Exploring Small Bite-Angle Ligands for the Rhodium-Catalyzed Intermolecular Hydroacylation of β-S-Substituted Aldehydes with 1-Octene and 1-Octyne

Abstract

A comparative study of seven crystallographically characterized rhodium precatalysts, which contain a variety of chelating diphosphine ligands, for the hydroacylation of 1-octyne or 1-octene with 2-(methylthio)benzaldehyde has been undertaken. These studies show that the best performing catalyst for 1-octyne, [Rh(L)(η6-C6H5F)][BArF 4], L = iPr2PNMePiPr2, delivers alkyne selective hydroacylation with high efficiencies at low loadings (1 mol %, 2.0 M aldehyde, 25 °C, ToN = 100, 97% conversion in 5 min), and also shows high selectivity for the linear product. Experiments suggest that the alkyne selectivity arises from the alkyne being more competitive for metal binding compared to the alkene. Labeling experiments using the [Rh(tBu2PCH2PtBu2)(η6-C6H5F)][BArF 4] system, that gives the final product in a linear:branched ratio of 6:1, indicate that the pathway that produces the branched product operates via an irreversible hydride insertion. Intermediate acyl hydride complexes, [Rh(L)(H)(COC6H4SMe)(acetone)][BArF 4], have been characterized by low temperature NMR spectroscopy, as have their subsequent reductive decarbonylation products, one of which has also been crystallographically characterized: [Rh(iPr2PNMePiPr2)(SMePh)(CO)][BArF 4].

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

“…Small-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 )(η 6 -C 6 H 5 F)][BAr F 4 ] [R= t Bu, 1 ; Ar F =3,5-C 6 H 3 (CF 3 ) 2 ] can be used at low catalyst loadings (e.g., 0.1 mol %) to couple terminal and activated internal alkenes with β-substituted aldehydes. [13] However, challenging internal alkenes are still out of reach with this system, as decarbonylation now outruns productive turnover. Others have since used similar ligands for intermolecular hydroacylation.…”

“…[17] The corresponding precatalysts[13] were prepared from sequential addition of ligand and H 2 to [Rh(cod) 2 ][BAr F 4 ] in C 6 H 5 F solution, to afford [Rh( 2 )(η 6 -C 6 H 5 F)][BAr F 4 ], 3 a – d (Scheme 3). The solid-state structures of 3 a and 3 d [16] (Figure 1 A shows 3 a ) show a Rh I complex and no close Rh⋅⋅⋅⋅O contact [for example, Rh1–O2 3.381(3) Å].…”

“…[13, 16] Subsequent addition of 4 a afforded a complex mixture of products, in which the intermediate acyl hydride [Rh( 2 a )(H)(κ,σ-1,2-SMe(CO)C 6 H 4 )(acetone)][BAr F 4 ] ( 8 ) was identified [ δ (H) −20.4, br]. [10, 13, 16] Over 2.5 h this reaction proceeded to give the corresponding decarbonylation product [Rh( 2 a )(SMePh)(CO)][BAr F 4 ] ( 9 ). This timescale for decarbonylation is similar to that for 1 ,[13b] suggesting that the OMe group does not offer strong stabilization to the acyl hydride in the absence of alkene.…”

Enantioselective hydroacylation of olefins with rhodium catalysts

“…Weller and Willis have established the utility of the thioether directing group by developing methods for its further elaboration, such as alkyne carbothiolation, cross-coupling, and desulfurization. 17 a-c Furthermore, Weller and Willis have recently developed a [Rh(C 6 H 5 F)(R 2 PCH 2 PR 2 )]BAr F 4 catalyst for general linear-selective hydroacylation with β-sulfur chelating aldehydes. 17d,e …”

“…17 a-c Furthermore, Weller and Willis have recently developed a [Rh(C 6 H 5 F)(R 2 PCH 2 PR 2 )]BAr F 4 catalyst for general linear-selective hydroacylation with β-sulfur chelating aldehydes. 17d,e …”