Alkyl selenide

Secondary alkyl selenides are reduced by (TMS)3SiH, as expected in view of the affinity of silyl radicals for selenium-containing substrates (Table 4.3) [40]. Reaction (4.23) shows the phenylseleno group removal from the 2 position of nucleoside [50]. Similarly to 1,3-dithiolanes and 1,3-dithianes, five- and six-membered cyclic selenoacetals can be monoreduced to the corresponding selenides in the presence of (TMS)3SiH [51]. The silicon hydride preferentially approached from the less hindered equatorial position to give transicis ratios of 30/70 and 25/75 for the five-membered (Reaction 4.24) and six-membered cyclic selenoacetals, respectively. 

The methods of synthesis of diaryl and aryl alkyl selenides have been recently reviewed.215 4-Nitro-phenyl methyl selenide was obtained in 93% yield from the reaction of 4-chloronitrobenzene in DMF with a suspension of MeSeLi in THF, prepared from powdered Se and MeLi.216 Other mixed RSeAr selenides were synthesized via alkylation with RI of ArSe resulting from the nucleophilic substitution of unactivated haloarenes ArCl with MeSeLi in DMF at 120 C, followed by MeSe -induced demethyl-ation.217... 

Secondary alkyl selenides are reduced by (TMS SiH (equation 9), as expected in view of the affinity of silyl radicals for selenium-containing substrates (Table 2)21. Similarly... 

The rate constant of R with (Me3Si)3SiH is about 105 M-1 s-1. The rate constants of (Me3Si)3Sf with alkyl halides and selenides are as follows 109M-1 s-1 for alkyl iodides 108 107 M-1 s-1 for alkyl bromides and alkyl methyl xanthates 107 M 1 s 1 for alkyl selenides 106 M 1 s-1 for alkyl sulfides. 

Me3Si)3SiH and Ph4Si2H2 systems initiated by AIBN or Et3B also reduce alkyl iodides, alkyl bromides, alkyl xanthates, and alkyl selenides to the corresponding reduction products as shown in eq. 2.12. 

The same photolytic treatment of O-acyl esters (2) in the presence of disulfides, diselenides, and ditellurides effectively produces the corresponding alkyl sulfides, alkyl selenides, and alkyl tellurides respectively, through SHi reaction on the chalcogen atoms by alkyl radicals, as shown in eq. 8.9. The reactivities somewhat depend on the kind of chalcogenides. Thus, the effective formation of alkyl sulfides requires 30 eq. of disulfides, that of alkyl selenides requires 10 eq. of diselenides, and that of alkyl tellurides requires 2 eq. of ditellurides [27, 28]. 

Alkyl selenides are most conveniently prepared by the dialkylation of Na2Se or by the monoalkylation of selenolates. Common routes to aryl derivatives include the reaction of a selenolate with a diazonium salt, the SrnI reactions described earlier, and the reaction of aryUithiums with aryl selenocyanates (ArSeCN). A different approach employs the reaction of alcohols with aryl selenocyanates or N-(phenylseleno)phthalimide (see Section 6) in the presence of tri-n-butylphosphine. Similar conditions can also be used to convert carboxylic acids to selenoesters (8). These reactions are illustrated in Scheme 3. 

Actually this elimination is general for primary and secondary alkyl selenides, and even potassium hydroxide can be used as base. ... 

Oxidation of the chiral (4-geranylseleno)-15-(4-toluenesulfonyl)[2.2]paracyclophane with 3-chloroperbenzoic acid gave (S )-linalool with 67% enantioselectivity39. Diethylamine was added before warming up to avoid side reactions by the generated selenic acid. On the basis of similar oxidation of paracyclophane-substituted alkyl selenides it was assumed that the asymmetric induction in the oxidation step is responsible for the modest selectivity. 

As previously observed for the diphenyl diselenide (Scheme 2) [24], Pandey has found that the phenyl alkyl selenides also can be transformed into the corresponding radical cations by photostimulated single electron transfer to 1,4-dicyanonaphthalene (DCN). These intermediates, as proposed above, suffer fragmentation to afford diphenyl diselenide and a carbocation, which can be trapped by a nucleophile. On the basis of these observations Pandey [121] described a sequential one-pot selenenylation-deselenenylation of alkenes in methanol using only catalytic amounts of diphenyl diselenide. An example is reported in Scheme 41 which illustrates the single steps of this process... 

