Strapped porphyrins

The incorporation of potential ligands into the porphyrin strap has three advantages. 

Thus the synthesis of a porphyrin strapped by an isophthalamide moeity imder conditions deliberately chosen to favour H-bonding between two of the strap units resulted in good yield of the corresponding his-porphyrin [2]cate-nane 53 as well as the expected singly-strapped derivative 54. 

High-resolution FTIR spectra have been reported for jet-cooled FeCO and Fe(CO)2, formed by photolysis of Fe(CO)5. For Fe(CO)2, V3 was found to be centred at 1928.18408(13) cm h Values of vCO for m. yo-5,15-diphenyl-porphyrins strapped by 2,2 -disubstituted biphenyl units, [P]Fe(CO)L, where L = py, imidazole or 7V-methylimidazole, decrease with increasing L binding strength, from py to ImH to A-MelmH. There was some evidence for the formation of regioisomers. ... 

Figure 18.13 Chemical structures of selected cofacial strapped diporphyrins (a), pillared diporphyrins (h), and pillared porphyrin/corrole, dicorrole, and diphthalocyanine derivatives (c) whose metal complexes have heen studied as ORR catalysts. Conventional notations for the structures are also hsted (in bold). Other molecular architectures of cofacial porphyrins are known, hut the corresponding complexes have not yet been studied as ORR catalysts.

In the complexes of Class 1, chiral auxiliaries are attached outside the porphyrin ring and distant from the metal center. To avoid this problem, a new class of complexes ((15) and (16)) bridged by chiral straps above and below the metal center has been synthesized (Figure 3).63,64 Limited success, in terms of enantioselectivity (up to 72% ee), has been achieved with these complexes. 

On the basis of the above result, the class 4 of chiral porphyrin complex (18) possessing a chiral strap, and facial chirality caused by it, has been introduced.66,67 Epoxidation with the complex (18) in the presence of 1,5-dicycohexylimidazole, which blocks the nonbridged side of the complex, shows good to high enantioselectivity when the substrates are conjugated mono- and m-di-substituted olefins (Scheme 11). 

Figure 9 A few schematic examples of mononuclear Fe(II) -porphyrin derivatives used to prevent the irreversible oxidation to oxygen-bridged binuclear Fe(III) derivatives. (a) Picket-fence complexes (b) strapped complexes (c) capped complexes (d) picnic-basket complexes (e) pocket complexes...

The molecular structure of a Co(II) macrocycle complex that in some way mimics the strapped-type metal-porphyrin complexes is illustrated in Figure 13.18 19... 

Chiral porphyrin metal complex catalysts have also received much attention. In this situation, the flat, symmetrical porphyrin structure must be modified dramatically in order to incorporate dissymmetry. This has been achieved through strapping techniques. " Some examples are shown in Figure 11.7. 

Dramatic shape selectivities in competitive olefin epoxidation was observed with picnic basket metalloporphyrins312 313 designed to exclude bulky axial ligands on one sterically protected porphyrin face. When oxidized with PhIO in acetonitrile in the presence of the rigid p-xylyl-strapped porphyrin, cis-2-octene reacted selectively versus ds-cyclooctene or 2-methyl-2-pentene, giving >1000 reactivity ratios.313,314 Some immobilized manganese(III) porphyrins proved to be as efficient as their homogeneous equivalents in epoxidation with PhIO.151,315... 

Cyclophanes are known to be efficient receptors for aromatic compounds in protic solvents. Thus, linking a cyclophane unit to a porphyrin, like in 193, provides an excellent way to study the oxidation of aromatic hydrocarbons [117]. The synthesis of 193 took advantage of an earlier protocol for the preparation of strapped porphyrins [118] using the bis-dipyrromethane 194 already linked to the cyclophane as a valuable precursor for an acid catalyzed condensation leading to the porphyrin in 9% yield (Fig. 32). 

The asymmetric epoxidation of unfunctionlized olefins has been investigated with a new family of Mn(III)porphyrins which also involve the concept of strapping the porphyrin on one face and, moreover, shielding the opposite... 

In the wake of this report, many chiral iron(III)- and Mn(III)-porphyrin complexes have been synthesized and applied to the epoxidation of styrene derivatives [20]. Because these asymmetric epoxidations are discussed in the first edition of this book [21], the discussion on metalloporphyrin-catalyzed epoxidation here is limited to some recent examples. Most chiral metallopor-phyrins bear chiral auxi Maries such as the one derived from a-amino acid or binapthol. Differing from these complexes is complex 6, which has no chiral auxiliary but is endowed with facial chirality by introducing a strap and has been reported by Inoue et al. [20f]. Epoxidation of styrene by using only 6 as the catalyst shows low enantioselectivity, but the selectivity is remarkably enhanced when the reaction is performed in the presence of imidazole (Scheme 6B.11). This result can be explained by assuming that imidazole coordinates to the unhindered face of the complex and the reaction occur on the strapped face [20f. ... 

Collman et al. have recendy reported that threitol-strapped manganese(IH)-porphyrin complex 7 shows high enantioselectivity in the epoxidation of a wide range of olefins when the reaction is carried out in the presence of 1,5-dicyclohexylimidazole (Scheme 6B. 12) [22], The substituted imidazole in this reaction appears to play the same role as the imidazole in Inoue s reaction. Detailed understanding of the mechanism of asymmetric induction by 7 needs further investigation, the major pathway should accommodate the steric interaction between the olefinic substituent and the inner oxygen atom of the threitol strap (Figure 6B.6). Thus, the pathway a is likely to be the major pathway,... 

The more sophisticated pocket and picnic basket porphyrins are also capable of mimicking properties such as 02/C0 binding selectivity in which the (normally high) CO affinity is reduced because of steric interactions between the CO molecule (which adopts a linear Fe C=0 conformation) and the capping or strapping moiety. Selectivity for small molecules such as 02 is also enhanced, relative to larger bases. 

Li s porphyrin rosettes (18) [22], Yagai s photoswitchable rosettes (19) [23], and Fenniri s nanotubes (20) [24] nicely illustrate the former approach. Thus, CA3M3 rosettes (Figure 3.7) endowed with a pyridine moiety at the melamine component and a strapped Zn-porphyrin (to enhance coordination from only one side of the ring) at the cyanuric acid component are more stable than the CA3 M3 model rosette initially reported by Whitesides. Even more complex versions of the same concept, involving one all-pyridine rosette and one all-porphyrin rosette to self-assemble into double rosettes were studied, although only NMR evidence was presented [22],... 

Other metal-centered catalysts that have been studied include (te)strapped chiral porphyrins derived from L-proline, which can induce modest (< 30%) enantioselectivity <02EJIOC1666>. Supported amidate-bridged platinum blue complexes, which have not yet been applied to chiral epoxidation, but which show promise of utilizing molecular oxygen as the terminal oxidant, have... 

To limit the formation of 6-coordinate iron(II) porphyrins some kind of steric hinderance is deliberately introduced, such as in the picket fences approach77 (17-E-IX) or the strapped (17-E-X) and roofed models (17-E-XI). 

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