Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold by midcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO2 emissions in the atmosphere demands that holding atmospheric CO2 levels to even twice their preanthropogenic values by midcentury will require invention, development, and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at a year-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form of chemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbon species.

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Powering the planet: Chemical challenges in solar energy utilization

Pick your Pd partners: A number of catalytic systems have been developed for palladium-catalyzed CH activation/CC bond formation. Recent studies concerning the palladium(II)-catalyzed coupling of CH bonds with organometallic reagents through a PdII/Pd0 catalytic cycle are discussed (see scheme), and the versatility and practicality of this new mode of catalysis are presented. Unaddressed questions and the potential for development in the field are also addressed.

In the past decade, palladium-catalyzed CH activation/CC bond-forming reactions have emerged as promising new catalytic transformations; however, development in this field is still at an early stage compared to the state of the art in cross-coupling reactions using aryl and alkyl halides. This Review begins with a brief introduction of four extensively investigated modes of catalysis for forming CC bonds from CH bonds: PdII/Pd0, PdII/PdIV, Pd0/PdII/PdIV, and Pd0/PdII catalysis. A more detailed discussion is then directed towards the recent development of palladium(II)-catalyzed coupling of CH bonds with organometallic reagents through a PdII/Pd0 catalytic cycle. Despite the progress made to date, improving the versatility and practicality of this new reaction remains a tremendous challenge.

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Palladium(II)-catalyzed C-H activation/C-C cross-coupling reactions: versatility and practicality.

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

Controlled/living radical polymerization: Features, developments, and perspectives

Transfer of ethylidene from .eta.5-C5H5(CO)2Fe:CHMe+ to para-substituted styrenes. Loss of stereochemistry about the C.alpha.-C.beta. double bond of cis-.beta.-deuterio-p-methoxystyrene

.pi. to .sigma. Rearrangements upon metal protonation of polyolefin-transition metal complexes

Exploring Oxidation of Half-Sandwich Rhodium Complexes: Oxygen Atom Insertion into the Rhodium–Carbon Bond of κ2-Coordinated 2-Phenylpyridine

Catalytic alkane transfer-dehydrogenation by PSCOP iridium pincer complexes

A mechanistic interpretation of the [(1,10-phenanthroline)Pd(CH(3))(CH(3)CN)](+)[BArF](-) (1a) and [(2,2'-bipyridine)Pd(CH(3))(CH(3)CN)](+)[BArF](-) (1b) (BArF = 3,5-(CF(3))(2)-C(6)H(3)) catalyzed perfectly alternating copolymerization of styrenes with CO is reported. The copolymerization in CH(2)Cl(2) or chlorobenzene has been found to be first order in styrene and inverse first order in CO concentrations. The microscopic steps involved in the catalytic cycle have been studied via low temperature NMR techniques. Palladium alkyl chelate complex [(2,2'-bipyridine)Pd(CHArCH(2)C(O)CH(3)](+)[BArF](-) (5b sigma) and [(2,2'-bipyridine)Pd(eta(3)-CH(CH(2)C(O)CH(3))Ar)](+)[BArF](-) (5b pi), existing in equilibrium, were prepared. Treatment of 5b sigma,pi with (13)CO followed by 4-tert-butylstyrene at -78 degrees C allowed for (13)C NMR monitoring of the alternating chain growth of a series of palladium acyl carbonyl complexes. The acyl carbonyl species, representing the catalyst resting state, is in equilibrium with a palladium acyl styrene complex. The equilibrium constant, K(4), measured between [(phen)Pd(CO)(C(O)CH(3)](+)[BArF](-) (3a) and [(phen)Pd(C(O)CH(3))-(C(6)H(5)C=CH(2))](+)[BArF](-) (8a), was determined to be 2.84 +/- 2.8 x 10(-7) at -66 degrees C. The barrier to migratory insertion in 8a was determined (DeltaG(double dagger) (-66 degrees C) = 15.6 +/- 0.1 kcal mol(-1)). From the experimentally determined kinetic and thermodynamic data for the copolymerization of styrene with CO a mechanistic model has been constructed. The ability of this model to predict catalyst turnover frequency (TOF) was used as a test of its validity. A series of para-substituted styrenes, p-XC(6)H(4)CH=CH(2) (X = -OCH(3), -CH(3), -H, -Cl), were copolymerized with CO. A Hammett treatment of TOF for the series showed that electron-donating groups increase the rate of copolymerization (rho p = -0.8). The ratio of chain transfer to chain propagation was found to increase with styrene concentration and decrease with CO concentration. Polymer end group analysis showed the presence of alpha, beta-enone end groups. The reactivity of model systems, coupled with a study of the effect of added acetonitrile, support a chain transfer mechanism involving beta-hydrogen transfer to monomer from a palladium alkyl styrene intermediate.

Maurice Brookhart

Papers

Transfer of ethylidene from .eta.5-C5H5(CO)2Fe:CHMe+ to para-substituted styrenes. Loss of stereochemistry about the C.alpha.-C.beta. double bond of cis-.beta.-deuterio-p-methoxystyrene

.pi. to .sigma. Rearrangements upon metal protonation of polyolefin-transition metal complexes

Exploring Oxidation of Half-Sandwich Rhodium Complexes: Oxygen Atom Insertion into the Rhodium–Carbon Bond of κ2-Coordinated 2-Phenylpyridine

Catalytic alkane transfer-dehydrogenation by PSCOP iridium pincer complexes

Palladium(II)-catalyzed copolymerization of styrenes with carbon monoxide: mechanism of chain propagation and chain transfer.