Silene-Type Species in Elimination-Addition Reactions

SILENE-TYPE SPECIES IN ELIMINATION-ADDITION REACTIONS 

The elimination-addition route [Eq. (36)] constitutes a rare but important dissociative mechanism for displacement at silicon (150). Along the reaction coordinate one must assume the formation of unsaturated silicon species, which are now of interest to great number of organosilicon chemists (151). 

Even in the late 1970s one could find the statement that stable compounds with double bonds from silicon to carbon, oxygen, and nitrogen were not known (152). In the 1980s knowledge about such systems has developed dramatically. Theoretical calculations and studies of gas-phase processes, though still numerous, are slowly giving way to the relatively new area of stable unsaturated molecules and their reactions in solution (153-157). 

The above ab initio and related calculations clearly reveal that unsaturated silicon compounds are less stable than their carbon analogs (156,170), but as gas-phase studies and the chemistry of sterically crowded silicon derivatives have proved (153,154,156,158,162,172-175) such compounds can no longer be labeled as nonexistent (176,177). A number of isolable double-bonded silicon derivatives have recently been synthesized and characterized in agreement with theoretical predictions (Table III) (169,173,178-195). The data illustrate well that the kinetic stability of multiply bonded silicon species can be greatly improved by substitution of sufficiently bulky groups across the p -pw bond. The ease of addition reactions is then severely restricted. The new target for organosilicon chemists appears to be silicon triple-bonded compounds (196,197). 

Elimination processes of the type depicted in Eq. (36) constitute an important pathway for generating unsaturated silicon compounds. 

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