Styrene epoxidation isotope effects

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... 

The relative position of the transition state along the reaction coordinate was evaluated directly by examining secondary kinetic isotope effects as a function of the electronic character of the catalyst. The relative rates of epoxidation of styrene and 3-deuteriostyrene were examined in competition experiments using... 

Mn(salen) catalysts 23a-e [70]. On transformation to the radical intermediate, the p-carbon of styrene undergoes a formal rehybridization from sp to sp which, in principle, should lead to the observation of an inverse secondary isotope effect (kY[/kj)transition state, whereby later transition states in which the p-carbon has more sp character should exhibit smaller values of A direct correlation between k lkj) and Gp was observed, indicating that the electronic character of the catalyst does indeed alter the degree of rehybridization at the p-carbon and thus the position of the transition state leading to formation of the radical intermediate (Fig. 8b). 

When imidazole or methanol was introduced to the dichloromethane solution of 36, instantaneous decomposition of 36 and the epoxide formation were observed. Thus, in the presence of either methanol or imidazole, the rate-determining step in the reaction of 14 and olefin was changed to the formation of 36. Under these conditions, secondary deuterium kinetic isotope effects on epoxi-dation were examined by a- and /3-deuterio-p-chlorostyrenes. For both the a- and the /3-positions of styrene, kn/fco = 1 was observed. The isotope effect and substituent effect on the formation of 36 suggest that both the a- and /8-carbons remain planar (sp hybridized) at the transition state and that a positive charge forms on the a-carbon. Accordingly, the formation of an olefin cation radical by an electron transfer from the olefin to 14 is indicated in the formation of 36 (Scheme XX). 

A detailed kinetic study using UV-vis, FTIR (Fourier-transform infrared), and NMR spectroscopy a Hammett plot with /0 = -h1.98 using para-substituted styrene oxides an inverse solvent kinetic isotope effect (KIE) (fcnHp/ HHP-d2) = 0.86 and nonlinear effects studies" have all shown that the (salen)Co- (J) and amidine-co-catalysed enantioselective ring opening of terminal- and mew-epoxides by fluoride ion (forming trans- -ttuoTO alcohols in a 42-89% yield with 84-99% ee) occurs by the mechanism shown in Scheme 5. /-BuOOH oxidizes the Co(H) to Co(III) in the (salen)Co(III) catalyst. PhCOF provides the fluorine for the reaction. 

I-Ph, or LNiIH-0-NiIUL) 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. 

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