Selenophenol addition

Although the mechanism of this reaction has not been studied as thoroughly as that of the thiol addition reaction, there appears to be considerable similarity in the main features of the reaction. Thus, although the transition state may well differ in a number of details, the transition state shown earlier (Figure 10) appears to predict the correct configuration for the products of the selenophenol addition as well as of the thiophenol addition to cyclohexenones. 

The higher yields of selenophenol are favored by exclusion of air, rapid stirring, and. not too rapid addition of selenium. 

The 1,4-addition of selenophenols to cycloalkenones proceeds smoothly under the same conditions used in the thiophenol addition (see Section V) (eq. [18]). Although the e.e. of the products is 40 to 65%, compared with the 60 to 80% achieved in the thiol addition reaction, the solid adducts are readily purified to enantiomeric purity by crystallization (63). 

Asymmetric induction of the Michael addition of thiols to electron-deficient alkenes (4.6.1) has been achieved in high overall conversion using both free [e.g. 12-20] and polymer-supported [e.g. 21, 22] cinchona alkaloids and their salts [23-25], but with varying degrees of optical purity. The corresponding asymmetric Michael addition of selenophenols to cyclohex-2-enones is promoted by cinchoni-dine to give a chiral product (43% ee) [26],... 

There seem to be very few examples of organocatalytic Michael additions of Se-nucleophiles, and these are limited to the addition of selenophenols to enones (Scheme 4.36) [3, 60]. As shown in the scheme, Wynberg and Pluim achieved... 

Michael reaction of selenophenols. Selenophenols (and other selenides) undergo enantioselective 1,4-addition to cyclohexenone in the presence of catalytic amounts of cinchona alkaloids. Chemical yields are high optical yields are 10-43%. Usually the optical yield can be enhanced by crystallization. In one case the addition product was converted into an optically active allylic alcohol by hydride reduction followed by selenoxide fragmentation. ... 

Selenophenols function as nucleophiles in 1,4-addition reactions to a,J -unsaturated ketones. When this addition is catalyzed by a cinchona alkaloid, chiral 3-(phenylseleno)cycloalkanones 5 are formed in quantitative yield and with up to 67% ee" 5. Moreover, the solid adducts are readily purified to 100% ee by crystallization. A variety of chiral 3-arylselenocyclohexanones have been prepared in this manner using quinine as a catalyst (Table 13). 

Water and hydroxyl compounds do not add spontaneously to alkynes, but do so in the presence of catalysts, as discussed fully in a standard texts on organic chemistry. Thiols add to alkynes in the presence of either basic or free radical catalysts. Under basic conditions the addition is trans, e.g., NaSC6H4CH3 adds to CgHsCCH in refluxing ethanol to produce cis-1-styryl-p-tolysulfide. Under the same conditions 2-butyne gives an excellent yield of the trans-2-butenyl-p-tolylsulfide . Selenophenol adds trans to phenylacetylene under the influence of strong bases, such as sodium methoxide. 

In this subsection, we describe a couple of examples taken from the recent literature, in which the Baylis-Hillman reaction has been employed for the construction of new carbon-carbon bonds. The Baylis-Hillman reaction proceeds in a catalytic cycle propagated by a nucleophilic catalyst (584). The nucleophilic catalyst initiates the cycle by Michael addition to a double bond bearing an EWG (586 or 590). The carbon a to the EWG is acidic and may react with an electrophile. Finally, the nucleophilic catalyst is eliminated, completing the cycle (Scheme 122). The most frequently used catalysts are quinuclidine, DABCO, phosphines, thiopheno-lates, and selenophenolates. The reaction rate of a catalytic Baylis-Hillman reaction approaches a maximum at a certain temperature and declines upon further heating, as the equilibrium concentration of (587) becomes very small. In the first example, the electrophilic component of the reaction was immobilized on a solid phase and the nucleophile was in solution, while in the other example the situation was reversed (Scheme 122). 

Compounds of selenium are highly toxic and in addition have a tendency, like their sulfur analogues, to have an unpleasant odour. Care is therefore necessary when handling selenium reagents, and it is advisable to avoid the more pungent chemicals where an alternative is available. In particular, selenophenol itself is extremely noxious and should be avoided if possible. As phenyl selenoglycosides are stable to both basic [41] and acidic [40] conditions it is often possible for the functionality to be introduced early in a given synthesis. 

The reduction of Schiff s bases is a well used and convenient method for the preparation of secondary amines. Selenophenol has now been found to be an excellent and advantageous reagent to effect this, and the reductive alkylation of amines with carbonyl compounds. The preparation of tertiary amines bearing three different substituents can also be achieved by the successive addition of carbonyl compounds to a primary amine in a one-pot reaction (Scheme 15). The... 

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