Selenides from alkyl halides

Since the starting tellurides are easily prepared from alkyl halides or epoxides by displacement with tellurolate aiuons (see Section 3.1.3.2), the overall sequence constitutes a mild reduction of these substrates and is advantageous over the analogous reductions of selenides, which require more severe conditions (a temperature of 120°C is necessary). 

Vinyl selenides have been lithiated at the a-position by LDA983,984 at —78 °C in THF to give a-(arylselanyl)vinyllithiums 680, a-(methylselanyl)vinyllithiums 681 being obtained by selenium-lithium transmetallation from l,l-bis(methylselanyl)alkenes with n-BuLi in THF or t-BuLi in ether at —78 °C985 986. These intermediates reacted with alkyl halides, epoxides, carbonyl compounds and DMF985, the final deprotection being performed by mercury(II) salts986. 

This chapter is devoted exclusively to the alkylation of the above mentioned organometallic reagents with alkyl halides, epoxides and oxetanes. It includes a large variety of organosulfur- and organoselen-ium-stabilized carbanions derived from saturated and unsaturated sulfides, selenides, sulfoxides, selen-oxides and sulfones as well as those carbanions bearing another heteroatomic moiety. The chapter excludes, however, those organometallics which can be viewed as a-thio and a-seleno enolates. 

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). 

Trimethylsilyl-l-alkenes. The lithium reagent is prepared from the corresponding selenide by treatment with BuLi. Its reaction with alkyl halides provides the allylsilanes. 

Alkyl halides can be prepared from selenides and selenoxides.60 Oxidations - Active manganese dioxide oxidations have been reviewed.61... 

Selenocyanates (RSeCN), which are employed for the synthesis of selenides and selenoesters, are usually derived from selenenyl halides and cyanide ion (or silyl cyanate) [38 k, 40]. Alkylation of potassium selenolate (KSeCN) with alkyl halides is an alternative means of preparation of aliphatic selenocyanates (Scheme 15.13)... 

Oxidations. Alkene formation from alkyl aryl selenides via oxidation is cleaner when the aryl group is ort/io-substituted (vs. para-substitution), for example, with a nitro group. In the presence of a phase-transfer catalyst, oxidation of aldehydes to carboxylic acids with H2O2 is successfully carried out without an organic solvent, halide, or metal ion. 

X = Ph-Se- From Amberlyst A-26 (BH ) by treatment with (PhSe),i9i Organyl phenyl selenides were prepared from alkyl iodides, allyl bromides, a-bromo esters or benzyl halides. ... 

Radicals can be generated by homolysis of weak a-bonds. Homolysis is effected by photochemical, thermal or redox (electron transfer) methods. A common method to initiate a radical reaction is to warm a peroxide such as benzoyl peroxide or azobi-sisobutyronitrile (AIBN) 1 (4.2). The radical -C(CN)Me2 generated from AIBN is rather umeactive, but is capable of abstracting a hydrogen atom from weakly bonded molecules such as tributyltin hydride (4.3). The resulting tributyltin radical reacts readily with alkyl halides, selenides and other substrates to form a carbon-centred radical. 

Preparation of Sulphides, Selenides, and Tellurides using the Elements Themselves, or Other Inorganic Sulphur, Selenium, and Tellurium Compounds.—Symmetrical sulphides can be obtained in 12—66% yields from an alkyl chloride, sulphur, and NaOH in DMSO. The same system with a bePxZyl cyanide R PhCHCN produces the unsymmetrical sulphides R PhC(CN)SR with an alkyl halide some... 

The coupling of arylzinc halides with alkyl (E)-2,3-dibromoalkenoates occurs at C-3. 3-Arylindoles are obtained from 3-indolylzinc halides with haloarenes." A synthesis of 1,4-disubstituted butadienes starts from the hydrozirconation of ethynyl phenyl selenide. After Pd(0)-catalyzed coupling with alkenyl bromides, the seleno group is replaced via Grignard reaction in the presence of (Ph3P)2NiCl2. 

The conversion of alcohols to iodides with inversion using triphenylphosphine-iodine-imidazole, or triphenylphosphine-2,4,5-tri-iodoimidazole, (4,157) has now been described in detail. The purity and yield of tertiary iodides (or bromides) prepared from the alcohols and aqueous HI (or HBr) at 0 can be improved by addition of lithium or calcium halides it is believed the metal salts retard competing hydrolysis of the tertiary halides. Secondary alkyl bromides have been prepared from the alcohols with retention of configuration via the inverted selenides (Scheme 35). Two other new reagents for the... 

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