Chem Soc Rev CRITICAL REVIEW

Chem Soc Rev Dynamic Article Links Cite this: Chem. Soc. Rev., 2012, 41, 3651–3678 www.rsc.org/csr CRITICAL REVIEW C–C, C–O and C–N bond formation via rhodium(III)-catalyzed oxidative C–H activation Guoyong Song,w Fen Wangw and Xingwei Li* Received 13th October 2011 DOI: 10.1039/c2cs15281a 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). 1. Introduction for the synthesis of value-added complex structures. Due to the high dissociation energy of C–H bonds (105 kcal molÀ1 for The demand for green and sustainable chemistry has inspired methane and 110 kcal molÀ1 for benzene), metal-mediation is chemists to seek efficient and economic ways to construct often necessary. Therefore, direct and catalytic functionaliza- chemical bonds during the synthesis of complex structures.1 tion of C–H bonds has been a highly intriguing research topic In particular, C–C, C–O, and C–N bonds are essential links in for the past two decades, and this topic has been extensively most organics, and the construction of these bonds constitutes reviewed.2–6 The strategy of metal-catalyzed C–H activation7 a fundamental aspect of synthetic chemistry. On the other is advantageous in that no prior activation of C–H bonds hand, C–H bonds are ubiquitous in organic molecules. Thus, is necessary, and the formation of reactive organometallic direct functionalization of C–H to C–E (E = C, O, N) bonds intermediates via C–H activation provides an eco-friendly becomes one of the most valuable and straightforward methods and step-economic alternative to conventional methods,8–13 for example, transmetalation using organo-main group reagents Dalian Institute of Chemical Physics, Chinese Academy of Sciences, or oxidative addition using organic halides. While the nature of Dalian 116023, P. R. China. E-mail: [email protected]; Fax: +86-411-84379089; Tel: +86-411-84379089 the cleavage of C–H bonds and the formation of a M–C species w These authors contributed equally. can significantly vary, depending on the substrate, solvent, Guoyong Song was educated Fen Wang received her BS in Chemistry at Lanzhou degree in Chemistry from University and in Lanzhou Yulin College in 2008. She Institute of Chemical Physics, obtained her MS degree from CAS. He received his doctoral the Northwest Normal Univer- degree from Nanyang Techno- sity in 2011, during which time logical University (Singapore) she was co-supervised by Prof. in 2009 with Prof. Xingwei Li. Xingwei Li at the Dalian After a postdoctoral stay in Institute of Chemical Physics, Roy A. Periana’s group CAS. In 2011 she joined Prof. (Scripps Florida), he joined Xingwei Li’s group as a Dalian Institute of Chemical Research Assistant, where she Physics, CAS as a visiting currently studies synthetic scientist in 2010. He now methods based on C–H bond Guoyong Song works in the Organometallic Fen Wang activation. Chemistry Laboratory of Riken as a JSPS Fellow. This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 3651–3678 3651 additives and nature of the transition metals and stabilizing salt additives and stabilizing ligands are frequently used; ligands, four general mechanisms have been invoked: oxidative otherwise, decomposition of the palladium catalyst to inert addition for electron-rich late metals, s-bond metathesis for metallic palladium is a typical deactivation pathway of the early metals, electrophilic C–H activation for electron-deficient catalyst. These issues have limited the practicability of palladium late metals, and Lewis base-assisted C–H activation.14,15 catalysis in the laboratory and in industry. These different pathways enabled the activation of C–H bonds Analogous to the Pd(II)/Pd(0) processes, the Rh(III)/Rh(I) in a plethora of substrates. Since various C–H bonds are cycles are widely present in catalysis, as in the well-known present in organic molecules, achieving regioselective C–H Monsanto acetic acid process. In line with the well-studied activation and functionalization often represents a big Wacker process, a rhodium-version of such process has been challenge. One of the most promising strategies to achieve extensively explored.19–21 However, rhodium-catalyzed oxida- high selectivity is to utilize a directing group. Following the tion reactions have been much less explored in contrast to the coordination of a directing group to a transition metal, the vast majority of reports on palladium-catalyzed reactions. proximal C–H bond is activated as a result of chelation Despite the generally high cost of rhodium compounds, assistance. By following this strategy, Murai pioneered in the rhodium catalysis will still be highly desirable if reaction highly