Esters selenenylation

Selenoxides are even more reactive than amine oxides toward [> elimination. In fact, many selenoxides react spontaneously when generated at room temperature. Synthetic procedures based on selenoxide eliminations usually involve synthesis of the corresponding selenide followed by oxidation and in situ elimination. We have already discussed examples of these procedures in Section 4.7, where the conversion of ketones and esters to their x,/J-unsatu rated derivatives was considered. Selenides can also be prepared by electrophilic addition of selenenyl halides and related compounds to alkenes (see Section... 

OrPhenybelenenylation of a,f -unsaturated esters.3 (F)-3r,/MJnsaturated esters are converted to a-phenylseleno-a,0-unsaturated esters in 20-65% yield by reaction with LDA followed by C6H5SeBr. The reaction is related to the reaction of a,/ -enones with pyridine and C H5SeCl (9, 28-29), and is also believed to involve conjugate addition of the base followed by selenenylation of the enolate. 

In a further development of this approach, the synthesis of cr,/J-acetylenic acyl silanes has been achieved as shown in Scheme 4514. Oxidation of the 3-selenenyl allenyl ethers (16) with m-chloroperbenzoic acid at —78 °C gave the corresponding unstable selenoxides, which underwent in situ [2,3] sigmatropic shift producing acetals (17). Loss of selenenyl ester on work-up gave the cr,/J-acetylenic acyl silanes in ca 50% yields. 

Similar reactions were also achieved by the formation of diastereomeric optically active selenoxides as intermediates in the elimination reaction. Optically active ferrocenyl diselenide 19 was used in selenenylations of alkynes generating vinyl selenides of type 164. Oxidation of the selenides was performed with mCPBA under various reaction conditions which afforded the corresponding chiral selenoxides, which, after elimination, afforded axial chiral allenecarboxylic ester derivatives 165 in high enantioselectivities (R = Me 89% ee, R=Et 82% ee, R = C3H7 85% ee) (Scheme 47)>85 87... 

Dibromoselenuranes 196 can be prepared from the corresponding selenides by reaction with elemental bromine. The a,/ -unsaturated ester 196 is converted into the a-bromo-/ ,7-unsaturated ester 197 by elimination of phenyl-selenenyl bromide (Scheme 58).338 Dibromoselenuranes from propargylic selenides undergo similar reactions leading to either allenes or propargylic bromides.339... 

It is possible to perform selenenylation-deselenenylation sequences with only catalytic amounts of selenium species. This reaction sequence provides double bond transpositioned allylic ethers, allylic esters, or allylic alcohols 240 from the corresponding alkenes (Scheme 71). This sequence can be performed electrochemically, and the selenium electrophile is generated from catalytic amounts of diphenyl diselenide.467,468 It has been shown that the electrophilic selenium species can also be generated using diselenides and peroxosulfates together with copper (ii)... 

An alternative one-step procedure using iV,A(-diethylbenzeneselenamide has been developed and, as illustrated in equation (10), this reagent is particularly suitable for differentiating between aldehytUc and ketonic moieties in the same molecule. The analogous morpholinoselenamide has been shown to selenenylate the a-keto ester (20 equation 11) but no other examples have been reported. ... 

An interesting variation for the introduction of selenenyl species, which has the advantage of using cheaper elemental selenium, has been described by Liotta and cowoiicers (Scheme 17). This reaction involves conversion of the lithium enolate (21) to the intomediate selenolate (22) which may be directly alkylated to give the selenenyl derivative (23) in high yield. The reaction worits well with ketones, esters and 3-dicarbonyl compounds, but has the disadvantage of requiring the use of HNiPA. ... 

Several related cross-couplings that involve both catalytic levels of Pd and Cu have also been reported, such as Hecklike reactions see Heck Reaction) of vinyl esters with arylstannanes (equation 6), aryl selenenylations between BusSnSeAr and aryl halides (equation 7), and thioarene or thioester couplings to aryl, vinyl, or alkyl stannanes (equation 8). ... 

