Shy Hannah, Mackin Paula, Orvieto Andrea S, Gharbharan Deepa, Peterson Geneva R, Bampos Nick, Hamilton Tamara D
Department of Physical Sciences, Barry University, 11300 NE 2nd Ave., Miami Shores, FL 33161, USA.
Faraday Discuss. 2014;170:59-69. doi: 10.1039/c3fd00140g.
Porphyrin synthesis under solvent-free conditions represents the "greening" of a traditional synthesis that normally requires large amounts of organic solvent, and has hindered the industrial-scale synthesis of this useful class of molecules. We have found that the four-fold acid-catalysed condensation of aldehyde and pyrrole to yield a tetra-substituted porphyrin is possible through mechanochemical techniques, without a solvent present. This represents one of the still-rare examples of carbon-carbon bond formation by mechanochemistry. Specifically, upon grinding equimolar amounts of pyrrole and benzaldehyde in the presence of an acid catalyst, cyclization takes place to give reduced porphyrin precursors (reversible), which upon oxidation form tetraphenylporphyrin (TPP). The approach has been found to be suitable for the synthesis of a variety of meso-tetrasubstituted porphyrins. Oxidation can occur either by using an oxidizing agent in solution, to give yields comparable to those published for traditional methods of porphyrin synthesis, or through mechanochemical means resulting in a two-step mechanochemical synthesis to give slightly lower yields that are still being optimized. We are also working on "green" methods of porphyrin isolation, including entrainment sublimation, which would hopefully further reduce the need for large amounts of organic solvent. These results hold promise for the development of mechanochemical synthetic protocols for porphyrins and related classes of compounds.
无溶剂条件下的卟啉合成代表了传统合成方法的“绿色化”，传统方法通常需要大量有机溶剂，这阻碍了这类有用分子的工业规模合成。我们发现，通过机械化学技术，在无溶剂存在的情况下，醛和吡咯经四倍酸催化缩合生成四取代卟啉是可行的。这是机械化学形成碳 - 碳键的仍较为罕见的例子之一。具体而言，在酸催化剂存在下研磨等摩尔量的吡咯和苯甲醛时，会发生环化反应生成还原型卟啉前体（可逆），该前体经氧化形成四苯基卟啉（TPP）。已发现该方法适用于多种中位四取代卟啉的合成。氧化反应既可以通过在溶液中使用氧化剂来进行，以获得与卟啉合成传统方法所报道产率相当的产率，也可以通过机械化学方法进行，从而实现两步机械化学合成，尽管产率略低但仍在优化中。我们还在研究卟啉分离的“绿色”方法，包括夹带升华法，有望进一步减少对大量有机溶剂的需求。这些结果为开发卟啉及相关类化合物的机械化学合成方案带来了希望。
