Deselenation Reactions

The three papers in this section are all concerned with the preparation of compounds such as (63). Compounds (69),74 (70a),74 (70b),75 and (71)76 were usually converted 

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Antoniadis, C.D., Hadjikakou, S.K., HadjUiadis, N. et al. (2006) Synthesis and structures of Se analogues of the antithyroid drug 6-n-propyl-2-thiouracil and its alkyl derivatives formation of dimeric Se—Se compounds and deselenation reactions of charge-transfer adducts of diiodine. Chem. Eur. J., 12, 6888-6897. 

Since the chemistry of the C-Se bond is very similar to the chemistry of the C-S bond, the deselenation procedures are also very similar to the desulfurization methods. Thus, Ra-Ni is an efficient reagent in hydrogenolytic desel-enations, but the outcome of the reaction depends on the preparation of the catalyst.426 28... 

The combination of the electrogenerated chloro cation [Cl] with diphenyldise-lenide is a typical example of the functionalization of an olefin. The electrochemical oxyselenation-deselenation of (1) to (2) proceeds in an MeOH-NaCl-(Pt) system (Scheme 1) [32]. The bromide salt-promoted oxyselenation of olefins is discussed in Sect. 15.2.2. Penicillin (3) can be converted into the oxazoline-azetidinone (4) by a chloride salt-promoted paired reaction in an MeOH-t-BuOH(5 l)-MgCl2-(Pt) system at —40°C in 74 93% yield (Scheme 2) [33, 34]. This conversion probably involves an initial attack of an... 

Silylated 1,4-cyclohexadienes were introduced as reducing agents in radical chain reactions such as dehalogenation, deoxygenation via thionocarbonate ester and deselenization [120]. Two examples are given in Reactions (4.75)... 

The advantage of this synthetic approach over the total synthesis of selenocysteine peptides is the reduced number of synthetic steps to which the Se-protected selenocysteine is exposed, thus, largely preventing deselenation as an undesired side reaction. It also allows access to larger peptides and even proteins containing selenocysteine to exploit the Se atom for X-ray crystallography, 13 or as 77Se nucleus for NMR spectroscopy. 60 ... 

Tellurium and many other impurities remain undissolved. The solution is filtered and cooled to reverse the reaction and to deposit solid selenium. Deselenized liquor is recycled to the dissolution step. 

Thermodynamically stable aromatic selenaheterocycles such as selenophene (1), 1,3-selenazole (3), 1,2,5-selenadiazole (5), and 1,2,4-selenadiazole undergo [l,4]cycloaddition followed by spontaneous deselenation, which is a convenient way for construction of nonselenium azaaromatic rings [3, 6, 42], 3,4-Diphenyl-1,2,5-selenadiazole reacted with DMAD to give methyl 2,3-diphenylpyrazine-5,6-dicarboxylate. The similar reaction of 1,2,4-selenadiazoles resulted in pyrimidine -5,6-dicarboxylate, while 2,1,3-benzoselenadiazoles reacted with DMAD to give the quinoxalines [6, 110]. (For deselenation see also Sect. 5.1). 

The valuable synthetic intermediates are benzo-2,l,3-selenadiazoles. They were used for preparation of. V-al ky I-1,2-benzenediamines, 3-nitro-1,2-benzenediamines, 3,4-diamino-2-nitrophenols, and 5-nitroqunoxalines. The key step of the reaction is their reductive deselenation with hydrogen iodide or, better, with ammonium hydrosulfide [233-238],... 

Selenazole and the 1,2,4- and 1,2,5-selenadiazoles serve as heterodienes in the Diels-Alder reaction. [l,4]Cycloaddition of the active dienophiles to these selenaheterocyclic compounds, followed by deselenation, is a convenient means of synthesis of a nonselenium azaaromatic ring. Cycloaddition of 2,4-disubstituted 1,3-selenazole (131) with DMAD forms a bicyclo intermediate (132) that undergo extrusion of elemental selenium liberating the pyridine product (133) (Scheme 42) [110]. 

Cyclizations by formation of carbon—selenium bonds represent a modern method with a high synthetic potential in the chemistry of cyclophanes. Selenocyanates such as 16 are accessible usually in excellent yields through the reaction of bromides with KSeCN [27], The reaction with benzylic bromides under reductive conditions using the dilution principle results in good to excellent yields of [3.3]di-selenacyclophanes which can be deselenized photochemically, pyrolytically (without previous oxidation), or by reaction with arynes, Stevens rearrangement and subsequent reaction with Raney nickel. [2.2]Metacyclophane (18), for example, is accessible in 47% total yield by using this sequence of reactions starting with... 

Selenides are also nucleophilic and produce isolable selenonium salts (9) when treated with alkyl halides. They are easily oxidized to selenoxides (10) and further to selenones (11) under more forcing conditions (see Section 4). Reduction of selenides to the corresponding hydrocarbons is most conveniently achieved with nickel boride,or with tri-n-butyl- or triphenyltin hydride under radical conditions. " Other reagents for reductive deselenization include Raney nickel, lithium triethylborohydride, and lithium in ethylamine (Scheme 4). Benzylic selenides undergo radical extrusion reactions under thermal or photolytic conditions to produce... 

Oxidative deselenizations have also been described. For example, reaction of triphenylphosphorus selenide 181 with perfluorooxaziridine 80 resulted in complete deselenization affording triphenylphosphine oxide 182 in 97% yield <1997JOC6401>. 

The deselenation of the selenadistibirane 20 ring results in the formation of the corresponding distibene in a reversible and nearly quantitative reaction as is shown in Scheme 6 <1998CL725>. 

Scheme 8 Deselenation and complexation reactions of the pentamethylcyclopentadienyl-substituted diphosphaselenirane 18.

These deselenations do not tolerate halides, which are generally reduced. Among the drawbacks in addition to side reactions inevitable with radical intermediates, the use of an excess of an expensive reagent and a high reaction temperature may make the procedure less useful, especially with sensitive substrates. Note that the generation of radicals using deselenation by tin hydrides is also very useful in organic synthesis. ... 

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