Mechanistic Study of the Pd/TOMPP-Catalyzed Telomerization of 1,3-Butadiene: Influence of Aromatic Solvents on Bis-Phosphine Complex Formation and Regioselectivity

A study of the influence of reaction conditions on the Pd/TOMPP-catalyzed (TOMPP = tris(2-methoxyphenyl)phosphine) telomerization of 1,3-butadiene with phenols revealed that the composition of the reaction medium strongly influences the regioselectivity of the telomer products. Mechanistic studies show this effect to be related to the stability of the key catalytic intermediate cis-[Pd((1-3 eta)-octa-2,7-dien-1-yl)(TOMPP)(2)](+) (E), which mainly depends on the solvent composition. The crotyl analogue cis-[Pd((1-3 eta)-but-2-en-1-yl)(TOMPP)(2)]BF4 (4) was synthesized as a model for E and was fully characterized, including by an X-ray crystal structure determination. Comparison of the crystal structures of 4 and [Pd((1-3,7,8 eta)-octa-2(E),7-dien-1-yl)(TOMPP)]BF4 (1), an analogue of intermediate C, shows that the difference in regioselectivity for nucleophilic attack of these complexes is not reflected in the structure of the allyl moiety. Furthermore, Pd/TOMPP-catalyzed n/iso equilibration reactions of telomers show that all product formation routes in the Pd-catalyzed telomerization mechanism are reversible. Telomer product equilibration studies resulted in complete conversion to 1,3,7-octatriene, with the selectivity changing over time in favor of formation of the Z isomer of octatriene.