Silyl enolates photoinduced electron transfer

Arisawa, M., Ramesh, N.G., Nakajima, M., Tohma, H., and Kita, Y, Hypervalent iodine(lll)-induced intramolecular cychzation of a-(aryl)alkyl-P-dicarbonyl compounds a convenient synthesis of benzarmulated and spirobenzannulated compounds, /. Org. Chem., 66, 59, 2001. Eberson, L., Reaction between organic and metal ion species, in Electron Transfer Reactions in Organic Chemistry, Reactivity and Structure, Vol. 25, Hafner, K., Lehn, J.-M., Rees, C.W., von Rague-Schleyer, R, Trost, B.M., and Zahradnik, R, Eds., Springer-Verlag, Berhn, 1987, chap. 7. Bockman, T.M., ShuMa, D., and Kochi, J.K., Photoinduced electron transfer from enol silyl ethers to quinone. Part 1. Pronounced effects of solvent polarity and added salt on the formation of a-enones, J. Chem. Soc., Perkin Trans. 2, 1623, 1996. 

Bockman, T.M. and Kochi, J.K., Photoinduced electron transfer from enol silyl ethers to quinone. Part 2. Direct observation of ion-pair dynamics by time-resolved spectroscopy, /. Chem. Soc., Perkin Trans. 2, 1633,1996. 

Mattay et al. examined the regioselective and stereoselective cyclization of unsaturated silyl enol ethers by photoinduced electron transfer using DCA and DCN as sensitizers. Thereby the regiochemistry (6-endo versus 5-exo) of the cyclization could be controlled because in the absence of a nucleophile, like an alcohol, the cyclization of the siloxy radical cation is dominant, whereas the presence of a nucleophile favors the reaction pathway via the corresponding a-keto radical. The resulting stereoselective cis ring juncture is due to a favored reactive chair like conformer with the substituents pseudoaxial arranged (Scheme 27) [36,37]. 

In addition to the former example, Pandey et al. achieved efficient a-aryla-tion of ketones by the reaction of silyl enol ethers with arene radical cations generated by photoinduced electron transfer from 1,4-dicyanonaphthalene. Using this strategy various five-, six-, seven-, and eight-membered benzannulated compounds are accessible in yields in the range 60-70% [39],... 

Similar to the deprotonation of enol radical cations, silyl enol ether radical cations can undergo loss of trialkylsilyl cations (most likely not as ionic silicenium ions [190]). Based on photoinduced electron transfer (PET), Gass-man devised a strategy for the selective deprotection of trimethylsilyl enol ethers in the presence of trimethylsilyl ethers [191]. Using 1-cyanonapthalene (1-CN) ( = 1.84 V) in acetonitrile/methanol or acetonitrile/water trimethylsilyl enol ether 93 ( j = 1.29 V) readily afforded cyclohexanone 64 in 60%. Mechanistically it was proposed that the silyl enol ether radical cation 93 undergoes O-Si bond cleavage, most likely induced by added methanol [192-194], and that radical 66 abstracts a hydrogen from methanol. Alternatively, back electron transfer from 1-CN - to 66 would yield the enolate of cyclohexanone which should be readily protonated by the solvent. 

Oxidative coupling of silyl enol ethers as a useful synthetic method for carbon-carbon bond formation has been known for a long time. Several oxidants have been successfully applied to synthesize 1,4-diketones from silyl enol ethers, e.g. AgjO [201], Cu(OTf)2 [202], Pb(OAc)4 [203] and iodosobenzene/BFj EtjO [204]. Although some of these reagents above are known to react as one-electron oxidants, the potential involvement of silyl enol ether radical cations in the above reactions has not been studied. Some recent papers, however, have now established the presence of silyl enol ether radical cations in similar C-C bond formation reactions under well-defined one-electron oxidative conditions. For example, C-C bond formation was reported in the photoinduced electron transfer reaction of 2,3-dichIoro-1,4-naphthoquinone (98) with various silyl enol ethers 99 [205], From similar reactions with methoxy alkenes [206,207] it was assumed that, after photoexcitation, an ion radical pair is formed. 

Alkylating agents. These selenides, activated through photoinduced electron transfer, react with enol silyl ethers, forming a-alkylated ketones. 

Although relatively short-lived, radical cations are the primary products of the electron transfer (ET) step. Evidence for the formation of radical cations has been presented for silyl enol ether systems that form stabilized radical cations. Kochi and coworkers investigated the photoinduced electron transfer between different silyl enol ethers and chloranil in detail.In a system suitable for the spectroscopic study of the radical cation intermediate, chloranil (CA) is electronically excited to its triplet state CA (10). Subsequent electron transfer generates the corresponding radical ion pair (11/12). All these transient species have been observed by time-resolved spectroscopy. 

The enol silyl ether (56) undergoes a photoinduced reaction in the presence of chloranil to give cyclohexenone and the adduct (57) and the currently available evidence suggests that the reaction proceeds by electron transfer to the photo-activated chloranil to give (56) A photophysical study has appeared of the chromophore-quencher compounds [Au(CCPh)(L )] (L = 10-[(diphenylphosphi-no)methyl]anthracene) and [Au(CCPh)(L )]BPh4 (L = l-[2-(diphenylphosphin-oxy)ethyl]pyridinium], 1 -[2-(diphenylphosphinoxy)ethyl]-4-methyl pyridinium, 1 -[2-(diphenylphosphinoxy)ethyl]-4-tert-butylpyridinium, and 1 -[2-(diphenylpho-sphinoxy)ethyl]triethylammonium in which the Au(CCPh) chromophore is linked by a flexible tether to the acceptors. 

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