Thionyl chloride with aromatic rings

Sulfurization of aromatic rings with thionyl chloride... 

In the reaction of ketones with thionyl chloride the intermediate sulfinyl (or sulfenyl) chloride can cyclize on to an aromatic ring located two carbon atoms away giving benzo[b]thiophenes (48) (Scheme 7).54 Additional... 

A totally different picture is presented by 3-phenylazo-2-naphthol. This unusual isomer cannot be prepared by a normal coupling procedure but has been obtained by reaction of 3-amino-2-naphthol with thionyl chloride to give the N-sulphinylamine (4-24), condensation of which with N-phenylhydroxylamine yields the desired product [59]. Here, assumption of a ketohydrazone form would entail loss of aromatic character in both rings of the naphthalene nucleus and the energetic unfavourability of this situation ensures that the compound exists solely in the hydroxyazo form. 

Diaryl sulfides can be prepared by treating aromatic compounds with SC12 and a Friedel-Crafts catalyst. Other reagents that can bring about the same result are S2C12, thionyl chloride, and even sulfur itself. A catalyst is not always necessary. The reaction has been used for ring closure ... 

The fully aromatic diquaternary system 81 is prepared by acid ring closure of the salt (79) obtained by quaternization of 1,10-phen-anthroline with bromoacetaldehyde followed by dehydration of the resulting hydroxy diquaternary salt (80) with thionyl chloride.310,311 The salt 81 is unstable in aqueous solution above a pH of about 5.0. In the pH range 3.3-5.0 it is reduced by a one-electron transfer to the corresponding radical cation at a potential (E0) of —0.12 volt.311 Its reduction in dimethylformamide solution has also been studied.15,307 Substituted derivatives of 81 have been prepared.312... 

A concerted elimination-cyclization mechansim, involving a sulfenyl halide in a 1,3-butadiene-1-thio system, is the most probable mechanism for the formation of benzo[6 Jthiophenes from cinnamic acids or 4-aryl-2-butanones by treatment with thionyl chloride. The reactions shown in Scheme 5 have been carefully worked out, and the intermediates isolated (75JOC3037). The unique aspect of this synthesis is the reduction of the sulfinyl chloride (a) by thionyl chloride to form the sulfenyl chloride (b). The intermediate (b) was isolated and converted in pyridine to the 3-chlorobenzo[6]thiophene-2-carbonyl chloride in 36% yield (73TL125). The reaction is probably initiated by a sulfenyl ion attack on the aromatic ring, since it is promoted by electron-releasing groups para to the site of ring closure. For example, when X in (36) was N02, a 23% yield of (37), a mixture of 5-and 7-nitro derivatives, was obtained, but when X in (36) was OMe, a 54% yield of (37) was obtained, contaminated with some 3,4-dichloro-5-methoxybenzo[6]thiophene-2-carboxylic acid. 

Most compounds of this type are cyclic sulfite and sulfate esters of aromatic 1,2-diols as well as anhydrides of aromatic 1,2-disulfonic acids. The simplest representatives with unsubstituted benzene rings are 1,3,2-benzodioxathiole 2-oxide (48) (catechol sulfite), the corresponding 2,2-dioxide (156) (catechol sulfate) and 2,1,3-benzoxadithiole 1,1,3,3-tetroxide (158) (1,2-benzenedisulfonic anhydride). Compound (48) was synthesized by refluxing catechol with thionyl chloride in the presence of pyridine. In a similar fashion, from 2-mercaptophenol 1,2,3-benzoxadithiole 2-oxide was prepared (81AG603). The dioxide (156) was obtained in two steps by reaction of catechol monosodium salt with sulfuryl chloride in benzene at 0-10 °C and subsequent reflux of the intermediate (155) in the presence of pyridine. 

Aromatic sulfonyl chlorides can be prepared directly, by treatment of aromatic rings with chlorosulfuric acid. Since sulfonic acids can also be prepared by the same reagent (11-7), it is likely that they are intermediates, being converted to the halides by excess chlorosulfuric acid. The reaction has also been effected with bromo- and fluorosulfuric acids. Sulfinyl chlorides (ArSOCl) have been prepared by the reaction of thionyl chloride and an aromatic compound on Montmorillonite KIO clay. ii... 

In a less frequently used technique, a silica surface is first chlorinated with, for instance, thionyl chloride. Next the Cl is displaced by a strong nucleophile such as benzyl- [21] or phenyllithium [22] (Figure le). Thus a new Si-C bond is formed containing an Si atom that initially was part of the silica support. This approach precludes the formation of surface-bound Si oligomers [12]. Eventual sulfonation of the aromatic rings leads to a useful catalyst. 

Phosphorus pentachloride is commonly used for the preparation of acid chlorides,along with thionyl chloride. Phosphorus pentabromide has been used to brominate aromatic rings, as in the conversion of 149 to 150 in 42% yield,but more vigorous conditions are required. 

The Friedel-Crafts acylation reaction places an acyl group (RC=0) onto an aromatic ring (Fig. 14.41). We will take up the chemistry of all manner of acyl derivatives in Chapter 18, but here we need to use acyl chlorides (RCOCl), which are more commonly known as acid chlorides. In practice, acid chlorides are rather easy to make from the relatively available carboxylic acids, RCOOH, by treatment with thionyl chloride (SOCI2) (Fig. 14.42). 

The cyclohexane ring in phenyl cyclo-hexanecarboxylates and in the PCH compounds, 2.5, usually enhances nematic character relative to that of the aromatic systems and even greater enhancement is achieved with cyclohexyl cyclohexanecarboxylates or a bicyclohexyl core unit such as 2.6. The esters are prepared as outlined in Scheme 6 and the bicyclohexyl systems are obtained by the route shown in Scheme 2 a similar reduction of the aromatic ring to that shown in Scheme 6 and equilibration with thionyl chloride to give the trans-acid [28, 29] allows the terminal cyano compound to be obtained. 

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