Selektive Metallierung von Bicyclo[3.2.1]octa-2,6-dien: I. Monostannylierte Derivate

Abstract

In contrast to earlier reports, bicyclo[3.2.1]octa-2,6-diene ( 1) can be deprotonated directly. The overall yield and the selectivity depend strongly on the reagent used n-BuLi/TMEDA metallates less than 5% of 1, whereas t-BuLi/TMEDA or n-BuLi/t-BuOK metallates 80 or 85% of 1. This was established from quenching of the carbanions with Me 3SnCl. The various stannylated isomers that have been obtained show that n-BuLi/t-BuOK attacks 1 exclusively at the allylic position 4 giving the exo/ endo-isomers 4a/ b. t-BuLi/TMEDA prefers the vinylic positions 6 and 7 and leads to the isomers 2a/ b (or 3a/ b having n-Bu 3Sn groups). No reaction occurs at the vinylic positions 2 and 3; this is shown by independent synthesis of the 3-stannylated derivative ( 7), 2a/ b, 3a/ b, 4a/ b and 7 were identified from the NMR parameter δ( 1H), δ( 13C), δ( 119Sn), n J( 1H 1H), 1 J( 13C 1H), n J( 119Sn 13C), and the isotope shifts n Δ 119Sn( 13C( i)). For 1 the 13C NMR signals were assigned unequivocally by analysis of the 13 C satellite spectrum which also gave 1 J( 13C 13C) and 1Δ 13C( i)( 13C( j)). The different reactivity shown by 1 is suggested to originate in the CH acidity and in the influence of Li + and K + on the transition state of the metallation. The isomer ratios could be traced back to the polarity of, and to torsional energies in 1.