Eliminations from 1-phenyl-2-alkyl tosylates promoted by MeONa in MeOH. Steric effects in alkene-forming elimination

Abstract

Reactions of PhCH₂CH(OTs)R [R = Me (1), Et (2), Prⁱ(3), Bu^s(4), Bu^t(5)] with MeONa in MeOH have been studied. The reactions produce both conjugated and unconjugated alkenes. The yields of the conjugated alkenes are nearly the same for 1–4, while the E/Z ratios depend strongly upon the α-alkyl group. The rates of eliminations forming the conjugated E alkenes are decreased by a bulkier alkyl group as indicated by the relative rate of 1, 0.8, 0.7, 0.6, 0.2 for 1, 2, 3, 4, 5, respectively. On the other hand, the relative rates for the formation of the unconjugated alkenes are 1, 1.7, 2.8, 1.9 for 1, 2, 3, 4 respectively, indicating that the rate increases with the double bond stabilizing ability of the alkyl groups and decreases with their steric effect. From these results, the relative steric effect of the α- and β-alkyl groups in alkene-forming elimination is assessed.