Aryl-substituted epoxides

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... 

Bhatia KA, Eash KJ, Leonard NM, Oswald MC, Mohan RS (2001) A facile and efficient method for the rearrangement of aryl-substituted epoxides to aldehydes and ketones using bismuth triflate. Tetrahedron Lett 42 8129-8132... 

Reaction of styrene oxide with tetraallyltin in the presence of Bi(OTf)3 (2 mol%) affords the corresponding l-phenyl-4-penten-2-ol (Fig. 5). In a similar fashion, various aryl substituted epoxides react smoothly with tetraallyltin to give the corresponding homoallylic alcohols. This method give generality as cycloalkyl oxiranes and sterically hindered ones give the corresponding homoallylic alcohols. 

Rough guidelines for the prediction of regioselectivity in epoxide ring openings are summarized in Scheme 4.60. Under neutral or basic reaction conditions alkyl-or aryl-substituted epoxides react with most nucleophiles at the less substituted carbon atom [248-253]. Under acidic reaction conditions, however, product mixtures or preferential attack at the most substituted carbon atom can be observed. Acids can usually be used to enhance the reactivity of epoxides and to promote substitution at the site of an epoxide which forms a carbocation more readily. 

The 2,4,6-triphenylpyrylium terafluoroborate (TPT)-sensitized electron transfer of aryl-substituted epoxides such as 250 leads to ring opening via selective C—O bond cleavage, while subsequent [3 + 2]-cycloaddition of the resultant carbonyl ylide with electron-rich olefins 251 leads to the synthesis of substituted THF derivatives 252 and 253 (Scheme 8.69) [109]. 

Hydration of aryl-substituted epoxides. Treatment of aryl-substituted oxiranes with copper(II) sulfate and pyridine in an aqueous phosphate buffer (pH 7) affords cis-diols by reaction of water at the benzylic position. Less than 15% of trans cleavage products obtain. In the absence of pyridine, the oxides are rapidly destroyed. Triethylamine cannot be substituted for pyridine. cis-Chlorohydrins are obtained when aryl-substituted oxides are treated with copper(II) sulfate, pyridine, and kiCl in THF, When methanol is used as solvent, the corresponding cis-glycol monomethyl ether is obtained. In all cases substitution occurs at the benzylic position. Alkyl oxiranes do not react under these conditions. ... 

Sodium borohydride (NaBH4), a relatively mild reducing agent, reduces epoxides only sluggishly except for nitro epoxides. In the mixed solvent f-butyl alcohol and methanol, sodium borohydride can reduce aryl-substituted epoxides, terminal epoxides and cyclohexene oxide, to the corresponding alco-hols. The regiochemistry of this reaction is nearly the same as that with LAH. For example the reduc-... 

An alkylmanganese complex has been examined reduction of aryl-substituted epoxides and cy-cloalkene oxides gives the corresponding alkenes in good yield with retention of stereochemistry (equation 43). 

Isomerization. Aryl-substituted epoxides are converted to benzylic aldehydes or ketones on brief exposure to PdfOAcj -BUjP (12 examples, 80-99%). ... 

Cycloaddition, Diels-Alder reactions, epoxidation of aromatic aldehydes, isomerization of aryl-substituted epoxides, and aziridination catalyzed by the iron Lewis acid [(j 5-C5H5)Fe(CO)2(THF)] 03ACA3. 

In Pd(0)-catalyzed isomerization reaction of epoxides, alkyl-substituted epoxides afford methyl ketones (path a, Scheme 11), whereas aryl-substituted epoxides give aldehydes or ketones via cleavage of the benzylic C-O bond (path b) [94]. 

Indium trichloride induces rearrangement of aryl-substituted epoxides to the respective aryl-substituted acetaldehydes via an exclusive hydride shift As phenyl group migration occurs more readily than hydride migration, stilbene oxide is converted to diphenylacetaldehyde (Scheme 8.140) [184]. N-Tosyl aziridines react smoothly with carboxylic acids in the presence of a catalytic amount of indium triflate to afford the corresponding -aminoacetates and benzoates (Scheme 8.141) [185]. Indium trichloride and indium bromide catalyze regio- and diastereoselective azidolysis, bromolysis, and iodolysis of a, -epoxycarboxylates in water (Scheme 8.142) [186]. 

Qi) Under acidic conditions, substitution is favored at the carbon that can better support a positive charge. Aryl substituents stabilize carbocations, making the benzylic position the one that is attacked in an aryl substituted epoxide. 

Regioselective reduction of aryl-substituted epoxides with NaBHj is catalyzed by PdCl2 in the presence of moist alumina (H2O content, 19 wt%) in hexane. The selectivity in reduction of aryl-substituted epoxides was the same as with NaBILj and diborane. In addition, the alumina can be easily recovered and reused without further treatment. 

Deoxygenation of Epoxides. The reagent deoxygenates epoxides in a stereospecific manner, although the reaction is limited to aryl-substituted epoxides (eq 6). A related reagent, [ -Bu3SnAlMe3] Li+, has broader applicability. ... 

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