Pinwheel-like tridentate phosphines for controlling divergent regioselectivity in Pd-catalyzed alkoxycarbonylation of alkenes

Abstract

The elaborate control of regioselectivity regulated by the involved ligands with specific structural skeleton and compositions is always demanded in alkoxycarbonylation of alkenes. Herein, a series of pinwheel-like tridentate phosphine ligands with rotatable flexibility (L3) or hard rigidity (L5–L7) were synthesized and characterized for the first time, in which the incorporated three PPh2-fragments are located at meta-positions of phenyl groups. It was proved that the tridentate phosphine of L3 with the flexible configuration favorably corresponded to the linear regioselectivity with l/b ratio of 98/2, whereas L7 with the rigid configuration absolutely did favor to the branched regioselectivity with l/b ratio of 1/99. The presence of the three PPh2-frgaments in L3 did contribute to the remarkable performance of L3-Pd(TFA)2 system in terms of activity, linear regioselectivity and stability, subjected to 10-run recycling uses in anti-Markovnikov’s methoxycarbonylation of styrene without activity loss and Pd-black precipitation. In comparison, although the optimal L7-Pd(MeCN)2Cl2 system could catalyze Markovnikov’s alkoxycarbonylation of alkenes to afford the branched esters with excellent regioselectivity in the fresh use, its stability was too poor to support the recycling experiments. L3-Pd(TFA)2 and L7-Pd(MeCN)2Cl2 catalytic systems were both applicable to a wide range of substrates in alkoxycarbonylation of alkenes to afford the target esters in the isolated yields of 41–91 %, with the well maintained divergent regioselectivity.