Wille Uta
ARC Centre of Excellence in Free Radical Chemistry and Biochemistry, University of Melbourne, VIC 3010, Australia.
J Org Chem. 2006 May 26;71(11):4040-8. doi: 10.1021/jo0520543.
The 1,2-nitroxyl and 1,2-acetoxyl rearrangement in beta-(nitroxy)vinyl and beta-(acetoxy)vinyl radicals 13a and 13b, respectively, has been studied for the gas phase with various ab initio and density functional methods. The energetically most favorable pathway for 13a is calculated to proceed via reversible fragmentation/radical addition through transition state I-19a. In the case of 13b, rearrangement through a five-membered ring transition state III-16b and the fragmentation/radical addition pathway via transition state I-19b are competing processes. Mulliken and natural population analysis reveal a certain degree of charge separation in III-16a/b that may indicate a potential solvent effect on the rearrangement rate. A stepwise group migration through a cyclic radical intermediate V-18a/b or rearrangement through a three-membered ring transition state II-15a/b can be ruled out for both vinyl radicals. A comparison of the results of the calculations with experimental findings provides important insights into the kinetics of "self-terminating radical oxygenations". A significant method dependence on the outcome of the calculations was observed, which revealed the unsuitability of the UHF, MP2, B3LYP, and mPW1PW91 methods for computing these radical rearrangement processes. The results from BHandHLYP/cc-pVDZ calculations showed the best agreement with single-point energy calculations performed at the QCISD and CCSD(T) levels of theory.
分别采用各种从头算和密度泛函方法对β-(硝氧基)乙烯基自由基13a和β-(乙酰氧基)乙烯基自由基13b中的1,2 - 硝氧基和1,2 - 乙酰氧基重排进行了气相研究。计算得出13a在能量上最有利的途径是通过过渡态I - 19a进行可逆的断裂/自由基加成。对于13b，通过五元环过渡态III - 16b的重排以及通过过渡态I - 19b的断裂/自由基加成途径是相互竞争的过程。Mulliken和自然布居分析揭示了III - 16a/b中存在一定程度的电荷分离，这可能表明溶剂对重排速率有潜在影响。对于这两种乙烯基自由基，都可以排除通过环状自由基中间体V - 18a/b的逐步基团迁移或通过三元环过渡态II - 15a/b的重排。将计算结果与实验结果进行比较，为“自终止自由基氧化”的动力学提供了重要见解。观察到计算结果对方法有显著依赖性，这表明UHF、MP2、B3LYP和mPW1PW91方法不适用于计算这些自由基重排过程。BHandHLYP/cc - pVDZ计算结果与在QCISD和CCSD(T)理论水平上进行的单点能量计算结果显示出最佳一致性。
