Reactions with electrophilic selenium

Addition Reactions with Electrophilic Sulfur and Selenium Reagents... 

Alkynes react with electrophilic selenium reagents such as phenylselenenyl tosylate.155 The reaction occurs with anti stereoselectivity. Aryl-substituted alkynes are regioselective, but alkyl-substituted alkynes are not. 

It is interesting to note that the latter result is exceptional since l,l-bis(phenyl-seleno)cyclopropane is the only selenoacetal derived from ketones to be at least partially cleaved under these conditions and even the homologous cyclobutyl derivative is inert under these conditions. This may be due to the extra stabilization introduced by the cyclopropyl ring. The case of 2-decy 1-1,1-bis(methylseleno)cyclo-propane merits further comment. It is difficult to assess whether the cleavage of the carbon-selenium bond occurs on the methylseleno moiety cis or tram to the alkyl group, since this organometallic leads to a mixture of the two possible stereoisomers on further reaction with electrophiles (Scheme 16). 

Starting from the symmetrical allylic diselenanyl compound 162, and through a selenium-lithium exchange with n-BuLi, methylselenanyl methallyllithium compound 163 was prepared, which after reaction with electrophiles afforded products 164 (Scheme 2.22) [137]. 

Various electrophilic selenium reagents such as those described in Scheme 4.3 can be used. V-Phenylselenylphthalimide is an excellent reagent for this process and permits the formation of large ring lactones." The advantage of the reagent in this particular application is the low nucleophilicity of phthalimide, which does not compete with the remote internal nucleophile. The reaction of phenylselenenyl chloride or V-phenylselenenylphthalimide with unsaturated alcohols leads to formation of (3-phenylselenenyl ethers. 

Selenium dioxide is a useful reagent for allylic oxidation of alkenes. The products can include enones, allylic alcohols, or allylic esters, depending on the reaction conditions. The mechanism consists of three essential steps (a) an electrophilic ene reaction with Se02, (b) a [2,3]-sigmatropic rearrangement that restores the original location of the double bond, and (c) solvolysis of the resulting selenium ester.183... 

This reaction depends upon the facile solvolysis of (J-haloselenides and the facile oxidative elimination of a selenoxide, which was discussed in Section 6.6.3. An alternative method, which is experimentally simpler, involves reaction of alkenes with a mixture of diphenyl diselenide and phenylseleninic acid.189 The two selenium reagents generate an electrophilic selenium species, phenylselenenic acid, PhSeOH. 

Methyl groups in the 2-, 4- or 6-position of pyrimidine are also more reactive. In addition to typical reactions such as condensation with benzaldehyde, selenium dioxide oxidation and halogenation, they can be converted into oximino groups by nitrous acid, and undergo Claisen condensation with (C02Et)2. In the reaction of 2,5-dimethylpyrimidine with benzaldehyde, only the electrophilic 2-methyl group reacts preferentially to yield the 2-styryl derivative (631). In quinazolines partial double bond fixation makes a methyl group in the 4-position more reactive than that in the 2-position. 

The lactonization of 5,7-dienoic acids with selenium and sulfur electrophiles generates 8-lactones by regioselective 1,4-addition to the diene (equation 36).s2a With substitution at the 8-position, reaction with PhSeCl or PhSCl gave a 1 1 mixture of rapidly interconverting 1,2- and 1,4-addition products. This inter-conversion, presumably via a 1,3-rearrangement,S2b was not observed with the alkylthio adduct. An example of conjugate cyclization of an enyne system to form an allenyl product has also been reported (equation 37).112... 

Fig. 12.10. Mechanism of the a-oxygenation of ketones in reactions with selenium dioxide an electrophilic substitution reaction (—> —> C) is followed by a /(-elimination at the C-0 single bond.

---