Boraalkenes Made by a Hydroboration Route: Cycloaddition and B=C Bond Cleavage Reactions

Abstract Hydroboration of styrene or vinylcyclohexane with the IMes(C 6 F 5 )BH + cation followed by deprotonation provided a convenient synthetic entry to the [B]=CHCH 2 R boraalkenes 9 a and 9 b . The in situ generated IMes(SCN)BH + system reacted similarly with 1,1‐diphenylethene followed by deprotonation to give the isothiocyanato substituted boraalkene 9 c . The boraalkenes underwent [2+2] cycloaddition reactions with a small series of heterocumulenes to give the respective four‐membered heterocycles. The [B]=CHCH 2 R+CO 2 cycloadducts 13 a and 13 b added the borane HB(C 6 F 5 ) 2 with cleavage of the central B−C σ‐bond. CS 2 underwent an unusual reaction with the boraalkenes, namely insertion into the B=C bond with formation of the borylated dithioketene acetal under complete rupture of the strong B=C double bond. The intermediate dithiobora‐β‐lactone type intermediate was isolated in the case of the isothiocyanato‐boraalkene reaction and characterized by X‐ray diffraction.