Routes to stannoles, stannolenes, and 1-stanna-4-bora-2,5-cyclohexadienes - crystal structure of a triorganotin cation stabilized by p-coordination.
Di-1-alkynyltin compds. (1), e.g., Me2Sn(C.tplbond.CH)2, react with triorganoboranes R3B [2a (R = Et), 2b (R = i-Pr)] to give (h2-alkynyl)tin compds. (3), e.g., I, in which a cationic three-coordinate tin center is stabilized by intramol. side-on coordination to the C.tplbond.C bond of an alkynylborate fragment. An x-ray anal. of 3e, prepd. from Me2Sn(C.tplbond.CiPr)2 (1e) and Et3B, proves the coordination of the tin atom to the C.tplbond.C bond as well as a weak B-C.tplbond. 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 temp. Most of the intermediates 3 react with an excess of Et3B by a second intramol. 1,1-ethyloboration to dialkenyltin derivs. which readily rearrange to the 3-stannolenes (8), e.g., II. Without an excess of Et3B, intramol. 1,1-vinylboration (leading to the stannoles (5), e.g., III ) competes with intramol. 1,1-ethyloboration (leading to 1-stanna-4-bora-2,5-cyclohexadienes (9), e.g., IV).
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