Both cis- and trans-olefin halides promote the overall retention of geometric configuration when coupled with alkyl Grignards. This observation is independent of other factors, including the choice of catalyst ligands and vinylic subsituents.
Conversely, a Kumada coupling using vinylic Grignard reagents proceeds without stereospecificity to form a mixture of cis- and trans-alkenes. The degree of isomerization is dependent on a variety of factors including reagent ratios and the identity of the halide group. According to Kumada, this loss of stereochemistry is attributable to side-reactions between two equivalents of the allylic Grignard reagent.
Other articles related to "stereoselectivity":
... One method of achieving the asymmetric MPV reduction is with the use of chiral hydride donating alcohols ... The use of chiral alcohol (R)-(+)-sec-o-bromophen-ethyl alcohol gave 82%ee (percent enantiomeric excess) in the reduction of 2-chloroacetophenone ...