Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 1135 122

the approaching peracid302. A similar explanation has been used to account for the syn selectivity in the epoxidation of some allylic alcohols303.

Syn selectivity in the epoxidation of allylic fluorides by peracids has been interpreted by electronic stabilization of the transition state with the electronegative fluorine atom oriented at the inside position304.

Molecular-orbital calculations indicate that the stereoselective epoxidation of the alkene 192 by peroxy acids arises from stereoelectronic control exerted by a CF3 C bond orientated anti to the alkene plane, in contrast to the previously proposed model for epoxidation of allylic fluoride in which the F C bond and alkene bonds are in a syn arrangement305.

Silyl enol ethers with stereogenic silicon atoms bearing chiral alkoxy groups on silicon, as in 193, induce modest stereoselectivity in peracid epoxidation of the enol

double bond306. Aryl participation has been observed in the epoxidation of the bicylooctene 194307.

The regioselectivity in the epoxidation of ethenylidenecyclopropanes by MCPBA strongly depends on the double-bond substituents. Thus, with alkyl substituents such as methyl 195, the adjacent electron-rich double bond is epoxidized, whereas the diphenyl analogue 196 reacts preferentially on the other double bond; both primary products undergo subsequent rearrangements308. The MCPBA oxidation of the hindered cumulene 197 gave the cyclopropanones 198 and 199309.

R

Ph O

O

(199)

B. Dioxiranes

Dioxiranes, such as dimethyldioxirane (200) and methyl(trifluoromethyl)dioxirane (201), are a class of reactive organic peroxides with great potential as oxidants310. The main advantage of this methodology is that it makes highly labile epoxides accessible. Thus, for instance, 203 can be obtained by epoxidation of ˇ-oxo enol ethers 202 with dimethyldioxirane311. Similarly, the reaction of the dimethyldihydropyran (204) with 200 gives the unstable epoxide 205; secondary deuterium isotope effects indicate a greater degree of rehybridization at the ˇ- than at the ˛-position in the transition state leading to the epoxide312.

Electronic and steric effects in the epoxidation of alkenes by dimethyldioxirane have been investigated313. Both mechanistic and synthetic aspects of the chemistry of dioxiranes have been reviewed314,315.

The relative rates of epoxidation of ethyl 4-substituted-(E)-cinnamates by 200 gave a Hammett value of 1.53, indicating an electrophilic oxygen-atom transfer316; the reaction rate is increased by protic solvents317.

Dioxiranes react with cholesterol and its acetate to give ca 1:1 mixtures of 5˛,6˛- and 5ˇ,6ˇ-epoxides (in contrast to peroxy acids, that are known to produce ca 5:1 mixture)318.

Dioxiranes have provided the first examples of direct epoxidation of a double bond bearing a trifluoromethyl group substituent by non-biochemical means319.

Some limitations have been discussed320 in the dimethyldioxirane oxidation of glucals321. Dimethyldioxirane induces epoxidation of enanimes (e.g. 206), which subsequently dimerize to 1,4-dioxanes (208)322. The stability of the ˛-amino epoxides 207 depends on the type of substitution at the nitrogen323.

Simple allenes (209) react with dimethyldioxirane (200) to give the corresponding spiro-dioxides 210; in instances where diastereoisomeric spiro-dioxides are possible, there is usually an acceptable stereochemical preference for epoxidation to occur anti to the alkyl substituents324,325. Allenic alcohol 211 yields the highly functionalized tetrahydrofuran 212 and tetrahydropyran derivatives by intramolecular nucleophilic addition of the hydroxy group to an intermediate allene diepoxide324.

less Katsuki procedure327,328, have appeared.

Studies of bis-tartrate esters and other tartrate ligands for titanium-mediated asymmetric epoxidation have provided evidence against the sole intermediacy of monomeric titaniumtartrate species in the parent system329,330. Other tartrate ligands have been studied in attempts to gain a better understanding of the mechanism of the Sharpless epoxidation330.

A systematic study of the Sharpless epoxidation led to the following conclusions: (1) an equimolar complex of titanium tetraalkoxide and tartrate diester [(tartrate)(RO)2Ti] is the

catalytically active template for asymmetric epoxidation; it is much more active than titanium tetraalkoxide alone or titanium tartrates of other than 1:1 stoichiometry and thus exhibits ligand-accelerated catalysis; (2) the rate is first order in substrate and oxidant, and inverse second order in inhibitor alcohol, under pseudo-first-order conditions in catalyst; this is characteristic of a system in which allylic alcohol and alkyl hydroperoxide bind to the same metal centre; (3) the rate of epoxidation is slowed by alkenes with electronwithdrawing substituents, indicating that the olefinic moiety is nucleophilic; (4) increased bulk at several positions in the epoxidation system (the alkyl group of either the tartrate ester of the hydroperoxide or the trans-olefinic substituent) results in increased epoxidation rates together with better kinetic resolution and asymmetric induction. Solution-phase and X-ray structure investigations on the detailed structure of [(tartrate)(RO)2Ti] have been reported331,332.

An alternative explanation for the enantioselectivity observed in the Sharpless epoxidation has been proposed333 but it does not seem to be compatible with the kinetic and other data.

Within limits, an increase in the steric bulk at the olefin terminus of allylic alcohols of the type R1CH(OH)CHDCHR2 causes an increase in the rate of epoxidation of the more-reactive enantiomer, and a decrease in the rate for the less-reactive enantiomer, resulting in enhanced kinetic resolution334. However, complexes of diisopropyl tartrate and titanium tetra-tert-butoxide catalyse the kinetic resolution of racemic secondary allylic alcohols with low efficiency335. Double kinetic resolution techniques can show significant advantages over the simple Sharpless epoxidation techniques336.

