Styrenes asymmetric epoxidation

Konishi et al.97 synthesized porphyrin compound 127. As shown in Scheme 4-44, asymmetric epoxidation of prochiral olefins such as styrene derivatives and vinyl naphthalene by iodosobenzene has been achieved by using this porphyrin complex as the catalyst in the presence of imidazole. The optically active epoxides were obtained with moderate ee. 

Better results for the porphyrin complex-catalyzed asymmetric epoxidation of prochiral olefins were achieved by Naruta et al.98 using iron complexes of chiral binaphthalene or bitetralin-linked porphyrin 128 as chiral catalysts. As shown in Scheme 4-45, asymmetric epoxidation of styrene or its analogs provided the product with good ee. Even better results were obtained with substrates bearing electron-withdrawing substituents. 

Scheme 4.12 Polymer-bound salen-complexes in asymmetric epoxidations of styrenes.

The earliest work, by Kureshy et al. used complexes of chiral Schiff bases, e.g. Ru(PPh3)(Hp) (SB= 0 (Fig. 1.41) Ru(PPh3)(H30) (SB = )/Phl0/CH3Cy4°C catalysed the asymmetric epoxidation of styrene in the dark under an inert atmosphere. Yields of epoxide were however relatively low [95], but later systems of this... 

In the present work, the Jacobsen s catalyst was immobilized inside highly dealuminated zeolites X and Y, containing mesopores completely surrounded by micropores, and in Al-MCM-41 via ion exchange. Moreover, the complex was immobilized on modified silica MCM-41 via the metal center and through the salen ligand, respectively. cis-Ethyl cinnamate, (-)-a-pinene, styrene, and 1,2-dihydronaphtalene were used as test molecules for asymmetric epoxidation with NaOCl, m-CPBA (m-chloroperoxybenzoic acid), and dimethyldioxirane (DMD) generated in situ as the oxygen sources. 

SCHEME 31. Asymmetric epoxidation of styrenes using iodosylbenzenes. 

The asymmetric epoxidation of an allylic alcohol in which the carbinol has been replaced by a silanol has been described [144]. As shown in Eq. 6A.10 [144], (3 )-phenylethenyldi-methylsilanol is converted to an epoxysilanol in 50% yield with 85-95% ee. Note that here the longer Si-C bonds appear to overcome the restriction to epoxidation associated with a fully substituted C-l atom in the allylic alcohol series. Fluoride cleavage of the silanol group gives (S)-styrene oxide. 

In the wake of this report, many chiral iron(III)- and Mn(III)-porphyrin complexes have been synthesized and applied to the epoxidation of styrene derivatives [20]. Because these asymmetric epoxidations are discussed in the first edition of this book [21], the discussion on metalloporphyrin-catalyzed epoxidation here is limited to some recent examples. Most chiral metallopor-phyrins bear chiral auxi Maries such as the one derived from a-amino acid or binapthol. Differing from these complexes is complex 6, which has no chiral auxiliary but is endowed with facial chirality by introducing a strap and has been reported by Inoue et al. [20f]. Epoxidation of styrene by using only 6 as the catalyst shows low enantioselectivity, but the selectivity is remarkably enhanced when the reaction is performed in the presence of imidazole (Scheme 6B.11). This result can be explained by assuming that imidazole coordinates to the unhindered face of the complex and the reaction occur on the strapped face [20f. ... 

In the course of their exploration of structure-activity relationships for ketone catalysts, Denmark et al. noted that oxoammonium salts such as 29-33 are very efficient catalysts of the epoxidation of olefins [34a]. Unfortunately, enantiomeric excesses achieved with this class of ketone catalyst have not yet exceeded 40% (30, epoxidation of tram-fl-rn eth yI styrene . With the fhiorinated oxoammonium catalyst 23 already mentioned, however, 58% ee was achieved in the asymmetric epoxidation of trans-stilbene [34b]. 

Some examples of asymmetric epoxidations of alkenes using chiral ruthenium porphyrins have been reported for example, the previously reported D4-sym-metrical chiral ruthenium porphyrin complex Run(D4-Por )(CO)(MeOH) [58], which produced (R)-styrene oxide in 57% ee with Cl2PyNO as a donor, was readily converted into the dichloro derivative A [59] (Fig. 11). This di-chlororuthenium porphyrin gave (R)-styrene oxide in 69% ee using Cl2PyNO and was highly active (875 TON in 1.5 h). The use of unsubstituted pyridine N-oxide or NMO as oxidants resulted in low substrate conversions as well as... 

Chloroperoxidase Enantioselective oxidation of sulfides Enantioselective oxidation of racemic epoxyalcohols Oxidation of benzyl alcohol Epoxidation of styrene Asymmetric oxidations Halogenation reactions [11, 15,77] [15, 48] [14] [78] [79] [80]... 

