Abstract
Ethers of the general formula CHR[2].OR' re-arrange under the influence of Lewis bases such as phenyl lithium into their isomeric alcohols CR[2].R'OH. [mathematical equation] This is a type of saturated electrophilic re-arrangement usually called the Wittig re-arrangement. An investigation of Wittig re-arrangement of ethers was undertaken employing optically active compounds because the mechanism of the closely allied Stevens re-arrangement has been established in this way. The following optically active ethers have been prepared and rearranged: (i) Ethyl mandelyl benzhydryl ether. (ii) Benzyl-B-butyl ether. (iii) Benzyl-l-phenyl ethyl ether. In ethyl mandelyl benzhydryl ether a symmetrical group has been allowed to migrate from oxygen to carbon atom which was already dissymmetric to constitute the centre of the molecule which was already optically active and it resulted in total racemisation of the products formed. In the other two ethers a dissymmetric and optically active group has been allowed to migrate from oxygen to a symmetric carbon, thereby converting it into a dissymmetric atom and thus a potential centre of optical activity. Benzyl-beta-butyl ether re-arranged to form an optically inactive carbinol while benzyl-l-phenyl ethyl ether formed an optically active carbinol on re-arrangement. It has been observed from the structure of the re-arranged product that the optical activity contributed to the re-arranged carbinol is due to the original asymmetric centre and not the new asymmetric centre generated on re-arrangement to which the hydroxyl group is attached. The results support the postulate of the earlier workers that the formation of carbinols from ethers was due to the intramolecular rearrangement of the metallated ethers. The results further confirm Hauser's view regarding the formation of benzaldehyde by beta-elimination and also that the re-arrangement is not followed by beta-elimination. The isomerisation of the metallated ethers seems to take place by migration of the organic group and the metal atom via intramolecular and intermolecular routes respectively. Some aspects in the mechanism remain obscure such as the retention of optical activity by the re-arranged carbinol which has only been observed in ethers containing a phenyl group attached to the potentially active carbon, and the necessity of two molecular equivalents of Lewis bases for effecting the re-arrangement of ethers.