Synthesis and characterization of tetraphenylammonium salts

Abstract

The phenyl (Ph) group is a representative substituent in the field of organic chemistry as benzene (the parent molecule) is of fundamental importance. Simple Ph-substituted compounds of common chemical elements are well known. However, extensive structural characterization of tetraphenylammonium (Ph₄N⁺) salts has not been reported. Herein, the synthesis of Ph₄N⁺ salts and their characterization data including the ¹H and ¹³C nuclear magnetic resonance (NMR) spectra and the single-crystal X-ray structure have been presented. An intermolecular radical coupling reaction between an aryl radical and a triarylammoniumyl radical cation was conducted to synthesize the target moieties. The Ph₄N⁺ salts described herein are the simplest tetraarylammonium (Ar₄N⁺) salts known. The results reported herein can potentially help access the otherwise inaccessible non-bridged Ar₄N⁺ salts, a new class of rigid and sterically hindered organic cations.

Fig. 1. Structures of Ph₄-substituted elements belonging to the groups 13–15, Ph₄Z^0 ± 1.

The number in the parentheses indicates the year of synthesis (reported in the literature).

Fig. 2. Ar₄N⁺ salts reported in the literature.

Counter anions are depicted in the general form X. a Pentatritiated Ph₄N⁺ salts (1) reported without proper structural data. b Synthesis of (N,N-diphenyl)carbazolium salts (2) from precursor 3 following the process of intramolecular N-arylation. The bridge moiety indicated in blue must be removed to obtain the Ph₄N⁺ salts. c Other bridged Ar₄N⁺ salts such as (N,N-diphenyl)carbazolium salts (4) bearing various substituents Y on the aryl groups, spirobicarbazolium salts (5), sulfide-bridged Ar₄N⁺ salts (6), and amide-bridged Ar₄N⁺ salts (7) prepared following the intramolecular N-arylation strategy. The bridge moieties are indicated in blue.

Fig. 3. Synthetic strategy followed for the construction of a non-bridged Ar₄N⁺ structure.

a Direct N-phenylation of 8 with a Ph cation is difficult as the N atom is a weak nucleophile. b Intermolecular radical coupling reaction between an aryl radical and the triarylammoniumyl salt 9 bearing bulky protecting groups that exert steric hindrance and block the reactions at the Ph groups.

Fig. 4. Synthetic scheme for the formation of the Ph₄N⁺ salts 23 and 24.

Triarylammoniumyl salt 9 was prepared from 10 over 2 steps. The intermolecular radical coupling reaction between 9 and aryl radical 12, formed in situ following the thermolysis of the diacyl peroxide 13, yielded the Ar₄N⁺ salt (15) along with various byproducts (11, 16, 17, and 18). Removal of the bromo and tert-butyl groups in 15 afforded 23, whose counter anion was exchanged to obtain 24.

Fig. 5. Structural characterization of Ph₄N⁺.

a Comparison of the ¹H NMR spectral profiles (600 MHz, (CD₃)₂CO) recorded for 24, 8, and 25. The number of protons is presented in the parentheses. b Comparison of the ¹³C NMR spectral profiles (150 MHz, (CD₃)₂CO) recorded for 24 and 8. c Single-crystal X-ray structure of 24. All ellipsoids are contoured at the 50% probability level. The counter anion was omitted for clarity.