Selenoxide, methyl phenyl

Methylphenylpropylphosphine, 387 Methyl phenyl selenoxide, 291 (2-Methyl-2-propenyl)trimethylsilane, 7 Methyl pseudomonate A, 66 Methyl pseudomonate C, 66 N-Methylpyrrolidone, 176 Methyl salicylates, 257 a-Methylseleno ketones, 345 1-Methylseleno-l-lithiocyclopropane, 318 3-Methyl-2-selenoxo-l,3-benzothiazole, 270... 

Sodium periodate is also frequently used as an oxidant for selenides, the reaction proceeds slowly in aqueous methanol. Various selenoxides such as methyl phenyl and benzyl phenyl selenoxides, (3B), (39),2 (40) and 2-azidocyclohexyl phenyl selenoxide have been isolated in this way. Other solvents and reaction conditions may also be employed. " °... 

Until quite recently the isolation of optically active selenoxides has been limited to those contained in steroids (isolated as diastereoisomers). The difficulty in obtaining these compounds was attributed to the racemization through the achiral hydrated intermediates. Simple optically active selenoxides (5-11% ee) were first prepared by kinetic resolution. Direct oxidation of selenides to selenoxides was first reported using optically active oxaziridine derivatives under anhydrous conditions, but the extent of the asymmetric induction was somewhat unsatisfactory with methyl phenyl selenide as substrate (8-9% ee). Recently much improved enantiomeric excesses (45-73%) were achieved with new oxaziridine reagents such as (70). An attempt at the asymmetric oxidation of more bulky selenides was independently carried out using Bu OCl in the presence of (-)-2-octanol (equation 55),2 but resulted in unsatisfactory enantioselectivities (ee 1%). Much better results were obtained by the oxidation of P-oxyalkyl aryl selenides (ee 18-40% equation 56) and alkyl aryl selenides (ee 1-28%) 2S using TBHP in the presence of (+)- or (-)-diisopropyl tartarate (DIPT) and titanium(IV) alkoxide. 

In the former case, almost complete stereoselective oxidation to the chiral selenoxides has been accomplished quite recently. The Davis oxidant, 3,3-di-chloro-l,7,7-trimethyl-2 -(phenylsulfonyl)spirobicyclol2.2.11heptane-2,3 -oxa-ziridine, was found to be the most efficient reagent for the enantioselective oxidation of a variety of prochiral alkyl aryl selenides [81. Asymmetric oxidation was accomplished by the treatment of the selenides with 1 molar equivalent of the Davis oxidant at 0°C to afford the corresponding chiral alkyl aryl selenoxides in quantitative yields with 91-95% ee (Scheme 1). The oxidation of methyl phenyl selenide was complete within 1 min, whereas that of triiso-propyl(a bulkier alkyl) phenyl selenide required a few hours. Typical results are... 

Like sulfides, selenides can be oxidized to the corresponding oxides, selenoxides, or to selenones. Oxidation to selenoxides is much faster and, with some oxidants, final. Reaction conditions are specified in equation 592 for the oxidation of methyl phenyl selenide [325, 711, 773, 1034]. 

Selenoxide elimination. The yield of alkenes from alky) phenyl selenoxides and alkyl methyl selenoxides under usual conditions (30% H2O2, O3, Oa) tends to be rather low because of formation of the original selenide. Much higher yields are obtained if the selenides are oxidized with /-butyl hydroperoxide (4 equiv.) in the presence of basic alumina (8 equiv.) in THF at 55°. No epoxida-tion is observed under these conditions. A less satisfactory method is ozonization in CH2CI2 in the presence of 1-3 equiv. of triethylamine. ... 

Phenyl methyl selenium dihydroxide, (C6H5)(CH3)Se(OH)2.— The dibromide (10 grams) is triturated with a suspension of 12 grams of silver oxide in 100 c.c. of water until the yellow colour disappears. Filtration, evaporation and desiccation yield about 8 c.c. of the hydroxide as a viscous oil. When heated at 100° C. at 15 mm., or at 170° C. at 760 mm., it decomposes, yielding phenyl methyl selenide, diphenyl diselenide and formaldehyde. The formation of formaldehyde is in agreement with the view that phenyl methyl selenoxide may exist in two isomeric forms, which, being unstable, decompose thus ... 

