Hydrogenation bifunctional catalysts

Feedstock Reactions Catalyst References Acetone Condensation-hydrogenation (bifunctional catalysis) Pd on sulfonated PS-DVB [6] Methanol, Raffinate II Condensation, hydrogenation Pd on sulfonated PS-DVB [61] Dioxygen dissolved in water Hydrogenation Pd on sulfonated PS-DVB [8]... 

The metallic component of HCK catalysts provides hydrogenation, dehydrogenation, hydrogenolysis, and isomerization. The number and nature of reactive hydrogen species created by the interaction of a bifunctional catalyst with hydrogen is not well understood [103], on the other hand, neither the action of those species on the catalytic sites is understood. The main limitation in this understanding is the dynamic character of the interaction however, now that in situ characterization techniques are becoming available, research would soon defeat the limitations. 

In summary, in order to reduce the content of toluene in gasoline while keeping a high octane number, toluene must undergo hydrogenation and ring contraction followed by SRO. The RC step can proceed via bifunctional catalysts and the SRO must use a metal catalyst (e.g. Ir/Si02) that is selective towards the dicarbene mechanism to cleave C-C bonds at unsubstituted positions. 

There are several examples of one-pot reactions with bifunctional catalysts. Thus, using a bifunctional Ru/HY catalyst, water solutions of corn starch (25 wt.%) have been hydrolyzed on acidic sites of the Y-type zeolite, and glucose formed transiently was hydrogenated on ruthenium to a mixture of sorbitol (96%), mannitol (1%), and xylitol (2%) [68]. Similarly a one-pot process for the hydrolysis and hydrogenation of inulin to sorbitol and mannitol has been achieved with Ru/C catalysts where the carbon support was preoxidized to generate acidic sites [69]. Ribeiro and Schuchardt [70] have succeeded in converting fructose into furan-2,5-dicarboxylic acid with 99% selectivity at 72% conversion in a one-pot reaction... 

Shortly thereafter, Terada demonstrated that the Mannich reaction between several N-Boc aryl imines and acetoacetone was effectively catalyzed by only 2 mol% of le (Scheme 5.2) [4]. In view of AMyama s work, this study is particularly significant because it suggested that le may act as a bifunctional catalyst [9] not only to form a chiral ion pair with the electrophile but also to activate the nucelo-phile through hydrogen bonding of the a-proton with Lewis basic phosphoryl oxygen. 

Several years later, Corey disclosed the C2 symmetric bicyclic guanidine 19 as an effective bifunctional catalyst for the Strecker reaction (Scheme 5.40) [74]. According to the catalytic cycle, HCN should hydrogen bond to the catalyst to form guanidinium-cyanide complex A. A subsequent increase in acidity of the catalyst N—H proton allows donation of a hydrogen bond to the aldimine to form TS assembly B. Enantiofacial attack of CN to the bound aldimine gives the Strecker product. 

M. Shi and Y.-L. Shi reported the synthesis and application of new bifunctional axially chiral (thio) urea-phosphine organocatalysts in the asymmetric aza-Morita-Baylis-Hillman (MBH) reaction [176, 177] of N-sulfonated imines with methyl vinyl ketone (MVK), phenyl vinyl ketone (PVK), ethyl vinyl ketone (EVK) or acrolein [316]. The design of the catalyst structure is based on axially chiral BINOL-derived phosphines [317, 318] that have already been successfully utilized as bifunctional catalysts in asymmetric aza-MBH reactions. The formal replacement of the hydrogen-bonding phenol group with a (thio)urea functionality led to catalysts 166-168 (Figure 6.51). 

In 2008, Tang and co-workers reported the utilization of tertiary amine-functionalized saccharide-thiourea 211 as a bifunctional hydrogen-bonding catalyst for the enantioselective aza-Henry [224] (nitro-Mannich) addition [72] of... 

Scheme 6.178 Typical products provided from the asymmetric aza-Henry addition of nitromethane to N-Boc-protected aldimines in the presence of saccharide thiourea 211 as bifunctional hydrogen-bonding catalyst.

Within the monohydridic route, apart from the already explained inner-sphere mechanisms, there is another possibility involving the concerted outer-sphere transfer of one hydride and one proton to the corresponding substrate (Scheme 4b). This mechanism is very common to the so-called bifunctional catalysts. This term was proposed by Noyori for those catalysts having one hydrogen with hydridic character directly bonded to the metal center of the catalyst, a hydride ligand, and another hydrogen with protic character bonded to one of the ligands of the metal complex (20). In Scheme 9, examples of bifunctional catalysts that are synthesized... 

The platforming catalyst was the first example of a reforming catalyst having two functions.43 44 93 100-103 The functions of this bifunctional catalyst consist of platinum-catalyzed reactions (dehydrogenation of cycloalkanes to aromatics, hydrogenation of olefins, and dehydrocyclization) and acid-catalyzed reactions (isomerization of alkanes and cycloalkanes). Hyrocracking is usually an undesirable reaction since it produces gaseous products. However, it may contribute to octane enhancement. n-Decane, for example, can hydrocrack to C3 and C7 hydrocarbons the latter is further transformed to aromatics. 

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