Properties of 4,4-dimethylcyclopent-2-enone: methylation of the enolate ion: base-catalysed and photochemical dimerisations

Abstract

(a) Methylation of the enolate ion of 4,4-dimethylcyclopent-2-enone (IV) occurs through the monocyclic valence tautomer, giving 4,4,5-trimethyl- and 4,4,5,5-tetramethyl-cyclopent-2-enone. The action of triphenylmethylsodium on (IV) is not proton abstraction, but Michael addition. A rationalisation for this is proposed. (b) Physical measurements are described which show the dimer of 4,4-dimethylcyclopent-2-enone, formed on treatment with base, to be endo-5,5,10,10-tetramethyltricyclo[5,2,1,0^2,6] decane-3,8-dione (X). Its mode of formation is discussed. (c) Structures for the two photodimers of 4,4-dimethylcyclopent-2-enone are assigned on the basis of physical data and by analogy with the photodimers of cyclopent-2-enone itself.