Organometallics 1995, 14, 1471-1477
1471
Photolysis of Vinyltris(trimethylsily1)silane. Formation and Observation of a Silene and a Silylene in Primary Steps S. Zhang, M. B. Ezhova, and R. T. Conlin" Center for Organometallic Research and Education, Department of Chemistry, University of North Texas, Denton, Texas 76203 Received November 15, 1994@
Photolysis of cyclohexane solutions containing vinyltris(trimethylsily1)silane produces both the isomeric silene and silylene fragmentation products (trimethylsily1)vinylsilylene and hexamethyldisilane as primary pathways. Both types of transients produced the expected five- and six-membered rings from reactions with dienes as shown by chemical trapping studies. In reactions with 2-trimethylsiloxybutadiene,the position of the trimethylsiloxy substituent on the n bond in both silacycloalkenes, initially allylic to silicon, isomerized at room temperature to the presumably more stable vinylic isomer. Low-temperature W spectra as well as chemical trapping studies with triethylsilane, methanol, and a variety of dienes support this mechanistic interpretation.
Introduction Polysilanes containing three consecutive silicon atoms and an additional chromophore on the central metalloid atom have been especially useful photochemical sources of divalent silicon.lI2 On the other hand, when the silicon atoms of the polysilane are arranged in a tris(trimethylsily1)silylgroup with an acyl chromophore on the central silicon, photochemical isomerization cleanly leads to 2-trimethylsiloxy-substitutedsilenes3 that are ~ > ~a typical silylene.6 significantly less r e a ~ t i v ethan R
I I
hv
MeaSiSiSiMe3
R2Si
+ Me3SiSiMe3
(refs 1 and 2)
R
0
-
Me3Si,
hv
RKSi(SiMe3)3
Me3Si
,O-SiMes
pi="\
(ref 3)
R
silicon-containingtransients, silenes and silylenes, from elementary steps.7 It may seem that numerous precursors are capable of such processes from absorption of a photon,8 but radical pathways can complicate the interpretati~n.~ For example, it has been pointed out that, from photolysis of aryldisilanes, radical pathways can intervene and provide products that appear t o require primary silene intermediates.1° When such silyl radicals, produced from Si-Si bond homolysis by absorption of a 254-nm photon,ll are formed in close proximity (solvent cage), they may initiate a variety of secondary reactions. Among these are (a) recombination of the geminate radicals, (b) disproportionation to hydridosilanes and the corresponding silene intermediates,12and (c) addition of one silyl radical to an unsaturated substituent of the other radical. This latter pathway might also proceed with a concerted component, geometry permitting.9913
Of interest to us has been the possibility of photolyzing a single precursor that yields both types of reactive
R
Me2Si* *SiMe3 Abstract published in Advance ACS Abstracts, January 15, 1995. (1)Ishikawa, M.; Kumada, M. Adu. Organomet. Chem. 1981,19, 51. (2)Michalczyk, M. J.; Fink, M. J.; De Young, D. J.; Carlson, C. W.; Welsh, K. W.; West, R.; Michl, J. Silicon, Germanium, Tin Lead Compd. 1986,9,75. West, R.;Fink, M. J.; Michl, J. Science (Washington, D.C.) 1981,214,1343. (3)For reviews of silene chemistry, see: (a) Brook, A. G.; Baines, K. Adu. Organomet. Chem. 1986,25, 1. (b) Raabe, G.; Michl, J . In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989;Vol. 2, p 1015. (c) Wiberg, N. J . Organomet. Chem. 1984,273,1984.(d) Gusel'nikov, L. E.; Nametkin, N. S. Chem. Rev. 1979,79,529. (4)For a discussion of stabilization of silenes by the 2-siloxy substituent, see: Apeloig, Y.; Karni, M. J.Am. Chem. SOC.1984,106, 6676. ( 5 )Zhang, S.;Conlin, R. T.; McGany, P. F.; Scaiano, J. C. Organometallics 1992,11, 2317. (6)For recent examples of solution kinetics of silylene reactions, see: (a) Conlin, R. T.; Netto-Ferreira, J. C.; Zhang, S.; Scaiano, J. C. Organometallics 1990, 9, 1332. (b) Konieczny, S.;Jacobs, S. J.; Braddock Wilking, J. K.; Gaspar, P. P. J. Organomet. Chem. 1988, 341,C17. (c) Levin, G.; Das, P. K.; Bilgrien, C.; Lee, C. L. Organometallics 1989,8,1206. Levin, G.; Das, P. K.; Lee, C. L. Organometallics 1988, 7 , 1231. (d) Shizuka, H.; Tanaka, H.; Tonokura, K.; Murata, IC;Hiratsuka, H.; Ohshita, J.;Ishikawa, M. Chem. Phys. Lett. 1988,143,225. (e) Gaspar, P. P.; Holten, D.; Konieczny, S.; Corey, J. Acc. Chem. Res. 1987,20,329.
