General eactivity base or acid or E + E - Keto-Enol Tautomerization (enol form usually very minor for simple ketones) - Can enhance rate / concentration by addition of acid or base + catalyzed + + + base induced base + + B
General eactivity base or acid or E + E X 2 'X X W 'C ' or ' W ' - ther functional groups can stabilize negative charge: -S 2, -C, 2, etc.
pka s of Some Common Carbon Acids compound pka compound pka C >45 (~60) C C 44 Ph C 37 2 40 S 2 C 27 25 2 35 C C 25 C 25 C 20 C 17 16 16 13 11 Ar 9-10 9 2 C 9 C 3 C 2 5
Effect of Substitution on Acidity W W alkyl halogen C 2 =C- Ph- S- pka effect 1-2 unit increase 1-2 unit decrease 5-7 unit decrease 5-7 unit decrease 3-5 unit decrease others: 2 > C > S 2 > C 2 > C > S = Ph
Generation of Enolates: bases!!! Metal ydrides: - examples: a, K - amorphous solids - very basic (conjugate acid is 2 ) - solubility may be a problem Alkyl Lithiums: - examples: MeLi, nbuli, sbuli, tbuli - souluble - very basic (conjugate acid is an alkane) - good nucleophiles side reactions possible Amides: - examples: a 2, Li 2, K 2 - strongly basic (conjugate acid is ammonia) - solubility may be a problem
Generation of Enolates: bases!!! Soluble Amine Bases: - examples: LDA, LICA, LTMP, MMDS (M = Li, a, K) - can be prepared by reaction of amine with nbuli - strongly basic (conjugate acid is an amine) - hindered, non-nucleophilic - amine by-product is typically easy to remove Li LDA (lithium diisopropylamide) hindered Li LICA (lithium isopropyl cyclohexyl amide) very hindered Li LTMP (lithium 2,2,6,6-tetramethylpiperidine) very hindered Si Si Li LiMDS (lithium bis(trimethylsilyl)amide) or (lithium hexamethyldisilylazide) very hindered, slightly less basic
Generation of Enolates: bases!!! Alkoxide Bases: - examples: Mea, tbuk - prepared by reaction of alcohol with M - weaker than amine bases (conjugate acid is an alcohol) - deprotonation is generally reversible
Enolate Concentration: Choice of Base + Mea + Me + Li +
egioselectivity of Deprotonation base base kinetic enolate thermodynamic enolate kinetic conditions: deprotonation under irreversible conditions; most accessible proton removed first LDA, LTMP, KMDS (1.05 equiv), TF, -78 C thermodynamic conditions: deprotonation is reversible; equilibrium conditions most stable enolate is formed tbu/tbu, ame/me or LDA (0.95 equiv), TF, 0-23 C
egioselectivity of Deprotonation base base kinetic enolate thermodynamic enolate - factors that influence kinetic vs. thermodynamic selectivity: kinetic aprotic solvents strong base; weakly nucleophilic oxophilic counterions (e.g. Li) low temperature (-78 C) short reaction times excess base irreversible deprotonation LDA LiMDS (a, K) LICA Ph 3 CLi, -78 C thermodynamic protic solvents weak base potassium counterions higher temperatures longer reaction times excess ketone reversible deprotonation a/et a, K, Li tbuk/tbu Ph 3 CLi, Δ LDA (0.95 equiv)
egioselectivity of Deprotonation base TF + LDA (1.05 equiv), -78 C 99 : 1 LDA (0.95 equiv), 0-23 C 10 : 90 Et 3, TMSCl Ph, 60 C TMS
egioselectivity of Deprotonation - kinetic selectivity sensitive to structure pka 26 (DMS) Ph LDA TF, -78 C Ph + Ph pka 20 (DMS) 99 : 1 Ph LDA TF, -78 C Ph + Ph 14 : 86 100 100 K 0 0 T
Under Kinetic Conditions 1. Direct Deprotonation LDA (1.05 equiv) TF, -78 C 2. egeneration from Silyl Enol Ethers TMS MeLi, TF or F - (dry) 3. Enone eduction Li / 3 or K(sBu) 3 B
Under Kinetic Conditions 4. Cuprate Addition 2 CuLi 5. Corey Enders Me 2 LDA Me 2 6. From α-alo Ketones Br Zn
rigin of Kinetic egioselectivity - Bulky base reacts most rapidly with less substituted C- C 3 > C 2 >> C - C- must be co-planar to C= π system 3 1 2 LDA; MeI 3 Me 2
Under Thermodynamic Conditions 1. LDA (0.95 equiv) LDA (0.95 equiv) TF, 0-23 C 2. Mg Counterion 1. ipr 2 MgBr TMS 2. TMSCl, Et 3 MPA
ther Enolates Et ' C 2 S Ph