Toward True Carbaporphyrinoids - Contraction of Benziporphyrins

A replacement of a single pyrrole of meso-tetraarylporphyrin by cyclopentadiene moiety seems to be a notion of the very first choice for the creation of meso-substituted or β-substituted 21-carbaporphyrins which ideally conserve the fundamental frame of porphyrin, seen also in 21-heteroporphyrins and in the first metallaporphyrin (21-pallada-23-tellura-porphyrin).^[3] The construction of a  true carbaporphyrin requires a replacement of the imine nitrogen of pyrrolene by a trigonally hybridized methine unit or amine NH of pyrrole by a tetrahedral methylene group affording a tautomeric couple (2 and 3) which is parental for all monocarbaporphyrinoids. It remains a challenge to create 5,10,15,20-tetraaryl-21-carbaporphyrins that are potentially amenable to diverse structural modifications.The remarkable, facile contraction of p-phenylene embedded in p-benziporphyrin 1 to cyclopentadiene, mediated by transition metal cation of 2-M produced complexes of 21-carbaporphyrins 2-M or 3-M.^[1,2] Thus these investigations afforded 5,10,15,20-tetraaryl- 21-carbaporphyrin - the first example of true carbaporphyrin complexes, which are firmly stabilized via coordination of trigonal (2-M) or tetrahedral (3-M) carbon donors. One can expect the true carbaporphyrins can prompt developments in carbaporphyrinoid and organometallic chemistry building on specific reactivity of cyclopentadienyl moiety. 

References
[1]  Szyszko, B.; Latos-Grazynski, L.; Szterenberg, L. Angew. Chem. Int. Ed. 2011, 50, 6587.
[2]  Szyszko, B.; Kupietz, K.; Szterenberg, L.; Latos-Grazynski, L. Chem. Eur. J. 2014, 20, 376.
[3]  Pacholska-Dudziak E.; Szczepaniak, M.; Ksiazek, A.; Latos-Grazynski, L. Angew. Chem. Int. Ed. 2013, 52, 8898.