Nucleophilic catalysis with rearrangement. The reactivity of pseudoaromatic compounds. Part VII. The 1-azabicyclo[2,2,2]octane-catalysed reaction of 2-iodocyclohepta-2,4,6-trien-1-one with piperidine

Abstract

The reactions of either 2-chloro- or 2-iodo-cyclohepta-2,4,6-trien-1-one with piperidine give, in a variety of media, including benzene, 2-piperidinocyclohepta-2,4,6-trien-1-one in quantitative yield, no rearrangement being involved. An added tertiary amine, such as 1-azabicyclo[2,2,2]octane (quinuclidine), catalyses the reaction of the chloro-compound in benzene only to a negligible extent, while the reaction of the iodo-compound is appreciably catalysed. It can be excluded that this reflects base catalysis by quinuclidine since the same reactions are not catalysed by other bases such as piperidine or 2,4-dinitrophenolate (whereas they undergo efficient acid catalysis by compounds such as phenol). Rather, quinuclidine catalysis can be accounted for in terms of two consecutive nucleophilic displacements at the troponoid carbon atom which we have encountered in previous studies. The first, displacement of iodine by quinuclidine from 2-iodo-cyclohepta-2,4,6-trien-1-one to give the reactive intermediate 2-(1-azoniabicyclo[2,2,2]oct-1-yl)cyclohepta-2,4,6-trien-1-one iodide, is a substitution at the carbon occupied by iodine. The second, of quinuclidine by piperidine from the reactive intermediate, is a nucleophilic substitution with rearrangement [substitution at C(7)]. This mechanism, which could be termed ‘nucleophilic catalysis with rearrangement’ is proved by labelling experiments. Thus, in the case of 2-iodo[3,5,7-²H₃]cyclohepta-2,4,6-trien-1-one, when quinuclidine is present, substitution by piperidine occurs at both C(2) and C(7) of the ring, substitution at C(7) occurring clearly via the reactive intermediate suggested above and substitution at C(2) occurring in a process in which quinuclidine does not take part.