Understanding a hydroformylation catalyst that produces branched aldehydes from alkyl alkenes
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This paper reports experimental and computational studies on the mechanism of a rhodium-catalysed 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. Labelling 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.
Dingwall , P , Fuentes , J A , Crawford , L , Slawin , A M Z , Buehl , M & Clarke , M L 2017 , ' Understanding a hydroformylation catalyst that produces branched aldehydes from alkyl alkenes ' Journal of the American Chemical Society , vol. 139 , no. 44 , pp. 15921–15932 . DOI: 10.1021/jacs.7b09164
Journal of the American Chemical Society
Copyright © 2017 American Chemical Society. This work has been made available online in accordance with the publisher’s policies. This is the author created accepted version manuscript following peer review and as such may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1021/jacs.7b09164
DescriptionThe authors thank the EPSRC for funding (EP/M003868/1).
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