Esters enol, hydrogenation

The DKR processes for secondary alcohols and primary amines can be slightly modified for applications in the asymmetric transformations of ketones, enol esters, and ketoximes. The key point here is that racemization catalysts used in the DKR can also catalyze the hydrogenation of ketones, enol esters, and ketoximes. Thus, the DKR procedures need a reducing agent as additional additive to enable asymmetric transformations. 

Eastman Chem. Co. has utilized a Ru(I) (R,R)-dimethyl-DuPhOS catalyst, based on singleisomer 2,5-dialkylphospholane ligands, to hydrogenate an enol ester such as 4-phenyl-1,3-butadien-2-yl acetate, to give 4-phenyl-3-buten-(2R)-yl acetate in 94% ee (Stinson, 1999). 

Enol esters have a similar structure as enamides. However, in contrast to many highly enantioselective examples on enantioselective hydrogenation of enamides, only a few successful results have been reported for the hydrogenation of... 

Hydrogenation of N-Acyl Enamides, Enol Esters and Enol Carbamates... 

The rhodium-catalyzed enantioselective hydrogenation of enol esters is an alternative to the asymmetric reduction of ketones. Although enol esters are accessible both from ketones and alkynes, the number of studies reporting successful asymmettic hydrogenation has been limited. It appears that, compared... 

Reetz and Goossen et al. reported recently the asymmetric hydrogenation of a series of enol esters using monodentate phosphite ligands 17 and 24 based on a combination of BINOL and carbohydrates or simple alcohols the results of these studies are shown in Table 28.6. 

Unprecedented e.e.-values were obtained using ligand 24b in the hydrogenation of aliphatic enol esters. A furyl substituent on the carboxylate is apparently... 

Table 28.6 Enantioselective hydrogenation of aliphatic enol esters. R2...

Asymmetric Hydrogenation of Enol Esters. Prochiral ketones represent an important class of substrates. A broadly effective and highly enantioselective method for the asymmetric hydrogenation of ketones can produce many useful chiral alcohols. Alternatively, the asymmetric hydrogenation of enol esters to yield a-hydroxyl compounds provides another route to these important compounds. 

While Rh-DuPhos mediated asymmetric hydrogenation of acyclic enol esters shows high levels of enantioselectivity, it does not provide the same high... 

The asymmetric hydrogenation of enol esters can also be catalyzed by chiral amidophosphine phosphinite catalysts derived from chiral amino acids, but the enantioselectivity of these reactions has thus far been only moderate.35... 

Although enol esters have a similar structure to enamides, they have proven more difficult substrates for asymmetric hydrogenation, which is evident from the significantly fewer number of examples. One possible explanation is the weaker coordinating ability of the enol ester to the metal center, as compared to the corresponding enamide. Some rhodium complexes associated with chiral phosphorous ligands such as DIPAMP [100, 101] and DuPhos [102] are effective for asymmetric hydrogenation of a-(acyloxy)acrylates. 

Anodic oxidation of cyclic enol esters with /1-hydrogens leads to allyl radicals, which then lose acyl radical to form a, /1-unsaturated ketones. When the electrolysis is performed in an undivided cell, these are converted by the cathode into enolate anion radicals, which then couple to form /1-dimers (Scheme 66)164. 

The asymmetric hydrogenation of enol esters is an alternative to asymmetric ketone hydrogenation. The precursors can be prepared from the ketones but also via ruthenium-catalyzed addition of the carboxylic acids to the 2-postion of terminal alkynes. This latter method allows the study of the effect of the carboxylate on the enantioselectivity of the asymmetric hydrogenation. A remarkable study by Reetz and colleagues established that it is possible to hydrogenate enolate... 

Not all /S-diketone arrangements remain unperturbed however. Compound (J) and other esters show significant 13C NMR chemical shift changes in going to the BF2 chelate and it is deduced that in (J) the H in the enol tautomer is located near the carbonyl shown 601. This result proves that Shapet ko s method is not insensitive to the proton s location. However in the light of what follows (Sect. III.C) it will be seen that the potential energy well of the cis enol tautomer hydrogen bond must be a double minimum. 

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