Triarylcarbenium Salts Highly Reducible by Primary Alcohols

Article abstract of DOI:10.1246/bcsj.68.243

A series of triarylmethanols bearing o-methoxyl groups (1a: <2,6-(MeO)2C6H3>3COH, 1b: (2-MeOC6H4)<2,6-(MeO)2C6H3>2COH, 1c: Ph<2,6-(MeO)2C6H3>2COH, 1d: <2,5-(MeO)2C6H3>3COH, 1e: Ph2<2,6-(MeO)2C6H3>COH) were prepared.Corresponding carbenium salts, X 2a-c, were isolated by short-period treatments of Ar3COH 1a-c with a slight excess of acid in some bulky alcohols, such as 2-propanol or 2-methyl-1-propanol.The high basicity of 1a-c was explained by through-space interactions of a pair of 2p electrons of an o-methoxyl oxygen with an empty 2p orbital of the resulting carbenium carbon.In ethanol, 1-butanol or 3-methyl-1-butanol, 1c, as well as 2c, was reduced under very mild conditions to triarylmethane, Ar3C-H 3c, and in other bulkier alcohols it was reduced under more forcing conditions.The formations of aldehyde or ketone, the by-product, were confirmed in some cases.Compounds 1a,b,d,e, as well as 2a,b, reacted in analogous manners under more forcing conditions.The reaction rates in several alcohols increased in the order 2a < 2b < 2c, and the reaction rates of alcohol for 2c decreased in the order ethanol > 3-methyl-1-butanol > 1-butanol > 2-propanol > methanol > 2-butanol > 2-methyl-1-propanol.Compounds 2b,c were also reduced in tetrahydrofuran, and formed xanthene derivatives in dimethyl sulfoxide or in water.