Over‐Oxidation as the Key Step in the Mechanism of the MoCl5‐Mediated Dehydrogenative Coupling of Arenes. Issue 3 (16th October 2015)

• Record Type: Journal Article
• Authors: Schubert, Moritz; Franzmann, Peter; Wünsche von Leupoldt, Anica; Koszinowski, Konrad; Heinze, Katja; Waldvogel, Siegfried R.
• Is Part Of: Angewandte Chemie international edition. Volume 55:Issue 3(2016)
• Page Start: 1156
• Page End: 1159
• Publication Date: 2015-10-16
• Subjects: arene coupling -- C−C coupling -- cyclization -- molybdenum -- oxidative coupling; Chemistry -- Periodicals; 540
• DOI: 10.1002/anie.201508035
• Languages: English
• ISSNs: 1433-7851

Abstract

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. Abstract : Over and out : Oxidation of arenes in the presence of MoCl5 does not stop at the biaryl but rather forms the over‐oxidized product which is protected from side reactions. In the presence of additional Lewis acids, MoCl5 acts as a two‐electron oxidant and the molybdenum containing "waste" serves as the reducing agent upon aqueous workup.