Rhodium-Catalyzed Hydroformylation and Deuterioformylation with Pyrrolyl-Based Phosphorus Amidite Ligands: Influence of Electronic Ligand Properties

Abstract

The influence of electronic ligand properties on the catalyst performance in the rhodium-catalyzed hydroformylation of alkenes has been investigated. Two bidentate phosphorus amidite and phosphinite ligands have been synthesized: 1,1‘-biphenyl-2,2‘-diyl-bis(dipyrrolylphosphoramidite) (3) and 1,1‘-biphenyl-2,2‘-diyloxy-bis(diphenylphosphinite) (4). Their monodentate analogues have also been studied: phenyldipyrrolylphosphoramidite (1) and phenyl diphenylphosphinite (2). These two sets of ligands have very similar steric properties but the amidites are much stronger π-acceptor ligands. Spectroscopic studies showed that under hydroformylation reaction conditions the monodentate ligands 1 and 2 form mixtures of HRhL2(CO)2 and HRhL3(CO) complexes depending on the ligand and rhodium concentrations and the carbon monoxide pressure. Depending on the reaction conditions, the bidentate ligands 3 and 4 form mixtures of HRh(L∩L)(CO)2 and HRh(L∩L)(L∩L‘)(CO), where L∩L‘ functions as a monodentate. All ligands have been tested in the hydroformylation reaction of oct-1-ene. A high π-acidity of the ligand resulted in a high rate of hydroformylation. The monodentate ligands 1 and 2 showed moderate selectivity for the linear aldehyde. The catalyst formed with the bidentate phosphorus amidite ligand 3 revealed high regioselectivity for the linear aldehyde (ratio l/b = ∼100) at a high rate together with a moderate selectivity for isomerization (∼7%). Deuterioformylation experiments of 1-hexene showed that the hydride (deuteride) migration is reversible in the hydroformylation system formed by 3. Surprisingly, both the linear rhodium−alkyl and the branched rhodium−alkyl complex undergo β-hydride elimination. Furthermore, the 2-hexylrhodium intermediate regenerates more often monodeuterated 1-hexene than 2-hexene. The rhodium hydride species formed this way reacts relatively slowly with the excess of D2 and as a result large amounts of monodeuterated heptanal (40% D1 vs 60% D2) and monodeuterated 1-hexene are formed. At higher conversions the latter gives trisdeuterated heptanal as well as bisdeuterated heptanal.