Over‐Oxidation as the Key Step in the Mechanism of the MoCl₅‐Mediated Dehydrogenative Coupling of Arenes

Abstract

Molybdenum pentachloride is an unusually powerful reagent for the dehydrogenative coupling of arenes. Owing to the high reaction rate using MoCl5, several labile moieties are tolerated in this transformation. The mechanistic course of the reaction was controversially discussed although indications for a single electron transfer as the initial step were found recently. Herein, based on a combined study including synthetic investigations, electrochemical measurements, EPR spectroscopy, DFT calculations, and mass spectrometry, we deduct a highly consistent mechanistic scenario: MoCl5 acts as a one-electron oxidant in the absence of TiCl4 and as two-electron oxidant in the presence of TiCl4, but leads to an over-oxidized intermediate in both cases, which protects it from side reactions. In the course of aqueous work-up the reagent waste (Mo(III/IV) species) acts as reducing agent generating the desired organic C-C coupling product.

Electrifying Organic Synthesis

“…[98] Nevertheless,t hese investigations mostly comprised uncontrolled electrolysis reactions of arenes and in particular phenols,l eading to unpredictable product mixtures.T herefore,t his Section will focus on as election of reports,s ummarizing strategies to achieve selective CÀHa ctivation reactions of arenes.G eneral problems for twofold anodic C À Hactivation include the formation of diverse product mixtures because of unselective oxidation processes,t he decomposition of oxidized intermediates,a nd high tendencies towards side reactions.F urthermore,t he formed biaryls can have lower oxidation potentials than the starting materials,m aking them prone to overoxidation. [99] Several strategies have been investigated to overcome these problems.…”

Selective Synthesis of Partially Protected Nonsymmetric Biphenols by Reagent‐ and Metal‐Free Anodic Cross‐Coupling Reaction

“…The yields achieved for the desired crosscoupling products are exceptionally high. [31] In addition, both aryl moieties are still electron rich and experience a strong solvation by HFIP. The rationale for the improved yields is a combination of two complementary effects: The installation of a bulky silyl group on component B leads to a strong twist of the biaryl axis.…”

“…Consequently, the aryl moiety serves as an electron-withdrawing group generating a less extended p-system in the final product which is therefore less prone to over-oxidation. [31] In addition, both aryl moieties are still electron rich and experience a strong solvation by HFIP. Therefore, these molecular entities exhibit a strongly reduced nucleophilicity, preventing participation in subsequent dehydrogenative couplings.…”

Allowing the direct interaction of N-aryl α-diimines with a high valent metal chloride: one-pot WCl6-promoted formation of quinoxalinium salts

“…However, all the described reactions have been performed with prior reduction, by one or two electrons, of the metal centre. This strategy quenches the activation ability of the strongly acidic metal chlorides, 13,14,15 so to straightforwardly afford coordination compounds with intact, redox active α-diimine ligands. A similar outcome has been achieved by allowing TiX 4 (X = Cl, Br) 2d,16 and MCl (M = 40 Nb, Ta) 16a,17 to react with dilithium(1,4-diaza-1,3-dienyl) salts.…”