Epoxidation aerobic enantioselective

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. 

In connection with the study on aerobic enantioselective epoxidation, /V,A - bis(3-oxobutyl-idene)diaminatomanganese(III) complex (25) is reported to be an effective catalyst for the reaction (Scheme 6B.24) [69]. It is noteworthy that epoxidations using 25 as the catalyst show higher enantioselectivity in the absence of a donor ligand rather than that in the presence of the ligand, which is different from the reaction using Mn-salen catalyst 22 (vide supra). This reaction... 

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. 

Introduction of bulky and chiral substituents at the 5,10,15,20 (meso)-positions of the porphyrin ring allows for aerobic, enantioselective epoxidation of olefins. Use of chiral Fe(III)- and especially Mn(ni)-... 

Subsequent to the development of the (salen)Cr-catalyzed desymmetrization of meso-epoxides with azide (Scheme 7.3), Jacobsen discovered that the analogous (salen)Co(n) complex 6 promoted the enantioselective addition of benzoic acids to meso-epoxides to afford valuable monoprotected C2-symmetric diols (Scheme 7.15) [26], Under the reaction conditions, complex 6 served as a precatalyst for the (salen) Co(iii)-OBz complex, which was fonned in situ by aerobic oxidation. While the enantioselectivity was moderate for certain substrates, the high crystallinity of the products allowed access to enantiopure materials by simple recrystallization. 

The complex Pd-(-)-sparteine was also used as catalyst in an important reaction. Two groups have simultaneously and independently reported a closely related aerobic oxidative kinetic resolution of secondary alcohols. The oxidation of secondary alcohols is one of the most common and well-studied reactions in chemistry. Although excellent catalytic enantioselective methods exist for a variety of oxidation processes, such as epoxidation, dihydroxy-lation, and aziridination, there are relatively few catalytic enantioselective examples of alcohol oxidation. The two research teams were interested in the metal-catalyzed aerobic oxidation of alcohols to aldehydes and ketones and became involved in extending the scopes of these oxidations to asymmetric catalysis. 

Oxidizing enzymes use molecular oxygen as the oxidant, but epoxidation with synthetic metalloporphyrins needs a chemical oxidant, except for one example Groves and Quinn have reported that dioxo-ruthenium porphyrin (19) catalyzes epoxidation using molecular oxygen.69 An asymmetric version of this aerobic epoxidation has been achieved by using complex (7) as the catalyst, albeit with moderate enantioselectivity (Scheme 9).53... 

Beller et al. [85] recently described the aerobic dihydroxylation of olefins catalyzed by osmium at basic pH, as mentioned above. When using the hydroquini-dine and hydroquinine bases, they were able to obtain reasonable enantioselectivities (54% ee to 96% ee) for a range of substrates. An alternative route towards enantiopure diols, is the kinetic resolution of racemic epoxides via enantioselec-tive hydrolysis catalyzed by a Co(III)salen acetate complex, developed by Jacob-... 

The possibility of asymmetric induction under the fluorous biphase conditions was first speculated upon by Horvath and Rabai [10], and this year has seen the first report of asymmetric catalysis in a fluorous biphase [69]. Two, C2 symmetric salen ligands (29a, b) with four C8Fi7 ponytails have been prepared (Scheme 5) and their Mn(II) complexes evaluated as chiral catalysts for the aerobic oxidation of alkenes under FBS-modified Mukaiyama conditions. Both complexes are active catalysts (isolated yields of epoxides up to 85%) under unusually low catalyst loadings (1.5% cf. the usual 12%). Although catalyst recovery and re-use was demonstrated, low enantioselectivities were observed in most cases. 

Stereochemical observations in the epoxidation of cholesterol derivatives mentioned above suggest that the manganese complex participates directly in the oxidation step and the enantioselective aerobic epoxidation should be realized by employing optically active manganese cojnplexes as catalysts. 

As the results of the screening of various aldehydes, pivalaldehyde worked quite effectively for the enantioselective aerobic epoxidation of unfunctionalized olefins such as 1,2-dihydronaphthlene derivatives (Scheme 11). It is noted that i) in the case of epoxidation catalyzed by Salen-type complex, addition of a catalytic amount of A -methylimidazole effectively improved the optical yield of epoxide and ii) bulkiness in ester moiety of P-diketone-type catalyst also influenced the optical yield to higher values. 

The present system was applied to the enantioselective epoxidations of various simple olefines. 1,2-Dihydronaphthalenes which contained no function groups were converted into the corresponding optically active epoxides in good yields with good enantioselectivities (52-72% ee. Entries 1-4). The enantioselective aerobic epoxidation of l,2-benzo-l,3-... 

Related V complexes show activity toward the oxidative decomposition of pinacol with C—C bond cleavage and aerobic oxidation of 4-methoxybenzylalcohol and other lignin model compounds." Other oxidovanadium(V) complexes with c 5-2,6-bis-(methanolate)-piperidine ligands of the type depicted on Scheme 3 were appHed as catalysts to convert prochiral alkenols into 2-(tetrahydrofiiran-2-yl)-2-propanols, 2-(tetrahydropyran-2-yl)-2-propanols, oxepan-3-ols and epoxides, upon oxidative alkenol cyclization with TBHP as oxidant (Scheme 3)." These catalysts are rather stable and possess improved chemoselectivity, e.g., epoxidation of geraniol occurs enantioselectively. It was ruled out the vanadium(V) ieri-butyl peroxy complex formation is a key step to activate peroxides. 

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