Meso-5- porphyrin formation

In 1989, the irradiation of (E,E)-2,4-hexadiene S3 sensitized by meso-porphyrin IX dimethyl ester led to the formation of cis-3,6-dimethyl-l,2-dioxene (62), which was the major product detected at — 78 °C in Freon 11 [69]. Endoperoxide 62 was purified under vacuum at 0.75 mmHg, and collected in a trap (98% isolated yield). Dienes that can adopt a cisoid conformation, such as 53 or ( , )-l,4-di phenyl butadiene, were photooxidized by the corresponding endoperoxides in high or quantitative yield in a suprafacial Diels-Alder reaction [60, 70], Dienes that cannot readily adopt cisoid conformations, such as (fc, Z)-2,4-hexadienes and (Z, Z)-2,4-hexadienes, lose their stereochemistry in the singlet oxygen [2 + 4]-cyclo-addition [60], (E,Z)- and (Z,Z)-dienes give a complex mixture of hydroperoxides and aldehydes, which suggests the intervention of intermediate zwitterions or 1,4-diradicals [71]. 

Ultimately, all porphyrin syntheses start from pyrroles (which themselves need to be made), so that a special section with this title might appear pointless. However, this section really deals with those porphyrins where the number of chemical manipulations that the pyrrole has to go through prior to porphyrin formation, is minimal the porphyrin is synthesised directly from the final pyrrole. This means that such synthetic routes are usually confined to those porphyrins with a symmetrical arrangement of peripheral substituents, e.g., 5,10,15,20-meso-tetrakis(aryl)porph)rrin (e.g. TPP) and 2,3,7,8,12,13,17,18-octa-substituted porphyrins (e.g. OEP). 

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

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. 

The zinc(II) complexes of meso-tetraphenyltetrabenzoporphyrin (38) and of meso-tetraphenyl-tetranaphthoporphyrin (39) have been prepared 173-175 some photophysical properties of this interesting series are given in Table 11176 Photooxidation of zinc(II) porphyrins causes cleavage of the macrocycle with formation of bilinone derivatives.122 206 207... 

A heta-substituted porphyrin, such as hefa-nitro-meso-tetraphenylporphyrin le, also reacts with o-benzoquinodimethane to give the corresponding nitrochlorin 2e, together with the naphthoporphyrin 4a and the bis-naphthoporphyrin (bisadduct) 7 the formation of these 4a and 7 derivatives imply HNO2 elimination and dehydrogenation reactions <06TL8437>. 

Figure 7.9 (A) Prevention of a-oxo dimer formation by distal-side porphyrin modifications. (Adapted with permission from the reference 19. Copyright 1985, Division of Chemical Edncation, Inc.) (B) Picket-fence porphyrin Fe(Tpjv)PP, meso-tetrakis(a, a, a, a-c>-pivalamidephenyl)porphyrin described in references 6,17,18, and 21.

Further work by Anson s group sought to find the effects that would cause the four-electron reaction to occur as the primary process. Studies with ruthenated complexes [[98], and references therein], (23), demonstrated that 7T back-bonding interactions are more important than intramolecular electron transfer in causing cobalt porphyrins to promote the four-electron process over the two-electron reaction. Ruthenated complexes result in the formation of water as the product of the primary catalytic process. Attempts to simulate this behavior without the use of transition-metal substituents (e.g. ruthenated moieties) to enhance the transfer of electron density from the meso position to the porphyrin ring [99] met with limited success. Also, the use of jO-hydroxy substituents produced small positive shifts in the potential at which catalysis occurs. 

The rhodium complex [CpRh(bipy)Cl2] is reported (162) to act as one-half of a redox couple that, in concert with a manganese porphyrin system, catalyzes the epoxidation of olefins by dioxygen. In this two-phase system, the aqueous phase contains sodium formate, and the organic phase is a trichloroethane solution of [Mnm(tpp)]1+ and the rhodium complex (tpp = meso-tetraphenylporphyrin). Apparently, the rhodium complex catalyzes the reduction of [Mnin(tpp)]1+ by formate, and the manganese(II) species thus formed binds dioxygen and reacts with the substrate olefin to form the epoxide. However, the intermedi-... 

For the synthesis of the porphyrin the formate ester of 3j3-hydroxy-5-cholenic acid 179 was coupled via amide bonds to the a,/ ,a,j -atropisomer of meso-tetrakis(o-aminophenyl)-porphyrin 170, using the mixed anhydride... 

The analogous reactions of Pd(OEP) or Pd(TTP) did not result in formation of Pd(IV) porphyrins, but in excessive chlorination of the meso [295] or peripheral (p-pyrrole) positions [296] of the porphyrin rings. The oxidation of Pt(P) in the absence of hydrogen chloride did not lead to isolatable products. Of course, an oxoplatinum(IV) porphyrin would have been a very interesting compound. 

Many examples are known in which multiple components are brought together about a metal ion to form macrocyclic complexes. Typical examples include the formation of meso-tetraphenylporphyrin (6.21) from benzaldehyde and pyrrole (Fig. 6-20, [4+4]), or phthalocyanine (6.6) from phthalonitrile (Fig. 6-21). The formation of the tetraphenyl-porphyrin is catalysed by a range of Lewis acids, and the facile preparation from aldehydes and pyrroles has obvious implications for the bioevolution of porphyrin pigments. Virtually any benzene derivative with ortho carbon-bearing substituents can be converted to a phthalocyanine complex on heating with a metal or metal salt in the presence of ammonia or some other nitrogen source. 

Scheme 2 Condensation of pyrrole with mixed aldehydes (ACHO BCHO/3 l). Formation of a mixture of meso-tetraary I porphyrins (A=B =Ar).

The metallation behavior of meso-tins ubstitu ted 21-thiaporphyrin 251 (Scheme 98) has been achieved using n-BuLi at 0°C. Subsequent oxidation with DDQ (1998ACI1107) resulted in the formation of meso-butyl porphyrin 252. 

Ng et al. first reported the axial ligation of zinc(II) l,8,15,22-tetrakis(3-pentyloxy) phthalocyanine (1) with meso-pyridyl porphyrins 2 and 3 in chloroform, which form the corresponding edge-to-face dyad and pentad, respectively [25], As shown by UV-Vis spectroscopy, the ground-state tt-tt interactions between the perpendicularly disposed macrocycles in these arrays are insignificant. Upon mixing of phthalocyanine 1, zinc(II) meso-tetra(/Molyl)porphyrin, and 4,4/-bipyridine in chloroform, the formation of a face-to-face hetero-dyad was also inferred by fluorescence quenching experiments. 

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