New Soluble Catalysts

New Soluble Catalysts. Trifluoromethansulfonic acid (triflic acid, TfOH)42 and acyl triflates, that is mixed anhydrides of carboxylic acids and triflic acid,43 44 were first reported to be effective for Friedel-Crafts acylation in 1972. Significantly lower yields ( 30%) were obtained with other Brpnsted acids. High activities were also observed for perfluorobutanesulfonic acid.37 

More recent studies reconfirmed the excellent properties of these acylating systems and showed that aroyl triflates are able to acylate aromatics without catalysts 

The acylation of alkylbenzenes with benzoyl chloride and a catalytic amount of per-fluorobutanesulfonic acid affords the corresponding alkyl-substituted benzophe-nones with unusually high amounts of ortho isomeric products (up to 30%) 38 

Even ferf-butylbenzene reacts satisfactorily (86% yield) without de-tert-butylation. Acylation of p-xylene with benzoic acid gave 71% yield with continuous removal of water. Water removal was also the decisive factor in producing anthraquinones with phthalic anhydride in satisfactory yields (52-89%). 

Triflates of boron, aluminum, and gallium were found to be efficient catalysts in Friedel-Crafts acylations.46 However, these are water-sensitive materials and were required to be used in equimolar quantities to be effective. More recently various water-tolerant and recyclable triflate salts, which were also tested in alkylation, were found to exhibit similar good characteristics in Friedel-Crafts acylations. Although benzene cannot be acylated, Sc triflate,47 48 lanthanum triflates,48-51 and Hf triflate52 usually give high yields of aryl ketones in acylation with acid anhydrides. In many cases, Li perchlorate was found to accelerate the reactions.48 52 

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The new soluble catalysts offer a solution to these three problems. First, the smaller size of the active site, and associated molecules, allows the growing chains to take advantage of a natural tendency for the growing polymer chain to form a regular helical structure (in comparison to polymers formed from solid state catalysts). 

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