Alkene epoxidation enantioselectivity

Compounds lb and 2b were the Urst fluorinated ligands tested in Mn-catalyzed alkene epoxidation [5,6]. The biphasic Uquid system perfluorooc-tane/dichloromethane led to excellent activity and enantioselectivity (90% ee) in the epoxidation of indene with oxygen and pivalaldehyde (Scheme 1, Table 1). In addition, the fluorous solution of the catalyst was reused once and showed the same activity and selectivity. This represents a considerable improvement over the behavior in the homogeneous phase, where the used catalyst was bleached and reuse was impossible. Unfortunately, indene was the only suitable substrate for this system, which failed to epoxidize other alkenes (such as styrene or 1,2-dihydronaphthalene) with high enantioselectivity. The system was also strongly dependent on the oxidant and only 71% ee was obtained in the epoxidation of indene with mCPBA at - 50 °C. 

Finally, with the aim of discovering novel chiral oxomolybdenum catalysts able to perform enantioselective alkene epoxidations, Kuhn et al. have reported the exploration of the catalytic behaviour of a series of dioxomolybdenum(VI) complexes with chiral cw-8-phenylthiomenthol ligands derived from ( + )-pulegone. Therefore, the epoxidation of c -p-methylstyrene using t-butyl-hydroperoxide as the oxidant and performed in the presence of ( + )-(2i ,5i )-2-[1-methyl-l-(phenylthio)ethyl]-5-methylcyclohexanone oxime as the ligand, did not produce, however, a significant optical induction in these conditions. 

Song et al. [62] reported poly-salen Co(III) complexes 18, 19 as catalyst for HKR (Figure 5) of terminal alkene epoxides. The polymeric catalysts provided product epoxides with excellent conversion (>49%) and high chiral purity (ee s, 98%) and the catalytic system could be recycled once with retention of activity and enantioselectivity. 

A chiral dichlororuthenium(IV) complex of a Z)4-symmetric porphyrin, [Ru (Z)4-por )(Cl)2], has been prepared by heating [Ru (Z>4-por )(CO)(MeOH)] in CCI4. The complex is characterized by NMR (paramagnetically shifted pyrrolic protons at = 52.3 ppm), FAB-MS, and magnetic susceptibility measurement (/.teff= 3.1/.tB). It is a very active catalyst for enantioselective alkene epoxidations using 2,6-dichloropyridine A-oxide as the terminal oxidant, with a turnover number of up to 2000 the ee of the epoxides is 50-80%. The complex can be incorporated into sol-gel and turnovers of over 10" can be achieved." ... 

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. 

Ordinary alkenes (without an allylic OH group) do not give optically active alcohols by the Sharpless protocol because binding to the catalyst is necessary for enantioselectivity. Simples alkenes can be epoxidized enantioselectively with sodium hypochlorite (NaOCl, commercial bleach) and an optically active manganese-complex catalyst. An important variation of this oxidation uses a manganese-salen complex with various oxidizing agents, in what is called... 

A. M. Daly, M. F. Renehan, D. G. Gilheany, High enantioselectivities in an (E)-alkene epoxidation by catalytically active chromium salen complexes. Insight into the catalytic cycle, Org. Lett. 3 (2001) 663. 

Key Words Platinum complexes. Epoxidation, Pentafluorophenyl, Hydrogen peroxide. Terminal alkenes, Regioselectivity, Enantioselectivity. 2008 Elsevier B.v. 

Lai, T.-S. Kwong, H.-L. Zhang, R. Che, C.-M. Stoichiometric enantioselective alkene epoxidation with a chiral dioxoruthenium(VI) D4-porphyrinato complex. Dalton Trans. 1998, 3559-3564. 

Lai, T.S., R. Zhang, K.K. Cheung, H.L. Kwong, and C.M. Che. Stoichiometric enantioselective alkene epoxidation with a chiral dioxoruthe-nium(VI) D-4-porphyrinato complex. J. Chem. Soc., Dalton Trans. 21, 3559-3564. 

The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 31 in combination with either iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trans-stilbene [130]. In the best result, using PhI(OAc)2 as oxidant, they obtained trans-stilbene oxide in 80% yield and 63% ee. More recently, Beller and coworkers have reexamined this catalytic system and found that asymmetric epoxidations could be performed using ruthenium catalysts 30 and 31 and 30% aqueous hydrogen peroxide [131-133]. A development of the pybox ligand led to ruthenium complex 32, which turned out to be the most efficient catalyst for asymmetric alkene epoxidation. Thus, using 5 mol% of 32 and slow addition of hydrogen peroxide, a number of aryl substituted alkenes were epoxidized in yields up >99% and enantioselectivity up to 84% (Scheme 2.25). 

Application of Ru-porphyrin complexes for catalytic, homogeneous alkene epoxidation is limited because of small turnover numbers, only moderate enantioselectivities in the case of chiral systems, the cost of porphyrin and the 0-atom donor (unless O2), and often limited stability of the porphyrin ring under the oxidizing conditions. In attempts to improve possible plicability, Ru-porphyrins have been heterogenized on various supports °, following methodology developed previously for Ee- and Mn-... 

Silva AR, Budarin V, Clark JH, Castro B de, Freire C. Chiral manganese(IIl) schiff base complexes anchored onto activated carbon as enantioselective heterogeneous catalysts for alkene epoxidation. Carbon 2005 43(10) 2096-2105. 

Finally, for this section is the study of Zhang et al. [33] who entrapped 5,10,15,20-tetralds[(lS,4R,5R,8S)-l,2,3,4,5,6,7,8-octahydro-l,2 5,8-dimethanoan-thrance-9-yl] porphyrin and showed that it exhibits remarkable enantioselective catalytic activity in alkene epoxidation using 2,6-dichloropyridine-A-oxide (Cl2pyNO) as an oxidant, with 69% ee and a very high turnover. 

MCPBA is the reagent most commonly used for alkene epoxidation. Payne oxidation (H O / CUjCN) is a convenient and inexpensive alternative. In the course of a study of the enantioselective enzymatic hydrolysis of 6, Takeshi Sugai of Keio University has described Tetrahedron Lett. 2007, 48, 979) a practical procedure for multigram Payne epoxidation of 5. 

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