Ring Synthesis and Destruction

Ring Synthesis and Destruction.—One method of preparing thiophens is by inserting a sulphur atom into an acyclic precursor. The sulphur-transfer reagent may be a simple inorganic compound, as illustrated by the formation of (11) from ethyl phenyl ketone (10) on treatment with pyridine and thionyl chloride. The simplest sulphur-transfer reagent is elemental sulphur, as in the production of the thiophen (13 X = S) from l,4-diphenyl-2,3-dibenzylbutadiene (12). The reaction of (12) with selenium dioxide affords the corresponding selenophen (13 X = Se).  

(Amsterdam), 1979, 3 (New Trends Heterocycl. Chem.), 250. 

Litvinov, in Topics in Organic Sulphur Chemistry , ed. M. Tisler, University Press, Ljubljana, 1978, p. 157 

Hydrogen sulphide reacts reversibly with many unsaturated or aromatic hydrocarbons, in the presence of mixed oxide catalysts, to produce thiophens these may be polycyclic, according to the complexity of the starting hydrocarbon. This reaction, which is believed to be the source of many of the sulphur compounds found in crude oil, has been studied in particular by Klemm and his co-workers, who have now published their detailed results. A typical example is the formation of the isomeric benzonaphthothiophens (15) and (16) from the reaction of 2-phenylnaphthalene (14) with hydrogen sulphide over a sulphided cobaltous oxide-molybdic oxide-alumina catalyst at 450—630 in a flow system. 

Thiophens may also be obtained from aldehydes in like manner. The kinetics and mechanism of these sulphurization reactions, and especially of the industrially important reverse reactions (hydrodesulphurization), have been examined in some detail thiols may be intermediates. 

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