Rieche conditions

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... 

An alternative method for dialkyl peroxide synthesis is the nucleophilic addition of an alkyl hydroperoxide to an alkene under acid catalysis reported by Davies and coworkers (Scheme 31, path B) ". A similar reaction is the nucleophilic addition of alkylhy-droperoxides to vinyl ethers under acid catalysis, producing perketals. Perketals can be deprotected under mild conditions (THF/water/acetic acid) and this hydroperoxide protection-deprotection sequence has been used by Dussault and Porter as a means for the resolution of racemic hydroperoxides (see also Section II.A.2) . In this respect more detailed studies were carried out with the perketals 75, which were prepared via reaction of alkyl hydroperoxides with vinyl ethers (Scheme 33). Weissermel and Lederer reported that in the presence of teri-butyl hypochlorite, a-chlorodialkyl peroxides can be formed in yields between 12% and 45% (Scheme 31, path C)". a-Alkoxydialkyl peroxides and diperoxyacetals were prepared by Rieche and coworkers via acid catalyzed reaction of one or two equivalents of alkyl hydroperoxides with acetals, ketals or aldehydes (Scheme 31, path D)" or by methylation of the corresponding a-alkoxy hydroperoxides with diazomethane (yields 11%, 27%)" . The diperoxyacetals 76 were isolated in yields ranging from 39 to 77%. 

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... 

---