To gain insights into the origin of the diastereoselectivity in the reaction between ketone 1i and COD (2a) selectively forming diastereomer 3i under Ni-Me IPr tBu catalysis, we examined reaction pathways leading to diastereomers 3i and 3i' using density functional computations (Fig. ). In line with previous studies involving chain walking on nickel catalysts , we computed free energy profiles of the major and minor diastereomers (see SI. 6 for full computational details).We began our study from Int1, in which 1a and COD is bound with only one of its olefin bonds to the nickel center. The relative orientation of the ketone and the bound olefin bond of COD leads to formation of two different possible structures for Int1 (blue vs orange pathways) that ultimately may give rise to the two experimentally observed diastereomers 3i and 3i'. While Int1 leading to minor isomer 3i' is slightly thermodynamically favored (by 5.2 kcal/mol), the transition state energy of TS1 that is associated with the ligandto-ligand hydrogen transfer (LLHT) favors major diastereomer 3i by 2.4 kcal/mol (30.3 vs 32.7 kcal/mol). As we observed formation of nickel hydride 10 in the absence of an olefin accepter, we invoked an alternative pathway involving discrete a nickel hydride intermediate to reach Int2. Although, no transition state could for the nickel hydride species could be located with the actual NHC nickel complex, it remains a possible pathway, especially as there is a continuum between both mechanisms (Fig. ). After the initial hydride onto the COD moiety, an immediate reductive elimination of Int2 is energetically disfavored under the current catalyst. Reaction progress is initiated by an olefin chain-walking process which proceeds through a series of low-energy intermediates and transition states (Int2-Int8) to arrive at an π-allyl-bound species Int9. At this stage, a final set of transition states (TS9) is associated with reductive elimination step and formation of 3a bound to the nickel complex (Int10). Like for TS1, the blue pathway associated with formation of the major diastereomer is both kinetically (TS9: 19.4 vs 23.2 kcal/mol) and thermodynamically (Int10: -5.0 vs 1.8 kcal/mol) favored over the orange pathway of the minor diastereomer. Moreover, the significant barrier height indicates that reductive elimination may also be the rate-determining step of the reaction. Thus, both of key transition states TS1 and TS9 along the reaction pathway energetically favor formation of the major observed diastereomer 3i. An interconversion between the orange and blue pathways could occur in principle during the chain walking process, but is unlikely due to the significant steric bulk of the ligand. In such case, the blue pathway leading to the experimentally observed major diastereomer remains favorable due to the lower transition state barrier TS9 of the reductive elimination.