C−H Activation of Inert Arenes using a Photochemically Activated Guanidinato‐Magnesium(I) Compound

Abstract

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.

Diverse Reactivity of a Ca(I) Synthon

“…As one of the Mg centers is still easily accessible for substrate binding, II reacts readily with CO. Likewise, polarization of the Mg−Mg bond can be realized by coordination of two different ligands but this concept so far has been hardly explored [4,5] …”

“…Thus preventing immediate Mg−Mg coupling, the highly reactive radical can reduce benzene. Jones and coworkers showed that the reactivity of I or related Mg−Mg bound complexes can also be increased by irradiation with UV or blue light resulting in bond cleavage and radical formation [4,10] . Such highly reactive low‐valent (BDI)Mg⋅ radical species were also obtained in the form of a deep purple powder by ball‐milling in the solid state [11] .…”

“…Likewise, polarization of the MgÀ Mg bond can be realized by coordination of two different ligands but this concept so far has been hardly explored. [4,5] Activation of Mg I complexes may also be achieved by breaking the MgÀ Mg bond which would result in formation of highly reactive metal-centered radicals. Attempts to break the MgÀ Mg bond using bulky β-diketiminate (BDI) ligands like DIPeP BDI or DIPeP BDI* led either to bond elongation (III) [6] or MgÀ N bond cleavage (IV); [7] ( DIPeP BDI = HC[C(Me)N-DIPeP] 2 , DIPeP BDI* = HC[C(tBu)N-DIPeP] 2 , DIPeP = 2,6-C(H)Et 2 -phenyl).…”

C-4 Regioselective Alkylation of Pyridines Driven by Mechanochemically Activated Magnesium Metal

“…Our recent work showed that alkyl radicals could be derived from the mechanochemical interaction of alkyl halides with freshly generated magnesium surfaces to accomplish Minisci-type alkylation of pyrimidine derivatives . In line with this work, and inspired by the elegant studies of Wakefield and Jones that showed the formation of a transient pyridine radical species from the reduction of pyridine by a low-valent dimagnesium­(I) complex (Scheme c), we envisioned that a halogenide radical XMg• (X = Cl, Br, or I) and/or dimagnesium­(I) complex might be concurrently formed with alkyl radicals by milling of the Mg(0) and alkyl halides . The resulting magnesium­(I) species could reduce pyridine to form the C-4 pyridine radical, thus allowing radical–radical coupling with alkyl radicals to access 4-alkylpyridines regioselectively.…”

Diverse Reactions of α‐Diimine‐Ligated MgI−MgI‐Bonded Compound with Carbodiimides

“…Moreover, the dimagnesium(I) species could also promote regioselective photochemical activation of the aryl CÀ H bond of toluene and xylenes through radical mechanisms. [21] Although the crystal data for compound 5 is not good enough and there is severe disorder of the solvent molecules, the preliminary structure can be unambiguously obtained (Figure S15). NMR spectra also indicate the presence of the ortho-dehydrogenated toluene at 2.29, 2.30, 7.00-7.25 ppm (for 1 H) and 21.4, 126.5-141.9 ppm for 13 C NMR (Figure S5, S6).…”