Dye-Sensitized Photooxygenation of the CN Bond. 5. Substituent Effects on the Cleavage of the CN Bond of <i>C</i>-Aryl-<i>N</i>-aryl-<i>N</i>-methylhydrazones<sup>1</sup>
作者:Ihsan Erden、Pinar Ergonenc Alscher、James R. Keeffe、Colin Mercer
DOI:10.1021/jo050014t
日期:2005.5.1
involved in the mechanism. We postulate a mechanism in which the initial event is exothermic electron transfer from the hydrazone to 1O2 leading to an ion−radical caged pair. Subsequent covalent bond formation between the hydrazone carbon and an oxygen atom is rate controlling. The transition state for this step also has a lower enthalpy than the starting reactants, but the free energy of activation
标题化合物在C干净地切割由单线态氧N键(1 Ò 2,1 Δ克得到芳醛)和ñ -芳基- Ñ -methylnitrosamines。这些反应在-78°C处比在室温下发生的速度更快。使用竞争性方法研究了取代基在C-芳基和N-芳基上的变化的影响。所得率比的良好的相关性与取代基常数(σ -或σ +所获得的)产生很小到非常小的ρ值,表明反应物和决定速率的过渡态之间的电荷分布从很小到很小。C-芳基部分上的吸电子基团通过preferential的优先稳定化而在某种程度上延迟了反应。另一方面,电子给体稍微稳定了决定过渡态的速率。N-芳基上的取代基几乎没有作用。未观察到3,5-二叔丁基苯酚的抑制作用,表明该机制未涉及自由基(未老化的)自由基中间体。我们假设一种机制,其中初始事件是放热电子从hydr到1 O 2的转移导致离子自由基笼对。carbon碳和氧原子之间随后的共价键形成是速率控制的。该步骤的过渡状态也