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((1–3η)-octa-2,7-dien-1-yl)­(TOMPP)2]+ (E), which mainly depends on the
solvent composition. The crotyl analogue cis-[Pd­((1–3η)-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((1–3,7,8η)-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.