Ch_322b_17.08

Ch_322b_17.08 - Chapter 17.8 lecture note

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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 ? In the classic aldol condensation , an aldehyde or ketone with enolizable H reacts with hydroxide ion : 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
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Alkylation of Enolate Anions The above equilibrium produces only a low concentration of the nucleophilic enolate anion, which undergoes an addition reaction with the carbonyl compound. While these reaction conditions work for the aldol reaction, a self-condensation reaction , they fail when the goal is alkylation of an enolate anion: Under standard aldol conditions, there is only a low concentration of the enolate anion, and the self-condensation reaction occurs. CH 3 C-CH 2 O - + R- X alkylation CH 3 CCH 2 R + X - O = X
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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
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Ch_322b_17.08 - Chapter 17.8 lecture note

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