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

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.

Stereoselective Synthesis of (E)- and (Z)-Disubstituted Vinyl Bromides via Organoboranes

Treatment of - and -disubstituted vinyldimethoxyboranes, prepared as described previously, with bromine, followed by a base, produced - and -disubstituted vinyl bromides in good yields (61-80percent) and excellent stereochemical purities (>/-97percent).In both cases, replacement of the boron moiety with bromine proceeds with inversion.