1,1-ORGANOBORATION OF TRI-1-ALKYNYLTIN COMPOUNDS - NOVEL TRIORGANOTIN CATIONS, STANNOLES, 3-STANNOLENES AND 1-STANNA-4-BORA-2,5-CYCLOHEXADIENES

Tri-1-alkynyltin compounds [R2Sn(C=CR1)3 (1), R2=Me, R1=Me (a), (n)Bu (b), (t)Bu (c), Me3Si (d), 1-(1-cyclohexenyl) (e); R2=Et, R1=Me (a(Et)), (n)Bu (b(Et)), (t)Bu (c(Et)), SiMe3 (d(Et)); R2=(n)Bu, R1=Me (a(Bu)), (n)Bu (b(Bu))] were prepared, and their reactivity towards trialkylboranes Et3B (2) and (i)Pr3B (3) in 1,1-organoboration reactions was studied. The first step in each reaction is an intermolecular 1,1-alkyloboration. Afterwards, intramolecular 1,1-vinyloboration or 1,1-alkyloboration compete with further intermolecular 1,1-alkyloboration. Various triorganotin cations (4-7), stabilized by intramolecular side-on coordination to the C=C bond of an alkynylborate moiety, were detected as highly fluxional intermediates prior to rearrangement into heterocyclic systems such as stannoles (9-11), 1-stanna-4-bora-2,5-cyclohexadienes (8, 12). The reactions between 1a or 1a(Bu) and an excess of Et3B (2) afford the tris(alkenyl)tin compounds 13 via threefold intermolecular 1,1-ethyloboration. 13 rearrange to the 3-stannolenes (14a or 14a(Bu)). The intermediates and final products were characterized by multinuclear one- and two-dimensional H-1, B-11, C-13, Si-29 and Sn-119 NMR.