Rieche formylation

While the Friedel-Crafts acylation is a general method for the preparation of aryl ketones, and of wide scope, there is no equivalently versatile reaction for the preparation of aryl aldehydes. There are various formylation procedures known, each of limited scope. In addition to the reactions outlined above, there is the Vdsmeier reaction, the Reimer-Tiemann reaction, and the Rieche formylation reaction The latter is the reaction of aromatic compounds with 1,1-dichloromethyl ether as formylating agent in the presence of a Lewis acid catalyst. This procedure has recently gained much importance. 

Reaction of pyrocatechol dichloromethylene acetaP (133) with dibenzothiophene in the presence of titanium tetrachloride yields o-hydroxjrphenyl-2-dibenzothiophenecarboxylate (134) (100%). This reaction is closely related to the Rieche formylation of dibenzothiophene (Section VI,F, 1) in which a mixture of 2- and 4-dibenzothiophene-carboxaldehyde was obtained. The complete absence of the 4-isomer in this reaction may be due to steric effects at the 4-position between the sulfur atom and the bulky reagent. Hydrolysis of 134 yields 2-dibenzo-thiophenecarboxylic acid (70%). 4-Methyl-3-dibenzothiophenecar-... 

Another ready source of a suitable 2-mercaptobcnzaldehyde derivative (12) stems from the Rieche formylation of di-/>-tolyl disulfide (Scheme 9). 

Benzo[h] thiophene undergoes selective Rieche formylation (65%) in the 3-position with dichloromethyl butyl ether and TiCl4.508 2-Bromo- and 3-methylbenzo[h]thiophene are similarly formylated in the vacant thiophene position.345 508 The diarylmethane compounds 121 (Ar = thiophene, benzo-[h]thiophene, or benzene) undergo Rieche formylation, but the resulting aldehyde cyclizes under the conditions of the reaction, to give the annelated products 122.509... 

Decarboxylation of the acids gave the parent compounds (541) and (542). The product of condensation between o-(methylseleno)phenylacetic acid and o-methylthio- or o-methylseleno-benzaldehydes (549) gave the tetracyclic derivatives (550). These were also obtained by Rieche formylation of (551). The pyrrole-fused selenophen (552) was prepared similarly to the... 

Friedel-Crafts acylation reactions of aromatics are promoted by Tilv complexes.104 In some cases, a catalytic amount of the titanium compound works well (Scheme 28). In addition to acyl halides or acid anhydrides, aldehydes, ketones, and acetals can serve as electrophile equivalents for this reaction.105 The formylation of aromatic substrates in the presence of TiCl4 is known as the Rieche-Gross formylation metalated aromatics or olefins are also formylated under these conditions.106... 

The need for different catalysts in the synthesis of 2-benzopyrylium salts (39) from compound 38 is determined by the introduced substituent R3, which is introduced at the first position of the cation. Thus, formylation of 38 (R1 = Me,Ar R2 = H) with dichloromethyl butyl ether in the presence of AICI3 [according to Rieche (60CB88)], followed by treatment with perchloric acid leads to 1-unsubstituted salts 39 (R1 = Me, Ar R2 = R3 = H) (70KGS278 70KGS1013). 

HaIogenobenzo[b]thiophenes are brominated and formylated (Rieche method) in the 3-position.243,345 Chlorination of 3-bromobenzo[6]thiophene in AcOH gives a mixture of 3-chloro-, 2,3-dichIoro-, 2,3-dibromo-,... 

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