M. P. Shurygina

Products and mechanisms of photochemical transformations of o-quinones

The photochemical transformations of quinones by the action of light at λ > 500 nm, namely, the photodecarbonylation and photoreduction reactions were studied with the use of a series of o-benzoquinones and 9,10-phenanthrenequinone as examples. The two-stage mechanism of the decarbonylation reaction of o-benzoquinones was established. At the first stage, rearrangement of a photoexcited quinone molecule into a bicyclic compound that spontaneously decomposes in the dark reaction into cyclopentadienone and CO takes place. It has been found that the formation of the photoreduction products of both o-benzoquinones and 9,10-phenanthrenequinone in the presence of various H donors (N,N-dimethylanilines and polymethylbenzenes) follows the same mechanism. In the first step, a phenol ether is produced, which subsequently undergoes quantitative transformation into pyrocatechol or ketol via the heterolytic mechanism. The stability of phenol ethers is determined by the structure and redox properties of the reactants.

The mechanism of photoinduced hydrogen transfer during photoreduction of carbonyl compounds

A model reaction of hydrogen transfer has been proposed that describes well the results of kinetic studies of the photoreduction of o- and p-quinones and fluorenone in the presence of para-substituted N,N-dimethylanilines and polyalkylbenzenes in various solvents.

Optically controlled distribution of o -quinonemethacrylate metal complexes in polymer materials

The complexation ability of photoreduction products of 2-(2,5-di-tert-butyl-3,4-dioxycyclohexa-1,5-dienyl)ethyl methacrylate in tetrahydrofuran (THF) solution and in its copolymer films with dimethacrylate and ethylene glycol monomethacrylate was studied. The product of photochemical reduction – catechol – reacts with (NH4)6Mo7O24·4H2O, NH4VO3 and (NH4)2Fe(SO4)2 with the formation of corresponding metal complexes in solution and in the polymer bulk. The concentration of the molybdenum complexes in the polymer matrix was proportional to the exposure and holding time in solution of molybdate salt. Polymer film with optically controlled distribution of o-quinonemethacrylate molybdenum complexes was prepared. A film containing antimony and molybdenum complexes together in the polymer structure in predetermined ratio was obtained by sequential treatment of irradiated copolymer film (50% conversion) by THF solution of SbPh3, and water solution of (NH4)6Mo7O24·4H2O.

Effect of solvent nature on the photoreduction kinetics of substituted benzoquinones

Effect of the solvent nature on the kinetics of photoreduction of substituted benzoquinones in the presence of hydrogen donors has been studied. It has been found that the effective photoreduction rate constant (kH) for quinones decreases with an increase in solvent polarity. For the 3,6-di-tert-butylbenzoquinone–1,2-N,N-dimethylaniline pair, the dependence of ln kH on the difference of the reciprocals of optical and static solvent permitivities (1/ε ∞–1/ε0) is stepwise with a break point corresponding to CH2Cl2. A similar relationship lnkH = f(1/ε ∞–1/ε0) is observed for the p-chloranil–mesitylene pair. In the study of the photoreduction kinetics for a series of seven o-benzoquinones in the presence of p-derivatives of N,N-dimethylaniline in CH2Cl2, it has been found that the dependence of kH on the free energy of electron transfer (ΔGe) has a maximum for the 3,6-di-tert-butylquinone-1,2–N,N-dimethylaniline pair at ΔGe = 0.11 eV.

Photoreduction of o -benzoquinone moiety in mono- and poly(quinone methacrylate) and on the surface of polymer matrix pores

The kinetics of photoreduction under the action of visible light of the 3,6-di-tert-butyl-o-benzoquinone moiety in methacrylic monomer, polymer, and in the graft layer on the surface of pores in the presence of different N,N-dimethylanilines has been studied. The reaction is accelerated with the increasing electron-donating ability of the amine. In the presence of N,N-dimethylaniline at 50-fold molar excess of the amine, the photoreduction in the porous polymer occurs three and four times faster than in the monomer and the polymer, respectively.

Effect of donor and acceptor properties of solvents on the kinetics of photoreduction of sterically hindered о-benzoquinones

Effect of the solvent nature on the kinetics of photoreduction of 3,6-di-tert-butyl-1,2-benzoquinone and its six derivatives in the presence of N,N-dimethylaniline and 4-(N,N-dimethylamino)benzaldehyde has been investigated. It has been found that for the о-quinone—amine pair, for which the free energy change of electron transfer is ΔGe > +0.11 eV, the rate constant of о-quinone photoreduction kH decreases proportionally to the increase in the acceptor number of the solvent. For the о-quinone—amine pair with ΔGe < +0.11 eV, the kH value decreases proportionally to the increase in the donor number of the solvent. It has been established that the enhancement of the electron-acceptor properties of the solvent leads to the emergence of kinetic isotope effect for the reactant pairs of 3,6-di-tert-butyl-1,2-benzoquinone and 4,5-dimethoxy-3,6-di-tert-butyl-1,2-benzoquinone with N,N-di-methylaniline (ΔGe = +0.11 and +0.22 eV, respectively).

The mechanism of the photochemical transformations of 2,5-di-tert-butyl-6-hydroxy-6-methylcyclohexa-2,4-dienone

When exposed to visible light, 2,5-di-tert-butyl-6-hydroxy-6-methylcyclohexa-2,4-dienone undergoes rearrangements in several steps. The first, photochemical step involves contraction of the six-membered ring to a five-membered cyclic ketol. Its subsequent rearrangements follow a number of parallel pathways. The main pathway involves the formation of an unstable intermediate bicyclo[3.1.0]hexenone derivative, whose further transformation leads to 4-acetylcyclopentenone via opening of the three-membered ring in a deaerated solution or to a bicyclic peroxide in the presence of air. In addition, a parallel bimolecular reaction of the five-membered ketol yields sterically hindered 2-acetylcyclopentenone.

Synthesis of sterically hindered benzoquinone methacrylates

Sterically hindered p- and o-benzoquinone methacrylates, viz., 2,5-di(tert-butyl)-3,6-di-oxocyclohexa-1,4-dienyl methacrylate and 2,5-di(tert-butyl)-3,4-dioxocyclohexa-1,5-dienyl methacrylate, were synthesized by O-acylation of the lithium and tetrabutylammonium salts of 3,6-di(tert-butyl)-2-hydroxy-p-benzoquinone with methacryloyl chloride. The influence of the nature of cation and solvent on regioselectivity of the acylation was studied. The o-benzoquinone derivative obtained can serve as a paramagnetic ligand in metal complexes.