Supported metalloporphyrin catalysts for alkene epoxidation

This review is devoted to the recent advances in the preparation of immobilised metalloporphyrins and their use as heterogeneous catalysts for alkene epoxidation. The wide range of supports, nature of attachments, and metalloporphyrins that have been reported is detailed and a comparison is made between the activities of the resulting catalysts in the epoxidation of different alkenes. The important issue of recyclability of the metalloporphyrins is also covered.     

Catalytic enantioselective alkene epoxidation using novel spirocyclic N-carbethoxy-azabicyclo[3.2.1]octanones

A general synthetic route allowing access to several spirocyclic N-carbethoxy-azabicyclo[3.2.1]octanones is developed. These novel ketones efficiently catalyse alkene epoxidation using Oxone^® with up to 91.5% ee.     

Highly efficient heterogeneous polymer-supported Sharpless alkene epoxidation catalysts

More widespread, and cost effective, use of soluble transition metal complex asymmetric catalysts would be achieved if highly efficient heterogeneous analogues could be developed. We now report the synthesis of branched/crosslinked poly(tartrate ester)s and their use as heterogeneous ligands for the complexation of Ti(OⁱPr)₄, and use of the resulting insoluble catalysts in the asymmetric epoxidation of a range of trans-allylic alcohols employing t-butylhydroperoxide (tBHP) as the oxidant. Isolated yields of epoxides of up to 80%, with enantiomeric excesses up to 98% have been achieved. Removal and recovery of the polymer catalyst is by simple filtration at the end of reactions. The influence of the ratio of polymer-ligand–titanium, and the polymer backbone molecular architecture on the activity and enantioselectivity of the catalysts has been assessed. Typically polymer branching ratios of 3–11% and a polymer-ligand:titanium ratio of 2:1 yield the optimum results.     

A chiral iron-sexipyridine complex as a catalyst for alkene epoxidation with hydrogen peroxide

A chiral iron-sexipyridine complex-hydrogen peroxide mixture is a highly efficient catalytic system for styrene epoxidation with excellent reactivity (3 min, 95% yield) and chemoselectivity (98%).     

A simple and versatile method for alkene epoxidation using aqueous hydrogen peroxide and manganese salophen catalysts

We describe a simple and versatile method for the catalytic epoxidation of a broad range of olefins (e.g., ketones, esters, and alkyl halides) with aqueous H₂O₂ using manganese salophen catalysts. Low catalyst loading, short reaction times, and a simple reaction setup (e.g., no pH buffer is required) are salient features of the system, which unites the benefits of H₂O₂ as an oxidant with the versatility and modularity of salen-based catalysts.     

Asymmetric alkene epoxidation catalysed by a novel family of chiral metalloporphyrins: effect of structure on catalyst activity,stability and enantioselectivity

A selection of alkenes has been epoxidised with iodosylbenzene, catalysed by three related iron(III) tetraarylporphyrins: 1*, 2* and 3* with four 2,6-di(1-phenylbutoxy)phenyl groups, with one pentafluorophenyl and three 2,6-di(1-phenylbutoxy)phenyl groups and with two pentafluorophenyl and two 2,6-di(1-phenylbutoxy)phenyl groups, respectively. 1* is very sterically hindered and prone to self-oxidation which makes it a relatively poor epoxidation catalyst. Introducing the smaller pentafluorophenyl groups, in place of 2,6-di(1-phenylbutoxy)phenyl, increases catalyst reactivity, stability and selectivity. This change allows easier access of the substrates to the active oxidant and also, by decreasing the electron density on the porphyrin ligand, increases the reactivity of the oxoiron intermediate and its stability towards self-oxidation. A family of five homochiral catalysts, 1, 2 and 3, [the analogues of 1*, 2* and 3*, prepared from (R,R)-2,6-di(1-phenylbutoxy)benzaldehyde] and catalyst 4 with three pentafluorophenyl and one (R,R)-2,6-di(1-phenylbutoxy)phenyl group and 5 the manganese(III) analogue of 3 have been used to epoxidise three prochiral alkenes. All the reactions give low enantioselectivities. Using styrene as the substrate, (S)-styrene epoxide is the major enantiomer obtained with all the catalysts except 1 which leads to the (R)-styrene epoxide being preferred. In contrast cis-hept-2-ene and 2-methylbut-2-ene give the same major epoxide enantiomer with all the catalysts. The dependence of the ee values on catalyst and substrate structure, temperature and solvent is examined and discussed.     

