Rhodium and iridium complexes of N-heterocyclic carbenes (3a-c and 4a-c) were obtained by transmetalation from the corresponding Ag(I) complexes. The structure of 3b was verified by X-ray diffraction. The compounds display restricted rotation about the metal-carbene bond, the rate of which can be controlled by altering the steric bulk of the auxiliary ligands. Infrared spectroscopy provides an estimate of the electron-donor power of the carbene ligands from ν(CO) of the carbonyl derivatives.

Rhodium(III)-catalyzed direct functionalization of C-H bonds under oxidative conditions leading to C-C, C-N, and C-O bond formation is reviewed. Various arene substrates bearing nitrogen and oxygen directing groups are covered in their coupling with unsaturated partners such as alkenes and alkynes. The facile construction of C-E (E = C, N, S, or O) bonds makes Rh(III) catalysis an attractive step-economic approach to value-added molecules from readily available starting materials. Comparisons and contrasts between rhodium(III) and palladium(II)-catalyzed oxidative coupling are made. The remarkable diversity of structures accessible is demonstrated with various recent examples, with a proposed mechanism for each transformation being briefly summarized (critical review, 138 references).

The possibility of developing new methods for the efficient construction of organic molecules via disconnections other than traditional functional group transformations has driven the interest in direct functionalization of C-H bonds. The ubiquity of C-H bonds makes such transformations attractive, but they also pose several challenges. The first is the reactivity and selectivity of C-H bonds. To achieve this, directing groups (DGs) are often installed that can enhance the effective concentration of the catalyst, leading to thermodynamically stable metallacyclic intermediates. However, the presence of a pendant directing group in the product is often undesirable and unnecessary. This may account for the limitation of applications of C-H functionalization reactions in more common and general uses. Thus, the development of removable or functionalizable directing groups is desirable. Another key problem is that the reactivity of the resulting M-C bond can be low, which may limit the scope of the coupling partners and hence limit the reaction patterns of C-H activation reactions. While the first Cp*Rh(III)-catalyzed C-H activation of arenes was reported only 7 years ago, significant progress has been made in this area in the past few years. We began our studies in this area in 2010, and we and others have demonstrated that diversified catalytic functionalization of arenes can be realized using Cp*Rh(III) complexes with high reactivity, stability, and functional group compatibility. This Account describes our efforts to solve some of these challenges using Rh(III) catalysis. We fulfilled our design and activation of the arene substrates by taking advantage of the nucleophilicity, electrophilicity, oxidizing potential, and properties of a participating ligand of the directing groups when the arenes are coupled with relatively reactive unsaturated partners such as alkenes and alkynes. These in situ funtionalizable roles of the DG allowed extensive chemical manipulation of the initial coupled product, especially in the construction of a diverse array of heterocycles. In the coupling of arenes with polar coupling partners, the polar Rh(III)-C(aryl) bond showed higher reactivity as both an organometallic reagent and a nucleophilic aryl source. The polar coupling partners were accordingly activated by virtue of umpolung, ring strain, and rearomatization. All of these transformations have been made possible by integration of the higher reactivity, stability, and compatibility of Rh(III)-C bonds into catalytic systems. We have demonstrated that to date some of these transformations can be achieved only under rhodium catalysis. In addition, by means of stoichiometric reactions, we have gained mechanistic insights into the interactions between the Rh-C bond and the other coupling partners, which have opened new avenues in future direct C-H functionalization reactions.