Exchange selenium-lithium

CuCl-catalyzed decomposition of iodonium ylides prepared from /3-keto esters and diacetoxyiodobenzene, has been developed (equation 151)331. 1-Methylbenzvalene is obtained in a good yield by treating a mixture of lithium cyclopentadienide and 1,1-dichloroethane with butyllithium332. The tandem cyclization substitution in l-selenyl-5-hexenyllithiums derived from corresponding selenacetals via selenium/lithium exchange produces bicyclo[3.1.0]hexane derivatives333. 

Lithiation next to selenium is by contrast impossible organolithiums attack the Se atom directly.92 a-Lithio selenides are best formed by selenium-lithium exchange (section 3.3.1)... 

Gilman noted in 1949 that butyllithium slowly displaces phenyllithium from phenylselenides in ether.134 But it was the work of Krief, who reported in 1974135 that in THF this reaction becomes much faster (complete in 30 min at 0 °C) that gave selenium-lithium exchange synthetic possibilities. The reaction of a dialkylselenide - or a trialkylselenonium salt - with an alkyllithium, displacing the most stable organolithium, is effective, but of rather limited use, and there is insufficient difference in stability between Me and Bu for clean displacement of one by the other ... 

With a sulfur electrophile, mixed thioselenoacetals 177 form which may then undergo a second selenium-lithium exchange, leading to fully substituted a-thio organolithiums 178.143... 

Tellurium-lithium exchange proceeds in much the same way as selenium-lithium exchange but is rarely used.144 Vinyltellurides such as 179 can be converted stereospecifically into vinyllithiums 180,145 and alkyltelluro-heterocycles have been used as precursors to lithio-species.146... 

Halogen-lithium exchange of vinylbromides retains double bond geometry.160 5.2.3 Selenium-lithium exchange... 

It is possible to form, and cyclise, tertiary alkyllithiums, provided they are benzylic or allylic, by starting with a selenide. Krief has used selenium acetals to construct the starting materials 249 and 252, and on treatment with n-BuLi an extremely rapid (less than 20 min even at -110 °C effectively instantaneous at -78 °C) selenium-lithium exchange ensues to give tertiary organolithiums 250 and 253. Cyclisation to give 251 or 254 takes half an hour at -78 °C, and... 

The lithiated compound derived from 120 and also other cr-phenylselanylcyclohexyllithium compounds are configurationally labile even at low temperatures.203,204 The diastereoselection in the selenium-lithium exchange... 

Krief and Barbeaux have also applied the selenium-lithium exchange methodology to some cascade carbolithiation reactions as shown in Scheme 3667. The l,l-(bis-methylseleno) derivative 128 undergoes a selenium-lithium exchange at —78 °C giving... 

Trying to carry out 3 -exo carbolithiation reactions with the tertiary benzyllithium 147, generated by selenium-lithium exchange, Krief and Barbeaux have reported66 an isolated example of the reaction of this homoallylic lithium reagent with ethylene and further intramolecular carbolithiation reaction of intermediate 148 onto the suitably positioned carbon-carbon double bond. The resulting 1,3-dimethyl-l-phenylcyclopentane was isolated in modest yield and as a 1 1 mixture of diastereoisomers (Scheme 41). 

Synthesis of a-selenoalkyllithiums by selenium-lithium exchange 2.6232 Synthesis of a-selenovinyl metals by selenium-metal exchange... 

For the synthesis of a-selenoalkyllithiums, the selenium-lithium exchange reaction is a good alternative to the almost impossible metallation of unactivated selenides.Thus it has been found that a large variety of selenoacetals, often readily available from carbonyl compounds and selenols, react with butyllithiums to provide a-selenoalkyllithiums - in very high yields (Scheme 2 see also Section 2.6.2.3). 

Procedures which utilize selenides are similar, but a-lithio selenides are not generally preparable via simple deprotonation chemistry, due to facile selenium-lithium exchange. - Selenium-stabilized anions are available, however, by transmetalladon reactions of selenium acetals and add readily to carbonyl compounds. The use of branched selenium-stabilized anions has been shown to result exclusively in 1,2-addidon to unhindered cyclohexenones, in contrast to the analogous sulfur ylides. The resulting 3-hydroxy selenides undergo elimination by treatment with base after activation by alkylation or oxidation (Scheme 10). An alternative method of activating either p-hydroxy selenides or sulfides toward elimination involves treatment of a chloroform solution of the adduct with thallium ethoxide (Scheme 11). A mechanism involving the intermediacy of a selenium ylide is proposed. 

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