Alkyl exchange reactions of organocobalt porphyrins. A bimolecular homolytic substitution reaction.
Alkyl exchange reactions of organocobalt porphyrins. A bimolecular homolytic substitution reaction.
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DOI：
10.1021/ja002954v
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发表时间：
2001-08
影响因子：
15
通讯作者：
Alan M. Stolzenberg and;Yang Cao
中科院分区：
文献类型：
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作者：
Alan M. Stolzenberg and;Yang Cao
有机钴（III）卟啉与可区分的卟啉或四吡咯的钴（II）配合物的反应导致有机轴位配体的可逆交换。在苯、甲苯、二氯甲烷、氯仿、吡啶等溶剂中交换反应容易；完全不受光的影响；在已知Co-C键离解能的情况下，比预期的均裂过程要快；并且在有机配体方面具有广泛的应用范围。甲基、苄基、伯烷基、仲烷基和酰基交换，但苯基没有交换。交换平衡点的位置与接近方向无关，并且是非统计性的。达到平衡的松弛似乎与二阶过程的松弛一致。反应速率随R基团的不同而变化，顺序为：Bzl >或= Me > Et约n-Pr > i-Pr > i-Bu >乙酰约新戊基约2-金刚烷基。然而，反应速率的总变化非常小。试图通过TEMPO或CO捕获或通过烷基交换过量的不同烷基的烷基卤化物来寻找自由基中间体的证据，在与交换反应的多个半衰期相当的时间尺度上是不成功的。此外，在5-己基交换反应中未检测到重排产物。在完全不光照的情况下，使用两面都有足够大的基团的钴卟啉反应物，以防止桥接结构的形成，Co-C-Co结构导致甲基交换速率降低5个或更多个数量级，如果不是完全停止的话。热交换速率的降低表明存在一个由室内光驱动的缓慢光化学交换过程。所有的证据都表明，双分子S(H)2的热交换机制是一致的。
Reaction of organocobalt(III) porphyrins with a cobalt(II) complex of a distinguishable porphyrin or tetrapyrrole resulted in the reversible exchange of the organic axial ligand. The exchange reaction was facile in such solvents as benzene, toluene, dichloromethane, chloroform, and pyridine; was unaffected by total exclusion of light; was faster than would be expected for a homolytic process given known Co-C bond dissociation energies; and was of broad scope with respect to the organic ligand. Methyl, benzyl, primary alkyl, secondary alkyl, and acyl groups exchanged, but phenyl groups did not. The position of the exchange equilibrium was independent of the direction of approach and was nonstatistical. The relaxation to equilibrium appeared to be consistent with that of a second-order process. The rate of the reaction varied with the identity of the R group in the order Bzl > or = Me > Et approximately n-Pr > i-Pr > i-Bu > acetyl approximately neopentyl approximately 2-adamantyl. However, the total variation in reaction rates was remarkably small. Attempts to find evidence of free-radical intermediates by trapping with TEMPO or CO or by alkyl group interchange with an excess of an alkyl halide of a distinct alkyl group were unsuccessful over a time scale comparable to multiple half-lives of the exchange reaction. In addition, no rearrangement products were detected in exchange reactions of the 5-hexenyl group. Use of cobalt porphyrin reactants that were sterically encumbered on both faces with groups large enough to prevent formation of a bridged, Co-C-Co structure resulted in a 5 or more order of magnitude decrease in the rate of methyl exchange, if not its outright cessation, when run with total exclusion of light. The decrease in the rate of the thermal exchange process revealed the existence a slow photochemical exchange process that was driven by room lights. All evidence was consistent with a bimolecular S(H)2 mechanism for the thermal exchange mechanism.