A laser flash photolysis and quantum chemical study of the fluorinated derivatives of singlet phenylnitrene
A laser flash photolysis and quantum chemical study of the fluorinated derivatives of singlet phenylnitrene
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DOI：
10.1021/ja9944305
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发表时间：
2001-03-07
影响因子：
15
通讯作者：
Platz, MS
中科院分区：
文献类型：
--
作者：
Gritsan, NP;Gudmundsdóttir, AD;Platz, MS
2-氟、4-氟、3，5-二氟、2，6-二氟和2，3，4，5，6-五氟苯基叠氮化合物的激光闪光光解（LFP，Nd：YAG激光，3,5 ps，266 nm，10 mJ或KrF准分子激光，10 ns，249 nm，50 mJ）产生相应的单线态氮烯。通过瞬态吸收光谱检测到单重态氮烯，其光谱特征在于在300-365 nm范围内具有最大吸收的尖锐吸收带。它们的衰减动力学作为温度的函数进行分析，得到所观察到的衰减速率常数k（OBS）。在惰性溶剂中所观察到的速率常数是k（R）+ k（ISC）的总和，其中kR是单线态氮烯重排为氮杂环丙烷的绝对速率常数，k（ISC）是氮烯系间穿越（ISC）的绝对速率常数。在假定k（ISC）与温度无关的前提下，导出了k（R）和k（ISC）的值。4-氟-、3，5-二氟-、2-氟-、2，6-二氟-和2，3，4，5，6-五氟苯基氮烯在惰性溶剂中的环化势垒分别为5.3 +/- 0.3、5.5 +/- 0.3、6.7 +/- 0.3、8.0 +/- 1.5和8.8 +/- 0.4 kcal/mol。母体单线态苯基氮宾的环化势垒为5.6 +/- 0.3 kcal/mol。所有这些值都与CASPT 2计算的氟取代的单线态芳基氮烯转化为苯并氮杂环丙烷的相对势垒高度具有良好的定量一致性（Karney，W. L.和Borden，W. T. J. Am.化学分析1997，119，3347）。单个邻氟取代基对起源上不是空间位阻的远程环化发挥小但显著的旁观者效应。两个邻位氟取代基对环化反应的影响是显著的。在单重态2-氟苯基氮烯系统的情况下，证据表明，苯并氮丙啶是一个中间体，相应的单重态氮烯和苯并氮丙啶相互转化。从头计算在不同的理论水平上的一系列苯并氮杂环丙烷，其异构体烯酮亚胺，和过渡态，转换苯并氮杂环丙烷烯酮亚胺进行。计算结果与实验结果在定性和定量上都有很好的一致性。
Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nin, 10 mJ or KrF excimer laser, 10 ns, 249 nm, 50 mJ) of 2-fluoro, 4-fluoro, 3,5-difluoro, 2,6-difluoro, and 2,3,4,5,6-pentafluorophenyl azides produces the corresponding singlet nitrenes. The singlet nitrenes were detected by transient absorption spectroscopy, and their spectra are characterized by sharp absorption bands with maxima in the range of 300-365 nm. The kinetics of their decay were analyzed as a function of temperature to yield observed decay rate constants, k(OBS) The observed rate constant in inert solvents is the sum of k(R) + k(ISC) where kR is the absolute rate constant of rearrangement of singlet nitrene to an azirine and k(ISC) is the absolute rate constant of nitrene intersystem crossing (ISC). Values of k(R) and k(ISC) were deduced after assuming that k(ISC) is independent of temperature. Barriers to cyclization of 4-fluoro-, 3,5-difluoro-, 2-fluoro-, 2,6-difluoro-, and 2,3,4,5,6-pentafluorophenylnitrene in inert solvents are 5.3 +/- 0.3, 5.5 +/- 0.3, 6.7 +/- 0.3, 8.0 +/- 1.5, and 8.8 +/- 0.4 kcal/mol, respectively. The barrier to cyclization of parent singlet phenylnitrene is 5.6 +/- 0.3 kcal/mol. All of these values are in good quantitative agreement with CASPT2 calculations of the relative barrier heights for the conversion of fluoro-substituted singlet aryl nitrenes to benzazirines (Karney, W. L. and Borden, W. T. J. Am. Chem. Sec. 1997, 119, 3347), A single ortho-fluorine substituent exerts a small but significant bystander effect on remote cyclization that is not steric in origin. The influence of two ortho-fluorine substituents on the cyclization is pronounced. In the case of the singlet 2-fluorophenylnitrene system, evidence is presented that the benzazirine is an intermediate and that the corresponding singlet nitrene and benzazirine interconvert. Ab initio calculations at different levels of theory on a series of benzazirines, their isomeric ketenimines, and the transition states; converting the benzazirines to ketenimines were performed. The computational results are in good qualitative and quantitative agreement with the experimental findings.