Photoactivation, chloranil

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. 

Exploitation of time-resolved spectroscopy allows the direct observation of the reactive intermediates (i.e., ion-radical pair) involved in the oxidation of enol silyl ether (ESE) by photoactivated chloranil (3CA ), and their temporal evolution to the enone and adduct in the following way.41c Photoexcitation of chloranil (at lexc = 355 nm) produces excited chloranil triplet (3CA ) which is a powerful electron acceptor (EKelectron-rich enol silyl ethers (Em = 1.0-1.5 V versus SCE) to the ion-radical pair with unit quantum yield, both in dichloromethane and in acetonitrile (equation 20). 

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). 

Detailed studies of the mechanism of these reactions have been performed by Mattay and by Kochi . The former has shown that the endo/exo regiochemistry of the ring closure reaction can be controlled either by variation of the silyl group or by addition of polar molecules such as alcohols (probably the source of hydrogen in equations 37a-c). Based on solvent and salt effects, Kochi has proposed that the oxidation of enols to ketones in the presence of activated chloranil proceeds via photoactivation of chloranil which reacts with the silyl enolate through two competing pathways, namely oxidative elimination to the ketone and oxidative addition to the adduct 51 (equation 38). Non-polar solvents such as dichloromethane favour the oxidative eliminations, while polar solvents such as acetonitrile direct the reaction towards the oxidative addition. More strikingly. 

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]. 

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