Nitration of Isopropylpyrenes. Strained Models for Protonation and Transfer-Nitration in the Condensed Phase

Article abstract of DOI:10.1021/jo00035a022

Protic mono- and dinitration of 1,3,5,7,9-pentaisopropylpyrene (1) occurred at the available α positions to give 2 and 3.Despite steric crowding, 2 and 3 did not exhibit a torsional barrier to i-Pr rotation at ambient temperatures; however, buttressing of the peri i-Pr groups was evident (1H NMR, UV, and force field energy minimizations).Persistent (dihydroxyiminium)pyrenium dications 2a⁺² and 3a⁺² were formed by low temperature protonation of 2 and 3 with CF3SO3H (TfOH)/SO2 or with FSO3H/SO2.Intramolecular cyclization of the nitro group of 2a⁺² gave the rearranged pyrenium ion 2c⁺. 1 reacted with NO2⁺BF4^- in acetonitrile solvent to give two pyrenium ions stable at rt, viz. the Wheland intermediate of α-nitration 2b⁺² and the (dihydroxyiminium)pyrenium dication 2a⁺²; the latter was also the predominant pyrenium ion formed in the reaction of 1 with NO⁺BF4^- in acetonitrile.Reaction of 1 with NO2⁺BF4^- in chloroform solvent gave α-nitration products and a persistent radical cation RC.The simultaneous presence of α-nitration products and a persistent pyrenium RC was also observed in the reaction of 1 with NO⁺BF4^- in chloroform, where broader NMR line widths and a stronger ESR signal suggested more extensive oxidation.Protic and aprotic nitrations of 1,3,6,8-tetraisopropylpyrene (10) occurred at the αβ positions; a minor addition product (26) was also found.A mixture of isomeric dinitropyrenes was obtained in NO2⁺ nitration of 1-isopropylpyrene (13).In line with low temperature protonation studies, aprotic nitrations of 2-isopropyl- and 4-isopropylpyrenes occurred predominantly at the α positions.The crowded pyrenium ion of 3 and 2 transfer nitrate to aromatics (toluene, mesitylene, benzene) under mild conditions in competition with a more facile transalkylation.