Abstract
The partial asymmetric synthesis occurring in the Reformatsky reaction has been studied with a view to elucidating what factors control the degree and direction of asymmetric synthesis. R'R"CO + BrZnCH[2]CO[2]R → R'R"C(OH)CH[2]CO[2]R → R'R"C(OH)CH[2]CO[2]H. The addition of the dissymmetric organometallic intermediate to the carbonyl group does indeed proceed asymmetrically, it having been shewn that "apparent" asymmetric synthesis does not occur as a result of the Reformatsky reaction being reversible, or as a result of preferential decomposition of one of the two diastereoisomeric B-hydroxyesters during the alkaline hydrolysis. The asymmetric addition of the organometallic intermediate to the carbonyl group probably proceeds by a polar mechanism rather than by a radical mechanism. Moreover, asymmetric addition does not occur as a result of reaction on the surface of the metallic zinc. With acetophenone or benzaldehyde, the degree of asymmetric synthesis was most marked using bromoesters of alicyclic alcohols such as (-)-menthol, (-)-borneol or (-)-alpha-fenchol, bromoacetates of aliphatic alcohols, such as (+)-butan-2-ol, leading to little asymmetric synthesis. In all cases, when aliphatic aldehydes were used, the degree of asymmetric synthesis was very small. Nevertheless, the presence of an aromatic nucleus in the carbonyl compound was shewn not to be essential for asymmetric synthesis to occur, hexahydrobenzaldehyde giving a product with a moderate degree of rotation. Moreover, the degree of asymmetric synthesis appears to depend, in some undefined way, upon both the type of solvent and its boiling point. Asymmetric synthesis has also been shewn to occur when organocadmium reagents, organozinc reagents or magnesium iodide are used as "condensing agents". Various mechanisms for the Reformatsky reaction have been considered in the light of the occurrence of asymmetric synthesis in this type of reaction. In this connection, the Reformatsky reaction has been compared with the Claisen Acylation reaction.