Diferrocenyl diselenide

Scheme 4.59 Synthesis of chiral allenecarboxylates 233 using chiral diferrocenyl diselenides 231.

The optical yield and stereoselectivity of the reaction were determined unambiguously by transforming the ring-opened products 68 into 2-cyclo-hexenyl benzoate (69). The obtained enantiomeric excess was up to 50% [90]. A similar protocol was employed by Nishibayashi et al., who synthesized a chiral diferrocenyl diselenide 70 and applied it to asymmetric ring opening of several meso-epoxides [91]. 

Asymmetric selenoxide elimination via a diastereomeric chiral selenoxide as a key intermediate was also achieved by Uemura and co-workers [23]. They prepared new optically active diferrocenyl diselenides from the corresponding chiral ferrocenes and used the optically active ferrocenyl group as a chiral aryl moiety since an arylselenium moiety can easily be introduced into organic molecules. 

The oxidation of the chiral ferrocenyl vinyl selenides, prepared from the optically active diferrocenyl diselenides and ethyl propiolate derivatives, with 1 molar equivalent of MCPBA under various conditions afforded the corresponding chiral selenoxides. The chiral selenoxides suffered in situ selenoxide elimination to afford the axially chiral allenecarboxyUc esters in moderate chemical yields with high enantioselectivities (Scheme 10). Typical results are shown in Table 5. The reaction temperature had a remarkable effect upon stereoselectivity and the lower temperature gave better results. The addition of molecular sieves (4 A) to the reaction system improved the stereoselectivity. Dichlo-romethane was revealed to be the solvent of choice. In other words, reaction conditions to suppress the racemization of a diastereomeric selenoxide intermediate were required. Asymmetric selenoxide elimination provides a new method for the preparation of the chiral allenecarboxyUc esters which have so far been prepared by optical resolution of the corresponding racemic acids. 

The chiral ferrocenylselenium reagents were found to act as the effective ligands for Rh(I)-catalyzed asymmetric hydrosilylation and transfer hydrogenation (see Sects. 2.1 and 2.2, respectively). Fukuzawa and Tsudzuki have found that the chiral ferrocenylselenium-based amino alcohols (DASF), prepared by treatment of the chiral diferrocenyl diselenide with NaBH4 in ethanol followed by the addition of epoxides (Scheme 11), efficiently catalyzed the diethylzinc addition to aldehydes to provide the corresponding secondary alcohols with up to 99% ee... 

The highest diastereoselectivity of asymmetric methoxyselenenylation of alkenes was achieved using the ferrocenylselenium triflates in excellent chemical yields [5gj. For example, the stoichiometric reaction of the chiral ferrocenylselenium triflate 41, prepared from the chiral diferrocenyl diselenide 2, with traus- -methylstyrene afforded the corresponding methoxyselenenylated product in high chemical yield with excellent diastereoselectivity (up to 98 % de). Fukuzawa and co-workers employed the diferrocenyl diselenide 2 for the catalytic asymmetric oxidation of, y-unsaturated esters and traus- -methylstyrene to the corresponding optically active allylic methyl ethers with moderate enantio-selectivity (Scheme 24) [27]. The allylic ethers were produced from 4-phenyl-3-butenoic acid esters in 70-78% yield with 17-22% ee. 

In 1997, Fukuzawa et al. reported the conversion of j8,y-unsaturated esters 472 into the optically active y-aUcoxy a, j3-nnsaturated esters 474 using catalytic amounts of the chiral diferrocenyl diselenide 473 in the presence of ammonium persulfate. A selected example is presented in Scheme 7.74 [316]. 

Scheme 7.74 Conversion of /J,y-unsaturated ester 472 into the optically active y-alkoxy a,ji-unsaturated ester 474 catalyzed by chiral diferrocenyl diselenides catalyst 473...

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