Oxidative Single-Electron Transfer Activation of -Bonds in Aliphatic Halogenation Reactions

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• 作者：Andrey A. Fokin ; Peter R. Schreiner ; Pavel A. Gunchenko ; Sergey A. Peleshanko ; Tat'yana E. Shubina ; Sergey D. Isaev ; Pyotr V. Tarasenko ; Natalya I. Kulik ; Hans-Martin Schiebel ; and Alexander G. Yurche
• 刊名：Journal of the American Chemical Society
• 出版年：2000
• 出版时间：August 2, 2000
• 年：2000
• 卷：122
• 期：30
• 页码：7317 - 7326
• 全文大小：313K
• 年卷期：v.122,no.30(August 2, 2000)
• ISSN：1520-5126

文摘

The reactions of a series of structurally related large-ring propellanes with iodine monochloridewere studied experimentally and computationally. In the case of 1,3-dehydroadamantane (1) and [3.3.1]propellane(2) free-radical addition was observed. [3.3.2]Propellane (3) and 3,6-dehydrohomoadamantane (4), which areless prone to radical attack, selectively form products of formal double nucleophilic (oxidative) addition, e.g.,dichloro (in ICl/CH₂Cl₂), dimethoxy (in ICl/CH₃OH), and diacetamino (in ICl/CH₃CN) derivatives underotherwise identical conditions. Single-electron transfer pathways involving the alkane radical cations are proposedfor the activation step for aliphatic hydrocarbons with relatively low oxidation potentials such as cage alkanes.Similar mechanisms are postulated for the activation of the tertiary C-H bonds of adamantane based on H/D-kinetic isotope effect data. The latter compare well to the k_H/k_D value for hydrogen atom loss from theadamantane radical cation (measured 2.78 ± 0.21 and computed 2.0) and differ considerably from the kineticisotope effects for electrophilic C-H bond activations (i.e., hydride abstraction) or for loss of a proton froma hydrocarbon radical cation (k_H/k_D = 1.0-1.4; computed 1.4). Hence, the reactions of alkanes with elementaryhalogens and other weak electrophiles (but strong oxidizers) do not necessarily involve three-centertwo-electron species but rather occur via successive single-electron oxidation steps. Upon C-C or C-Hfragmentation, the incipient alkane radical cations are trapped by nucleophiles.