Rhodium complexes enantioselective aldol reaction

Morken and co-workers have reported the highly enantioselective version of this reaction, albeit with low efficacy in the aldol-type coupling [8d, e]. Unfortunately, we obtain low enantioselectivity ee 2-4%) using chiral rhodium complexes under our reaction conditions. An intramolecular adaptation has led to new opportunities in cobalt-catalyzed carbocyclizations, wherein the use of PhSiHs was essential for smooth ring formation (Eq. 4) [9]. The identical products were also formed by a combination of [Rh(COD)2]OTf/(p-CE3Ph)3P and molecular hydrogen [10]. 

Alkylative aldol reactions with aryl- or vinylboron reagents are also catalyzed by rhodium complexes.404 4043 4041 Equations (54) and (55) show examples of enantioselective reactions. 

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. 

Preliminary examples of catalytic enantioselective aldol additions using rhodium complexes with chiral phosphine ligands attached, e.g. (42), have been disclosed by Reetz and Vougioukas (equation 15) although the low level of asymmetric induction so far obtain in the silylated aldol adduct (43) requires substantial enhancement before this reaction has any synthetic value. However, there is ample scope for future improvement by the use of more effective chiral diphosphine- odium complexes. 

Later, Eilbracht et al. developed a related enantioselective domino hydroformylation-aldol reaction catalysed by a rhodium complex in combination with a chiral amine derived from L-proline. In this process, the reaction of acetone with vinylbenzene afforded the corresponding chiral p-hydrojgr ketone in 69% yield, a moderate diastereoselectivity of 74% de, and a high enantioselectivity of 93% ee, as shown in Scheme 7.50. ... 

The enantioselective intermolecular reductive aldol reaction with enone instead of ester is also an important C-C bond formation reaction. The coupling reaction of cyclopent-2-enone and benzaldehyde with hydrosilane in the presence of 1 mol% of the phebox-Rh complex 5-Ph smoothly proceeded to give the corresponding anti-p-hydroxyketone 29 with the high ee value (Scheme 20) [25], On the basis of the absolute configuration of 29, the transition state of the reaction might involve the Re face attack of the Rh enolate to the Re face of the aldehyde captured on the rhodium atom leading to form 2S, l R-diastereomeric product. 

By tethering a,p-enones with another carbonyl-containing moiety, Rrische developed rhodium-catalyzed asymmetric 1,4-addition/aldol cyclization reactions." These reactions proceed with high diastereo- and enantioselectivity, furnishing structurally complex cyclic compounds in a single step [Eqs. (3.11-3.13)]. 

---