High diastereoselectivity is found in the epoxidation of fluoroallylic alcohols with titanium(IV) isopropoxide and tert-butyl hydroperoxide337. The anomalous Sharpless asymmetric epoxidation has been used in the synthesis of L-erythro- and D-threo- sphingosines338.

The epoxidation of alkenylsilanols parallels that of allylic alcohols in exhibiting good enantioselectivities339. Kinetic resolution of the alkenylsilanol 213 by the Sharpless asymmetric epoxidation has been accomplished, with the rate difference for the oxidation of the enantiomers of 213 being unusually high (>11)340.

Sharpless epoxidation of the alkenylsilanol 214 gave, after protodesilylation of the silyl epoxide 215, styrene epoxide 216 in 95% e.e.; the stereochemical course of the reaction follows that predicted by Sharpless for allylic alcohols341.

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-1-ol (218) via an asymmetric [2,3] sigmatropic shift (Scheme 4)343. For other titanium-catalysed epoxidations, see Section V.D.1 on vanadium catalysis.

(217)ButOOH

H2 O

OH

Ph

(218)

SCHEME 4

D. Other Metal-catalysed Epoxidation Reactions

1. Vanadium

The importance of 1,2- rather than 1,3-allylic strain in the vanadium(V)-catalysed ButOOH epoxidation directed by hydroxy groups has been assessed using (Z)-3-en-2-ols as model substrates344.

Epoxidation of both syn- and anti-5-(tosylamino)-hex-3-en-2-ol derivatives 219 with t- butyl hydroperoxide with vanadium or titanium catalysts has been shown to exhibit little stereocontrol ( 3:1)345.

NH.TS

R′

R

OH

(219)

Stereoselective epoxidation of ˇ-cis-homoallylic alcohols by vanadium, tungsten and molybdenum oxo species has been used for the construction of intermediates with four adjacent asymmetric centres346.

The epoxidation of electron-deficient alkenes with either vanadium or titanium catalysts give syn-epoxides347; a free hydroxy group and a ketone or ester function are necessary for the reaction to take place, and a modest level of asymmetric induction can be achieved with -hydroxyenone substrates and chiral titanium catalysts348.

In the photo-oxygenation of the diene 220, use of titanium isopropoxide as oxygentransfer catalyst afforded exclusively the epoxy-alcohol 225, whereas a vanadiumperoxo complex gave exclusively the isomeric product 226; oxygen transfer with a titanium reagent outweighs regio-isomerization whereas the reverse is true for the vanadium catalyst349. This procedure has been applied to the regioand stereo-controlled oxygenation of cholesterol350.

(220)

X X

2. Molybdenum and tungsten

The molybdenum complex 227 is an effective catalyst for epoxidation of alkenes and has allowed the development of the polystyrene-supported peptide-linked epoxidation catalyst 228351.

Studies on the epoxidation of unsaturated acids by hydrogen peroxide in the presence of phosphotungstic acid352 and of alkenes by alkyl hydroperoxides in the presence of molybdenum complexes353,354 and vanadium oxide355 have appeared.

In the epoxidation of alkenes with tert-butyl hydroperoxide and a molybdenum oxide catalyst, addition of an aliphatic amine first accelerates the formation of the intermediate 229 and also favours the production of epoxide in favour of the alternative fragmentation to carbonyl compounds (Scheme 5)356.

3. Manganese

The optically active manganese-salen complexes 230 and 231 are effective catalysts for the enantioselective epoxidation of unfunctionalized alkenes357 361. The yield of epoxide

diminishes with increasing concentration of the catalysts in the epoxidation of alkenes with iodosylbenzene as oxidant, indicating that rigid coordination of alkene to oxomanganese(V) species does not take place in the rate-determining step360.

Asymmetric epoxidation of conjugated dienes and enynes catalysed by the chiral manganese(III) complex 232 give monoepoxides exclusively; reactions of cis-enynes give trans-alkynyl epoxides as the major products with a high level of asymmetric induction362.

In the catalytic epoxidation of alkenes by a manganese porphyrin with phase-transfer catalysis and hypochlorite, the yield of epoxide also decreases with decreasing alkene concentration363; dibenzo-18-crown-6 has been shown to have an effect on the reaction364.

The two-phase epoxidation of alkenes by hydrogen peroxide in water dichloromethane system, catalysed by manganese(III) porphyrin, is strongly accelerated by addition of catalytic amounts of a carboxylic acid and lipophilic imidazole or pyridine axial ligand365,366. Manganese(III) porphyrin bound to colloidal anion-exchange particles is more active in the selective epoxidation of styrene by aqueous hypochlorite than the same catalyst in aqueous solution367.

The mechanism of the epoxidation of alkenes by the cytochrome P450 model, sodium hypochlorite manganese(III) tetraarylporphyrins, involves rate-determining formation of an active species 234 from a hypochlorite-manganese complex 233 (Scheme 6); pyridine or imidazole derivatives, as axial ligands, accelerate this step by electron donation, although the imidazoles are destroyed under the reaction conditions368.

O

(233)(234)

L = Porphyrine

SCHEME 6

4. Rhenium

The oxidation of 5-hydroxyalkenes 235 with rhenium(VII) oxide gives 2- (hydroxymethyl)-tetrahydrofurans 236; the stereoselectivity has been rationalized by an initial [2 C 2] cycloaddition followed by reductive elimination369. The yield and stereoselectivity of such oxidations are the same in both stoichiometric and periodatecatalysed reactions370.

5. Miscellaneous

Sodium perborate in acetic anhydride has been reported to oxidize alkenes to expoxides371.