Absolute and relative rate constants for radical reactions are critically important for synthetic planning and for mechanistic studies. Although rate constants for a number of unimolecular and bimolecular reactions of alkyl radicals are known, kinetic data for reactions of nitrogen-centered radicals is substantially more limited.
Mechanistic aspects of the radical cyclizations and C-N β-fragmentations by the intramolecular addition of aminyl radicals to carbonyl groups(A→B : $(C=O)^n$) have been studied. For the kinetic studies, tributylstanane as the trapping agent and alkyl azides as the radical precusor were employed. Product distributions are reported as a function of $Bu_3SnH$ concentration. Rate constants have been determined as follows. $k_1(CHO)^6=4.0\times10^5s^{-1}$,$k_1(CHO)^5=4.5\times10^4s^{-1}$, $k_1(COCH_3)^6=1.6\times10^5s^{-1}$ for n-exo ring closures and $k_2(CHO)^6=6.7\times10^8s^{-1}$,$k_2(CHO)^5=9.4\times10^6s^{-1}$, $k_2(COCH_3)^6=1.3\times10^7s^{-1}$,$k_2(COCH_3)^5=1.1\times10^6s^{-1}$ for C-N β-scissions.
Kinetic analyses of the reactions of aldehyde-azido and keto-azido derivatives indicate that 6-exo closures are faster than 5-exo closures and that C-N β-scissions of cycloalkyloxy intermediates are much faster in 6-membered rings than in 5-membered rings.
