Hetero-π-systems from 2+2 cycloreversions.: Part 1.: Gusel'nikov -: Flowers route to silenes and origination of the chemistry of doubly bonded silicon

The review concerns the production of the unsaturated compounds of a doubly bonded silicon via the four-membered silacycles' 2+2 cycloreversion, a reaction which allowed 35 years ago the evidence of 1,1-dimethylsilene (DMSE), (CH3)(2)Si=CH2, the first compound containing the silicon-carbon double bond, and inspired a renewed interest to the chemistry of the multiply bonded silicon compounds at that time believed to be non-existent. Since then a great many of the doubly bonded silicon compounds have been obtained by various methods. Of them sterically non-hindered compounds are kinetically unstable because of their unprecedented reactivity in the bimolecular reactions, whereas those ones kinetically stabilized by the bulky groups, the 'crowded' compounds of the multiply bonded silicon, are relatively stable. Thermal and photochemical 2+2 cycloreversions occurring both in gas and liquid phases are reviewed. A special section is devoted to the gas-phase 2+2 thermocycloreversion of 1,1-dimethyl-1-silacyclobutane as a clean reaction of the transient DMSE production for the study of its chemistry as well as for its characterization by physicochemical and spectroscopic methods. Scope and limitations of 2+2 cycloreversion affected by the numbers and the position of silicon atoms in the four-membered ring, by the presence of Groups 15 and 16 elements, by the substituents and by the reactions conditions are evaluated. (C) 2003 Elsevier B.V. All rights reserved.