Styrene epoxidation transition states

Styrenes [103], conjugated aT-dienes [107], and aT-enynes [108] are also epoxidized with ketones 57 in high ees (Table 5, entries 9-14). No isomerization of the epoxides was observed therefore only c/x-epoxides were obtained from cis-olefins. Alkenes and alkynes appear to be effective directing groups to favor the desired transition states T and V (Fig. 19). 

FIGURE 10.7 Transition states for the epoxidation of styrenes with ketone 16. 

Similar to di-olefins, the epoxidation of styrenes with ketone 16 proceeds mainly through transition state spiro G. One difference for styrenes is that planar transition state I could also be competing (R = H) in addition to spiro H, whereas the corresponding planar I for d.v-olefins would be less competitive because of the steric effect (Figure 10.7). As a result, the enantioselectivities obtained for di-olefins with 16 are usually higher than styrenes.74-75 Reducing the competition from planar I would be important to further improve the enantioselectivity for terminal olefins. 

Embedded quantum/classical calculations were used to study the epoxidation reaction of styrene catalyzed by Mn-porphyrins. Optimized geometries were obtained for the Mn-porphyrin, reaction intermediates, and transition state structures along the proposed reaction path. A polarizable continuum model (PCM) was used to study solvent effects, with dichloromethane as the solvent. While it has been shown previously that the concerted intermediate between the oxidized porphyrin and the alkene is the lowest energy configuration, a transition state to directly form the concerted intermediate without the prior formation of a radical could not be found. A stepwise mechanism, in which a radical intermediate is formed before the concerted intermediate, is proposed. 

The present study builds on the previous DFT and QM/MM results, where stable intermediates were identified [30], to explicitly map the reaction coordinates of individual steps in the catalytic cycle. Transition state structures are identified and characterized, and energy barriers are calculated. The effect of the solvent, dichloromethane, on the energy barriers is also analyzed. Based on the transition state structures identified, a plausible mechanism for styrene epoxidation by Mn-porphyrins is proposed. 

FIGURE 19.4 Mechanism of styrene epoxidation with iodosylbenzene catalyzed by Mn-porphyrins proposed based on transition state structures identified. 

FIGURE 19.6 Electronic energy as a function of the reaction coordinate for the epoxidation of styrene using a Mn-porphyrin catalyst. Numbering of the transition states (TS) corresponds to the steps in Fig. 19.4. 

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). 

Though the detailed mechanism of olefin epoxidation is still controversial, Scheme 8 depicts possible intermediates, metallacycle (a), K-cation radical (b), carbocation (c), carbon radical (d), and concerted oxygen insertion (e) [2, 216, 217]. As discussed above, the intermediacy of metallacycle has been questioned. One of the most attractive mechanism shown in Scheme 8 is the involvement of one electron transfer process to form the olefin 7C-cation radicals (b). Observation of rearranged products of alkenes, known to form through the intermediacy of the alkene cation radicals, in the course of oxidation catalyzed by iron porphyrin complexes is consistent with this mechanism [218, 219]. A -alkylation during the epoxidation of terminal olefins is also well explained by the transient formation of olefin cation radical [220]. A Hammett p value of -0.93 was reported in the epoxidation of substitute styrene by Fe (TPP)Cl/PhIO system, suggesting a polar transition state required for cation radical formation [221] Very recently, Mirafzal et al. have applied cation radical probes as shown in Scheme 9 to... 

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