Treatment of the hydronitrates in aqueous solution with sodium carbonate causes evolution of carbon dioxide. Evaporation to dryness, followed by extraction with alcohol or benzene, then yields oils which are probably the selenoxides. These oils with concentrated hydrochloric acid are converted into white solids, crystallisable from benzene, xylene, alcohol or dry ether. These solids are the dichlorides of the original selenides, and when prepared by this method their melting-points are as follows Phenyl methyl selenium dichloride, M.pt. 122° C. phenyl ethyl selenium dichloride, M.pt. 64° to 65° C. diphenyl selenium dichloride, M.pt. 142° C. 

In another simple procedure, deprotonation of methoxy bis(trimethylsilyl)methane with butyl lithium and addition of the resulting anion to aldehydes induces Peterson elimination (Scheme 27). The product methyl enol ethers could be hydrolysed to the parent acyl silanes with hydrochloric acid-THF or could be treated with electrophiles such as M-halosuccinimides to give a-haloacyl silanes105. Alternatively, treatment with phenyl selenenyl chloride, oxidation at selenium and selenoxide elimination afforded a,/3-unsaturated acyl silanes. 

The isolable and thermally stable selenoxides are, therefore, rather limited. Stable examples are as follows those derived from selenides which have no hydrogen atoms on the -carbon, such as dimethyl sel-enide, aryl methyl selenides, diaryl selenides and benzyl phenyl selenides, those with an intramolecular hydrogen bonding, such as (36) and (37), and those leading to an unfavorable double bond such as (38). Vinylic selenoxides (39) and (40) are also generally isolable. 

Thus, when cyclohexyl selenides 1, prepared from the corresponding 4-sub-stituted cyclohexanone via the selenoketals, were oxidized with various Davis and Sharpless oxidants, the chiral alkyl aryl 4-substituted cyclohexylidenemethyl ketones were obtained in excellent chemical yields with high enantiomeric excesses. Typical results are summarized in Table 4. In this asymmetric induction, of the substrate and the chiral oxidant employed were revealed to show a remarkable effect upon the enantioselectivity of the product. The use of a methyl moiety as instead of a phenyl moiety gave a higher ee value, probably due to the steric difference between the two groups bonded to the selenium atom of the substrate. The results indicate that the titanium complex of the Sharpless oxidant may promote the racemization of the chiral selenoxide intermediate by acting as a Lewis acid catalyst, whereas the racemization in the case of the Davis oxidant, which is aprotic in nature, is slow. 

Phenyl vinyl selenoxide was expected to be ccmsiderably more acidic than the corresponding seletiide and it was thought that it might be deprotonated more easily and with greater selectivity at the vinylic carbon. Deprotonation with LDA occurs" at -78 C as well as at -90 C (Scheme 49), but the decomposition of the resulting a-lithiovinyl selenoxide is so rapid ha = 0.5 h at -78 C in THF) tiiat the methylation product is formed in less than 50% yield (Scheme 49)."... 

In the selenium series, methyl phenylselenide in chtoroform at 25°C upon portionwise treatment with 1 mole of 2-benzenesulphonyl-3-(4-nitro-phenyl)oxaziridine and reaction for 1 min. only, gave the selenoxide in 97% yield (ref. 183). 

Syntheses of Alkylidene cyclopropanes Via the Selenonium route The selenonium route proved to be more valuable. It has been specifically designed by us to replace the deficient selenoxide route (Scheme 38). It was expected to produce alkylidene cyclopropanes by a mechanism which mimics the selenoxide elimination step but which involves a selenonium ylide in which a carbanion has replaced the oxide. Cyclopropyl selenides are readily transformed to the corresponding selenonium salts on reaction with methyl fluorosulfonate or methyl iodide in the presence of silver tetrafluoroborate in dichloromethane at 20 °C and, as expected, methylseleno derivatives are more reactive than phenyl-seleno analogs. Alkylidene cyclopropanes are, in turn, smoothly prepared on reaction of the selenium salts at 20 °C with potassium tert-butoxide in THF (Scheme 38). Mainly alkyl cyclopropenes form at the beginning of the reaction. They then slowly rearranges, in the basic medium, to the more stable alkylidene cyclopropanes( 6 kcal/mol). In some cases the complete isomerisation requires treatment of the mixture formed in the above reaction with potassium fcrt-butoxide in THF. The reaction seems to occur via a selenonium ylide rather than via a P-elimina-tion reaction promoted by the direct attack of the /crt-butoxide anion on the P-hydrogen of the selenonium salt, since it has been shown in a separate experiment that the reaction does not occur when a diphenylselenonium salt (imable to produce the expected intermediate) is used instead of the phenyl-methyl or dimethyl selenonium analogs. It has also been found that the elimination reaction is the slow step in the process, since styrene oxide is formed if the reaction is performed in the presence of benzaldehyde which traps the ylide intermediately formed... 

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