Hydroarylation reactions of olefins are catalyzed by the octahedral Ru(II) complex TpRu-(CO)(NCMe)(Ph) (1) (Tp ) hydridotris(pyrazolyl)borate). Experimental studies and density functional theory calculations support a reaction pathway that involves initial acetonitrile/ olefin ligand exchange and subsequent olefin insertion into the ruthenium-phenyl bond. Metal-mediated C-H activation of arene to form a Ru-aryl bond with release of alkyl arene completes the proposed catalytic cycle. The cyclopentadienyl complex CpRu(PPh 3 ) 2 (Ph) produces ethylbenzene and styrene from a benzene/ethylene solution at 90 °C; however, the transformation is not catalytic. A benzene solution of (PCP)Ru(CO)(Ph) (PCP ) 2,6-(CH 2 P t -Bu 2 ) 2 C 6 H 3 ) and ethylene at 90 °C produces styrene in 12% yield without observation of ethylbenzene. Computational studies (DFT) suggest that the C-H activation step does not proceed through the formation of a Ru(IV) oxidative addition intermediate but rather occurs by a concerted pathway.

Reactions that interconvert strong covalent bonds, generally known as dynamic covalent chemistry (DCC), offer a powerful approach for the thermodynamically controlled synthesis of organic molecules with interesting structures and/or properties. DCC involving esters, thioesters, imines, and disulfides, among other functional groups, has provided access to interesting new molecules. [1,2] Such efforts to date have focused on bonds that were previously known to be readily exchangeable. Extension of the DCC approach to other types of functional groups will require advances in organic reactivity and catalysis. It would be valuable, for example, to implement DCC with carboxamide-containing molecules, [1c] but the low intrinsic reactivity of the carboxamide group has hampered efforts to achieve this goal. A fundamental challenge is the identification of catalysts that induce amide metathesis, that is, the interconversion of carboxamides based on cleavage and formation of the N-acyl bonds [Eq. (1)]. We recently described metal-catalyzed transamidation reactions [Eq. (2)], [3] which in principle offer a pathway to amide metathesis. Subsequent studies in our lab, however, revealed that metathesis of secondary amides is not successful under the original transamidation conditions.[4] Herein we describe an alternative and mechanistically novel strategy for catalytic amide metathesis that involves imide-mediated transacylation. The results provide a foundation for future efforts to implement amide-based DCC.The only previous example of amide metathesis, to our knowledge, involved the use of proteases under conditions compatible with both peptide hydrolysis and synthesis. [5] Drawbacks associated with these reactions include the limited substrate scope and long reaction times, and these prompted us to pursue the development of small-molecule catalysts for amide metathesis. As initial efforts using transamidation catalysts were unsuccessful, we sought an alternative strategy.We postulated that substoichiometric quantities of an acyclic imide and a Brønsted base (the latter to generate amidate species) might promote acyl-group exchange between secondary amides [Eq. (3)]. Successive reactions of this type should enable equilibrium-controlled metathesis of secondary amides. [6] Initial efforts to promote the metathesis of N-benzylheptanamide and acetanilide [Eq. (4); Bn = benzyl] with the imide N-benzyldiacetamide (5) established the feasibility of the proposed strategy.[7] The effectiveness of several different bases was evaluated by comparing the ratio of the amides 3/1 obtained when the reaction was conducted in both the forward and reverse directions. Reactions that achieve equilibrium produce a 3/1 ratio that is independent of the reaction direction. The most effective bases were found to be NaN(SiMe 3 ) 2 , KN(SiMe 3 ) 2 , and KH ( Figure 1); equilibrium was achieved in all three cases. Significant amide exchange was observed also for MeMgCl, KOtBu, and LiN(SiMe 3 ) 2 . Metal complexes previously shown to promote transamidat...