D. A. Evans An Introduction to Frontier Molecular Orbital Theory-1 Chem 206 Universal Effects Governing Chemical Reactions Stereoelectronic Effects There are three ■ Steric Effects Geometrical constraints placed upon ground and transition states by orbital overlap considerations Nonbonding interactions (Van der Waals repulsion) between substituents within a molecule or between reacting molecules Fukui postulate for reaction During the course of chemical reactions, the interaction of the highest filled(HOMO) and lowest unfilled(antibonding) J I Brauman, Science, 2002, 295, 2245 molecular orbital (LUMO)in reacting species is very important to the stabilization of the transition structure General Reaction Types Radical reactions(-10%):A·+B·—A-B Electronic Effects(Inductive Effects) Polar Reactions(90%): A(+ B(+)-A--B rate decreases as r becomes more electronegative Lewis Base Lewis Acid Inductive Effects: Through-bond polarization FMO concepts extend the donor-acceptor paradigm to Field Effects: Through-space polarization non-obvious families of reactions Your thoughts on this transformation ■ Examples to consider H2+2Li()→2LH R3Sio Lewis acid EtO2 C\ CH3-1+ Mg(O)- CH3-MgBr OSiR3 Danishefsky, JOC 1991, 56, 38 diastereoselection >94.6RO H O C Br Me R R SN2 C R R Me Br Me2CuLi O OSiR3 R3SiO EtO C Me R R RO H O Me H C R R Me Nu RO H O H Me OSiR3 OSiR3 EtO2C H2 CH3–I A(:) A· B(+) B· 2 LiH CH3–MgBr A B A B D. A. Evans An Introduction to Frontier Molecular Orbital Theory-1 Chem 206 + Br:– minor major Br: – Nu: Nonbonding interactions (Van der Waals repulsion) between substituents within a molecule or between reacting molecules ■ Steric Effects Universal Effects Governing Chemical Reactions There are three: ■ Electronic Effects (Inductive Effects): + SN1 rate decreases as R becomes more electronegative Inductive Effects: Through-bond polarization Field Effects: Through-space polarization Danishefsky, JOC 1991, 56, 387 Lewis acid diastereoselection >94 : 6 Your thoughts on this transformation "During the course of chemical reactions, the interaction of the highest filled (HOMO) and lowest unfilled (antibonding) molecular orbital (LUMO) in reacting species is very important to the stabilization of the transition structure." Geometrical constraints placed upon ground and transition states by orbital overlap considerations. ■ Stereoelectronic Effects Fukui Postulate for reactions: ■ General Reaction Types Radical Reactions (~10%): + Polar Reactions (~90%): + Lewis Base Lewis Acid FMO concepts extend the donor-acceptor paradigm to non-obvious families of reactions ■ Examples to consider + 2 Li(0) + Mg(0) J. I. Brauman, Science, 2002, 295, 2245