Chapter 18 (Lecture #19)

Chapter 18 (Lecture #19) - Lithium Enolates in Organic...

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Unformatted text preview: Lithium Enolates in Organic Synthesis The extent to which a carbonyl compound with an !-H is converted to its enolate anion depends on the acidity of the !-H and the strength of the base. C-C O = H + B:- C-C O = :- + B-H ? CH 3 CCH 3 O = pKa = 20 weaker acid + HO- CH 3 C-CH 2 O- weaker base stronger base + H 2 O pKa = 15.7 stronger acid Preparation of Stoichiometric Amounts of Enolate Anions The lithium amides may be prepared by reacting the secondary amine with a stoichiometric amount of an alkyl lithium (such as butyl or methyl) in ethyl ether or tetrahydrofuran at -78 o C. CH 3 CCH 3 O = + N CH CH 3 CH 3 CH CH 3 CH 3 : :- Li + pKa = 20 stronger acid stronger base lithium diisopropylamide ("LDA") CH 3 C-CH 2 O- Li + + N i-Pr Pr-i H : weaker base weaker acid pKa = 38 diisopropylamine 100% enolate anion CH 3 CH 2 CH 2 CH 2 Li + N: (CH 3 ) 2 CH CH(CH 3 ) 2 H ether-78 o C 100% pKa = 38 N: (CH 3 ) 2 CH CH(CH 3 ) 2 :- Li + CH 3 CH 2 CH 2 CH 3 + pKa > 50 LDA In the 1970s, it was found that nitrogen bases derived from highly branched secondary amines are powerful bases but poor nucleophiles . Stoichiometric amounts of enolate anions could be prepared by reactions as shown below: Note: The conjugate base of the amine is called an "amide." Do not confuse this name with the acid derivative called an amide. Other hindered secondary "amide" bases: Note: Hindered "amides" are strong bases , but poor nucleophiles. N: CH CH 3 CH 3 :- Li + N: :- Li + CH 3 CH 3 CH 3 CH 3 lithium cyclohexylisopropylamide lithium 2,2,6,6-tetramethylpiperidide C-Alkylation Reaction of an enolate anion with alkyl halides usually occurs by way of c-alkylation.c-alkylation....
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Chapter 18 (Lecture #19) - Lithium Enolates in Organic...

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