Unusual Stabilities and Reactivities of Tris- and Bis(2,4,6-trimethoxyphenyl)carbenium Salts

Abstract

2,4,6-Trimethoxyphenyllithium, Φ′Li[Φ′ = 2,4,6-(MeO)3C6H2], reacted with diphenyl carbonate and with ethyl formate to give tris(2,4,6-trimethoxyphenyl)methanol, Φ′3COH (1), and bis(2,4,6-trimethoxyphenyl)methanol, Φ′2CHOH (2), respectively. Compound 1 reacted with a slight excess of perchloric acid in 2-propanol to give very dark-purple crystals of the triarylcarbenium salt, [Φ′3C]X (3, X = ClO4), which were inert even in hot ethanol. However, a treatment of 3 (X = Cl), prepared in situ in 1 M (1 M = 1 mol dm−3) hydrochloric acid, with 1 M sodium hydroxide resulted to give 4-bis(2,4,6-trimethoxyphenyl)methylene-3,5-dimethoxy-2,5-cyclohexadienone. Compound 2 also reacted with a slight excess of perchloric acid in methanol to give dark-red crystals of the diarylcarbenium salt, [Φ′2CH]X (5, X = ClO4). Compound 5 was considerably inert even in hot methanol, but was easily reduced in primary and secondary alcohols to give bis(2,4,6-trimethoxyphenyl)methane, Φ′2CH2. Measurements of the half-lives of 5 in these alcohols showed that 5 was more labile in secondary alcohols, such as 2-propanol and 2-butanol, than in primary alcohols. When a suspension of 5 (X = Cl), prepared in situ in 1 M hydrochloric acid, was heated, tris(2,4,6-trimethoxyphenyl)methane, Φ′3CH (7), was obtained. Analogous reactions to give 7 were observed in methanol containing a slight excess of hydrochloric acid or nitric acid in place of perchloric acid. The possible paths of these reactions are discussed.

Modification of bismaleimide with allyl compound and N‐phenylmaleimide

“…Recently, it has been shown that readily prepared tris(2,4,6‐trimethoxyphenyl) methyl ( 1 ) and tris(2,6‐dimethoxy‐ phenyl)methyl ( 2 ) carbenium ions18–22 react with alkyl amines to generate very stable, permanently charged, nitrogen‐containing carbocations. The products, that result from sequential S N Ar reactions of methoxy substituents by the nucleophilic amines, can be studied—along with their derivatives—for photochemical, photophysical, electrochemical, stereochemical, topological and synthetic applications 18–33 …”

“…In the particular case of air‐ and moisture‐stable [ 1 ][BF 4 ], primary and secondary alkyl amines substitute effectively one, two or even three para ‐methoxy substituents of the carbenium ion in a polar solvent (DMF, acetonitrile) at room temperature 18–22. However, as a significant decrease in reactivity follows each consecutive substitution, it is generally facile to substitute a single p ‐MeO group of 1 by using a lone equivalent of amine and a short reaction time (shown in Scheme ).…”

“…Derivatization reagent [ 1 ][BF 4 ] was synthesized following the procedure reported by Wada and collaborators 18–22. The preparative protocol was only slightly modified; the tris(2,4,6‐trimethoxyphenyl)carbinol precursor to carbocation 1 being obtained in analytically pure form by a solubilization of the crude mixture obtained at the end of the synthesis in a minimum of CH 2 Cl 2 and a subsequent precipitation by addition of n ‐hexane rather than by recrystallization as previously described.…”

“…The preparative protocol was only slightly modified; the tris(2,4,6‐trimethoxyphenyl)carbinol precursor to carbocation 1 being obtained in analytically pure form by a solubilization of the crude mixture obtained at the end of the synthesis in a minimum of CH 2 Cl 2 and a subsequent precipitation by addition of n ‐hexane rather than by recrystallization as previously described. Cation 1 can also be prepared without prior isolation of the carbinol following the one‐step procedure described by Laursen and collaborators 18–22…”

Synthesis and properties of sulfur-functionalized triarylmethylium, acridinium and triangulenium dyes

“…2) [17]. Starting from the readily available tris(2,4,6-trimethoxyphenyl)methylium salt (TMP) 3 C + [18,29], stepwise replacement of the para- methoxy groups by dialkylamines provides access to a wide variety of symmetric and asymmetric triarylmethylium dyes [18,30–31]. Replacement of two o- methoxy groups by one primary amine gives acridinium-type ring systems (Fig.…”