UV irradiation of solutions of a guanidinate coordinated dimagnesium(I) compound, [{(Priso)Mg} 2 ] 3 (Priso = [(DipN) 2 CNPr i 2 ] À , Dip = 2,6-diisopropylphenyl), in either benzene, toluene, the three isomers of xylene, or mesitylene, leads to facile activation of an aromatic CÀ 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 CÀ H activations proceed with little regioselectivity, though they are considerably faster. Reaction of 3 with an excess of the pyridine, p-NC 5 H 4 Bu t (py But ), gave [(Priso)Mg(py But H)(py But ) 2 ] 10, presumably via reduction of the pyridine to yield a radical intermediate, [(Priso)Mg(py But* )(py But ) 2 ] 11, which then abstracts a proton from the reaction solvent or a reactant. DFT calculations suggest two possible pathways to the observed arene CÀ H activations. One of these involves photochemical cleavage of the MgÀ 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 CÀ 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 CÀ H activation process and formation of an alkylbridged magnesium(II) species, [{Mg(μ-Priso À 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 ] À ), effects a "scrambling" reaction, and the near quantitative formation of an unsymmetrical dimagnesium(I) compound, [(Priso)MgÀ 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.
<div> <div> <div> <p>Correlation of catalyst structure with activity is foundational to the rational design of transition metal catalysts. While X-ray crystallography routinely provides structural characterization of kinetically stable pre-catalysts and intermediates, experimental elucidation of the structures of reactive intermediates, which are the species intimately engaged in bond-breaking and -making in catalysis, is generally not possible due to the transient nature of these species. Here, we demonstrate in crystallo synthesis of Rh2 nitrenes that participate in catalytic C–H amination, and characterization of these transient intermediates as triplet adducts of Rh2. Further, we observe the impact of coordinating substrate, which is present in excess during catalysis, on the structure of transient Rh2 nitrenes involved in C–H amination. By providing structural characterization of authentic C–H functionalization intermediates, and not kinetically stabilized model complexes, these experiments provide the opportunity to define critical structure-activity relationships. </p> </div> </div> </div>
<div> <div> <div> <p>Correlation of catalyst structure with activity is foundational to the rational design of transition metal catalysts. While X-ray crystallography routinely provides structural characterization of kinetically stable pre-catalysts and intermediates, experimental elucidation of the structures of reactive intermediates, which are the species intimately engaged in bond-breaking and -making in catalysis, is generally not possible due to the transient nature of these species. Here, we demonstrate in crystallo synthesis of Rh2 nitrenes that participate in catalytic C–H amination, and characterization of these transient intermediates as triplet adducts of Rh2. Further, we observe the impact of coordinating substrate, which is present in excess during catalysis, on the structure of transient Rh2 nitrenes involved in C–H amination. By providing structural characterization of authentic C–H functionalization intermediates, and not kinetically stabilized model complexes, these experiments provide the opportunity to define critical structure-activity relationships. </p> </div> </div> </div>
Small molecule redox mediators convey interfacial electron transfer events into bulk solution and can enable diverse substrate activation mechanisms in synthetic electrocatalysis. Here we report that 1,2-diiodo-4,5-dimethoxybenzene (1a) is an efficient electrocatalyst for C-H/E-H coupling that operates at as low as 0.5 mol% catalyst loading. Spectroscopic, crystallographic, and computational results indicate a critical role for a three-electron I-I bonding interaction in stabilizing an iodanyl radical intermediate (i.e., formally I(II) species). As a result, 1a operates at more than 100 mV lower potential than related monoiodide catalysts, which results in improved product yield, higher Faradaic efficiency, and expanded substrate scope. The isolated iodanyl radical is chemically competent in C-N bond formation. These results represent the first examples of substrate functionalization at a well-defined I(II) derivative and bona fide iodanyl radical ca-talysis and demonstrate one-electron pathways as a mechanistic alternative to canonical two-electron hypervalent iodine mechanisms. The observation establishes I-I redox cooperation as a new design concept for the development of metal-free redox mediators.
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