Characterization Raney nickel

If, for the purpose of comparison of substrate reactivities, we use the method of competitive reactions we are faced with the problem of whether the reactivities in a certain series of reactants (i.e. selectivities) should be characterized by the ratio of their rates measured separately [relations (12) and (13)], or whether they should be expressed by the rates measured during simultaneous transformation of two compounds which thus compete in adsorption for the free surface of the catalyst [relations (14) and (15)]. How these two definitions of reactivity may differ from one another will be shown later by the example of competitive hydrogenation of alkylphenols (Section IV.E, p. 42). This may also be demonstrated by the classical example of hydrogenation of aromatic hydrocarbons on Raney nickel (48). In this case, the constants obtained by separate measurements of reaction rates for individual compounds lead to the reactivity order which is different from the order found on the basis of factor S, determined by the method of competitive reactions (Table II). Other examples of the change of reactivity, which may even result in the selective reaction of a strongly adsorbed reactant in competitive reactions (49, 50) have already been discussed (see p. 12). 

Enzymic syntheses are considered next. Xylitol is a substrate for sheep-liver L-iditol dehydrogenase, a NAD-linked enzyme. 1-Deoxy-D-xylitol, prepared by Raney nickel reduction of D-xylose diethyl dithioacetal in a 27% overall yield from D-xylose, was also reported31 to be a substrate, although with a higher Km and lower Vmax. The product was assumed to be l-deoxy-D-f/ireo-pentulose because of the appearance of a yellowish fluorescent spot when a chromatogram was sprayed with acidic 3,5-aminobenzoic acid, resembling that formed from 1 -deoxyfructose. There was no more-rigorous characterization. 

Several extraction techniques have also been described that use enzymatic or chemical reactions to improve extraction efficiency. A technique that has been used to increase the overall recovery of the marker residue is enzymatic hydrolysis to convert specific phase II metabolites (glucuronides or sulfates) back into the parent residue. Cooper etal used a glucuronidase to increase 10-fold the concentration of chloramphenicol residues in incurred tissue. As an example of a chemical reaction, Moghaddam et al. used Raney nickel to reduce thioether bonds between benomyl and polar cellular components, and as a result achieved a substantially improved recovery over conventional solvent extraction. In choosing to use either of these approaches, thorough characterization of the metabolism in the tissue sample must be available. 

Thus, 5-amino-2-mercapto-l-methylimidazole (96 R = Me, R2 = SH) (65%) was obtained (48JCS2028) from 5-amino-2-methylaminothiazole (113 R1 = Me) on treatment with aqueous sodium carbonate. The aminoim-idazole (96 R1 = Me, R2 = SH) was found to be unstable in air, but gave a stable hydrochloride salt and on treatment with Raney nickel was desulfurized to give 5-amino-1-methylimidazole (96 R = Me, R2 = H) (58%), which was also unstable in air and was characterized as its picrate salt. 

Reduction of the nitro group of 545-547 in the presence of Raney nickel catalyst respectively afforded the corresponding 4-amino-pento-, -hexo-, and -hepto-pyranosides 548-550. Methyl 4-amino-2,3,4,6-tetradeoxy-a- and -/3-DL-en/t/iro-hexopyranoside (549), characterized as the A -benzoyl derivative, was identical in its H-n.m.r.-speetral data with the analogous derivative of the natural, antibiotic sugar tolyposamine. On the other hand, reductive demethyl-ation of 549 with formaldehyde-Raney nickel (under 3.5 kg/cm2 pressure of hydrogen) was effected, to yield another antibiotic sugar, methyl DL-forosaminide (551). 

D-Galactose was converted by ethanethiol and hydrochloric acid into crystalline D-galactose diethyl dithioacetal, which was acetonated with acetone-zinc chloride. The product (15) was reduced to the L-fu-citol derivative (16) with Raney nickel. The overall yield of 16 was 29%, and it was characterized as the crystalline 6-p-toluenesulfonate 17. Oxidation of 16 by the Pfitzner-Moffatt reagent55 proceeded readily and, after O-deacetonation, and purification of the product by chromatography on a column of silica gel, L-fucose (18 13% overall yield from D-galactose) and L-fucitol (19 1% yield) were isolated (see Scheme 3). 

An approach to the synthesis of angularly substituted polycyclics through the Diels-Alder cycloaddition of dihydrothiophenes has been devised (69JA7780). The easily prepared 2,5-dihydro-4-methoxycarbonyl-2-thiopheneacetic acid methyl ester (316) was heated at 180 °C with excess butadiene to yield (317). Desulfurization and double bond reduction of the cycloadduct with W-5 Raney nickel gave (318) which was characterized by conversion to the corresponding diacid and comparison with an authentic sample. Dieckmann cyclization of (318) is known to lead to the 5-methyl-1-hydrindanone (319 Scheme 68). The use of other dienes in the [4 + 2] cycloaddition process will, of course, produce more highly functionalized hydrindanones. 

