Relay catalysis over Pd-catalysts induced by two divergent ligands for cascade bis-alkoxycarbonylation of alkynes

The mono-metal relay catalysis is a good strategy for one-pot multi-step cascade process in which the specific catalysis can be switched upon the ligand exchange to match the requirements for each independent reaction but without mutual interference and quenching problem. Herein, the one-pot cascade bis-alkoxycarbonylation of terminal alkynes was achieved by taking advantages of the mono-metallic (Pd) relay catalysis, wherein the two kinds of divergent di-phosphines of L1 and DTBPMB were involved. In this Pd(OAc)₂-L1-DTBPMB system, due to the steric bulkiness of L1 and DTBPMB, the two type of catalysts including Pd(OAc)₂-L1 and Pd(OAc)₂-DTBPMB co-existed independently and compatibly. Pd(OAc)₂-L1 system was only in charge of the first-step carbonylation of alkynes towards the branched α,β-unsaturated esters with gem-positioned double bond. Subsequent, Pd(OAc)₂-DTBPMB system took over the carbonylation of the branched α,β-unsaturated esters to afford the target diesters. It was indicated Pd(OAc)₂-L1 system was an efficient and stable catalyst which could be recycled upon 15 runs for the first-step carbonylation. Anyway, Pd(OAc)₂-DTBPMB system easily underwent deactivation, which degraded the recyclability of the Pd(OAc)₂-L1-DTBPMB coupling system for this cascade bis-alkoxycarbonylation.