Collision-induced dissociations of enolate negative ions. Deprotonated cyclohexanones

Abstract

The retro reaction of the cyclohexanone enolate ion is specific, involving loss of ethene from the 3,4 (4,5) positions. Similar reactions occur for all alkyl-substituted cyclohexanone enolates. 3-Substituted cyclohexanones form two enolate ions and, in these cases, the predominant retro reaction is that in which the larger olefin is eliminated. Loss of H 2 is the major fragmentation of all cyclohexanone enolate ions. There are two losses of H 2, from the 3,4 and 3,6 positions with the former being the more pronounced. 3-Alkylcyclohexanone enolates also lose RH (R is the 3-alkyl substituent); there are two discrete mechanisms directly analogous to the losses of H 2. 3-Substituted cyclohexanones undergo a unique reaction which involves two specific proton transfers; for example, the 3-methylcyclohexanone ion → MeCOCH 2 − + C 4H 6, with the two transferring protons coming from the methyl group and the 4 position. 2-Substituted cyclohexanone ions also undergo retro reactions and loss of H 2, but when the substituent ⩾ Et, characteristic elimination of an olefin (with proton transfer from the 1′ position) occurs from the side chain.