Epoxides catalytic reactions

This class of substrate is the only real problematic substrate for the AE reaction. The enantioseleetivity of the AE reaction with this class of substrate is often variable. In addition, rates of the catalytic reactions are often sluggish, thus requiring stoichiometric loadings of Ti/tartrate. Some representative product epoxides from AE reaction of 3Z-substituted allyl alcohols are shown below. 

Metzner et al. also prepared the selenium analogue 17 of their C2 symmetric chiral sulfide and tested it in epoxidation reactions (Scheme 1.6) [8]. Although good enantioselectivities were observed, and a catalytic reaction was possible without the use of iodide salts, the low diastereoselectivities obtained prevent it from being synthetically useful. 

There are, however, numerous cases where electronegative additives can act as promoters for catalytic reactions. Typical examples are the use of Cl to enhance the selectivity of Ag epoxidation catalysts and the plethora of electrochemical promotion studies utilizing O2 as the promoting ion, surveyed in Chapters 4 and 8 of this book. The use of O, O8 or O2 as a promoter on metal catalyst surfaces is a new development which surfaced after the discovery of electrochemical promotion where a solid O2 conductor interfaced with the metal catalyst acts as a constant source of promoting O8 ions under the influence of an applied voltage. Without such a constant supply of O2 onto the catalyst surface, the promoting O8 species would soon be consumed via desorption or side reactions. This is why promotion with O2 was not possible in classical promotion, i.e. before the discovery of electrochemical promotion. 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

In the same spirit DFT studies on peroxo-complexes in titanosilicalite-1 catalyst were performed [3]. This topic was selected since Ti-containing porous silicates exhibited excellent catalytic activities in the oxidation of various organic compounds in the presence of hydrogen peroxide under mild conditions. Catalytic reactions include epoxidation of alkenes, oxidation of alkanes, alcohols, amines, hydroxylation of aromatics, and ammoximation of ketones. The studies comprised detailed analysis of the activated adsorption of hydrogen peroxide with... 

Catalytic turn-over [59,60] in McMurry couplings [61], Nozaki-Hiyama reactions [62,63], and pinacol couplings [64,65] has been reported by Fiirst-ner and by Hirao by in situ silylation of titanium, chromium and vanadium oxo species with McaSiCl. In the epoxide-opening reactions, protonation can be employed for mediating catalytic turn-over instead of silylation because the intermediate radicals are stable toward protic conditions. The amount of Cp2TiCl needed for achieving isolated yields similar to the stoichiometric process can be reduced to 1-10 mol% by using 2,4,6-collidine hydrochloride or 2,6-lutidine hydrochloride as the acid and Zn or Mn dust as the reduc-tant (Scheme 9) [66,67]. 

Attempts have been made to exploit the intrinsic C2 symmetry of the phenolate-based dinickel core in enantioselective catalytic reactions. Therefore, enantiomerically pure C2-symmetric ligands such as (736a) and the corresponding dinickel systems (736b) have been prepared ( Equation (27)),1890 and (736b) was tested in the epoxidation of unfunctionalized alkenes with sodium hypochlorite as the oxidant. The catalytic reaction was found to be highly pH dependent with an optimum at a pH of 9. While the complex is catalytically active, significant enantioselectivity was not achieved. 

By 1990, most of the catalytic reactions of TS-1 had been discovered. The wide scope of these reactions is shown in Fig. 6.1.35 Conversions include olefins and diolefins to epoxides,6,7 12 16 19 21 24 34 36 38 13 aromatic compounds to phenols,7,9 19 25 27 36 ketones to oximes,11 20 34 46 primary alcohols to aldehydes and then to acids, secondary alcohols to ketones,34-36 42 47-30 and alkanes to secondary and tertiary alcohols and ketones.6 34 43 31 52... 

These catalytic reactions are distinguished from the homolytic reactions in that no evidence exists for paramagnetic intermediates. The epoxidation is stereospecific, trans- and c/.v-alkenes yielding trans- and o.v-epoxides, respectively. Under the same conditions, complexes of Cu, Mn, and Fe give no yields or... 

It was observed that no leaching of Ti occurs during the catalytic reaction in the anhydrous medium. The acidity of the catalysts (which gave rise to many side products) was evaluated by a comparison of their reaction rates in the acid-catalyzed conversion of citronellol into isopulegol (Scheme 7). The acidity of the catalysts decreased in the following order A>C>D>B = E. The catalytic activity and epoxidation selectivities are compared in Table XIII. 

Therefore, we derided to initiate a program directed towards the development of a tita-nocene-catalyzed epoxide opening [3c]. Since titanocene dichloride is formed in the stoichiometric reaction after the protic quench, the challenge to be met is the regeneration of the redox-active species in situ, the fundamental requirement for a catalytic reaction. This underlying problem is depicted in Scheme 12.12. 

