To experimentally investigate the hypothesis that isomerization occurs prior to transmetalation, diastereomerically pure (Z)-1a was subjected to the reaction conditions with stoichiometric amounts of Ni(COD) 2 and ligands L1 and L2 to access the oxidative addition complexes 1a'-L1 or 1a'-L2 (Scheme 4A). The reaction was then quenched with 1 M HCl and analyzed by 1 H-NMR spectroscopy. With L1 as the ligand in THF, the protodenickelated trisubstituted alkene 3ak was produced with 68:32 E/Z selectivity and 93% yield. Using L2 as the ligand in t-AmOH, 3ak was obtained as a diastereomeric mixture of 47:53 E/Z and 83% yield. Hence, these results illustrate that the postulated isomerization step indeed precedes the transmetalation (cf. Figure ). To elucidate the potential role of transmetalation as the stereodetermining step, we subjected the oxidative addition complex 1a'-L1 formed in situ to the transmetalation conditions by addition of pinacol boronate 2a and K 3 PO 4 (Scheme 4B). Thus, product 3aa was afforded in 23% yield and notably with a diastereoselectivity as high as 92:8 Z/E, which is comparable to the selectivity observed in the catalytic reaction (97:3 Z/E, Scheme 3). Of note, the major product in this reaction was the trisubstituted alkene 3ak with 55% yield and a selectivity of 70:30 E/Z, which is similar to the previously obtained selectivity of 68:32 E/Z after acidic quench (Scheme 4A). The analogous transmetalation reaction with 1a'-L2 in t-AmOH did not yield measurable amounts of product 3aa (see SI for details). This could potentially be rationalized in terms of a less stable oxidative addition complex 1a'-L2 and potential com-or disproportionation degradation pathways. In addition to the stoichiometric isomerization experiments, product isomerization under both optimized reaction conditions was excluded, as no significant isomerization nor decomposition was detected upon subjecting (Z)-3aa to the (E)-selective conditions C2 nor (E)-3aa to the (Z)-selective conditions C1 (see SI for details).