Rhodium enolates aldol reaction

Scheme 3-129. Rhodium-catalyzed aldol reaction of silyl enol ethers with...

In the reductive aldol condensation of an ,/J-unsaturated ester and an aldehyde shown in Eq. 291, the initial step is believed to be the addition of an in situ formed rhodium hydride to the a,/Tunsaturated ester, followed by reaction of the resulting rhodium enolate with the aldehyde.470 The reaction has been carried out both inter-470 and intramolecularly471,472 as well as in an asymmetric fashion (Eq. 291). 

Krische and co-workers have revealed efficient reductive generation of rhodium enolates under hydrogenation conditions.403 40311-403 Both inter- and intramolecular reductive aldol reactions proceed smoothly and stereoselec-tively, although intermolecular reactions generally show low diastereoselectivity (Equations (52) and (53)). 

For example, rhodium catalyzed hydroformylation of 2-formyl-N-allyl-pyrrol gives an approx. 1 1 mixture of iso- and u-aldehydes. The latter cyclizes immediately in an aldol reaction followed by dehydration giving 7-formyl-5,6-indolizine in up to 46% (Scheme 29) [83]. Since here only one of the aldehyde groups can act as the enolate nucleophile this cyclization proceed with high regioselectivity (Scheme 29). 

Enol silyl ethers undergo aldol reactions with aldehydes, acetals and their equivalents with the aid of a Lewis acid catalyst. These reactions are discussed in Volumes 2-4. Enol silyl ethers prepared by hydro-silylation of a, 3-unsaturated ketones with a rhodium catalyst can be used for aldol reactions with aldehydes or ketones in situ under neutral conditions (equation 60)." ... 

In 1986, Reetz et al. reported that chiral Lewis acids (B, Al, and ll) promoted the aldol reaction of KSA with low to good enantioselectivity [115]. The following year they also introduced asymmetric aldol reaction under catalysis by a chiral rhodium complex [116]. Since these pioneering works asymmetric aldol reactions of silyl enolates using chiral Lewis acids and transition metal complexes have been recognized as one of the most important subjects in modern organic synthesis and intensively studied by many synthetic organic chemists. 

The aldol reactions of rhodium enolates have limited synthetic utility when employed stoichiometri-cally. 0-Bound rhodium enolates of ketones, e.g. (36), have been prepared in high yield by reaction of preformed potassium enolates with carbonyldi(trimethylphosphine)ihodium(I) chloride or fluoride. However, the basic nature of these particular enolates restricts their aldol reactions to nonenolizable aldehydes. 

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-... 

Crossed aldol reactions of enol silyl ethers with aldehydes have been successfuly performed with the aid of catalytic amounts of the rhodium complex ((COD)Rh(DPPB)]+X (X = PFs or CIO4) or Rh4(CO)i2. Although the intermediacy of rhodium enolate has been suggested for these reactions, the fact that the same rhodium catalysts can promote the condensations of acetals as well (Scheme 38) tends to indicate that the reactive species may not be a metal enolate. 

Aluminium montmorilIonite and rhodium carbonyl have been reported as catalysts for the aldol reaction of enol silanes with aldehydes, but in neither case are the stereoselectives at all high. The reaction has also been found to occur without Lewis acid catalysis in water or in 1 1 water oxolane. Although the yields are not high, the method is of interest in that syn-products were... 

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