Quinones photochemical reaction

Photochemical Reactions. Increased knowledge of the centraUty of quinone chemistry in photosynthesis has stimulated renewed interest in their photochemical behavior. Synthetically interesting work has centered on the 1,4-quinones and the two reaction types most frequentiy observed, ie [2 A 2] cycloaddition and hydrogen abstraction. Excellent reviews of these reactions, along with mechanistic discussion, are available (34,35). 

DDQ ( red = 0.52 V). It is noteworthy that the strong medium effects (i.e., solvent polarity and added -Bu4N+PFproduct distribution (in Scheme 5) are observed both in thermal reaction with DDQ and photochemical reaction with chloranil. Moreover, the photochemical efficiencies for dehydro-silylation and oxidative addition in Scheme 5 are completely independent of the reaction media - as confirmed by the similar quantum yields (d> = 0.85 for the disappearance of cyclohexanone enol silyl ether) in nonpolar dichloromethane (with and without added salt) and in highly polar acetonitrile. Such observations strongly suggest the similarity of the reactive intermediates in thermal and photochemical transformation of the [ESE, quinone] complex despite changes in the reaction media. 

From the Contents J.L.R. Williams Photochemical Reactions of Polymers. M.B. Rubin Photochemistry of o-Quinones and -Diketones. L.B. Jones, V. K. Jones Photochemical Reactions of Cycloheptartrienes. C. V. Sonntag Strahlenchemie von Al koholen. 

On the planet Earth, the most important photoreaction occurs in green plants or in green or purple organisms. Their photochemical reaction centers contain a special pair of chlorins (cf. the purple bacterium Rhodobacter sphaeroides. Fig. 6.2). Solar photons cause electron transfer and generate a radical ion pair. Within two picoseconds, the negative charge is transferred to a second chlorin, and from it to a quinone. ... 

Photochemical reactions of quinones with allenes have also been studied and in some cases cyclobutane formation occurs, although in competition with products derived from attack of the allene on the carbonyl oxygen. Thus, photocycloaddition of tetramethyl-l,4-benzoquinone with 1,1-dimethylallene affords the four-membered carbocycle 6 in good yield.12... 

Another variant of the photochemical reaction between A-hetero-cyclic o-quinones and olefins has been described by Mustafa et al.196 200 Stilbene reacts with CXLIX and with CL to give the photoproducts CLVIII and CLIX, respectively. Similar photoaddition products were obtained by the interaction of phenanthraquinone with a-stilbazole245 and with l,2-di-(4 -pyridyl)ethylene.241 Although the process has been suggested as involving diradicals, it is not clear whether the quinone or the olefin undergoes photoexcitation. 

A new synthetic route to alkyl-substituted quinones has relied on the photochemical reaction of 2,3-dichloro-l,4-naphthoquinone with a thiophene derivative (77CL851). Irradiation of a benzene solution of the quinone and thiophene by a high pressure mercury lamp gave photoadduct (295) in 56% yield. Desulfurization of this compound over Raney nickel (W-7) gave the 2-butyl-1,4-naphthoquinone derivative (296 Scheme 62). Alkyl-substituted quinones such as coenzyme Q and vitamin K, compounds of important biological activity, could possibly be prepared through such methodology. 

Photoinitiation of polymerization can be obtained through a variety of photochemical reactions which produce reactive free radicals. These radicals then lead to the formation of the polymer chains through the addition of further monomer units to the end of a chain in a sequence of radical addition reactions (Figure 6.10). A photoinitiator of polymerization is therefore a molecule which produces free radicals under the action of light. Benzo-phenone and other aromatic ketones can be used as photoinitiators, since a pair of free radicals is formed in the hydrogen abstraction reaction. Some quinones behave similarly, for example anthraquinone in the presence of hydrogen donor substrates such as tetrahydrofuran. 

A number of other general photochemical reactions have been applied to heterocyclic systems, and these are worthy of mention. o-Quinone diazides on irradiation undergo ring contraction with loss of nitrogen to give a carboxylic acid this has been observed in a... 

The combination of neutral non-aromatic and zwitterionic aromatic contributing valence bond structures confers a distinctive chemical reactivity to quinone methides, which has attracted the interest of a tremendous number of chemist and biochemists. This chapter reviews reactions that generate quinone methides, and the results of mechanistic studies of the breakdown of quinone methides in nucleophilic substitution reactions. The following pathways for the formation of quinone methides are discussed (a) photochemical reactions (b) thermal heterolytic bond... 

