Mn-Catalyzed Aromatic C–H Alkenylation with Terminal Alkynes

Abstract

The first manganese-catalyzed aromatic C-H alkenylation with terminal alkynes is described. The procedure features an operationally simple catalyst system containing commercially available MnBr(CO)(5) and dicyclohexylamine (Cy(2)NH). The reaction occurs readily in a highly chemo-, regio-, and stereoselective manner delivering anti-Markovnikov E-configured olefins in high yields. Experimental study and DFT calculations reveal that (1) the reaction is initiated by a C-H activation step via the cooperation of manganese and base; (2) manganacycle and alkynylmanganese species are the key reaction intermediates; and (3) the ligand-to-ligand H-transfer and alkynyl-assisted C-H activation are the key steps rendering the reaction catalytic in manganese.

Manganese-Mediated C-C Bond Formation via C-H Activation: From Stoichiometry to Catalysis

“…22 DFT calculations further demonstrated that, through the cooperation of MnBr(CO) 5 and Cy 2 NH, the C-H bond cleavage only needed to overcome a barrier of 12.5 kcal/mol, which are in good line with the experimental results. We believed that the C-H activation proceeded via a base-assisted deprotonation mechanism, which was further confirmed by deuterium-labeling experiments.…”

“…Though manganese-mediated stoichiometric reactions of arenes bearing auxiliary groups with alkynes have been reported by Liebeskind, Nicholson, Woodgate, and others since 1989, 12-17 manganese-catalyzed aromatic C-H alkenylation with alkynes remained elusive before our work (Scheme 14). 22 We found that 2-phenylpyridines reacted smoothly with terminal alkynes affording E-alkenylated product 61 in the presence of catalytic amounts of MnBr(CO) 5 and dicyclohexylamine (Cy 2 NH). This reaction is noteworthy in the following aspects: First, the alkenylation process exhibited a high chemo-, regio-, and stereoselectivity with neither dialkenylated products nor Markovnikov adducts detected in the reactions, while only a small amount of Z-isomeric products were detected in some cases.…”

“…24 Finally, a closed catalytic cycle for manganese was established by experimental study and DFT calculations. 22 It is widely accepted that five-membered manganacycle 62 can undergo insertion of an alkyne to afford seven-membered manganacycle 64 via the intermediacy of manganese-alkyne complex 63 (Scheme 16, right). Nevertheless, the ensuing transformation of 64 to the alkenylated product 61 and regenerating the active manganese species to enter into the next catalytic cycles are unprecedented.…”

Substrate‐Directed C‐H Functionalization of 2‐Aryl Pyridines by Transition Metal Complexes

“…In this reaction, delivering anti-Markovnikov E-coffingured olefins (43) in good yields (Scheme 20). [35] In 2014, Kakiuchi et alreported alkenylation reactions of 2-aryl pyridines with the alkenyl esters using Ru(cod)(cot) as a catalyst. [36] The reaction was performed on various 2-aryl pyridi < nes (3) and related aromatic compounds with a variety of alkenyl esters (44) including 1-or 2-monosubstituted and 1,2-or 2,2-disubsituted vinyl esters to obtain the alkenylated products (45) (Scheme 21).…”

Manganese‐Catalysed C−H Activation: A Regioselective C−H Alkenylation of Indoles and other (hetero)aromatics with 4‐Hydroxy‐2‐Alkynoates Leading to Concomitant Lactonization

“…[3] Among these catalysts, Mn-catalysts in CÀ H functionalization reactions have received momentum in recent days. [7] Later, several groups have independently employed terminal alkynes as well as internal alkynes as coupling partners with various substituted aromatics. [7] Later, several groups have independently employed terminal alkynes as well as internal alkynes as coupling partners with various substituted aromatics.…”