Selenenyl halides are relatively stable, though moisture sensitive, compounds that are generally prepared by the reactions shown in Scheme 7 and behave as electrophihc selenium species. " They react with ketones and aldehydes via their enols or enolates to afford a-seleno derivatives (e.g. (17) in equation 11). Similar a-selenenylations of /3-dicarbonyl compounds, esters, and lactones can be performed, although the latter two types of compounds require prior formation of their enolates. Moreover, the a-selenenylation of anions stabilized by nitrile, nifro, sulfone, or various types of phosphorus substituents has also been reported (equation 12). In many such cases, the selenenylation step is followed by oxidation to the selenoxide and spontaneous syn elimination to provide a convenient method for the preparation of the corresponding a ,/3-unsaturated compound (e.g. 18 in equation 11). Enones react with benzeneselenenyl chloride (PhSeCl) and pyridine to afford a-phenylselenoenones (equation 13). 

Selenenylations of ketones, esters, lactones and lactams are usually effected by the reaction of the corresponding lithium enolates with PhSeCl, PhSeBr and PhSeSePh (with the exception of ketones) at low temperature. Aldehydes have not been selenenylated in this manner. Table 4 illustrates some typical products that have been made in this way. Selenenylation has been especially useful in natural piquet synthesis for the formation of a-methylenelactones from the parent a-methyl compounds (Scheme 15 and Table 4), and has significant advantages over the more traditional methods for ef-... 

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. 

The preparation of a-selenoketones, esters, nitriles and related compounds can easily be performed via alkylation of the corresponding enolates or stabilized carbanions [21]. These compounds have found many synthetic applications in radical chemistry. In Eq. (9), a typical example involving a ketone is depicted [22]. The stability of a-selenoketones such as 41 is remarkable. Similar reactions with lactones have been performed. For instance, this approach has been applied to the stereoselective synthesis of oxygen-containing rings to either faces of a bicyclic structure [23]. The approach based on a-selenenylation/radical allyla-tion compares favorably with classical enolate allylation procedures, which usually leads to mixture of mono- and diallylated compounds. Furthermore, this strategy is excellent for the preparation of quaternary carbon centers [24] as shown by the conversion of 43 to 45, a key intermediate for the synthesis of fredericamycin A, [Eq. (10)] [25]. Similar reactions with sulfoxides [26] and phosphonates [27] have also been reported. 

Reacting the cyclic seleninate ester 15 with an excess (three fold) of pentanethiol gave the selenenyl sulfide 104 (Equation 18) <2005JOC9237>. Treating 105 with benzyl thiol provided the selenenyl sulfide 106 via the intermediates depicted in Scheme 5 <2002JA12104>. 

Symmetrical monothio- and monoselenopyrophosphate esters (36) have been prepared in high yields from dialkyl trimethyl-silyl phosphites and the sulphenyl or selenenyl chlorides (37)... 

Aryl selenides have also proven to be excellent reagents in group transfer reactions. Photolysis of selenides in an inert solvent such as benzene can initiate chain reactions. Various substituted radicals can be generated in this manner by using a-selenenyl derivatives of esters, nitriles, malonates, P-ketoesters, a-methoxyesters, and phosphonates. The resulting radicals undergo addition to alkenes to generate y-seleno derivatives. 

Bromodeselenenylation. y-Phenylseleno a,j3-unsaturated esters undergo bro-mination at the a-carbon while detaching the selenenyl group and deconjugating the double bond. 

Preparation of Sulphides Selenides, and TeUurides using Other Sulphenylation, Selenenylation, and Tellurenylation Reagents.—Sulphenamides are effective reagents for cr-heteroarylsulphenylation of ff-keto-esters. Sulphenamide intermediates are thought to be involved in the asymmetric 2-phenylsulphenylation of 4-alkylcyclohexanones in moderate optical yields, using a benzenesulphenyl halide with a chiral secondary alkylamine. ... 

References to addition reactions of sulphenyl halides with alkenes and alkynes are given in the Sulphides section. An incidental bonus from a study of the addition of PhSCl to adamantylideneadamantane is the formation of the 4e-chloro-derivative and PhSSPh, via an intermediate thiiranium salt. Sulphenyl and selenenyl chlorides have been used for the dehydration of aldoximes to nitriles. Whereas sulphinyl chlorides yield sulphinate esters with alkoxytri-methylsilanes, no reaction occurs with sulphenyl chlorides. ... 

Deiodination of T4 model compounds, 2,6-diiodo-4-phenoxyphenol (56a) and N-butyrylthyronine methyl ester 56b by selenol 49 was examined (Scheme 11.32). When diiodophenol 56a was treated with an equimolar amount of selenol 49 in the presence triethylamine, deiodination proceeded slowly, and after 1 week 50% of 56a was converted to monoiodophenol 57a. Concomitantly, the formation of selenenyl iodide 55 (39%) was confirmed. 

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