Reaction of anthracenide A˙– with 1-pivaloyl-2,3-diphenylaziridines cis-4a and trans-4a yield the same products, namely PhCH2CHPhNHCOCMe3 (11) and AH–CHPhCHPhNHCOCMe3 (6) (AH = dihydroanthryl). The steric differentiation is lost when the two ketyls 12 formed from 4a undergo homolytic ring cleavage forming the same anionic radical 13. Extremely short reactions (⩽10 s) give 6 as the erythro isomer exclusively or nearly so. Coupling of 13 with A˙– does not form the precursor (threo-8) of threo-6 owing to electrostatic repulsion between A˙– and the anionic tail of 13 in the preferred conformation of the latter. Radical coupling is not completed within this short time so 11 can be formed directly from 13 via 10, the amide anion of 11. Reduction of 13 to carbanion 9 by outer-sphere electron transfer or via 8 and its benzylic fragmentation (BFR) is the other path to 11. Extending the time to 1 or 2 min has the following effects. Coupling of 13 with A˙– is completed at the expense of 11. Second, more than a trace of threo-6 is detected indicating that BFR 8→9 + A (A = anthracene) is reversible and that addition of dianion 9 to A proceeds without pronounced stereochemical preference. With even more time the erythro∶threo ratio changes in favour of threo-6 and finally can even reach a value slightly less than 1. Simultaneously the amount of 11 increases slowly at the expense of the total 6 indicating that part of BFR which becomes irreversible by carbanion protonation 9 + THF→10. With much longer reaction times imidate ion 10 eliminates (E)-stilbene. Both isomers of 6 have been independently synthesized from the two isomers of 4a and anthracene hydride AH–.
蒽化合物 A˙– 与 1-
新戊酰基-2,3-二苯基
氮丙啶顺式-4a 和反式-4a 反应产生相同的产物,即 PhCH2CHPhNHCOCMe3 (11) 和 AH–CHPhCHPhNHCOCMe3 (6) (AH = 二氢
蒽基)。当由 4a 形成的两个羰基 12 经历均裂环裂解形成相同的阴离子自由基 13 时,空间差异消失。极短的反应(⩽10 s)仅产生或几乎如此的赤式异构体 6。 13 与 A˙– 的偶联不会形成 threo-6 的前体 (threo-8),因为 A˙– 与 13 的阴离子尾部之间的静电排斥力处于后者的首选构象中。自由基偶联不会在这么短的时间内完成,因此 11 可以直接从 13 通过 10(11 的酰胺阴离子)形成。通过外层电子转移或通过 8 将 13 还原为负碳离子 9,其苄基断裂(BFR)是其他路径为 11。将时间延长至 1 或 2 分钟具有以下效果。 13 与 A˙– 的偶联以 11 为代价完成。其次,检测到超过痕量的 threo-6,表明 BFR 8→9 + A(A =
蒽)是可逆的,并且二价阴离子 9 添加到 A进行时没有明显的立体
化学偏好。随着时间的推移,赤式:苏式比率会向有利于 threo-6 的方向变化,最终甚至可以达到略小于 1 的值。同时,11 的数量缓慢增加,但以总 6 的损失为代价,这表明 BFR 的一部分变得不可逆通过碳负离子质子化9+THF→10。随着反应时间的延长,
亚胺酸根离子 10 消除了 (E)-
二苯乙烯。 6的两种异构体都是由4a和
蒽氢化物AH–的两种异构体独立合成的。