Cp*Co(III)-Catalyzed C–H Alkylation with Maleimides Using Weakly Coordinating Carbonyl Directing Groups

Abstract

A novel protocol for ortho-C-H alkylation of aromatic and heteroaromatic ketones and esters under Cp*Co(III) catalysis has been developed for the first time. The reaction proceeds through initial cyclometalation via weak chelation-assisted C-H bond activation, followed by coordination of activated alkene, insertion between Co-C, and protodemetalation.

Weakly Coordinated Cobaltacycles: Trapping Catalytically Competent Intermediates in Cp*CoIIICatalysis

“…We therefore sought to utilize 3 ket ‐I , 3 CONHtBu ‐I , and 3 acetanilide ‐I as a platform for determining the intermediacy of these species in representative catalytic reactions. Importantly, these directing groups have been utilized in Cp*Co‐catalyzed C−H functionalization reactions . As shown in Figure , these cyclometalated complexes proved to be suitable catalysts for representative C−C coupling reactions, thus supporting the feasibility of the corresponding cationic intermediates in these transformations…”

“…After demonstrating the accessibility and stability of various Co III complexes supported by catalytically relevant scaffolds, we next pursued detection of the direct analogues of the reactive species formed after the C−H metalation step (Figure ) –. In this context, we have previously described the employment of coordinating ligands to access otherwise highly reactive cationic metallacyclic Cp*Co III species.…”

Rh(III)‐Catalyzed [4+2] Cyclization of 2‐Aryl‐1H‐benzo[d]imidazoles with Maleimides via C‐H Activation

“…[15] Maleimides 2 f, 2 g, 2 h were prepared according to published procedures. [16] 2-Aryl-1H-phenanthro [9,10-d] imidazoles 6 a-6 c were prepared according to published procedures. [14] General procedure for the synthesis of compound 3, 4 or 6…”

Cp*Co(III)‐Catalyzed C−H Alkenylation of Aromatic Ketones with Alkenes

“…Furthermore, the low value of kinetic isotope effect (KIE) from both the intermolecular competition experiment ( k H / k D =1.9) and parallel experiment ( k H / k D =1.3) indicate that the C−H bond cleavage is probably not involved in the rate‐limiting step (Scheme d) –. To eliminate the possibility of alkylation followed by subsequent oxidation to the corresponding alkene, first we prepared the alkylated product 12 from 3 h by hydrogenation, and then subjected to the standard reaction conditions. The absence of any alkenylated product 3 h in the reaction mixture further support the β ‐hydride elimination pathway (Scheme e).…”