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2 Perfluoroalkylation Using Perfluorocarboxylic Acids and Anhydrides
Shintaro Kawamuraand Mikiko Sodeoka
1Catalysis and Integrated Research Group, RIKEN Center for Sustainable Resource Science, 2‐1 Hirosawa, Wako, Saitama, 351‐0198 Japan
2Synthetic Organic Chemistry Laboratory, RIKEN Cluster for Pioneering Research, 2‐1 Hirosawa, Wako, Saitama, 351‐0198 Japan
2.1 Introduction
In recent years, increasing numbers of perfluoroalkyl group‐containing medicines, agrochemicals, and functional materials have been reported [1], in large part due to the development of sophisticated perfluoroalkylating reagents, such as the Ruppert–Prakash, Togni, Umemoto, and Langlois reagents, as reviewed elsewhere in this book. Although synthetic routes to many perfluoroalkylated molecules can now be designed based on reported perfluoroalkylations using these reagents, it remains preferable to employ readily available, inexpensive, multipurpose perfluoroalkyl compounds that can be conveniently stored in the laboratory. In particular, perfluoroalkyl group‐containing building blocks that can be prepared by means of scalable perfluoroalkylation reactions are needed. In this context, we focus here on perfluoroalkylation reactions utilizing perfluorocarboxylic acids and anhydrides as user‐friendly perfluoroalkyl sources. Although several excellent secondary reagents, including the corresponding esters prepared from perfluorocarboxylic acids and anhydrides, are known [2], we will review only methods directly using the carboxylic acids and anhydrides themselves.
2.2 Perfluoroalkylation with Perfluorocarboxylic Acids
The history of perfluoroalkylation reactions of organic compounds using perfluorocarboxylic acids began in the 1970s, and since then various methods for the generation of perfluoroalkyl radicals or perfluoroalkyl metals as reactive species for perfluoroalkylations have been developed. In this section, we describe perfluoroalkylation reactions with perfluorocarboxylic acids, classified according to the following reaction modes: electrochemical reactions (2.1), reactions using XeF2 (2.2), reactions using copper and silver salts (2.3), photochemical reactions (2.4), and other methods (2.5).
2.2.1 Electrochemical Reactions
Electrolysis of perfluorocarboxylic acids and their metal salts can generate a perfluoroalkyl radical by anodic reaction, which was inspired by Kolbe electrosynthesis (Scheme 2.1) [3, 4]. The perfluoroalkyl radical can be electrophilically added to alkenes, alkynes, and aromatic compounds, and some examples are described below.
Scheme 2.1 Electrochemical generation of perfluoroalkyl radical.
2.2.1.1 Reactions of Alkenes and Alkynes
In 1973, Renaud and Sullivan found that electrochemical oxidation of a mixture of sodium trifluoroacetate and propionate gave a mixture of 3,3,3‐trifluoropropene, 1,2‐bis(trifluoromethyl)ethane, 1,1,2‐tris(trifluoromethyl)ethane, and 1,2,4‐tris(trifluoromethyl)butane (Scheme 2.2) [5]. These products were considered to be produced by addition of the CF3 radical to ethylene formed in situ from propionic acid under electrolysis conditions; this was confirmed by the reaction of ethylene gas with trifluoroacetate [6].
Scheme 2.2 Anodic trifluoromethylation of ethylene formed in situ.
The group of Brookes, Coe, Pedler, and Tatlow reported perfluoroalkylation of alkenes with perfluorocarboxylic acids [7, 8]. They determined the structures of several products obtained by the reaction of alkenes such as non‐substituted terminal alkenes, methyl acrylate, acrylonitrile, and methyl 3‐butenoate with perfluorocarboxylic acid. For example, methyl acrylate gave dimethyl succinate derivatives as major products in most cases via homocoupling of alkyl radicals generated by the reaction of alkene and perfluoroalkyl radical (Scheme 2.3). Notably, β‐perfluoroalkylated methyl acrylate was also isolated as a major product when pentafluorobutanoic