Reaction oftrans-Pd(styryl)Br(PMePh2)2with Styryl Bromide Affording 1,4-Diphenylbutadiene. An Unexpected Homocoupling Process Induced by P−C Reductive Elimination

Reaction of ( Z )-styryl bromide ( 1 ) with PhB(OH) 2 in toluene at 80 °C for 1 h in the presence of Pd(PPh 3 ) 4 (1.5 mol %) and an aqueous solution of K 2 CO 3 (3 equiv) affords ( Z )-stilbene as the cross-coupling product in 99% yield. On the other hand, the same reaction of the ( E )-isomer of 1 forms considerable amounts of homocoupling products (1,4-diphenylbutadiene ( 2, 22%) and biphenyl (27%)) in addition to the cross-coupling product (( E )-stilbene, 73%). The formation of 2 was examined by kinetic experiments using trans -Pd(CH═CHPh)Br(PMePh 2 ) 2 ( 3 ) as a model of the presumed intermediate. Complex 3 reacts with 1 at 50 °C for 5 h, giving 2 (91%) and trans -PdBr 2 (PMePh 2 ) 2 ( 4, 92%), together with a small amount of Pd(η 2 -PhCH═CHPMePh 2 )Br(PMePh 2 ) ( 5, 8%). The reaction rate shows first-order dependence on the concentration of 3 and 5, respectively, but is independent of the concentration of 1 . A novel homocoupling process induced by P−C reductive elimination from 3 giving 5 is proposed.