Push-pull alkenes: structure and π-electron distribution
Push-pull alkenes: structure and π-electron distribution
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DOI：
10.2298/jsc0601001k
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发表时间：
2006-01-01
影响因子：
1
通讯作者：
Kleinpeter, E
中科院分区：
文献类型：
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作者：
Kleinpeter, E
推拉式烯烃是指在C=C双键的一端具有一个或两个供电子取代基，在另一端具有一个或两个受电子取代基的取代烯烃。允许π电子离域导致中心C=C双键变得更加极化，并且随着推挽特性的增加，该双键的π键级降低，并且相反地，C-Don和C-Ace键的相应π键级相应地增加。这种推拉效应对这类化合物的动力学行为和化学反应性都有决定性的影响，因此确定和量化固有的推拉效应是相当有意义的。先前，围绕C=C、C-Don和/或C-Acc部分双键的旋转的障碍（Δ G（不相等），通过动态NMR光谱测定）或极化C=C部分双键的C-13化学位移差（Δ Δ（C=C））用于此目的。然而，这些参数可能具有严重的局限性，即，屏障在NMR时间尺度上可能是不可测量的（无论是太高还是太低，存在严重偏向的构象异构体等）或Δ Δ（C=C）相对于四个取代基的组合以非加和方式表现。因此，目前还没有量化推拉效应的一般参数。从头算分子轨道计算的化合物的集合，连同NBO分析，提供了宝贵的信息，结构，键能，电子占位和键/反键相互作用。除了Delta G（C=C）（不相等）（实验测定或理论计算）和Delta delta（C=C）之外，还检查了C=C部分双键的键长，并且证明它是量化推拉效应的可靠参数。同样，中心C=C部分双键的反键π轨道和成键π轨道的占据数的商（π *（C=C）/π（C=C））也可以用于此目的。
Push-pull alkenes are substituted alkenes with one or two electron-donating substituents on one end of C=C double bond and with one or two electron-accepting substituents at the other end. Allowance for pi-electron delocalization leads to the central C=C double bond becoming ever more polarized and with rising push-pull character, the pi-bond order of this double bond is reduced and, conversely, the corresponding pi-bond orders of the C-Don and C-Ace bonds are accordingly increased. This push-pull effect is of decisive influence on both the dynamic behavior and the chemical reactivity of this class of compounds and thus it is Of Considerable interest to both determine and to quantify the inherent push-pull effect. previously, the barriers to rotation about the C=C, C-Don and/or C-Acc partial double bonds (Delta G(not equal), as determined by dynamic NMR spectroscopy) or the C-13 chemical shift difference of the polarized C=C partial double bond (Delta delta(C=C)) were employed for this purpose, However, these parameters can have serious limitations, viz. the barriers can be immeasurable on the NMR timescale (either by being too high or too low-, heavily-biased conformers are present, etc.) or Delta delta(C=C) behaves in a non-additive manner with respect to the combination of the four substituents. Hence, a general parameter to quantify the push-pull effect is not yet available. Ab initio MO calculations on a collection of compounds, together with NBO analysis, provided valuable information on the structure, bond energies, electron occupancies and bonding/antibonding interactions. In addition to Delta G(C=C)(not equal) (either experimentally determined or theoretically calculated) and Delta delta(C=C), the bond length of the C=C partial double bond was also examined and it proved to be a reliable parameter to quantify the push-pull effect. Equally so, the quotient of the occupation numbers of the antibonding and bonding pi orbitals of the central C=C partial double bond ( pi*(C=C)/pi(C=C) ) could also be employed for this purpose.