Nuclear magnetic resonance and UV spectral probe of photochemical steady states of vitamin K1

Intramolecular Photoredox Reaction Mechanism of Naphthoquinone Compounds: Combined Time-Resolved Spectroscopies and DFT Calculations

Time-resolved spectroscopies and DFT calculations were utilized to investigate the photoredox mechanisms of naphthoquinone compounds. 5-Methoxy-8-tetrahydropyrane-1,4-naphthoquinone (NQ) and 2-methyl-3-(3-methylbut-2-en-1-yl) 1,4-naphthoquinone (MNQ) were excited to singlet excited species (labeled NQ(S₁) and MNQ(S₁), respectively). NQ(S₁) underwent intersystem crossing to produce a triplet NQ, which further underwent hydrogen atom transfer to form a biradical intermediate. The biradical underwent electron transfer to form a zwitterion, followed by cyclization and proton transfer to generate a photoproduct. MNQ(S₁) underwent a 1,4-proton transfer process to produce a quinone methide intermediate (1,3-QM) with zwitterionic character, which tautomerized to 1,2-QM. Then, 1,2-QM underwent electrocyclization. The substituent on the parent naphthoquinone is the key factor leading to the different reaction processes for NQ and MNQ.

Photoprocesses of p-naphthoquinones and vitamin K(1): effects of alcohols and amines on the reactivity in solution

The photochemistry of 1,4-naphthoquinone (NQ), the 2-methyl, 2,3-dichloro and 2-bromo derivatives, and vitamin K(1) was studied in non-aqueous solvents by time-resolved UV-vis spectroscopy after ns laser pulses at 248 and 308 nm. The triplet state of the NQs reacts with alcohols and amines, e.g. triethylamine (TEA) and DABCO, yielding semiquinone radicals (HQ(*)/Q (*)(-)). They are the major intermediates and their second-order decay kinetics depend on the properties of the additives and the medium. Transient conductivity measurements suggest the occurrence of photoinduced electron transfer from amines to the triplet state of NQs in acetonitrile. The photoconversion lambda (irr)= 254 nm) of NQs to the 1,4-dihydroxynaphthalenes (H(2)Q) was measured in the absence and presence of varying concentrations of electron and H-atom donors, and the quantum yield was found to increase with increasing electron- or proton-donor concentration. The mechanisms of photoreduction of NQs by propan-2-ol and TEA in acetonitrile exhibit a number of similarities. Oxygen quenches the triplet state, thereby forming singlet molecular oxygen. Oxygen also reacts with the semiquinone radical, thereby forming HO(2)(*)/O(2) (*) (-) radicals, and reacts with H(2)Q, thereby re-forming the quinone. A different pattern, involving intramolecular H-atom transfer, holds for vitamin K(1), where 1,3-quinone methide (1,3-QM) diradicals were observed in acetonitrile prior to formation of two 1,2-QM tautomers, but a triplet was not. The decay of the 1,3-QM intermediates becomes faster in the presence of alcohols and amines due to proton-transfer reactions.

Primary photoreactions of phylloquinone (vitamin K1) and plastoquinone-1 in solution

The photoreactions of electron transport quinones vitamin K1 (1) and plastoquinone-1 (2) were studied by picosecond pump-probe spectroscopy, nanosecond flash photolysis, step-scan FTIR spectroscopy, and by irradiation in glassy solvents at 77 K with optical and EPR detection. In polar solvents, charge transfer from the beta,gamma-double bond to the quinone moiety initiates intramolecular proton transfer from the side chain, k = 2.7 x 10(11) s-1, yielding 1,3-quinone methide diradicals, which establish a metastable equilibrium between the singlet and the triplet state. Subsequent proton transfer through the solvent forms 1,2-quinone methides. In apolar solvents the predominant primary photoreaction is formation of cyclic 'preoxetane' diradicals (kform > 3 x 10(11) s-1, kdecay approximately 1 x 10(9) s-1), which revert to a photostationary E/Z mixture of the starting materials, unless they are trapped by oxygen. The beta,gamma-double bond in the isoprenoid side chain thus provides two efficient deactivation processes, which prevent the intermolecular photoreactions commonly observed with the parent quinones. A combined mechanistic scheme rationalizes the known photoreactions of 1, 2 and related compounds in solution.

Photooxidations Initiated or Sensitized by Biological Molecules: Singlet Oxygen Versus Radical Peroxidation in Micelles and Human Blood Plasma¶

Biomolecules common in blood plasma, including 2-methyl-1,4-naphthoquinone (vitamin K-0, 2), 2,3-dimethoxy-5-methyl-1,4-benzoquinone (ubiquinone-0, 3), bilirubin, 4, and urocanic acid, 5, were used as photoactivators for the photooxidation of methyl linoleate (ML) in 0.50 M sodium dodecyl sulfate micelles to mimic a bioenvironment. UV irradiation of 2 in this system initiated H-atom abstraction from ML (Type-I mechanism). The evidence includes kinetics of oxygen uptake, inhibition of oxidation by an antioxidant ((R)-(+)-6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid [Trolox], 7) and the analysis of four geometric hydroperoxides formed (cis, trans to trans, trans ratio of 0.5). In contrast, irradiation with a singlet-oxygen sensitizer, 3,5-di-t-butyl-1,2-benzoquinone, 1, formed six isomers by a Type-II mechanism, yielding a cis, trans to trans, trans isomer ratio of 6. Peroxidation activated by 3 or 4 with visible light occurred by a singlet-oxygen pathway (Type-II mechanism), as shown by kinetics of oxygen uptake and the effect of quenchers. In contrast, peroxidation in the presence of 5 in this system initiated H-atom abstraction from ML as shown by oxygen uptake and inhibition by Trolox. A comparison of thermal free-radical peroxidation with direct photooxidation of human blood plasma samples showed important differences. Blood plasma resisted thermal peroxidation because of natural antioxidants or on the addition of Trolox. In contrast, direct photooxidation involved singlet oxygen, according to the effect of quenchers and the lack of inhibition by antioxidants.

1995 Merck Frosst Award Lecture Quinone methides: relevant intermediates in organic chemistry

ortho and para-Quinone methides (2-methylene-3,5-cyclohexadien-1-one and 4-methylene-2,5-cyclohexadien-1-one, respectively) are intermediates in a variety of important chemical systems. In particular, o-quinone methides are useful in synthesis for the construction of chroman ring systems. A brief account of the relevance of quinone methide chemistry will be provided. This is followed by a review of recent studies from our laboratory on efficient methods for the photogeneration of quinone methides, concentrating on the use of hydroxy-substituted benzyl alcohols in aqueous media. It is shown that this method is general since it provides access to o-, m-, and p-quinone methide isomers. When appropriately substituted, all of these quinone methide isomers have been spectroscopically characterized by laser flash photolysis, making this technique the one of choice for studying the dynamics of these reactive intermediates. The mechanism of photochemical generation from hydroxybenzyl alcohols and extensions of the reaction to photogeneration of fluorenyl and biphenyl quinone methides will also be presented. Key words: quinone methide, biphenyl quinone methide, carbocation, photosolvolysis, photodehydroxylation, hetero-Diels–Alder reaction.