ABSOLUTE KINETICS OF AMIDYL RADICAL REACTIONS

An absolute kinetic scale for amidyl radical reactions was established by a combination of direct laser flash photolysis (LFP) and indirect competition-kinetic studies. Six amidyl radicals were studied by LFP. Arrhenius parameters were determined for 6-exo cyclizations of the N-b utyl-6,6-diphenyl-5-hexenamidyl radical (2) and the -butyl-6-(trans-2- phenylcyclopropyl)-5-hexenamidyl radical (5) and for 1,5-hydrogen tran sfer in the N-(6,6-diphenyl-5-hexenyl)acetamidyl radical (3); rate con stants for these reactions at 20 degrees C are 3.0 x 10(7) s(-1) (2), 5.5 x 10(6) s(-1) (3), and 1 x 107 s(-1) (5). Kinetic limits were esta blished by LFP for the fast cyclizations of the N-methyl-5,5-diphenyl- 4-pentenamidyl radical (1) and the N-methyl-5 -(trans-2-phenylcyclopro pyl)-4-pentenamidyl radical (4) (k > 2 x 10(8) s(-1) at ambient temper ature) and for the slow fragmentation of the N-(2,2-diphenylethyl)acet amidyl radical (6) (k = 2 x 10(4) s(-1) at 48 degrees C). Rate constan ts for amidyl radical reactions with Bu3SnH and PhSH were determined b y competition kinetics; respective rate constants are 1.3 x 10(9) M-1 s(-1) at 20 degrees C and 9 x 10(7) M-1 s(-1) at 23 degrees C. Cycliza tions of simple amidyl radicals were determined from competition kinet ic studies by employing Bu3SnH and N-(phenylthio)amide radical precurs ors using data from the literature and from studies in this work. Rate constants ac 65 degrees C for 5-exo cyclizations of the N-butyl-4-pen tenamidyl radical and the N-(4-pentenyl)butanamidyl radical and for 6- exo cyclization of the N-ethyl-5-hexenamidyl radical are 3 x 10(9) s(- 1), 7 x 10(8) s(-1), and 1.0 x 10(7) s(-1), respectively. The kinetic values determined in-this work can be employed in synthetic planning i nvolving amidyl radicals, and the simple amidyl radical clocks can be used for measuring rate constants of bimolecular reactions. A compilat ion of the kinetics of nitrogen-centered radical cyclizations and bimo lecular reactions of nitrogen radicals with Bu3SnH and PhSH is present ed.