Kinetics and Stereochemistry of LiNR2-Induced 1,2-Elimination of Homoallylic Ethers

Article abstract of DOI:10.1021/jo00265a013

The cis and trans isomers of 2-ethenyl-1-methoxycyclohexane were prepared and treated with LiNR2 in the solvents ether, tetrahydrofuran, and hexane.The diene 1-ethenylcyclohexene was formed in high yield from both substrates.In the ethereal solvents, syn elimination is favored over anti, although both pathways are viable and the k_syn/K_anti ratio is only 3-5.In contrast, anti elimination is favored when lithium diisopropylamide is employed in hexane solvent.This inversion in selectivity is associated with depression of k_syn rather than enhancement of k_anti, relative to reactions in ether and THF.Syn elimination is favored in all three solvents for reactions of lithium tetramethylpiperidide, again by modest factors (3-5).Rates were measured under excess base conditions and, with few exceptions, first-order losses of the homoallylic ether substrates were observed.The LiNR2 concentration was varied to examine the effect on rate.The calculated second-order rate constants drifted downward with increasing concentration in ethereal solvents, suggesting a net kinetic order in base between 1/2 and 1.The k_syn/k_anti ratios were only moderately sensitive to base concentration.A novel method for the preparation of cis-enriched homoallylic ether is described.Treatment of cyclohexene in methanol with N-bromosuccinimide gives trans-1-bromo-2-methoxycyclohexane, which, when subjected to vinylmagnesium bromide/CuI catalyst, affords 2-ethenyl-1-methoxycyclohexane in good yield, as a mixture of stereoisomers containing ca. 85percent of the cis product.