The two main systems of this kind used in the deselenation of selenides and diselenoketals are Li-and Na in liquid NH3. Na in HMPA or DMA has also been used with some special aryl alkyl selenides. = This procedure does not deserve special comment since, broadly speaking, the behavior of the reagents towards selenated substrates was very similar to that observed with sulfur-containing substrates (see Section 4.3.1.4). As expected from the difference of bond energies, C—Se bonds are more easily cleaved than C—S bonds. However, few selective deselenations have been performed in the presence of sulfur groups. C—O bonds vicinal to C—Se bonds are, of course, removed with alkene formation, and this property has found interesting applications. ... 

HYDROXY ALKYL SELENIDES AS VALUABLE SYNTHETIC INTERMEDIATES... 

The alkylations of a-selenoalkylmetals share some similarities with those of their thio analogs. Thus a-methylseleno- and a-phenylseleno-alkyllithiums, available by cleavage of the corresponding sele-noacetals by n-butyllithium in THF or s-butyllithium in ether, usually react with methyl iodide and primary alkyl halides to produce the corresponding alkylated selenides. - - - "° 2- ... 

The parent compounds, methylselenomethyllithium and phenylselenomethyllithium, are less reactive towards alkylating agents than their higher homologs. They require higher temperatures (-20 C instead of -78 C) for a reasonable reaction rate irrespective of whether ether or THF is used. Unfortunately they produce, along with the expected alkylated selenides, appreciable amounts of the selenide resulting from substitution of the alkyl halide by the selenolate ion (Scheme 8, entry a). Apparently, under these conditions, the selenoalkyllithiums are in equilibrium with methylene and lithium selenolates. This side reaction can be avoided if the reactions are performed in THF-HMPA, under which conditions alkylation proceeds at temperatures as low as -78 C (Scheme 8, compare entry b to a). In several cases HMPA aids the alkylation of ot-selenoalkyllithiums. For example HMPA has been used for the alkylation of 1-phenylseleno-l-hexyllithium, and also for 1-selenocyclopropyl- and 1-selenocyclobutyl-lithiums (Scheme 8, entries c and e). 

Among the functionalized selenides, P-hydroxy-alkyl-selenides 3-9,11,12) and allylselenides 3 24-411 are those which possess a typical reactivity. 

P-hydroxy-alkyl-selenides are also very powerful precursors of allyl alcohols 3 9, 11 12). The transformation requires the oxidation of the P-hydroxy-alkyl-selenides to P-hydroxy-alkyl-selenoxides which usually collapse to the allyl alcohol below 70 °C and often at room temperature. Hydrogen peroxide supported on alumina in THF are, among the conditions reported, the ones which can be recommended (Schemes 8Aa, BBa). 

The first approach to the seleno series taken advantage of the wide availability of 1-(l-seleno)cyclopropyl carbinols and the small tendency (discussed in Section Syntheses of alkylidene cyclopropanes from 3-hydroxy alkyl selenides), of the phenylseleno derivatives to produce alkylidene cyclopropanes on reaction 37), with for example, thionyl chloride/triethylamine mixture (Scheme 51). However, the best results have... 

Alkyl sulfides, aryl sulfides, alkyl selenides and acyl selenides... 

The regioselechvity of the ehmination depends on the nature of the substituents. The presence of oxygen-containing funchonal groups such as hydroxy, alkoxy, and acetoxy leads to the selechve formation of allyhc compounds, whereas P-chloro alkyl selenides afford a mixture of vinyhc and allyhc chlorides, as shown in Scheme 15.95 [38a]. Asymmetric selenoxide eliminahon using optically active fer-rocenyl selenides has also been reported ]183]. 

Scheme 20. Phenyl selenide transfer reactions of alkyl selenides...

Dialkyl diselenides. A con ei KSeCN in aq DMF in the presence < and the resulting alkyl selenide 10ns... 

Clearly, more research needs to be done. For instance, there is a real need for more detailed product studies. The initial products with oxygen and sulphur atoms are the epoxides and episulphides. With selenium and tellurium the cyclic adducts are apparently only transient, so there is no accumulation of initial product to serve as a measure of the reaction rate. Callear and Tyerman [123] measure the selenium—olefin reaction rate by photometric methods, in which the appearance of bands attributed to an alkyl selenide are observed. These products are not stable, and decay according to... 

Among the functionalizal selenides, P-hydroxy-alkyl-selenides and... 

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