Reactions of 1,8-Dimethoxy-9-phenylxanthen-9-ol in the Presence of an Acid, and Its Basicity

Abstract

In the presence of a catalytic amount of acid, the title xanthenol (1) reacted in acetone to give 1,8-dimethoxy-9-phenylxanthen-9-ylmethyl methyl ketone. Analogous reactions were observed for ethyl methyl ketone and acetophenone, but not for diethyl ketone. Propanal and butanal also reacted to give 2-(1,8-dimethoxy-9-phenylxanthen-9-yl)propanal and 2-(1,8-dimethoxy-9-phenylxanthen-9-yl)butanal, respectively. In hot primary and secondary alcohols, 1 was reduced to give 1,8-dimethoxy-9-phenylxanthene. 1 also reacted with methoxybenzenes, phenol, and N-alkylanilines to give 9-aryl-1,8-dimethoxy-9-phenylxanthenes. The basicity of 1, or the stability of the carbenium ion (pKR+ = −0.81), was measured in hydrochloric acid and compared with those of related 9-arylxanthen-9-ols, such as 1,8-dimethoxy-9-(2,6-dimethoxyphenyl)xanthen-9-ol (1.14), 9-(2,6-dimethoxyphenyl)xanthen-9-ol (4.80) (8), and 9-phenylxanthen-9-ol (0.75). The drastic differences in the reactivities and the basicity between 1 and 8 were attributed to a steric effect rather than an electronic effect.

Conformational flexibility of xanthene‐based covalently linked dimers

“…Figure B compares the pH‐dependent changes of the absorption at 378 nm (where the Xan + moiety absorbs) for two model compounds: the monomeric model Xan + (9‐phenyl‐xanthylium cation) and the dimer (Xan+) 2 , in both form of their perchlorate salts. The monomer Xan + converts to Xan‐OH at pH = 1.3, which is consistent with the previously reported pseudobase pKa value for this compound . This low pseudobase pKa value for Xan + reflects the large stability of the aromatic cation.…”

“…The monomer Xan + converts to Xan-OH at pH = 1.3, which is consistent with the previously reported pseudobase pKa value for this compound. [34,35] This low pseudobase pKa value for Xan + reflects the large stability of the aromatic cation. In the case of the dication, two pseudobase pKa values were observed at pKa 1 = 0.2 and pKa 2 = 2.6, which were assigned to the formation of the monohydroxylated and dihydroxylated species, respectively (Scheme 1).…”

Cage‐Shaped Borate Esters with Tris(2‐oxyphenyl)methane or ‐silane System Frameworks Bearing Multiple Tuning Factors: Geometric and Substituent Effects on Their Lewis Acid Properties

“…The ligated pyridine showed downfield-shifted signals around d( 1 H) = 9.24, 8.20, and 7.78 ppm (free pyridine: d( 1 H) = 8.61, 7.61, and 7.28 ppm). Other cage-shaped complexes 1 b-fB·L showed analogous chemical shifts for their 1 H, 13 C, and 11 B NMR spectra, as shown in Table 1. The order of the downfield shifts Dd( 1 H) of the ligated THF on 1 a-dB as compared to free THF was as follows: 1 bB·THF (4.85 and 2.35 ppm) > 1 cB·THF (4.53 and 2.28 ppm) > 1 dB·THF (4.52 and 2.26 ppm) > 1 aB·THF (4.46 and 2.13 ppm).…”

“…The treatment of 3 a with p ‐toluenesulfonic acid in THF/MeCN directly gave the reduced compound 4 a . The in situ‐generated carbenium cation, which was stabilized by electron‐donating groups,13 was reduced by THF probably by means of either an ionic or a single‐electron transfer (SET) mechanism 14. The desired compound 1 a H 3 was obtained after deprotection of 4 a by BBr 3 .…”

Cage-Shaped Borate Esters with Enhanced Lewis Acidity and Catalytic Activity

“…The treatment of 2 with p -toluenesulfonic acid in THF/MeCN directly gave the reduced compound 3 . The in situ-generated carbenium cation stabilized by electron-donating groups can be reduced by THF probably in either an ionic or SET mechanism . The desired compound 4 H 3 was obtained after treatment of 3 with BBr 3 .…”