This invention relates to a process for the preparation of 2-alkenoic acid esters, especially esters of .alpha.-halocinnamates.
Typical syntheses of esters of .alpha.-halocinnamates involve base-catalyzed eliminations of the cinnamate dihalides such as those reported by Lown et al., Can. J. Chem., 51, pp. 856-59 (1973) and Berlin et al., Tett. Letts., pp. 873-76 (1968). The implementation of this methodology requires esterification of the acid, halogenation of the resultant ester, and base-catalyzed elimination of the isolated dihalide ester (a three-step operation).
The synthesis of .alpha.-halocinnamic acids has been demonstrated by Zakharkin, Acad. Sci. USSR Bull. Div. Chem. Sci., pp. 303-10 (1956) and employs an allylic rearrangement of a trichlorovinylphenyl carbinol using an acid catalyst. It is further taught in the art that these carbinol starting materials can be economically produced through the free radical condensation of benzyl alcohol, benzyl chloride or toluene with perchloroethylene. L. Schemerling and J. P. West, J. Amer. Chem. Soc., 75, pp. 6216-7 (1953).
Nonetheless, the known method for rearranging these carbinols leaves much to be desired. For example, the Zakharkin method gives a 4 percent isolated yield of .alpha.-chlorocinnamic acid after heating a hydrogen chloride saturation solution of 2,3,3-trichloro-1-phenyl-2-propene-1-ol in acetic acid for 4 hours at 120.degree. C. The difficulty encountered in catalyzing this rearrangement has been noted by others (F. Pochat and E. Levas, Bull. Chem. Soc. Fr., 10, pp. 3846-55 [1972]) who have demonstrated a multistep synthesis of .alpha.-chlorocinnamic acid starting from perchloroacrolein. Lastly, the latter two procedures yield the free acid and conversion to the cinnamate ester requires another process operation.
In view of the deficiencies of the prior art, it would be desirable to have a high yield, one-step process for the preparation of .alpha.-halocinnamate esters and related compounds which would employ inexpensive starting materials.