Porphyrin and salen complexes

Meta11ophtha11ocyan1nes, porphyrines and salen complexes encaged In mainly Y zeolites have been reported to be active and shape selective in the oxidation of alkanes and alkenes. 

Figure 6B.7. Basic structures of metallo-porphyrin and -salen complexes,...

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. 

C-H Hydroxylation Using Metallo-Porphyrin and -Salen Complexes as Catalysts its Mechanism and Stereochemistry... 

Dioxygen binding to cobalt complexes is well known and has been reviewed extensively [74]. The most widely studied systems are cobalt porphyrin and salen complexes [salen, N,N -bis(salicylidene)-l,2-ethylene-diaminato(2-)], with the latter dating back to the time of Werner [75]. The reactions of these systems most often involve Co(II) complexes and the formation of stable cobalt-(// -superoxo) complexes or species with Co(III)(/t-l,2-peroxo)Co(III) cores. Recent work has focused on more reactive cobalt-dioxygen species and will be highlighted here. 

At present, the best enantiomeric excess with different olefins is achieved by chiral salen ligands100 108. Contrary to the porphyrin systems, salen complexes (5) [i.e. N,N -ethylene-bis(salicylidene-aminate) ligands] bear tetravalent and potentially stereogenic carbon centers in the vicinity of the metal binding site. 

A characteristic feature is the formation of oxo and/or hydroxo bridges.42 Binu-clear systems may be (a) doubly bridged and (b) singly bridged with a linear or bent FeOFe. The bent ones are known with 4-, 5-, and 6-coordinate iron(III) an example is [(salen)Fe]2(/x-0). Porphyrin and phthalocyanine complexes are linear bridged species of the type (porph Fe)2/x-X where X = O, N, and C. 

It should be emphasized that tetradentate ligands, such as porphyrin and salen, are dominating in this chemistry, since they are nonoxidizable ligands and tend to form square-pyramidal complexes with bent nitrosyls at axial positions that facilitate the oxidation by molecular oxygen to form the desired nitro ligands. 

Apart from porphyrin and salen catalysts, manganese complexes of N-alkylated 1,4,7-triazacyclononane (e.g., TMTACN, 6) have been found to catalyze the epoxida-... 

It has been noted (2) that many oxygen-carrying cobalt(II) complexes involve ligands that provide four donor atoms lying in the same plane as the cobalt atoms, such as salen , porphyrin and dimethylglyoxime, DMG, and related ligands. 

Extensive work has been carried out on microsensors built from electropolymerized nickel porphyrin films.328,329 Films of Prussian blue (Fe4[Fe(CN)6]3) 345 metal-salen complexes (M = Co, Fe, Cu, Mn)346 or the ferrocene-containing Nin-tetraaza[14] annulene (24),347 also exhibit interesting activity for NO electrooxidation and sensing. 

The asymmetric epoxidation of /i-alkenes and terminal alkenes proved to be more difficult, though a recent finding, describing the use of a modified salen complex to epoxidize ( )-0-methylstyrene to form the corresponding epoxide in 83% ee, represents another important step forward. Alternatively, chiral (D2-symmetric) porphyrins have been used, in conjunction with ruthenium or iron, for efficient asymmetric oxidation of trans- and terminal alkenes[92]. 

Table 102 Some Synthetic Details and Properties of Porphyrin and Crv-Salen Complexes...

More examples are found for varied oxidation processes mainly for various epoxidations carried out by metal catalysts bearing F-modified ligands, such as porphyrins,139 Ru perfluoroacetylacetonate salt,140 or salen complexes,141 142 or using the 3 selenium compound as catalyst.143 The potential for enantioselective transformations offering an easy way to recover precious chiral reagents and catalysts was demonstrated in enantioselective epoxidation using fluorous chiral salen... 

Chromium complexes (continued) organohalide activation, 5, 381 with phosphine-based supporting ligands, 5, 365 porphyrin-ligated derivatives, 5, 364 pyridinebis(imine) ligand derivatives, 5, 361 salen-ligated complexes, 5, 379 surface chemistry on oxides, 12, 525 tetradentate [04] compounds, 5, 352 tetradentate thioether derivatives, 5, 365 thiolate-bridged, Fe- and Rh complexes, 5, 308 with Ti(IV), 4, 627... 

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