Unexpected E -to- Z Isomerizations during the Negishi-Type Homocoupling of E- Iodoalkenes.

The direct insertion of Zn into olefin-halide bonds is a challenge. When ( E )-alkenyl iodides were treated with a very large excess of Zn nanoparticles, in the presence of Pd(PPh 3 ) 4 , the dimerization was observed but, unexpectedly, yielding mainly Z,E -1,3-dienes. This apparently contrathermodynamic E -to- Z isomerization of organometallic intermediates is predicted to be general and is explained with the aid of DFT [principally M06/6-311+G(d,p)], MP2, and CCSD(T) calculations.