Routes to Stannoles, Stannolenes and 1‐Stanna‐4‐bora‐2,5‐cyclohexadienes – Crystal Structure of a Triorganotin Cation Stabilized by π1‐Coordination

Di‐1‐alkynyltin compounds (1) react with triorganoboranes R3B [2a (REt), 2b (RiPr)] to give (η21‐alkynyl)tin compounds 3 (REt) and 4 (RiPr) in which a cationic three‐coordinate tin centre is stabilized by intramolecular side‐on coordination to the CC bond of an alkynylborate fragment. An X‐ray analysis of 3e, prepared from Me2Sn(CCiPr)2 (1e) and Et3B, proves the coordination of the tin atom to the CC bond as well as a weak BC bond. An important dynamic process, i.e. exchange of the alkynyl group between boron and tin, is shown by 11B and 119Sn NMR at variable temperature. Most of the intermediates 3 react with an excess of Et3B by a second intramolecular 1,1‐ethyloboration to dialkenyltin derivatives 7 which readily rearrange to the 3‐stannolenes 8. Without an excess of Et3B, intramolecular 1,1‐vinyloboration (leading to the stannoles 5) competes with intramolecular 1,1‐ethyloboration (leading to 1‐stanna‐4‐bora‐2,5‐cyclohexadienes 9). The latter route is preferred in the case of most of the intermediates 4 (intramolecular 1,1‐isopropyloboration) and affords the six‐membered rings 10 (RiPr). 11B‐and 13C‐NMR data indicate that the heterocycles 9 (REt) adopt a flatter conformation than 10 (RiPr).