Detection of Intermediates in Cobalt-Catalyzed Hydroformylation Using para-Hydrogen-Induced Polarization

Abstract

para-Hydrogen-induced polarization methods are shown to enable the in situ detection of linear and branched monophosphine-containing intermediates during hydroformylation when Co(eta3-C3H5)(CO)2(PCy3) is the catalyst precursor. The NMR signal characteristics of the alkyl arms of these species provide direct evidence for the rapid interconversion of linear and branched cobalt alkyls prior to the CO insertion step. The observation of additional para-hydrogen-enhanced signals for the corresponding linear and branched aldehydes enables the reactions selectivity to be rapidly monitored as a function of H2 and CO pressure or reaction temperature.

Parahydrogen-based NMR methods as a mechanistic probe in inorganic chemistry

“…The parahydrogen enhancement, in conjunction with extensive 13 C labelling experiments, allowed these intermediates to be fully characterised, and their reaction pathways determined. The authors also reported a similar study into hydroformylation using Co( 3 -C 3 H 5 )(CO) 2 (PCy 3 ), in which cobalt acyl intermediates with both linear and branched acyl substituents were detected, in addition to the corresponding aldehydes [87]. The enhancement provided by the parahydrogen was found to be transferred into the organic ligand framework, thus allowing the characterisation of intermediates which do not contain discrete hydride ligands, mirroring the contemporary development reported by Duckett et al in the field of hydrogenation catalysis (vide supra, Section 2.2).…”

Selective detection of hyperpolarized NMR signals derived from para-hydrogen using the Only Para-hydrogen SpectroscopY (OPSY) approach

“…The signal enhancements PHIP provides have been used extensively to detect elusive reaction intermediates based on metal dihydrides by NMR spectroscopy [6,7]. It has also been used to generate hyperpolarized organic molecules by hydrogenation of unsaturated substrates [5].…”

Carbonylation of Alkenes and Dienes

“…Each of the previously proposed steps in cobalt-catalyzed hydroformylation was confirmed by the detection of intermediates using para-hydrogen-induced polarization [8]. Thermodynamic parameters relevant to the phosphane-modified cobalt-catalyzed hydroformylation reaction were reported.…”

“…The cobalt-catalyzed hydroformylation of 1-octene was investigated using bis{tri(3fluorophenyl)phosphane}hexacarbonyldicobalt and Co 2 (CO) 8 as precatalysts in supercritical carbon dioxide. The catalytic performance in scCO 2 was compared to the one in toluene as a conventional solvent.…”

“…Similar activities and selectivities were obtained in both reaction media. In scCO 2 , a substantial improvement in the selectivity for aldehydes was found by using the phosphane-containing complex in comparison to the unmodified catalyst Co 2 (CO) 8 [115]. The formation of cobalt complexes in supercritical carbon dioxide derived from Co 2 (CO) 6 [P(p-CF 3 C 6 H 4 ) 3 ] 2 under hydroformylation conditions has been investigated by in situ high-pressure NMR spectroscopy.…”