Spirocyclic stannole derivatives by 1,1-organoboration of 2,2-bis(1-alkynyl)-1,3-di-tert-butyl-1,3,2-diazastannacyclohexanes

Abstract

With the exception of 1c, the 2,2-bis(1-alkynyl)-1,3-di-tert-butyl-1,3,2-diazastannacyclohexanes ( 1) [CCR 1: R 1=Me ( a), Bu ( b), tBu ( c), Ph ( d), SiMe 3 ( e), CH 2NMe 2 ( f), CH 2OMe ( g)] react with triethylborane, Et 3B ( 2), to give the spirocyclic stannole derivatives 4a, b, d, e, f, g by intermolecular 1,1-ethylboration, followed by intramolecular 1,1-vinylboration. In the cases of 1f and 1g, zwitterionic intermediates 7f, g, in which the tin atom is coordinated to an alkynylborate CC bond, were detected by NMR spectroscopy. In the case of the reaction of 1e with triphenylborane, Ph 3B ( 3), exchange reactions involving the SnN bonds compete with 1,1-organoboration, and the spirocyclic stannole derivative 5e is only part of a complex reaction mixture. In the reaction of 1e with Et 3B, an intermediate 8e with the wrong stereochemistry for ring closure was detected by NMR spectroscopy. Owing to the reversibility of 1,1-organoboration, the spirocyclic stannole 4e is finally formed. Products and intermediates were characterised by 1H, 11B, 13C, 15N, 29Si and 119Sn NMR spectroscopy. Several absolute signs of coupling constants in 4e were determined.