Simple Metalated Epoxides

Hodgson and coworkers have demonstrated that the use of diamine ligands in combination with s-BuLi allows the direct deprotonation/electrophile trapping of 

---

Electrophile trapping of simple metalated epoxides (i. e., those not possessing an anion-stabilizing group) is possible. Treatment of epoxystannane 217 with n-BuLi (1 equiv.) in the presence of TMEDA gave epoxy alcohol 218 in 77% yield after trapping with acetone (Scheme 5.51) [76], In the absence of TMEDA, the non-stabilized epoxides underwent dimerization to give mixtures of enediols. 

Both benzothiazolyl and berizolriazoly] units have been employed as heteroaromatic anion-stabilizing groups for metalated epoxides (Scheme 5.47) [71]. The successful use of a simple alkyl bromide as electrophile with 200 is notable. 

Metalated epoxides are a special class of a-alkoxy organometallic reagent. Unstabilized oxiranyl anions, however, tend to undergo a-elimination. On the other hand, attempts to metalate simple unfunctionalized epoxides may lead to nucleophilic ring opening. The anion-stabilizing capability of a trimethylsilyl substituent overcomes these problems. Epoxysilanes 22 were... 

The deoxygenation of simple unfunctionalized epoxides has already been investigated with titanocene [17-20] and samarium [27] reagents. Usually both metal complexes give mixtures of the isomers with low selectivity. Epoxide 7 investigated here is mechanistically more interesting because the... 

Initially, the chentical compoimds termed precursors, which contained the catimis M from which an oxide gel was made, were essentially metallic salts. The sodium metasUicate Na2Si03, initially used by Kistler [1], was cheap. Hence, an industrial process based on this precursor was developed for some time by BASF [19]. Simple metallic salts also remained interesting when a more elaborate precursor was not easily available. More recently, the use of metallic salts as sol-gel precursors has seen a renewed interest when hydrolyzed in solution in an organic solvent, in which a slow proton scavenger such as an epoxide was added [3] (Chap. 8). Nice aerogel monoliths were obtained in this way with Cr, Fe, Al, Zr, and other cations. 

Cationic zirconium complexes (88) having diketonato ligands were examined for selective epoxide ring opening (Equation 40) [45]. The catalysts showed higher activity than simple metal halides such as TiCU and ZrCU, and various epoxides (87) were converted into (89) and/or (90) effectively. When R and R" in (87) were alkyl groups, the product selectivity of the catalyst (88) were moderate to low. But with styrene oxide (R =Ph, R" =H), alkoxy alcohol (89) was obtained with high selectivity. 

The phenomenon that early transition metals in combination with alkyl hydroperoxides could participate in olefin epoxidation was discovered in the early 1970s [30, 31]. While m-CPBA was known to oxidize more reactive isolated olefins, it was discovered that allylic alcohols were oxidized to the corresponding epoxides at the same rate or even faster than a simple double bond when Vv or MoVI catalysts were employed in the reaction [Eq. (2)] [30]. 

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

In a recent screening of different metal salts, Lane and Burgess found that simple manganese(n) and (in) salts catalyzed the formation of epoxides in DMF or t-BuOH in the presence of aqueous hydrogen peroxide (Scheme 6.7) [68]. It was further established that the addition of bicarbonate was of importance for the epoxidation reaction. 

In the following we will concentrate on three important cases, i.e. CO oxidation on alkali doped Pt, ethylene epoxidation on promoted Ag and synthesis gas conversion on transition metals. We will attempt to rationalize the observed kinetic behaviour on the basis of the above simple rules. 

The catalysts which have been tested for the direct epoxidation include (i) supported metal catalysts, (ii) supported metal oxide catalysts (iii) lithium nitrate salt, and (iv) metal complexes (1-5). Rh/Al203 has been identified to be one of the most active supported metal catalysts for epoxidation (2). Although epoxidation over supported metal catalysts provides a desirable and simple approach for PO synthesis, PO selectivity generally decreases with propylene conversion and yield is generally below 50%. Further improvement of supported metal catalysts for propylene epoxidation relies not only on catalyst screening but also fundamental understanding of the epoxidation mechanism. 

Enantiomerically pure 3-amino alcohols which are important intermediates for many bioactive compounds can be directly synthesized by the ARO reaction of readily accessible racemic and meso epoxides with appropriate amines. Indeed, some simple and multifunctional p-amino alcohols have been obtained using this strategy by the promotion of chiral BINOL [30-32,88,89], salen [35,52], bipyridine [33,40,90-94] and proline-A,JV-dioxide based metal complexes [95]. However, none of these systems demonstrated the recyclability of the precious chiral catalyst. 

An alternative method for the epoxidation of enones was developed by Jackson and coworkers in 1997 , who utilized metal peroxides that are modified by chiral ligands such as diethyl tartrate (DET), (5,5)-diphenylethanediol, (—)-ephedrine, ( )-N-methylephedrine and various simple chiral alcohols. The best results were achieved with DET as chiral inductor in toluene. In the stoichiometric version, DET and lithium tert-butyl peroxide, which was generated in situ from TBHP and n-butyllithium, were used as catalyst for the epoxidation of enones. Use of 1.1 equivalent of (-l-)-DET in toluene as solvent afforded (2/f,35 )-chalcone epoxide in 71-75% yield and 62% ee. In the substo-ichiometric method n-butyllithium was replaced by dibutylmagnesium. With this system (10 mol% Bu2Mg and 11 mol% DET), a variety of chalcone-type enones could be oxidized in moderate to good yields (36-61%) and high asymmetric induction (81-94%), giving exactly the other enantiomeric epoxide than obtained with the stoichiometric system (equation 37). 

The direct epoxidation of simple aikenes by hydrogen peroxide or alkyl hydroperoxides is a longstanding goal in oxidation chemistry. The reaction is usually catalyzed by suitable high-valent metals, mainly belonging to group 5 and 6, through formation of metal peroxo species. ... 

---