Hydroxy-substituted triarylcarbenium bromides. Synthesis, structure, derivatization and facile conversion to highly substituted xanthenes

The tris(3,5-dialkyl-2-hydroxyphenyl)carbenium bromides (2a) (3-tert-butyl, 5-methyl) and 2b (3,5-di-tert-butyl) have been prepared by the oxidation o f the corresponding tris(3,5-dialkyl-2-hydroxyphenyl)methanes with bromine. Compound 2a has been fully characterized by a single crystal X-ray study, which shows that the bromide, atypically, does not bind to the central carb on atom. The materials 2 are reactive, and the trityl alcohol (4a), methyl (7a, 7b) and phenyl (8a) derivatives can be readily prepared in good yield by reaction of 2 with the appropriate anions; X-ray structures for 4a, 7b ( as its lithium salt) and 8a are reported. In solution, 2a and 4a undergo a facile, uncatalyzed, controllable intramolecular dehydration to produce the corresponding xanthene derivatives; X-ray structures for the xanthene carb ocation (bromide salt) (3a) and the 9-hydroxy derivative (5a) are also repo rted.