9-Phenylxanthen-9-ylium and 9-Phenylthioxanthen-9-ylium Ions: Comparison of o- and p-Substitutions in the 9-Phenyl Group by Cyclic Voltammetry and Visible Spectra

Abstract

9-Arylxanthen-9-ylium (3a–i) and 9-arylthioxanthen-9-ylium (4a,b,e–i) perchlorates [aryl = 2,4,6-(MeO)3C6H2 (a), 2,6-(MeO)2C6H3 (b), 2-MeOC6H4 (c), 4-MeOC6H4 (d), 3-Br-2,6-(MeO)2C6H2 (e), 2,4,6-Me3C6H2 (f), 2-MeC6H4 (g), 4-MeC6H4 (h), C6H5 (i)] were prepared by the reactions of 9-arylxanthen-9-ols or 9-arylthioxanthen-9-ols with perchloric acid. Their LUMO and HOMO levels were estimated from the redox potential (E0) in cyclic voltammetry and λmax in the UV–visible spectra measured for a 1,2-dichloroethane solution, and were compared with those of 9-aryl-1,8-dimethoxyxanthen-9-ylium ions (8b,i). We found that 1) both the LUMO and HOMO levels varied almost in the same order of substituent on the 9-phenyl group; 2) the MeO-group on the 9-phenyl group was more effective to raise both the HOMO and LUMO levels than the Me-group; 3) the HOMO levels of 3 and 4 were more sensitive than the LUMO levels to the change in the 9-aryl group; 4) p-substitution by MeO- or Me-groups was more effective to raise the HOMO and LUMO levels than o-substitution; 5) the presence of two o-MeO groups was more effective to raise the HOMO and LUMO levels than one o-MeO group; 6) a m-bromination of 9-aryl group in 3b or 4b greatly lowered both LUMO and HOMO levels, as observed for 3e or 4e; 7) both the HOMO and LUMO levels of 8b and 8i were higher than those of 3b and 3i, respectively; 8) the LUMO level of 3b was higher than that of 8i, the isomer.

Friedel–Crafts approach to the one-pot synthesis of methoxy-substituted thioxanthylium salts

“…Initially, we screened the reaction of bis(3,5-dimethoxyphenyl) sulfide ( 1a ) with benzoyl chloride ( 2a ) in the presence of Brønsted acids in chlorobenzene at several temperatures (Table 1). When we used a strong Brønsted acid such as trifluoromethanesulfonic acid (TfOH) at room temperature, the desired thioxanthylium salt 3a was obtained with 21% yield while other typical Brønsted acids did not work efficiently (Table 1, entries 1–8) [2,9–10]. At 60 °C, the yield effectively improved to 60% (Table, entry 9).…”

“…Owing to these useful properties, several research groups have developed methodologies to synthesize them. The typical synthetic methods for thioxanthylium salts include the reaction of thioxanthone with aryl bromide in the presence of n -butyllithium or Grignard reagents followed by dehydration by acids such as hexafluorophosphoric acid (Scheme 1 and 1b) [3–4 9–10], oxidation of thioxanthene in the presence of PbO 2 followed by dehydration by tetrafluoroboric acid [1], the reaction of 4,4’-bis(dimethylamino)diphenylmethane with sulfur in the presence of ZnCl 2 [11], and the ring-closure reaction of diaryl sulfide in the presence of a Lewis acid such as SnCl 4 and AlCl 3 [12–14]. While these reactions were proven to be useful, they require the use of stoichiometric amounts of metals and/or toxic metal reagents.…”

p‐Quinoid Compounds by Nucleophilic Aromatic Substitution with Hydride as Leaving Group

“…Triarylmethyl alcohols 1 are easily synthesized from the corresponding diaryl ketone and a metallated arene, generally with yields in the range of 60 to 80 %, even for the sterically rather crowded mesityl-substituted xanthenol 1.1 [4] (Scheme 1). The transformation to the tetrafluoroborate 2 also follows the standard procedure with tetrafluoroboric acid as reagent.…”

Modern Spin on the Electrochemical Persistence of Heteroatom-Bridged Triphenylmethyl-Type Radicals

“…As represented qualitatively by [XS] + in Figure 3, the return oxidation E pa(1) of the [XS] • radical decreases in magnitude as air enters the cell, and after ∼80 s, the redox wave is irreversible, likely due to aerial O 2 . The other radicals [X NCH 3 ] • and [XO] • exhibit the same order of sensitivity, and this behavior is observed on both glassy carbon and Pt electrodes, suggesting that surface-specific effects do little to The expedient reaction of these radicals with O 2 is such that even an incomplete purge or insufficient blanketing with Ar will yield the appearance of an EC mechanism to the cation/radical redox couple, which prompted others 36,38 to rely on radical dimerization even when such explanations were inappropriate. Evidence for a chemical (C) reaction mechanism appears when scanning anodically after the solution has been saturated with air; an irreversible oxidation wave (E pa(O2) ) appears (Table 1), which was absent when the solutions were purged with Ar (Figures S18, S21, and S24).…”

“…This irreversibility is due to a reaction with the in situ generated radicals, 22−24 which we suggest is what many authors have been measuring in their experiments instead of radical−radical dimerization. 36,38 Due to the conflicting reports on the persistence of the electrochemically generated radicals, we feel that this class of structure may have been overlooked as scaffolds for radical-based materials. Preparation of the three triflate salts [XNCH 3 ] + , [XO] + , and [XS] + is effected by treating the tertiary alcohols obtained from Grignard reactions of the corresponding diaryl ketones with triflic acid, as summarized in Scheme 2.…”