Ilya M. Magin

CIDNP study of the third spin effect on the singlet–triplet evolution in radical pairs

Abstract The time-resolved 1 H photo-CIDNP formed upon photocleavage of diisopropylketone in the presence and absence of stable radical (TEMPO) and dodecanetiol-1 as radical traps has been studied. It was found that TEMPO affects CIDNP formation in a primary radical pair not only as a radical acceptor but also through spin exchange. The rate constant of the recombination of isopropyl radicals and TEMPO in acetonitrile at ambient temperature was determined as (2.4±0.5)×10 8 M −1 s −1 .

Modeling magnetic field effects in multispin systems

A model is proposed for calculating magnetic field effects formed in a radical triad composed of a biradical and a paramagnetic particle. To describe the influence of the “third” spin on the spin evolution in a biradical, the electron spin exchange interaction of the added spin with one of the paramagnetic centers of the biradical has been considered. Calculating the field dependence of the recombination probability of the biradical-oxygen complex revealed both an increase in recombination probability earlier attributed to spin catalysis and the influence of the values and signs of the exchange interaction in the complex on the shape of the magnetic-field effect dependence. Calculation results are in agreement with the experimental data on the photolysis of 7,7’-dimethyl-silanorbornadiene in aerated and deaerated solution.

Time-Resolved Fluorescence Study of Exciplex Formation in Diastereomeric Naproxen−Pyrrolidine Dyads

The influence of chirality on the elementary processes triggered by excitation of the (S,S)- and (R,S)- diastereoisomers of naproxen-pyrrolidine (NPX-Pyr) dyads has been studied by time-resolved fluorescence in acetonitrile-benzene mixtures. In these systems, the quenching of the (1)NPX*-Pyr singlet excited state occurs through electron transfer and exciplex formation. Fluorescence lifetimes and quantum yields revealed a significant difference (around 20%) between the (S,S)- and (R,S)- diastereomers. In addition, the quantum yields of exciplexes differed by a factor of 2 regardless of solvent polarity. This allows us to suggest a similar influence of the chiral centers on the local charge transfer resulting in exciplex and full charge separation that leads to ion-biradicals. A simplified scheme is proposed to estimate a set of rate constant values (k1-k5) for the elementary stages in each solvent system.

The mechanism of oxidation of NADH analogues 5: Photooxidation of N-methyl substituted 1,4- and 1,2-dihydropyrimidines in the presence of quinones

Abstract The photoinduced interaction of the 1-methyl-4-phenyl-5-carbomethoxy-6-methyl-1,4(1,2)-dihydropyrimidines (1,4- and 1,2-DHPm) with quinones has been studied by means of CIDNP method in polar media. It was established that 1-methyl-4-phenyl-5-carboxymethyl-6-methylene-1,6-dihydropyrimidine (1,6-MDHPm) is a main reaction product. It has been found that radical cation of DHPm is more reactive species than earlier studied radical cations of related N -methyl substituted dihydropyridines and is disposed to proton loss resulting in neutral pyrimidinyl radical. The 1,6-MDHPm was formed by hydrogen atom loss from 6-CH 3 group of pyrimidinyl radical. Oxidation of the pyrimidinyl radical to pyrimidinium cation was not detected. Formation of pyrimidinium cation was detected in the presence of a high concentration of acetic acid in the reaction mixture.

Spin Selectivity in Chiral Linked Systems

This work has shown spin selectivity in electron transfer (ET) of diastereomers of (R,S)-naproxen-(S)-N-methylpyrrolidine and (R,S)-naproxen-(S)-tryptophan dyads. Photoinduced ET in these dyads is interesting because of the still unexplained phenomenon of stereoselectivity in the drug activity of enantiomers. The chemically induced dynamic nuclear polarization (CIDNP) enhancement coefficients of (R,S)-diastereomers are double those of the (S,S)-analogue. These facts are also interesting because spin effects are among the most sensitive, even to small changes in spin and molecular dynamics of paramagnetic particles. Therefore, CIDNP reflects the difference in magnetoresonance parameters (hyperfine interaction constants (HFIs), g-factor difference) and lifetimes of the paramagnetic forms of (R,S)- and (S,S)-diastereomers. The difference in HFI values for diastereomers has been confirmed by a comparison of CIDNP experimental enhancement coefficients with those calculated. Additionally, the dependence of the CIDNP enhancement coefficients on diastereomer concentration has been observed for the naproxen-N-methylpyrrolidine dyad. This has been explained by the participation of ET in homo-(R,S-R,S or S,S-S,S) and hetero-(R,S-S,S) dimers of dyads. In this case, the effectivity of ET, and consequently, CIDNP, is supposed to be different for (R,S)- and (S,S)-homodimers, heterodimers, and monomers. The possibility of dyad dimer formation has been demonstrated by using high-resolution X-ray and NMR spectroscopy techniques.

Role of association in chiral catalysis: from asymmetric synthesis to spin selectivity

The origin of biomolecules in the pre-biological period is still a matter of debate, as is the unclarified nature of the differences in enantiomer properties, especially for the medically important activity of chiral drugs. With regards to the first issue, significant progress was made in the last decade of the 20th century through experimental confirmation of Franks popular theory on chiral catalysis in spontaneous asymmetric synthesis. Soai examined the chiral catalysis of the alkylation of achiral aldehydes by achiral reagents. Attempts to model this process demonstrated the key role of chiral compounds associates as templates for chiral synthesis. However, the elementary mechanism of alkylation and the role of free radicals in this process are still incompletely understood. Meanwhile, the influence of external magnetic fields on chiral enrichment in the radical path of alkylation has been predicted. In addition, the role of chiral dyad association in another radical process, electron transfer (ET), has been recently demonstrated by the following methods: chemically induced dynamic nuclear polarisation (CIDNP), NMR spectroscopy, XRD and photochemistry. The CIDNP analysis of ET in two dyads has revealed a phenomenon first observed for chiral systems, spin selectivity, which results in the difference between the CIDNP enhancement coefficients of dyad diastereomers. These dyads are linked systems consisting of the widespread drug (S)-naproxen (NPX) or its R analogue and electron donors, namely, (S)-tryptophan and (S)-N-methylpyrrolidine. Because NPX is one of the most striking examples of the difference in the therapeutic properties of enantiomers, the appearance of spin selectivity in dyads with (S)- and (R)-NPX and S donors can shed light on the chemical nature of these differences. This review is devoted to discussing the chemical nature of spin selectivity and the role of chiral associates in the chiral catalysis of an elementary radical reaction: ET in chiral dyads.