Epoxidation catalytic system

Djordjevic et al.464 have described the synthesis and characterization of amino acid complexes MoO(02)2Leq(OH2) (192) and the X-ray structures of the Gly, Ala, and Pro derivatives. Chiral ligands such as (R)/(S)-(194), (R)/(S)-(195), and (RR,S)/(R,SR)-(196) form Mimoun complexes, MoO(02)2L and MoO(02)2L(OH2). IR and 31P NMR data, as well as the X-ray structure of pentagonal bipyramidal MoO(02)2(R,R,.S -196)(OH2), indicate the presence of equatorial phos-phoryl donors.465,466 Enantioselectivity in the stoichiometric epoxidation of pro-chiral olefins was marginal (<10%) except in the case of the binaphthyl derivatives (L - 195) where e.e. s of up to 39% were recorded.4 Related structures are observed for phosphine oxide458,467 and chelate complexes such as MoO(02)2 OE(iPr)2CH2CH2OMe (197, E = P, As) 458 468 An efficient bipha-sic catalytic epoxidation system based on MoO(02)2(OPR3) (R = -dodecyl) has been developed and the activity of related complexes assessed.460 Earlier attempts to produce aqueous oxidants included the synthesis of water-soluble bpy derivatives.469,470... 

This highly active epoxidation system, based on the controlled hydrolysis of BTSP with a catalytic amount of water, maximizes the formation of the Re monoperoxide complex at the expense of the more thermodynamically stable bis (peroxide) (Scheme 12.8). BTSP is very stable and can be prepared in molar amounts... 

The catalytic epoxidation of ethylene on silver has been studied extensively over the last thirty years. The literature in this area is very broad and has been reviewed by several authors (2>2 3). In recent years considerable progress has been made towards a satisfactory understanding of the mechanism of this important and complex catalytic system. 

Mimoun and coworkersdescribed the first well-defined example of a d° metal aUtylperoxidic species 49 which epoxidized simple olefins with high selectivity. Several features of the epoxidation performed by 49 resemble those of the Halcon catalytic epoxidation process " . Novel tungsten complexes containing 2 -pyridyl alcoholate ligands like 50 have been synthesized and tested as catalysts in the epoxidation of cw-cyclooctene with TBHP in the absence of solvent . The system displayed modest catalytic activity (100% conversion in 60 h) but excellent product selectivity. 

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... 

Warnmark et al. [12] have reported the formation of a dynamic supramolecular catalytic system involving a hydrogen bonding complex between a Mn(ll I) salen and a Zn(II) porphyrin (Figure 1.4). The salen sub-unit acts as the catalytic center for the catalytic epoxidation of olefins while the Zn-porphyrin component performs as the binding site. The system exhibits low selectivity for pyridine-appended styrene derivatives over phenyl-appended derivatives in a catalytic epoxidation reaction. The... 

Jonsson, S., Odille, F.G.J., Norrby, P.-O. and Wammark. K. (2005) A dynamic supramolecular system exhibiting substrate selectivity in the catalytic epoxidation of olefins. Chem. Commun., 549-551,... 

Jonsson, S., Odille Fabrice, G.J., Norrby, P.-O. and Warnmark, K. (2006) Modulation of the reactivity, stability and substrate- and enantioselectivity of an epoxidation catalyst by noncovalent dynamic attachment of a receptor functionality - aspects on the mechanism of the Jacobsen-Katsuki epoxidation applied to a supramolecular system. Org. Biomol. Chem., 4, 1927-1948 Jonsson, S., Odille Fabrice, G.J., Norrby, P.-O. and Warnmark, K. (2005) A dynamic supramolecular system exhibiting substrate selectivity in the catalytic epoxidation of olefins. Chem. Commun., 549-551. 

Scheme 6. Epoxidation of 1-alkenes with 30% H2O2 by addition of aminomethylphosphonic acid to the catalytic oxidation system shown in Scheme 5 [25],...

Denmark has developed a practical dioxirane-mediated protocol for the catalytic epoxidation of alkenes, which uses Oxone as a terminal oxidant. The olefins studied were epoxidized in 83-96% yield. Of the many reaction parameters examined in this biphasic system, the most influential were found to be the reaction pH, the lipophilicity of the phase-transfer catalyst, and the counterion present. In general, optimal conditions feature 10 mol% of the catalyst l-dodecyl-l-methyl-4-oxopiperidinium triflate (30) and a pH 7.5-8.0 aqueous-methylene chloride biphasic solvent system [95JOC1391]. 

