Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany.
Chirality. 2013 Apr;25(4):224-9. doi: 10.1002/chir.22131. Epub 2013 Feb 7.
Diaziridines are very interesting representatives of organic compounds containing stereogenic nitrogen atoms. In particular, 1,2-dialkyldiaziridines show extraordinarily high stereointegrity. The lone electron pairs of the nitrogen atoms are in trans configuration, avoiding a four-electron repulsive interaction. Furthermore, the trans configuration of the substituents at the nitrogen atoms is energetically favored because of reduced steric interactions. Therefore only two stereoisomers (enantiomers) are observed. At elevated temperatures the enantiomers are interconverting because of the limited stereointegrity of the chirotopic nitrogen atoms. The enantiomerization rate constants and the activation parameters of interconversion are of great interest. Here, we investigated the stereodynamics of a set of small 1,2-dialkyldiaziridines bearing short substituents (Me, Et, iPr, tBu), using enantioselective dynamic gas chromatography (DGC). Separation of enantiomers of all compounds, including the highly volatile 1,2-dimethyldiaziridine, was achieved using heptakis(2,3-di-O-ethyl-6-O-tert-butyldimethylsilyl)-β-cyclodextrin in 50% PS086 (w/w) as chiral stationary phase in fused silica capillaries with a length of up to 50 m. Measurements at variable temperatures were performed and reaction rate constants were determined using the unified equation of chromatography implemented in the software DCXplorer. The activation barriers at room temperature for 1-(tert-butyl)-2-ethyldiaziridine, ΔG(╪)(298K) = 123.8 kJ mol(-1) (ΔH(╪) = 115.5±2.9 kJ mol(-1), ΔS(╪) = -28±1 J mol(-1) K(-1)), and 1-ethyl-2-isopropyldiaziridine, ΔG(╪)(298K) = 124.2 kJ mol(-1) (ΔH(╪) = 113.1±2.4 kJ mol(-1), ΔS(╪) = -37±2 J mol(-1) K(-1)), were determined, representing some of the highest values observed for nitrogen inversion in diaziridines.
重氮化合物是含有手性氮原子的有机化合物非常有趣的代表。特别是，1,2-二烷基重氮化合物表现出极高的立体完整性。氮原子上的孤对电子处于反式构型，避免了四电子排斥相互作用。此外，由于空间位阻相互作用的降低，氮原子上取代基的反式构型在能量上是有利的。因此，只观察到两种立体异构体（对映体）。在较高温度下，由于手性氮原子的立体整合性有限，对映体相互转化。对映体转化的速率常数和活化参数非常有趣。在这里，我们使用手性动态气相色谱法（DGC）研究了一系列带有短取代基（Me、Et、iPr、tBu）的小 1,2-二烷基重氮化合物的立体动力学。使用庚基（2,3-二-O-乙基-6-O-叔丁基二甲基硅基）-β-环糊精在 50% PS086（重量/重量）中作为手性固定相，在长达 50 m 的熔融石英毛细管中实现了所有化合物的对映体分离，包括高挥发性的 1,2-二甲基金属重氮化合物。在可变温度下进行了测量，并使用统一的色谱方程在软件 DCXplorer 中确定了反应速率常数。在室温下，1-（叔丁基）-2-乙基重氮化合物的活化能垒，ΔG(╪)(298K) = 123.8 kJ mol(-1)（ΔH(╪) = 115.5±2.9 kJ mol(-1），ΔS(╪) = -28±1 J mol(-1) K(-1)）和 1-乙基-2-异丙基重氮化合物，ΔG(╪)(298K) = 124.2 kJ mol(-1)（ΔH(╪) = 113.1±2.4 kJ mol(-1），ΔS(╪) = -37±2 J mol(-1) K(-1)），是氮原子反转在重氮化合物中观察到的一些最高值。
