The Effect of Bulky Substituents on the Formation of Symmetrically Trisubstituted Triptycenes

A series of 1,8-dichloroanthracene precursor molecules with substituents in C-10 position of different steric demand (cyclohexyl, tert-butyl, methyl, isopropyl, n-butyl, phenyl, benzyl, trimethylsilylethinyl) were synthesised and subjected to electrocyclic cycloadditions with chlorobenzyne generated from 3-chloroanthranilic acid. The aim was to steer the regioselectivity of the addition reaction by the steric repulsion between this C-10 substituent and the chlorine substituent at the benzyne intermediate. With H as C-10 substituent the reaction leads to 23% syn and 77% anti form. With the small methyl group a syn/anti ratio of 37:63 was achieved. Contrary to our expectations the large C-10 substituent tert-butyl leads to 100% selectivity for the anti form. The best results were achieved for the C-10 substituent n-butyl with a syn/anti ratio of 40:60. The crystal structures of five 1,8-dichloroanthracenes with C-10 substituents were determined, namely those with tert-butyl, cyclohexyl, n-butyl and phenyl groups. As expected the backbone of the tert-butyl compound shows marked distortion of the planarity. Additionally, the crystal structures of three triptycenes were determined, namely those of the syn compound 1,8,13-trichlorotriptycene and those of the anti compounds 10-tert-butyl-1,8,16-trichlorotriptycene and 1,8,16-trichloro-10-methyltriptycene.