Selenides, benzyl

This displacement of an allylic QH5Se group by a (CH,)3Sn group is observed also with allenyl, propargyl, and benzyl selenides, and in all cases results in the less substituted of the two possible trimethylstannyl compounds. 

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. 

Thermolysis or photolysis of the benzyl selenide 315 gave the selenapenam 316 (Equation 34) <20040BC2612>. 

A related radical transformation can be used to prepare 2,2-disubstituted selenanes. Treatment of the alcohol 124 with oxalyl chloride followed by addition of 2-mercaptopyridine iV-oxide and heating afforded the selenane 125 in moderate yield (Scheme 11) <2001JOC6286>. The required alcohol precursor for this reaction can be readily prepared from the bromo ester and in ////-generated sodium benzyl selenide, followed by Grignard addition. 

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... 

Selenolates prepared from diphenyl or dimethyl diselenide by reduction with NaBH4 smoothly transform various benzylic alcohols 24 into the corresponding selenides 25 in the presence of aluminum chloride (Scheme 30 a) [41]. AICI3 is considered to activate the alcohol substrate by coordinating to the oxygen. Similar transformations are possible by the reaction of alcohols with phenyl selenocyanate in the presence of tributyl phosphine [52]. When the selenolate is reacted with aromatic aldehydes or ketones 26 in the presence of AICI3, the corresponding benzylic selenides 27 are obtained in moderate yields (Scheme 30b) [41]. 

The halogen-metal exchange of secondary alkyl iodides leads mainly to hydrocarbons containing Wurtz-type coupling or elimination products [44], except in the case of benzyllithium derivatives which are easily generated from benzyl selenides [45]. 

The 6-exo-trig cyclization of a 6-heptenylmetal to a (cyclohexylmethyl)metal is much slower than the analogous 5-exo-trig carbocyclization. Indeed, as shown in Scheme 7-64, the 6-exo rearrangement of 6-heptenyllithium requires the presence of TMEDA to facilitate the process [44]. However, in the case of 6-alkenyl benzyl selenides, the formation of the six-membered rings is possible by carbocyclization [70] as well as by the use of Ziegler-Natta conditions [71],... 

Benzyl selenides exhibit a similar pattern of leactivi. Thus, although the parent aryl benzyl selenides are successfiilly metallated with LDA at -78 their higher homologs, as well as benzyl mediyl... 

Benzyl selenides Direct conversion of benzylic alcohols to the selenides is achieved using the MCjSiSePh-AlBtj combination. 

Benzyl selenides. PhSeNa derived from PhSeSePh and NaBH,j converts benzyl alcohol to BnSePh in the presence of AlCl,. The direct transformation of benzaldehydes into... 

Nucleophilic and radical chemistry of benzyl selenides has been used in the preparation of 93 and 94 <0ITL4737>. 

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

Applications Based on Deprotonation of Benzyl Selenides PhSeCHRPh R =H or alkyl). Only a low degree of stereoselection is observed in the reaction of PhSeCH(Li)Ph with benzaldehyde and propanal. The derived alcohols of RS,SR) and (RR,SS) configuration were converted into olefins (using MsCl and EtjN), the former giving the (Z)-olefin and the latter the (jB)-isomer. In further experiments the salt PhSeCH(Li)Ph was alkylated with several alkyl halides and with propylene oxide. ... 

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