Selenation Reactions

Positively polarized selenation reagents for oxyselenation reactions can be substituted by an indirect electrochemical procedure in which the bromide ion acts as the redox catalyst. The active bromine species generates the phenylselenyl cation from the added diphenyl diselenide (Eq. (48) Table 4, No. 21-26) 

In many cases even the oxyselenation-deselenation sequence can be performed electrochemically in one step (Eq. (49), Table 4, No. 27-31) 

This allyloxylation was used to produce d,l-rose oxide from citronellol (Eq. 50) 1 -Similarly d, 1-dihydroactinidiolide was formed in one step by the intramolecular oxyselenation-deselenation sequence (Eq. (51)) 

Bromide and iodide ions are not only effective as redox catalyst in the indirect electrochemical formation of carbon-hetero atom bonds but also of hetero-hetero atom bonds. Thus, the nitrogen-sulfur bond in sulfenimides (Eq. (53) Table 4, No. 33) and sulfenimines (Eq. (54) Table 4, No. 34) can be generated. The electrogenerated active bromine species forms a positively polarized sulfenyl intermediate from disulfides. 

The last procedure is important as it can be used for the synthesis of 6- and 7-methoxy penicillins or cephalosporins which are potent antibiotics against Gramnegative bacteria (Eq. (55)) 

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A. Bouali, G. Descotes, D. F. Ewing, A. Grouiller, J. Lefkidou, A.-D. Lespinasse, and G. Mackenzie, Derivatization of 1,3,4,6-tetra-O-benzoyl-a-D-fructofuranose at the anomeric site O-Alkylation, O-acylation, O-arylation, amination, and selenation reactions, J. Carbohydr. Chem., 11 (1992) 159-169. 

The mechanism of the catalytic hydrothiolation and hydroselenation reactions differs from the bis-thiolation and bis-selenation reactions at the product formation stage. For the Z—H bond addition to alkynes the last stage of the catalytic cycle is protonolysis or C—H reductive elimination. It was found that, independently of the catalyst precursor (either Pd or Pd derivatives), the same catalytically active species [Pd(ZR)2] were formed (Scheme 3.81) [147, 148]. 

Selenosulfate reacts with ethyleneknine ia the same way as thiosulfate to give 2-ainiQoethaiieselenosulfuric acid. However, the reaction of ethyleneimine using selenous acid does not yield a stable product (137). 

Oxidative rearrangement of 5a-cholestan-3-one (62) with hydrogen peroxide and a catalytic amount of selenic acid affords 2a-carboxy-A-nor-5a-cholestane, isolated in about 35 % yield as the methyl ester (63)." However, the reaction gives a complex mixture of A-nor- and seco-acids, and under... 

The ease with which the dipolarophile interacts with vinylacetylenes depends mainly on a spatial factor. The study of the reactions of alkylthiobuten-3-ynones-l and their selenic and telluric analogs with DPNT shows that, in this case, nitrilimine also acts as a nucleophilic agent with a nucleophilic center on the carbon atom of the 1,3-dipole and always adds to the terminal carbon of the enyne system to form l,3-diphenyl-5-/ -2-pyrazolenines. The oxidation of the latter with chloranil leads to alkynylpyrazoles (65ZOR51). 

In an effort to make productive use of the undesired C-13 epimer, 100-/ , a process was developed to convert it into the desired isomer 100. To this end, reaction of the lactone enolate derived from 100-) with phenylselenenyl bromide produces an a-selenated lactone which can subsequently be converted to a,) -unsaturated lactone 148 through oxidative syn elimination (91 % overall yield). Interestingly, when 148 is treated sequentially with lithium bis(trimethylsilyl)amide and methanol, the double bond of the unsaturated lactone is shifted, the lactone ring is cleaved, and ) ,y-unsaturated methyl ester alcohol 149 is formed in 94% yield. In light of the constitution of compound 149, we were hopeful that a hydroxyl-directed hydrogenation52 of the trisubstituted double bond might proceed diastereoselectively in the desired direction In the event, however, hydrogenation of 149 in the presence of [Ir(COD)(py)P(Cy)3](PF6)53 produces an equimolar mixture of C-13 epimers in 80 % yield. Sequential methyl ester saponification and lactonization reactions then furnish a separable 1 1 mixture of lactones 100 and 100-) (72% overall yield from 149). 

Two mechanisms have been suggested for the reaction with Se02. One of these involves a selenate ester of the enol ... 

The observed complexity of the Se(IV) electrochemistry due to adsorption layers, formation of surface compounds, coupled chemical reactions, lack of electroactivity of reduction products, and other interrelated factors has been discussed extensively. Zuman and Somer [31] have provided a thorough literature-based review with almost 170 references on the complex polarographic and voltammetric behavior of Se(-i-IV) (selenous acid), including the acid-base properties, salt and complex formation, chemical reduction and reaction with organic and inorganic... 

