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

Once considered the 'holy grail' of organometallic chemistry, synthetically useful reactions employing C-H bond activation have increasingly been developed and applied to natural product and drug synthesis over the past decade. The ubiquity and relative low cost of hydrocarbons makes C-H bond functionalization an attractive alternative to classical C-C bond forming reactions such as cross-coupling, which require organohalides and organometallic reagents. In addition to providing an atom economical alternative to standard cross - coupling strategies, C-H bond functionalization also reduces the production of toxic by-products, thereby contributing to the growing field of reactions with decreased environmental impact. In the area of C-C bond forming reactions that proceed via a C-H activation mechanism, rhodium catalysts stand out for their functional group tolerance and wide range of synthetic utility. Over the course of the last decade, many Rh-catalyzed methods for heteroatom-directed C-H bond functionalization have been reported and will be the focus of this review. Material appearing in the literature prior to 2001 has been reviewed previously and will only be introduced as background when necessary. The synthesis of complex molecules from relatively simple precursors has long been a goal for many organic chemists. The ability to selectively functionalize a molecule with minimal pre-activation can streamline syntheses and expand the opportunities to explore the utility of complex molecules in areas ranging from the pharmaceutical industry to materials science. Indeed, the issue of selectivity is paramount in the development of all C-H bond functionalization methods. Several groups have developed elegant approaches towards achieving selectivity in molecules that possess many sterically and electronically similar C-H bonds. Many of these approaches are discussed in detail in the accompanying articles in this special issue of Chemical Reviews. One approach that has seen widespread success involves the use of a proximal heteroatom that serves as a directing group for the selective functionalization of a specific C-H bond. In a survey of examples of heteroatom-directed Rh catalysis, two mechanistically distinct reaction pathways are revealed. In one case, the heteroatom acts as a chelator to bind the Rh catalyst, facilitating reactivity at a proximal site. In this case, the formation of a five-membered metallacycle provides a favorable driving force in inducing reactivity at the desired location. In the other case, the heteroatom initially coordinates the Rh catalyst and then acts to stabilize the formation of a metal-carbon bond at a proximal site. A true test of the utility of a synthetic method is in its application to the synthesis of natural products or complex molecules. Several groups have demonstrated the applicability of C-H bond functionalization reactions towards complex molecule synthesis. Target-oriented synthesis provides a platform to test the effectiveness of a method in unique chemical and steric environments. In this respect, Rh-catalyzed methods for C-H bond functionalization stand out, with several syntheses being described in the literature that utilize C-H bond functionalization in a key step. These syntheses are highlighted following the discussion of the method they employ.

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Rhodium-Catalyzed C-C Bond Formation via Heteroatom-Directed C-H Bond Activation

N-Heterocyclic Carbenes in Late Transition Metal Catalysis

The area of transition-metal-catalyzed direct arylation through cleavage of CH bonds has undergone rapid development in recent years, and is becoming an increasingly viable alternative to traditional cross-coupling reactions with organometallic reagents In particular, palladium and ruthenium catalysts have been described that enable the direct arylation of (hetero)arenes with challenging coupling partners—including electrophilic aryl chlorides and tosylates as well as simple arenes in cross-dehydrogenative arylations Furthermore, less expensive copper, iron, and nickel complexes were recently shown to be effective for economically attractive direct arylations

Transition-metal-catalyzed direct arylation of (hetero)arenes by C-H bond cleavage.

Ruthenium(II)-Catalyzed C–H Bond Activation and Functionalization

An efficient method for the synthesis of symmetrical diselenides is described. Reductive selenation of aromatic and heterocyclic aromatic aldehydes (ArCHO) with Se/CO/H2O in DMF afforded diselenides (ArCH2SeSeCH2Ar) in yields up to 94% under atmospheric pressure without use of a base.

Efficient Reductive Selenation of Aromatic Aldehydes to Symmetrical Diselenides with Se/CO/H2O under Atmospheric Pressure

Di- and trisubstituted furan derivatives have been efficiently synthesized via palladium(II)-catalyzed intramolecular carbonylative cycloisomerization of gamma-propynyl-1,3-diketones with aryl iodides and carbon monoxide. The mechanism suggests that in situ generated acylpalladium species from the carbonylation of aryl iodide initiates the reaction followed by cyclization of the enolized isomer of a 1,3-diketone substrate via carbon-carbon triple bond activation.

Palladium-catalyzed carbonylative cycloisomerization of gamma-propynyl-1,3-diketones: a concise route to polysubstituted furans.

Highly efficient arylations of b-chloro ke- tones and their ester and amide derivatives were achieved by means of domino dehydrochlorination/ Rh(I)-catalyzed conjugate addition. In situ generat- ed vinyl ketones and their analogues were identified as the reaction intermediates. The present synthetic protocol provides a concise route to (chiral) b-aryl ketones, esters, and amides.

Zhengkun Yu

Papers

Efficient Reductive Selenation of Aromatic Aldehydes to Symmetrical Diselenides with Se/CO/H2O under Atmospheric Pressure

Palladium-catalyzed carbonylative cycloisomerization of gamma-propynyl-1,3-diketones: a concise route to polysubstituted furans.

Rhodium(I)‐Catalyzed Arylation of β‐Chloro Ketones and Related Derivatives through Domino Dehydrochlorination/ Conjugate Addition

Photoinduced, Copper-Catalyzed Three-Component Annulation of gem-Dialkylthio Enynes.

Palladium-Catalyzed Synthesis of Highly Substituted Endocyclic Enol Lactones via a Three-Component Coupling Reaction in an Ionic Liquid