Laali Kenneth K, Arrica Maria A, Okazaki Takao, Harvey Ronald G
Department of Chemistry, Kent State University, Kent, Ohio 44242, USA.
J Org Chem. 2007 Aug 31;72(18):6768-75. doi: 10.1021/jo070936r. Epub 2007 Aug 4.
A series of novel carbocations were generated from isomeric monoalkylated and dialkylated benz[a]anthracenes (BAs) by low-temperature protonation in FSO(3)H/SO(2)ClF. With the monoalkyl derivatives (5-methyl, 6-methyl, 7-methyl, and 7-ethyl) as well as the D-ring methylated analogues (9-methyl, 10-methyl, and 11-methyl), the C-7 or the C-12 protonated carbocations were observed (as the sole or major carbocation) in all cases. Protonation of the 12-methyl derivative (9) gave the C-7 protonated carbocation (9H+) as the kinetic species and the ipso-protonated carbocation (9aH+) as the thermodynamic cation. With the 12-ethyl derivative (10), relief of steric strain in the bay-region greatly favors ipso-protonation (10aH+). With 3,9-dimethyl (14), C-7 protonation (14H+) is strongly favored (with <10% protonation at C-12), and with 1,12-dimethyl (15) the sole species observed is the C-7 protonated carbocation (15H+). For 7-methyl-12-ethyl, 7-ethyl-12-methyl, and 7,12-diethyl derivatives (16, 17, and 18), two ipso-protonated carbocations were initially formed (C-7/C-12), rearranging in time to give the C-12 protonated carbocations exclusively (16aH+, 17aH+, and 18aH+). Protonation outcomes are compared with the computed relative energies by DFT. Charge delocalization paths in the resulting carbocations were deduced based on the magnitude of Deltadelta13C values. For the thermodynamically more stable C-12 protonated carbocations, the charge delocalization path is analogous to those derived based on computed NPA charges for the benzylic carbocations formed by 1,2-epoxide (bay-region) and 5,6-epoxide (K-region) ring opening. Nitration (and bromination) of the 4-methyl, 7-methyl, 7-ethyl, 3,9-dimethyl, and 1,12-dimethyl derivatives resulted in isolation and characterization of several novel derivatives. Excellent agreement is found between low-temperature protonation selectivities and the regioselectivities observed in model substitution reactions.
通过在FSO(3)H/SO(2)ClF中进行低温质子化反应，从异构的单烷基化和二烷基化苯并[a]蒽（BAs）生成了一系列新型碳正离子。对于单烷基衍生物（5-甲基、6-甲基、7-甲基和7-乙基）以及D环甲基化类似物（9-甲基、10-甲基和11-甲基），在所有情况下均观察到C-7或C-12质子化的碳正离子（作为唯一或主要的碳正离子）。12-甲基衍生物（9）的质子化反应生成了作为动力学物种的C-7质子化碳正离子（9H+）和作为热力学阳离子的本位质子化碳正离子（9aH+）。对于12-乙基衍生物（10），湾区空间位阻的缓解极大地有利于本位质子化（10aH+）。对于3,9-二甲基（14），强烈倾向于C-7质子化（14H+）（C-12处质子化小于10%），而对于1,12-二甲基（15），观察到的唯一物种是C-7质子化碳正离子（15H+）。对于7-甲基-12-乙基、7-乙基-12-甲基和7,12-二乙基衍生物（16、17和18），最初形成了两个本位质子化的碳正离子（C-7/C-12），随着时间的推移重排，最终仅生成C-12质子化的碳正离子（16aH+、17aH+和18aH+）。将质子化结果与通过密度泛函理论（DFT）计算的相对能量进行了比较。根据Δδ13C值的大小推导了所得碳正离子中的电荷离域路径。对于热力学上更稳定的C-12质子化碳正离子，电荷离域路径类似于基于通过1,2-环氧化物（湾区）和5,6-环氧化物（K区）开环形成的苄基碳正离子计算的自然键轨道（NPA）电荷所推导的路径。对4-甲基、7-甲基、7-乙基、3,9-二甲基和1,12-二甲基衍生物进行硝化（和溴化）反应，得到了几种新型衍生物并对其进行了表征。在低温质子化选择性与模型取代反应中观察到的区域选择性之间发现了极好的一致性。
