Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan.
Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Kita 21, Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan.
J Am Chem Soc. 2021 Aug 11;143(31):12355-12360. doi: 10.1021/jacs.1c06331. Epub 2021 Jul 28.
A long-standing question in porphyrin chemistry is why pyrrole monomers selectively form tetrapyrrolic macrocycles, whereas the corresponding tripyrrolic macrocycles are never observed. Calix[3]pyrrole, a tripyrrolic porphyrinogen-like macrocycle bearing three sp-carbon linkages, is a missing link molecule that might hold the key to this enigma; however, it has remained elusive. Here we report the synthesis and strain-induced transformations of calix[3]pyrrole and its furan analogue, calix[3]furan. These macrocycles are readily accessed from cyclic oligoketones. Crystallographic and theoretical analyses reveal that these three-subunit systems possess the largest strain energy among known calix[]-type macrocycles. The ring-strain triggers transformation of calix[3]pyrrole into first calix[6]pyrrole and then calix[4]pyrrole under porphyrin cyclization conditions. The present results help explain the absence of naturally occurring three-pyrrole macrocycles and the fact that they are not observed as products or intermediate during classic porphyrin syntheses.
卟啉化学中长期存在的一个问题是，为什么吡咯单体选择性地形成四吡咯大环，而相应的三吡咯大环从未被观察到。杯[3]吡咯是一种具有三个 sp 碳键的三吡咯卟啉原样大环，是一种缺失的链接分子，可能是解开这个谜团的关键；然而，它一直难以捉摸。在这里，我们报告了杯[3]吡咯及其呋喃类似物杯[3]呋喃的合成和应变诱导转化。这些大环可以很容易地从环状寡酮中获得。晶体学和理论分析表明，在已知的杯[]型大环中，这三个单元系统具有最大的应变能。环应变在卟啉环化条件下引发杯[3]吡咯首先转化为杯[6]吡咯，然后转化为杯[4]吡咯。目前的结果有助于解释为什么自然界中不存在三吡咯大环，以及为什么它们在经典卟啉合成过程中不是作为产物或中间体被观察到。
