Canadian Journal of Chemistry, 78, 1459-1468 (2000).

National Research Council of Canada, 2000

The One- and Two-Photon Photochemistry of Benzylsilacyclobutanes, Acyclic Benzylsilanes, and 1,1,2-Triphenylsilacyclobutane

William J. Leigh* and Thomas R. Owens

Contribution from the Department of Chemistry, McMaster University, Hamilton, ON  Canada L8S 4M1

Abstract: The photochemistry of several a-silylbenzyl compounds has been investigated in hexane and in methanol solution. Direct photolysis of 1-benzyl-1-methylsilacyclobutane (1) in methanolic hexane solution produces 1-propyl-1-methyl-2,3-benzosilacyclobutene (6) in quantitative yield, by a sequential two-photon process involving the photoactive isotoluene derivative 1-methylene-6-(1-methylsilacyclobutyl)-2,4-cyclohexadiene (13a), which has been identified on the basis of its nmr and UV absorption spectra. In contrast, direct irradiation of 1-benzyl-1-phenylsilacyclobutane (2) under similar conditions results in the formation of a complex mixture of products consistent with the competing formation of 1-benzyl-1-phenylsilene and benzyl- and 1-phenylsilacyclobutyl radicals. The silene is a transient which has been detected directly by laser flash photolysis of 2 (lmax = 315 nm; t ~ 4.5 ms). Free radical formation is shown to be due to secondary photolysis of a second primary product, 1-methylene-6-(1-phenylsilacyclobutyl)-2,4-cyclohexadiene (13b), which has also been detected and identified by static UV absorption (lmax = 335 nm) and 1H nmr spectroscopy. In a reaction analogous to the acid-catalyzed desilylation of allylsilanes, both 13a and 13b can be intercepted in neutral or acidic methanol solution to yield toluene and 1-methyl- or 1-phenyl-1-methoxysilacyclobutane, respectively. Direct photolysis of benzyltrimethylsilane (3) and benzyldimethylphenylsilane (4) also leads to the formation of isotoluene derivatives. The endocyclic benzylsilane 1,1,2-triphenylsilacyclobutane (5) is shown to undergo competing [2+2]-cycloreversion and [1,3]-silyl migration to yield a bicyclic isotoluene analogue, which explains a few unusual results that have been reported previously for 2-phenylsilacyclobutane derivatives. Relative quantum yields for isotoluene formation from photolysis of 1-5 and absolute rate constants for methanolysis of the five isotoluenes in 1:1 THF:MeOH have also been determined.


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