Strapped complexes

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 mono-strapped complex 98, which is prepared by the ethyl benzoate displacement method (Section II. C), has planar chirality owing to the presence of different 1,3-substituents on the r 6-arene and can be separated into its enantiomers by chiral HPLC. Each enantiomer reacts with AgOTf/H20 to give the corresponding di-strapped aqua cation in which a second chiral centre is present because there are now three different groups in the tripodal set. However, each... 

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.

The constraints imposed by the additional trimethylene strap in the macrocyclic diamide complex of (623) causes the Ni11 ion to be pushed out of the N4 plane and results in a lower ligand field strength than in the Ni11 complex of the corresponding monocyclic dioxocyclam ligand.1 71... 

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. 

Complex (17) of Class 3 has no chiral auxiliary, but is endowed with facial chirality by the presence of a bridging strap (Figure 4).65 Treatment of (17) with oxidant generates metal oxo bonds, preferentially on the sterically less hindered (nonbridged) side of the complex, and epoxidation with (17) is low in enantioselectivity (Scheme 10). However, the enantioselectivity is considerably improved by the addition of imidazole. The imidazole has been considered to coordinate the metal center from the nonbridged side and to force the formation of metal oxo bonds on the bridged (chiral) side, thus enhancing enantioselectivity. 

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). 

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. 

These nebulas are similar in some respects to the Hll regions. The difference is that here the source of ionisation is an ageing star (white dwarf) in its death throes rather than a strapping young blue star. The fluorescent region is both denser and chemically more complex for it includes those atoms expelled from the envelope of the dying star in the form of a stellar wind. 

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,... 

A tridentate (r 6 ri1 r 1) strapped arene complex of ruthenium(II), 20, containing a pair of auxiliary nitrogen donor atoms derived from (R, R)-1,2-diphenylethylenedi-amine, has been prepared by a sequence in which the functionalized arene is first coordinated to ruthenium(II), as shown in Scheme 5.25 The structure of 20 has been... 

A large number of strapped arene complexes of ruthenium(ll) have been prepared by treatment of [RuCl2(il6-arene)]2 complexes with tertiary phosphines of the type R2P Ar in which an aromatic group is separated from the phosphorus atom by a two- or three-atom chain. In the first step, which usually occurs at or just above... 

A series of cationic, strapped, r)6 ri1 ri1-arene complexes of ruthenium(II) (82) containing both carbon and phosphorus as the auxiliary atoms has been obtained by a coupling reaction of the iminophosphorane complexes 83 with an excess of 1, l-diphenyl-2-propyn-l-ol, the p-cymene in 83 being displaced by one of the phenyl... 

In a reaction that similarly relies on the acidity of benzylic protons in arene-ruthenium(II) complexes, the r 6-mesitylene complex 92 containing bidentate (C6F5)2PCH2CH2P(C6F5)2 loses two molecules of HF on treatment with proton sponge to give the di-strapped salt 93 in which each tether contains three carbon atoms [Eq. (18)]).83... 

So far as we know, this approach has been used only once, although it should be capable of extension. The octahedrally coordinated rhodiacyclopentadiene complex 94, which results from the reaction of [RhCl(PPh3)3] with 2 mol of (2-phenyl-ethynyl)diphenylphosphine, reacts with diphenylacetylene to give the di-strapped arene-rhodium(I) complex 95, from which the p-terphenyl-based bis (diphenylphosphine) can be released by heating with NaCN [Eq. (19)].84... 

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