Diastereoselective and Branched-Aldehyde-Selective Tandem Hydroformylation–Hemiaminal Formation: Synthesis of Functionalized Piperidines and Amino Alcohols

A Resorcin[4]arene-Based Phosphite-Phosphine Ligand for the Branched-Selective Hydroformylation of Alkyl Alkenes

Synthesis of a chelating phosphite-phosphine ligand from a tris(quinoxaline) extended resorcin[4]arene and its application in the rhodium-catalyzed hydroformylation of terminal alkyl alkenes are reported. Rhodium complexes are formed within the cavity of the macrocycle and branched-selective hydroformylation of 1-octene with a b/l ratio of 5.9 has been achieved at 60 °C under 1:1 H2/CO (20 bar).

Ligand Hydrogenation during Hydroformylation Catalysis Detected by In Situ High-Pressure Infra-Red Spectroscopic Analysis of a Rhodium/Phospholene-Phosphite Catalyst

Phospholane-phosphites are known to show highly unusual selectivity towards branched aldehydes in the hydroformylation of terminal alkenes. This paper describes the synthesis of hitherto unknown unsaturated phospholene borane precursors and their conversion to the corresponding phospholene-phosphites. The relative stereochemistry of one of these ligands and its Pd complex was assigned with the aid of X-ray crystal structure determinations. These ligands were able to approach the level of selectivity observed for phospholane-phosphites in the rhodium-catalysed hydroformylation of propene. High-pressure infra-red (HPIR) spectroscopic monitoring of the catalyst formation revealed that whilst the catalysts showed good thermal stability with respect to fragmentation, the C=C bond in the phospholene moiety was slowly hydrogenated in the presence of rhodium and syngas. The ability of this spectroscopic tool to detect even subtle changes in structure, remotely from the carbonyl ligands, underlines the usefulness of HPIR spectroscopy in hydroformylation catalyst development.

Branch-Selective Addition of Unactivated Olefins into Imines and Aldehydes

Radical hydrofunctionalization occurs with ease using metal-hydride hydrogen atom transfer (MHAT) catalysis to couple alkenes and competent radicalophilic electrophiles. Traditional two-electron electrophiles have remained unreactive. Herein we report the reductive coupling of electronically unbiased olefins with imines and aldehydes. Iron catalysis allows addition of alkyl-substituted olefins into imines through the intermediacy of free radicals, whereas a combination of catalytic Co(Sal ^{t-Bu, t-Bu}) and chromium salts enables a branch-selective coupling of olefins and aldehydes through the formation of a putative alkyl chromium intermediate.

Understanding a Hydroformylation Catalyst that Produces Branched Aldehydes from Alkyl Alkenes

This paper reports experimental and computational studies on the mechanism of a rhodium-catalyzed hydroformylation that is selective for branched aldehyde products from unbiased alkene substrates. This highly unusual selectivity relies on a phospholane-phosphite ligand prosaically called BOBPHOS. Kinetic studies using in situ high pressure IR (HPIR) and the reaction progress kinetic analysis methodology suggested two steps in the catalytic cycle were involved as turnover determining. Negative order in CO and positive orders in alkene and H₂ were found and the effect of hydrogen and carbon monoxide partial pressures on selectivity were measured. Labeling studies found rhodium hydride addition to the alkene to be largely irreversible. Detailed spectroscopic HPIR and NMR characterization of activated rhodium-hydrido dicarbonyl species were carried out. In the absence of H₂, reaction of the rhodium-hydrido dicarbonyl with allylbenzene allowed further detailed spectroscopic characterization of four- and five-coordinate rhodium-acyl species. Under single-turnover conditions, the ratios of branched to linear acyl species were preserved in the final ratios of aldehyde products. Theoretical investigations uncovered unexpected stabilizing CH-π interactions between the ligand and substrate which influenced the high branched selectivity by causing potentially low energy pathways to become unproductive. Energy span and degree of TOF control analysis strongly support experimental observations and mechanistic rationale. A three-dimensional quadrant model was built to represent the structural origins of regio- and enantioselectivity.