Mechanistic Study of the Pd/TOMPP-Catalyzed Telomerization of 1,3-Butadiene with Biomass-Based Alcohols: On the Reversibility of Phosphine Alkylation

Abstract

Liquid-chromatography electrospray-ionisation mass spectrometry (LC-ESI-MS) studies on reaction mixtures of the telomerization of 1,3-butadiene with biomass-based polyols revealed that the TOMPP (TOMPP = tris(2-methoxyphenyl)phosphine) ligand is converted towards the corresponding (2,7-octadienyl) phosphonium species during catalysis. The extent of ligand alkylation is substrate dependent and was identified as the primary cause of deactivation for carbohydrate substrates with anomeric hydroxyl groups. Coordination studies of the phosphonium cation with [Pd(dba)(2)] (dba=dibenzylideneacetone) gave insight into the alkylation mechanism and showed that the formation of the phosphonium cation is fully reversible. The reaction yields the key cationic intermediate [Pd(1,2,3,7,8-eta(5)-octa-2,7- dien-1-yl)(TOMPP)](+), which, in the presence of the iodide anion, results in the formation of [Pd(1,2,3-eta(3)-octa-2,7-dien-1-yl)(I)(TOMPP)]. Both complexes were fully characterized by various techniques including single crystal X-ray crystallography. Based on these results, an extension to the Pd/TOMPP-catalyzed telomerization mechanism was formulated to include the 2,7-octadienylphosphonium cation as a ligand reservoir. Catalytic tests show that the use of [Pd(dba)(2)] as precatalyst improves the telomerization of glucose and xylose.