Controlling Selectivity in Intermolecular Alkene or Aldehyde Hydroacylation Reactions Catalyzed by {Rh(L₂)}⁺Fragments

Abstract

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 .

Cationic iridium complexes of the Xantphos ligand. Flexible coordination modes and the isolation of the hydride insertion product with an alkene

“…These data are consistent with rapid inversion of the Xantphos ligand on the NMR timescale in solution, possibility via a k 3 eintermediate [12]. Similar NMR data are observed for the analogous Rhenorbornadiene complex [13].…”

“…We have recently reported upon this unusual k 3 coordination mode using Rh-based systems, e.g. A [13], and were interested in extending this to iridium to afford complexes that are directly analogues to IrePNP systems that have found much recent application in CeH activation chemistry [14e18]. We report here a preliminary study into their synthesis and reactivity.…”

Rh–POP Pincer Xantphos Complexes for C–S and C–H Activation. Implications for Carbothiolation Catalysis

“…Cooling of 3 (183 K, CD 2 Cl 2 ) did not result in a 1 H NMR spectrum that reflects the solid-state structure (i.e., inequivalent hydrides were not observed). This is in contrast to [Rh(κ 3 -P,O,P -Xantphos)(H) 2 (acetone)][BAr F 4 ], 15 in which the hydrides are equivalent at 298 K but inequivalent at low temperature, which is suggested to be due to reversible decoordination of acetone. Given these data, we suggest that complex 3 exists in C 6 H 5 Cl or CD 2 Cl 2 solution predominantly as [Rh(κ 3 -P,O,P -Xantphos-3,5-C 6 H 3 (CF 3 ) 2 )(H) 2 ][BAr F 4 ], 2b, but can be recrystallized from C 6 H 5 Cl solvent to give the solvent adduct 3.…”

Well‐Defined and Robust Rhodium Catalysts for the Hydroacylation of Terminal and Internal Alkenes

“…[6] An alternative approach is to increase the rate of reductive elimination of ketone product, as tep that is often (although not exclusively [7] )t he turnover-limiting process in hydroacylation. [3a,8] Wide-bite-angle,o rs terically bulky,l igands have this effect, but also change the ratio of alkene versus aldehyde hydroacylation [9] or the linear/branched selectivity.…”

“…[6] An alternative approach is to increase the rate of reductive elimination of ketone product, as tep that is often (although not exclusively [7] )t he turnover-limiting process in hydroacylation. [3a,8] Wide-bite-angle,o rs terically bulky,l igands have this effect, but also change the ratio of alkene versus aldehyde hydroacylation [9] or the linear/branched selectivity. [10] Additionally,these systems in general still require high loadings,a nd activated alkenes or terminal alkynes as substrates.S mall-bite-angle phosphine ligands,R 2 PCH 2 PR 2 (e.g.,R= t Bu, Cy), initially developed by Hofmann et al, [11] have been shown to favor the products of reductive elimination [12] and we recently demonstrated that catalyst systems exemplified by [Rh(R 2 PCH 2 PR 2 )(h…”