Table 1 Composition and characterization of Raney-nickel catalysts...

Kiros, Y. Majari, M. Nissinen, T. A. Effect and characterization of dopants to Raney nickel for hydrogen oxidation. Journal of Alloys and Compounds 2003 360(1-2) 279-285. 

Peripheral double bonds of porphyrins and hydroporphyrins may be hydrogenated catalytically. Zn-OEP on treatment with hydrogen at 180 and 90°C in dioxane solution gave 8% cis-OEC (12) after demetallation (69TL1145). Low yields of TPBC (34) and TPiBC (35) were obtained from a Raney nickel reduction of TPP (33) in ether and dioxane, respectively. Tetrahydroporphyrins, however, were not isolated, and characterized only on the basis of their optical spectra (52JA6101). 

Methyl 4,6-0-benzylidene-3-deoxy-a-D-ribo-hexopyranoside (56) was benzoylated, debenzylidenated, and partially p-toluenesulfon-ylated to 57 this was converted into 58 by reaction with sodium iodide, followed by catalytic reduction. The methanesulfonate of 58 was converted into 59 by reaction with sodium azide in N,N-dimethylformamide, and 59 was converted into 4-azido-3,4,6-trideoxy-a-D-xylo-hexose (60) by acetolysis followed by alkaline hydrolysis. Reduction of 60 with borohydride in methanol afforded 61, which was converted into 62 by successive condensation with acetone, meth-anesulfonylation, and azide exchange. The 4,5-diazido-3,4,5,6-tetra-deoxy-l,2-0-isopropylidene-L-ara/uno-hexitol (62) was reduced with hydrogen in the presence of Raney nickel, the resultant diamine was treated with phosgene in the presence of sodium carbonate, and the product was hydrolyzed under acidic conditions to give 63. The overall yield of 63 from 56 was 4%. The next three reactions (with sodium periodate, the Wittig reaction, and catalytic reduction) were performed without characterization of the intermediate products, and gave (+)-dethiobiotin methyl ester indistinguishable from an authentic sample thereof prepared from (+)-biotin methyl ester. 

ESMS has been used to characterize the intermediate Nin-complexes formed in the coupling reaction of 2-bromo-6-methylpyridine in the presence of Raney nickel (Scheme 1) [45]. The composition of the intermediate had already been determined previously by elemental analysis, but the ES mass spectra, showing a strong peak for the ion [Ni2(dmbp)2Br3]+, pointed to a dimeric structure. It was concluded that this ion was formed by the loss of Br from the dimeric structure 1. An alternative explanation is that the intermediate has the more common four-coordinate structure 2, and that the observed peak was due to the ion-paired species [Ni2(dmbp)2Br2]2++Br. The dimeric nature of the intermediate was confirmed by a cross experiment when mixtures of differently substituted pyridines were reacted, mixed ligand dinickel species were observed in the ES mass spectra. 

Using data from other alkaloids, the NMR data enabled substantial progress to be made in the placement of the esterifying ester groups, but in these compounds the cathate bridge provided a novel feature. In the cases of both E3 and E4 this dilactone bridge could be opened by hydrogenolysis over palladium or by Raney nickel treatment to form 55 and 56, respectively. A new methyl resonance appeared in the NMR spectrum, and the new phenolic hydroxyl was characterized by a reversible bathochromic shift in the UV spectrum on the addition of... 

High r factors are, however, not without some other complications since they imply porosity of materials. Porosity can lead to the following difficulties (a) impediment to disengagement of evolved gases or of diffusion of elec-trochemically consumable gases (as in fuel-cell electrodes 7i2) (b) expulsion of electrolyte from pores on gas evolution and (c) internal current distribution effects associated with pore resistance or interparticle resistance effects that can lead to anomalously high Tafel slopes (132, 477) and (d) difficulties in the use of impedance measurements for characterizing adsorption and the double-layer capacitance behavior of such materials. On the other hand, it is possible that finely porous materials, such as Raney nickels, can develop special catalytic properties associated with small atomic metal cluster structures, as known from the unusual catalytic activities of such synthetically produced polyatomic metal clusters (133). 

Ring closures to 2-oxa-6-aza-adamantanes starting from bicyclic dienes were also accomplished by hydroxybromination of the 9-aza-diene 62 with N-bromo-succinimide (-> 63 53%) and by reaction of the 9-oxa-diene 1 with N -dibromo-p-tolylsulfonamide (- 64 25%). Both dibromides were converted to 63 (raney-nickel/Hj), which itself by treatment with sodium in liquid ammonia, yielded unsubstituted 2-oxa-6-aza-adamantane (5S), also characterized as its hydrochloride 38 HCl and N-benzoyl-derivative 66. 

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