Porphyrinic azomethine ylides

In 1995, Boyd and co-workers <95TL7971 > covalently linked a porphyrin to fullerene Cgo through a 1,3-dipolar cycloaddition reaction involving the porphyrinic azomethine ylide 28 (Scheme 8). The ylide was generated in situ from befa-formyl-meso-tetraphenylporphyrin 27 and A -methylglycine, and provided the porphyrin-C6o diad 29 in good yield. 

Scheme 8. 1,3-DC reaction of the porphyrinic azomethine ylide 28 with Cv,n.

More recently, porphyrinic azomethine ylide 28 (M = Ni) has been trapped with isatin to give a spiro porphyrin derivative 32 (Scheme 9) <06SC2655>. 

It has also been demonstrated that in the absence of dipolarophiles, porphyrinic azomethine ylide 28 can participate in 1,5-electrocyclization reactions, to yield pyrroloporphyrins, e.g. 33 (Scheme 10) <01JCS(PT1)2752>. 

In terms of synthetic methodologies for the preparation of porphyrinic azomethine ylides, the porphyrin moiety, in the examples above, was the carbonyl component. However, there are also examples where the porphyrin is used as the a-amino acid component. 

Scheme 9.1,3-DC reactions of porphyrinic azomethine ylide 28 with different...

Besides the use of porphyrins as azomethinic ylide derivatives, the porphyrin macrocycle can also be used to generate porphyrinic nitrile oxides 55 (Scheme 17) <04RCB(E)2192>. Thus, the treatment of oxime 54 with /V-bromosuccinimide in the presence of triethylamine, led to the formation of nitrile oxide 55, which was trapped in 1,3-DC reactions with dimethyl maleate and 2,5-norbomadiene to afford 56 and 57, respectively. In the reaction with 2,5-norbomadiene, if an excess of 55 was used, then the corresponding bis-adduct was obtained in good yield. 

Our first entry to the use of porphyrins as dipolarophiles in 1,3-DC reactions involved the reaction of porphyrins with azomethinic ylides, generated in situ from a-amino acids and aldehydes, to yield chlorins and isobacteriochlorins (bisadducts) <99CC1767, 05JOC2306>. 

In the particular scenario of the reaction of ie.vo-tctrakis(pcntafluorophenyl)porphyrin Id with the azomethine ylide, generated in situ from /V-mcthylglycine and paraformaldehyde, in refluxing toluene during 15 hours, the pyrrolidinochlorin derivative 62 was obtained as the main product (Scheme 19), together with a small amount of isobacteriochlorin 63 (bis-adduct, Figure 5). 

Scheme 19.1,3-DC Reactions of porphyrin Id with azomethine ylide.

Reaction of porphyrins with nitrones has also been studied and the results obtained showed that this is a versatile approach leading to the synthesis of isoxazolidine fused-chlorins (Scheme 26). For instance, chlorin 74 was successfully prepared from the reaction of the jV-methylnitrone, generated in situ from JV-methyl hydroxylamine and paraformaldehyde, with porphyrin Id . It is important to note that bis-addition also took place, yielding exclusively bacteriochlorin type derivatives 76 and 77 (Figure 6). This result contrasts with those obtained in 1,3-DC reactions with azomethinic ylides where isobacteriochlorins were obtained preferentially. 

In an attempt to explain the different behaviour of azomethinic ylides and N-methylnitrones in 1,3-DC reactions with porphyrins and chlorins, a theoretical study has been carried out. The results obtained showed that while in the cycloadditions of porphyrins and chlorins with azomethinic ylides the processes are irreversible and consequently are kinetically controlled, the cycloadditions of such macrocycles involving JV-methylnitrone are clearly reversible, showing that the products from such reactions should be thermodynamically controlled <07MI1>. 

Several organofullerene donor-acceptor molecular material hybrid systems have been synthesized via 1,3-dipolar cycloaddition reactions of azomethine ylides, via Bingel cyclopropanation and methanofullerene formation intermediates as well as via cycloaddition reactions, that have already been discussed in previous sections. The majority of such hybrid systems possess always as acceptor unit the fullerene core and as donor moieties porphyrins, tetrathiafulvalenes, ferrocenes, quinones, or electron-rich aromatic compounds that absorb visible light [190-193]. The most active research topic in this particularly technological field relies (i) on the arrangement of several redox-active building blocks in... 

Some representative examples of fullerene-porphyrin dyads are shown in Scheme 9. In other examples, porphyrin analogs such as phthalocyanines and subphthalocyanines have been used for the construction of efficient dyads. Again, the most straightforward approach for their synthesis involved 1,3-dipolar cycloaddition of the appropriate azomethine ylides to C60 [203-205]. Also, with the aid of the Bingel reaction, other phthalocyanine-fullerene systems have been prepared [206,207] with the most prominent example being the one that contains a flexible linker possessing an azacrown subunit [208]. The novelty of this dyad can be found in the nature of the linker that could, in principle, induce conformational changes in the multicomponent system when certain ions (e.g., alkaline ions) are present. As a direct consequence this would potentially allow an external control over the electronic interactions between the phthalocyanine and fullerene units. 

Cavaleiro et al. developed methodologies based on cycloaddition transformations of porphyrins leading to the synthesis of compounds like chlorins and bacteriochlorins which have strong absorptions in the visible region near or above 650 nm. The reactive diene and dipolarophile species have been o-quinodimethanes [159], azomethine ylides [160] and nitrones [161]. The 1,3-dipolar cycloaddition approach led to the synthesis of glycoderivatives of the chlorin and bacteriochlorin types (Scheme 45) [162]. 

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