The first example of a catalytic enantioselective epoxidation by cyclohexanone monooxygenase was shown with a fosfomycin-related model compound [75]. The efficient asymmetric epoxidation of styrene to (S)-styrene oxide by recombinant styrene monooxygenase has been achieved by increasing biocatalyst concentrations and reducing the exposure time of the biocatalyst to the product [76]. 

Aqua(phosphine)ruthenium(II) complexes [121] are useful for activation of molecular oxygen, and catalytic oxidation of cyclohexene can be carried out with 1 atm of O2 [121a,bj. The ruthenium catalyst bearing perfluorinated 1,3-diketone ligands catalyzes the aerobic epoxidation of alkenes in a perfluorinated solvent in the presence of i-PrCHO [122]. Asymmetric epoxidations of styrene and stilbene proceed with 56-80% e.e. with ruthenium complexes 38-40 (Figure 3.2) and oxidants such as PhI(OAc)2, PhIO, 2,6-dichloropyridine N-oxide, and molecular oxygen [123-125]. 

Jacobsen reported in 1990 that Mnm complexes of chiral salen ligands (41) were the most efficient catalysts available for the enantioselective epoxidation of alkyl- and aryl-substituted olefins.118 This stimulated a rapid development in the chemistry and applications of chiral SB complexes, which offer promising catalytic applications to several organic reactions, such as enantioselective cyclopropanation of styrenes, asymmetric aziridination of olefins, asymmetric Diels-Alder cycloaddition, and enantioselective ring opening of epoxides.4,119... 

Complete conversions and good enantiomeric excesses (64-100%) were achieved in the asymmetric epoxidation of chromenes and indene using UHP as oxidant and a novel dimeric homochiral Mn(III) Schiff base as catalyst. The reactions were carried out in the presence of carboxylate salts and nitrogen and oxygen coordinating co-catalysts. However, the epoxidation of styrene unfortunately proceeded with incomplete conversion and only 23% ee. Modification of the catalyst and use of pyridine 7V-oxide as cocatalyst allowed improvement of the ee to 61% (Scheme 18). ... 

Finally, Beller and coworkers introduced a new approach towards the optimisation of activity and selectivity in Ru-catalyzed asymmetric epoxidations. Their catalysts contained pyridine dicarboxylate (pydic) as the second ligand. Although excellent results (e.g., 85% yield and 59% ee for styrene) were obtained using 5 mol% of catalyst, the authors reported that a similar level of efficiency could be achieved with only 0.5 mol% of catalyst (Scheme 21)." ... 

S. Bhattcharjee, J. A. Anderson, Novel chiral sulphonato-salen-manganese(lll)-pillared hydro-talcite catalysts for the asymmetric epoxidation of styrenes an cyclic alkenes, Adv. Synth. Catal. 348 (2006) 151. 

Axial bonding of manganese-salen complexes to polystyrene containing phen-oxide or sulfonate groups yielded catalysts for the asymmetric epoxidation of styrene and styrene derivatives with NaClO/PPNO [35]. Catalyst performances were similar to the corresponding nonsupported systems and epoxide yields reached 93% with 70% ee using 1-phenylcyclohexene as substrate. Catalyst recyclability was investigated and the catalysts could be used up to three times in the epoxidation of ot-methylstyrene. 

Since 2000 a few catalysts for asymmetric epoxidation based on polymer-anchored chiral l,l -bi-2-naphthol (BINOL) have been developed. Polystyrene-supported BINOL was prepared by radical copolymerization of styrene with BINOL, bearing 4-vinylbenzyloxy groups in the 3- or 6-position [96]. Immobilization of lanthanum or ytterbium was accomplished by treatment of the polymers... 

C. Borriello, R. Del Litto, A. Panunzi, P. Ruffo, A supported Mn(III) catalyst based on D-glucose in the asymmetric epoxidation of styrenes, Inorg. Ghem. Gommun. 8 (2005) 717. 

Asymmetric epoxidation of unfunctionalized aUcenes catalyzed by chiral Mn(III)(salen) complexes has proven to be a useful solution-phase reaction [88]. To simplify product isolation and to avoid degradation of the Mn(salen) complex through formation of i-oxo-manganese(lV) dimers by spatial redistribution, the polymer-supported catalyst 112 was prepared by co-polymerization of complex 113, styrene 58, and divinylbenzene as a cross-linker (Scheme 20) [89]. As a stoichiometric oxidant, a combination of meta-chlor-operbenzoic acid (mCPBA) and N-methyl-morpholine N-oxide (NMO) in acetonitrile was used. Yields and rates of conversion were satisfactory for the epoxidation of styrene 58 and of methyl styrene, but only low enantioselectivities were obtained. Nevertheless, the catalyst retained its efficiency in terms of yields and enantioselectivities after repetitive use. Similar results have been described by other researchers [90]. 

Although a large number of chiral porphyrin catalysts has been developed for study in asymmetric epoxidation chemistry [12], to date only one example has been applied to aziridination. In that instance, Lai and coworkers reported the use of the Halterman porphyrin catalyst in the aziridination of styrene derivatives with moderate enantioselectivity (Scheme 5) [13]. 

---