Epoxides aryl substitution effect

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... 

FIGURE 24. Proposed transition stmcture for epoxidation of an aryl-substituted styrene (4-vinyl-biphenyl, left) and the transition stmcture for epoxidation of 1,3-butadiene (right, calculated at the QC1SD/6-31G level). The theoretical isotope effects were calculated at the MP2/6-31G level using the Bell tunneling correction. For a discussion see Reference 19b. Bond lengths are given in A... 

Alumina also serves as the support for reagents effecting the epoxidation of alkenes efficiently under mild conditions. This is the case with triphenylsilyl hydroperoxide even aryl-substituted alkenes are eas-... 

Baeyer-Villiger oxidation of alkyl- and aryl-substituted C -C, cycloalkanones, steroid ketones and branched chain aliphatic ketones is catalysed by arsonated polystyrene resins [53], Larger size cycloalkanones and linear ketones react much slower. Water miscible and immiscible solvents can be used. With the latter, the resin behaves as an effective catalyst and a phase-transfer agent (triphase catalysis). The same compounds are also epoxidation catalysts. More recently, a method for the preparation of phenols by the oxidation of aromatic aldehydes and ketones has been reported. The most convenient catalysts are nitro-substituted arylseleninic acids and corresponding diselenides [54]. 

Ni(II) complexes of cyclam and oxocyclam derivatives catalyze the epoxidation of cyclohexene and various aryl-substituted alkenes with PhIO and NaOCl as oxidants, respectively. In the epoxidation catalyzed by the Ni(II) cyclam complex using PhIO as a terminal oxidant, the high-valent nickel-oxo complexes (e.g., LNi -0, LNi=0, LNi -0-I-Ph, or LNi -0-Ni L) have been proposed as the active oxidant (92). In the reaction, E olefins are more reactive than the corresponding Z isomers, and a strong correlation was observed between the electron-donating effect of the para substituents in styrene and the initial reaction rate (91). Isotope labeling studies have shown that the epoxide oxygen is derived from PhIO. 

Encouraged by the enantioselectivity obtained with ketone 44a, a series of N-aryl-substituted ketones (examples shown in Scheme 3.12) were investigated to further understand the N-substituent effect on the epoxidation and to search for more practical ketone catalysts [24e,i]. Ketones such as 44b,c can be readily prepared in four steps in large quantities from inexpensive glucose and anilines [241],... 

Scheme 67 Effect of aryl substitution on the spirocyclization of glycal epoxides 267 [140]...

In contrast to SMOs, which exclusively catalyze the formation of (S)-epoxides, CPOs are able to yield chiral (P)-epoxides from styrene derivatives, which could serve as a stereocomplementary method (Scheme 13.11). However, the substrate spectrum appeared to be very limited [85,109]. Two ds-disubstituted aryl-substituted alkenes, czs-a-methylstyrene and 1,2-dihydronaphthalene, were effectively epoxi-dized in the presence of H Oj, and the later underwent spontaneous hydrolytic ring opening to afford the corresponding trans-diol. The enantiomeric excesses of the products were as high as 96% and 97% ee, respectively [83]. Terminal alkenes such as styrene and derivatives with a halide substituent on the benzene ring [109] or an alpha-alkyl substituent [85] could be accepted by CPO, and underwent epoxidation using f-BuOOH or HjOj as the oxidant, but the (P)-epoxides were achieved with only low to medium enantiopurity [83,85,109]. 

Alkyl- and aryl-hydrazones of aldehydes and ketones readily peroxidise in solution and rearrange to azo hydroperoxides [1], some of which are explosively unstable [2], Dry samples of the p-bromo- and p-fluoro-hydroperoxybenzylazobenzenes, prepared by oxygenation of benzene solutions of the phenylhydrazones, exploded while on filter paper in the dark, initiated by vibration of the table or tapping the paper. Samples were later stored moist with benzene at —60°C to prevent explosion [3], A series of a-phenylazo hydroperoxides derived from the phenyl-or p-bromophcnyl-hydrazones of acetone, acetophenone or cyclohexanone, and useful for epoxidation of alkenes, are all explosive [4], The stability of several substituted phenylazo hydroperoxides was found to be strongly controlled by novel substituent effects [5],... 

RB3LYP calculations indicate that the s-cis conformer of peroxy acids is more stable than the s-trans conformer. Calculations on the reaction of prop-2-enol with some peroxy acids showed that trans-transition states collapse to the epoxide via a 1,2-shift, whereas a 1,4-shift is operable for cis-transition states.195 Quantum mechanical calculations have been performed on the migration step of the Baeyer-Villiger rearrangements of some substituted acetophenones with m-chloroperbenzoic acid (m-CPBA). The energy barriers, charge distributions and frontier molecular orbitals, determined for the aryl migration step, have been used to explain the effects of substituents on the reactivity of the ketones.196... 

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