Styrene epoxidation electronic effects

Kureshy et al. have reported that ruthenium-Schiff base complexes 27 serve as a catalyst for enantioselective epoxidation of styrene derivatives (Scheme 6B.26) [71], An electronic effect similar to that described in the Mn-salen-catalyzed epoxidation (vide supra) is observed in this epoxidation, that is, an electron-donating group on the catalyst and electron-withdrawing group on the substrate lead to higher enantioselectivity. For example, the epoxidation of styrene with 27c shows modest enantioselectivity (38% ee), whereas that of m-nitrostyrene with 27a exhibits much higher enantioselectivity (80% ee). 

During our further studies of ketone catalysts, ketone 16 was found to be highly enantioselective for a number of acyclic and cyclic d.s-olefins (Table 10.6).73-74 It is important to note that the epoxidation is stereospecific with no isomerization observed in the epoxidation of acyclic systems. Ketone 16 also provides encouragingly high ee s for certain terminal olefins, particularly styrenes.74-75 In general, ketones 1 and 16 have complementary substrate scopes. In our subsequent study of the conformational and electronic effects of ketone catalysts on epoxidation, ketone 17, a carbocyclic analog of 16, was found to be highly enantioselective for various styrenes (Table 10.7).76... 

Ru(0)(biqn)(tmtacn)](C10 )2 and [Ru(0)(diopy)(tmtacn)](C10 )2 (biqn=C2 symmetric 1,T-biisoquinoline, diopy=(R,R)-3,3 -(l,2-dimethylethylenedioxy)-2,2 -bipyridine) aremadefrom [RuCl(L)(tmtacn)] + (L=biqn, diopy) and(NH )2[Ce(N03)J with Li(ClO ). Electronic and IR spectra were measured (v(Ru=(0) bands lie at 760 and 795 cm" respectively). The (diopy) complex is paramagnetic with 2.88 B.M. As stoich. [Ru(0)(biqn)(tmtacn)] + and [Ru(0)(diopy)(tmtacn)] VCH3CN they oxidised alkenes (styrene, cis and fran.y-P-methylstyrenes, fran -stilbene, nor-bomene, cyclohexene) to mixtures of aldehydes and epoxides. Conttary to expectation the (diopy) complex did not effect enantioselective epoxidations except with fran -stilbene, for which a moderate e.e. of 33% was observed [623]. 

Ru(CO)(den-por) (den-por=dendritic porphyrins). These are made by condensation of Ru(CO)L (L=5,10,15,20-tetrato(4 -hydroxyphenyl)porphyrin or 5,10,15,20-tetrakis(3, 5 -dihydroxy-phenyl)porphyrin), themselves made from RUjlCO) and (L), with poly-(benzylether) dendrons H, C NMR, electronic and IR spectra of the products were measured. Epoxidation of cyclic alkenes, cis- and tranx-sfilbene, styrene and its 3-methyl, methoxy, chloro and bromo derivatives, dimethylchromene and of cholesteryl esters in high yields and turnovers was effected by RulCOKden-porl/lCl pyNOl/CHjClj [871]. 

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. 

Naruta et al. [225, 226] designed the twin-coronet porphyrin ligands (62) and (63) with binaphthyl derivatives as chiral substituents (Figure 13). Each face of the macrocycle is occupied by two binaphthyl units and the ligand has C2 symmetry. Iron complexes of these compounds can be very effective catalysts in the epoxidation of electron-deficient alkenes. Thus, nitro-substituted styrenes are readily epoxidized in 76-96% ee [226]. The degree of enantioselectivity can be explained on the basis of electronic interactions between the substrate aromatic ring and the chiral substituents rather than on the basis of steric interactions. 

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). 

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