Photochemistry of silylcarbene precursors and mechanisms of metallaene reactivity

Abstract

<p>The photochemistry of a series of α-silylcarbene precursors has been studied by steady state and nanosecond laser flash photolysis (NLFP) techniques. Direct irradiation of (trimethylsilyl)ketene and -diazirine in hydrocarbon solution results in the formation of 1,1,2-trimethylsilene. Flash photolysis of (trimethylsilyl)ketene and -diazirine yield singlet trimethylsilylcarbene as the direct precursor to the silene, as illustrated by pyridine trapping experiments. On the other hand, irradiation of (trimethylsilyl)diazomethane results in photoisomerization to the diazirine or silene formation from the excited state of the precursor, depending on the wavelength of irradiation. Photolysis of (pentamethyl)(trimethylsilyl)diazomethane or -ketene in methanolic hexane solution yields four methoxysilane isomers. 1,1-Dimethyl-2,2-bis(trimethylsilyl)silene (11a ) is formed in the highest yield and is accompanied by the formation of three other silenes: E - and Z -1,2-dimethyl-1,2-bis(trimethylsilyl)silene and 1,1,2-trimethyl-2-pentamethyldisilanylsilene. Silene 11a is the most electrophilic silene studied to date and its absolute kinetics with a number of nucleophiles in hexane and THF solution at 24°C were determined. The results were compared with relative rate data for the same silene determined by N. Wiberg in diethyl ether at 100°C. 1,1-Diphenylsilene, and its germanium analogue 1,1-diphenylgermene, were generated by photolysis of the corresponding 1,1-diphenylmetallacyclobutanes in hexane solution. Laser flash photolysis techniques were used to determine the absolute rate constants and Arrhenius parameters for the head-to-tail dimerizations of these species. The dimerizations occur at nearly the diffusion limit and are among the fastest reactions known for these two species under these conditions. The photochemistry of the 1,1,3,3-tetraphenyl-1,3-dimetallacyclobutane dimers has been done in an effort to propose a possible mechanism for the dimerization of metallaenes.</p>