Porphyrin metal complex catalysts, alkene

Supercritical CO2 is a non-polar, aprotic solvent and promotes radical mechanisms in oxidation reactions, similar to liquid-phase oxidation. Thus, wall effects might occur as known, e.g. from olefin epoxidation with 02 or H202 which may decrease epoxide selectivities. The literature covers the synthesis of fine chemicals by oxidation either without catalysts (alkene epoxidation, cycloalkane oxidation, " Baeyer-Villiger oxidation of aldehydes and ketones to esters ), or with homogeneous metal complex catalysts (epoxidation with porphyrins, salenes or carbonyls ). Also, the homogeneously catalysed oxidation of typical bulk chemicals like cyclohexane (with acetaldehyde as the sacrificial agent ), toluene (with O2, Co +/NaBr ) or the Wacker oxidation of 1-octene or styrene has been demonstrated. 

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. 

Reactions of rhodium porphyrins with diazo esters - According to Callot et al., iodorhodium(III) porphyrins are efficient catalysts for the cyclopropanation of alkenes by diazo esters [320,321], The transfer of ethoxycarbonylcarbene to a variety of olefins was found to proceed with a large syn-selectivity as compared with other catalysts. In their study to further develop this reaction to a shape-selective and asymmetric process [322], Kodadek et al. [323] have delineated the reaction sequences (29, 30) and identified as the active catalyst the iodoalkyl-rhodium(III) complex resulting from attack of a metal carbene moiety Rh(CHCOOEt) by iodide. 

Simple alkenes do not normally react with IOB, unless there is catalysis by metal porphyrins or related metal complexes, in which case epoxidation occurs [1,2]. A great deal of work has been done in this field, especially with the relatively simple catalyst Fe(TPP)Cl (TPP is tetraphenylporphyrin) in some instances this approach can be used advantageously in comparison with other well-known methods of epoxidation. The Fe(TPP)Cl catalysed IOB epoxidation of alkenes is stereospecific, with cis substrates being considerably more reactive than trcrns. Several alkenes underwent efficient epoxidation with this system, e.g. cyclooctene (84% of epoxide), norbomene (67% exo-epoxide, accompanied by 3% of the enc/o-isomer) and... 

N-substituted iron porphyrins form upon treatment of heme enzymes with many xenobiotics. The formation of these modified hemes is directly related to the mechanism of their enzymatic reactivity. N-alkyl porphyrins may be formed from organometallic iron porphyrin complexes, PFe-R (a-alkyl, o-aryl) or PFe = CR2 (carbene). They are also formed via a branching in the reaction path used in the epoxidation of alkenes. Biomimetic N-alkyl porphyrins are competent catalysts for the epoxidation of olefins, and it has been shown that iron N-alkylporphyrins can form highly oxidized species such as an iron(IV) ferryl, (N-R P)Fe v=0, and porphyrin ir-radicals at the iron(III) or iron(IV) level of metal oxidation. The N-alkylation reaction has been used as a low resolution probe of heme protein active site structure. Modified porphyrins may be used as synthetic catalysts and as models for nonheme and noniron metalloenzymes. 

Similarly, ra 5-cyclopropanes were obtained from alkenes, such as styrene and 2,5-dimethyl-hexa-2,4-diene, with relative yields > 90% when a diazoacetate bearing a bulky ester group was decomposed by a copper catalyst with bulky salicylaldimato ligands. Several metal complexes with bulky Cj-symmetrlc chiral chelating ligands are also suitable for this purpose, e.g. (metal/ligand type) copper/bis(4,5-dihydro-l,3-oxazol-2-yl)methane copper/ethyl-enediamine ruthenium(II)/l,6-bis(4,5-dihydro-l, 3-oxazol-2-yl)pyridine cobalt(III)/ salen. The same catalysts are also suited for enantioselective reactions vide infra). For the anti selectivity obtained with an osmium-porphyrin complex, see Section 1.2.1.2.4.2.6.3.1. 

The more widely accepted mechanism for oxo-transfer involves direct substrate attack at the oxo ligand with concerted or sequential C-0 bond formation. In 1985, Groves proposed a transition state geometry for epoxidation by porphyrin complexes involving a side-on, perpendicular approach of the olefin to the metal-oxygen bond [17]. This trajectory accounted for the enhanced reactivity of cis- over frans-alkenes (4a vs. 4b) in porphyrin and other metal oxo catalyst systems (Fig. 2). This model has also helped explain the observed enantioselec-tivities in AE reactions with successful chiral catalysts [18,19]. 

Metal complexes of fluorous tetraarylporphyrins (1-5) have been used as catalysts in the epoxidation of alkenes under FB [9] or more traditional conditions [10], depending on their affinity for perfluorocarbons. Free base porphyrins 1-5 were readily metaUated with transition metal cations under standard conditions normally employed for their nonffuorous coimterparts. In particular, porphyrins 1-4 were metalated with Mn(OAc)2 4 H2O in boiling DMF to give their respective Mn(III) complexes Mn-l-Mn-4 [10], whereas the perffuorocarbon-soluble porphyrin 5 was similarly converted into the cobalt(II) complex Co-5 by treatment with Co(OAc)2 -4H20 [9],... 

Due to their importance for research but also for industrial chemistry, transition metal based catalysts are intensively investigated. Ananikov et al. [684] reviewed various appUcatimis of hybrid ONIOM methods within this field. This review involves reaction mechanisms and enantioselective reactions of transition metal complexes, e.g. Ti-catalyzed cyanation of benzaldehyde [685], Cu-catalyzed cyclopropanation [686], Mn-porphyrin catalyzed epoxidation of alkene [687], and Mo-catalyzed nitrogen activation [688]. These approaches involve QM/QM as well as QM/MM approaches. 

In addition to copper and rhodium catalysts commonly used in the generation of metal carbene complexes, other transition metals have also been explored in the diazo decomposition and subsequent ylide generation.Che and co-workers have recently studied ruthenium porphyrin-catalyzed diazo decomposition and demonstrated a three-component coupling reaction of a-diazo ester with a series of iV-benzylidene imines and alkenes to form functionalized pyrrolidines in excellent diastereoselectivities (Scheme 20). ... 

The catalytic cycle proposed for the rhodium-porphyrin-based catalyst is shown in Fig. 7.18. In the presence of alkene the rhodium-porphyrin precatalyst is converted to 7.69. Formations of 7.70 and 7.71 are inferred on the basis of NMR and other spectroscopic data. Reaction of alkene with 7.71 gives the cyclopropanated product and regenerates 7.69. As in metathesis reactions, the last step probably involves a metallocyclobutane intermediate that collapses to give the cyclopropane ring and free rhodium-porphyrin complex. This is assumed to be the case for all metal-catalyzed diazo compound-based cyclo-propanation reactions. 

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