Regioselectivity Control in Pd-Catalyzed Telomerization of Isoprene Enabled by Solvent and Ligand Selection

Controlling the selectivity in palladium-catalyzed telomerization of nonsymmetric dienes represents a formidable challenge since up to 12 isomers can be obtained, and a general method for selective synthesis is still lacking. We select isoprene (2-methylbutadiene) as a representative and relevant example of a nonsymmetric diene. A combined experimental-computational study on a large set of phosphine-modified palladium catalysts and reaction conditions aiming to understand the factors governing the selectivity shows that it can be controlled by selecting the protic solvent pK(a) and by the ligand. Atomistic and kinetic simulations reveal that the solvent switches the selectivity-determining step as a function of pK(a) from C-C oxidative coupling at low pKa values (preference for telomer head-to-head) to protonation at high pK(a) values (preference for telomer tail-to-tail). The selectivity toward tail-to-head telomer can be directed in moderately acidic solvents by selection of the appropriate ligand, which exerts steric control of the protonation step. Thus, using Et2NH as a nucleophile, it was possible to synthesize 3 of the 4 main isomers in very high yields and selectivities and to provide a complete mechanistic picture of Pd-catalyzed telomerization of nonsymmetric dienes.