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. 

As in the uncatalyzed reactions with enamines (vide supra), there is potentially more than one point where stereochemical differentiation can occur (Scheme 59). Selectivity can occur if the initial addition of the enol ether to the Lewis acid complex of the a,/J-unsaturated acceptor (step A) is the product-determining step. Reversion of the initial adduct 59.1 to the neutral starting acceptor and the silyl enol ether is possible, at least in some cases. If the Michael-retro-Michael manifold is rapid, then selectivity in the product generation would be determined by the relative rates of the decomposition of the diastereomers of the dipolar intermediate (59.1). For example, preferential loss of the silyl cation (or rm-butyl cation for tert-butyl esters step B) from one of the isomers could lead to selectivity in product construction. Alternatively, intramolecular transfer of the silyl cation from the donor to the acceptor (step D) could be preferred for one of the diastereomeric intermediates. If the Michael-retro-Michael addition pathway is rapid and an alternative mechanism (silyl transfer) is product-determining, then the stereochemistry of the adducts formed should show little dependence on the configuration of the starting materials employed, as is observed. 

The role of stoichiometric amount of zinc compounds in the aldol reaction was studied 30 years ago (107). The first study of asymmetric zinc-catalyzed aldol reaction was carried out by Mukaiyama and co-workers the chiral zinc catalyst was prepared from diethylzinc and chiral sulfonamides and was effective in the reaction of ketene silyl ethers with aldehydes (108). Among the subsequent studies on zinc-catalyzed aldol reactions, Trost s group gave important contribution to zinc/prophenol ligand complexes (109,110). The chiral dinuclear zinc catalyst promotes the direct aldol reaction of ketones, including a-hydroxyketones, and aldehydes with excellent enantioselectivity (Scheme 17). It is proposed that one zinc metal coordinated different substrates to form zinc enolate, and another zinc metal center provided the bridge between the interaction of donor and acceptor. 

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