Barrier to Internal Rotation in 1,2-Bis(bromochloromethyI)-3,4,5,6-tetraisopropyl-benzene

The tightly gear meshed arrangement of isopropyl and dichloromethyl groups in hexaisopropyl- and hexakis(di-chloromethyl)benzene, respectively, imparts a high barrier to internal rotation. For hexaisopropylbenzene,¹ a lower limit of 22 kcal mol^-1 was estimated for the topomerization barrier from NMR measurements on a d₂₈ isotopomer and a barrier of ca. 35 kcal mol^-1 was calculated by use of the empirical force field (EFF) method. For hexakis(di-chloromethyl) benzene,² the calculated (EFF) topomerization barrier was 33.9 kcal mol^-1. Consistent with these findings is the lower limit of 24 kcal mol?¹ estimated (NMR) for the enantiomerization barrier in (1′RS,2′SR)-1,2-bis(l-bromoethyl)-3,4,5,6-tetraisopropyl-benzene.³.