Metalation of 9‐Oxabicyclo[3.3.1]nonadiene – the Deprotonation Path to Bridgehead Olefins

Abstract

AbstractA new route to bridgehead olefins based on a deprotonation reaction has been established. The deprotonation of 9‐oxabicyclo[3.3.1]nonadiene (1) with tBuLi/TMEDA occurs selectively in the allylic position. Whilst 1 has four allyl positions, only one bridgehead proton was removed, as demonstrated by quenching with Me3SnCl, Me3SiOTf, and Me3PbCl. With a threefold excess of deprotonating agent and subsequent treatment with Me3SnCl, three stannylated derivatives – 3, 4a, and 4b – were obtained. Compound 3 is the starting diene 1 substituted at the bridgehead position 1 in the skeleton, and when smaller quantities of deprotonating agent were used it was the only stannylated derivative of 1 formed. Compounds 4a and 4b had each undergone a shift of one double bond to give bridgehead olefins exo‐ and endo‐stannylated at position 3 in the skeleton. The common intermediate precursor of compounds 3, 4a, and 4b was a bridgehead allyl anion, which can also be generated from 3 and could be observed directly by 6Li and 13C NMR spectroscopy. After quenching of the anion with Me3SiOTf, and Me3PbCl, two silylated and plumbylated derivatives of 1 were obtained in each case: the bridgehead‐substituted compounds and the 3‐exo‐substituted bridgehead olefins. The stannylated bridgehead olefins 4a and 4b rearranged slowly to give 3 in what is formally a 1,3‐stannatropic shift reaction. The kinetics of the reactions are zero‐order in 4a and 4b, respectively, pointing to intermediate light‐promoted homolytic cleavage of the Sn–C bond and the formation of a tight radical pair.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)