Palladium-Catalyzed Telomerization of Isoprene with Amines: Ligands and Solvents Working Together to Improve the Selectivity

Abstract

Controlling the selectivity in palladium-catalyzed telomerization of nonsymmetric dienes represents a challenge as several isomers can be obtained, although the four resulting from the attack of the nucleophile at the terminal carbon atom are the main products. Concerning telomerization of isoprene using amines as nucleophiles, the selectivity for three of these four isomers, tail-to-head, head-to-head, and tail-to-tail (1–3), were previously optimized as a result of solvent (pKa) and ligand control. However, the head-to-tail telomer (4) was always obtained in low selectivity. In this paper, we test different Pd catalytic systems with phosphine, phosphite, and phosphoramidite ligands under different solvents and pKa conditions to improve the selectivity for telomers 3 and 4. Results showed that, in the telomerization of isoprene with diethyl amine using non-protic solvents and phosphite ligands with large cone angle afforded telomer 3 in high yield and selectivity (up to 90% yield and 90% selectivity for 3). Phosphite ligands with small cone angle provided telomer 4 with selectivity of 43% (80% yield), which was raised to 48% (23% yield) with ⁱPr₂NH as nucleophile. The significant effect of residual water on the solvent was quantitatively accessed, and greener solvents were tested as alternative for this reaction.