Benzyl phenyl selenides

Enantioselective conversions of aryl benzyl selenides to N-tosylselenimides 28 with [(tosylimino)iodo]benzene, cuprous triflate, and the chiral bis(oxazo-line) 22 have recently been demonstrated (Scheme 16) [37,38]. Because benzyl phenyl selenide undergoes uncatalyzed imidation with Phi = NTs in acetonitrile (46 % yield) or dichloromethane (trace yield), toluene was selected as the solvent for the asymmetric imidation reactions. Furthermore, in order to avoid racem-ization of 28 by moisture, molecular sieves were added to the reaction medium. 

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. 

We have shown that benzylation and allylation at the selenium atom can be partially operative and depend on the enolate structure [38] (Scheme 32). With silyl enol ethers, derived from a-phenylselanyl ketones as substrates, the formation of ally or benzyl phenyl selenide was observed but the characterization of the carbene, which must be formed at the same time, was not achieved. In the course of the allylation or benzylation of phenylselanyl enolates, derived from a-phenylselanyl aldehydes, the amount of 0-allylated products increased with the size of the substituent. In both reactions, phenylselanyl enolates were... 

Diphenyl selenide is oxidized with peroxyacetic acid at room temperature to diphenyl selenoxide hydrate, C6H5Se(OH)2, in 43% yield after 2 h [1198]. Benzyl phenyl selenide is oxidized to benzyl phenyl selenoxide by sodium periodate in aqueous methanol at 0 °C in 95% yield and by iodobenzene dichloride in aqueous pyridine at -40 °C in 85% yield [773]. 

Treatment of benzylmethylphenylselenonium tetrafluoroborate with potassium r-butoxide gave a 9 1 mixture of methyl phenyl and benzyl phenyl selenides but no products which would have resulted had the appropriate ylide been formed (Scheme 66, a). This demonstrated that at least this salt is extremely susceptible to nucleophilic attack, even by nucleophiles as hindered as the r-butoxide anion (Scheme 66, a). ... 

The photo-induced cyclizations of the selenol esters (20) and (21) to give (22) and (23), respectively, have been described. Photolysis of benzyl phenyl selenide (24a) or of the phenyl ribosyl derivative (24b) yield the corresponding diselenides (25). The photoreduction of acetophenone by H2Se has been investigated. ... 

Benzyl phenyl selenide allowed to react at 0 with a slight excess of Na-meta-periodate in methanol-water -> benzyl phenyl selenoxide. Y 95%. - lodobenzene dichloride can be used in cases in whidi periodate is not effective sudi as diaryl selenides with electron-withdrawing groups. F. e. s. M. Cinquini, S. Colonna, and R. Giovini, Chem. Ind. 1969, 1737. 

An ethanolic soln. of benzyl bromide and Na-phenylselenide refluxed 2 hrs. -> benzyl phenyl selenide (Y almost 100%) treated with Li-diisopropylamide and ethyl bromide in tetrahydrofuran, the resulting crude mixture treated with 30%-H2O2, and the product isolated after 24 hrs. -> product. Overall Y 80%. F. e. s. R. H. Mitchell, Chem. Commun. 1974, 990 syntheses of ethylene derivs. via a-metalation of selenoxides cf. H. J. Reich and S. K. Shah, Am. Soc. 97, 3250 (1975). 

Scheme 1.9 Osmium-catalyzed dihydroxylation using Oj and benzyl phenyl selenide.

Deprotonation of Benzyl Selenides PhSeCHRPh (/ = alkyl). Deprotonation of benzyl phenyl selenide is readily effected (81%) with LDA at —78 but... 

Singlet oxygen (air, visible light and sensitizer) and benzyl phenyl selenide [which may be oxidized by O2 to the selenoxide, which may in turn oxidize Os(VI) back to... 

Coupled redox systems have also been successfully employed in asymmetric catalysis. Krief and coworkers were the first to use air as a terminal oxidant with the Sharpless dihydroxylation catalyst by using benzyl phenyl selenide as a photosensitizer [32-34]. The dihydroxylation of a-methylstyrene using the selenide sensitizer decreases the oxidant waste by over 50-fold compared to the standard oxidation conditions using AD-Mix (KsFelCNJg is the terminal oxidant in AD-Mix) with similar enantiomeric excess and yield [34]. This example of a coupled reaction showcases the potential waste reduction provided by this approach. 

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. " °... 

Phenylseleneny lotion of acetates and lactones. In the presence of zinc iodide, 1 reacts with alkyl acetates to form alkyl phenyl selenides. The reaction is particularly facile with acetates of tertiary, ailylic, and benzylic aicohols. Acetates of primary and secondary alcohols react only at 110°. 

Reactions with Nucleophiles. The epoxide is, by far, the more reactive site and a wide variety of nucleophiles have been used (eq 2) to open the ring at C-3 such as HCl (96%), HOAc (>50%), H2S (65% as cyclized product 3-thietanol), HCN (66%), ethanol (90%), t-butanol (86%), phenyl or benzyl thiol (99% or 93%, respectively), and phenyl selenide (generated in situ from the diselenide and sodium hydroxymethyl sulfite) (>55%). If desired, the epoxide is easily formed from the chlorohydrin by treatment with excess KOH or Et3N. 

One of the major drawbacks of these reactions is clearly the unavailability of the whole set of alkyl phenyl selenones due to side reactions which occur during their preparations. Methyl, n-alkyl and some cyclopropyl phenyl selenones are available from the corresponding selenides and potassium permanganate, peroxycarboxylic acids or hydrogen peroxide/seleninic acids mixtures, whereas s-alkyl phenyl selenones, 2,2,3,3-tetramediylcyclopropyl selenone and benzyl phenyl selenone are not available in these ways. 

Carbon-carbon bond forming radical reactions of phenyl selenides have also provided a wealth of synthetically useful methodology. Phenylselenomalonates [47] and malononitriles (Scheme 16) [48] can be added to olefins upon photolysis or thermolysis in the presence of AIBN. Phenylselenomalononitriles are the more reactive of the two, as expected, based on the I-transfer evidence. For example, phenylselenomalonates will not add to styrene [47], This is presumably because of the inability of the stable benzylic radical formed upon malonate radical addition to carry out the atom transfer step with another phenylselenomalonate. The phenylselenoma-... 

Phenyl selenide transfer radical addition reactions can be limited by the lack of reactivity observed in some precursors. Simple alkyl phenyl selenides do not undergo inter- or intramolecular radical additions to olefins. Phenylselenotrichloromethane will add to olefins upon photolysis, and the products formed can be elaborated into a, ff-unsaturated carboxylic acids (Scheme 20) [57], Benzyl phenylselenides have been observed to undergo atom transfer cyclization (Scheme 20) [58]. 

Organolithiums. Allylic and benzylic alcohols undergo deoxygenative lithiation by treatment of their lithium alkoxides or phenyldimethylsilyl ethers with LDTBB. Alkyl phenyl selenides are also cleaved to give organolithium species that can react with aldehydes and allyl bromide. Some special alkyllithiums have been prepared from (2-pyridylthio)alkanes, which are available from carboxylic acids. ... 

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. ... 

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