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

Abstract

Abstract Di‐1‐alkynyltin compounds ( 1 ) react with triorganoboranes R 3 B [ 2a (REt), 2b (R i Pr)] to give (η 2 1‐alkynyl)tin compounds 3 (REt) and 4 (R i Pr) 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 Me 2 Sn(CC i Pr) 2 ( 1e ) and Et 3 B, 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 11 B and 119 Sn NMR at variable temperature. Most of the intermediates 3 react with an excess of Et 3 B by a second intramolecular 1,1‐ethyloboration to dialkenyltin derivatives 7 which readily rearrange to the 3‐stannolenes 8 . Without an excess of Et 3 B, 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 i Pr). 11 B‐and 13 C‐NMR data indicate that the heterocycles 9 (REt) adopt a flatter conformation than 10 (R i Pr).