Kinetics of 5-exo Cyclizations of N-Alkyl-4-pentenaminyl Radicals and -Fragmentations of 
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- 作者：Martin Newcomb ; Osama M. Musa ; Felix N. Martinez ; and John H. Horner
- 刊名：Journal of the American Chemical Society
- 出版年：1997
- 出版时间：May 21, 1997
- 年：1997
- 卷：119
- 期：20
- 页码：4569 - 4577
- 全文大小：291K
- 年卷期：v.119,no.20(May 21, 1997)
- ISSN：1520-5126
文摘
The kinetic conclusions of a recent report by Maxwell andTsanaktsidis (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, weredetermined at 50 and 80 
C bycompetitive Bu3SnH trapping. Rate constants for5-exo cyclization of a dialkylaminyl radical and for
-fragmentationof a -(dialkylamino)ethyl radical were measured by direct laserflash photolysis (LFP) methods. In contrast to theconclusions of Maxwell and Tsanaktsidis, all of these radical reactionswere facile with rate constants of at least 1× 104 s-1. The claim byMaxwell and Tsanaktsidis that bis(tributyltin oxide) catalyzesdialkylaminyl radical reactionswas investigated by LFP kinetic studies of the 5-exocyclization of the N-methyl-5,5-diphenyl-4-pentenaminylradical(20) in the presence of the additive which demonstrated that(Bu3Sn)2O does not have a catalytic effecton thereaction. Computations of the energies of the N-methylanalogs of radicals 1 and 2 with a high level oftheory(fourth-order M
ller-Plesset perturbation theory) and by a hybriddensity functional theory with a very large basisset indicate that the cyclization reaction is expected to be slightlyexergonic at 298 K. This work demonstrates thatthe kinetic results reported by Maxwell and Tsanaktsidis were spurious.We speculate that impurities of dichalcogensin their radical precursor samples were reduced byBu3SnH to highly reactive chalcogen hydrides(arylthiols andbenzeneselenol) in their kinetic studies.
C bycompetitive Bu3SnH trapping. Rate constants for5-exo cyclization of a dialkylaminyl radical and for-fragmentationof a -(dialkylamino)ethyl radical were measured by direct laserflash photolysis (LFP) methods. In contrast to theconclusions of Maxwell and Tsanaktsidis, all of these radical reactionswere facile with rate constants of at least 1× 104 s-1. The claim byMaxwell and Tsanaktsidis that bis(tributyltin oxide) catalyzesdialkylaminyl radical reactionswas investigated by LFP kinetic studies of the 5-exocyclization of the N-methyl-5,5-diphenyl-4-pentenaminylradical(20) in the presence of the additive which demonstrated that(Bu3Sn)2O does not have a catalytic effecton thereaction. Computations of the energies of the N-methylanalogs of radicals 1 and 2 with a high level oftheory(fourth-order Mller-Plesset perturbation theory) and by a hybriddensity functional theory with a very large basisset indicate that the cyclization reaction is expected to be slightlyexergonic at 298 K. This work demonstrates thatthe kinetic results reported by Maxwell and Tsanaktsidis were spurious.We speculate that impurities of dichalcogensin their radical precursor samples were reduced byBu3SnH to highly reactive chalcogen hydrides(arylthiols andbenzeneselenol) in their kinetic studies.