Desulfurization of thiophene

Through the work of especially Goldfarb et al. but also of Buu-Hoi et al., Badger et al., and Wynberg et al., the Raney nickel desulfurization of thiophenes has been developed to an important tool for the synthesis of aliphatic compounds. During the last 5 years over fifty papers have appeared describing applications of this method. 

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. 

Raney nickel desulfurization has been applied especially to the synthesis of different kinds of amino acids. a-Amino acids have been prepared by the Strecker synthesis of substituted thiophenealdehydes, followed by desulfurization of the thiophene a-amino acids. a-Amino-n-enantic acid, a-amino-n-caprylic acid, and norleucin have been obtained in about 50% yield from the appropriate thiophene aldehydes. From the desulfurization of thiophene -amino acids, obtained from the reaction of thiophenealdehydes with malonic acid in ammonia, aliphatic j8-amino acids, isolated as acetates, have been obtained in high yields. The desulfurization of 3-nitrothiophenes, such as (232), in ammonia leads to y-substituted amino acids (233). ... 

Progress and prospects in hydrogenation, hydrogenolysis, and desulfurization of thiophenes with soluble metal complexes 98ACR109. 

An important special case of RSR reduction is desulfurization of thiophene derivatives. This proceeds with concomitant reduction of the double bonds. Many compounds have been made by alkylation of thiophene, followed by reduction ... 

Raney nickel desulfurization of thiophenes is an important technique of chain extension. Ring fission is accompanied by saturation of the ring carbon atoms and chain extension by four carbon atoms is effected. This method has been widely used to prepare alkanes, ketones and carboxylic acids and their derivatives (B-74MI30200). An illustrative example is given in Scheme 38 see also CHEC 1.16 and CHEC-II(2)522. 

Raney nickel reduction of 2-benzyl-5-ethylselenophene (71) yields 1-phenylheptane (72), a conversion analogous to the much used reductive desulfurization of thiophenes (73JGU871). The electrochemical reduction of selenophene-2-carboxylic acid gives a mixture of dimeric products the major product is compound (73). This is in contrast to the 2,5-dihydro derivatives obtained by electrochemical reduction of thiophene and furan carboxylic acids (82CS( 19)95). Wolff-Kishner reduction of 2-selenienyl 2 -thienyl ketone gives, in addition to the expected methylene derivative, 2-(pentenyl)thiophene (72ZOB1780). 

A pertinent example of steric hindrance on the desulfurization of thiophenes has been reported by a Gulf investigator (1). Alkyl or ring addition at the 2 position can reduce the reaction rate by a factor of 100. 

As complex as the desulfurization of thiophene might appear, projection of the kinetic picture to benzothiophene and dibenzothiophene, and to their derivatives, is even more complex. As has already been noted for bond energy data, kinetic data derived from model compounds cannot be expected to include contributions from the various steric effects that are a consequence of complex molecules containing three-dimensional structures. Indeed, such steric effects can lead to the requirement of additional catalyst and process parameters for sulfur removal (Isoda et al., 1996a, 1996b). 

Results of experiments with benzothiophene and pure NaOH serve to emphasize the difference between NaOH and KOH. Even at reaction times of 6 hours, no benzothiophene decomposition was observed with NaOH. These results not only emphasize the role of KVKOH but suggest that KVKOH is a necessary part of the hydroxide mixture for decomposition and ultimate desulfurization of thiophene-ring systems. 

Desulfurization of thiophene on metal surfaces and with organometalic complexes 92CRV493. 

Figure 2. Effect of preadsorbed H2 on per cent desulfurization of thiophene shots...

Thiophenes are desulfurized to give products with a C4 carbon chain (see eqs. 13.60 and 13.63), and hence the desulfurization of thiophene derivatives has been applied for syntheses of compounds with longer alkyl chains, in particular, for long-chain compounds, which are not easily obtainable by conventional synthetic methods. Examples are shown in eqs. 13.87-13.90. 

The insertion of a metal atom into the C-S bond might constitute the first stage in the ultimate desulfurization of thiophene derivatives. It is therefore of great interest that several reactions have been discovered from which such metallothiacycles could be isolated. 

Joffre, J., P. Geneste, and D. A. Lerner (1986). A quantum chemical study of site modeling for the adsorption and desulfurization of thiophene. J. Catalysis 97, 543. 

A progressive loss in catalytic activity occurs in the plant reactor from small amounts of thiophene present in the feed. The sulfur acts as an irreversible poison of the metallic catalyst, ft has been suggested that hydrogen sulfide is the sulfiding agent, and mechanistically this occurs from desulfurization of thiophene to hydrogen sulfide on the catalyst (4). 

Desulfurization of thiophenes 88MI17 89CRV459 90MI13. Electrochemical functionalization of halothiophenes 90S369. Electrophilic substitution of thiophenes 86HC(44,2)1. 

From comparative studies, it appears that a number of G=C double bonds are tolerated only when Raney Ni has been deactivated.It has been claimed" that desulfurization of thiophenes could lead to acceptable yields of unsaturated products, but this result seems to be due to steric hindrance in the unsaturation formed. Curiously it has been found that Raney Ni desulfurization of unsaturated lithium thio-lates resulting from Li-EtNH2 desulfurization (vide infra) takes place easily, while direct reduction of the starting heterocycles is unsatisfactory (Scheme 1). ... 

The ET-induced desulfurization of thiophene and related sulfur heteroaromatics by the action of metal naphthalenides [108], metal hydride-metal complexes [109], or transition metals [110] is a process with high practical significance, in view of its implication in the desulfurization of coal. 

X-ray photoelectron spectroscopy and first-principles DF calculations were used to investigate the desulfurization of thiophene (a typical test molecule in HDS studies) and... 

As previously mentioned, the stoichiometric desulfurization of thiophenes has been achieved with a relatively large number and variety of metal complexes. In general, polynuclear complexes containing both component (Mo or W) and promoter (Ni, Co, Ir, Ru) metals turn out to be more active than mononuclear complexes containing promoter metals [1, 2]. A paradigmatic case has been reported in which the hydrogenolysis of BT to either 2-vinylthiophenol or 2-ethylthiophe-nol is a facile process for the promoter (Rh), but the desulfurization step to ethylbenzene requires the compulsory assistance of a component metal (W) to take place [33]. 

RING OPENING, HYDROGENOLYSIS AND DESULFURIZATION OF THIOPHENES BY METAL COMPLEXES. 

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