Ene reactions of allenes. Part 5. Regio- and stereo-selective ene reactions of a trialkylallene

Abstract

Thermal ene insertion reactions have been achieved between 2-methylpenta-2,3-diene and the reactive enophiles diethyl azodiformate, hexafluorobut-2-yne, dimethyl acetylenedicarboxylate, and 1,3-dichlorotetrafluoroacetone. In each case the ene adduct isolated arises by regioselective attack at the more substituted allenic π-bond, and the ¹H n.m.r. spectra of the adducts also suggest that the reaction is stereoselective. Whereas the azodiformate converts the allene cleanly to the dienyldicarbamate (1d), the other enophiles also cause numerous side-reactions to occur, leading to the formation of [2 + 2] cycloadducts, rearrangement of the allene to 2-methylpenta-1,3-diene, and Diels–Alder cycloaddition. Related reactions between dichlorotetrafluoroacetone and 2,4-dimethylpenta-2,3-diene and 3-methylbuta-1,2-diene, and between hexafluoroacetone and 2,4-dimethylpenta-2,3-diene are discussed.