Synthesis, Characterization, and Reactivity of Arene-Stabilized Rhodium Complexes

Merging Visible Light Photocatalysis and P(III)-Directed C-H Activation by a Single Catalyst: Modular Assembly of P-Alkyne Hybrid Ligands

Metal-catalyzed C-H activation strategies provide an efficient approach for synthesis by minimizing atom, step, and redox economy. Developing milder, greener, and more effective protocols for these strategies is always highly desirable to the scientific community. In this study, the utilization of a single rhodium complex enabled the visible-light-induced late-stage C-H activation of biaryl-type phosphines with alkynyl bromides, employing inherent phosphorus atoms as directing groups. This chemistry combines P(III)-directed C-H activation with visible light photocatalysis, under exogenous photosensitizer-free conditions, offering a unique platform for ligand design and preparation. Furthermore, this study also explores the asymmetric catalysis and coordination chemistry of the resulting P-alkyne hybrid ligands with specific transition metals. Experimental results and density functional theory calculations demonstrate the mechanistic intricacies of this transformation.

Catalytic direct hydrocarboxylation of styrenes with CO2 and H2

A three-component hydrocarboxylation of an olefin with CO₂ and H₂ could be regarded as a dream reaction, since it would provide a straightforward approach for the synthesis of aliphatic carboxylic acids in perfect atom economy. However, this transformation has not been realized in a direct manner under mild conditions, because boosting the carboxylation with thermodynamically stable CO₂ while suppressing the rapid hydrogenation of olefin remains a challenging task. Here, we report a rhodium-catalysed reductive hydrocarboxylation of styrene derivatives with CO₂ and H₂ under mild conditions, in which H₂ served as the terminal reductant. In this approach, the carboxylation process was largely accelerated by visible light irradiation, which was proved both experimentally and by computational studies. Hydrocarboxylation of various kinds of styrene derivatives was achieved in good yields without additional base under ambient pressure of CO₂/H₂ at room temperature. Mechanistic investigations revealed that use of a cationic rhodium complex was critical to achieve high hydrocarboxylation selectivity.

Rhodium(I) Complexes Bearing an Aryl-Substituted 1,3,5-Hexatriene Chain: Catalysts for Living Polymerization of Phenylacetylene and Potential Helical Chirality of 1,3,5-Hexatrienes

Unique and bench-stable rhodium(I) complexes bearing an aryl-substituted 1,3,5-hexatriene chain have been synthesized by the reactions of bicyclo[2.2.1]hepta-2,5-diene-rhodium(I) chloride dimer ([Rh(nbd)Cl]2) with aryl boronic acids and diphenylacetylenes in the presence of a 50% aqueous solution of KOH. X-ray crystallographic analysis of the isolated complexes indicated a square-planar structure stabilized by a strong interaction with one of the aryl groups on the 1,3,5-hexatriene chain, which has a helical structure. The helical chirality of the isolated rhodium complexes was confirmed to be sufficiently stable to be resolved by chiral HPLC at room temperature into enantiomers, which showed mirror-imaged CD spectra. It was confirmed that the isolated rhodium complex worked as an initiator for living polymerization of phenylacetylene to give cis-stereoregular poly(phenylacetylene) with a well-controlled molecular weight.

Rhodium-catalysed direct hydroarylation of alkenes and alkynes with phosphines through phosphorous-assisted C-H activation

Biarylphosphines have been widely applied as ligands in various synthetic methods, especially in transition-metal-catalysed carbon-carbon and carbon-heteroatom bond cross-coupling reactions. Based on the outstanding properties of the parent scaffolds, a general method for in situ modification of the commercial tertiary phosphine ligands to access a series of ligands is in high demand. Here we show that a rhodium-catalysed system is introduced for the hydroarylation of alkenes and alkynes with tertiary phosphines through P(III)-chelation assisted C-H activation. A series of ligand libraries containing alkyl and alkenyl substituted groups with different steric and electronic properties are obtained in high yields. Furthermore, several experimental studies are performed to uncover the key mechanistic features of the linear-selective hydroarylation of alkenes and branch-selective hydroarylation of alkynes.

Dialkyl biaryl phosphines find wide application in catalysis as ligands. Here, the authors report a rhodium-catalysed hydroarylation of alkenes and alkynes with tertiary phosphines through P(III)-chelation assisted C-H activation affording a library of functionalized phosphines which hold potential in catalytic applications.

A comparison of MOP-phosphonite ligands and their applications in Rh(i)- and Pd(ii)-catalysed asymmetric transformations

Six chiral MOP-phosphonites have been synthesised and compared via experimental and computational methods in an effort to quantify their differing structural and electronic profiles. They were found to be electron-poor ligands in comparison to their arylphosphine analogues and have a larger trans influence in square planar Pt(ii) complexes. Four [Rh(L^P)(η²:η²-cod)Cl] complexes were synthesised and characterised by NMR, HRMS and X-ray crystallography. Two [Rh(L^P)₂]BF₄ complexes were prepared where one ligand acts as a chelating P,C-π-donor; detailed NMR studies demonstrated a hemilabile η⁶-coordination mode, which in one case was confirmed by X-ray crystallography. Rh(i) complexes were used as catalysts in asymmetric hydrogenation and hydroformylation reactions and in the addition of phenyl boronic acid to an isatin. Pd(ii) complexes were successfully employed in asymmetric Suzuki-Miyaura cross-coupling reactions yielding binaphthyl products. Two [Pd(L^P)₂Cl₂] complexes were synthesised and characterised by X-ray crystallography, both adopting cis orientations, with one of the complexes crystallising as two pseudo-polymorphs.

Rhodium-Catalyzed meta-C-H Functionalization of Arenes

Rhodium-catalyzed ortho-C-H functionalization is well known in the literature. Described herein is the Xphos-supported rhodium catalysis of meta-C-H olefination of benzylsulfonic acid and phenyl acetic acid frameworks with the assistance of a para-methoxy-substituted cyano phenol as the directing group. Complete mono-selectivity is observed for both scaffolds. A wide range of olefins and functional groups attached to arene are tolerated in this protocol.

General Route to Cyclobutadiene Rhodium Complexes

Cyclobutadiene rhodium complexes bear high potential for applications in organometallic synthesis and catalysis. We have found that the cyclobutadiene complexes with substitutionally labile p-xylene ligands [(C4 R4 )Rh(p-xylene)](+) can be synthesized in one step from the commercially available bis(ethylene) complex [{(C2 H4 )2 RhCl}2 ], p-xylene, and internal alkynes. The replacement of p-xylene by various ligands provides a general access to other [(C4 R4 )Rh] compounds, such as [(C4 R4 )RhCl]x , [(C4 R4 )RhL3 ](+) , [(C4 R4 )Rh(C5 H5 )], and [(C4 R4 )Rh(arene)](+) . Complex [(C4 Et4 )Rh(p-xylene)](+) also catalyzes an unusual cycloisomerization of a 1,11-dien-6-yne into a bicyclic diene.

A new dirhodium catalyst with hemilabile tropolonato ligands for C-H bond functionalization

Don't hold on too tightly! In a new dirhodium catalyst for C-H functionalization reactions, two tropolonato ligands are introduced as hemilabile chelating ligands (see scheme). Only two bridges hold the Rh-Rh core together. The tropolonato ligands can liberate a binding site in the equatorial coordination sphere of the catalyst. This opens a doorway to new mechanistic channels in C-H functionalization.