Generation and reaction of nitrogen-centered radicals

Abstract

Cyclization of nitrogen-centered radicals generated from azido compounds and the free radical rearrangements of nitrogen centered radicals were studied. For mechanistic and synthetic studies of the aminyl radicals, rate constants of alkyl radical cyclizations onto imines and azides were measured. The intramolecular addition of a N-tributyltin substituted aminyl radical onto oximes afforded the N-heterocycles. Especially, the intramolecular addition of α-carbonyl and phosphoryl substituted aminyl radicals to olefins gave an amino alcohol and diamine precursors. Radical cyclization of azides onto aziridinylimines was studied and bicyclic N-heterocycles were obtained. 1,5-Hydrogen transfer from carbon to nitrogen occurred when the resulting carbon centered radicals could be stabilized by the neighboring group after migration and 1,6-hydrogen transfer was much inferior to 1,5-hydrogen transfer. Rate constants of alkyl radicals addition onto imines and azides have been measured by indirect tributyltin hydride method and competitive addition of carbon radicals to azido and alkenyl groups. The rate constant for the 5-exo addition of primary alkyl radical onto imines was $k(C=NBn)^5= 6.2×10^6 s^{-1}$, whereas the rate constant for the corresponding 6-exo addition was $k(C=NBn)^6= 6.7×10^5 s^{-1}$ at 80℃. In case of the azides, the rate constant for the 1,5-ring closure was $k(-N_3)^5 = 4.9×10^4 s^{-1}$ and the rate constant for the 1,6-ring closure was $k(-N_3)^6 = 1.3×10^4 s^{-1}$ at 80℃.