Garner Marc H, Corminboeuf Clemence
Laboratory for Computational Molecular Design, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de Lausanne (EPFL), 1015 Lausanne, Switzerland.
Org Lett. 2020 Oct 16;22(20):8028-8033. doi: 10.1021/acs.orglett.0c02980. Epub 2020 Sep 30.
Helical frontier molecular orbitals (MOs) appear in disubstituted allenes and even- cumulenes. Chiral molecules are optically active, but while these molecules are single-handed chiral, π-orbitals of both helicities are present. Here we computationally examine whether the optical activity of chiral cumulenes is controlled by the axial chirality or the helicity of the electronic structure. We exploit hyperconjugation with alkyl, silaalkyl, and germaalkyl substituents to adjust the MO helicity without altering the axial chirality. For the same axial chirality, we observe an inversion of the helical MOs contribution to the electronic transitions and a change of sign in the electronic circular dichroism and optical rotation dispersion spectra. While the magnitude of the chiroptical response also increases, it is similar to that of chiral cumulenes without helical π-orbitals. Overall, helical π-orbitals correlate with the big chiroptical response in cumulenes, but are not a prerequisite for it.
螺旋型前沿分子轨道（MOs）出现在二取代丙二烯和偶数累积多烯中。手性分子具有旋光性，但这些分子虽然是单手性的，但两种螺旋度的π轨道都存在。在这里，我们通过计算研究手性累积多烯的旋光性是由轴向手性还是电子结构的螺旋度控制。我们利用与烷基、硅烷基和锗烷基取代基的超共轭作用来调节MO螺旋度，而不改变轴向手性。对于相同的轴向手性，我们观察到螺旋型MOs对电子跃迁的贡献发生反转，并且电子圆二色性和旋光色散光谱中的符号发生变化。虽然手性光学响应的幅度也增加了，但它与没有螺旋π轨道的手性累积多烯的幅度相似。总体而言，螺旋π轨道与累积多烯中的大手性光学响应相关，但不是其先决条件。