Molybdenum Schiff base-polyoxometalate hybrid compound: A heterogeneous catalyst for alkene epoxidation with tert-BuOOH

The hybrid compound consisting of molybdenum(salen) [salen = N,N′-bis(salicylidene)ethylnediamine] complex covalently linked to a lacunary Keggin-type polyoxometalate, K₈[SiW₁₁O₃₉] (POM), was synthesized and characterized by elemental analysis, FT-IR, ¹H NMR and diffuse reflectance UV–Vis spectroscopic methods and BET analysis. The complex, [Mo(O)₂(salen)–POM], was studied, for the first time, in the epoxidation of various alkenes with tert-BuOOH and in 1,2-dichloroethane as solvent. This catalyst can catalyze epoxidation of various olefins including non-activated terminal olefins. The effect of the other parameters such as solvent, oxidant and temperature on the epoxidation of cyclooctene was also investigated. The interesting characteristic of this catalyst is that, in addition to being a heterogeneous catalyst, it gives higher yields towards epoxidation of olefins in comparison to the corresponding homogeneous [Mo(O)₂(salen)] complex.     

Direction of epoxidation of esters of polyunsaturated acids

Conclusions The direction of the epoxidation of a number of polyunsaturated acids was studied and it was shown that the epoxidation proceeds at the double bonds furthest away from the carbethoxyl group.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 943–947, April, 1977.     

An improved protocol for the ruthenium(pybox)-catalyzed asymmetric alkene epoxidation

A considerable rate enhancement in the ruthenium-catalyzed asymmetric epoxidation of olefins in the presence of PhI(OAc)₂ is reported. By the addition of H₂O, the rate of the reaction was increased by two orders of magnitude. Reactions of both aliphatic and aromatic olefins were realized for the first time and enantioselectivities up to 71% ee were obtained. In addition an in situ generation of ruthenium pybox catalysts for faster screening of oxidation catalysts was also developed.     

Ruthenium bis(bipyridine) sulfoxide complexes: new catalysts for alkene epoxidation

The ruthenium bis(bipyridine) sulfoxide complexes Ru-1 and Ru-2 exhibit high catalytic activity for epoxidation of unfunctionalized olefins in the presence of [bis(acetoxy)iodo]benzene; with the chiral catalyst, Ru-2, asymmetric induction up to 94% was observed for beta-methylstyrene.     

New polydentate ligand and catalytic properties of the corresponding ruthenium complex during sulfoxidation and alkene epoxidation

Bis(diimine)-ruthenium complexes constitute a class of catalysts with good activity for oxidation reactions, such as sulfoxidation and epoxidation. The synthesis and the full characterization of a new ruthenium complex bearing an original pentadentate ligand (L5pyr for 2,6-bis-(6-ethyl-2,2'-bipyridyl)-pyridine) is reported. Comparison of its activity with regard to[Ru(bpy)2(CH3CN)2](2+) and [Ru(bpy)2(py)(CH3CN)](2+) during alkene and sulfide oxidation allowed us to conclude that the addition of a fifth pyridine ligand in the coordination sphere improves the efficiency of the catalyst. Moreover, under these oxidation conditions a hydroxylation of the ligand L5pyr led to a better activity than its analogue [Ru(bpy)2(py)(CH3CN)](2+), especially during epoxidation of alkenes by PhI(OAc)2.     

