Meso-Aryl porphyrins

The structure of N-confused porphyrin (41 in Figure 9.11) was first proposed by Aronoff and Calvin in 1943 when they reexamined Rothemund s synthesis of tetraphenylporphyrin [43]. Because of lack of supporting experimental studies, the concept was under hibernation until the 5,10,15,20-tetraphenyl-substituted 41 was isolated as the byproduct in the Lindsey synthesis of meso-aryl porphyrins, 50 years later [44]. Currently, many more N-confused and X-confused porphyrins have been synthesized and characterized [37,45]. There are indeed innumerable analogs, in addition to the common derivatives of porphyrin, that constitute a huge molecular library. Every individual porphyrinoid has its own characteristic properties. Their aromaticity can be controlled by means of ingenious structural modifications. This encourages researchers to synthesize novel porphyri-noids with tuned properties as required for the destined applications. 

The Lewis acid-Lewis base interaction outlined in Scheme 43 also explains the formation of alkylrhodium complexes 414 from iodorhodium(III) meso-tetraphenyl-porphyrin 409 and various diazo compounds (Scheme 42)398), It seems reasonable to assume that intermediates 418 or 419 (corresponding to 415 and 417 in Scheme 43) are trapped by an added nucleophile in the reaction with ethyl diazoacetate, and that similar intermediates, by proton loss, give rise to vinylrhodium complexes from ethyl 2-diazopropionate or dimethyl diazosuccinate. As the rhodium porphyrin 409 is also an efficient catalyst for cyclopropanation of olefins with ethyl diazoacetate 87,1°°), stj bene formation from aryl diazomethanes 358 and carbene insertion into aliphatic C—H bonds 287, intermediates 418 or 419 are likely to be part of the mechanistic scheme of these reactions, too. 

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 ... 

Oxidation of the porphyrin 7r-system of iron(IH) (45) and iron(IV) tet-raarylporphyrins (8) leads to large chemical shifts for the meso-aryl protons. This is a result of spin density on the raeso-carbon, which is delocalized into the aryl rings via a 7r-spin delocalization mechanism (50). Accordingly, the chemical shifts of ortho-, meta-, and para-protons have alternating signs. Coupling of the porphyrin spin to the metal spin in both a ferro- and antiferromagnetic manner has been demonstrated (45). 

The large paramagnetic contribution to the chemical shifts for the meso-aryl protons is consistent with the presence of a porphyrin 7r-radical species (45). The shift pattern for these signals (o-H upfield, m-H and p-CH3 downfield) is similar to that observed for (TPP )Fein(Cl04)2 (o-H, p-H upfield, m-H downfield), which exhibits ferromagnetic coupling between the porphyrin 7r-radical spin and the metal spin (45). The small... 

Atropisomers arising from hindered rotation of meso-aryl have been used in studies of metalloporphyrins. Barriers for a series of para and meta aryls in titanyl porphyrins and... 

A few other triads and more complex systems eonsisting of a porphyrin linked to two or more fullerene moieties have been reported [150, 159, 221]. The most complex to date is a tetraarylporphyrin linked to four fullerene moieties through double ester-containing bridges to the porphyrin meso aryl groups [151]. This molecule is rendered soluble in organie solvents via 16 twelve-carbon aliphatic chains. The porphyrin fluorescence is strongly quenehed by the attached Ceo moieties. 

In carotenoporphyrin 24 an ester group with a methylene spacer serves to link the two chromophores together. Note that this linkage is from the para position of the meso aryl ring of the porphyrin to the carotenoid. The absorption spectrum of 24 is the sum of those of the individual components... 

As expected, carotenoporphyrin species in which the carotenoid is linked to the porphyrin meso aryl ring by an amide linkage in which there is no methylene group, such as 3 and most of the other compounds discussed in the section on electron transfer, exhibit extremely fast triplet-triplet energy transfer. The rise time for the carotenoid triplet is < 4 ns, which was the time response of the instrument employed for the measurement [29b],... 

Fig. 3.24 Conformational changes in the porphyrin macrocyclic skeleton (for meso-substituted porphyrins only) on dication formation. The pyrrole subunits hinge about carbons 1, 4, 6, 9,11,14,16, and 19 (two up and two down) while the meso-aryl imits rotate more into the mean macrocyclic plane. [From E. Meyer and D.L. Cullen, in The Porphyrins, ed. D. Dolphin, Vol. 3, Academic Press, New York (1978), pp. 513-529.]...

According to the definition of multiporphyrin assembly given in the Introduction (Sect. 1), a dimeric hetero-assembly can be formed by A—A and B—B monomers only if both contain porphyrin subunits. An example is given by the dimer 24 (meso-aryl groups omitted) whose stability constant, iCsa = 3x10 mol 1, was determined in CH2CI2 by a fluorescence titration... 

Figure 56. a, Metalloporphyrins 99 have meso aryl substituents bearing crown ether subunits b. Addition of 4 equiv of produces the porphyrin dimers 100 c. Schematic view of the dimer the squares are the porphyrin macrocycles and the arrows represent the coordinating units. M is the bridging metal. 

George, R.G. and M.P.D.H. Padmanabhan (2003). Studies on iron(III) and man-ganese(III) derivatives of new meso-aryl substituted and brominated porphyrins. Transit Met. Chem. 28, 858-863. 

Boron and phosphorus complexes of meso-aryl expanded porphyrins 13H(87)31. 

Scheme 16 Indirect method for the isolation of dimethylamine adducts to fluorinated porphyrin meso aryl groups...

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