Strong protic acids

Cleavage - with strong protic acid (3M HCl, CF3COOH)... 

The stmcture of the ketones produced from unsymmetrical internal perfluoroepoxides has been reported (5). The epoxide ring may also be opened by strong protic acids such as fluorosulfonic acid or hydrogen fluoride at elevated temperatures (23—25). The ring opening of HFPO by sulfur trioxide at 150°C has been interpreted as an example of an electrophilic reaction (26) (eq. 3). 

However, strong protic acid catalysts are needed when 7T- or CJ-donor alkylating agents are used to produce carbocationic or highly polarized donor-acceptor-complexes as the reactive alkylating iatermediates ... 

Cationic polymerization of coal-tar fractions has been commercially achieved through the use of strong protic acids, as well as various Lewis acids. Sulfuric acid was the first polymerization catalyst (11). More recent technology has focused on the Friedel-Crafts polymerization of coal fractions to yield resins with higher softening points and better color. Typical Lewis acid catalysts used in these processes are aluminum chloride, boron trifluoride, and various boron trifluoride complexes (12). Cmde feedstocks typically contain 25—75% reactive components and may be refined prior to polymerization (eg, acid or alkali treatment) to remove sulfur and other undesired components. Table 1 illustrates the typical components found in coal-tar fractions and their corresponding properties. 

Lactams can also be polymerized under anhydrous conditions by a cationic mechanism initiated by strong protic acids, their salts, and Lewis acids, as weU as amines and ammonia (51—53). The complete reaction mechanism is complex and this approach has not as yet been used successfully in a commercial process. 

Early studies of the condensation reaction on the monomer level did not give the full picture of this process and only in the 1980s was polycondensation of siloxanols studied by using oligomeric model compounds (76,77). These studies revealed that in the presence of strong protic acids three processes must be considered linear condensation (eq. 14), cyclization (eq. 15), and disproportionation (eq. 16). 

Anhydrides are reduced with relative ease. McAlees and McCrindle 20) established the following increasing order of difficulty for various carbonyls acid chlorides > aldehydes, ketones > anhydrides > esters > carboxylic acids > amides. Reduction may proceed by 1,2-addilion of hydrogen or by cleavage of an oxygen-carbonyl bond. If 1,2-addition to the carbonyl occurs, as in the presence of strong protic acids over palladium, 1,1-diesters are formed by acylation 26). 

Oxazolines undergo polymerization upon exposure to a variety of cationic initiators such as strong Lewis acids or strong protic acids. Copolymerization between different oxazolines of defined composition can be carried out in a random manner or in a controlled fashion resulting in block polymers. Alternatively, oxazolines can also be grafted onto other types of polymers. It is beyond the scope of this chapter to review in detail this enormous and important subject. Instead, the... 

The most striking differences within this class of compounds (fi-methylene complexes) are encountered upon attempted protonation. First-row metal derivatives such as (jU,-CH2)[(i75-C5H5)Mn(CO)2]2 (3a) and (M-CH2)[Fe2(CO)g] (35) prove to be completely resistant to even strong protic acids (e.g., HBF4, CF3S03H), whereas second-row rhodium gives rise to some intriguing chemistry, both synthetically and mechanistically, when attacked by a proton (49, 51, 52, 61). 

More recently, the reactivity of SZ has been assigned to its oxidizing ability,129-131 which should not be surprising because it has often been considered as SO3 adsorbed on zirconium oxide. However, that sulfated zirconia is not only an oxidant but also a strong protic acid has been demonstrated by Sommer, Walspurger, and co-workers132 on the basis H/D exchange experiments with neopentane. 

A powerful electrophilic species is obtained from C120 and a strong protic acid xuch as trifluoroacetic acid. This reagent effects exclusive ring chlorination of deactivated arenes in high yield with the usual regioselectivity. 

Despite the fact that H+ (in terms of electrophilic reactions of polyfluorinated compounds only) is a relatively mild electrophile, strong protic acids (H0S02F, H0S02CF3, anhydrous HF) are widely used for generating electrophilic species. Thus, protonation of fluorine in HgF2 by anhydrous HF results in formation of the corresponding metal centered cation 3 [ 17,20] ... 

Reactivity of sulfur chlorides significantly increases when reaction with fluoroolefin is carried out in fluoro- or chlorosulfonic acids, since strong protic acids are able to increase electrophilicity of the molecule by protonation [145]. For example, HFP reacts with solution of SC12 in H0S02F at 40-60°C and atmospheric pressure to give a mixture of products from which the corresponding sulfenyl chloride was isolated in 75% yield ... 

Aza-Cope rearrangement.7 This reaction proceeds readily when catalyzed by Pd(0) and a strong protic acid, particularly trifluoroacetic acid or methanesul-fonic acid. This isomerization is involved in a direct synthesis of S,e-unsaturated imines from allylamines with carbonyl compounds (equation I). The unsaturated... 

Since Hausmanrfs and Kipping s initial studies, the synthesis of truxene from 1-indanone has been carried out many times under a wide variety of conditions. Reagents that have been reported to promote this trimerization are generally either strong protic acids or Lewis acids and include hydrochloric acid [15], hydroiodic acid [15], phosphorus pentoxide [15], zinc dust [15], sulfuric acid [17], phosphorus pentachloride [17], zinc chloride in acetic acid [18],... 

Percarboxylic acids have been used widely in Baeyer-Villiger oxidation. Peracetic acid is one of the most commonly used peracids distilled peracetic acid is employed commercially by Solvay Interox for the production of e-caprolactone.246 The use of distilled peracetic acid is essential, as it contains no strong protic acids which can catalyse polymerization of the resulting lactone and cause other side reactions. Figure 3.53 illustrates the use of pre-formed peracetic acid for the Baeyer-Villiger oxidation of ketones.247,248... 

Strong protic acids cleave phenolic methyl ethers. Thus hydrogen bromide accomplished the same double demethylation as that depicted in Scheme 4.104.189 The final step in a Merck synthesis of the potent dopamine agonist (A,R)-4-pro-pyl-9-hydroxy naphthoxazine (106 2), a phenolic ether deprotection was accomplished on a large scale using methanesulfonic acid in the presence of methionine as the nucleophile [Scheme 4.106]. 190 191... 

Strong protic acids will also cleave benzyl ethers. A highly stereoselective synthesis of the sex pheromone of the drugstore beetle Stegobium paniceum by Matteson and co-workers277 conveniently accomplished the debenzylation of the benzyl ether 151.1 with methanesulfonic add in chloroform [Scheme 4.151]. 

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