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J. Am. Chem. SOC.1990, 112, 9131-9140 9131 Alkyl-and Silyl-Substituent Effects on Keto-Enol Equilibria

Summary: J. Am. Chem. SOC.1990, 112, 9131-9140 9131
Alkyl- and Silyl-Substituent Effects on Keto-Enol Equilibria
and the Structures of Simple Aliphatic Enols. A Theoretical
ab Initio Study8
Yitzhak Apeloig,*,+Dorit Arad,+and Zvi Rappoportt
Contribution from the Department of Chemistry, Technion-Israel Institute of Technology,
Haifa 32000, Israel. and the Department of Organic Chemistry, The Hebrew University,
Jerusalem 91904, Israel. Received January 10, 1990
Abstract: Ab initio molecular orbital theory was used to study the following enol-carbonyl pairs: H,C=C(OH)R (4)-
CH3C(=O)R (5) [R = H, Me, Et, i-Pr, t-Bu, SiH,, SiMe'], CH,CH=C(OH)R (6Eand 6Z)-CH,CH2C(=O)R (7) [R
= H, Me, Et, i-Pr, t-Bu], (CH3)2C=C(OH)R (8)-(CH3),CHC(=O)R (9) [R = H, Me]. Geometries were optimized with
thc 3-21G basis set, and for most moleculessingle-point6-31G* calculations were also performed. For 4 and 5 (R = H, Me)
gcomctryoptimizationswere carried out alsoat 6-31G* and 6-31G**and for R = H also at MP2/6-31G*. In the most stable
conformation of the carbonyl compoundsa C-H or preferably a C-C bond (Si-H and Si-C bonds, respectively,in the a-silyl
kctoncs) cclipscs the carbonyl bond. All the enols adopt the syn conformation (Le., LH-O-C=C = OO), but AE(syn-anti)
is smallcr in thc 62series than in the 6Eor the 4 series. In the 62series the C-H bond of the @-methylgroup eclipses the
C=C bond. Our best calculated AE(4a-5a) and AE(4b-5b) energy differences are 14.9 (at MP4SDTQ/6-31G**//6-
31G**+ZPE) and 17 kcal mol-', respectively,but a larger MP2/6-311++G**//6-31G* calculation gives AE(4a-5a) = 11.1
kcal mol-' (ref 30a). Addition of polarization functions on hydrogen and further splitting of the valence shell lowers AE by
ca. 7.3 kcal mol-', but electron correlation has a small effect on PE. The best theoretical AE values are higher by 2-3 kcal


Source: Apeloig, Yitzhak - Department of Chemistry, Technion, Israel Institute of Technology


Collections: Chemistry