Unsaturated model substrates hydrogenation

The catalytic properties of the sulfonated diphosphine-stabilized RuNPs and sulfonated diphosphine/cyclodextrin-stabilized RuNPs were compared in the hydrogenation of unsaturated model substrates (styrene, acetophenone, and w-methylanisole) in biphasic liquid-hquid conditions (i.e., ruthenium aqueous colloidal solution and organic substrate no added solvent). Whilst all of these RuNPs displayed suitable performances in catalysis, different activities and selec-tivities were observed. This highhghted that supramolecular interactions on the metallic surface in the presence of a cyclodextrin control the catalytic reactivity of the nanocatalysts. Interestingly the CD acts as a phase-transfer promotor, which... 

The catalytic properties of the sulfonated diphosphine-stabilized Ru NPs and sulfonated diphosphine/CD-stabilized Ru NPs were compared in the hydrogenation of unsaturated model substrates (styrene, acetophenone, and m-methylanisole) in biphasic liquid-liquid conditions (i.e., ruthenium aqueous colloidal solution and organic substrate no added solvent). 

Aluminium alkoxides were anchored in the pores of siliceous MCM-41 type materials. The resulting catalysts were used in the hydrogen transfer reduction of a,p-unsaturated ketones to the corresponding allylic alcohols. The most active material is obtained by exposure of MCM-41 to a toluene solution of Al(OPr )3. With benzalacetone as a model substrate, optimum reaction conditions are cyclopentanol (hydride donor), toluene (solvent), and addition of 5A molecular sieve (water trapping). 

The conducted experiments [70,71] demonstrate that PVP-Pd complexes are active, selective and stable catalysts. The composition of such catalysts represents a composite system including Pd(II) and Pd(0). The role of the polymer ligands evidently consists in the stabilization of the particular valent states of palladium which are optimum for the substrate hydrogenation. One can assume that in the given catalytic system, Pd(0) promotes the activation of hydrogen, whereas complex-bound Pd(II) promotes the formation of a tr-allyl complex with unsaturated double bonds of the substrate and thus its activation. Furthermore, pyridine rings promote substrate orientation. This assumption enables polymer-metal heterogenized catalysts to be considered as models of catalytic enzyme systems. 

Ca.ta.lysis, Iridium compounds do not have industrial appHcations as catalysts. However, these compounds have been studied to model fundamental catalytic steps (174), such as substrate binding of unsaturated molecules and dioxygen oxidative addition of hydrogen, alkyl haHdes, and the carbon—hydrogen bond reductive elimination and important metal-centered transformations such as carbonylation, -elimination, CO reduction, and... 

Subsequently, List reported that although the method described above was not applicable to the reduction of a,P-unsaturated ketones, use of a chiral amine in conjunction with a chiral anion provided an efficient and effective procedure for the reduction of these challenging substrates [210]. Transfer hydrogenation of a series of cyclic and acyclic a,P-unsaturated ketones with Hantzsch ester 119 could be achieved in the presence of 5 mol% of valine tert-butyl ester phosphonate salt 155 with outstanding levels of enantiomeric control (Scheme 64). A simple mechanistic model explains the sense of asymmetric induction within these transformations aUowing for reliable prediction of the reaction outcome. It should also be noted that matched chirality in the anion and amine is necessary to achieve high levels of asymmetric induction. 

For our initial studies we chose to evaluate the hydrogenation of two unsaturated carbonyl model prochiral substrates with rhodium complexes of chiral ferrocene diphosphine and tetraphosphine ligands using a standard set of conditions. The substrates screened were methyl a-acetamido cinnamate (MAC) and dimethyl iticonate (DIMI). The substrates, catalysts, conditions, and experimental results are shown in Table 1. 

A similar approach has been chosen also in the evaluation of the effect of solvents on the reactivity and adsorptivity of unsaturated substrates. Parameters of solvents, formally resembling those of substituents used in the evaluation of the effect of structure, were defined. These parameters adequately described the effect of solvents on the course of hydrogenation in systems of similar compounds, but became unsatisfactory for other model series. A detailed analysis of these parameters revealed that they could not be freed from the effect of the structure of substrates, which obviously is the cause of their nontransfertibility. 

Wang et al. further used pyrrolidine-sulfonamide 3a to develop a highly enantioselective Michael reaction of cyclic ketones to a,p-unsaturated ketones (chalcones). The synthetically useful 1,5-dicarbonyl compounds were obtained in good yields and with high stereoselectivities (>40 1 dr, up to 97% ee). The most satisfactory results were achieved for six-membered cyclic ketones, whereas cyclopentanone appeared to be a more challenging substrate for this reaction (Scheme 9.26). A possible transition-state model was presented to rationalise the stereochemical outcome in which hydrogen-bond formation between the NH in 3a and the carbonyl group of chalcone caused the increase in reactivity. 

In the early studies of the conjugate addition of aromatic thiols to a,p-unsaturated cyclohexenones, cinchonidine 2 gave the best enantioselectivity (Scheme 6.1), while somewhat lower ee s were obtained with quinine 1 or quinidine 4 [9j. The mechanistic model advanced in these studies involved activation of the thiol by proton transfer to the nitrogen atom of the quinucUdine moiety and hydrogen bonding between the hydroxyl group at the C9 and the carbonyl group of the unsaturated substrate (Scheme 6.1). 

Other empirical models for the prediction of the diastereoselectivity in dihydroxylation reactions of acyclic substrates have been proposed. Thus, the dihydroxylation of certain y-hydroxy-a,j3-unsaturated esters led Stork to suggest the inside alkoxy" model 273 (Scheme 9.35) [197]. The results observed by Stork in the dihydroxylation of (E)-enoate 272 are in good agreement with Houk s related inside alkoxy model for diastereoselective reactions of acyclic substrates [198]. However, for (2)-isomer 276 the overriding consideration are A, 3 interactions, and thus Stork hypothesized that in the reactive conformation 277, the allylic hydrogen is eclipsed with C=C, leading to dihydroxy-lated lactone 279. 

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