REGIOSELECTIVITY OF THE INSERTION OF DIENES INTO PD-R BONDS - DIASTEREOSELECTION IN THE ISOMERIZATION OF AN (ETA(1)-ETA(2)-ENYL)PALLADIUM COMPLEX TO AN (ETA(3)-ALLYL)PALLADIUM COMPLEX

The regioselectivity of the olefin insertion reaction into Pd-pentafluorophenyl bonds has been examined by studying the reactions of [Pd(C6F5)Br(NCMe)(2)] and dienes with two nonequivalent double bonds, namely, isoprene, vinylcyclohexene, R-(+)-limonene, alpha-terpinene, and gamma-terpinene. Analysis of the structure of the final (eta(3)-allyl)palladium derivatives formed shows that in each case attack occurs selectively on the least substituted carbon of the least substituted double bond. The selectivity is very high except in the case of isoprene which leads to two products as a result of insertion of the least and the most substituted double bond in a 3:1 ratio, respectively. Total regioselective attack an the least substituted carbon of each double bond is still maintained for this diene. Vinylcyclohexene and R-(+)-limonene give at low temperature eta(1)-eta(2)-enyl derivatives as a 1:1 mixture of two diastereoisomers in each case. These eta(1)-eta(2)-enyl derivatives are intermediates in the allyl formation and isomerize to the corresponding eta(3)-allyl derivatives via Pd migration. Isomerization occurs simultaneously for the two diastereomeric eta(1)-eta(2)-enyl complexes derived from vinylcyclohexene, but the eta(1)-eta(2)-enyl derivatives from R-(+)-limonene form the corresponding eta(3)-allyl complexes with different activation energies. This selective isomerization is attributed to the difference in stability of the two diastereomeric eta(1)-eta(2)-enyl complexes formed from R-(+)limonene.