Stabilization of triorganotin cations - competition between intramolecular coordinative O-, S- or N-Sn bonds and side-on p coordination to C:C bonds.
Dialkynyltin compds. alkynyltin of the type Me2Sn(C.tplbond.CL)2 [1; L = 2-pyridyl (a), Me2NCH2 (b), MeOCH2 (c), MeOCH:CH (d), EtSCH2 (e)] were prep. and the 1,1-organoboration of 1 with triethyl- (2a) and triisopropylborane (2b) has been studied. The first detectable intermediates were in all cases zwitterionic compds. 3 (I, R2 = Et) (from Et3B) and 4 (I, R2 = Me2CH) [from (Me2CH)3B] with a cationic triorganotin fragment, stabilized by coordinative s L-Sn bonds and by side-on coordination to the C.tplbond.C bond of an alkynylborate moiety. The nature of this stabilization has been established by three x-ray analyses (3b, 4b, and 4d) in the solid state and by multinuclear NMR (1H, 11B, 13C, 15N, 119Sn NMR) in soln. The comparison of the mol. structure of 3b, 4b, and 4d with that of intermediates without functional groups shows that the distance of the tin atom to the C.tplbond.C bond is elongated. However, the planar arrangement of all relevant atoms proves that the side-on coordination of the tin atom to the C.tplbond.C bond is still present. These intermediates 3 and 4 rearrange to stannoles 5, 6 (II) and/or to 1-stanna-4-bora-2,5-cyclohexadienes 9, 10 (III). In the case of 3b and 3e, 119Sn NMR indicates that dialkenyltin compds. 7b and 7e are also formed as minor products. In the case of 3e, the 3-stannolene 8e is one of the final products. In the stannoles 5 and 6 a coordinative L-B bond is evident, and there is NMR spectroscopic evidence of a weak coordinative L-Sn bond.
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