Electron donor-acceptor complexes silyl enolate reactions

Recently, Kochi et al. described a novel photochemical synthesis for a-nitration of ketones via enol silyl ethers. Despite the already well-known classical methods, this one uses the photochemical excitation of the intermolecular electron-donor-acceptor complexes between enol silyl ethers and tetranitrometh-ane. In addition to high yields of nitration products, the authors also provided new insights into the mechanism on this nitration reaction via time-resolved spectroscopy, thus providing, for instance, an explanation of the disparate behavior of a- and (3-tetralone enol silyl ethers [75], In contrast to the more reactive cross-conjugated a-isomer, the radical cation of (3-tetralone enol silyl ether is stabilized owing to extensive Tr-delocalization (Scheme 50). 

The oxidation of silyl enol ethers184 (105) to enone (107) by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (106) is illustrated in Scheme 42. The electron donor-acceptor complex (red, Xmax = 520 nm) precedes the formation of the adduct 109, which is unreactive, and 108, which is the intermediate of the reaction. At 22 °C, the reaction mixture affords a mixture of 108 and of 109. At 100 °C, 108 is transformed into the final enone 107. 

Various a-nitro ketones, widely used as synthetic intermediates, have been prepared by reaction of silyl enolates with tetranitromethane in the dark at room or low temperature or under photochemical conditions. The highly coloured solutions are due to intermolecular 1 1 electron donor-acceptor complexes formed between the enolate and tetranitromethane. The formation of similar vividly coloured complexes with electron acceptors such as chloranil, tetracyanobenzene and tetracyanoquinonedimethane readily establishes silyl enolates as electron donors. The formation of radical cations as reactive intermediates has been confirmed. 

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. 

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