90645-70-8Relevant articles and documents
Kinetics and Stereochemistry of LiNR2-Induced 1,2-Elimination of Homoallylic Ethers
Tobia, David,Rickborn, Bruce
, p. 777 - 782 (1989)
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 ksyn/Kanti 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 ksyn rather than enhancement of kanti, 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 ksyn/kanti 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.
