Ferrocenyl selenides

Allylic animation was also effected in a chiral cinnamyl ferrocenyl selenide (PhCH=CHCH2Fc ), with formation of the chiral amine PhCH(NHTs)CH=CH2 in up to 87% ee [50],... 

Asymmetric [2,3]sigmatropic rearrangements can proceed via optically active selenoxides. It has been shown that the Davis oxidant 158 can be used for the oxidation of selenides such as 172. The reaction product, after oxidation and rearrangement, is the allylic alcohol 173 formed with 35% ee (Scheme 50).279,282 Also Sharpless conditions (Ti(/ -PrO)4, (+)-DIPT, /-BuOOH) have been applied to this reaction and the product has been obtained in 69% ee. When, however, the phenyl selenide moiety in 172 is replaced with an or/ < -nitrophenyl selenide, the selectivity is increased to 92% ee in the allylic alcohol 173 using Sharpless conditions.296 Other selenides such as 2 -pyridyl or ferrocenyl selenides gave much lower selectivities. 

Uemura and co-workers have succeeded in diastereoselective imidation of optically active allylic ferrocenyl selenides to give the corresponding optically active allylic amines [40]. Thus, the treatment of optically active cinnamyl ferrocenyl selenides with TsN = IPh or TsNClNa in dichloromethane at 0°C afforded the allylic amines in moderate yields with high enantiomeric excesses (Scheme 26). 

Table 8. Diastereoselective imidation of optically active allylic ferrocenyl selenides...

Nishibayashi and Uemura attempted the diastereoselective [2,3]sigmatropic rearrangement via a chiral selenium ylide bearing an optically active ferrocenyl-selenium moiety, but the selectivity was low (up to 25% de) [54]. The treatment of an allylic chiral ferrocenyl selenide with ethyl diazoacetate in the presence of a catalytic amount of Rh2(OAc)4 afforded a diastereomeric mixture (around 2 1) of ethyl 2-ferrocenylseleno-3-phenylpent-4-enoates (III) with low diastereo-selectivity (Scheme 35). 

Similar reactions were also achieved by the formation of diastereomeric optically active selenoxides as intermediates in the elimination reaction. Optically active ferrocenyl diselenide 19 was used in selenenylations of alkynes generating vinyl selenides of type 164. Oxidation of the selenides was performed with mCPBA under various reaction conditions which afforded the corresponding chiral selenoxides, which, after elimination, afforded axial chiral allenecarboxylic ester derivatives 165 in high enantioselectivities (R = Me 89% ee, R=Et 82% ee, R = C3H7 85% ee) (Scheme 47)>85 87... 

Treatment of allylic selenides, bearing a chiral ferrocenyl moiety at selenium, with 3-chloroper-benzoic acid afforded allylic alcohols in 28-82% chemical yields with 64-89% ee40. 

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 high degree of diastereoselection in the oxidation of chiral ferrocenyl vinyl selenides was supported by the oxidation of the chiral p-tolyl ferrocenyl sulfide, instead of the selenide, with MCPBA at low temperature. Thus, an almost complete diastereoselective oxidation occurred either at - 20 °C or - 78 °C to give the corresponding chiral sulfoxide (Scheme 11). On the other hand, the oxidation of p-tolyl 1 -(M,M-dimethylamino)ethylphenyl sulfide with MCPBA afforded... 

A shielding of the phosphorus atom and an increase of the PSe coupling from 746 to 765 Hz have been observed by Jakob et al upon progressive replacement of phenyl by ferrocenylethynyl in phosphane selenides (FcC=Q Ph3 P = Se (n=l-3 and Fc = ferrocenyl = ri -C5H4)(ri -C5H5)Fe). 

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