Aldehydes enantioselective epoxidation

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. 

Ru(0)(biqn)(tmtacn)](C10 )2 and [Ru(0)(diopy)(tmtacn)](C10 )2 (biqn=C2 symmetric 1,T-biisoquinoline, diopy=(R,R)-3,3 -(l,2-dimethylethylenedioxy)-2,2 -bipyridine) aremadefrom [RuCl(L)(tmtacn)] + (L=biqn, diopy) and(NH )2[Ce(N03)J with Li(ClO ). Electronic and IR spectra were measured (v(Ru=(0) bands lie at 760 and 795 cm" respectively). The (diopy) complex is paramagnetic with 2.88 B.M. As stoich. [Ru(0)(biqn)(tmtacn)] + and [Ru(0)(diopy)(tmtacn)] VCH3CN they oxidised alkenes (styrene, cis and fran.y-P-methylstyrenes, fran -stilbene, nor-bomene, cyclohexene) to mixtures of aldehydes and epoxides. Conttary to expectation the (diopy) complex did not effect enantioselective epoxidations except with fran -stilbene, for which a moderate e.e. of 33% was observed [623]. 

The enantioselective epoxidation of prochiral alkenes with an aldehyde dioxirane was achieved by Bez and Zhao . ... 

Masakatsu Shibasaki of the University of Tokyo reports (J. Am. Chem. Soc. 2004,126, 7559) that use of a BINOL-derived catalyst with cumyl hydroperoxide enables the enantioselective epoxidation of unsaturated N-acyl pyrroles such as 7. The pyrroles 7, prepared from the precursor aldehydes such as 5 with the reagent 6, can be used directly, without further purification. 

It should be noted that the nucleofugal group in component XII (Scheme 6.83) might also be a cationic sulfonium moiety. In this circumstance the nucleophile attacking the aldehyde is a sulfur ylide. Catalytic enantioselective versions of the transformation of aldehydes to epoxides involving sulfur ylides are covered in Section 6.8. 

The —N=C— bond in imines, especially Schiff bases formed from aromatic aldehydes and amines, can be epoxidized by peracids to form oxaziridines [N(0)C].326 Unlike epoxides, oxaziridines will oxygen-transfer.327 Chiral oxaziridines have been used to carry out enantioselective epoxidations,328 although these compounds are often prepared by non-peroxygen routes.329 Oxaziridines can also be rearranged to oximes or nitrones (Figure 3.85).330... 

Fig. 4.102 Enantioselective epoxidation of phenyl-conjugated olefins employing aldehyde and molecular oxygen as the oxidant.

Chiral bis(oxazolines) 51 with an oxalylic acid backbone were used for the Ru-catalyzed enantioselective epoxidation of tran5-stilbene yielding franx-l,2-diphenyloxirane in up to 69% ee [24]. The asymmetric addition of diethylzinc to several aldehydes has been examined with ferrocene-based oxazoline ligand 52 [25], resulting in optical yields from 78-93% ec. The imide 53 derived from Kemp s triacid containing a chiral oxazoline moiety was used for the asymmetric protonation of prochiral enolates [26]. Starting from racemic cyclopentanone- and cyclohexanone derivatives, the enantioenriched isomers were obtained in 77-98 % ee. 

Oxiranes may also be prepared by the cooxidation of aldehydes and olefins. There are two assumptions as regards the mechanism the oxidation occurs via either an acylperoxy radical or a peracid. The peracid oxidation is stereospecific. Experiments carried out with a view to assessing the radical versus nonradical mechanism indicate that the extent of the radical epoxidation depends on the structure of the olefin and the olefin/aldehyde ratio. Cooxidation in the presence of oxygen was achieved by irradiating the aldehyde and carrying out the reaction with the alkene after a suitable quantity of peracid had been obtained. Enantioselective epoxidation has been described in the reaction of (1-phenyl-alkylidene)malonitriles 63 catalyzed by optically active tertiary amines. ... 

The bulk of oxidations with tert-butyl hydroperoxide consists of epoxidations of alkenes in the presence of transition metals [147, 215, 216, 217, 218]. In this way, a,p-unsaturated aldehydes [219] and ketones [220] are selectively oxidized to epoxides without the involvement of the carbonyl function. Other applications of tert-butyl hydroperoxide such as the oxidation of lactams to imides [225], of tertiary amines to amine oxides [226, 227], of phosphites to phosphates [228], and of sulfides to sulfoxides [224] are rare. In the presence of a chiral compound, enantioselective epoxidations of alcohols are successfully accomplished with moderate to high enantiomeric excesses [221, 222, 223]. 

