Chiral macrocycles porphyrins

Given the observation of catalysis of alkene epoxidation by iron N-alkyl porphyrins, it is likely that these complexes may yield synthetically useful catalysts (32, 65). The possibility of chiral induction by using either a chiral N-alkyl group (66) or a chiral macrocycle such as N-Me etioporphyrin (67) is an area that should prove fruitful in the near future. 

The following discussion is intended to provide an introduction to the latest innovations in the design and synthesis of chiral carbon-rich macrocycles while providing some historical context of their development and applications. The systems under review are limited primarily to chiral macrocycles that are conjugated or that possess highly conjugated substructures. Excluded from the discussion are, for example, porphyrins [2, 3], cyclodextrins [4] and calixarenes [5], either because they do not fit within the context of our discussion or because they have been extensively reviewed elsewhere. 

In addition to these pyridine-containing helical macrocycles, there are numerous reports of polyaza-macrocyclic species that are chiral due to the adaptation of a non-planar conformation. Some current examples included the ruffled benzimidazole-based ligands and the distorted phthalocyanines recently prepared by the groups of Chan [40] and Kobayshi [41], respectively. These systems do not fall within the context of this Chapter due to their resemblance to porphyrinic systems but they nevertheless represent an interesting class of chiral macrocycles based upon the incorporation of heterocyclic rings. 

Woo et al. [54] prepared new chiral tetraaza macrocyclic hgands (48 in Scheme 23) and their corresponding iron(II) complexes and tested them, as well as chiral iron(II) porphyrin complexes such as Fe (D4 -TpAP) 49, in asymmetric cyclopropanation of styrene. 

In 12, the donor and acceptor moieties are chiral, and, as noted by the authors, the molecule therefore exists as a pair of diastereomers which are separable by chromatography. However, rotation about the linkages between the porphyrin macrocycle and the attached aryl rings must be very slow on the time scale of electron transfer. Thus, non-interconverting diastereomers should be present, with slightly different separations and orientations between the aniline donor and the quinone acceptor. This distribution would be expected to influence the decay kinetics of D+-P-QT if charge recombination is via direct electron transfer. If a... 

Naruta et al. [225, 226] designed the twin-coronet porphyrin ligands (62) and (63) with binaphthyl derivatives as chiral substituents (Figure 13). Each face of the macrocycle is occupied by two binaphthyl units and the ligand has C2 symmetry. Iron complexes of these compounds can be very effective catalysts in the epoxidation of electron-deficient alkenes. Thus, nitro-substituted styrenes are readily epoxidized in 76-96% ee [226]. The degree of enantioselectivity can be explained on the basis of electronic interactions between the substrate aromatic ring and the chiral substituents rather than on the basis of steric interactions. 

Chiral iron catalysts form a relatively new type of catalysts for asymmetric cyclopropanation. The first reported catalysts derived from chiral tetraaza macrocycles gave moderate enantioselectivities (up to 79% ee) but good diastere-oselectivities t/c — 13.3 1) (95). With the use of a chiral porphyrin catalyst (23b when M = Fe, X = Cl in Scheme 15), improvements in the enantioselective cyclopropanation from styrene and EDA was observed, with high turnover of more than 1200, 86% ee and high trans-selectivity t/c of 23 1) (96). Other types of iron catalysts derived from chiral terpyridines were also found to catalyze asymmetric cyclopropanation. After reacting with AgOTf, [Fe(14)Cl2] (14, R = re-Bu in Scheme 11) gave 67% ee of the same cyclopropane (97). 

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