Porphyrinoid systems

Just recently a [2+2] methodology has been described (07JA13800, 08M11), which enables the synthesis of fld/-diazuliporphyrins (dicarba-porphyrinoid systems with two adjacent azulene subunits) by the condensation of dipyrrylmethanes with diazulene dialdehydes. 

Borovkov VV, Inoue Y (2006) Supramolecular Chirogenesis in Host-Guest Systems Containing Porphyrinoids. 265 89-146... 

Following the standard system of nomenclature, the names of porphyrinoids consist of three parts (i) a number in the square bracket corresponds to the number of jt-electrons in the shortest conjugation pathway (ii) a core name representing the number of pyrroles or other heterocycles in the overall system and (iii) numbers in round brackets specify the number of bridging C-atoms between each pyrrole subunit, starting with the largest. 

Abstract Porphyrins and their analogues constitute one of the most important families of aromatic macrocycles. The present review discusses aromaticity of porphyrinoids, focusing mainly on non-expanded systems. The effect of structural modifications on the aromaticity-dependent properties of porphyrin-like macrocycles is described. It is shown that delocalization modes observed in porphyrinoids can be classified using a simple valence-bond approach. Aromaticity of porphyrinoids is further discussed as a function of tautomerism, coordination chemistry, and the oxidation state of the macrocycle. 

Variations on the basic structural theme of 1 have led to a plethora of unusual macrocyclic systems, collectively known as porphyrin analogs or porphyrinoids. These molecules, often nontrivial to synthesize, exhibit remarkable physical and chemical properties and their chemistry has been extensively reviewed [2-18], With the impressive range of structural modifications introduced so far, the term porphyrinoid has ultimately expanded to encompass a wide range of often exotic macrocycles, some of which contain no pyrrole rings at all, or have a structural outline barely resembling that of porphyrin. Some of the generic modification types are shown in Fig. 1. Combination of these design concepts provides a virtually inexhaustible source of structural diversity. 

A great number of porphyrin analogs possess circular conjugation pathways, and often exhibit aromaticity comparable with that of the parent system 1. Moreover, the JT-electron delocalization of many porphyrin-like molecules is strongly affected by structural detail, redox chemistry, and prototropic tautomerism. The aim of the present review is to provide a description of porphyrinoid aromaticity and its connection with tautomeric equilibria. Our main focus will be on the physical manifestations of aromaticity, with a special emphasis on NMR spectroscopy. The reactivity of porphyrin analogs, including their coordination chemistry, will be discussed only to the extent it has a bearing on their aromaticity. 

Most of the porphyrinoid molecules synthesized so far possess a number of peripheral substituents. In fact, many of the important macrocyclic systems were obtained with several distinct substitution patterns. Usually they correspond to one of the generic substitution types of regular porphyrins meso-aryl or p-alkyl, as exemplified by 5,10,15,20-tetraphenylporphyrin and 2,3,7,8,12,13,17,18-octaethylporphy-rin, respectively (Fig. 2). For the sake of brevity we will assume the following conventions in the entire manuscript ... 

Many of the above deficiencies were removed in further refinements of the Hiickel method, which was anyway made obsolete by the development of ab initio and DFT techniques. Still, organic chemists adhere to the original Hiickel description, which is often sufficient to make qualitative predictions about the nature of n-conjugated systems. In particular, the Hiickel model finds widespread use in porphyrinoid chemistry. The so-called annulene model, which will be used throughout this review, is outlined below for the parent porphyrin macrocycle. 

Aromaticity of ir-electron systems is a complex phenomenon which is defined in terms of a variety of physical and chemical properties [26], The following criteria are of particular value in porphyrinoid chemistry ... 

According to a recently proposed definition, expanded porphyrins possess more than 16 atoms in the inner or, more precisely, smallest circuit of the macrocycle (porphyrin 1 contains exactly 16 atoms in the smallest macrocyclic circuit) [11], In addition to porphyrinoid macrocycles traditionally recognized as expanded, i.e., possessing more than four cyclic subunits or more than four meso bridges, this new definition encompasses some other systems, such as p-benziporphyrin 150. Regardless of the exact definition, expanded porphyrins constitute an unusually rich and diverse family of structures, and have been the subject of several excellent reviews [2, 5, 11, 146], The relationship between aromaticity and tautomerism in... 

The N-confusion concept can be generalized to heterocyclic rings other than pyrrole. The resulting systems, known as X-confused porphyrinoids (Fig. 30), structurally... 

The X-confused systems 100 and 101 exhibit borderline macrocyclic aromaticity, which results from the onium-type contributions 100 and 101, characterized by the presence of 18-electron aromatic circuits. For instance, the NH protons in 100 resonate at ca. 5.8 ppm, while those in the corresponding dication are observed at 5.3—4.4 ppm. These chemical shifts are intermediate between values expected of a nonaromatic porphyrinoid (S > 10 ppm) and those typical of aromatic systems (S < 0 ppm). The macrocyclic aromaticity is fully restored in the 3-substituted derivatives containing an sp3 carbon (104-105) or a carbonyl group (102-103), which is an additional reason for the enhanced reactivity of 100 and 101. In comparison, the pyrrolyl-substituted derivative 106 shows only moderate diatropicity, which is noticeably enhanced upon protonation [247], This effect is explained by cross-conjugation of the pyrrolyl substituent in the dication 107, which enables an 18-electron aromatic circuit in the macrocycle. 

Chapter 3 by Marcin Slcpien and Lechoslaw Latos-Grazynski deals with jt-elec-tron delocalization in relation to tautomerism and chemical properties in porphy-rines and porphyrinoids. An important and very interesting problem discussed here is that for the title systems aromaticity may be concerned either locally (for one or a few rings) or for a whole macrocycle. The body of the chapter is based on analyses of II NMR and I IWVis spectroscopies and takes into account the relations between n-clectron delocalization and tautomeric equilibria. 

Supramolecular Chirogenesis in Host-Guest Systems Containing Porphyrinoids... 

So far, several reviews have been published, some of which refer to supramolecular chirogenic porphyrinoid-containing systems to a greater or lesser extent. Therefore, before moving to specific examples of this area, we shall give an overview of some of these articles. 

---