Methoxyselenenylation

The mechanism of the asymmetric methoxyselenenylation of alkenes has been investigated using competition experiments and computational methods (Scheme 8). The experiments have demonstrated that the formation of the intermediate seleniranium ion (48) is reversible. Ions of type (49), generated in the addition of chiral selenium electrophiles to alkenes, are the key intermediates in the asymmetric methoxyselenenylation their stability is strongly dependent on the strength of the selenium-heteroatom interaction. Calculations have been carried out to determine the relative stabilities of the diastereoisomeric seleniranium ions (49). The results obtained from the calculations support the experimental flndings. ... 

Regio- and diastereo-selective methoxyselenenylation of cinnamylamines attached (g) to a chiral perhydrobenzoxazine (17) proceeds in high yields in dichloromethane-methanol. The diastereoselection is dependent on the temperature and the nature of the substituent at C(2) and can be rationalized by accepting a 1,4-asymmetric induction process after coordination of the selenium to the nitrogen atom of the allylamine... 

Methoxyselenenylation of alkenes.1 This reaction can be effected by reaction of 1 with Br, in CH2C12 to form a red solid that is considered to be a selenenyl bromide. This intermediate reacts with an alkene in CH,OH to give a methoxy- denenylated alkene. The reaction with styrene gives only the Markovnikov product in a yield of 49% and 49% de. 

For a better comparison of the different chiral reagents shown in Figure 2, the methoxyselenenylation of (it)-l-phenylpropene leading to addition products 42 is shown as a representative example in Table 1. After oxidative elimination, ether 43 can be obtained the absolute configuration of 43 is given in Table 1 as well (Scheme 7). 

Double differentiation by a binaphthylselenenyl bromide bearing a chirally modified amide group in the methoxyselenenylation of ( )-l-phenyl-1-propene has been reported and substantially enhanced diastereoselectivities were found in some cases (Table 4)33. 

The behavior of the sulfate counter anion is noteworthy. A considerably important advantage in using sulfates is that the reactions can be effected at room temperature. From the data collected in Tables 1 and 2 it can be seen that in the methoxyselenenylation of alkenes the diastereomeric excesses thus obtained are comparable with those observed with other selenenylating agents,... 

The stereospecific conversion of cyclohexene into the corresponding amido selenide 54 is illustrated in Scheme 8. These amidoselenenylation reactions are commonly employed for the preparation of allylic and saturated amides by oxidative or reductive deselenenylation. Propionitrile, butyronitrile, benzonitrile and ethyl cyanoacetate may be used in place of acetonitrile. Styrene gave poor results and other electron-rich olefins such as 1-methylcyclohexene or 2,3-di-methylbut-2-ene did not give the amidoselenenylation products. The reaction can also be effected starting from the hydroxy- or methoxyselenenylation products of alkenes, in the presence of water and trifluoromethanesulfonic acid in this case the nitriles are used in stoichiometric amounts [48c]. This methodology was employed to prepare the amidoselenenylation products of styrene, 55, and of electron-rich olefins. It was necessary, however, to replace the phenyl-... 

Quite recently, Tiecco [46a, 128] reported the asymmetric version of the one-pot conversion of, y-unsaturated esters and nitriles 261 (Scheme 42) into the enantiomerically enriched allylic ethers and alcohols 276 (Scheme 45). The reactions were effected with the selenenyl sulfate produced from the camphor diselenide 26. Unfortunately, in the present case, this diselenide must be employed in stoichiometric amounts. However, it can be partially recovered at the end of the reaction. Good chemical yields and enantiomeric excesses (up to 86%) were obtained in the methoxyselenenylation-elimination reactions. Lower ee was observed when the reactions were run in ethylene glycol or in water. In the case of the hydroxyselenenylations, reaction yields were low because the addition products 275 gave rise to the lactones, which were then deselenenylated to the butenolides. These were isolated in about 30% yield. 

Scheme 44. Catalytic Asymmetric Methoxyselenenylation-Elimination Reactions of )8-Methyl-styrene...

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. 

Asymmetric methoxyselenenylation. A chirally constituted Ar SeBr reagent induces asymmetric addition to double bonds. Chiral allyl ethers are accessible after oxidation and selenoxide elimination. 

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