Porphyrin, Chiral

Binding of organic nitroso compounds to metalloporphyrins 99ACR529. Design and applications of chiral porphyrins 98YGK201. 

Multifunctional and chiral porphyrins as model receptors for chiral recognition 98ACR81. 

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

Very few examples have been described for the non-covalent immobilization of chiral porphyrin complexes (Fig. 26). In the first case, the porphyrin-dichlororutheninm complex was encapsulated in silica, which was prepared around the complex by a sol-gel method [78], in an attempt to prevent deactivation observed in solution in the epoxidation of different alkenes with 2,6-dichloropyridine N-oxide. In fact, the heterogeneous catalyst is much more active, with TON up to 10 800 in the case of styrene compared to a maximum of 2190 in solution. Enantioselectivities were about the same imder both sets of conditions, with values aroimd 70% ee. 

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). 

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. 

There have also been significant advances in the imido chemistry of ruthenium and osmium. A variety of imido complexes in oxidation states +8 to +6 have been reported. Notably, osmium (VIII) imido complexes are active intermediates in osmium-catalyzed asymmetric aminohydroxyl-ations of alkenes. Ruthenium(VI) imido complexes with porphyrin ligands can effect stoichiometric and catalytic aziridination of alkenes. With chiral porphyrins, asymmetric aziridination of alkenes has also been achieved. Some of these imido species may also serve as models for biological processes. An imido species has been postulated as an intermediate in the nitrite reductase cycle. " ... 

Chiral porphyrins are also effective in the asymmetric epoxidation of alkenes. For example, a Cj-symmetiic iron porphyrin (29) <99JA460> catalyzes the efficient epoxidation of terminal alkenes, such as 30, with very good ee s and up to 550io turnovers. Similarly, trons-disubstituted... 

Callot and co-workers established in 1982 that iodorhodium(III) porphyrin complexes could be used as cyclopropanation catalysts with diazo esters and alkenes with c/.s-disubstituted alkenes these catalysts provide preferential production of cis(syn) disubsdtutcd cyclopropancs (syn/anti up to 3.3 with 1,4-cyclohexadiene) [72], More recently, chiral porphyrins have been designed and prepared by Kodadek and co-workers [73], and their iodorhodium(lll) complexes have been examined for asymmetric induction in catalytic cyclopropanation reactions [74,751. The intent here has been to affix chiral attachments onto the four porphyrin positions that are occupied in tetraphenylporphyrin by a phenyl group. Iodorhodium(III) catalysts with chiral binaphthyl (27, called chiral wall porphyrin [74]) and the structurally analogous chiral pyrenyl-naphthyl (28,... 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that mam group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pah. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the //-product were seen but the use of chiral porphyrins gave only low ees. 

Chiral porphyrins, prepared in different ways84,85 (chiral units attached to preformed porphyrins84, chiral substituents introduced during the synthesis of porphyrins86 or chiral porphyrins synthesized without the introduction of chiral groups69,87-90), proved to be effective as asymmetric epoxidation catalysts. 

The steric requirements of the tetramesityl porphyrin ligand are likewise determining the optimal geometry of the transition states in these reactions. Therefore, it was consequent to look for a chiral porphyrin ligand which could add enantioselectivity to the processes already described. Simonneaux et al. [370] have synthesized a set of chiral porphyrins derived from the four atrop-isomers of tetrakis(o-aminophenyl) porphyrin, H2(ToAPP), by acylation with (R)-( + )-a-methoxy-a-trifluormethylphenylacetyl chloride. This mixture of chiral porphyrins H2(P ) was metallated with Ru3(CO)i2 in o-dichlorobenzene and the resulting chiral isomers of RuCO(P )THF separated by thin layer... 

The representative examples of chirogenic assemblies on the basis of achiral monomeric porphyrinoids exposed to a chiral influence clearly show their great importance and wide applicability in various fields. The major chiroptical properties of these relatively simple supramolecular systems include transferring the chiral information and controlling the induced asymmetry by different factors. However, another type of chirogenic process— modulation of inherent chirality is going to be illustrated for chiral porphyrins in the next sections. 

Zhang s group developed highly active chiral (porphyrin)cobalt(II) complexes 327b-d, which catalyzed the cyclopropanation reactions of styrenes [356-358] and even of ot,(3-unsaturated esters or nitriles [358, 359] by diazoacetates. Nitrodi-azoacetates [360] or sulfonyldiazomethane [361] also proved to be useful in asymmetric cyclopropanation reactions of styrenes, acrylic derivatives, and in some cases even simple olefins with good to high de and moderate to excellent ee (highlight [362]). 

Intramolecular cyclopropanation of allyl diazoacetates gives rise to interesting cyclopropane-fused y-butyrolactones. A chiral ruthenium bis(oxa-zolinyl)pyridine complex 85 was employed for the catalytic cyclization of trans-cinnamyl diazoacetate 83 at room temperature to obtain an optically active lactone 84 in 93% yield with 86% ee (Eq. 34, Fig. 2) [85]. Chiral porphyrin and salen complexes of ruthenium 86 [86] and 87 [87] also catalyzed the asymmetric intramolecular cyclopropanation of 83 to afford 84 in similar yields and enantiomeric excess. 

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... 

Scheme 2 Picture of the pH-induced disassembly of the chiral porphyrin aggregate and of the two possible reassembling paths. Route a the chiral aggregate is completely disassembled after the pH-jump. Route b some undetectable chiral seed survive the pH-jump. Modified from [45]...

The non-covalent synthesis of the chiral porphyrin aggregate has been performed as discussed in the previous paragraph. Addition of an equimolar amount of H6TPyP4+ and CuTPPS (Figs. 14 and 1, respectively) to a solution at pH 2.3 (by HC1) of L-Phe or D-Phe leads to mirror images induced CD signal in the... 

Now the last and most important steps to validate experimentally our hypothesis can be performed erase and rewrite the chiral information using solutions of the amino acid-free chiral porphyrin assemblies. 

In order to demonstrate that the chiral porphyrin reassembly is due to the presence of the (spectroscopically silent) chiral seeds of the porphyrin aggregate, various solutions of the chiral aggregates were kept at pH 9.0 (that is in the disassembled state) for different time intervals before reassembling them (lowering the pH at 2.3) [49]. In fact, if the chiral reassembly is driven by the presence of inert chiral seeds, then (at pH 9.0) there must be a time interval after which the chiral seeds will disassemble. Then chirality would not be reversible anymore and the system will reassemble in a non-chiral fashion (see route a in Scheme 2). Indeed, after about 24 h at pH 9, the CD at pH 2.3 is not restored anymore because the chiral seeds also disassembled. 

Onouchi H, Miyagawa T, Morino K et al (2006) Assisted formation of chiral porphyrin homoaggregates by an induced helical poly(phenylacetylene) template and their chiral memory. Angew Chem Int Ed Engl 45 2381-2384... 

Bellacchio E, Lauceri R, Gurrieri S et al (1998) Template-imprinted chiral porphyrin aggregates. J Am Chem Soc 120 12353-12354... 

Matassa R, Carbone M, Lauceri R et al (2007) Supramolecular structure of extrinsically chiral porphyrin hetero-assemblies and achiral analogues. Adv Mater 19 3961-3967... 

Figure 1. Schematic view of chiral porphyrins designed to recognize alkyl residues of different sizes attached to a common oxidizable functional group.

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