Transition metal catalyzed allylic substitution often exhibits complex product selectivity patterns which has been primarily attributed to η1→η&lt;span style="font-size:10.8333px"&gt;3&lt;/span&gt;→η1 isomerization of the η1 and η3 intermediates. Product selectivity may be even further complicated if the η1 and η3 intermediates share a single transition state (TS) leading to their formation resulting in a post-transition state bifurcation (PTSB). In this work, DFT (B3LYP-D3) calculations combined with ab-initio molecular dynamics (AIMD) have been carried out that suggests a PTSB does indeed exist in Ni, Pd, and Pt catalyzed allylic halide activation and may directly influence product selectivity. The AIMD results initiated from the TS for Pd catalyzed allylic fluoride activation predict the η1 intermediate to be favored in the gas phase and low dielectric solvent (ε&lt;2.6) while the preference shifts to η3 in higher dielectric solvents ((ε&gt;2.6). The bifurcation is minimal in allylic chloride activation. A PTSB was also discovered to be present in Ni and Pt allylic activation, but the η3 product is favored by &gt;90:10 in the gas and solution phase, so a bifurcation is predicted to be likely inconsequential. The greater polarizability of the ionizing Cl ion compared to the hard localized F ion is used to rationalize the preference for formation of η3 and η1 intermediates respectively. The results of this investigation are expected to profoundly impact the mechanistic model of transition metal allylic substitution.

Computational investigations into post-transition state bifurcations in allylic halide activation by Ni, Pd, and Pt.

ACS Spring Meeting 2021

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