Photoactivated quinone

Fig. 9. (A) Free-energy dependence of the second- the formation product (/

Whereas in organic solvents the oxidation of toluene with permanganate (Mn04 ) and chromyl chloride (Cr02Cl2) has been established to occur by HAT [157], Kochi showed that in the reaction of methylarenes with photoactivated quinones such as chloranil intermediate aromatic radical cations are formed which then undergo side-chain deprotonation [158]. 

Thus, electron transfers from a series of unhindered, partially hindered, and heavily hindered aromatic electron donors (with matched oxidation potentials) to photoactivated quinone acceptors are kinetically examined by laser flash photolysis, and the free-energy correlations of the ET rate constants are scrutinized [31]. The second-order rate constants of electron transfers from hindered donors such as hexaethylbenzene or tri-icrt-butylbenzene strongly depend on the temperature, the solvent polarity and salt effects, and they follow the free-energy correlation predicted by Marcus theory (see Figure 20A). Moreover, no spectroscopic or kinetic evidence for the formation of encounter complexes (exciplexes) with the photo-activated quinones prior to electron transfer is observed. 

In contrast, electron transfers from unhindered (or partially hindered) donors such as hexamethylbenzene, mesitylene, di-ferr-butyltoluene, etc. to photoactivated quinones exhibit temperature-independent rate constants that are up to 100 times faster than predicted by Marcus theory, poorly correlated with the accompanying free-energy changes (see Figure 20A), and only weakly affected by solvent polarity and salt effects. Most importantly, there is unambiguous (NIR) spectroscopic and kinetic evidence for the pre-equilibrium formation K c) of long-lived encounter complexes (exciplexes) between arene donor (ArH) and photoexcited quinone acceptor (Q ) prior to electron transfer (A et) [20] (Eq. 95). 

The interaction of arenes or alkylarenes, ArCH2R, with some oxidants, Ox, [27] for example, with cerium(IV) salts [27c] or photoactivated quinones [27d], gives rise to the formation of the ion-radical pair ... 

Various enol silyl ethers and quinones lead to the vividly colored [D, A] complexes described above and the electron-transfer activation within such a donor/acceptor pair can be achieved either via photoexcitation of charge-transfer absorption band (as described in the nitration of ESE with TNM) or via selective photoirradiation of either the separate donor or acceptor.41 (The difference arising in the ion-pair dynamics from varied modes of photoactivation of donor/acceptor pairs will be discussed in detail in a later section.) Thus, actinic irradiation with /.exc > 380 nm of a solution of chloranil and the prototypical cyclohexanone ESE leads to a mixture of cyclohexenone and/or an adduct depending on the reaction conditions summarized in Scheme 5. 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

In summary, it would appear that the oxidation of a catecholamine probably first involves the formation of a semi-quinone radical (this can be brought about by an one-electron transfer, e.g. from Cu++ ions,14 or by photoactivation 1) which rapidly undergoes further oxidation (e.g. with atmospheric oxygen) to an intermediate open-chain quinone (such as adrenaline-quinone) and then cyclizes by an oxidative nucleophilic intramolecular substitution to the amino-chrome molecule. Whilst the initial formation of a leucoaminochrome by non-oxidative cyclization of the intermediate open-chain quinone in some cases cannot be entirely excluded at the moment (cf. Raper s original scheme for aminochrome formation72), the... 

Time-resolved (fs/ps) spectroscopy revealed that the (singlet) ion-radical pair is the primary reaction intermediate and established the electron-transfer pathway for this Paterno-Buchi transformation. The alternative pathway via direct electronic activation of the carbonyl component led to the same oxetane regioisomers in identical ratios. Thus, a common electron-transfer mechanism applies involving quenching of the excited quinone acceptor by the stilbene donor to afford a triplet ion-radical intermediate which appear on the ns/ps time scale. The spin multiplicities of the critical ion-pair intermediates in the two photoactivation paths determine the time scale of the reaction sequences and also the efficiency of the relatively slow ion-pair collapse ( c=108/s) to the 1,4-biradical that ultimately leads to the oxetane product 54. 

It is possible that the hydroquinone-reduced species with its trapped Mn(II) corresponds to an intermediate in the photoactivation process. There appears to be one oxidized Mn dimer in the hydroquinone-reduced sample, as indicated by the 2.7-A feature in the EXAFS. This feature could correspond to the Mn that is photooxidized in the photoactivation process. Oxidation and photoligation of the first two Mn are the rate-limiting steps in photoactivation. Thus, as long as hydro-quinone reduction does not affect these two critical Mn, the effects of hydroquinone should be easily and rapidly reversed, as is observed. In... 

Johnson, G.N., Rutherford, A.W., Krieger, A. (1995). A change in the midpoint potential of the quinone Q-A in photosystem associated with photoactivation of oxygen evolution. Biochim. Biophys. Acta 1229 202-7. 

FIGURE 4. Restoration of the multiline EPR signal by photoactivation. NH2OH-treated PS II membranes were incubated with 0.1 mM Mn /20 mM Ca VlSO mM Cl /S /i M DCIP/0.4 M sucrose/25 mM Mes-NaOH (pH 6.5) under continuous illumination or in darkness for 30 min as in Fig. 1. Then, the membranes were collected and resuspended in medium A containing 5 mM CaCl2/2 mM EDTA/0.3 mM phenyl-p-benzo-quinone. The suspension was illuminated at 210 K for 4 min and the EPR spectrum was measured at 6 K [83, (a), NaCl-treated membranes,... 

Light can profoundly affect in vivo and in vitro toxin bioassays, as has been documented by photosensitization experiments using livestock and poultry (Towers 1980 Ivie 1982). Photoactivation of quinones, certain alkaloids, and other compounds can be assessed by, for example, exposure of agar plates used in diffusion assays to UV-A light (320-400 nm, 5 W/m ) for 2 h while keeping control plates in the dark (Taylor et al. 1995). 

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