Hetero-Atom Effects on Kinetic Acidities: The Metalation of 1-Methyl-cyclohexene and its 4-Aza- and 4-Oxa-Analogs

Abstract

Hexane suspensions of trimethylsilylmethylpotassium or butyllithium/potassium tert-butoxide deprotonate 1-methyl-cyclohexene almost exclusively at the methyl group. In contrast 1,4-dimethyl-1,2,3,6-tetrahydropyridine and 4-methyl-5,6-dihydro-2H-pyran preferentially undergo hydrogen/metal-exchange at the nitrogen-distant and, respectively, oxygen-adjacent allylic methylene group. Strong amide bases convert 5,6-dihydro-2H-pyrans into (Z)-pentadienols. The latter ring-opening reaction follows a concerted and not a stepwise pathway (E2 rather than E1cb-irrev, as a novel «common intermediate»-criterion reveals).