(trimethylsily1)2,5-diphenylsilole (1) underwent a reversible, thermally induced 1,5-migration of trimethylsilyl from silicon to carbon t o generate a transient silole 2 which could be removed from the equilibrium with great efficiency by aldehydes, ketones, olefins, and acety1enes.l (1)Barton, T. J.; Wulff, W. D.; Arnold, E. V.; Clardy, J. J.Am. Chem. SOC.1979, 101, 2733.
0276-7333/83/2302-0004$01.50/0
,100 ' C
Ph-+Ph
'
Me
phQ Me3S~
'SIMe,
1
I
Me
2
Since 1,5-sigmatropic rearrangements are entropically more On cyclopentadienes~ it was not a foregone certainty t h a t acyclic l-disilanyl-1,3-butadieneswould
0 1983 American Chemical Society
