Stable [M₂F₁₁]^– (M = Nb, Ta) Salts of Protonated 1,3‐Dimethoxybenzene

Abstract

The 3:1 molar reactions of MF 5 (M = Nb, 1a; M = Ta, 1b) with 1,3-dimethoxybenzene in CHCl 3 led to the formation of the arenium salts [2,4-(OMe) 2 C 6 H 5 ][M 2 F 11 ] (M = Nb, 3a; M = Ta, 3b) in good yields. The reactions took place with the intermediate formation of (MF 5 ) 2 [κ 2 -1,3-(OMe) 2 C 6 H 4 ] (M = Nb, 2a; M = Ta, 2b), followed by single-electron oxidation of the arene and production of MF 4 (identified by means of EPR spectroscopy as the acetonitrile adduct). Compounds 3a and[a]

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

“…In previous work, we obtained clear evidence that the dehydrogenative arene coupling starts with an inner-sphere single-electron transfer (SET) forming aradical cation [Ar] + C and MoCl 5 À .T his finding alone does not confirm that the radical-cation pathway is followed, however, because the initially formed radical cation could also abstract ahydrogen atom from the solvent and, thus,e nter the arenium-cation pathway. [5,6] To gain further insight into the reaction mechanism, we performed squarewave voltammetric studies,e lectrospray ionization-mass spectrometry (ESI-MS), and electron paramagnetic resonance (EPR) experiments of reaction mixtures of substrates 1a-c with MoCl 5 in dichloromethane (Scheme 2), as well as density functional theory (DFT) studies.T he combined results clearly support that the expected products 3a-c are further oxidized to their respective radical cations 3a + C-c + C. [7,8] Thee lectrochemical experiments indicate that the products 3a-c are indeed more easily oxidized to the radical cations 3a + C-c + C than the respective starting arenes 1a-c,a se xpected (Table 1). Hence,M o V (or even Mo IV )s hould be able to oxidize 3a-c to 3a + C-c + C.…”

“…[5,6] To gain further insight into the reaction mechanism, we performed squarewave voltammetric studies,e lectrospray ionization-mass spectrometry (ESI-MS), and electron paramagnetic resonance (EPR) experiments of reaction mixtures of substrates 1a-c with MoCl 5 in dichloromethane Scheme 1. Arenium-cation mechanism (a) and radical cation mechanism (b).…”

Überoxidation als Schlüsselschritt im Mechanismus der MoCl5‐ vermittelten dehydrierenden Arenkupplung

“…Dieser Befund belegt aber keinen Radikalmechanismus, da das zunächst gebildete Intermediat ein Wasserstoffatom z. B. vom Lösungsmittel abstrahieren und anschließend den Weg des Areniummechanismus einschlagen könnte …”

MoV Reagents in Organic Synthesis

“…In the course of our mechanistic studies, we were able to show that the oxidative coupling of aryl ethers is initiated by an inner‐sphere single‐electron transfer (SET) resulting in a radical cation [Ar] + · and MoCl 5 – 25,34,35. However, this observation alone does not confirm that the radical‐cation pathway takes place, because Pampaloni and colleagues reported that the initially formed radical cation could also abstract a hydrogen atom from the solvent and, thereby, enter the arenium‐cation pathway 36. To gain further insight into the course of the reaction, we performed electrospray ionization‐mass spectrometry (ESI‐MS), electron paramagnetic resonance (EPR) experiments and squarewave voltammetric (SWV) studies of several reaction mixtures of MoCl 5 with a variety of different substrates 34,35…”