Epoxidations using catalysts

Aymmetric ring-opening reactions of other meso epoxides using catalysts derived from trimethylsilyl azide with titanium isopropoxide are described (3). 

TABLE 5.2 Catalytic asymmetric epoxidation using catalysts (17) and (24) ... 

Scheme 19.13 Proposed catalytic cycle for asymmetric epoxidation using catalyst 34.

Initially, a new iminium salt catalyst was prepared the 1,3-dioxane catalyst 44, derived from amine 42, and also gave up to 60% ee in the epoxidation of 1-phenylcyclohexene, interestingly this series of catalysts were far more reactive than the related dihydroisoquinolinium catalysts, for example epoxidation using catalyst 44 at 5 mol% loading was complete after only 3 min (Scheme 1.40) compared to the 60 min required for catalyst 43. 

Reaction of olefin oxides (epoxides) to produce poly(oxyalkylene) ether derivatives is the etherification of polyols of greatest commercial importance. Epoxides used include ethylene oxide, propylene oxide, and epichl orohydrin. The products of oxyalkylation have the same number of hydroxyl groups per mole as the starting polyol. Examples include the poly(oxypropylene) ethers of sorbitol (130) and lactitol (131), usually formed in the presence of an alkaline catalyst such as potassium hydroxide. Reaction of epichl orohydrin and isosorbide leads to the bisglycidyl ether (132). A polysubstituted carboxyethyl ether of mannitol has been obtained by the interaction of mannitol with acrylonitrile followed by hydrolysis of the intermediate cyanoethyl ether (133). 

Although trans epoxides ean be obtained via epoxidation of aeyelie cti-eonjugated olefins under speeified eonditions, a direet method based on the epoxidation of trans-olefins would be valuable. The Katsuki group reeently identified eatalyst 15 as an effieient catalyst for the direet epoxidation of trans-olefins. Crucial to the sueeess of the eatalyst is the inherent adoption of a deeply folded eonformation eoupled with the use of ehlorobenzene as solvent. While only a limited number of substrates have been examined to date using catalyst 15, the results are very promising. For example, trans- -methyl styrene is epoxidized in 91% ee, trans-P-n-butyl styrene in 95% ee, and trans-stilbene in 87% ee. 

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

Chiral epoxides and their corresponding vicinal diols are very important intermediates in asymmetric synthesis [163]. Chiral nonracemic epoxides can be obtained through asymmetric epoxidation using either chemical catalysts [164] or enzymes [165-167]. Biocatalytic epoxidations require sophisticated techniques and have thus far found limited application. An alternative approach is the asymmetric hydrolysis of racemic or meso-epoxides using transition-metal catalysts [168] or biocatalysts [169-174]. Epoxide hydrolases (EHs) (EC 3.3.2.3) catalyze the conversion of epoxides to their corresponding vicinal diols. EHs are cofactor-independent enzymes that are almost ubiquitous in nature. They are usually employed as whole cells or crude... 

Compounds lb and 2b were the Urst fluorinated ligands tested in Mn-catalyzed alkene epoxidation [5,6]. The biphasic Uquid system perfluorooc-tane/dichloromethane led to excellent activity and enantioselectivity (90% ee) in the epoxidation of indene with oxygen and pivalaldehyde (Scheme 1, Table 1). In addition, the fluorous solution of the catalyst was reused once and showed the same activity and selectivity. This represents a considerable improvement over the behavior in the homogeneous phase, where the used catalyst was bleached and reuse was impossible. Unfortunately, indene was the only suitable substrate for this system, which failed to epoxidize other alkenes (such as styrene or 1,2-dihydronaphthalene) with high enantioselectivity. The system was also strongly dependent on the oxidant and only 71% ee was obtained in the epoxidation of indene with mCPBA at - 50 °C. 

Asymmetric epoxidation is another important area of activity, initially pioneered by Sharpless, using catalysts based on titanium tetraisoprop-oxide and either (+) or (—) dialkyl tartrate. The enantiomer formed depends on the tartrate used. Whilst this process has been widely used for the synthesis of complex carbohydrates it is limited to allylic alcohols, the hydroxyl group bonding the substrate to the catalyst. Jacobson catalysts (Formula 4.3) based on manganese complexes with chiral Shiff bases have been shown to be efficient in epoxidation of a wide range of alkenes. 

Until recently only few examples on asymmetric epoxidation using iron-based catalysts were reported in the literature (Scheme 6) [42-44]. With [Fe(BPMCN) (CF3S03)2] 10, 58% of the epoxide with 12% ee was obtained in the oxidation of frans-2-heptene [42]. 

The hydrolytic kinetic resolution (HKR) of terminal epoxides using Co-salen catalysts provides a convenient route to the synthesis of enantioemiched chiral compounds by selectively converting one enantiomer of the racemic mixture (with a maximum 50% yield and 100% ee) (1-3). The use of water as the nucleophile makes this reaction straightforward to perform at a relatively low cost. The homogeneous Co(III) salen catalyst developed by Jacobsen s group has been shown to provide high... 

Oxidizing enzymes use molecular oxygen as the oxidant, but epoxidation with synthetic metalloporphyrins needs a chemical oxidant, except for one example Groves and Quinn have reported that dioxo-ruthenium porphyrin (19) catalyzes epoxidation using molecular oxygen.69 An asymmetric version of this aerobic epoxidation has been achieved by using complex (7) as the catalyst, albeit with moderate enantioselectivity (Scheme 9).53... 

A number of additional metal-catalyzed epoxidations have been reported in the past year. Platinum is a rarely used catalyst in oxidation reactions. The use of chiral Pt-catalyst 2 in the epoxidation of terminal alkenes provides the epoxide products in moderate yield and enantiomeric excess <06JA14006>. The chiral hydroxamide 3 is used with a Mo catalyst to provide the epoxide product in excellent yields and moderate enantioselectivity <06AG(I)5849>. A bis-titanium catalyst, 4, has also been used to epoxidize the usual set of alkenes with H202 as the oxidant <06AG(I)3478>. 

B. Lygo, P. G. Wainwright, Asymmetric Phase-Transfer Mediated Epoxidation of a,p-Unsaturated Ketones using Catalysts Derived from Cinchona Alkaloids , Tetrahedron Lett. 1998,39,1599-1602. 

Since tetranuclear platinum-blues are oxidized by 02 to Pt(III) dinuclear complexes and are reversively reduced to the platinum-blues and further to the Pt(II) dinuclear complexes, an attempt was made to use these complexes as catalysts for olefin oxidation to ketones and epoxides. The catalysts used were a-pyrrolidonato-bridged Pt-tan [Pt4(NH3)8(C4H6N0)4](N03)6 -2H20 (19), pivalamidato-bridged Pt-blue [Pt4(NH3)8(C5H10NO)4](NO3)5 (57), a-pyrrolidonato-tan [Pt4(NH3)8... 

Although the Sharpless catalyst was extremely useful and efficient for allylic alcohols, the results with ordinary alkenes were very poor. Therefore the search for catalysts that would be enantioselective for non-alcoholic substrates continued. In 1990, the groups of Jacobsen and Katsuki reported on the enantioselective epoxidation of simple alkenes both using catalysts based on chiral manganese salen complexes [8,9], Since then the use of chiral salen complexes has been explored in a large number of reactions, which all utilise the Lewis acid character or the capacity of oxene, nitrene, or carbene transfer of the salen complexes (for a review see [10]). 

---