Trimethoxysilyl enolates

An aldol reaction of a trimethoxysilyl enol ether, catalysed by a lithium binaphtholate, shows anti diastereoselectivity and modest ees under dry conditions, but addition of water brings about syn adduct formation, with higher ee.131... 

Independently, Yamamoto, Yanagisawa, and others reported the asymmetric aldol reaction using trimethoxysilyl enol ethers.19 The reaction was conducted with aldehydes and trimethoxysilyl enol ethers in the presence of Tol-BINAP-AgF to give the corresponding adducts with high enantioselectivities and diastereoselectiv-ities. They obtained vyra-aldol adducts as major products even when silyl enol ethers derived from cyclic ketones were used. Moreover, when a,(3-unsaturated aldehydes were employed as substrates, 1,2 adducts were obtained exclusively (Table 9.10). From an NMR study and correlation between the E Z ratio of the enol ethers and diastereoselectiviy, they proposed a cyclic transition state (Fig. 9.5). Thus, the reaction of E enol ethers proceeded via a boat form, whereas the reaction of Z enol ethers took place via a chair form. 

TABLE 9.10. Aldol Reaction Using Trimethoxysilyl Enol Ethers... 

Scheme 2.8 Enantioselective aldol reaction of aldehydes with trimethoxysilyl enol ethers with the use of chiral lithium(i) binaphtholate.

In contrast, Yanagisawa, Yamamoto, and coworkers have studied diverse combinations of BINAP-silver(I) catalysts and silyl enolates and found that high levels of asymmetric induction and isolated product yields were attained in the p-Tol-BINAP-AgF-catalyzed aldol reaction of trimethoxysilyl enolates (38) with aldehydes (39) in methanol (Scheme 18.15) [54]. Table 18.2 summarizes the results... 

Table 18.2 Diastereo- and enantioselective aldol reaction of trimethoxysilyl enolates (38) with aldehydes (39) catalyzed by (R)-/9-ToI-BINAP- AgF Complex.

Catalytic enantioselective protonation of prochiral ketone enolates is a beneficial route to optically active carbonyl compounds possessing a tertiary asymmetric carbon at the a-position. In the asymmetric protonation of trimethylsilyl enolates with methanol, BINAP-AgF has been found to act as a chiral catalyst [90,91], which is also known to catalyze asymmetric allylation of aldehydes with allylic trimethoxysilanes [42] as well as asymmetric aldol reaction with trimethoxysilyl enolates [54]. This protonation can be most effectively performed using 6 mol% ofBINAP and 10 mol% of... 

The BINAP-AgOTf complex was also applied to Mukaiyama aldol reactions using trimethoxysilyl enol ethers instead of tin enol ethers. This reaction was only efficient in the presence of catalytic KF and 18-crown-6. Such conditions were also applied to asymmetric allylation (eq 65), increasing thus the scope of the AgOTf-catalyzed aUylations. ... 

Stereoselectivities of aldol reactions of trimethoxysilyl enol ethers catalysed by lithium binaphthoate are greatly affected by the presence of water, which may induce a change from anti- to iyn-adduct formation for those derived from cyclohexanone, for example. " Direct anti- and regio-specific aldol reactions of cyclododecanone with 0 benzaldehyde in NaOH/MeOH have provided building blocks for helical construction of supramolecules. ... 

Simple chiral bisphosphine silver(I) complexes have been reported by Yamamoto to catalyze enantioselective addition reactions of trimethoxysilyl enolates [149, 150] and diketenes [150] to aldehydes. The aldol addition reaction with diketene 301 in Equation 28 is illustrative [150]. The requisite tin enolates are generated in situ from the corresponding enol acetates or diketenes upon exposure to Me3SnOMe or Bu2Sn(OMe)2. This represents a noteworthy example of a transition-metal-catalyzed process involving aldol addition that can be carried out successfully in a polar, protic solvent (methanol). 

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