Coupling reactions, silyl enol ether radical cations

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. 

The two reaction channels described represent the most important steps following the generation of the initial radical cation and can be directly incorporated into synthetic applications involving silyl enol ether radical cations. Deprotonation of the radical cation is a way to conduct a ketone-enone transformation via the silyl enol ether. Other synthetic applications utilizing the radical cation or the a-carbonyl radical are coupling reactions of silyl enol ethers, intramolecular addition to double bonds, or introduction of substituents other than carbon at the a-carbonyl position, respectively. Examples for these synthetic transformations will be presented in the following sections. 

Coupling reactions of silyl enol ether radical cations with double bonds other than silyl enol ethers have been investigated as well. Reactions with butadiene, ethyl vinyl ether, and allylic silanes have been reported. 

This vanadium method enables the cross-coupling only in combinations of silyl enol ethers having a large difference in reactivity toward radicals and in their reducing ability. To accomplish the crosscoupling reaction of two carbonyl compounds, we tried the reaction of silyl enol ethers and a-stannyl esters based on the following consideration. a-Stannyl esters (keto form) are known to be in equilibrium with the enol form such as stannyl enol ethers, but the equilibrium is mostly shifted toward the keto form. When a mixture of an a-stannyl ester such as 45 and a silyl enol ether is oxidized, it is very likely that the stannyl enol ether will be oxidized preferentially to the silyl enol ether. The cation radical of 45 apparently cleaves immediately giving an a-keto radical, which reacts with the silyl enol ether selectively because of the low concentration of the stannyl enol... 

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. 

When identical silyl enol ethers are used in the coupling reaction (Rj = R/, R = R ), homocoupling to symmetrical 1,4-diketones can be achieved (Scheme 11). For the synthesis of unsymmetrical 1,4-diketones, the two silyl enol ethers must differ significantly in terms of their oxidation potentials. This can be realized by selecting monosubstituted silyl enol ethers (R = H) and 1,2-disubstituted silyl enol ethers for the coupling reaction. Another possible way to reduce the oxidation potential is by the use of mesomeric stabilization vide supra).In the coupling reactions presented so far, the reactivity of silyl enol ethers is twofold. The component that is more easily oxidized forms the radical cation and consequently the a-carbonyl radical. In contrast, the second component acts as an electron-rich double bond in the radical addition reaction. 

Since enol silyl ethers are readily accessible by a number of methods in a regioselective manner and since the trialkylsilyl moiety as a potential cationic leaving group facilitates the termination of a cyclization sequence, unsaturated 1-trialkylsilyloxy-1-alkenes represent very promising substrates for radical-cation cyclization reactions. Several methods have been reported on the synthesis of 1,4-diketones by intermolecular oxidative coupling of enol silyl ethers with Cu(II) [76, 77], Ce(IV) [78], Pb(IV) [79], Ag(I) [80] V(V) [81] or iodosoben-zene/BFa-etherate [82] as oxidants without further oxidation of the products. 

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