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Abstract

The possible transition state conformations (chair (S), boat (W), and twist (T), respectively cross (K) forms) and methods for their determination in the thermal ortho-CLAISEN rearrangement of allyl aryl ethers are discussed.

Crotyl 3,5-dimethylphenyl ether (11) gives a mixture of 2-(α-methylallyl)-3,5-dimethyl-phenol (12) and 4-crotyl-3,5-dimethyl-phenol (13) on heating in N, N-diethylaniline. Values of 3 and 31 were obtained for the ratio of 12/13 for trans-11 and cis-11, respectively. It therefore follows that both ethers rearrange steroselectively ( > 90%) by the S or W forms of the activated complex.

αMethylallyl 6-alkylphenyl ethers rearrange on heating in various solvents to a mixture of trans-and cis-2-crotyl-6-alkyl-phenols. The amount of the cis-phenols in the rearrangement products decreases with the increasing bulk of the 6-alkyl substituent. This result is only obvious if the chair form of the transition state during the rearrangement of these ethers is highly favoured.

trans-Crotyl 2,6-dimethylphenyl ether (trans-33) rearranges highly steroselectively (94%) on heating to trans-4-crotyl-2,6-dimethyl-phenol (trans-34). In the case of the corresponding cis ether 33, the rapid cis [RIGHTWARDS ARROW] trans isomerisation of this ether and the cis/trans ratio of the phenol 34 indicate that the reverse rearrangement of the intermediate ortho-dienone to the ether 33 and the further rearrangement to 4-crotyl-2,6-dimethyl-phenol (34) has little stereoselective character.