A-Phenylseleno carbonyl compounds

A method for the conversion of alkenes to a-phenylseleno carbonyl compounds involves the use of benzeneselenenic anhydride. This reagent, which has a relatively short lifetime, is prepared in situ from diphenyl diselenide and r-butyl hydroperoxide. The alkene is converted to a phenylseleniranium ion... 

From enolates via selenoxidesA Lithium enolates derived from ketones, lactones, and esters react with PhSe-SePh or with phenylselenyl bromide or chloride (PhSeX) to form a-(phenylseleno)carbonyl compounds. These can be oxidized to the corresponding selenoxides with subsequent 5yn-elimination of benzeneselenic acid to form enones. 

Phenylselenoalkenes, 25-26 a-Phenylseleno carbonyl compounds, 28 Phenylseleno-l-dodecene, 294 Phenylselenoetherification, 26-28 Phenylselenoimines, 29-30 a-Phenylseleno ketones, 26, 209 Phenylsclcnolactonization, 26... 

Elimination of selenoxides takes place through an intramolecular, syn elimination pathway. The carbon—hydrogen and carbon—selenium bonds are coplanar in the transition state. The reaction is highly traws-selective when acyclic a-phenylseleno carbonyl compounds are employed. The formation of conjugated double bonds is favored. Endocyclic double bonds tend to predominate over exocyclic ones, unless there is no syn hydrogen available in the ring. Some examples of selenoxide-mediated syn eUmination reaction are given in Scheme 6.23. 

The reaction with a, -epoxy carbonyl compounds 12 leads to the corresponding reductive ring-opened products, -hydroxy carbonyl compounds 13, in good yields (Scheme 22). Electrochemically generated benzeneselenolate [21,22] and sodium phenylseleno(triethoxy)borate (1) [35, 36], have been applied for this type of reaction as a nucleophilic selenolate. In the latter case, the reaction mechanism was suggested as shown in Scheme 23 [36]. Reaction of 1 with 12 first produces the ring-opened adduct 14, which is then reacted with an excess amount of 1 to produce the final product 13. This method is important as a simple synthetic procedure to aldols and -hydroxy esters that are rather difficult to obtain by other methods. The reactions have been extended to the reduction of more functionalized a, -epoxy carbonyl compounds [37] and have been successfully applied for the synthesis of several natural products [38]. 

The cyclizations of 85 to 86 and of 87 to 88 represent the simple cases in which the internal nucleophile is the OH group of an alcohol [64,65]. An in situ generated hydroxy group, as in the addition of alcohols to carbonyl compounds, can also participate in phenylseleno-etherification reactions. This is examplified by the conversion of 89 into 90 in the presence of benzyl alcohol [66]. Another type of OH, which gives rise to these reactions is the enolic OH of /1-dicarbonyl compounds. Thus, Ley reported that compounds like 91 and 93 can be transformed into the cyclic derivatives 92 and 94 by treatment with N-PSP 11 in the presence of zinc iodide [67]. The cyclization of 95 to 96 represents a simple example of the selenolactonization process [68, 69]. It is interesting to note that the various cyclization reactions indicated in Scheme 14, which require different electrophilic selenenylating agents, can all be effected with phenyselenyl sulfate [70]. 

Tri(organoseleno)boranes 35 are prepared from boron trihalides and organic selenolates as stable compounds (Scheme 35) [63]. These selenoboranes have been shown to be useful for the conversion of carbonyl compounds into seleno-acetals 36 [64] and the selective ring opening of epoxides [65]. Recently, it was reported that tri(phenylseleno)borane reacts with cyclic ethers to produce m-hydroxyalkyl phenyl selenides 37 in the presence of a catalytic amount of Lewis acid [43]. 

Phenylselenomethyl phosphonates and (phenylseleno)chloromethane have been metallated by NaH and Bu OK, respectively (Scheme 28, a and b Scheme 29). The former organometallics react with carbonyl compounds and dirratly produce vinyl selenides (Scheme 28, a and whereas the second decomposes to phenylselenomethylene which adds stereospecifically to alkenes and leads to phenyl-selenocyclopropanes in reasonably good yields (Scheme 29). 

C, 1-phenylseleno- and 1-methylseleno-l-alkenyllithiums in almost quantitative yields (Scheme 101). The stereochemistry of the reaction is not well defined and the only available results are those concerning the stereochemistry of the compounds resulting from further reaction of these organo-metallics with electrophiles. Both stereoisomers are formed if the methylselenoalkenyllithium is hydrolyzed or reac with an aldehyde or a ketone, whereas only one stereoisomer of an a, -un-saturat carbonyl compound is found from l-methylseleno l-alkenyllithiums and and from the... 

Phenylseleno- and methylseleno-alkyllithiums usually exhibit a closely related reactivity towards carbonyl compounds whether the reaction is performed in THF or in ether. As expected, the nudeophilicity of such species often decreases by increasing the substitution around the ct anionic center (6 and 8 Scheme 112), but interestingly they are often far more nucleophilic than the corresponding alkyl-lithiums. However, in some rare cases employing particularly hindered reaction partners there is a significant difference of reactivity between phenyl- and methyl-selenoalkyllithiums when the reactions are performed in THF, since the former reagents are much less nucleophilic than the latter (compare 11 and 12 in Scheme 113 and 17a-17f in Scheme 114). As general trends, a-methylselenoalkyllithiums are more nucleophilic in ether than in THF (compare 15 in Scheme 113.17d and 17e in Scheme 114 and 24 in Scheme 116). 

One of the more useful methods used in enolate ion chemistiy is the finding that carbonyl compounds can be selenenylated. That is, a selenium atom can be introduced onto the position of a carbonyl compound. Selenenylation is accomplished by allowing the carbonyl compound to react with LDA to generate an enolate ion, followed by addition of ben-zeneselenenyl bromide, CeHsSeBr. Immediate -substitution reaction yields an a-phenylseleno-substituted product. 

Phenyl propargyl sulfide " and selenide are rapidly deprotonated by 2 equiv. of base. The resulting dilithio derivatives react at their allylic rather than at their alkynic carbon centers (Scheme 34, entries c and d). Reduction of the sulfides allows the synthesis of 1,5-enynes (Scheme 34, entry b), whereas oxidation of the selenides leads to a-phenylseleno-a,P-unsaturated carbonyl compounds (Scheme 34, entry... 

Phenylseleno-%(l-unsaturated carbonyl compounds.8 a-Phenylselenenylation of, /l-cnoncs and a,/f-enals can be effected with C HsSeCl pyridine (1 1). A related... 

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