In searching to formulate a mechanism of CuInSc2 phase formation by one-step electrodeposition from acid (pH 1-3) aqueous solutions containing millimolar concentrations of selenous acid and indium and copper sulfates, Kois et al. [178] considered a number of consecutive reactions involving the formation of Se, CuSe, and Cu2Se phases as a pre-requisite for the formation of CIS (Table 3.2). Thermodynamic and kinetic analyses on this basis were used to calculate a potential-pH diagram (Fig. 3.10) for the aqueous Cu+In-i-Se system and construct a distribution diagram of the final products in terms of deposition potential and composition ratio of Se(lV)/Cu(ll) in solution. 

Arsenite is also an intermediate in the fungal biomethylation of arsenic (Bentley and Chasteen 2002) and oxidation to the less toxic arsenate can be accomplished by heterotrophic bacteria including Alcaligenes faecalis. Exceptionally, arsenite can serve as electron donor for chemolithotrophic growth of an organism designated NT-26 (Santini et al. 2000), and both selenate and arsenate can be involved in dissimilation reactions as alternative electron acceptors. 

Specific examples include treatment of o-stannyl benzyl alcohols with TFA,55 reactions of ketones and aldehydes with Lewis acids,56 and electrophilic selenation of styrenes.57... 

The formation of silaneselone 57 was evidenced by the trapping reaction with mesitonitrile oxide leading to the corresponding cycloadduct 58 and was also supported by the observation of a remarkably downfield 29Si chemical shift (8Si = 174) indicative of the Si=Se double bond of 57. Although this direct selenation of silylene 55 with an equimolar amount of selenium was not reproducible, the use of excess amount of elemental selenium resulted in the formation of a new cyclic diselenide, diselenasilirane 59, as a stable compound (8Si = -44 and... 

In a similar way, a-nitroselenides are prepared via the reaction of nitronates with phenylse-lenyl bromide, which gives a new synthetic method of l-nitroalkenes from nitroalkanes.130 The sequence of a-selenation, nitro-aldol reaction, and oxidation provides a useful method for the preparation of nitroalkenes with a hydroxymethyl group (Eq. 5.81).131... 

A similar reaction of methyl P-D-ribopyranoside (5) with P(NMe2)3 in dioxane gave 2,3,4-bicyclophosphite of P-methyl-D-ribopyranoside, 6a in 90% yield. It was sulfurated and selenated to give the corresponding thiophosphate 6b and selenophosphate 6c in 89 and 81% yields, respectively. Oxidation of 6a with hydrogen peroxide gave cyclic phosphate 6d (92%), whereas its hydrolysis in aqueous... 

Brus and co-workers produced nanoparticles of CdSe from the pyrolysis of the single-molecular cadmium selenate precursor [Cd(SePh)2].392 The similar metal(benzylthiolate) compounds [M(SCH2C6i Is)2] (M = Zn, Cd) were also used in the solid-state preparation of ZnS and CdS particles and produced mixtures of the hexagonal and cubic phases of the crystallites, with sizes of 5nm, on thermolysis between 200 °C and 400 °C under nitrogen.393 The bis(trialkylsilyl)chalco-genides [(Me3SiE)2Cd] (E = S, Se), prepared by heterocumulene metathesis as in Equation (11), were used to produce nanoparticles. If the reactions are carried out in TOPO, TOPO-passivated CdE nanoparticles can be obtained although there was little control over particle size 394... 

Although selenic and telluric acids contain the central atom in the +6 oxidation state, they are very different. The properties of selenic acid, H2Se04, are very similar to those of H2S04, and many of their salts are similar. The oxyacid that contains Te in the +6 state is HsTeOs which can also be written as Te(OH)s. This acid can be prepared from Te or Te02 by suitable oxidation reactions, and it can be also obtained as a solid hydrate. As expected from the formula, telluric acid is a weak acid, although some salts can be obtained in which one or more protons are replaced. 

Primary selenoamides are prepared by reaction of nitriles with appropriate selenating reagent, such as phosphorus (V) selenide (P2Se5), hydrogen selenide (H2Se), Al2Se3, NaSeH, trw(trimethylsilyl)monoselenophosphate or potassium selenobenzoate (Scheme 81).248 252... 

Common routes for the synthesis of selenates and tellurates(IV and VI) are the reactions of metal oxides or carbonates with the repsective acids (see Section 4). The disadvantage of this procedure is, that one usually obtains hydrates or, at higher acid concentrations, acidic compounds. Because the oxides E03 and E02 (E=Se, Te) are solids under ambient conditions, solid-state reactions with the respective metal oxides are an alternative route to prepare the anhydrous compounds. 

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