Porphyrinato ruthenium complexes

Although the chiral ketoiminatomanganese(lll) complexes were reported to catalyze the asymmetric aerobic alkene epoxidations, an aldehyde such as pivalaldehyde is required as a sacrihcial reducing agent. Groves reported that the dioxo(porphyrinato)ruthenium complexes 31, prepared with m-chloroperoxyben-zoic acid, catalyzed the aerobic epoxidation without any reductant. " On the basis of these reports, Che synthesized the optically active D4-porphyrin 35 and applied it to the truly aerobic enantioselective epoxidation of alkenes catalyzed by the chiral frani-dioxo (D4-porphyrinato)ruthenium(Vl) complex. The dioxoruthenium complex catalyzed the enantioselective aerobic epoxidation of alkenes with moderate to good enantiomeric excess without any reductant. In the toluene solvent, the turnovers for the epoxidation of T-(3-methylstyrene reached 20 and the ee of the epoxide was increased to 73% ee. 

This chapter is essentially a review of those ruthenium complexes which have been used as oxidation catalysts for organic substrates, emphasis being placed on such species which have been chemically well-defined and are effective catalysts. Of all the ruthenium oxidants dealt with here those which have the greatest diversity of use are RuO, [RuO ] , [RuO ], the tetramesityl porphyrinato (TMP) complex fran.y-Ru(0)2(TMP), RuCl3(PPh3)3, and cw-RuCl3(dmso). Many other catalysts are covered, and the uses of two principal starting materials, RuO. nH O and RuClj. nH O as precursors for a number of catalysts, discussed. 

Autoxidadon of Bare ruthenium( II) and osmium(II) porphyrins - A resonance Raman study of the intermediates formed during the reaction of Ru(TPP) (which was obtained according Scheme 1, paths — f, — j, — k) in toluene [258] proved the anticipated [205] reaction scheme of the inner-sphere autoxidation, the first step of which is the formation of a p-peroxobis[porphyrinato-ruthenium(III)] complex which is split into two oxoruthenium (IV) fragments. These species precede the formation of /r-oxobisruthenium(IV) porphyrins (reaction 16) for P = TPP, OEP for P = TMP, a disproportionation (17) is indicated, the resulting Ru(P) itself is further autoxidized. 

Lai, T.S., H.L. Kwong, R. Zhang, and C.M. Che (1998). Aerobic enantioselective alkene epoxidation by a chiral trans-dioxo(D-4-porphyrinato) ruthenium(VI) complex. J. Chem. Soc. Chem. Commun. 15, 1583-1584. 

Metalloporphyrin complexes have been shown to catalyze epoxidation of alkenes (Fig. 12), and in this case metal oxo intermediates have been implicated [19-21]. In most cases strong oxidants are required however, it has recently been reported that a ruthenium trans-dioxo complex catalyses direct epoxidation of an alkene with molecular oxygen as the oxidant [22]. It is believed that porphyrinato metal complex-catalyzed epoxidations are related to alkene epoxidations catalyzed by cytochrome P-A50 or other active enzymatic systems. It has recently been shown that creating site-isolation by anchoring (tetra-phenylporphinato)manganese(lll) acetate to a rigid polymer support considerably enhances the rate of epoxidation of cyclohexene by sodium hypochlorite [23]. 

These bis(amido)ruthenium(IV) and bis(amido)osmium(IV) porphyrin complexes show well-resolved H NMR spectra with the signals of the porphyrinato ligands appearing at normal fields, indicating that they are all diamagnetic. 

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