Rhodium(III) dihydrido complexes [Rh(L 2 )(H) 2 (acetone)][BAr F4 ] (Ar F =C 6 H 3 (CF 3 ) 2 ) containing the potentially hemilabile ligands L 2 = 9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene (Xantphos) and [Ph 2 P(CH 2 ) 2 ] 2 O (POP 0 ) have been prepared from their corresponding norbornadiene rhodium(I) precursors. In solution these complexes are fluxional by proposed acetone dissociation, which can be trapped out by addition of MeCN to form [Rh(L 2 )(H) 2 (NCMe)][BAr F 4 ], which have been crystallographically characterized. Addition of alkene (methyl acrylate) to these complexes results in reduction to a rhodium(I) species and when followed by addition of the aldehyde HCOCH 2 CH 2 SMe affords the new acyl hydrido complexes [Rh(L 2 )(COCH 2 CH 2 SMe)H][BAr F 4 ] in good yield. The solid-state and solution structures show a tight binding of the POP 0 and Xantphos ligands, having a trans-arrangement of the phosphines with the central ether linkage bound. This is similar to the previously reported complex [Rh(DPEphos)-(COCH 2 CH 2 SMe)H][BAr F 4 ] (DPEphos=[Ph 2 P(C 6 H 4 )] 2 O). Unlike the DPEphos complex, the Xantphos and POP 0 ligated complexes are not effective catalysts for the hydroacylation reaction between methyl acrylate and HCOCH 2 CH 2 SMe. This is traced to their inability to dissociate the central ether link in a hemilabile manner to reveal a vacant site necessary for alkene coordination. Consistent with this lack of availability of the vacant site, these complexes also are stable toward reductive decarbonylation. Complexes [Rh(Ph 2 P(CH 2 ) n PPh 2 )(acetone) 2 ][BAr F 4 ] (n = 2-5) have also been studied as catalysts for the hydroacylation reaction between methyl acrylate and HCOCH 2 CH 2 SMe at 22°C. As found previously, for n=2 this affords the product of alkene hydroacylation, but as the chain length is progressively increased to n=5, the reaction also progressively changes to favor the product of aldehyde hydroacylation. This is suggested to occur by a decrease in the accessibility of the metal site on increasing the bite angle of the chelate ligand, so that alkene coordination to a putative Rh(III)-acyl hydrido intermediate is progressively disfavored and aldehyde coordination (followed by hydride transfer) is progressively favored. These, and previous, results show that the overall conversion in the hydroacylation reaction can be controlled by the hemilabile nature of the chelating phosphine in the catalyst (e.g., DPEphos versus Xantphos), and the course of the reaction can also be tuned by changing the bite angle of the phosphine, cf. Ph 2 P(CH 2 ) 2 PPh 2 and Ph 2 P(CH 2 ) 5 PPh 2 .

The isolation of the branched alkenyl intermediate that directly precedes reductive elimination of the final α,β-unsaturated ketone product is reported for the hydroacylation reaction between the alkyne HCCArF (ArF = 3,5-(CF3)2C6H3) and the β-S-substituted aldehyde 2-(methylthio)benzaldehyde: [Rh(fac-κ3-DPEphos)(C(CH2)ArF)(C(O)C6H4SMe)2][CB11H12]. The structure of this intermediate shows that, in this system at least, hydride migration rather than acyl migration occurs. Kinetic studies on the subsequent reductive elimination to form the crystallographically characterized ketone-bound product [Rh(cis-κ2-DPEphos)(η2:η2,κ1-H2CC(ArF)C(O)(C6H4SMe)][CB11H12] yield the following activation parameters for reductive elimination, which follows first-order kinetics (k obs = (6.14 ± 0.04) × 10–5 s–1, 324 K): ΔH ⧧ = 95 ± 2 kJ mol–1, ΔS ⧧ = −32 ± 7 J K–1 mol–1, ΔG ⧧(298 K) = 105 ± 4 kJ mol–1. Mechanistic studies, including selective deuteration experiments, show that hydride insertion is not reversible and also reveal that an interesting isomerization process is occurring between the two branched alkenyl protons that is suggested to occur via a metallocyclopropene intermediate. During catalysis, the consumption of substrates and evolution of products follow pseudo zero-order kinetics. The observation of both linear and branched products under stoichiometric and catalytic regimes, in combination with kinetic modeling, allows for an overall mechanistic scheme to be presented. Partitioning of linear and branched pathways at the hydride insertion step occurs with an approximate 2:1 selectivity, while reductive elimination of the linear product is at least 3 orders of magnitude faster than that from the branched. An explanation for the large difference in rate of reductive elimination in this system, as recently outlined by Goldman, Krogh-Jespersen, and Brookhart, is that steric crowding in branched intermediates can slow C–C reductive elimination even though such species are higher in energy than their linear analogues, if the rotation of the vinyl group to the appropriate orientation is inhibited by steric crowding in the branched isomers.

It's all in the ligand: By choice of the appropriate diphosphine ligand a previously linear‐selective alkyne hydroacylation process can be “switched” to be highly branched‐selective (see scheme, l=linear, b=branched). Structural data for the ortho‐iPr‐dppe–rhodium catalyst suggest restricted rotation of the phosphine aryl units may be responsible for the observed selectivity.

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Alles eine Frage des Liganden: Durch die Wahl des geeigneten Diphosphanliganden kann eine vormals linear(l)‐selektive Alkinhydroacylierung in eine verzweigt(b)‐selektive Reaktion „umgeschaltet“ werden (siehe Schema). Strukturdaten des o‐iPr‐dppe‐Rh‐Katalysators lassen vermuten, dass eine gehinderte Rotation um die Phosphanaryl‐Einheiten für die Selektivität verantwortlich ist.