Newly developed oxo-tethered Ru amido complexes (R,R)-1 and their HCl adducts (R,R)-2 exhibited excellent catalytic performance for both asymmetric transfer hydrogenation and the hydrogenation of ketonic substrates under neutral conditions without any cocatalysts to give chiral secondary alcohols with high levels of enantioselectivity.

A concise asymmetric transfer hydrogenation of diaryl ketones, promoted by bifunctional Ru complexes with an etherial linkage between 1,2-diphenylethylenediamine (DPEN) and η(6)-arene ligands, was successfully developed. Because of the effective discrimination of substituents at the ortho position on the aryl group, unsymmetrical benzophenones were smoothly reduced in a 5:2 mixture of formic acid and triethylamine with an unprecedented level of excellent enantioselectivity. For the non-ortho-substituted benzophenones, the oxo-tethered catalyst electronically discerned biased substrates, resulting in attractive performance yielding chiral diarylmethanols with >99% ee.

Enantioselective protonation of prochiral enolates and enantioselective alkylation of enolates are efficient methods to prepare optically active carbonyl compounds with a tertiary asymmetric carbon center at the a-position. [1,2] The catalytic enantioselective protonation reactions of metal enolates already reported are performed either under basic or under acidic conditions. The method under basic conditions involves, for example, the protonation of a reactive metal enolate, such as lithium enolate, with a catalytic amount of a chiral acid and an excess of an achiral acid. [3] In contrast, the method under acidic conditions uses silyl enolates or ketene silyl acetals as substrates which, in the presence of a chiral Lewis acid or a chiral Brønsted acid catalyst, are converted into optically active carbonyl compounds.[4] Binap·AgF is an efficient chiral catalyst for the asymmetric aldol reaction of silyl enolates [5] and also for the asymmetric allylation of aldehydes with allylsilane.[6] Because the activation of a trimethoxysilyl group by the fluoride ion is remarkable, we envisioned that the silver fluoride complex could also act as a chiral catalyst for the asymmetric protonation of silyl enolates with an appropriate achiral proton source such as methanol. We report here a new catalytic asymmetric protonation of silyl enolates with methanol using binap·AgF as a chiral catalyst [Eq. (1)].We initially examined the protonation of a 2-methyl-1-tetralone-derived trimethylsilyl enolate with methanol to find the optimal reaction conditions. We attempted the reaction employing diverse ratios of binap and AgF at À20 8C and found that a 0.6:1 mixture yielded a nonracemic product with higher enantioselectivity than a 1:1 mixture [Eq. (2)]. As we have reported previously, various ratios of binap and AgF were examined by 1 H NMR spectroscopy and the 0.6:1 mixture was found to give the desired 1:1 complex without formation of the unreactive 2:1 complex. [5] We then studied the influence of the solvent on yield and enantioselectivity (Table 1). Among the solvents tested, THF or chlorinated hydrocarbons gave better enantioselctivities than methanol, and dichloromethane was the solvent of choice. When the protonation was performed in a 20:1 mixture of dichloromethane and methanol, (S)-enriched 2-methyl-1-tetralone was obtained with 56 % ee (entry 7). A further improvement in the enantiomeric ratio was achieved when twice as much solvent was used (entries 8 and 9). We also investigated the enantioselectivity of this catalytic protonation with other achiral alcohols but methanol proved most efficient (entries 7, 10, and 11).This asymmetric protonation was applied to a variety of trimethylsilyl enolates; the results with 2-methyl-1-tetralone and related ketone derivatives are summarized in Table 2. Both the 5-methoxy and the 2-ethyl derivatives gave good optical purities similar to that of 2-methyl-1-tetralone (entries 1-3). However, to our surprise, use of the 2,2,6-trimethylcyclohexanone-derived silyl enolate resulted in a

Protecting-group-free transformation is a challenging and important issue in atom-economical organic synthesis. The η(6)-arene/N-Me-sulfonyldiamine-Ru(II)-BF4 complex-catalyzed asymmetric hydrogenation of 2-substituted unprotected indoles in weakly acidic hexafluoroisopropanol gives optically active indoline compounds with up to >99% ee. Under mild reaction media, halogen atoms and synthetically important protecting groups (e.g., silyl ether, acetal, benzyl ether, and ester) on indoles are maintained, which is advantageous for the synthesis of further complex indoline molecules.