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Published October 2009 | public
Journal Article

Late Metal Carbene Complexes Generated by Multiple C−H Activations: Examining the Continuum of M═C Bond Reactivity


Unactivated C_(sp^3)−H bonds are ubiquitous in organic chemicals and hydrocarbon feedstocks. However, these resources remain largely untapped, and the development of efficient homogeneous methods for hydrocarbon functionalization by C−H activation is an attractive and unresolved challenge for synthetic chemists. Transition-metal catalysis offers an attractive possible means for achieving selective, catalytic C−H functionalization given the thermodynamically favorable nature of many desirable partial oxidation schemes and the propensity of transition-metal complexes to cleave C−H bonds. Selective C−H activation, typically by a single cleavage event to produce M−C_(sp^3) products, is possible through myriad reported transition-metal species. In contrast, several recent reports have shown that late transition metals may react with certain substrates to perform multiple C−H activations, generating M ═ C_(sp^2) complexes for further elaboration. In light of the rich reactivity of metal-bound carbenes, such a route could open a new manifold of reactivity for catalytic C−H functionalization, and we have targeted this strategy in our studies. In this Account, we highlight several early examples of late transition-metal complexes that have been shown to generate metal-bound carbenes by multiple C−H activations and briefly examine factors leading to the selective generation of metal carbenes through this route. Using these reports as a backdrop, we focus on the double C−H activation of ethers and amines at iridium complexes supported by Ozerov's amidophosphine PNP ligand (PNP = [N(2-P^(i)Pr_(2)-4-Me-C_(6)H_(3))_2]^−), allowing isolation of unusual square-planar iridium(I) carbenes. These species exhibit reactivity that is distinct from the archetypal Fischer and Schrock designations. We present experimental and theoretical studies showing that, like the classical square-planar iridium(I) organometallics, these complexes are best described as nucleophilic at iridium. We discuss the classification of this reactivity in the context of a scheme originally delineated by Roper. These "Roper-type" carbenes perform a number of multiple-bond metatheses leading to atom and group transfer from electrophilic heterocumulene (e.g., CO_2, CS_2, PhNCS) and diazo (e.g., N_(2)O, AdN_3) reagents. In one instance, we have extended this methodology to a process for catalytic C−H functionalization by a double C−H activation-group transfer process. Although the scope of these reactions is currently limited, these new pathways may find broader utility as the reactivity of late-metal carbenes continues to be explored. Examination of alternative transition metals and supporting ligand sets will certainly be important. Nonetheless, our findings show that carbene generation by double C−H activation is a viable strategy for C−H functionalization, leading to products not accessible through traditional C_(sp^3)−H activation pathways.

Additional Information

© 2009 American Chemical Society. Received on March 31; publication Date (Web): July 22, 2009. We gratefully acknowledge funding by BP through the MC^2 program. Special thanks go to Nigel Brookes (University of Tasmania) and Dr. Patricio Romero for valuable discussions and help in advancing this work. We are also appreciative for advice and collaboration from Prof. Oleg Ozerov (Texas A&M University) and Prof. Brian Yates (University of Tasmania), who have been immensely helpful as we have sought to understand and apply the work described herein.

Additional details

August 19, 2023
October 19, 2023