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 [R 2 Sn(CCR 1 ) 3 ( 1 ), R 2  Me, R 1  Me ( a ), n Bu ( b ), t Bu ( c ), Me 3 Si ( d ), 1-(1-cyclohexenyl) ( e ); R 2  Et, R 1  Me ( a (Et)), n Bu ( b (Et)), t Bu ( c (Et)), SiMe 3 ( d (Et)); R 2  n Bu, R 1  Me ( a (Bu)), n Bu ( b (Bu))] were prepared, and their reactivity towards trialkylboranes Et 3 B ( 2 ) and i Pr 3 B ( 3 ) in 1,1-organoboration reactions was studied. The first step in each reaction is an intermolecular 1,1-alkytoboration. 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 Et 3 B ( 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 1 H, 11 B, 13 C, 29 Si and 119 Sn NMR.