Reactions and reactivity of allylic and benzylic carbamates.

Abstract

The purpose of this investigation was to study the effect of the carbamate group on the reactions and reactivity of substituted 1,1-diphenylmethane and a,ro-diphenyl allylic compounds. A series of carbamates were prepared and reacted with a variety of electrophiles and nucleophiles. l-(O-N,N-dimethylcarbamoyloxy)-l,l-diphenylmethane (i) reacted with a variety of electrophiles to afford a-substituted carbamate products (H). Reactions of allylic carbamates with electrophiles proceed with substitution at the carbon atom a or y to the carbamate. 1-(O-N,N-diethylcarbamoyloxy)-1,3-diphenyl-2-propene (iii) only reacted with methyl iodide to afford the y-substituted product (iv). Reactions of 1-(O-N,N-diethylcarbamoyloxy)-l ,5-diphenyl-2,4-pentadiene (v) with electrophiles were all unsuccessful. Nucleophilic substitution reactions were performed with carbamates (i), (iii) and (v). No success was achieved in the reactions of (i) with nucleophiles. Carbamates (iii) and (v) reacted with a few oxygen nucleophiles to afford allylic ethers with simultaneous elimination of the carbamate group. Several properties ofterminal-diphenyl carbamates have been revealed: (a) Benzylic carbamate (i) reacted successfully with a variety of electrophiles. Nucleophilic substitution is not favoured with the benzylic carbamate. This indicates that SN2 elimination of the carbamate does not occur in this molecule. (b) Allylic carbamate (iii) only reacts with methyl iodide indicating that the bulk of the diethyl substituents on the carbamate group, the bulk of the incoming electrophile and the size of the phenyl groups are fundamental to the success of reaction. Methylation occurred only at the y-position. (c) Conjugated allylic carbamate (v) did not react with any electrophiles, suggesting that, in addition to the steric factors, the stability offered in the retention of conjugation in the molecule prevented the formation of the electrophilic substitution products. (d) Unsymmetrical allylic carbamates (iii) and (v), in which the carbamate occupies a benzylic position, are more stable than symmetrical allylic carbamates (vi) and (vii) which decompose to the corresponding alcohol. (e) Nucleophilic substitution of (iii) and (v) occurred with SN2' elimination of the carbamate group, the reaction proceeding in a way which faciliates the formation of the most conjugated product possible. Nucleophilic substitution of (iii) with the phenoxide anion resulted in the allyl aryl ether (viii). Several attempts were made to promote the Claisen rearrangement to afford the allylic phenol (ix) but without success. Exploitation of the migrational ability and the leaving group ability of the carbamate moiety were extended to the synthesis of /13-flavene (xi) from allylic alcohol (x). Several attempts were made to synthesise (x) via the Aldol condensation and Grignard reactions. The synthesis of (xi) was hindered by the failure to produce allylic alcohol (x). During the course of these investigations an unexpected decomposition reaction was discovered. The purification by distillation of allylic carbamate (xii) afforded allylic amine (xiii) in quantitative yield. The driving force for the reaction is the intramolecular SN2' elimination of the carbamate with the simultaneous loss of carbon dioxide. This reaction may have scope in the synthesis of allylic amines.