Kinetics of 5-exo Cyclizations of N-Alkyl-4-pentenaminyl Radicals and β-Fragmentations of β-(Dialkylamino)alkyl Radicals

Abstract

The kinetic conclusions of a recent report by Maxwell and Tsanaktsidis (J. Am. Chem. Soc. 1996, 118, 4276) were investigated. The kinetics of ring opening of the (N-butyl-2-pyrrolidinyl)methyl radical (2) to the N-butyl-4-pentenaminyl radical (1) and the reverse reaction, 5-exo cyclization of 1 to 2, were determined at 50 and 80 °C by competitive Bu 3 SnH trapping. Rate constants for 5-exo cyclization of a dialkylaminyl radical and for β-fragmentation of a β-(dialkylamino)ethyl radical were measured by direct laser flash photolysis (LFP) methods. In contrast to the conclusions of Maxwell and Tsanaktsidis, all of these radical reactions were facile with rate constants of at least 1 × 10 4 s -1 . The claim by Maxwell and Tsanaktsidis that bis(tributyltin oxide) catalyzes dialkylaminyl radical reactions was investigated by LFP kinetic studies of the 5-exo cyclization of the N-methyl-5,5-diphenyl-4-pentenaminyl radical (20) in the presence of the additive which demonstrated that (Bu 3 Sn) 2 O does not have a catalytic effect on the reaction. Computations of the energies of the N-methyl analogs of radicals 1 and 2 with a high level of theory (fourth-order Møller-Plesset perturbation theory) and by a hybrid density functional theory with a very large basis set indicate that the cyclization reaction is expected to be slightly exergonic at 298 K. This work demonstrates that the kinetic results reported by Maxwell and Tsanaktsidis were spurious. We speculate that impurities of dichalcogens in their radical precursor samples were reduced by Bu 3 SnH to highly reactive chalcogen hydrides (arylthiols and benzeneselenol) in their kinetic studies.

The reaction of benzothiazole sulfenamide with (TMS)3SiH: An example of degenerate-branched chain process

“…The two main methods that are also compatible with radical chain reactions for synthetic purposes make use of N-hydroxypyridine-2-thione carbamates and arenesulfenamides. Newcomb and coworkers [2,3] measured rate constants for a series of dialkylaminyl radical reactions, including cyclizations and hydrogen abstraction from Bu 3 SnH, t-BuSH, PhSH and PhSeH, using N-hydroxypyridine-2-thione carbamates. Tsanaktsidis and coworkers [4,5] reported on the preparation of arenesulfenamides and their utility as precursors of dialkylaminyl radical in the presence of Bu 3 SnH.…”

“…However, in these reactions the presence of small quantities of (Bu 3 Sn) 2 O significantly improved the reaction performance by increasing the yield of cyclization products. The initial suggestion that this additive catalyzes dialkylaminyl radical reaction [4] was found to be incorrect [3], and later its role was found to be that of scavenging the thiol 2 [3,5], which is a good hydrogen donor and may be produced in situ from the reaction of adventitious disulfide and Bu 3 SnH. In the recent years, we have reported two kinetic studies on the reaction of aminyl radicals with (TMS) 3 SiH.…”

“…The initial suggestion that this additive catalyzes dialkylaminyl radical reaction [4] was found to be incorrect [3], and later its role was found to be that of scavenging the thiol 2 [3,5], which is a good hydrogen donor and may be produced in situ from the reaction of adventitious disulfide and Bu 3 SnH. In the recent years, we have reported two kinetic studies on the reaction of aminyl radicals with (TMS) 3 SiH. In particular, the hindered 2,2,6,6-tetra-0022-328X/$ -see front matter Ó 2005 Elsevier B.V. All rights reserved.…”

“…doi:10.1016/j.jorganchem.2005.01.031 methylpiperidinyl radical [6] and a variety of diarylaminyl radicals [7] were used in connection with some applications of silicon hydrides as process stabilizers for organic polymeric materials subject to oxidative degradation [8]. Since the removal of PhS moiety from a variety of RSPh by (TMS) 3 SiH could be an efficient reaction depending on the stabilization of the forming radical R Å [8], we tested the reaction of sulfenamide 1 with (TMS) 3 SiH in order to verify its potentiality as dialkylaminyl radical precursor. Based on the available kinetic data for the reaction of R 2 N Å radicals with group 14 hydrides [9,10], we are expecting (TMS) 3 SiH to be a relatively good hydrogen donor towards these radicals.…”

“…Since the removal of PhS moiety from a variety of RSPh by (TMS) 3 SiH could be an efficient reaction depending on the stabilization of the forming radical R Å [8], we tested the reaction of sulfenamide 1 with (TMS) 3 SiH in order to verify its potentiality as dialkylaminyl radical precursor. Based on the available kinetic data for the reaction of R 2 N Å radicals with group 14 hydrides [9,10], we are expecting (TMS) 3 SiH to be a relatively good hydrogen donor towards these radicals. We anticipate that the reaction is indeed an efficient chain process producing the corresponding dialkylamine quantitatively and, more importantly, it is a rare example of radical chain-branching process.…”