I
MepSiSiMe3
recombination disproportionation radical or concerted addition to R
A possible distinction between reaction paths suggested from product studies and those paths supported ~~~~~~
(7)Some stable silenes have been reported to rearrange on photolysis: Baines, K. M.; Brook, A. G.; Ford, R. R.; Lickiss, P. D.; Saxena, A. K.; Chatterton, W. J.; Sawyer, J. F.; Behnam, B. A. Organometallics 1989,8, 693. (8)Miller, R. D.;Michl, J. Chem. Rev. 1989,89,1359. (9)Slugget, G. W.; Leigh, W. L. J . A m . Chem. SOC.1992,114,1195. More recent results from Professor Leigh's group suggest that both concerted and stepwise eliminations can occur in competition: Slugget, G. W.; Leigh, W. Organometallics 1992,11,3731.A possible silatriene intermediate has been identified by nanosecond flash photolysis experiments on phenylpentamethyldisilane: Shizuka, H.; Okasaki, K.; Tanaka, M.; Ishikawa, M.; Sumitani, M.; Yoshihara, K. Chem. Phys. Lett. 1985,113,89. (10)Sakurai, H.; Nakadaira, Y.; Kira, M.; Sugiyama, H.; Yoshida, K.; Takiguchi, T. J. Organomet. Chem. 1980,184,C36. (11)The amount of energy available to 1 from a 254-nm photon corresponds 112 kcdmol and clearly is sufficient to achieve siliconsilicon bond homolysis. (12)Hawari, J. A.; Griller, D.; Weber, W. P.; Gaspar, P. P. J . Organomet. Chem. 1987,326,335.
0276-7333/95/2314-1471$09.00/00 1995 American Chemical Society
Zhang et al.
1472 Organometallics, Vol. 14,No.3, 1995 by spectral observation must be recognized. The difficulty is the sometimes tenuous connection between the spectral signal and the reaction pathway attributed to the reactive intermediate. The anticipated correspondence between the macro- and microscopic experiments can be misleading when the method of spectral detection (electronictransitions as U V , ESR, LIF, etc.) is typically many orders of magnitude more sensitive than the techniques used for product analysis (NMR, IR, etc.). Within these constraints, however, direct observation of the transient species in combination with chemical trapping studies can offer valuable support for a reaction mechanism. Vinyltris(trimethylsily1)silane (l),a structure containing both the polysilanyl and vinyl chromophores, was a likely photochemical precursor to such reactive intermediates. Previous photolyses of 1,2-divinyltetramethyldisilanes14-16 indicated an uncomplicated isomerization to a transient silene that was captured by a variety of trapping agents. At the outset of this study, several questions pertinent to mechanisms of the photolysis of functionalized polysilane were addressed. For example, what primary intermediates can be observed by W/visible spectroscopy using a low-temperature organic glass for matrix isolation of the reactive species? Of additional importance was the possibility of supporting the spectral assignments with unambiguous chemical trapping studies.
Reactions. Vinyltris(trimethylsily1)silane (1) was synthesized in good yield by addition of vinylmagnesium bromide to tris(trimethylsily1)chlorosilane and isolated by distillation at reduced pressure. Cophotolysis of hexane solutions of 1 was done with different trapping agents, such as 1,3-dienes, and those containing a heteroatom, such as alcohols and also silyl hydrides. It has been shown previously that the first two groups of reagents yield characteristically different products from silylenes and silenes while silyl hydrides are unreactive with silenes. Photolysis of 1 (0.28 M) in a 20-fold excess of butadiene in hexane yielded adducts of 1,l-bidtrimethylsilyl)-2-(trimethylsilyl)methylsilene (2) and of vinyl(trimethylsily1)silylene (3)to butadiene. Three new products, silacyclohex-3-ene4 (58%yield), silacyclopent3-ene 5 (27%), and hexamethyldisilane (HMDS, 40%) were detected by analytical gas chromatography after 16 h photolysis based on -80% decomposition. Products were isolated and characterized by NMR and mass spectroscopy.
/' hv
SiMe3 I
r S i M e 3+
MesSi,
?="\H
Me3Si
2
I
Q+ si
I
Me3Si
3 (40%)
1 Me&
I
4 (58%)
w 5 (27%)
1
+
hv
q OSiMe3
Me&
SiMe3
IS( Me3SiO"
a (54%) R = CHnSiMe3
Results and Discussion
Me3SiSiSiMe3
The five-membered ring 5, from silylene addition to butadiene, is also photosensitive and undoubtedly subject t o the secondary photochemical reactions of vinyldisilanes described above.17 This suggestion is supported by detection ofa product,
