Chromium(III)-Catalyzed C(sp2)-H Alkynylation, Allylation, and Naphthalenation of Secondary Amides with Trimethylaluminum as Base.

Among base metals used for C-H activation reactions, chromium(III) is rather unexplored despite its natural abundance and low toxicity. We report herein chromium(III)-catalyzed C(sp2)-H functionalization of an ortho-position of aromatic and α,ß-unsaturated secondary amides using readily available AlMe3 as a base and using bromoalkynes, allyl bromide, and 1,4-dihydro-1,4-epoxynaphthalene as electrophiles. This redox-neutral reaction taking place at 70-90 °C, requires as low as 1-2 mol % of CrCl3 or Cr(acac)3 as a catalyst without any added ligand, and tolerates functional groups such as aryl iodide, boronate, and thiophene groups. Stoichiometric and kinetics studies as well as kinetic isotope effects suggest that the catalytic cycle consists of a series of thermally stable but reactive intermediates bearing two molecules of the amide substrate on one chromium atom and also that one of these chromate(III) complexes takes part in the alkynylation, allylation, and naphthalenation reactions. The proposed mechanism accounts for the effective suppression of methyl group delivery from AlMe3 for ortho-C-H methylation.

Manganese-Catalyzed Directed Methylation of C(sp2)-H Bonds at 25 °C with High Catalytic Turnover.

We report here a manganese-catalyzed C-H methylation reaction of considerable substrate scope, using MeMgBr, a catalytic amount of MnCl2·2LiCl, and an organic dihalide oxidant. The reaction features ambient temperature, low catalyst loading, typically 1%, high catalytic turnover reaching 5.9 × 103, and no need for an extraneous ligand and illustrates a unique catalytic use of simple manganese salts for C-H activation, which so far has relied on catalysis by manganese carbonyls.