After reduction of the enal with diisobutylaluminium hydride, the Wittig olefination of D-glycer-aldehyde acetonide (7 )-24 with Ph3P=CHCHO gives the ( )-allylic alcohol 129. The Katsuki-Sharpless enantioselective epoxidation [89] applied to 129 allows the preparation of D-arabinitol (= D-lyxitol) and ribitol, a meso alditol (Scheme 13.47). Similarly, Wittig olefination of R)-2A with Ph3P=CHCH(OEt)2, followed by acidic hydrolysis of the diethyl acetal and subsequent reduction of the enal with diisobutylaluminium hydride, provides the (Z)-allylic alcohol 130. Diastereoselective epoxidation and hydrolysis leads to D-arabinitol or xylitol, another meso alditol [90a]. 

Bez, G., Zhao, C.-G. First highly enantioselective epoxidation of alkenes with aldehyde/Oxone. Tetrahedron Lett. 2003, 44, 7403-7406. 

Chiral sulfides based on camphor (3a-g) have been tested in the catalytic process for the preparation of non-racemic epoxides (Scheme 6) [15]. It was found that high enantioselectivity could be obtained provided that the thioa-cetal was substituted at the 2 position (R, 3b-g). Sterically hindered or electron-withdrawing groups resulted in lower yields in the epoxidation process. The optimum sulfide in terms of yield (73%) and enantioselectivity (93%) was 3b (R= Me) which underwent highly enantioselective epoxidations with a range of both aromatic and aliphatic aldehydes (Scheme 7). Aliphatic aldehydes gave lower yields (paraformaldehyde did not work) compared to aromatic aldehydes and resulted in a mixture of tram- and ds-epoxides whereas aromatic aldehydes only... 

The direct rhodium-catalysed cyclopropanation is limited to relatively electron-rich alkenes, as the metallocarbenoids formed in situ are electrophilic, j garwal and coworkers have developed a strategy for the catalytic enantioselective epoxidation of aldehydes utilising catalytic quantities of enantiomerically pure sulfides such as... 

An enantioselective epoxidation in a fluorous biphasic system was reported in 1998. The fluorous manganese complex 17 was tested for the enantioselective epoxidation of alkenes in the presence of molecular oxygen and sacrificial aldehydes. The characteristic ability of fluorocarbons to dissolve large quantities of... 

Based on enantioselective epoxidation and subsequent ring opening and closing, the so-called Achmatowicz reaction was developed. This is an organocatalytic one-pot cascade for the annulation of a,(J-unsaturated aldehydes, hydrogen peroxide, p-carbonyl compounds and NBS, which furnish optically active 3-pyrones. Other chiral heterocycles were also assembled by organocatalytic cascade reactions using diaiylprolinol silyl ethers as catalysts. ... 

Scheme 20.3 Sulfide-catalyzed enantioselective epoxidation of aldehydes.

Enantioselective Suljur Ylide Catalysis SS3 Table 20.1 Enantioselective epoxidation of aldehydes with sulfide 13. 

Besides the success obtained in the epoxidation of enones by either phase transfer catalysts or polyamino acid derivatives, there was not any example of the related reaction with aldehydes 13. Recently, chiral amine 22 was deployed as a soluble catalyst for the enantioselective epoxidation of a, 3-unsaturated aldehydes 13 to give the expected epoxide 55 (Scheme 4.9). 

The aforementioned epoxidation has been also performed using imidazolidinone 58 in combination with perchloric acid and (nosylimino)iodo benzene as hyperva-lent iodine oxidant. In this manner, several a,P-unsaturated aliphatic aldehydes were epoxidized in high yields and enantioselectivities (72-93%, 87-97% ee) [65], with only enals possessing an electron-withdrawing group being unreactive under these reaction conditions. 

Enantioselective epoxidation of ,/ -unsaturated aldehydes can be achieved with chiral amines as catalysts in combination with hydrogen peroxide [104]. Jorgensen reported a particularly notable example as depicted in Equation 19 [105]. Use of prolinol derivative 106 promotes the enantioselective epoxidation of enals with aqueous (30%) hydrogen peroxide. Under these conditions, epoxide 107 was isolated in 94% ee and dr =96 4. 

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