Epoxidation, phenyl-substituted

Regioselective reduction of terminal epoxides.2 By appropriate choice of i-Bu2 AIH or 1-BU3AI, terminal epoxides can be reduced with marked regioselcctivity to the primary or the secondary alcohol. Alkyl-substituted epoxides are reduced by f-Bu2AIH mainly to a secondary alcohol, whereas use of i-BujAl favors rcduclion to the primary alcohol. This regioselcctivity is reversed in the case silyl-substitutcd epoxides. Phenyl-substituted epoxides are reduced by i-Bu3AI mainly to primary alcohols. Reduction of these epoxides to secondary alcohols is best effected with LiAIH4 in THF. 

Our syntheses of these new materials showed that both hydrogenation and epoxidation occurred exclusively from the exo face of both VN and < >VX. This is consistent with extensive literature precedent for a variety of reactions on norbornenyl double bonds (9). H1 NMR spectroscopy of these materials also revealed an interesting pattern. All phenyl substituted compounds with endo imide rings (< >VN, HYVN, VNE) evidenced signals due to one of the ten aromatic protons (five on each phenyl ring) being shifted to unusually high field (6.4 +. 16). This is consistent with a field effect on a proton of one of... 

In conclusion, the ability to abstract the -proton is correlated to its degree of substitution. However, substituents other than simple aUcyl groups may modify this facility of proton removal and influence the course of the -elimination. As anticipated, the presence on the adjacent carbon of the epoxide of an ally lie or benzylic group results in a dramatic enhancement of the -elimination rate and modifies its regioselectivity . As shown in Scheme 19, the rearrangement of the phenyl-substituted epoxide 34 is complete within a few minutes, whereas several hours are required in the case of the corresponding alkyl-substituted epoxide 35. 

Compared to metal-catalyzed asymmetric epoxidation reactions, asymmetric versions of this reaction without the need of a catalyst (apart from a base) are rarely known. In 2000 Adam and coworkers reported a method for the asymmetric Weitz-Scheffer epoxidation of substituted enones 91 by the secondary, optically active hydroperoxide (5 )-(l-phenyl)ethyl hydroperoxide (equation 27, Table 10). ... 

TABLE 10. Enantioselective Weitz-Scheffer epoxidation of substituted enones p-XC6H4C(0)CH= CR with (5)-(l-phenyl)ethyl hydroperoxide... 

Table 17) with two substituents in position C3 the oxygen transfer by the chiral hydroperoxides occurred from the same enantioface of the double bond, while epoxidation of the (ii)-phenyl-substituted substrates 142c,g,i resulted in the formation of the opposite epoxide enantiomer in excess. In 2000 Hamann and coworkers reported a new saturated protected carbohydrate hydroperoxide 69b , which showed high asymmetric induction in the vanadium-catalyzed epoxidation reaction of 3-methyl-2-buten-l-ol. The ee of 90% obtained was a milestone in the field of stereoselective oxygen transfer with optically active hydroperoxides. Unfortunately, the tertiary allylic alcohol 2-methyl-3-buten-2-ol was epoxidized with low enantioselectivity (ee 18%) with the same catalytic system . 

Epoxides also participate in the Ritter reaction with nitriles. An investigation of the ring opening of several alkyl-substituted glycidic esters and amides 181 showed that this transformation occurs with inversion and is completely regiospecific. ° Esters appeared to be somewhat more reactive than amides. However, phenyl-substituted glycidic esters and amides 184 are almost totally nonstereoselective. In addition, the oxazolines 186 are isolated in low yield due to the propensity of intermediate 185 to generate an aldehyde byproduct 187 (Scheme 8.53). 

Among the phenyl-substituted ethylenos convertible into epoxides l>y treatment with pwrbetixoie arid are styrene, [Pg.347]

The polymer-bound Mn and Cr complexes were used as catalysts for epoxidations of six phenyl-substituted olefins with m-CPBA/NMO and for dihydropyranone formation from the Danishefsky diene and aldehydes. There are several remarkable features of the novel immobilized salens ... 

Analysis of the second-order rate constants for various methyl and phenyl substituted cyclopropenes shows effects similar to those observed with alkenes undergoing epoxidation, again in agreement with the intermediacy of the oxabicyclo[1.1.0]butane in cyclopropene oxidation 263 ... 

It was found that the ring opening of phenyl-substituted epoxides, as well as of aliphatic epoxides, can be achieved in high yield in a regio- and stereoselective manner with a mixture of 1 3 of diisopropylamine and hydrogen fluoride (diisopropylamine trishydrofluoride). d.s-... 

Table 16. Ring Opening of Phenyl-Substituted and Cyelie Aliphatic Epoxides with Alkylaminc Hydrofluorides ...

A summary of bimolecular rate constants for the acid-catalyzed hydrolysis of a series of alkyl-, vinyl- and phenyl-substituted epoxides is given in Table 1. Propylene oxide (7) is 6.6 times more reactive than ethylene oxide, and from a study of its reaction in H2018, it was shown that 70% of the glycol product results from addition of solvent to the secondary carbon and 30% from addition of solvent to the primary carbon. The reactivity per primary carbon of ethylene oxide is one-half of the observed reactivity of ethylene oxide, and thus the introduction of a methyl group on ethylene oxide results in an increase in reactivity at the primary carbon by a factor of 4 and an increase in reactivity at the secondary carbon by a factor of 9. These results are consistent with A-2 mechanisms for the acid-catalyzed hydrolyses of ethylene oxide and propylene oxide, in which some amount of positive charge generated on carbon at the transition state is stabilized by a methyl group. 

The acid-catalyzed hydrolysis of p-phenyl-substituted cyclohexene oxides 49a-d yields diols resulting from cis and "trans addition of water to the epoxide group (Scheme 14). It was initially reported that the cis/trans diol ratio correlates well with the electronic effect of the / -substituent, and varied from 7.5 93.5 for p-nitro-sub-stituted oxide 49d to 95.3 4.7 for p-methoxy-substituted epoxide 49a.56 58 Later work established that methoxy-substituted diols 50a and 51a underwent isomerization under the conditions of acid-catalyzed epoxide hydrolysis, and that the cis/ trans diol ratios for hydrolysis of 49a-c are quite similar.59... 

W. Adam, K. J. Roschmann, C. R. Saha-Moller, A novel counterion effect on the diastereoselectivity in the Mn (salen)-catalyzed epoxidation of phenyl-substituted cis-alkenes, Eur. J. Org. Chem. (2000) 3519. 

Dendritic and nondendritic polystyrene-boimd manganese-salen complexes were described by Seebach and coworkers [30]. The supported catalysts were prepared by suspension copolymerization of styrene with the vinyl-substituted complexes and employed in the epoxidation of phenyl-substituted alkenes by m-CPBA/NMO. Activities and selectivities were similar to those obtained with the monomeric complexes. High catalyst stabilities were observed and it was demonstrated that the immobilized catalysts can be recycled up to 10 times without loss of performance. Laser ablation inductively coupled plasma mass spectrometry was used to monitor the manganese content in repeatedly used polystyrene beads and a correlation between metal leaching from the support and catalytic activity was disclosed [31]. 

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