To overcome this issue Kureshy et al. [55, 56] reported dimeric form of Jacobsen s catalysts 3, 4. They used the concept of solubility modification by altering the molecular weight of the catalyst so that in a post catalytic work-up procedure the catalyst is precipitated, filtered and used for subsequent catalytic runs. The complexes 3, 4 (0.2 mol % of Co(lll)-salen unit) (Figure 2) were effectively used for HKR of racemic epoxides, e.g., styrene oxide, epichlorohydrin, 1,2-epoxypropane, 1,2-epoxyhexane, 1,2-epoxyoctane, and 1,2-epoxydodecane to achieve corresponding epoxides and 1,2-diols in high optical purity and isolated yields. In this process, once the catalytic reaction is complete the product epoxides were collected by reduced pressure distillation. Addition of diethylether to the residue precipitated the catalyst which was removed by filtration. However, the recovered catalyst was required to be reactivated by its treatment with acetic acid in air. The catalysts were reused 4 times with complete retention of its performance. 

Nugent, W. A. (1998) Desymmetrization of meso-epoxides with halides A new catalytic reaction based on mechanistic insight, J. Am. Chem. Soc., 120 7139-7140. Bruns, S. Haufe, G. (1999) Catalytic asymmetric ring opening of epoxides to chlorohydrins with mild chloride donors and enantiopure titanium complexes.. 

The three-coordinated unsaturated structure of the Ru complex remained unchanged after 1000 cycles of the stilbene epoxidation as suggested by XPS, DR-UV/Vis and XAFS. The unsaturated Ru complex 5 is active for the epoxidation reaction. Notably, the three-coordinate Ru complex 5 is quite stable under the reaction conditions and also in air despite its unsaturated structure. This remarkable stability and durability made the immobilized catalyst 5 recyclable for the catalytic reactions, maintaining 100% conversion with selectivity higher than 80% [17]. 

The most relevant catalytic reactions approached by SOMC are olefin polymerization (and depolymerization), alkane activation (including a new reaction, discovered thanks to SOMC-alkane metathesis), alkene metathesis and epoxidation. All these reactions are discussed in this chapter. 

Styrene epoxide on reaction with 3-buten-l-ol in the presence of a catalytic amount of BiCl3 gave two possible isomers, of which the m-isomer was found to be the major one [25] (Fig. 1). The scope and versatility of the method is depicted in... 

TBHP and the molybdenum catalysts are soluble in imidazohiun-based RTlLs. The system becomes biphasic when the olefinic substrate is added. In all cases, the TOFs of the catalytic reactions are considerably lower with the ionic solvent than when performed without the ionic solvent (data reported in Table 9). This slower catalytic reaction may be due to dilution effects and phase transfer problems, especially with the olefin, which is quite insoluble in the RTIL. The conversion appears to be strongly temperature-dependent, as decreasing the temperature from 55 °C to 35 °C reduces the conversion by ca. 50% (entries 7 and 8, Table 9). With the dioxomolybdenum complexes 1 and 2, the epoxidation reaction proceeds with 100% selectivity (Table 9), whereas some diol is formed with the catalyst 3. 

This 6-hydrogen elimination in 2-rhoda oxetanes is apparently favored over reductive elimination to an epoxide. Moreover, the reverse step, i.e., the oxidative-addition of epoxides to Rh and Ir results in 2-rhoda oxetanes [85] and/or hydrido formylmethyl complexes [86]. Therefore, assuming that 2-metalla oxetanes are intermediates in the oxygenation of alkenes by group VIII transition metals, the reported reactivity would account for selectivity to ketones in the catalytic reactions based on these metals. 

Catalytic Reactions. Certain catalytic reactions show a considerable departure from the linear relationship between the ee of a chiral source and the extent of the asymmetric induction (Scheme 42) 68). The Sharpless epoxidation of geraniol using Ti(IV) tetraisopropoxide modified by enantiomerically pure diethyl tartrate (DET) (Ti DET =1 1) gives the epoxide in 94% ee. Kagan and Agami first found that, by using the tartrate auxiliary in 50% ee, the optical yield varied to 70% ee. This optical yield is considerably higher than the expected value, 47 % ee... 

Both reagents transfer a methylene group in efficient and selective pathways. So it is not surprising that sulfur ylides have been widely used as synthetic tools for the preparation of epoxides. The reactions can make use of sulfonium salts under phase transfer catalytic conditions, and the cheap and easily accessible trimethyl sulfonium methyl sulfate and triethylsulfonium ethyl sulfate were found to show a high reactivity under such conditions [450]. 

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