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

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.