Several remarkable selectivities in competitive epoxidations using a ruthenium porphyrin / 2,6-disubstituted pyridine N-oxide system were observed. The pro fp sal that the active intermediate of this system differed from the trarrsdioxo complex of ruthenium porp yrin was indicated.
Much research has been conducted on the reactivities and the mechanisms of metalloporphyrin catalyzed oxidation systems, to elucidate the biological reaction mechanisms of cytochromes. or to develop synthetically useful catalytic oxidation systemsl).
We recently described the highly efficient epoxidation of oletins with 2ddisubstituted pyridine Noxides catalyzed by ruthenium porphyrin complexesz). This system offered some advantages for use as a synthetic reagent such as high chemo-selectivity, mild conditions, and simple handling procedures; therefore, we investigated whether our system showed several remarkable selcctivities which are known in epoxidations catalyzed by porphyrins having bulky substituents on their rings3)v4).
Table 1 presents results for the competitive epoxidation of cis-and rruns-olefins using this system. A mixture of cis-and trans-stilbene (170 mM, respectively) was reacted with 2,64ttidine N-oxide (170 mM) in the presence of a catalytic amount of RuTMP(O)z [dioxo(tetramesitylporphyrinato)ruthenium(VI)]5) (1 mM) to afford cis-stilbene oxide in an 87% yield based on cis-stilhcne used, but only 1% of franr-stilbene was converted into its epoxide6). The competitive epoxidation of cis-and lrun@methylstyrene under the same conditions proceeded with the similar results. The reaction of frans,cis.trans-1.5,9-cyclododecatriene with this system also showed that cis-olefin was more highly reactive than tram ones.
Some terpenes in their acetate form were epoxidized using this system to give the results summarized in Table 2. In all cases, 6,7double bonds were epoxidized selectively. The steric and electronic effects of acetoxy groups may cause the high-selective epoxidation.
RuTMP(O)z was recently revealed to catalyze the epoxidation of oletins with molecular oxygen by Groves et a1.7) In their system, RuTMP(O)z itself has been regarded as the active oxidant for olefins. While the turnover number of above aerobic epoxidation is about 50, in our system using 2,6disubstituted pyridine N-oxides as oxidants, ruthenium porphyrins work very efficiently as catalysts for a turnover reaching 17000. The different reactivity between both systems suggests that another active intermediate exists during epoxidation in our syqtem.