Aryl asymmetric epoxidation with chiral

A heterobimetallic BINOL-Ga/Li complex 53 has been developed for the enantioselective ARO of meso-cpoxides (BINOL = l,T-bi(2-naphthol)).278 Using />-methoxyphenol as the nucleophile, this etherification reaction was observed to take place with a high level of asymmetric induction. An improved catalyst 54 has also been reported that exhibits greater stability under the reaction conditions and delivers higher yields and ee s (Equation (78)).279 A simple catalyst derived from Sc(OTf)3 and the chiral bipyridine ligand 52 has been shown to be effective for the ARO of aryl-substituted /// -epoxides with aliphatic alcohols to give high ee s (Equation (79)).280... 

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... 

This chapter begins by classifying the combinations of oxidation/reduction processes with subsequent cationic transformations, though to date the details of only two examples have been published. The first example comprises an asymmetric epoxidation/ring expansion domino process of aryl-substituted cyclopropyl-idenes (e. g., 7-1) to provide chiral cyclobutanones 7-3 via 7-2, which was first described by Fukumoto and coworkers (Scheme 7.1) [2]. 

Chiral (salen)Mn(III)Cl complexes are useful catalysts for the asymmetric epoxidation of isolated bonds. Jacobsen et al. used these catalysts for the asymmetric oxidation of aryl alkyl sulfides with unbuffered 30% hydrogen peroxide in acetonitrile [74]. The catalytic activity of these complexes was high (2-3 mol %), but the maximum enantioselectivity achieved was rather modest (68% ee for methyl o-bromophenyl sulfoxide). The chiral salen ligands used for the catalysts were based on 23 (Scheme 6C.9) bearing substituents at the ortho and meta positions of the phenol moiety. Because the structures of these ligands can easily be modified, substantia] improvements may well be made by changing the steric and electronic properties of the substituents. Katsuki et al. reported that cationic chiral (salen)Mn(III) complexes 24 and 25 were excellent catalysts (1 mol %) for the oxidation of sulfides with iodosylbenzene, which achieved excellent enantioselectivity [75,76]. The best result in this catalyst system was given by complex 24 in the formation of orthonitrophenyl methyl sulfoxide that was isolated in 94% yield and 94% ee [76]. 

A very different type of chemistry occurs when sulfur ylids add to carbonyl compounds. Epoxides are formed and recent progress with chiral sulfur ylids allows good asymmetric induction in this reaction. The easily prepared C2 symmetric sulfide 96 reacts with alkyl halides and then with aryl and alkyl aldehydes to give good yields of trans epoxides 97 with reasonable ees.19... 

Manganese and iron porphyrins substituted by chiral atropisomeric groups are models for cytochrome P-450 and several of such catalysts have been recommended by Groves, Kodadek, Mansuy and their coworkers in asymmetric epoxi-dations of olefins with hypochlorites or ArlO [957, 970], Recently, asymmetric epoxidation of 2-mtrostyrene has been accomplished with 89% enantiomeric excess, but this result lacks generality. A preliminary report on the use of threitol-strapped manganese poiphyrins in enantioselective epoxidations of unfunctionalized aryl substituted olefins appeared in 1993 [971] enantiomeric excesses in the range of 80% were observed. 

Table 3.1 Representative Asymmetric Epoxidation of Aryl Alkenes with Chiral Ketones.

The asymmetric oxidation of sulphides to chiral sulphoxides with t-butyl hydroperoxide is catalysed very effectively by a titanium complex, produced in situ from a titanium alkoxide and a chiral binaphthol, with enantioselectivities up to 96%342. The Sharpless oxidation of aryl cinnamyl selenides 217 gave a chiral 1-phenyl-2-propen-l-ol (218) via an asymmetric [2,3] sigmatropic shift (Scheme 4)343. For other titanium-catalysed epoxidations, see Section V.D.l on vanadium catalysis. 

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