Substitution reactions occuring via the elimination-addition mechanism. Part I. The reactions of 1-chlorocyclopentene and 1-bromocyclobutene with phenyllithium. Part II. The coupling reaction of phenyllithium and cyclopropyl chloride and the action of strong bases on 1-bromoadamantane
PART I: Phenyllithium and 1-chlorocyclopentene couple at 150° to give 1-phenylcyclopentene in 30% yield. The mechanism of this reaction was investigated by reacting phenyllithium and 1-chlorocyclopentene-1-14C under similar conditions. Extensive rearrangement of the 14C label occurred; 1 phenylcycl...
| Summary: | PART I:
Phenyllithium and 1-chlorocyclopentene couple at 150° to give 1-phenylcyclopentene in 30% yield. The mechanism of this reaction was investigated by reacting phenyllithium and 1-chlorocyclopentene-1-14C under similar conditions. Extensive rearrangement of the 14C label occurred; 1 phenylcyclopentene-x-14C was formed with 48.9%, 36.2%, and 14.9% of the 14C distributed among the 1-, 2-, and 5-positions, respectively. The coupling reaction proceeds by way of the elimination-addition mechanism with the intervention of an intermediate possessing the symmetry properties of cyclopentyne. The 14.9% of 1-phenylcyclopentene-5-14C most probably arises from a phenyllithium-induced allylic rearrangement of the double bond of the first-formed 1-phenylcyclopentene-2-14C. Alternative substitution mechanisms are discussed.
Piperidine catalyzes the coupling reaction of phenyllithium and 1-chlorocyclopentene. The catalysis is similar in magnitude to that observed for the coupling of phenyllithium and aryl halides.
The reaction of 1-bromocyclobutene and phenyllithium has been studied at temperatures from 42-135°. Phenylacetylene and cyclobutene are formed as reaction products, but no 1-phenylcyclobutene was observed. The mechanism of this unusual phenylacetylene-forming fragmentation reaction has been investigated, and several possible mechanistic courses have been ruled out.
PART II:
Phenyllithium and cyclopropyl chloride couple under mild conditions to give phenylcyclopropane in low yield. The poor nucleophilicity of phenyllithium and the known reluctance of cyclopropyl derivatives to undergo typical unimolecular (SN1) and bimolecular (SN2) nucleophilic substitution reactions make it doubtful that the substitution proceeds by either of these courses. The possibility that substitution occurs via the elimination-addition mechanism was investigated briefly, but experimental difficulties prevented a unique interpretation of the results.
Bridgehead halides are known to be inert to nucleophilic substitution reactions. The possibility that substitution reactions of bridgehead halides might be achieved by way of elimination-addition is very interesting, since this mechanistic route would require the formation of an intermediate in violation of Bredts rule. 1-Bromoadamantane did not react with potassium amide in liquid ammonia but was converted to adamantane by potassium piperidide in refluxing piperidine. |
|---|