Process for preparing dienoic acids

A process for the preparation of dienoic acids having double bonds in a conjugated position, which process comprising reacting a vinyl halide of the formula:

• R--CH.dbd.CHX, wherein R is selected from the group consisting of hydrogen, hydrocarbylic groups having from 1 to 20 carbon atoms, and said hydrocarbylic groups substituted by substituents inert under the reaction conditions, and X is chlorine or bromine, with a salt of 3-butenoic acid of the formula:
• CH.sub.2 .dbd.CH--CH.sub.2 .dbd.COOM, wherein M is a metal selected from the group consisting of Na, K, 1/2 Ca, 1/2 Mg or the group NR'.sub.4 wherein (R').sub.4 are alkyl groups having from 1 to 12 carbon atoms, in an organic medium and under an inert atmosphere, at a temperature above 50.degree. C. and at atmospheric pressure, in the presence of a catalyst selected from the group consisting of phosphinic complexes of rhodium and nickel having the formulae:
• RhCl(PR".sub.3).sub.x.(ol).sub.y and Ni(PR".sub.3).sub.z
• wherein R" is a hydrocarbylic group having from 1 to 8 carbon atoms;
• "ol" is an olefin selected from the group consisting of olefins having a number of carbon atoms comprised between 2 and 8 and chelating olefins having 6 to 8 carbon atoms, and where x is an integer comprised between 1 and 3, y is an integer comprised between 0 and 2 and such that x+y=3, and z is a number comprised between 3 and 4.