Activation of Silicon- and Tin-Carbon Bonds. 1,1- Carboboration of Alkynylsilanes and -stannanes

In the course of 1,1-carboborations, the activation of polar Si-C? and Sn-C? bonds by electrophilic triorganoboranes, BR3 (R = alkyl. aryl, C6F5) is used to build new C-C bonds. Expectedly, alkynyltin compounds were found to be much more reactive than the corresponding silanes. Monoalkynyl derivatives lead to organometallic-substituted alkenes, quantitatively and stereoselectively in most cases. Dialkynylsilanes R 2 2Si(C?C-R 1 )2 (R1 = alkyl, aryl, silyl; R2 = H, alkyl, allyl, vinyl, aryl, Cl) react with BR3 via twofold 1,1-carboboration by selective formation of siloles. In the case of dialkynylstannanes R2 2Sn(C?C-R 1 )2, (R1 = alkyl, aryl, silyl; R2 = alkyl, benzyl, aryl, amino) the analogous reactions lead mainly to stannoles or alternatively to 1-stanna-4- bora-cyclohexa-2,5-diene derivatives, depending in a complex way on substitutents R at boron and R1 at the C?C bond. The high reactivity of the Sn-C? bonds allows the reactions to be conducted under mild conditions enabling the isolation and structural characterisation of intermediates. These possess a zwitterionic structure, in which typically an almost trigonal planar surrounded tin atom is side-on coordinated to the C?C bond of an alknylborate unit. Extending these reaction principles to tetraalkynylsilanes Si(C?C-R 1 )4, one obtains 1,1'-spirobisiloles via fourfold 1,1- organoboration. Similarly, 1,1'-spirobistannoles can be prepared starting from some tetraalkynylstannanes Sn(C?C-R 1 )4 (R1 = i Pr, tBu, SiMe3).