Supported hydrogen-bonded

An alternative to the supported liquid phase (SLP) method was recently proposed using the same type of solid the supported hydrogen bonded (SHB) catalysts. First demonstrated with the zwitteronic Rh1 catalysts [(sulphos)Rh(cod)] (1) and [(sulphos)Rh(CO)2] (2) (sulphos = 03S(Cr,I I4)CI I2C(CH2PPh2) ). the immobilization procedure is based on the capability of the sulfonate tail of sulphos to link the silanol groups of the support via hydrogen bonds (Figure 12). 

Figure 3.12. Silica grafted catalysts for hydroformylation and hydrogenation, the immobilisation is based on hydrogen bonding between the surface and the sulfonate group (supported hydrogen-bonded (SHB)...

For this reason, these catalysts are also known under the name of supported hydrogen-bonded (SHB) catalysts and, in conjunction with Pd° particles on the same support material, have contributed to generate active heterogeneous systems for the hydrogenation of benzenes in aprotic solvents. Irrespective of the substrate, the combined single-site/dispersed-metal catalyst RhI-Pd0/SiO2 shown in Figure 16.5a was from four- to six-fold more active than supported palladium... 

Fig. 16.5 Some examples of supported hydrogen-bonded catalysts (as described by Bianchini).

Weak Chemisorption Supported Hydrogen-Bonded (SHB) Catalysts... 

Whitman et al. also evaluated the effect of pretreatments of zirconia with a number of Lewis bases on normal phase selectivity. Chemometric methodology was used to characterize the similarities and differences between the acid - and base -washed supports. Hydrogen bonding interactions are quite pronounced for the Lewis base-modified zirconia, the extent of which differs greatly among the various Lewis bases used to modify the zirconia. 

The formation of an complex has also been proposed to occur in the solid-liquid hydrogenation of alkenes with the supported hydrogen-bonded (SHB) catalyst [(sulphos)Rh(cod)]/Si02 (sulphos = 03S(C6H4)CH2C(CH2PPh2)3 cod = cyclo-octa-1,5-diene) (Scheme 10) [55-56]. 

Epoxidation with a Supported Hydrogen-Bonded Catalyst. 64... 

One limitation of octanol as a solvent is the hydroxyl group and in addition its relatively high (4%) aqueous content (Smith et al 2012). Thus octanol supports hydrogen bonding and by itself will not completely characterize the behavior of a molecule. Alternative portioning systems such as cyclohexane are experimentally difficult, so the characterization of hydrogen bonding is normally carried out by simple calculations. 

Evidence supporting hydrogen bonding between cyanide ions bound to ferric porphyrins and the solvent has been obtained (see, e.g., refs. 176 and 177), and the effects of aqueous and reversed micelles on the interaction of surfactant-solubilized monomeric haemin with imidazole in water, cyclohexane, and benzene have been reported. The reduction of haemin (Hm) by dithionite has been studied in sodium dodecyl sulphate micelles (added to render the haemin monodisperse) and the following scheme proposed ... 

This elaborated methodology should provide an easy access to glycopeptide mimetics. To demonstrate that we reacted maltose 124 as an example for the deployment of disaccharides in these cascade reactions (Scheme 2.25), the expected lactone 125 was isolated with 33% yields after 3 h in refluxing methanol. These yields are comparable with those obtained in reactions of glucose with L-proline and ethyl isocyanoacetate (114 36% Scheme 2.23). But an increase in syn-selectivity was detected, indicating further supporting hydrogen bonds (compare 114 60/40 with 125 83/17). 

A thorough DPT investigation of diverse pathways in the intramolecular hydroamination of (177), catalysed by the iridium chloropyrazole complex (178) and affording pyrrolidine (179), has defined the mechanism that features turnover-limiting reductive cycloamine elimination from an iridium-hydrido-alkyl intermediate. The vital role of a supportive hydrogen-bonded network (180) and cooperative activation of aminoalkenes with iridium-pyrazolato complexes have also been identified. " ... 

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