Wittig, Michael, and Related Reactions

Wittig, Michael, and Related Reactions.—Carbanions generated under PTC conditions can undergo reactions other than alkylation, and an amount of interest has been shown in adapting the Wittig reaction (Equation 3), and related processes. 

An interesting finding in this area is that no added catalyst is necessary in the reactions of equation (4). The assumption made was that phosphonate systems could, like their phosphonium ion counterparts, act as their own phase-transfer catalysts, and this is borne out by the use of a-phosphoryl sulphoxides and sulphones (13) and related compounds as the mediators of some two-phase car-banion alkylations. 

Mikolajczyk, S. Grzejszczak, A. Zatorski, F. Montanari, and M. Cinquini, Tetrahedron Letters, 1975, 3757. 

Simple aldol processes do not seem to have been undertaken by PTC in any serious way. Some of the papers reporting alkylations of ketones, or the Horner-Wittig reactions mentioned above, describe aliphatic aldehydes and ketones as undergoing undesired self-condensations, and it is presumably a lack of control over such processes that has deterred researchers. 

Miscellaneous Processes.—Various types of reaction other than those discussed above have been investigated under PTC conditions, and only some recent instances are cited here. A ready debromination of 1,2-dibromides to alkenes (Equation 7) by aqueous iodide uses catalytic quantities of both a phosphonium salt (to transfer iodide ion) and iodide ion itself. The latter species is regenerated continuously by reduction of the halogen products with sodium thiosulphate in the 

---