D.A. Evans Ground state Torsional effects Chem 206 Torsional Effects Relevant Orbital Interactions: Torsional Strain: the resistance to rotation about a bond Torsional energy: the energy required to obtain rotation about a bond Torsional Angle: also known as dihedral angle Torsional steering: Stereoselectivity originating from transition stat torsional energy considerations GC-H& T electrons are C-H's properly aligned for T* overlap AG =+3 kcal mol destabilizing Dorigo, A E Pratt, D W; Houk, K N. JACS 1987, 109, 6591-6600 Conformational Preferences: Acetaldehyde Torsional Strain(Pitzer Strain): Ethane The eclipsed conformation(conformation A)is preferred Polarization of the carbonyl decreases the 4 electron destabilizing Houk. JACS 5,59805988 +2.0 kcal/mol eclipsed Conformational Preferences conformation 1-Butene(X= CH2); Propanal (X=O) Wiberg K B. Martin, E.J. Amer. Chem. Soc. 1985, 107, 5035-5041 A M H X+Me B See Lecture 5 for previous discussion Conformation A is preferred. There is little steric repulsion between theTorsional Angle: also known as dihedral angle H H H H H O H Me X H H C C H H H H C C H H H H H H C C H H H C C H H H H H O H H H H H H X H H H Me H H B A B C H H H CH2 CH2 H A B A H C H H H D. A. Evans Ground State Torsional Effects Chem 206 Torsional Strain: the resistance to rotation about a bond Torsional energy: the energy required to obtain rotation about a bond Torsional steering: Stereoselectivity originating from transition state torsional energy considerations DG = +3 kcal mol-1 Torsional Strain (Pitzer Strain): Ethane staggered eclipsed Relevant Orbital Interactions: Dorigo, A. E.; Pratt, D. W.; Houk, K. N. JACS 1987, 109, 6591-6600. Wiberg K. B.; Martin, E. J. Amer. Chem. Soc. 1985, 107, 5035-5041. s C–H's properly aligned for p* overlap hence better delocalization s C–H & p electrons are destabilizing Conformational Preferences: Acetaldehyde The eclipsed conformation (conformation A) is preferred. Polarization of the carbonyl decreases the 4 electron destabilizing Rotational barrier: 1.14 kcal/mol Houk, JACS 1983, 105, 5980-5988. Conformational Preferences 1-Butene (X = CH2); Propanal (X = O) Conformation A is preferred. There is little steric repulsion between the methyl and the X-group in conformation A. Torsional Effects eclipsed conformation staggered conformation +2.0 kcal/mol See Lecture 5 for previous discussion