Closely related to the ketone/Oxone epoxidation system is the use of iminium salts as promoters. As isolated oxaziridinium salts are known to effect alkene epoxidation [38], these are presumed to be the reactive intermediates in this catalytic system (see Scheme 12.1 X = NR.2+). The first asymmetric example used the dihydroisoquinolinium-based system 15 (Fig. 12.7), which afforded 33% ee for the epoxidation of F-stilbene [39]. 

Neumann and Miller (360) reported catalytic epoxidations with analogous P-W materials in a triphasic mode. The activity in the solvent-free system is influenced by the length of the hydrocarbon spacer between the silica and the ammonium group. Cyclooctene, for example, is epoxidized with only 10% conversion when a trimethyl propyl ammonium salt is used, whereas a conversion of 45% can be obtained in the presence of an immobilized octyldimethyl benzyl ammonium salt. The enhanced conversion is probably the result of a nearly ideal hydrophilic-lipophilic balance at the active site. 

Unlike epoxides, these five-membered heterocyclics have received scant attention from organic chemists. But the recent catalytic asymmetric dihydroxylation of alkenes (14, 237-239), which is now widely applicable (this volume), and the ready access to optically active natural 1,2-diols has led to study of these compounds, including a convenient method for synthesis. They are now generally available by reaction of a 1,2-diol with thionyl chloride to form a cyclic sulfite of a 1,2-diol, which is then oxidized in the same flask by the Sharpless catalytic Ru04 system, as shown in equation I.1... 

The study of mixed-ligand 0x0 derivatives is closely related to the use of these species in catalytic oxidation systems (including the epoxidation of aUcenes and the oxidation of alkanes. See Oxidation Catalysis by Transition Metal Complexes). In such complexes, the 0x0 group can be terminal, doubly bridging, or triply bridging. 

Then, the catalytic action is performed under homogeneous conditions and, at the end of the reaction, H2O2 being completely consumed, the precatalyst precipitates and can be easily filtered off and recovered. Both conversions and selectivities of this method are very good. Finally, as in the case of TS-1, this epoxidation system was combined with the 2-ethylanthra-quinone (EAQ)/2-ethylanthrahydroquinone (EAHQ) process for hydrogen peroxide formation, and good conversion and selectivity were obtained for propylene oxide in three consecutive cycles. The catalyst was recovered and reused in between every cycle (Scheme 5) ... 

Recently, 3,5-bis(trifluoromethyl)benzeneseleninic acid has been used in a tandem catalytic epoxidation." The concept of tandem catalysis has been applied to oxidation reactions by Backvall and co-workers for the direct dihydroxylation of olefins using a couple catalytic system and hydrogen peroxide as the terminal oxidant." In this context, the seleninic acid was used in combination with a trifluoromethyl oxaziridine catalyst (Scheme 17), using urea hydrogen peroxide as the terminal oxidant." This system showed... 

M. R. Bukowski, P. Comba, A. Lienke, C. Limberg, C. Lopez de Laorden, R. Mas-Balleste, M. Merz, L. Que Jr., Catalytic epoxidation and 1,2-dihydroxylation of olefins with bispidine-ironJllJ/HiOz systems, Angew. Chem. Int. Ed. Engl. 45 (2006) 3446. 

So far, catalyst design has aimed mainly at oxidant activation and little attention has been paid to the interaction between the metal center and the alkene. A requirement for a successful epoxidation system of wide scope for simple terminal alkenes would seem to be a new catalyst design focusing on activation of the substrate instead of the oxidant. This suggests noble metals as applicable catalytic centers, because of their affinity for terminal alkenes versus internal ones. This results in a change of role for the catalyst from electrophile to nucleophile in the system. 

For this comparative study, the catalytic epoxidation of cyclooctene was used as a representative transformation, since it allows for accurate determinations of conversion and yield because cyclooctene is not prone to allylic oxidation and its corresponding epoxide is not prone to hydrolysis. Conversion-time profiles for each catalyst system were measured under identical conditions, that is, a catalyst loading corresponding to 0.1 mol% W, 1.5 mol equiv 50% H2O2, and toluene as solvent at 60 °C. The results are shown in Fig. 16.2. 

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