Catalysis under excess hydrogen

Soon after these initial reports, the groups of Antilla [92] and You [93] indepen dently applied the chiral phosphoric acid catalysis to the enantioselective hydro genation of a imino esters. The method provides an alternative route to the enantioselective synthesis of a amino esters. Antilla and coworkers employed a new type of axially chiral phosphoric acid (9) derived from VAPOL originally developed by his research group (Scheme 3.42), whereas lg was used in You s case. In both cases, excellent enantioselectivities were achieved. You and coworkers further applied the method to the enantioselective reduction of a imino esters having an alkynyl substituent at the a position (Scheme 3.43) [94]. Both alkyne and imine moieties were reduced under transfer hydrogenation conditions with an excess amount of... 

The reaction takes place extremely rapidly and if D2O is present in excess all the alcohol is con verted to ROD This hydrogen-deuterium exchange can be catalyzed by either acids or bases If D30 is the catalyst in acid solution and DO the catalyst in base wnte reasonable reaction mech anisms for the conversion of ROH to ROD under conditions of (a) acid catalysis and (b) base catalysis... 

The enantioselective hydrogenation of prochiral substances bearing an activated group, such as an ester, an acid or an amide, is often an important step in the industrial synthesis of fine and pharmaceutical products. In addition to the hydrogenation of /5-ketoesters into optically pure products with Raney nickel modified by tartaric acid [117], the asymmetric reduction of a-ketoesters on heterogeneous platinum catalysts modified by cinchona alkaloids (cinchonidine and cinchonine) was reported for the first time by Orito and coworkers [118-121]. Asymmetric catalysis on solid surfaces remains a very important research area for a better mechanistic understanding of the interaction between the substrate, the modifier and the catalyst [122-125], although excellent results in terms of enantiomeric excesses (up to 97%) have been obtained in the reduction of ethyl pyruvate under optimum reaction conditions with these Pt/cinchona systems [126-128],... 

Mikami et al. [18] demonstrated that under Lewis acid catalysis silyl enol ethers, bearing at least one hydrogen atom in the allylic position, form carbonyl-ene products. They succeeded in using the titanium catalyst 32 for the asymmetric catalysis of this reaction (Scheme 11b). If the aldehyde contains an activating substituent, as in the case of the glyoxolate 62, an excess of the enecompound is not necessary. For example, the reaction of 62 with the silyl enol ether 27 to the carbonyl-ene adduct 63 still proceedes with good stereoselectivity, but yields drop to a moderate value. 

Lu and co workers found that the asymmetric hydrogenation of the tri substituted or di substituted exocyclic double bond of N tosyl 4 alkylidene 1,3 oxazolidin 2 ones (74), which are enamides in nature, proceeds smoothly under the catalysis of neutral [Rh(COD)Cl]2 and (S) (+) DTBM SEGPHOS (73) with nearly quantitative yields and high enantioselectivities (Scheme 9.20) [64]. This method provided a novel way to prepare chiral N tosyloxazolidinones (75) with high enantiomeric excess, which can be converted into amino acids, amino alcohols, and piperidine derivatives. 

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