Weber J, Clennan E L
Department of Chemistry , University of Wyoming , Laramie , Wyoming 82071 , United States.
J Org Chem. 2019 Jan 18;84(2):817-830. doi: 10.1021/acs.joc.8b02671. Epub 2018 Dec 27.
The regiochemistry of four bis-Mallory photocyclization substrates has been examined from experimental and computational perspectives. Formation of all three possible regioisomers was only observed in the reaction of one of the substrates. In the other three substrates, only the two C-symmetric products, but not the C product, were formed. In the three reactions that only formed two products, the photocyclization temperature could be used to select for exclusive formation of one or the other regioisomer. The use of temperature to select between two regioisomers also worked in the photocyclization of the substrate that formed three products. However, no temperature was located for exclusive formation of the third component, one of the C-symmetric products, which always formed alongside either one or both of its regioisomers. B3LYP/6-311+G(2d,p) calculations were used to determine the energies of all of the dihydrophenanthrene (DHP), tetrahydrophenanthrene (THP), and mono-Mallory photocyclization intermediates. The oscillator strengths of the DHP precursors to the helicene products were a factor of 4.8-9.2 smaller than those of competitively formed DHPs. This observation suggests that establishment of a photostationary state is responsible for the preferential formation of helicenes that has been observed as a unique and useful feature of many Mallory photocyclizations.
已从实验和计算的角度研究了四种双马洛里光环化底物的区域化学。仅在其中一种底物的反应中观察到了所有三种可能区域异构体的形成。在其他三种底物中，仅形成了两种C对称产物，而未形成C产物。在仅形成两种产物的三个反应中，光环化温度可用于选择排他性地形成一种或另一种区域异构体。利用温度在两种区域异构体之间进行选择，在形成三种产物的底物的光环化反应中也有效。然而，未找到能排他性地形成第三种组分（一种C对称产物）的温度，该产物总是与其一种或两种区域异构体一起形成。采用B3LYP/6-311+G(2d,p)计算来确定所有二氢菲（DHP）、四氢菲（THP）和单马洛里光环化中间体的能量。向螺旋烯产物的DHP前体的振子强度比竞争性形成的DHP的振子强度小4.8 - 9.2倍。这一观察结果表明，光稳态的建立是导致螺旋烯优先形成的原因，螺旋烯的优先形成已被视为许多马洛里光环化反应的一个独特且有用的特征。
