摘要：

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 CH activation is an attractive and unresolved challenge for synthetic chemists. Transition-metal catalysis offers an attractive possible means for achieving selective, catalytic CH functionalization given the thermodynamically favorable nature of many desirable partial oxidation schemes and the propensity of transition-metal complexes to cleave CH bonds. Selective CH activation, typically by a single cleavage event to produce MC_(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 CH 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 CH functionalization, and we have targeted this strategy in our studies.\ud\udIn this Account, we highlight several early examples of late transition-metal complexes that have been shown to generate metal-bound carbenes by multiple CH 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 CH 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.\ud\udWe 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 CH functionalization by a double CH activation-group transfer process.\ud\udAlthough 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 CH activation is a viable strategy for CH functionalization, leading to products not accessible through traditional C_(sp^3)H activation pathways.

展开