Hydrogenation of the Ring

Manufacture. Furfuryl alcohol has been manufactured on an industrial scale by employing both Hquid-phase and vapor-phase hydrogenation of furfural (56,57). Copper-based catalysts are preferred because they are selective and do not promote hydrogenation of the ring. 

A high endo selectivity is observed in the reaction of (phenylsulfonyl)allene (112) with furan (157) (equation 113)108. The endo adduct 158 can be readily transformed into highly substituted cyclohexenol 160 upon treatment with n-butyllithium after hydrogenation of the ring double bond (equation 114)108. 

The isolation of benzene and cyclohexane from chlorobenzene and thiophenol, and cyclohexane from fluorobenzene, suggests the preferential reductive cleavage of the substituent prior to hydrogenation of the ring. However, fluorocyclohexane decomposes slowly to cyclohexene, which could give rise to the cyclohexane higher yields of fluorocyclohexane are obtained at lower temperatures. 

Hydroxymethylfurfural. 2,5-Di(hydroxymethyl)furan can be synthesized from 5-HMF via hydrogenation (Scheme 16). Under a hydrogen pressure of 7 MPa at 140 °C in the presence of platinum or copper catalysts, practically quantitative yields of 2,5-di(hydroxymethyl)furan can be obtained.However, under similar conditions but with palladium or nickel as catalyst, hydrogenation of the ring system occurs so that 2,5-di(hydroxymethyl)-tetrahydrofuran is obtained as the predominant product. 

Benzyl alcohol readily undergoes the reactions characteristic of a primary alcohol, such as esterification and etherification, as well as halide formation. In addition, it undergoes ring substitution. In the presence of acid, polymerization is observed, and the alcohol can be thermally dehydrated to toluene [108-88-3], Catalytic oxidation over copper oxide yields benzaldehyde benzoic acid is obtained by oxidation with chromic acid or potassium permanganate. Catalytic hydrogenation of the ring gives cyclohexylmethanol [100-49-2]. 

Method a is in several ways the easier to apply (it uses well known radical prefixes and leads to no problems in deciding on the degree of hydrogenation of the ring system), and was given as an alternative to method b in the 1957 IUPAC Rules. However, it was removed from the 1969 version and is not used by Chemical Abstracts. Method b has the advantage that the polycyclic system is considered as a whole its use is more general than that of method a, and it seems advisable for method a to be abandoned. 

Furthermore, two of the hydrogens of the methyl group of A are uncomfortably close to two axial hydrogens of the ring. 

Examination of the reaction products indicated that the primary products of reaction were probably butadiene and H2S. The rates of hydrogenation of butadiene and butene were found to be consistent with the amounts appearing in the reaction products (provided, in the case of cobalt molybdate catalyst, that H2S was present to simulate reaction conditions). The results support the view that C-S bond cleavage is the first step in thiophene desulfurization, rather than hydrogenation of the ring. 

The addition reaction to the furan ring which has been by far the most studied is certainly the catalytic hydrogenation of the ring. It has been shown in the numerous publications on this reaction that various classes of compounds can be obtained—alcohols, ketones, hydrocarbons, ethers, and others. The extent of the literature on this subject severely limits an exhaustive treatment, which is not possible within the framework of this chapter. We are able to mention only a small part of the work. A complete review, with particular reference to recent Russian work, has been given by Bel skii and Shuikin.184... 

The decreased EPD properties of pyridine may be due to steric hindrance between methyl groups and the a-hydrogens of the ring. Contrary to Fig. 3, conductivities measured for Sn(CH3)gI in the pure... 

Treatment of the salt of a phenol with carbon dioxide brings about substitution of the carboxyl group, COOH, for hydrogen of the ring. This reaction is known as the Kolbe reaction its most important application is in the conversion of phenol itself into o-hydroxybcnzoic acid, known as salicylic acid. Although some p-hydroxybenzoic acid is formed as well, the separation of the two isomers can be... 

The methine protons and ring carbons in 5-methyl or 5-protonated thiiranium ions appear at lower field ( H, 4.0-5.0 ppm 13C, 74.0-78.0 ppm) as shown in Table 22. The methine hydrogens of the ring are coupled to the S—H proton, 3Jh h — 6-8 Hz—of the same magnitude as observed in acyclic protonated sulfides. 

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