Asymmetric Oxyselenenylation Reactions

It can be observed that in several cases the addition reaction proceeds with excellent diastereomeric excesses. Although the reaction temperature, the solvent and the nature of the counter anion are different from case to case, nevertheless the data reported in the table can be used to have preliminary indications about the ability of the various electrophilic reagents to transfer the chirality to the newly generated stereocenters. The information gained from these experiments, however, must be used with caution since the stereoselectivity is also a function of the alkene employed and examples are known in which a reagent which gives unsatisfactory results with styrene can, in contrast, be efficient with other alkenes. This is clearly evident from the data reported in Table 2 in which the diastereomeric excesses measured for the reactions of the electrophilic reagents derived from the diselenides 24 and 26 with various alkenes are reported. In both cases the results obtained with other alkenes are much better than those observed with styrene. 

Quite recently, a new asymmetric addition reaction has been described by Tiecco. The oxidation of the diselenide 26 with ammonium persulfate produces the camphorselenyl sulfate, which reacts with alkenes in acetonitrile in the presence of water, to afford the hydroxyselenenylation products in good yields and with moderate to good diasteroselectivities [46b]. The results of these experiments are collected in Table 3. Moderate diastereomeric ratios were observed in the hydroxyselenenylation of cyclohexene, styrene and -methylstyrene. Good facial selectivity was observed in all the other cases. The two diastereomeric addition products thus obtained could be separated in most cases. Enantiomerically pure saturated or allylic alcohols 

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, 

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Scheme 45. Asymmetric Oxyselenenylation-Elimination Reactions of )8,y-Unsaturated Esters and Nitriles...

If the optically active organoselenium compounds can be used for Tomoda s or Tiecco s catalytic system using diselenide and persulfate (see Sect. 4.1), a catalytic asymmetric oxidation reaction should be possible. The enantioselectivity of the produced allylic compounds may depend on the stereoselectivity of the oxyselenenylation step of chiral selenium electrophiles with prochiral alkenes. Several groups have reported diastereoselective oxyselenenylation using a variety of chiral diselenides in moderate to high diastereoselectivity [5 f, g, i, 25]. The detailed results are reviewed in Chap. 2. 

The first catalytic asymmetric oxyselenenylation-oxidative deselenenylation reaction was reported by Tomoda and co-workers in 1994 [26]. The treatment of... 

Wirth et al. reported a catalytic asymmetric oxyselenenylation-elimination reaction using 10 mol% of chiral nitrogen-containing selenide 475. Trans-P-methylstyrene (469) gives the allyhc ether 471 with 75% ee but in only 23% yield (Scheme 7.75) [317]. 

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