Unsaturated ketones, hydrogenation chemoselective

Bianchini and coworkers [126] found a difference in the chemoselectivity between the metals Fe, Ru, and Os in the complexes [M(H2)H(P(CH2CH2PPh2)3)]-BPh4 in the hydrogenation of benzylideneacetone by transfer from iso-propanol. The Fe and Ru catalysts reduced the 0=0 bond to give the allyl alcohol, with Ru more active than iron (TOF 79 IT1 at 60°C for Ru versus 13 IT1 at 80°C for Fe), while the Os catalyst first reduced the 0=0 bond but then catalyzed isomerization of the allyl alcohol to give the saturated ketone (TOF 55 IT1 at 80°C). The difference in reactivity was attributed to the weak binding of the alkene of the allyl alcohol to Fe and Ru relative to Os in these complexes. A variety of selec-tivities was noted for other unsaturated ketones, whereas unsaturated aldehydes were not hydrogenated. 

Electrocatalytic hydrogenation has the advantage of milder reaction conditions compared to catalytic hydrogenation. The development of various electrode materials (e.g., massive electrodes, powder cathodes, polymer film electrodes) and the optimization of reaction conditions have led to highly selective electrocatalytic hydrogenations. These are very suitable for the conversion of aliphatic and aromatic nitro compounds to amines and a, fi-unsaturated ketones to saturated ketones. The field is reviewed with 173 references in [158]. While the reduction of conjugated enones does not always proceed chemoselectively at a Hg cathode, the use of a carbon felt electrode coated with polyviologen/Pd particles provided saturated ketones exclusively (Fig. 34) [159]. 

Chemoselective catalytic reduction of a,/3 unsaturated ketones to allylic alcohols is a challenging problem since, but a few exceptions [1-3], this reaction generally proceeds with formation of saturated ketones or saturated alcohols [4]. This reduction indeed is best carried out with stoicheiometric hydrides [4] but even in this case overreduction products are often obtained [5]. Recently, we reported in a preliminary communication [6] the unprecedented observation that a,/3 unsaturated ketones are reduced to the corresponding allylic alcohols by hydrogen transfer from propan-2-ol over MgO as catalyst according to the following scheme ... 

Table 9.4 Chemoselective hydrogenation of unsaturated ketones to secondary alcohols.

The Baeyer-Villiger rearrangement of cyclohexanone and acetophenone with TS-I/H2O2 proved to be poorly selective [117]. Notably, Ti-P and Sn-P have different chemoselectivities in the oxidation of unsaturated ketones, leading selectively to corresponding epoxides and lactones, respectively [118]. The different oxidation pathways were attributed to the preferential adsorption of hydrogen peroxide on Ti-sites and of the carbonyl group on Sn-sites. 

We recently reported on the different reactivity of copper catalysts when supported on pure silica or on mixed oxides. Thus, the use of very pure, pyrogenic silica, allowed to obtain a catalyst showing excellent chemoselectivity in the hydrogenation of a,P-unsaturated ketones containing also an isolated olefmic bond [Ij. 

Ephedrine was used as chiral modifier of Pd in the hydrogenation of some cyclic a,y9-unsaturated ketones, tetralone and indanone derivatives, affording 50-93 % chemoselectivity for the saturated ketones and up to 36% ee [48]. Hydrogenation of the ( ) isomer was much faster and afforded higher ee than that of the (Z) isomer, though both reactions provided the same major enantiomer. 

Chemoselective Hydrogenation of Unsaturated Ketones in the Presence of Copper Catalysts... 

During our studies on the use of supported copper catalysts in selective hydrogenation reactions, we found that these systems can be effectively applied to the chemoselective hydrogenation of a,P-unsaturated ketones giving quantitative yields in the corresponding saturated ketone also when another olefinic bond is present in the molecule [3]. 

Sn-beta (68,151,235) shows extraordinary chemoselectivity for the BV oxidation of unsaturated ketones using aqueous hydrogen peroxide as... 

A number of bench-scale applications of Ru(P P)(CF3C02)2 complexes were also reported as summarized in Fig. 10. Roche [54] succeeded in the chemoselective C=C hydrogenation of an unsaturated ketone (a vitamin E intermediate)... 

Chen JX, Daeuble JF, Brestensky DM, Stryker JM (2000) Highly chemoselective catalytic hydrogenation of unsaturated ketones and aldehydes to unsaturated alcohols using phosphine-... 

More recently, the Noyori group described an organic solvent- and haUde-free oxidation of alcohols with aqueous H202 . The catalyst system typically consists of Na2W04 and methyltrioctylammonium hydrogen sulfate, with a substrate-to-catalyst ratio of 50-500. Secondary alcohols are converted to ketones, whereas primary alcohols, in particular substituted benzyUc ones, are oxidized to aldehydes or carboxylic acid by selecting appropriate reaction conditions This system also catalyzed the chemoselective oxidation of unsaturated alcohols, the transformation exemplified in equation 65, with a marked prevalence for the hydroxy function. 

Rh and Ir complexes stabilized by tertiary (chiral) phosphorus ligands are the most active and the most versatile catalysts. Although standard hydrogenations of olefins, ketones and reductive aminations are best performed using heterogeneous catalysts (see above), homogeneous catalysis becomes the method of choice once selectivity is called for. An example is the chemoselective hydrogenation of a,/ -unsaturated aldehydes which is a severe test for the selectivity of catalysts. 

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