Silafunctional Compounds in Organic Synthesis. 32. Stereoselective Halogenolysis of Alkenylsilanes: Stereochemical Dependence on the Coordination State of the Leaving Silyl Groups

Article abstract of DOI:10.1021/jo00382a023

Halogenation of (E)-1-octenyltrimethylsilane (1), -trifluorosilane (2), and -pentafluorosilicate(2-) (3) by iodine chloride, bromine, and N-bromosuccinimide has been examined in nonpolar solvents, mostly carbon tetrachloride, and in polar solvents, mostly dimethylformamide, in the dark.Cleavage reactions of 1 proceed uniformly with inversion of configuration to form (Z)-1-octenyl halides, while 3 is cleaved with retention of configuration to form (E)-1-octenyl halides.A dual selectivity is observed with 2: inversion in nonpolar solvents and retention in polar solvents.Further spectral and mechanistic studies on 2 have provided a general rule for correlation between the stereochemical courses of the halogen cleavage reactions and the coordination states of the silicon center at the stage of attacking by electrophiles.Thus, while attack on ordinary tetracoordinate silicon compounds (trans addition) followed by nucleophilic attack on silicon (anti elimination) leads to inversion, attack on extracoordinate silicon species immediately followed by cleavage of the activated silicon-carbon bond results in retention of the olefin geometry.On the basis of the new aspect, one-pot synthesis of (E)- and (Z)-alkenyl bromides from acetylene has been achieved in combination with hydrosilation by triethoxysilane.