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2784 J. Am. Chem. SOC.1983, 105, 2784-2793 The Role of Hyperconjugation In Determining the

Summary: 2784 J. Am. Chem. SOC.1983, 105, 2784-2793
The Role of Hyperconjugation In Determining the
Stereochemistry of Nucleophilic Epoxidation and
Cyclopropanation of Electrophilic Olefins
Yitzhak Apeloig,*2PMiriam Karni," and Zvi Rappoport*2b
Contribution from the Department of Chemistry, Technion-Israel Institute of Technology,
Haifa 32000, Israel, and the Department of Organic Chemisty, The Hebrew University,
Jerusalem 91904, Israel. Received July 30, 1982
Abstract: The barriers to rotation in a number of @-substitutedethyl anions of the general formula ZXCH2CYY'- where
X = 0,CH2, and S, Z = CI, F, H, OH, OCH3,OCHO, SiH3,and SH2+,and Y and/or Y' = H, NO2,and CN were calculated
by standard ab initio methods using the STO-3G, 3-21G, 4-31+G, and 6-31G' basis sets. All the carbanions are more stable
in the perpendicular conformation (14) where the carbanionic lone pairs and the C-X bonds are in the same plane. The barriers
to rotation around the C-C- bonds in the parent ZXCH2CH2-anions, which are a measure of the hyperconjugating ability
(HCA) of the C-XZ bond, are as follows (in kcal mol-', STO-3G): X = 0,Z = C1 (29.4) >> OCHO (19.8) > F (17.3) -OCH3(17.1) > OH (16.4) > SiH3(14.8) > H (12.4). With X = S the barriers are similar, Le., Z = C1 (26.1) > H (15.1).
The barriers to rotation are much lower for X = CHI, Le., Z = H2S+(21.5) >CI (10.0) > F (4.6) > H (2.3). Electron-withdrawing
a-substituents reducesignificantlythe rotation barriers, Le., CIOCH2CHCN-(11.8)>CIOCH2C(CN)Y(6.3) E C10CH2CHN0y
(6.6) > CICH2CH2CHCN-(3.9). Similar rotation barriers are obtained at higher levels of theory. This order of HCA is
rationalized by PMO theory. The results of the calculationsare used to analyze the stereochemistry of nucleophilic vinylic
epoxidation and cyclopropanation. The following conclusions are obtained: (a) the higher the HCA of the C-XZ bond is
the higher the stereospecificity; (b) a-electron-withdrawing substituents Y or Y' lower the stereospecificity; (c) the better the


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


Collections: Chemistry