Alkylation with Alcohols and Cyclic Ethers

Data for the use of alcohols as alkylating agents in superacids are scarce. A study of the alkylation of phenol and naphthols with ferf-butyl alcohol has shown198 that triflic acid adsorbed on aminopropyl-modified silica is the most selective to yield monoalkylated products compared to solid acids (triflates immobilized in silica). 

Sarca and Laali199 have used triflic acid in butylmethylimidazolium hexafluor-ophosphate BMIM][PF6 ionic liquid for the benzylation of various arenes with benzyl alcohol [Eq. (5.76)]. When compared with Yb(OTf)3, triflic acid proved to be a better catalyst showing higher selectivity (less dibenzyl ether byproduct) by exhibiting similar activity (typically complete conversion). Of the isomeric products, para isomers dominate. Experimental observations indicate that dibenzyl ether originates from less complete protonation of benzyl alcohol and, consequently, serves as a competing nucleophile. Both substrate selectivity (kT/kB) and positional selectivity (ortho/para ratio) found in competitive benzylation with a benzene-toluene mixture (1 1 molar ratio) are similar to those determined in earlier studies, indicating that the nature of the electrophile is not affected in the ionic liquid. 

Triflic acid is also efficient in the alkylation of electron-rich aromatics (anisole, 1,3-dimethoxybenzene, 2-methylfurane, pyrrole, benzofurane, indole) with secondary benzylic alcohols and 3-phenylallyl alcohols (50°C, 1-9 h, 66-95% yield).201 Benzene, toluene, and halobenzenes are also alkylated with hydroxy-biindantetraone 53 in triflic acid within 1-2h202 [Eq. (5.78)]. Suprisingly, however, the primary products (with the exception of the 4-methylphenyl-substituted compound) undergo rearrangement upon prolonged treatment to yield alkenes 

Kotsuki et al.203 have reported high yields of alkylated products formed in the presence of triflic acid using cyclohexyl methanesulfonate [Eq. (5.79)]. Similar performance was found for mesitylene, durene, and naphthalene, as well as for other secondary alcohol methanesulfonates. 

When olefins are used as alkylating agents, the catalytic activity of Nafion-H slowly decreases, most probably due to some polymerization on the surface, which deactivates the catalytic sites. The activity decreases faster when more reactive branched alkenes are used. The use of alcohols instead of olefins as the alkylating agents improves the lifetime of the catalyst. With alcohols, no ready polymerization takes place, since water formed as byproduct inhibits polymerization of any olefin formed (by dehydration) but does not affect the acidity of the catalyst at the reaction temperatures. 

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