Iron-Catalyzed C–H Activation for Heterocoupling and Copolymerization of Thiophenes with Enamines

C–H/C–H coupling via C–H activation provides straightforward synthetic access to the construction of complex π-conjugated organic molecules. The palladium-catalyzed Fujiwara–Moritani (FM) coupling between an arene and an electron-deficient olefin presents an early example but is not applicable to enamines such as N-vinylcarbazoles and N-vinylindoles. We report herein iron-catalyzed C–H/C–H heterocoupling between enamines and thiophenes and its application to copolymerization of bisenamine and bisthiophene using diethyl oxalate as an oxidant and AlMe3 as a base, as a result of our realization that synthetic limitations in oxidative C–H/C–H couplings imposed by the high redox potential of the Pd(II)/Pd(0) catalytic cycle can be circumvented by the use of iron, which has a lower Fe(III)/Fe(I) redox potential. The trisphosphine ligand provides a coordination environment for iron to achieve the reaction’s regio-, stereo-, and chemoselectivity. The reaction includes C–H activation of thiophene via σ-bond metathesis and subsequent enamine C–H cleavage triggered by nucleophilic enamine addition to the Fe(III) center, thereby differing from the FM reaction in mechanism and synthetic scope. The copolymers synthesized by the new reaction possess a new type of enamine-incorporated polymer backbone

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,β-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