Selenation, of aldehydes

Wang and co-workers recently developed an efficient strategy for the a-selenation of aldehydes and ketones using the proline derivatives as catalysts under mild conditions, the products were obtained in good yield (Eq. 10) [35]. 

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... 

Selenolates such as Na2Se, NaSeH, PhSeNa, etc., and tellurolates such as NapTe, NaTeH, PhTeNa, etc., are excellent nucleophiles and can reduce a variety of functional groups by nucleophilic attack or single electron-transfer. On treatment with alkali metal selenolates (or amine salts of HpSe and PhSeH), reduction or reductive selenation of ketones and aldehydes, C=C reduction of a,/l-unsaturated compounds, and reduction of nitrogen compounds such as nitro compounds occur successfully [118, 176]. Compared with these selenolate anions, the corresponding tellurium compounds are highly reactive not only toward the same substrates but also toward halo compounds such as a-bromo ketones and vic-dibromoalkanes [46, 52, 177]. 

A second major application of selenium dioxide is the oxidation of aldehydes and ketones to the corresponding l,2-dicarbonyl.533 7 55 conversion proceeds by reaction of the enol form of the carbonyl (386) with selenium dioxide to give the selenous enol ester, 387. Oxidative rearrangement to 388 is followed by loss of... 

The transformation of saturated ketones into a,/8-unsaturated ketones is often carried out by the introduction of an a-seleno-substituent, followed by oxidation and elimination. An alternative method for selenation of ketones (and esters) has been described wherein the lithium enolate reacts with selenium metal (rather than a selenyl halide) followed by methyl iodide (Scheme 29). Yields are comparable to the classical method. Phenylselenyl chloride reacts with enamines derived from aldehydes to give a-phenylselenoaldehydes, and hence o(,/3-unsaturated aldehydes by oxidative elimination [equation (21)]. ... 

The oxidation is regarded as taking place by an electrophilic attack of selenium dioxide (or selenous acid, H2Se03, the hydrate) on the enol of the ketone or aldehyde. This is followed by hydrolytic elimination of the selenium.258... 

Selenals, selones, and tellurals generated in situ via [3,3] sigmatropic rearrangements of allyl alkenyl selenides and tellurides 167 can be trapped by 2,3-dimethyl-l,3-butadiene affording the expected cycloadducts (Equation 68) <1995CL135>. Higher yields were noted in less-hindered cases where the selenium and tellurium aldehyde... 

The above schemes work reasonably well for certain enzyme reactions, especially for substrates where oxygen addition/loss occurs at a main group element (e.g., N, S, Se, Cl, see Table I). In addition to SO and nitrate reductase, key examples are DMSOR, trimethylamine oxide reductase, chlorate reductase, and selenate reductase. In the case of enzymes catalyzing C-based redox reactions of organic molecules, notably XDH and aldehyde oxidase, a direct OAT step is unlikely and is replaced by mechanistic steps typical of hydro-xylation (2). The essential features of the mechanism are shown in Fig. 10 for xanthine dehydrogenase/oxidase. 

Nakayama and co-workers established that carbonyl-stabilized sulfur ylide (83) reacted with elemental selenium to afford 1,3-oxaselenole (85) in good yield (85TL2201). The reaction was presumed to proceed through the intermediacy of selenal (84). Subsequently, support for this mechanism has come from the isolation of adduct 86 when the reaction is performed in the presence of dimethylbutadiene (87TL4423). Other Diels-Alder reactions of intermediate a-oxoseleno-aldehydes and -ketones were also found... 

Addition of phenylethynethiolate or -selenate to the carbene complex 185 (M = Cr or W, R = Me or Et, E = S or Se) affords an anionic adduct (186) which, unexpectedly, contains a reactive thioketene function. Electrophiles can attack either at the sulfur or at the carbon a thereto to produce coordinated carbenes or thio- and seleno aldehydes and esters [H4J45). 

Phosphorus-containing Ring Systems. - A range of new chiral oxazaphospholidine oxides 266 and 267 have been synthesised and used as catalysts in asymmetric reductions of ketones with diborane. Mannich-type cyclisation reactions of 5-amino-3-benzylthio-4-cyano(ethoxycarbonyl)pyrazoles with dichlorophenylphosphine and aromatic aldehydes in the presence of cation exchange resin have been used to prepare a number of 6-oxo-6-phospha-4,5,6-trihydroimidazolo[l,2-b]pyrazoles, e.g. 268. Some of these compounds have herbicidal activity and this report is typical of a number of similar ones in the Chinese literature. A number of metallocycles, e.g. 269, have been reported as products from reactions of transient zirconocene-benzyne intermediates with phosphaimines followed by sulfuration or selenation. ... 

SELENIC ANHYDRIDE (13768-86-0) OgSe Noncombustible solid. Violent reaction with water, forming selenic acid. A strong oxidizer. Violent reaction with many substances including reducing agents, hydrides, nitrides, and sulfides cyanides, esters, combustible materials, active metals, organic substances, aldehydes, alkenes, carboxylic acids, isocyanates, and thiocyanates. Attacks most metals in the presence of moisture. 

It might also be noted that loss of sulphur occurs when 0,5-dialkyl methylphospho-nothioates react with thionyl chloride or PCl5 The ability of certain carbonyl-containing compounds, such as amides, aldehydes and some heterocyclic systems, to undergo oxygen-selenium exchange represents an oxidative deselenation of PhP(Se)Cl2, a reagent used for selenation purposes ". ... 

The ab initio quantum-chemical method on the 6-31G basis has been used to study the mechanism of selenium oxide and selenous acid oxidation of 6-methyluracil to orotic aldehyde. The mixed anhydride of acetic and selenous acids, which possessed high activity and steric accessibility in electrophilic attack on position 5 of the pyrimidine ring, was formed with a gain of energy. The three-stage mechanism of the oxidation of the Me group in 6-methyluracil by the mixed anhydride of selenous and acetic acids had been analysed. ... 

Electrochemically induced reactions have been a feature of selenium chemistry in the past two or three years. This year a potentially useful synthesis of ap-unsaturated aldehydes (279) has been reported and involves electrochemical generation of the active selenating species from diphenyl diselenide. Propargyl alcohols are thus selenated to give (279) in good to excellent yields. The ap-unsaturated aldehydes (279) are also formed when 1-lithioselenoalkenes, RCH=C(Li)SePh, react with DMF. Various other a-substituted alkenes are obtained in a similar fashion when DMF is replaced by other electrophiles. 

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