Aldol reactions hydrosilanes

In this context, lipshutz et al. reported in 2000 a catalytic reductive aldol reaction of enones and aldehydes with [PhsPCuHjs (5 mol%) and PhMe2SiH (150mol%) [46]. The two-step reaction was carried out in one pot, without isolation of the intermediate sUyl enol ethers, efficiently providing the b-hydroxyketones in high yield. Lewis acids such as BF3 or TiCLj are used to promote the second step involving aldol reaction of the enol silane. In place of hydrosilanes, dialkylboranes could be employed as hydride sources, circumventing the need to introduce additional Lewis acids. Here, the aldol products are formed via intermediacy of the boron-enolates, with 5y -selectively for acychc enones and antz-selectively for cycHc enones [47-50]. 

Reductive aldol reaction of a,(5-unsaturated esters and enones with aldehyde mediated by a transition metal hydride complex and a hydride source, such as hydrosilane, is a versatile process to produce p-hydroxy carbonyl compounds (Scheme 15a) [21]. This reaction is thought to be an alternative transformation of Lewis acid-catalyzed Mukaiyama-type aldol reaction with silyl enol ethers or silyl ketene acetals (Scheme 15b). 

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. 

Leaving precise mechanistic arguments aside, it should be stressed that an a,/9-unsatu-rated carbonyl compound can behave as a latent nucleophile with the assistance of a hydrosilane and a low-valence rhodium complex. With this simplification, isocyanates 18 and aldimines 20 were used as electrophiles for a similar protocol. Both were successfully incorporated with the aid of [Rh(COD)(P(OPh)3)2]OTf to afford 19 and 21, respectively (Scheme 6.5) in a reaction that was similar to the aldol-type coupling [11]. 

We have explored two types of carbon-carbon bond forming reactions operated under almost neutral conditions. Both reactions are initiated by the formation of an H-Rh-Si species through oxidative addition of a hydrosilane to a low-valence rhodium complex. Aldol-type three-component couphngs are followed by the insertion of an a,yS-unsatu-rated carbonyl compound into a Rh-H bond, whereas silylformylation is accomplished by the insertion of an acetylenic moiety into a Rh-Si bond. 

---