Reactions of cis-2-Boryl-1-stannylalkenes with Carbodiimides, Thiocyanates, Isothiocyanates, and Isocyanates

Abstract

The reactions of (E)-2-diethylboryl-1-trimethylstannylbut-1-ene and (Z)-3-diethylboryl-2-trimethylstannylpent-2-ene with carbodiimides, methyl thiocyanate, thioisocyanates, and isocyanates were studied, and the products were characterized by multinuclear magnetic resonance spectroscopy (¹H, ¹¹B, ¹³C and ¹¹⁹Sn NMR). It was found that carbodiimides bearing tert-butyl or trimethylsilyl groups at the nitrogen atoms do not react with (E)-2-boryl-1-stannylalkenes, in contrast to dicyclohexylcarbodiimide. There was also no reaction in the case of MeSCN. All other reactions proceed via a weak N→B adduct formation in the initial step, followed by different rearrangements, depending on the structure of reagents as well as on the substitution pattern at the C=C bond in alkenes. New heterocycles are formed, in which the boron atom is either tricoordinated (1,2-azaborolenes, 1,2-azaborolan! es), a nd one ethyl group has been transferred to the neighbor olefinic carbon atom, or the boron atom is tetracoordinated (1,2-azaboratoles, 1,2-oxoniaboratoles), and the trimethylstannyl group has migrated to one of the heteroatoms of the heterocumulenes.