DFT Conformational Studies of Chiral Bis-Binaphthyl Porphyrins and Their Metal Complexes Employed as Cyclopropanation Catalysts

Three C2-symmetrical chiral porphyrins,
derived from α,α,β,β-tetrakis­(2-aminophenyl)­porphyrin
and holding two binaphthyl handles through dimethylene, methylene,
or direct connections between the binaphthyl moieties and the amidophenyl
pickets, have been submitted to a theoretical study to investigate
their conformational properties. The porphyrin with no methylene spacer
was shown to be a very rigid molecule, whereas its higher homologues
showed a certain degree of conformational freedom resulting in two
molecular arrangements that give significant contributions to the
equilibrium population of each compound. The effect of complexation
with zinc­(II) was studied through the modeling of the corresponding
complexes as well as the role of two N-methylimidazoles
(NMI) coordinating the apical positions of zinc in these complexes.
The computational data showed that only one of the two molecular architectures
accessible for the free ligands can easily accommodate NMI, indicating
that the presence of an additional group on the apical coordination
positions selects the geometry most suitable to host this group. Finally,
the supposed intermediate radical active species in the cyclopropanation
catalyzed by a cobalt­(II) porphyrin complex, in which the central
metal ion coordinates one NMI and the CHCOOEt carbene, were modeled
together with the transition states leading to them. It was shown
that the cavity originated by the binaphthyl moiety surmounting the
porphyrin is not large enough to host the carbene ethyl group, suggesting
the opportunity to increase the size of the cavity in order to confer
it hosting capability toward the alkyl group. With the ethyl group
outside the cavity, the relatively high mobility of CHCOOEt allows
it to assume conformations exposing both the carbene Re and Si faces to the approaching alkene, thus leading
to a poorly selective process.

三种C₂对称手性卟啉（C2-symmetrical chiral porphyrins），衍生自α,α,β,β-四(2-氨基苯基)卟啉，其两个联萘基（binaphthyl）手性手柄分别通过二亚甲基、亚甲基或直接与卟啉上的酰胺苯基侧臂相连，已被用于理论研究以探究其构象性质。无亚甲基间隔基的卟啉被证明是刚性极强的分子，而其高级同系物则具备一定程度的构象自由度，可形成两种分子构型，这两种构型对各化合物的平衡分布均有显著贡献。本研究通过对相应配合物的建模，探究了锌(II)配位作用的影响，同时分析了两个N-甲基咪唑（NMI）在该类配合物中配位锌原子轴向位点的作用。计算数据表明，游离配体可形成的两种分子结构中，仅有一种能轻松容纳NMI，这说明轴向配位位点上额外基团的存在会筛选出最适合承载该基团的几何构型。最后，本研究对钴(II)卟啉配合物催化环丙烷化反应（cyclopropanation）中假想的自由基活性中间体进行了建模，该配合物的中心金属离子配位一个NMI与乙氧羰基甲基卡宾（carbene），同时还建模了生成该中间体的过渡态。研究结果表明，由覆盖于卟啉环上方的联萘基所形成的空腔尺寸不足以容纳卡宾上的乙基，这提示可通过扩大空腔尺寸来赋予其承载烷基基团的能力。当乙基处于空腔外部时，CHCOOEt具备较高的运动自由度，可采取多种构象，使卡宾的Re面与Si面均能暴露给趋近的烯烃，最终导致反应选择性较差。