Toward a catalytic cycle for the Mn-salen mediated alkene epoxidation: a computational approach

BP86 density functional calculations for the title reaction are presented, where a model catalyst with hypochlorite as oxygen-containing counter ligand, (ClO)(O)Mn(acacen') (acacen' = -O(CH)3N-C2H4-N(CH)3O-), is employed. The epoxidation reaction on potential energy surfaces corresponding to an overall spin-density of two and four unpaired electrons is investigated. The presence of the hypochlorite ligand is found to cause the reaction to proceed under conservation of spin. Further, the oxygen-containing counter ligand causes reoxidation of the Mn-center, thus closing the catalytic cycle. A catalytic scheme is therefore proposed, which includes a step of regeneration of the catalytically active species. Energetic estimates including corrections for solvent effects are presented for the relevant steps constituting the proposed catalytic scheme.     

Dendritic ruthenium porphyrins: a new class of highly selective catalysts for alkene epoxidation and cyclopropanation

Attachment of Fréchet-type poly(benzyl ether) dendrons [G-n] to carbonylruthenium(II) meso-tetraphenylporphyrin (5) using covalent etheric bonds forms a series of dendritic ruthenium(II) porphyrins 5-[G-n](m) (m=4, n=1, 2; m=8, n=0-2). The attachment was realized by treating the carbonylruthenium(II) complex of 5,10,15,20- tetrakis(4'-hydroxyphenyl)porphyrin or 5,10,15,20-tetrakis(3',5'-dihydroxyphenyl)porphyrin with [G-n]OSO(2)Me in refluxing dry acetone in the presence of potassium carbonate and [18]crown-6. Complexes 5-[G-n](m) were characterized by UV/Vis, IR, and NMR spectroscopy and mass spectrometry. All of the dendritic ruthenium porphyrins are highly selective catalysts for epoxidation of alkenes with 2,6-dichloropyridine N-oxide (Cl(2)pyNO). The chemo- or diastereoselectivity increases with the generation number of the dendron and the number of dendrons attached to 5, and complex 5-[G-2](8) exhibits remarkable selectivity or turnover number in catalyzing the Cl(2)pyNO epoxidation of a variety of alkene substrates including styrene, trans-/cis-stilbene, 2,2-dimethylchromene, cyclooctene, and unsaturated steroids such as cholesteryl esters and estratetraene derivative. The cyclopropanation of styrene and its para-substituted derivatives with ethyl diazoacetate catalyzed by 5-[G-2](8) is highly trans selective.     

新型手性氨基酸烯砜的合成

以苯亚磺酸钠和3-氯-2-甲基-1丙烯为原料,经偶联、氯化、溴化,之后再与氨基酸胺化反应合成得到了两个新型手性氨基酸烯砜化合物。所获得化合物的结构经HR-MS,1H,13C和/或DEPT NMR验证。     

Polymer‐supported Ti(IV) and Mn(III) asymmetric alkene epoxidation catalysts

Epoxides represent a very important group of speciality and fine chemicals because they are derived directly from alkenes, a primary petrochemical source, and because of the breadth of opportunity they offer the organic synthetic chemist in terms of the highly selective reactions they undergo, often requiring only very mild conditions. Since most epoxides also bear at least one stereogenic centre the strategic importance of these molecules in synthesis is even higher. The most important asymmetric alkene epoxidation catalyst systems that have been discovered are those reported by Sharpless and his co‐workers utilising tartrate ester complexed Ti(IV) centres¹ and by Jacobsen and his co‐workers utilising chiral Mn(III) salen complexes.² The former system provides high conversions and high enantioselectivity (enantiomeric excess, ee%) in the case of allylic alcohol substrates, while the latter is likewise effective in the case of non‐functional cis‐internal alkenes, especially cyclic systems. Both catalytic systems are homogeneous and exploitation of both involve rather laborious work‐up procedures. Generally no attempt is made to recover and re‐use these catalysts. The potential advantages in converting a process catalysed by a homogeneous metal complex into one involving a heterogeneous polymer‐supported analogue have been well rehearsed.³ Suffice to say that on a laboratory scale supported metal complex catalysts considerably facilitate product work‐up and isolation, while on a large scale such heterogeneous species allow processes to be run continuously using packed or fluidised bed columns with considerable financial advantages both in terms of capital expenditure on plant and with regard to recurrent costs.