efficient and selective ruthenium-catalyzed ortho C–H systems that are inaccessible under palladium catalysis can activation of aryl ketones, followed by functionalization with be efficiently developed and if different reaction selectivity can alkenes and alkynes, where the carbonyl group acts as a be executed under rhodium catalysis. Indeed, the last five years 16 5,22 directing group. Ever since this work, a large volume of has witnessed drastic progress in this field. Rhodium(III) 2 reports have appeared, and in most cases sp C–H bonds were catalysts, in particular [RhCp*Cl2]2 (Cp*=pentamethylcyclo- 2–6 2+ functionalized. pentadienyl) and [RhCp*(MeCN)3] , stand out in the func- Construction of C–E (E = C, N, and O) bonds under tionalization of C–H bonds via a C–H activation pathway oxidative conditions is of great significance not only in funda- owing to the high efficiency, selectivity, and functional group mental research but also in the pharmaceutical industry and tolerance. Thus this area has been increasingly explored, and in the production of chemical feedstock. For example, the facile construction of C–E (E = C, O, and N) bonds via C–H well-known Wacker process17 and the Fujiwara reaction18 activation should find widespread applications in the synthesis allowed the efficient construction of C–O and C–C bonds of natural products, organics, and materials. using palladium catalysts and oxidants. Inspired by these In 2010, Satoh and Miura reviewed the most recent progress in pioneering works, various research groups have succeeded in this field.22 However, much exciting process has been made in this constructing C–C, C–O, and C–N bonds under oxidative rapidly growing field. Thus reports after mid 2010 fall beyond this conditions, and many useful synthetic methodologies have review. We herein summarize the most recent findings on Rh(III)- been developed starting from substrates with or without catalyzed oxidative C–E (E = C, N, and O) coupling reactions chelation assistance, in which the C–H bond is typically using both external and internal oxidants. The versatility and coupled with alkenes, alkynes, arenes and heteroarenes.2–6,8–13 practicability of these reactions in their current forms are evaluated These are important alternatives to traditional palladium- in terms of catalytic efficiency, substrate scope, mechanistic aspects catalyzed redox-neutral C–E (E = C, N, and O) coupling and problems. This has been done by categorizing the substrates. reactions. Despite such exciting progress, palladium-catalyzed oxidative coupling reactions suffer from limited substrate 2. General reaction patterns and mechanisms of the oxidative scope, limited functional group compatibility, and high coupling of arenes with alkenes and alkynes catalyst loading (often 45 mol%). In addition, acids, metal In line with the well-studied active organopalladium species in coupling reactions, active organorhodium intermediates can also be functionalized, but so far the coupling partner is Xingwei Li obtained his BS mostly limited to unsaturated molecules such as alkenes and degree from Fudan University in 1996 and his PhD from Yale alkynes. We noted that palladium and rhodium differ at least University in 2005 with Prof. in the following aspects in catalytic oxidative coupling reactions. Robert H. Crabtree, after (1) Rh(III)-catalyzed C–H activation is mostly limited to 2 3 which he did postdoctoral C(sp )–H bonds, while catalytic activation of C(sp )–H bonds studies with Prof. John E. is quite common under palladium catalysis;23 (2) formation of Bercaw at Caltech. In 2006 Rh–C bonds via C–H activation is generally limited to chelation he took an Assistant Professor assistance. In contrast, palladation of simple arenes and position at Nanyang Techno- heteroarenes (such as indoles and pyridines) without chelation logical University, Singapore assistance is well known in palladium-catalyzed oxidation and in 2008 he became 24 reactions, and this can be the 1st step in a catalytic cycle; an Assistant Professor of and (3) the coupling partner that serves to functionalize Rh–C Catalysis at the Scripps species is mostly limited to unsaturated molecules such as Xingwei Li Research Institute in Florida. He has served as a Professor alkenes and alkynes, while the scope of the coupling partner is at the Dalian Institute of Chemical Physics, CAS since 2011. much broader under palladium catalysis. Comparisons and His research interests include organometallic chemistry and contrasts between rhodium and other metals in catalytic metal-catalyzed organic reactions, particularly C–H activation.

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