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

Abstract

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.

Dimethylcuprate Undergoes a Dyotropic Rearrangement

“…À , [20][21][22] or could involve a true dyotropic rearrangement proceeding via a single transition state. Attempts to Figure 2 c).…”

C–H Activation in Diiron Bridging Vinyliminium Ligands: Reaction with CS2 to Form New Zwitterionic Complexes Acting as Organometallic Ligands

“…Examples are numerous and include insertion into M-O and M-N, [19] M-S, [20] M-H, [21] M-C [22] and M-M bonds in early-late heterodinuclear transition-metal complexes and bonds. [23] The reactions of CS 2 and SNCPh with M=N (double bond) [24] and M=C (metalcarbene interaction) [25] have also been reported. In all cases, the new ligands generated by insertion and rearrangement of XCS (X = S, NPh) are invariably coordinated to the metal centre through S (or N) atoms, more frequently as S,S-or S,N-chelating ligands.…”

Activation of C–O and C–F Bonds by Pincer–Iridium Complexes

“…Grubbs and Ozerov [46] have investigated the reactions of (PNP)Ir and anthraphos-based (PCP)Ir complexes with methyl t-butyl and methyl benzyl ether, resulting in methoxy C-H activation and the formation of alkoxycarbene complexes (Scheme 4.5) found to be active for multiple-bond metatheses with various electrophilic heterocumulenes (e.g., CO 2 , CS 2 , and AdN 3 ). Remarkably, the formation of a carbonyl ligand results from the cleavage of every bond to the original ether methoxy carbon except the C-O bond [47,48]. To our knowledge, however, the only example before this work of "simple" intermolecular oxidative addition of an unactivated C(sp 3 )-O bond was reported by Ittel and Tolman [34,35] who observed cleavage of the C-O bond of anisole by the highly nucleophilic Fe(dmpe) 2 (Scheme 4.6).…”