D. A. Evans Acidity Trends for Carbonyl Related Compounds Chem 206 a Carbanion Stabilization by 2rd-Row Atoms: SR, SO2R, PR3 etc a Conjugative Stabilization of Conjugate Base 17.2 pKA(DMSO) S-CH H3C-S-CH 18.2 (DMSO) 26.5 H CH3 (JAcS1976,98,7498;:JAcs1977,99,5633:MAcs1978.100,200 C-C≡N C=C=N 31.5 The accepted explanation for carbanion stabilization in 3rd row elements is delocalization into vicinal antibonding orbitals For efficient conjugative stabilization, rehybridization of carbanion orbital from nsp3 to np is required for efficient overlap with low-lying t-orbital of stabilizing group. However, the cost of rehybridization must be considered sXo(empty I Stereoelectronic Requirement for Carbanion Overlap nolization of Carbonyl Stereoelectronic Requirements: The a-CH bond must be able to overlap with C-O This argument suggests a specific orientation requirement. This has been noted: Anti (or syn) periplanar orientation of Carbanion-orbital and orbital mandatory Rates for deprotonation with n-BuLi pKA(DMSO) ZIshe He Ha=8.6 JAcs1974,96,1811) 5.2 Ph3C-H He: Ha=30 Acs1978100,200) C-HS He Ha H O O H He : Ha = 30 H Ph3C–H O O H S S Ha He Me Me H H Hb Hc R C O CH3 R O Hc Hb R O – Ha C S X C H H C N C H H C N P CH3 Ph Ph Ph C H H C O CH3 C H H C O CH3 S S H H H3C S CH3 O O S CH3 Me Me H3C S CH3 O C H H NO2 C H H N O O D. A. Evans Acidity Trends for Carbonyl & Related Compounds Chem 206 ■ Carbanion Stabilization by 2rd–Row Atoms: SR, SO2R, PR3 etc + 18.2 (DMSO) + 22.5 31 31 35 The accepted explanation for carbanion stabilization in 3rd row elements is delocalization into vicinal antibonding orbitals (JACS 1976, 98, 7498; JACS 1977, 99, 5633; JACS 1978, 100, 200). Cn S–Xs* E Cn (filled) S–Xs* (empty) This argument suggests a specific orientation requirement. This has been noted: Anti (or syn) periplanar orientation of Carbanion-orbital and s* orbital mandatory for efficient orbital overlap. He : Ha = 8.6 (JACS 1978, 100, 200) (JACS 1974, 96, 1811) pKA (DMSO) 17.2 26.5 31.5 For efficient conjugative stabilization, rehybridization of carbanion orbital from nsp3 to np is required for efficient overlap with low-lying p*-orbital of stabilizing group. However, the cost of rehybridization must be considered. ■ Conjugative Stabilization of Conjugate Base ■ Stereoelectronic Requirement for Carbanion Overlap: Enolization of Carbonyl Compounds pKA 5.2 C-H acidity not detectable pKA (DMSO) 31.5 47.7 Rates for deprotonation with n-BuLi Stereoelectronic Requirements: The a-C-H bond must be able to overlap with p* C–O – Ha + base p* C–O