1,1‐Carboboration through Activation of Silicon–Carbon and Tin–Carbon Bonds

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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. As would be expected, organotin compounds were found to be much more reactive than the corresponding silanes, and Sn–C(sp) bonds were more reactive than other tin–carbon bonds. Monoalkynyl derivatives lead to organometallic‐substituted alkenes, quantitatively and stereoselectively in most cases. Dialkynylsilanes R22Si(C≡C–R1)2 (R1 = alkyl, aryl, silyl; R2 = H, alkyl, allyl, vinyl, aryl, Cl) react with BR3 by twofold 1,1‐carboboration through selective formation of siloles. In the case of dialkynylstannanes R22Sn(C≡C–R1)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, in a way that depends in a complex manner on the substituents R at boron and R1 at the C≡C bond. Vinyltin compounds and triorganoboranes also react by 1,1‐carboboration. The high reactivity of the Sn–C(sp) bonds allows the reactions to be conducted under mild conditions, enabling the isolation and structural characterisation of intermediates. These are of a zwitterionic structure, in which typically an almost trigonal‐planar surrounded tin atom is coordinated "side‐on" to the C≡C bond of an alknylborate unit. On extending these reaction principles to tetraalkynylsilanes Si(C≡C–R1)4, one obtains 1,1′‐spirobisiloles through fourfold 1,1‐carboboration. Similarly, 1,1′‐spirobistannoles can be prepared by starting from tetraalkynylstannanes Sn(C≡C–R1)4 (R1 = iPr, tBu, SiMe3). Combination of 1,2‐hydroboration and 1,1‐carboboration opens the way to numerous silicon heterocycles, not readily available by any other method.