Nickel-Catalyzed Telomerization of 1,3-Butadiene Promoted by Triarylphosphines: Unveiling Mechanistic Details for Selective Production of 1-Methoxy-2,7-octadiene

Abstract

The telomerization of 1,3-butadiene (BD) with methanol represents a highly atom-economical route to 1-octene, an essential precursor for linear low-density polyethylene (LLDPE). While palladium-based catalysts are used commercially, their high cost and recovery challenges motivate the search for Earth-abundant alternatives. Herein, we report the selective telomerization of BD with methanol catalyzed by a nickel/triarylphosphine system, providing mechanistic insight into the role of ligand structure in determining reactivity and selectivity. Among the twelve phosphine ligands examined in this study, the mono-ortho-methoxy-substituted triphenylphosphine (L3) afforded the highest yield (∼40%) and selectivity (∼66%) toward 1-methoxy-2,7-octadiene (MOD-1) under mild conditions (50 °C, 0.6 mol % Ni(cod)2). Kinetic analyses revealed first-order dependence in Ni and a bifurcated order in L3, implicating parallel mono- and bis(phosphine) manifolds that govern linear (MOD-1) versus branched (MOD-3) selectivity. Nonlinear Eyring analysis gave activation parameters of ΔH‡ = 10.9 kcal/mol and ΔS‡ = –35.8 cal/(mol K) for MOD-1 formation and ΔH‡ = 10.6 kcal/mol and ΔS‡ = –38.9 cal/(mol K) for MOD-3 formation, both consistent with an associative turnover-limiting step. In contrast, 1,3,7-octatriene formation proceeds via an enthalpically demanding, near-zero-entropy, unimolecular-like step. Comparative studies with triphenylphosphine analogs suggest that hemilabile P–O coordination in L3 could play a role in lowering the activation barrier, potentially directing the system toward the linear telomer. These results establish a mechanistic framework for rational ligand design in Ni-catalyzed telomerization and demonstrate the feasibility of replacing precious-metal catalysts in C4-to-C8 coupling processes.