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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. [ABSTRACT FROM AUTHOR]

The scope of carbon monoxide‐free Asymmetric Transfer HydroFormylation (ATHF) procedures using a highly active single catalyst system derived from 1,2‐bis‐((2,5)‐diphenylphospholano)ethane as chiral ligand has been studied. This reveals some highly successful reactions, but also significant limitations. The development of a new protocol in which a catalyst for formaldehyde decomposition to CO and H2 is combined with the catalyst of choice for the subsequent asymmetric hydroformylation is described. This enables ATHF reactions that were problematic to be significantly improved. The new method has been used in the synthesis of several key precursors to biologically active molecules. [ABSTRACT FROM AUTHOR]

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 rhodiumhydrido 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • In situ spectroscopic study of hydroformylation catalysts at typical reaction concentrations. • Two ligands in the literature found to not coordinate to rhodium in the presence of syngas. • The highly active Rh/1,3,5,7-Tetramethyl-6-phenyl-2,4,8-trioxa-6-phosphaadamantane catalyst system studied by in situ HPIR spectroscopy and found to be mixture of resting states, with the composition tuneable by reaction conditions. The paper describes the composition of the resting states of several catalysts for alkene hydroformylation derived from bulky monophosphorus ligands. The results presented assess how bulky ligands compete with CO for the rhodium, and hence the role of 'unmodified' catalysts in alkene hydroformylation in the presence of these ligands. High Pressure Infra-Red (HPIR) spectroscopy was carried out at the rhodium and syngas concentrations typically used during catalysis experiments. These HPIR studies revealed that two ligands previously studied in Rh-catalysed hydroformylation react with [Rh(acac)(CO) 2 ] and H 2 /CO to give the unmodified rhodium cluster, [Rh 6 (CO) 16 ], as the only detectable species. Both less bulky phosphoramidites, and 1,3,5,7-Tetramethyl-6-phenyl-2,4,8-trioxa-6-phosphaadamantane, on the other hand, do not show the presence of [Rh 6 (CO) 16 ], and hence catalysis proceeds by purely ligand modified species under normal conditions. In the case of the Rh/phosphaadamantane catalysts, anecdotal evidence that this only forms a particularly useful catalyst above a certain pressure threshold can be understood in terms of how the catalyst composition varies with pressure. The ligands discussed have all been assessed in the hydroformylation of propene to separate their innate branched selectivity from their ability to isomerise higher alkenes to internal isomers. [ABSTRACT FROM AUTHOR]