Tetrahydrothiophene, basicity

The resulting tetrahydropyrroles and tetrahydrothiophenes could be easily aromatized under basic conditions (Scheme 36), thus allowing a convenient access to a new class of pyrrole and thiophene derivatives, which have found application in the preparation of new organic materials [304-308]. 

The basic poly(phenylene vinylene) (PPV) polymer is commonly prepared by the sufonium prepolymer route developed by WessUng and Zimmerman in 1968 but much modified by subsequent workers. The synthesis starts from 1,4-bis(chloromethyl)benzene, via the bis-sulfonium salt formed by reaction with tetrahydrothiophene, and then polymerisation is effected to give the prepolymer by reaction with lithium hydroxide (Figure 3.39). Because of the inherent insolubility of PPV it is this prepolymer that is used to form the film coating on the substrate, for example by using a doctor blade technique. The prepolymer is converted into PPV on the substrate by heating in an oven under vacuum at 200 °C for 8-10 h. 

The phosphine-mediated desulfurization of substituted 1,2-dithianes to the corresponding tetrahydrothiophenes proceeds stereospecifically for the corresponding reactions of cis- and /ra r, 5-dihydroxy-l,2-dithianes 124 and 125/126, three different phosphines R3P (R=Et, Ph, (CH2)2COOH-HCl) were employed <2003H(60)47>. The reaction is pH-dependent under mildly acidic conditions, the thiols 127 and 128 were obtained under neutral or moderately basic conditions, however, the 4-hydroxy-3-mercaptotetrahydrothiophenes 129-131 were formed (Scheme 32). Erom 124 and 125 racemic 129 and 130 were obtained, while for 126 the stereospecific product 131 was isolated the identity of... 

These show marked similarities to their acyclic counterparts, e.g. tetrahydrofuran closely resembles diethyl ether. Minor differences arise due to the less sterically hindered nature of the heteroatoms in the cyclic compounds. The basicities are tetrahydropyrrole (pKa 10.4), tetrahydrofuran (— 2.1) and tetrahydrothiophene (thiolane) ( — 4.5). 

Hydrogen peroxide has been estimated to be more than 106 times less basic than H20. However, on addition of concentrated H202 to tetrafluoroboric acid in tetrahydrothiophene 1,1-dioxide (sulfolane) the conjugate cation (H302) can be obtained. The solutions are very powerful, but unselective, oxidants for benzene, cyclohexane, and other organic materials. 

Aksenov D.G., Klimov O.V., Startsev A.N. (1997) Alumina supported sulfide catalysts. VI. Hydrogenolysis of tetrahydrothiophen. Kinet. Katal 38, 903-7. Yukelson 1.1., (1969) Methods of the Basic Organic Synthesis, Nauka, Moscow. Startsev A.N. (1995) The Mechanism of HDS Catalysis. Catai. Rev.-Sci. Eng., 37, 353-423. 

The ring exchange of tetrahydrofuran to tetrahydrothiophene is also catalyzed by basic sites and showed a decrease in activity with addition of HCl [8, 235]. However, pyridine adsorption also decreased the catalytic activity of the reaction on alkali-exchanged zeolites. In this case, acid and basic sites seem to be necessary for catalysis. 

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