Allylindation of 6-substituted 2-hydroxy-1-tetralones in aqueous and organic media. Stereochemistry and competition studies

A systematic investigation of the stereoselectivity associated with coupling reactions involving the allylindium reagent-and 6-substituted 2-hydroxy-1-tetralones is presented. In each instance, the allylations were carried out in water, 50% aqueous THF, and dry THF. The extent of 1,2-induction was found to be highest-in THF-H2O (1:1) and to favor the trans diol isomer. Somewhat lower levels of stereochemical bias in the same direction were observed in pure water. However, further erosion of this trend was noted for those reactions performed in THF, such that a modest crossover in product distribution became apparent in certain examples. On the basis of competition experiments, both reaction trajectories give evidence of proceeding via chelated intermediates. The extent of stereoinduction was found to be in Line with the normal predilection of 2-cyclohexenones for axial attack, which is the process believed to be beset with minimal torsional effects. The substituents situated para to the ketone carbonyl do not exert a very large influence on product distribution, although electronic effects are evident in the context of the competition studies. A mechanistic model which integrates these findings is offered.