Ferrocenyl diselenide

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... 

Selenoalkoxylation and -lactonizauo and 8, the ferrocenyl diselenide 9 . i processes. The diselenides are activated... 

Selenoalkoxylation and -lactonization using chiral areneselenenyl derivatives such as 7 and 8, the ferrocenyl diselenide 9 , and the camphor-based species (10) are efficient processes. The diselenides are activated by Br to form reactive selenenyl bromides. 

The first stereoselective selenenylation reactions were reported using the binaphthyl-based diselenide 16 as the reagent precursor.71-76 After that, several other diselenides have been prepared initial attempts used quite long syntheses to obtain these compounds, for example, the C2 symmetrical diselenides 1777-79 and 2380 and the mannose-derived diselenide 18.81-84 The ferrocenyl-based diselenide 1985-89 is using expensive starting materials and in 1995 more simple and easy accessible diselenides like 20 and 21 have been described in literature.90-93 Camphor-based diselenides such as 2294-98 have been prepared and, based on the success of diselenides 20 and 21, structural variants 2499,100 and diselenides with different coordinating heteroatoms such as sulfur and selenium 25101-108 and 26109 have been described recently. 

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. 

---