21 triarylamines (1 n -1 z, 1 za, 1 zb) and triarylamine analogs (2a, 2b, 3a, 3b, 4a, 4b) with substituents in at least all three p positions and some of their cation-radical hexachloroantimonates have been synthesized. The electrochemical behavior has been studied by cyclic voltammetry. Most of the cornpounds show chemically and electrochemically reversible first oxidation waves in the formation of the cation radicals. With the exception of 4 a and 4b, the second wave for the formation of the dication is chemically irreversible. The UV spectra of the triarylamine cation radicals have been obtained in the presence of a slight excess of SbC15. A good Hammett correlation between the first anodic potential of only p-substituted triarylamines and the U/G' values has been established. Some redox-catalytic properties of triarylamine cation radicals are described.Triarylamine cation radicals are homogeneously reacting electrontransfer systcms with oxidation potentials which can be varied ovcr a wide rangc by proper selection of the ortho and para substituent~ [~,~]. Stablc cation radicals are formed, if at least all puru positions are blocked by substituents from immediate attack of nucleophilesL'-il. Therefore, in numerous cases they have been applied as mild and selective oxidizing agents either in stoichiomctric amounts as stable cation-radical salts or in catalytic amounts with electrochemical generation and in situ regeneration (indirect electrooxidations). Examples are deprotcction of alcohols~4~s1 and carboxylic acidsL6' from their substituted benzyl ethers and esters, cleavage of carbon -sulfur bonds in dithioacetals ['] A prerequisite for the successful application of triarylamine cation radicals as redox catalysts is the availability of a large spectrum of triarylamines so that the properties of the catalyst, for example the oxidation potentials, can be matched to those of the substrate to be transformed. In addition, the stability of the cation radicals must be high so that the catalyst is not lost in side reactions, a n d a sufficient solubility must be ensured. Therefore, we have synthesized a large number of differently substituted triarylamines in addition t o those already known in the literature^'^*] and have published some preliminary r e~u l t s [~-'~J .In this paper we report on the synthesis, electrochemical, spectroscopical, and Hammett behavior, as well as redoxcatalytic properties in detail.
Syntheses of Substituted Triarylamines and their Cation Radical HexachloroantimonatesThe following compounds containing triarylamine struc- To obtain triarylamines with oxidation potentials between those of tris(4-bromopheny1)amine (lc) (E" = 1.3 V vs. NHE) and tris(2,4-dibromophenyl)amine (1 fj (E' = 1.72 V vs. NHE)[',*I triphenylamines containing four o r five bromo substituents have been used (lc, 1 d)'']. However, they have to be prepared by careful reaction with the appropriate amount of bromine. Therefore, we tried to brominate the known compounds tris(4-acetylpheny1)amine (1 g) (E" = 1.5 V vs. NHE) or...
