Hydrogenation thiophene derivatives

At lower temperatures oxygenated compounds may be isolated thus the treatment of diethyl acetylsuccinate (2) with hydrogen sulfide yields the thiophene derivative (3).6... 

A thiophene derivative of acrylic acid (21) has been hydrogenated to 88% ee with a DIOP catalyst, the thiophene ring remaining unreduced (243). 

In the absence of a dipolarophile, thiocarbonyl ylide 84 undergoes a 1,4-hydrogen shift to give the naphthoannelated thiophene derivative 85. Many examples of related syntheses have been reported (139-143). The photolysis of tetraarylthiiranes in the presence of tetracyanoethene represents an approach to tetrahydrothiophenes via a SET mechanism (75,76). 

Reported relative rates ( hs/ hp,) are quite varied. As shown in Fig. 11, this ratio has been estimated to be from 1.5 (55) to as little as 0.13 (25) for some multiaromatic thiophene derivatives. Phenylnaphthalene was also reported to have a very different preference for the position of naphthalene ring hydrogenation in independent studies as compared to being an intermediate in the reaction matrix (56, 57). 

Two excellent reviews <71AHC(13)235, 72IJS(C)(7)6l) have dealt with quantitative aspects of electrophilic substitution on thiophenes. Electrophilic substitution in the thiophene ring appears to proceed in most cases by a mechanism similar to that for the homocyclic benzene substrates. The first step involves the formation of a cr-complex, which is rate determining in most reactions in a few cases the decomposition of this intermediate may be rate determining. Evidence for the similarity of mechanism in the thiophene and benzene series stems from detailed kinetic studies. Thus in protodetritiation of thiophene derivatives in aqueous sulfuric and perchloric acids, a linear correlation between log k and —Ho has been established the slopes are very close to those reported for hydrogen exchanges in benzene derivatives. Likewise, the kinetic profile of the reaction of thiophene derivatives with bromine in acetic acid in the dark is the same as for bromination of benzene derivatives. The activation enthalpies and entropies for bromination of thiophene and mesitylene are very similar. 

Selected substituted thiophene derivatives may also react with nucleophiles. Primary and secondary amines react with 2-bromo-3-substituted-5-nitrothiophenes to produce 11 <95T5403>. Vicarious nucleophilic substitution of hydrogen (VNS) works generally for both 2-nitro- and 3-... 

Another interesting group of heterocyclic compounds formed in the reaction of 2,4-decadienal and cysteine is the thiophenes. Thiophenes having butyl to heptyl groups substituted at the 2-position were identified. Mechanisms have been suggested for the production of thiophene derivatives by the action of hydrogen sulfide on 1,4-dicarbonyl compounds (24). The exact mechanism for the formation of thiophenes in this system is not clear and deserves more study. 

Thiophenium ions not only play a key role in electrophilic substitution reactions but they also allow some addition reactions to be possible, such as the ionic hydrogenation of thiophene derivatives proceeding by repeated protonations and additions of hydride ion (Scheme 16) (73IZV1918, 75T311, 78T1703, 79MI1). 

Sulfur-containing heteroaromatics can be smoothly hydrogenated with [Co2(CO)s] under carbon monoxide and a hydrogen atmosphere (238-272 atm), as demonstrated by the reduction of thiophene derivatives (71). ... 

When 111a is treated with CO or Ce(IV)salts, cleavage of the PRj group takes place with almost quantitative formation of the thiophene derivative 112 (Scheme 39). Under catalytic hydrogenation conditions with Raney nickel, ring contraction via selective sulfur extrusion to the bicy-clo[2.1.1]hexene 113 occurs. - ... 

Special syntheses. Middleton prepared 2,5-diamino-3,4-dicyanothiophene by passing hydrogen sulfide into a stirred solution of tetracyanoethylene in acetone and carbon disulfide at 0-5° and added pyridine rapidly. The solution became clear and then the yellow thiophene derivative began to separate. Hydrogen sulfide... 

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