An NMR study of cobalt-catalyzed hydroformylation using para-hydrogen induced polarisation

Abstract

The syntheses of Co(eta3-C3H5)(CO)2PR2R' (R, R' = Ph, Me; R, R' = Me, Ph; R = R' = Ph, Cy, CH2Ph) and Co(eta3-C3H5)(CO)(L) (L = dmpe and dppe) are described, and X-ray structures for Co(eta3-C3H5)(CO)(dppe) and the PPh2Me, PCy3 derivatives reported. The relative ability of Co(eta3-C3H5)(CO)2(PR2R') to exchange phosphine for CO follows the trend PMe2Ph < PPh2Me < PCy3 < P(CH2Ph)3 < PPh3. Reactions of the allyl complexes with para-hydrogen (p-H2) lead to the observation of para-hydrogen induced polarisation (PHIP) in both liberated propene and propane. Reaction of these complexes with both CO and H2 leads to the detection of linear acyl containing species Co(COCH2CH2CH3)(CO)3(PR2R') and branched acyl complexes Co(COCH(CH3)2)(CO)3(PR2R') via the PHIP effect. In the case of PPh2Me, additional signals for Co(COCH2CH2CH3)(CO)2(PPh2Me)(propene) and Co(COCH(CH3)2)(CO)2(PPh2Me)(propene) are also detected. When the reactions of H2 and diphenylacetylene are studied with the same precursor, Co(CO)3(PPh2Me)(CHPhCH2Ph) is seen. Studies on how the appearance and ratio, of the PHIP enhanced signals vary as a function of reaction temperature and H2 : CO ratio are reported. These profiles are used to learn about the mechanism of catalysis and reveal how the rates of key steps leading to linear and branched hydroformylation products vary with the phosphine. These data also reveal that the PMe2Ph and PPh2Me based systems yield the highest selectivity for linear hydroformylation products.

Using para hydrogen induced polarization to study steps in the hydroformylation reaction

“…Mechanistic studies underpin this process and normally involve a combination 1-3 of chemical, [4][5][6][7] analytical, [8][9][10][11][12] and theoretical work. [13][14][15][16][17][18][19] In situ NMR methods are gaining in importance 1, [20][21][22][23][24] in this regard despite the techniques inherent low sensitivity. This change is a result of improved hardware, 25 novel in situ high pressure methods 20,21,26 and in situ photochemical approaches, 27,28 with hyperpolarization methods in the solid and solution phases adding a further level of refinement.…”

“…This change is a result of improved hardware, 25 novel in situ high pressure methods 20,21,26 and in situ photochemical approaches, 27,28 with hyperpolarization methods in the solid and solution phases adding a further level of refinement. 23,24 Hydroformylation reflects a widely used industrial reaction that converts a range of alkenes into aldehydes, esters and other important chemical feedstocks. [29][30][31][32] In hydroformylation, both linear and branched products form and their ratio is an important parameter in maximising commercial return.…”

“…Earlier studies in our group used an array of Co(η 3 -C 3 H 5 )(CO) 2 (phosphine) complexes in conjunction with the para-hydrogen induced polarization (PHIP) effect to assist in the NMR spectroscopy examination of hydroformylation. 24,55 We employ the PHIP effect here to detect further species in this iridium study. This works because when p-H 2 is introduced into a material, the resulting PHIP effect facilitates the detection of low concentration reaction intermediates, whilst also provides a route to obtain additional kinetic information on their role in a reaction.…”

Relayed hyperpolarization from para-hydrogen improves the NMR detectability of alcohols

“…Its two nuclear spins are aligned antiparallel in a singlet state and were predicted to deliver strong 1 H NMR signals when added to an unsaturated material. 10,11 Experimentally, this hypothesis has since been validated many times and the resulting enhanced signals have been used for the examination of numerous reaction mechanisms [12][13][14][15][16][17][18] and for the determination of low concentration intermediates. [19][20][21] A major barrier to the widespread application of this approach is the requirement that an unsaturated p-H 2 acceptor is needed to create a magnetic environment where the singlet symmetry of p-H 2 is broken and hence allows the products of chemical change to be detected.…”

Following palladium catalyzed methoxycarbonylation by hyperpolarized NMR spectroscopy: a parahydrogen based investigation

“…[49][50][51] We are therefore seeking to detect active reaction intermediates in this process, which contrasts with the situation faced in some of the earlier p-H 2 based studies of the hydroformylation reaction where a series of resting states were detected. 52,53 From the perspective of this paper, we harness the known NMR parameters of these previously reported palladium complexes to support the speciation enunciated here and build on the fact that PHIP-NMR has already been shown to facilitate the detection of a number of related intermediates during studies of palladium catalysed hydrogenation under mild conditions. For example when chelating 17,54,55 In this study, we use Pd(OTf) 2 (bcope) (1) to drive the methoxycarbonylation of diphenylacetylene, and both 13 CO and C 6 D 5 -13 C C C 6 D 5 are employed to aid in the characterization of a series of reaction intermediates that are seen through the use of p-H 2 under mild conditions.…”