Yates and coworkers have examined the mechanism for photohydration of o-OH-8. The addition of strong acid causes an increase in the rate of quenching of the photochemically excited state of o-OH-8, and in the rate of hydration of o-OH-8 to form l-(o-hydroxyphenyl)ethanol. This provides evidence that quenching by acid is due to protonation of the singlet excited state o-OH-8 to form the quinone methide 9, which then undergoes rapid addition of water.22 Fig. 1 shows that the quantum yields for the photochemical hydration of p-hydroxystyrene (closed circles) and o-hydroxystyrene (open circles) are similar for reactions in acidic solution, but the quantum yield for hydration of o-hydroxystyrene levels off to a pH-independent value at around pH 3, where the yield for hydration of p-hydroxystyrene continues to decrease.25 The quantum yield for the photochemical reaction of o-hydroxystyrene remains pH-independent until pH pAa of 10 for the phenol oxygen, and the photochemical efficiency of the reaction then decreases, as the concentration of the phenol decreases at pH > pAa = 10.25 These data provide strong evidence that the o-hydroxyl substituent of substrate participates directly in the protonation of the alkene double bond of o-OH-8 (kiso, Scheme 7), in a process that has been named excited state intramolecular proton transfer (ESIPT).26... 

Unfortunately, turnover control of PSII is more complicated than the above description would indicate. Because turnover of the S states is achieved via a photochemical reaction, the yield of the reaction depends on both the electron donors and the electron acceptors. The overall picture of electron transfer in PSII is shown in Figure 2 (II). Light induces a series of electron-transfer reactions that lead to the formation of progressively more stable charge-separated states. The dominant reaction under physiological conditions leads to a one-step advancement of the S state and reduction of the secondary quinone electron acceptor (Qb). In purified PSII preparations, however, the quinones are depleted and the QB site will mostly be unoccupied unless exogenous quinones are added. 

Other examples of electron polarization transfer in photochemical systems include the reaction of polarized isopropanol radicals with a ground-state quinone (reaction 55) and the polarization transfer from the primary amine radical to biacetyl (97). These examples serve to emphasize how CIDEP and polarization transfer can be used to follow complex photochemical reaction mechanisms. 

The applications of CIDNP to mechanistic studies of organic photochemical reactions are numerous, but only a few systems, such as the photoreduction of quinones, have been fully examined by both CIDEP and CIDNP methods. Instead of repeating some of the well-known CIDNP mechanistic studies summarized in other reviews, we shall go into a relatively new area of CIDNP studies involving metallorganic compounds. 

The classical Friedel-Crafts reaction has functional restrictions for the starting materials, as highly electronegative atoms cannot be present. The photochemical reaction has no such restrictions, and a variety of substituted aldehydes and quinones have been successfully utilized. 

The construction and properties of monolayers has been well documented by Kuhn (1979) and the photochemical reactions which occur in such systems reviewed (Whitten et al., 1977). Molecules in monolayers are usually ordered and in the case of rru/i -azastilbenes irradiation of the ordered array produces excimer emission and dimers (Whitten, 1979 Quina et al, 1976 Quina and Whitten, 1977). This contrasts with what is found when the fra/jj-isomers of such compounds are incorporated into micelles. In such systems the predominant reaction is cis-trans isomerisation excimer emission is lacking. It is suggested that the lack of isomerisation in the fatty acid monolayers is due to the tight packing and consequent high viscosity of such systems. Styrene also dimerises in a fatty acid monolayer. Interestingly, the products formed on photo-oxidation of protoporphyrins are dependent upon whether the reaction is carried out in a monolayer or a micelle (Whitten et al., 1978). Zinc octa-ethylporphyrin exhibits excimer emission in monolayers (Zachariasse and Whitten, 1973). Porphyrins are photoreduced by amines in monolayers (Mercer-Smith and Whitten, 1979). Electron-transfer reactions have been carried out with monolayers of stearic acid containing chlorophyll and electron acceptors such as quinones (Janzen et al., 1979 Janzen and Bolton, 1979). 

All photosynthetic eubacteria contain photochemical reaction centers (RCs) containing one or more chlorophyll molecules. Each reaction center consists of a primary electron donor P (bacteriochlorophyll), an initial electron acceptor I (bac-teriochlorophyll or bacteriopheophytin), and one or more secondary acceptors (Fe-S centers, quinones). Sometimes a secondary electron donor D (Cyt c) is tightly bound to the RC. 

Redox catalysis is the catalysis of redox reactions and constitutes a broad area of chemistry embracing biochemistry (cytochromes, iron-sulfur proteins, copper proteins, flavodoxins and quinones), photochemical processes (energy conversion), electrochemistry (modified electrodes, organic synthesis) and chemical processes (Wacker-type reactions). It has been reviewed altogether relatively recently [2]. We will essentially review here the redox catalysis by electron reservoir complexes and give a few examples of the use of ferrocenium derivatives. 

The photolysis of quinone derivatives with secondary amines was presumed to involve an aziridine derivative as an intermediate. The photochemical reaction of 3-chloro-2-bis(ethoxycarbonyl)methyl-l,4-naptho-quinone (246) with 6-(4-bromophenyl)-3,6-diazabicyclo[3.1.0]hexane (247) gave aziridinopyrrolo[l,2-fl]benz[/]indoloquinone (248) as a model compound of mitomycin by a one-pot reaction in 63% yield (Scheme 44). ... 

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