Metalloporphyrin olefin epoxidation

Sterically Hindered Metalloporphyrins Capable of Direct Aerobic Oxygenation. The catalytic aerobic olefin epoxidation system of Quinn and Groves, (tetramesitylporphyrinato)Ru/02/olefin substrate, effects equations 3-6, that is, the direct oxygenation of substrate using O2 as the oxidant without consumption of reducing agent 14), The (tetramesitylporphyrinato)Ru complex sterically... 

These findings prompted the development of chiral metalloporphyrin-catalyzed epoxidation of simple olefins. 

The olefin epoxidation reaction has also been extensively studied with model metalloporphyrin systems. The epoxidation of olefins by Fe" porphyrin/PhIO systems generally proceeds by a stereo-retention pathway (Table VIII) (10, I20a,c). Asymmetric epoxidations of prochiral olefins have also been demon-... 

De Poorter, B. and B. Meunier (1985). Metalloporphyrin-catalyzed epoxidation of terminal olefins with hypochlorite salts or potassium hydrogen persulfate. Perkin Trans. U,J. Chem. Soc. 1735-1740. 

Groves, J.T. and K.V. Shalyaev (1998). Paramagnetic H-NMR relaxation probes of stereoselectivity in metalloporphyrin catalyzed olefin epoxidation. Chirality 10, 106-119. 

Metalloporphyrin catalyzed olefin epoxidations using single oxygen donors... 

The main features of olefin epoxidation with metalloporphyrin catalysts and single oxygen donors are as follows ... 

The different mechanistic proposals for epoxidation reflect a complex oxidation pattern dependent on a variety of factors. At this time it does not seem either possible or necessary to accommodate all the observed features of metalloporphyrin catalyzed epoxidation within a single mechanism. The crucial importance of oxoiron(IV) porphyrin cation radicals has been established beyond doubt but the net 0-atom transfer to the olefin may perhaps occur via more than one kind of intermediate. 

Another kind of metalloporphyrin catalyzed olefin epoxidation utilizes aldehydes, which generate peroxy radicals and subsequently peroxy acids [95,98,258]. The latter are complexed by the metalloporphyrin and serve as sources of 0-atoms for epoxidation either as the peroxy acid complex or as the oxidized ir-cation radical (oxometal species) [96,97]. The catalysis of propene epoxidation by... 

Collman and his coworkers proposed a metallacycle intermediate in the olefin epoxidation by metalloporphyrin/hypochlorite systems due to saturation kinetics [206-209]. According to Nolte et al. these saturation kinetics were explained by a dimmer formation [210-212]. Bruice et al. also rather suggested involvement of comproportiona-tion process of higher valent 0x0 complex [213]. In addition, Bruice et al. prepared sterically hindered Fe (TDBPP)Cl, TDBPP tetrakis(2,6-dibromophenyl)porphyrin, and used as the catalyst for epoxidation of nine structurally different alkenes [214]. Fe (TDBPP) was found to be an extremely efficient catalyst for epoxidation and nearly quantitative yields of epoxide formation were obtained in all cases. A computer graphics docking study (Polygen programs Quanta and CHARMm) questioned possible intermediacy of metallacycle. A recent report by Collman indicated a direct reaction of OCl and olefins could also explain the saturation kinetics [215]. 

Epoxidation.1 This combination is known to oxidatively cleave double bonds but to effect epoxidation when catalyzed by a metalloporphyrin. Epoxidation of alkenes can also be effected by catalysis with a simple amine. The choice of the amine depends on the olefin. N,N-Dimethylethylenediamine is the most efficient ligand for epoxidation of a 1-alkene (68% yield). Pyridine is the best ligand for epoxidation of stilbene (93%), and imidazole is preferred for epoxidation of QH5CH=CHCH, (71% yield). 

A new trend in the field of oxidations catalyzed by metalloporphyrin complexes is the use of these biomimetic catalysts on various supports ion-exchange resins, silica, alumina, zeolites or clays. Efficient supported metalloporphyrin catalysts have been developed for the oxidation of peroxidase-substrates, the epoxidation of olefins or the hydroxylation of alkanes. 

Metallosalen complex [salen = N, A-ethylenebis(salicyldeneaminato)] has a structure similar to metalloporphyrin, and these two complexes catalyze the epoxidation of olefins. For example, Kochi et al. have found that metallosalen complexes such as (salen )manganese(III) [25] and (salen)chromium(IIl) complexes [26] (hereafter referred to as Mn- and Cr-salen complexes, respectively) serve as catalysts for the epoxidation of unfunctionalized olefins by using iodosylbenzene [25] or sodium hypochlorite [27], In particular, cationic Mn-salen complex is a good catalyst for epoxidation of unfunctionalized olefins, which proceeds through an oxo(salen)manganese(V) species (Scheme 6B.14) [25,28], The presence of oxo-Mn(V)-salen... 

Metalloporphyrins catalyze the autoxidation of olefins, and with cyclohexene at least, the reaction to ketone, alcohol, and epoxide products goes via a hydroperoxide intermediate (129,130). Porphyrins of Fe(II) and Co(II), the known 02 carriers, can be used, but those of Co(III) seem most effective and no induction periods are observed then (130). ESR data suggest an intermediate cation radical of cyclohexene formed via interaction of the olefin with the Co(III) porphyrin this then implies possible catalysis via olefin activation rather than 02 activation. A Mn(II) porphyrin has been shown to complex with tetracyanoethylene with charge transfer to the substrate (131), and we have shown that a Ru(II) porphyrin complexes with ethylene (8). Metalloporphyrins remain as attractive catalysts via such substrate activation, and epoxidation of squalene with no concomitant allylic oxidation has been noted and is thought to proceed via such a mechanism (130). Phthalocyanine complexes also have been used to catalyze autoxidation reactions (69). 

Rose E, Andrioletti B, Zrig S, Quelquejeu-Etheve, M. Enantioselective epoxidation of olefins with chiral metalloporphyrin catalysts. Chem. Soc. Rev. 2005 34 573-583. 

C. L. Hill, R. B. Brown Jr., Sustained epoxidation of olefins by oxygen donors catalyzed by transition metal-substituted polyoxometalates, oxidatively resistant inorganic analogs of metalloporphyrins, J. Am. Chem. Soc. 108 (1986) 536. 

The group of Masui first attempted the direct epoxidation of olefins by using oxygen and NHPI with metalloporphyrins, but they obtained poor results [15]. Ishii and coworkers proposed two different methods. In the first protocol [16,17], the epoxidizing agent is obtained in situ by the aerobic oxidation of a suitable alcoholic (benzhydrol) compound in the presence of catalytic amounts of NHPI. The resulting oxidant, which is not able to promote the epoxidation by itself, is then activated in the presence of an olefin by catalytic amounts of hexafluoroacetone (HFA) (Scheme 6.1). 

Enantioselective epoxidation of unfunctionalized olefins is an important area of asymmetric synthesis Metalloporphyrins represent... 

---