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

Abstract

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.

The Telomerization of 1,3‐Dienes – A Reaction Grows Up

“…The reaction of the intermediate with methanol yielded the respective telomers and it could be demonstrated that the electronic (ν CO ) as well as the steric (θ) properties of the phosphorus ligand greatly influence the reactivity. It could later be shown that the η 3 ‐allyl species (similar to complex D in Scheme , but with an additional P‐ligand instead of the coordinated double bond) is stabilized by aromatic solvents, thus leading to the conclusion that this is the key intermediate for aromatic substrates such as phenols . The non‐chelated species, which can also be favored by a high ligand excess, leads to lower selectivities to the linear product .…”

Palladium‐Catalyzed Cross‐Coupling of Alkenyl Carboxylates

“…Further work is underway to fully explore the role of L6 in promoting catalysis, including the potential for supporting cationic Pd II intermediates. [66][67][68] In summary, we have demonstrated that Pd catalysis is effective for the cross-coupling of alkenyl carboxylates. This observation, contrary to the often observed and generally accepted reactivity patterns for Pd-catalyzed cross-coupling, opens the door to discovery of new methods complementary to those involving other transition metals.…”

Palladium‐Catalyzed Cross‐Coupling of Alkenyl Carboxylates

“…While the poor reactivity of Pd 2 dba 3 / L6 relative to both Pd(OAc) 2 / L6 and Pd(Ph)(OAc)( L6 ) is not definitive, the balance of current evidence supports a Pd II ‐mediated mechanism when using L6 rather than one involving Pd 0/II . Further work is underway to fully explore the role of L6 in promoting catalysis, including the potential for supporting cationic Pd II intermediates …”