Non-Electronic Activation of Anthracenes Using Steric Repulsion of the 9-Substituent with Chloro Groups at the peri-Positions

We investigated how the introduction of substituent at the 9- or 10-position of the 1,8-dichloroanthracene framework affects the horizontal and vertical distortions of the anthracene rings. The 9-substituted anthracenes showed higher distortions than their 10-substituted counterparts due to the steric repulsion with two chloro groups at the peri-positions. The distortion of anthracene framework affected their reactivity. Indeed, 9-substituted anthracenes exhibited higher reactivity to undergo the Diels–Alder reaction with maleic anhydride, which was a result of non-electronic activation.