Palladium-mediated cross-coupling reactions involving 3-substituted alkyl (E)-2,3-dibromopropenoates and arylzinc or aryltin derivatives

Abstract

Stereodefined unsymmetrically 3,3-disubstituted alkyl 2-bromopropenoates, 5, were regioselectively prepared by Pd-mediated reactions between 3-alkyl, 3-aryl and 3-alkoxycarbonyl substituted ( E)-2,3-dibromopropenoates, (E)- 7 , and aryl or 1-alkynylzinc chlorides. The stereospecificity of these reactions was found to be dependent on the type of substituent present in the 3-position of (E)- 7 . The ( E)-stereochemistry of compounds 5b, 5d and 5i so prepared was confirmed by their conversion into the corresponding 4-substituted 3-bromocoumarins. Tetrasubstituted α,β-unsaturated esters 10 were then synthesized by Pd-mediated reactions either of (E)- 7 with a molar excess of an arylzinc chloride or an aryltributylstannane, or of a compound of general formula 5 with an aryltributylstannane. An examination of the parameters which influence the stereochemistry and the yields of these arylations was made.