Salts as Lewis Acids

Whereas the Prins-type cyclizations reported in this and the preceeding chapter were performed using stoichiometric amounts of Fe salts as Lewis acids, a breakthrough in the field of catalysis was reported in 2009 when the first iron-catalyzed Prins- and aza-Prins cyclization was reported. The catalytic system, which is obtained by combining catalytic amounts of an iron salt with trimethylsilyl halides as a halide source, is widely applicable and promotes the construction of substituted six-membered oxa- and aza-cycles (Scheme 33) [44]. 

Amino Acid Salts as Lewis Acids for Asymmetric Diels-Alder Reactions... 

By 1989 Mukaiyama had already explored the behaviour of phosphonium salts as Lewis acid catalysts. It was possible to show that the aldol-type reaction of aldehydes or acetals with several nucleophiles and the Michael reaction of a,j3-unsatu-rated ketones or acetals with silyl nucleophiles gave the products in good yields with a phosphonium salt catalyst [116]. In addition, the same group applied bisphosphonium salts as shown in Scheme 45 in the synthesis of ]3-aminoesters [117]. High yields up to 98% were obtained in the reaction of A-benzylideneaniline and the ketene silyl acetal of methyl isobutyrate. Various analogues of the reaction parteers gave similar results. The bisphosphonium salt was found to be superior to Lewis acids like TiCl and SnCl, which are deactivated by the resulting amines. 

Due to the continuous need for efficient and environmentally benign alkylation processes, the Rueping group decided to develop new C-C bond forming reactions using cheap, nontoxic and air-stable Bi(III) salts as Lewis acid catalysts. In this... 

The role of bismuth(III) salts as Lewis acids has only been studied since the late 1980 s. Pioneering work by Dubac, Wada and others paved the way to wide and general methods using bismuth(III) catalysts. The versatile use of bismuth salts in synthesis has clearly been highlighted by the increasing number of publications in the field. The low toxicity of bismuth salts, associated with low cost, make them attractive and practical catalysts to use. Synergistic effects with other Lewis acids have also been recently highlighted. 

This chapter focuses on the use of silver(I) salts as Lewis acids in the ring-opening reactions of halocyclopropanes. Both the classical and most recent examples of this type of chemistry are examined with emphasis on the use of this simple transformation to build up complex intermediates that are potentially useful in the construction of natural products. 

This review has detailed the utility of copper salts as Lewis acids. Their ready availability, or ease of preparation, low toxicity, and counter ion-dependent tuning of the strength of Lewis acidity makes them ideal for use in a variety of organic transformations. The last decade has witnessed dramatic progress in catalytic methods for the preparation of enantiomerically pure compounds. Copper Lewis acids have played a key role in their development. The future holds a lot of promise for the identification of other processes for which copper will be the Lewis acid of choice. 

These Diels-Alder reactions were performed in dichloromethane, but the efficiency of imidazolium salts as Lewis acid catalysts suggested that ionic liquids could improve the rate of such transformations. Indeed, Diels-Alder reactions result in greater stereoselectivity when performed in a 5M solution of LiClOq in diethyl ether. 

Alkali Metal Salts as Lewis Acid Reaction Promoter... 

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