Unexpected CO Dependencies, Catalyst Speciation, and Single Turnover Hydrogenolysis Studies of Hydroformylation via High Pressure NMR Spectroscopy

Rhodium bis­(diazaphospholane) (BDP) catalyzed hydroformylation of styrene is sensitive to CO concentration, and drastically different kinetic regimes are affected by modest changes in gas pressure. The Wisconsin High Pressure NMR Reactor (WiHP-NMRR) has enabled the observation of changes in catalyst speciation in these different regimes. The apparent discrepancy between catalyst speciation and product distribution led us to report the first direct, noncatalytic quantitative observation of hydrogenolysis of acyl dicarbonyls. Analysis and modeling of these experiments show that not all catalyst is shunted through the off-cycle intermediates and this contributes to the drastic mismatch in selectivities. The data herein highlight the complex kinetics of Rh­(BDP) catalyzed hydroformylation. In this case, the complexity arises from competing kinetic and thermodynamic preferences involving formation and isomerization of the acyl mono- and dicarbonyl intermediates and their hydrogenolysis to give aldehydes

Backbone-Modified Bisdiazaphospholanes for Regioselective Rhodium-Catalyzed Hydroformylation of Alkenes

A series of tetraaryl bisdiazaphospholane (BDP) ligands were prepared varying the phosphine bridge, backbone, and substituents in the 2- and 5-positions of the diazaphospholane ring. The parent acylhydrazine backbone was transformed to an alkylhydrazine via a borane reduction procedure. These reduced ligands contained an all sp³ hybridized ring mimicking the all sp³ phospholane of (<i>R,R</i>)-Ph-BPE, a highly selective ligand in asymmetric hydroformylation. The reduced bisdiazaphospholane (red-BDP) ligands were shown crystallographically to have an increased C–N–N–C torsion anglethis puckering resembles the structure of (<i>R,R</i>)-Ph-BPE and has a dramatic influence on regioselectivity in rhodium catalyzed hydroformylation. The red-BDPs demonstrated up to a 5-fold increase in selectivity for the branched aldehyde compared to the acylhydrazine parent ligands. This work demonstrates a facile procedure for increased branched selectivity from the highly active and accessible class of BDP ligands in hydroformylation

Interception and Characterization of Catalyst Species in Rhodium Bis(diazaphospholane)-Catalyzed Hydroformylation of Octene, Vinyl Acetate, Allyl Cyanide, and 1‑Phenyl-1,3-butadiene

In the absence of H₂, reaction of [Rh­(H) (CO)₂(BDP)] [BDP = bis­(diazaphospholane)] with hydroformylation substrates vinyl acetate, allyl cyanide, 1-octene, and <i>trans</i>-1-phenyl-1,3-butadiene at low temperatures and pressures with passive mixing enables detailed NMR spectroscopic characterization of rhodium acyl and, in some cases, alkyl complexes of these substrates. For <i>trans</i>-1-phenyl-1,3-butadiene, the stable alkyl complex is an η³-allyl complex. Five-coordinate acyl dicarbonyl complexes appear to be thermodynamically preferred over the four-coordinate acyl monocarbonyls at low temperatures and one atmosphere of CO. Under noncatalytic (i.e., no H₂ present) reaction conditions, NMR spectroscopy reveals the kinetic and thermodynamic selectivity of linear and branched acyl dicarbonyl formation. Over the range of substrates investigated, the kinetic regioselectivity observed at low temperatures under noncatalytic conditions roughly predicts the regioselectivity observed for catalytic transformations at higher temperatures and pressures. Thus, kinetic distributions of off-cycle acyl dicarbonyls constitute reasonable models for catalytic selectivity. The Wisconsin high-pressure NMR reactor (WiHP-NMRR) enables single-turnover experiments with active mixing; such experiments constitute a powerful strategy for elucidating the inherent selectivity of acyl formation and acyl hydrogenolysis in hydroformylation reactions