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 CNPrⁱ 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^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 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.

Keywords: C−H activation; low oxidation state; magnesium(I); photochemical; radical.

Scheme 1

C−H activation of arenes by a photochemically activated β‐diketiminate ligated dimagnesium(I) compound (Dip=2,6‐diisopropylphenyl).

Scheme 2

Arene C−H activation products derived from UV irradiation of solutions of 3 in toluene, xylenes, or mesitylene. The ratios of the isomers obtained are shown, and were derived from I₂/THF quenching of the total reaction mixtures.

Scheme 3

Synthesis of compound 10.

Figure 1

Molecular structure of 4 (20 % thermal ellipsoids are shown; hydrogen atoms, except the hydride, omitted; Dip groups shown as wireframe for clarity). Selected bond lengths (Å) and angles (°): Mg(1)−N(3) 2.0428(10), Mg(1)−N(1) 2.0593(10), Mg(1)−C(1) 2.2830(12), Mg(1)−H(1) 1.779(16), C(1)−Mg(2) 2.2335(12), Mg(2)−N(6) 2.0214(10), Mg(2)−N(4) 2.0616(10), Mg(2)−H(1) 1.830(16), N(3)−Mg(1)−N(1) 66.02(4), C(1)−Mg(1)‐H(1) 89.8(5), N(6)−Mg(2)−N(4) 66.18(4), C(1)−Mg(2)−H(1) 90.1(5).

Figure 2

Molecular structure of 5 (3‐Me) (20 % thermal ellipsoids are shown; hydrogen atoms, except the hydride, omitted; Dip groups shown as wireframe for clarity). Selected bond lengths (Å) and angles (°): Mg(1)−N(2) 2.0220(15), Mg(1)−N(1) 2.0625(14), Mg(1)−C(101) 2.212(4), Mg(1)−H 1.85(2), Mg(2)−N(4) 2.0419(14), Mg(2)−N(5) 2.0543(14), Mg(2)−C(101) 2.266(4), Mg(2)−H 1.83(2), N(2)−Mg(1)−N(1) 66.18(6), C(101)−Mg(1)−H 92.3(7), N(4)−Mg(2)−N(5) 66.02(6), C(101)−Mg(2)−H 91.3(7).

Figure 3

Molecular structure of 10 (20 % thermal ellipsoids are shown; hydrogen atoms, except H(21), omitted; Dip groups shown as wireframe for clarity). Selected bond lengths (Å) and angles (°): Mg(1)−N(3) 2.046(3), Mg(1)−N(4) 2.0741(18), Mg(1)−N(2) 2.165(2), Mg(1)−N(6) 2.2234(16), Mg(1)−N(1) 2.268(2), C(20)−C(21) 1.424(4), C(21)−C(22) 1.444(4), N(3)−Mg(1)−N(2) 102.12(9), N(4)−Mg(1)−N(6) 62.27(6), N(3)−Mg(1)−N(1) 96.14(10).

Figure 4

Computed Gibbs free energy profile for the reaction of [{(Priso)Mg}₂] 3 with benzene. Energies are presented as kcal/mol.

Scheme 4

Synthesis of compound 12.

Figure 5

Molecular structure of 12 (20 % thermal ellipsoids are shown; hydrogen atoms, except methylene protons, omitted; Dip groups shown as wireframe for clarity). Selected bond lengths (Å) and angles (°): Mg(1)−N(1) 2.0326(16), Mg(1)−N(2) 2.0575(16), Mg(1)−C(25)’ 2.2265(19), Mg(1)−C(25) 2.304(2), N(1)−Mg(1)−N(2) 66.14(6), C(25)’−Mg(1)−C(25) 108.10(6).

Scheme 5

Synthesis of compound 13.

Figure 6

Stack plot of ¹H NMR spectra of a 1 : 1 mixture of 3 and [{(^DipNacnac)Mg}₂] (A) in C₆D₆ which has been irradiated by blue light (λ=456 nm) for the time shown on each spectrum (*=signal for C₆D₅H; #=residual toluene).

Figure 7

Molecular structure of 13 (20 % thermal ellipsoids are shown; hydrogen atoms omitted; Dip groups shown as wireframe for clarity). Selected bond lengths (Å) and angles (°): Mg(1)−N(2) 2.0365(11), Mg(1)−N(1) 2.0436(12), Mg(1)−Mg(2) 2.7935(6), Mg(2)−N(4) 2.0586(11), Mg(2)−N(5) 2.0692(11), N(2)−Mg(1)−N(1) 93.49(5), N(4)−Mg(2)−N(5) 65.46(4).

Figure 8

EPR spectrum of a solution of 3 in a 3/1 mixture of 2,2‐dimethylbutane and tert‐butylbenzene, which has been irradiated for 1 h with UV light (λ=335 nm) at 77 K.