19F NMR studies of the Diels-Alder adduct of N-pentafluorophenylmaleimide with phencyclone.: Hindered rotation about a N-C6F5-bond

N-pentafluorophenyl maleimide acts as an efficient Diels-Alder dienophile to form an adduct with phencyclone. The adduct was found to exhibit five distinct F-19 NMR signals (282 MHz, ambient temperature, CDCl3) consisting of two gross doublets and three gross triplets. We interpret this as consistent with a severely hindered rotation about the N-C6F5 bond in the adduct, leading to a slow exchange limit (SEL) spectrum. The C6F5 group is thought to lie, on average, on the effective mirror plane of the adduct, perpendicular to the pyrrolidinedione ring system, to reduce steric interactions of the ortho fluorines with the imide N(CO)(2) carbonyls. F-19-F-19 COSY45 NMR allowed assignment of vicinal fluorines in the C6F5. Comparative data is presented for the precursors, N-pentafluorophenyl maleamic acid and N-pentafluorophenyl maleimide. Variable temperature 19F NMR results for the maleamic acid are presented. (C) 1998 Elsevier Science S.A. All rights reserved.