C−H Activation of Inert Arenes using a Photochemically Activated Guanidinato‐Magnesium(I) Compound. Issue 65 (26th September 2022)

• Record Type: Journal Article
• Authors: Mullins, Jeremy C.; Yuvaraj, K.; Jiang, Yixiao; Van Trieste, Gerard P.; Maity, Asim; Powers, David C.; Jones, Cameron
• Is Part Of: Chemistry. Volume 28:Issue 65(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-09-26
• Subjects: C−H activation -- low oxidation state -- magnesium(I) -- photochemical -- radical; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.202202103
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: UV irradiation of solutions of a guanidinate coordinated dimagnesium(I) compound, [{(Priso)Mg}2 ] 3 (Priso=[(DipN)2 CNPr i 2 ] −, Dip=2, 6‐diisopropylphenyl), in either benzene, toluene, the three isomers of xylene, or mesitylene, leads to facile activation of an aromatic C−H bond of the solvent in all cases, and formation of aryl/hydride bridged magnesium(II) products, [{(Priso)Mg}2 (μ‐H)(μ‐Ar)] 4 –9 . In contrast to similar reactions reported for β‐diketiminate coordinated counterparts of 3, these C−H activations proceed with little regioselectivity, though they are considerably faster. Reaction of 3 with an excess of the pyridine, p ‐NC5 H4 Bu t (py Bu t ), gave [(Priso)Mg(py Bu t H)(py Bu t )2 ] 10, presumably via reduction of the pyridine to yield a radical intermediate, [(Priso)Mg(py Bu t ⋅)(py Bu t )2 ] 11, which then abstracts a proton from the reaction solvent or a reactant. DFT calculations suggest two possible pathways to the observed arene C−H activations. One of these involves photochemical cleavage of the Mg−Mg bond of 3, generating magnesium(I) doublet radicals, (Priso)Mg⋅. These then doubly reduce the arene substrate to give "Birch‐like" products, which subsequently rearrange via C−H activation of the arene. Circumstantial evidence for the photochemical generation of transient magnesium radical species includes the fact that irradiation of a cyclohexane solution of 3 leads to an intramolecular aliphatic C−H activation process and formation of an alkyl‐bridged magnesium(II) species, [{Mg(μ‐Priso −H )}2 ] 12 . Furthermore, irradiation of a 1 : 1 mixture of 3 and the β‐diketiminato dimagnesium(I) compound, [{( Dip Nacnac)Mg}2 ] ( Dip Nacnac=[HC(MeCNDip)2 ] − ), effects a "scrambling" reaction, and the near quantitative formation of an unsymmetrical dimagnesium(I) compound, [(Priso)Mg−Mg( Dip Nacnac)] 13 . Finally, the EPR spectrum (77 K) of a glassed solution of UV irradiated 3 is dominated by a broad featureless signal, indicating the presence of a doublet radical species. Abstract : A radical approach . Activations of arene C−H bonds by a photochemically excited dimagnesium(I) compound are reported (see picture, Dip=2, 6‐diisopropylphenyl). Experimental and theoretical evidence for the intermediacy of magnesium radicals in these reactions is presented. Such transition‐metal‐free C−H bond activations hold considerable potential for organic synthesis.