Enol ethers, trimethylsilyl rhodium enolates

Ketone and ester enolates have historically proven problematic as nucleophiles for the transition metal-catalyzed allylic alkylation reaction, which can be attributed, at least in part, to their less stabilized and more basic nature. In Hght of these limitations, Tsuji demonstrated the first rhodium-catalyzed allylic alkylation reaction using the trimethly-silyl enol ether derived from cyclohexanone, albeit in modest yield (Eq. 4) [9]. Matsuda and co-workers also examined rhodium-catalyzed allylic alkylation, using trimethylsilyl enol ethers with a wide range of aUyhc carbonates [22]. However, this study was problematic as exemplified by the poor regio- and diastereocontrol, which clearly delineates the limitations in terms of the synthetic utihty of this particular reaction. 

Rhodium(II)-catalyzed decomposition of ethyl 2-diazo-3,3,3-trifluoropropanoate in the presence of excess trimethylsilyl enol ethers gave ethyl l-trifluoromethyl-2-(trimethylsiloxy)cyclo-propane-l-carboxylates 7 which were converted directly to 2-trifluoromethyl-4-oxoalkanoates 8. 1... 

If rhodium enolates are used in a catalytic cycle they can promote aldol reactions under reasonably mild conditions. For example, the aldol reactions of trimethylsilyl enol ethers and ketene silyl acetals (37) with aldehydes can be catalyzed by various rhodium(I) complexes, under essentially neutral conditions, to give p-trimethylsiloxy ketones and esters (38 equation 14 and Table 6). The study of Matsuda and coworkers suggests that use of the rhodium complex Rlu(CO)i2 (39 at 2 mol %) in benzene at 100 C gives best results for the formation of adduct (38 Table 6, entries 1-7). There is negligible diastereoselectivity in most cases. Various cationic ihodium complexes such as (40) also catalyze the reaction. Reetz and Vougioukas have found that this aldol reaction proceeds well with the more reactive ketene silyl acetals, (37) for R = OMe or OEt, in CH2CI2 at room temperature (Table 6, entries 8-13). The intermediacy of an ti -O-bound rhodium enolate, such as (41), in the catalytic cycle is like-... 

If rhodium enolates are used in a catalytic cycle they can promote aldol reactions under reasonably mild conditions. For example, the aldol reactions of trimethylsilyl enol ethers and ketene silyl acetaU (37) with aldehydes can be catalyzed by various rhodium(I) complexes, under essentially neutral conditions, to give p-trimethylsiloxy ketones and esters (38 equation 14 and Table The study of... 

---