Catalytic dehydrogenative cross-coupling: forming carbon-carbon bonds by oxidizing two carbon-hydrogen bonds

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).

C–C, C–O and C–N bond formation via rhodium(III)-catalyzed oxidative C–H activation

C-H bonds are ubiquitous in organic compounds. It would, therefore, appear that direct functionalization of substrates by activation of C-H bonds would eliminate the multiple steps and limitations associated with the preparation of functionalized starting materials. Regioselectivity is an important issue because organic molecules can contain a wide variety of C-H bonds. The use of a directing group can largely overcome the issue of regiocontrol by allowing the catalyst to come into proximity with the targeted C-H bonds. A wide variety of functional groups have been evaluated for use as directing groups in the transformation of C-H bonds. In 2005, Daugulis reported the arylation of unactivated C(sp(3))-H bonds by using 8-aminoquinoline and picolinamide as bidentate directing groups, with Pd(OAc)2 as the catalyst. Encouraged by these promising results, a number of transformations of C-H bonds have since been developed by using systems based on bidentate directing groups. In this Review, recent advances in this area are discussed.

Catalytic Functionalization of C(sp2) ? H and C(sp3) ? H Bonds by Using Bidentate Directing Groups

Over the last decade, substantial research has led to the introduction of an impressive number of efficient procedures which allow the selective construction of CC bonds by directly connecting two different CH bonds under oxidative conditions. Common to these methodologies is the generation of the reactive intermediates in situ by activation of both CH bonds. This strategy was introduced by the group of Li as cross-dehydrogenative coupling (CDC) and discloses waste-minimized synthetic alternatives to classic coupling procedures which rely on the use of prefunctionalized starting materials. This Review highlights the recent progress in the field of cross-dehydrogenative C sp 3C formations and provides a comprehensive overview on existing procedures and employed methodologies.

The Cross-Dehydrogenative Coupling of C sp 3H Bonds: A Versatile Strategy for CC Bond Formations

The use of coordinating moieties as directing groups for the functionalization of aromatic CH bonds has become an established tool to enhance reactivity and induce regioselectivity. Nevertheless, with regard to the synthetic applicability of CH activation, there is a growing interest in transformations in which the directing group can be fully abandoned, thus allowing the direct functionalization of simple benzene derivatives. However, this approach requires the disclosure of new strategies to achieve reactivity and to control selectivity. In this review, recent advances in the emerging field of non-chelate-assisted CH activation are discussed, highlighting some of the most intriguing and inspiring examples of induction of reactivity and selectivity.

Beyond directing groups: transition-metal-catalyzed C-H activation of simple arenes.

Organic transformations that involve direct functionalization of C–H bonds represent an attractive synthetic strategy that maximizes atom- and step-economy. With the generally high stability of C–H bonds, these processes have mostly required harsh reaction conditions, in combination with the necessity of activation of the C–H substrates and/or the coupling partners. As a class of activated coupling partners, strained or reactive rings exhibited high activity in the coupling with aryl and alkyl C–H bonds. Such a high reactivity of the rings allowed the facile construction of various new structural platforms via coupling with scission of the ring structures. The combination of C–H activation and scission of the rings allowed for applications of a broader scope of C–H bonds, including those less reactive alkyl ones. This synthetic diversity of these rings has been realized owing to the intrinsically different mechanisms of the interactions of transition metal catalysts and the strained/reactive rings.

Transition metal-catalysed couplings between arenes and strained or reactive rings: combination of C-H activation and ring scission.

Rh(III)-catalyzed tandem oxidative olefination-Michael reactions between aryl carboxamides and alkenes.

Co(III)-Catalyzed Synthesis of Quinazolines via C–H Activation of N-Sulfinylimines and Benzimidates

Making C-C from C-H: [{RhCp*Cl(2)}(2)]/AgSbF(6) (Cp*=pentamethylcyclopentadienyl) can regioselectively catalyze the C-C coupling of arenes with aziridines by a C-H activation pathway. An eight-membered rhodacyclic intermediate resulting from the insertion of the Rh-C bond into the aziridine was isolated.

Fen Wang

Papers

C–C, C–O and C–N bond formation via rhodium(III)-catalyzed oxidative C–H activation

Transition metal-catalysed couplings between arenes and strained or reactive rings: combination of C-H activation and ring scission.

Rh(III)-catalyzed tandem oxidative olefination-Michael reactions between aryl carboxamides and alkenes.

Co(III)-Catalyzed Synthesis of Quinazolines via C–H Activation of N-Sulfinylimines and Benzimidates

Rhodium(III)‐Catalyzed CC Coupling between Arenes and Aziridines by CH Activation