Oxidation of silyl enol ethers

The oxidation of silyl enol ethers 111 with palladium(n) acetate is a convenient nnethod for the preparation of synthetically useful 2,6-disubstituted 2,3-dihydro-4-pyridones 112 <95TL(36)9449>. 

The oxidation of silyl enol ethers with the osmium tetroxide-amine oxide combination also leads to a-hydroxyketones in generally good yields.147... 

In 1992, Thornton et al. reported that Mn(salen) (43) catalyzed the asymmetric oxidation of silyl enol ethers to give a mixture of a-siloxy and a-hydroxy ketones, albeit with moderate enantioselectivity (Scheme 28).135 Jacobsen et al. examined the oxidation of enol esters with Mn(salen) (27) and achieved good enantioselectivity.136 Adam et al. also reported that the oxidation of enol ethers with (27) proceeded with moderate to high enantioselectivity.137 Good substrates for these reactions are limited, however, to conjugated enol ethers and esters. Based on the analysis of the stereochemistry,137 enol ethers have been proposed to approach the oxo-Mn center along the N—Mn bond axis (trajectory c, vide supra). 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

Oxidation of silyl enol ethers. Oxidation of silyl enol ethers to a-hydroxy aldehydes or ketones is usually effected with w-chloroperbenzoic acid (6, 112). This oxidation can also be effected by epoxidation with 2-(phenylsulfonyl)-3-( p-nitrophenyl) oxaziridine in CHC1, at 25-60° followed by rearrangement to a-silyloxy carbonyl compounds, which are hydrolyzed to the a-hydroxy carbonyl compound (BujNF or H,0 + ). Yields are moderate to high. Oxidation with a chiral 2-arene-sulfonyloxaziridine shows only modest enantioselectivity. 

Coumaran-3-one without substitution at the 2-position can be synthesized by oxidation of silyl enol ether 62 with iodosobenzene under Lewis acid conditions. a-Hydroxyketone 63 is formed as a side product (86SC1239). 

Oxidation of silyl enol ethers leading to carbon-carbon bond formation [85JCS(CC)420 87JCS(P1)559] finds an interesting application in the synthesis of furans. For example, l,4-di(3-thienyl)-l,4-butanedione (65), which... 

Since silyl enol ethers have a silyl group ji to the jr-system, anodic oxidation of silyl enol ethers takes place easily. In fact, anodic oxidation of silyl enol ethers proceeds smoothly to provide the homo-coupling products, 1,4-diketones (equations 37 and 38)42. This dimerization of the initially generated cation radical intermediate is more likely than the reaction of acyl cations formed by two electron oxidation of unreacted silyl enol ethers in these anodic reactions. 

A related reaction is the oxidation of silyl enol ethers to enones. This requires stoichiometric pal-ladium(II), though reoxidation of Pd(0) with benzoquinone can cut that down to about half an equivalent, but does ensure that the alkene is on the right side of the ketone. The first step is again oxypalladation and p elimination puts the alkene in conjugation with the ketone chiefly because there are no P hydrogens on the other side. 

Enones. Two groups have reported the oxidation of silyl enol ethers to o,/3-unsaturated ketones using DDQ. This method is most suitable for cyclohexanone derivatives. Fleming and Paterson report that yields are generally improved by addition of collidine (1.5 equiv.), which presumably removes the by-product DDQHj. This last communication also reports improvements in the preparation of either the kinetic or thermodynamic silyl enol ethers by House s method (3, 310-311). By a combination of the two reactions, carvone (6) can be prepared from 2-methylcyclohexanone (1) in an overall yield of 28% (equation I). 

Hegedus and co-workers isolated a cyclobutane-fused epoxide intermediate in the Rubottom oxidation of silyl enol ether 17 (Scheme 9) <1998JOC4691>. 

This chapter describes a new type of oxidation of silyl enol ethers for the synthesis of a,p-unsaturated carbonyl compounds. 

Davis, F. A., Sheppard, A. C. Oxidation of silyl enol ethers using 2-sulfonyloxaziridines. Synthesis of a-siloxy epoxides and a-hydroxy carbonyl compounds. J. Org. Chem. 1987, 52, 954-955. 

Adam, W., Fell, R. T., Saha-Moller, C. R., Zhao, C.-G. Synthesis of optically active a-hydroxy ketones by enantioselective oxidation of silyl enol ethers with a fructose-derived dioxirane. Tetrahedron Asymmetry 1998, 9, 397-401. 

Stankovic, S., Espenson, J. H. Facile Oxidation of Silyl Enol Ethers with Hydrogen Peroxide Catalyzed by Methyltrioxorhenium. J. Org. Chem. 1998, 63, 4129 130. 

Solladie-Cavallo, A., Lupattelli, P., Jierry, L., Bovicelli, P., Angeli, F., Antonioletti, R., Klein, A. Asymmetric oxidation of silyl enol ethers using chiral dioxiranes derived from a-fluoro cyclohexanones. Tetrahedron Lett. 2003,44, 6523-6526. 

Ito, Y., Suginome, M. Palladium-catalyzed or -promoted oxidation via 1,2- or 1,4-elimination oxidation of silyl enol ethers and related enol derivatives to a,P-unsaturated enones and other carbonyl compounds. Handbook of Organopalladium Chemistry for Organic Synthesis 2002, 2, 2873-2879. 

Finally, the pioneering work of Snider et al. demonstrated that copper(II)-mediated radical reactions can be applied for an elegant tandem strategy [19]. Thus, oxidation of silyl enol ether 40 by copper(II) triflate generates electrophilic radicals 41, which undergo a 5-exo-trig cyclization to the intermediate 42. Oxidation to a... 

Enol ethers are interesting substrates for epoxidations since a-hydroxy ketones or the corresponding acetals are isolated, depending on the choice of solvent. Kat-suki has used enol ethers as substrates, including the cyclic enol ether (4.67), which affords the hydroxy acetal product (4.68). ° Adam has used silyl enol ethers and silyl ketene acetals as substrates. A typical example is provided by the asymmetric oxidation of silyl enol ether (4.69), generating the oi-hydroxy ketone (4.70) after a suitable work up. ... 

A similar intramolecular oxidation of silyl enol ether 158 affords oxygen heterocycle 159... 

The r-nucleophilicity and electron-transfer oxidation of silyl enol ethers and ketene silyl acetals has been studied by DFT, focusing on local softness and local nucle-ophilicity index as parameters for intramolecular reactivity and the corresponding group parameters for intermolecular reactivity. ... 

VIII.3.1 Oxidation of Silyl Enol Ethers and Related Enol Derivatives to a,]8-Unsaturated Enones and Other Carhonyl Compounds... 

Some synthetic modification for Pd(II)-catalyzed oxidation of silyl enol ethers is also developed. Silyl enol ethers prepared from aldehydes and ketones are converted to the corresponding a,/3-unsaturated carbonyl compound in good yields by 10 mol % of palladium(II) acetate in the presence of 1 atm pressure of O2 in DMSO as solvent (Scheme... 

VIII.3.1 OXIDATION OF SILYL ENOL ETHERS TO ENONES... 

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