Raney nickel desulfurization catalyst

Reduction. Raney Nickel desulfurization of 1,3-dithiolanes effects overall reduction of C=0 to CH2, as does Sodium-Ammonia in THF (eqs 4 and 5). Na/hydrazine is an alternative reagent (eq 6). Peptidic sulfoxides are reduced to thioethers with (1) and an electrophilic catalyst. ... 

Single-bond cleavage with molecular hydrogen is termed hydrogenolysis. Palladium is the best catalyst for this purpose, platinum is not useful. Desulfurizations are most efficiently per-formed with Raney nickel (with or without hydrogen G.R. Pettit, 1962 A or with alkali metals in liquid ammonia or amines. The scheme below summarizes some classes of compounds most susceptible to hydrogenolysis. 

Active Raney nickel induces desulfurization of many sulfur-containing heterocycles thiazoles are fairly labile toward this ring cleavage agent. The reaction occurs apparently by two competing mechanisms (481) in the first, favored by alkaline conditions, ring fission occurs before desul-, furization, whereas in the second, favored by the use of neutral catalyst, the initial desulfurization is followed by fission of a C-N bond and formation of carbonyl derivatives by hydrolysis (Scheme 95). 

Two different sets of experimental conditions have been used. Buu-Hoi et al. and Hansen have employed the method introduced by Papa et using Raney nickel alloy directly for the desulfurization in an alkaline medium. Under these conditions most functional groups are removed and this method is most convenient for the preparation of aliphatic acids. The other method uses Raney nickel catalysts of different reactivity in various solvents such as aqueous ammonia, alcohol, ether, or acetone. The solvent and activity of the catalyst can have an appreciable influence on yields and types of compounds formed, but have not yet been investigated in detail. In acetic anhydride, for instance, desulfurization of thiophenes does not occur and these reaction conditions have been employed for reductive acetylation of nitrothiophenes. Even under the mildest conditions, all double bonds are hydrogenated and all halogens removed. Nitro and oxime groups are reduced to amines. 

Catalytic hydrogenation of thiophene poses a problem since noble metal catalysts are poisoned, and Raney nickel causes desulfurization. Best catalysts proved to be cobalt polysulfide [425], dicobalt octacarbonyl [426], rhenium heptasulfide [5i] and rhenium heptaselenide [54]. The last two require high temperatures (230-260°, 250°) and high pressures (140, 322 atm) and give 70% and 100% of tetrahydrothiophene (thiophane, thiolene), respectively. 

Thioketals, unlike ordinary ketals, are formed readily from ketones and thiols (RSH) in the presence of acid catalysts. The desulfurization procedure usually goes well, but the product is rather difficult to separate by extraction from the large excess of Raney nickel required for optimum yields. 

The activity of the Raney nickel catalyst greatly affects the yield of the desulfurized pyrimidine. A catalyst described by Brown 3 gave very satisfactory results. A Raney nickel C described by Hurd and Rudner 4 is perhaps a more reactive catalyst however, the yield of desulfurized pyrimidine was not sufficiently better to warrant its use. 

The carbon-nitrogen double bond of phenanthridine can be reduced selectively by hydrogenation over Raney nickel, and attempted reductive dechlorination of 6-chloro derivatives in the presence of this catalyst normally results in the formation of the corresponding 5,6-dihydro compounds.106 Hydrogenations over palladium catalysts are more successful.203 325 Desulfurization of phenanthridinthione... 

Desulfurization (13, 158-159).1 In addition to desulfurization of thiols, thioethers, sulfoxides, and sulfones, Cp2Ni-LiAlHi (1 1) can serve as a hydrogenation catalyst for reduction of alkenes it also reduces enones to ketones, but in low yield. In general, it is similar in reactivity to Raney nickel and to (2,2 -bi-... 

Raney nickel is very probably the most commonly used nickel catalyst. It is also the most versatile of catalysts. In one or other of its several modifications it has been used for hydrogenations over a wide range of pressures varying from high to subatmospheric, for desulfuration, for dehalogenation, and for very many other reactions. 

Phenylthioalkylation of silyl enol ethers. Silyl enol ethers of ketones, aldehydes, esters, and lactones can be alkylated regiospecifically by a -chloroalkyl phenyl sulfides in fhe presence of a Lewis acid. Zinc bromide and titanium(IV) chloride are the most effective catalysts. The former is more satisfactory for enol ethers derived from esters and lactongs. ZnBr2 and TiCL are about equally satisfactory for enol ethers of ketones. The combination of TiCL and Ti(0-f-Pr)4 is more satisfactory for enol ethers of aldehydes. Since the products can be desulfurized by Raney nickel, this reaction also provides a method for alkylation of carbonyl compounds. Of more interest, sulfoxide elimination provides a useful route to a,B-unsaturated carbonyl compounds. 

Alkylidene-l,3-thiazolin-5-ones (215) are also reduced by the action of sodium borohydride, but in this case only the exocyclic double bond reacts. Similarly, this reagent may be used to reduce carbonyl groups in the side chains of thiazoles without any effect on the heterocycle itself. The thiazole ring also survives hydrogenation over Raney nickel catalyst at atmospheric pressure, but the fully reduced thiazole ring system is vulnerable to desulfurization. This occurs under quite mild conditions, and thus when... 

Phenothiazines, like phenoxazines (Section 3.8.3.2.2i), are relatively stable compounds and unsaturated substituents attached to the carbocycles can be reduced by the usual reagents, leaving the nuclei intact. There is one important difference, however, namely that phenothiazines may be desulfurized. An early example is provided by the degradation of phenothiazines (239) to diphenylamines (240) by hydro-genolysis over Raney nickel catalyst. 

Aldehyde synthesis [1, 730, before references]. Gottstein et a/.48 developed an efficient method for the transformation of a tertiary amine salt of a penicillin (1) according to Scheme I into the thioacid salt (2) through the ethoxy formic anhydride. The salt is then desulfurized with No. 28 Raney nickel catalyst (W. R. Grace Co.)... 

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