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Unformatted text preview: Chem 206 D. A. Evans D. A. Evans Monday, September 15, 2003 http://www.courses.fas.harvard.edu/~chem206/ Reading Assignment for week: Kirby, Stereoelectronic Effects Carey & Sundberg: Part A ; Chapter 1 Fleming, Chapter 1 & 2 Fukui, Acc. Chem. Res. 1971 , 4 , 57. (pdf) Curnow, J. Chem. Ed. 1998 , 75 , 910 (pdf) Alabugin & Zeidan, JACS 2002 , 124 , 3175 (pdf) Chemistry 206 Advanced Organic Chemistry Lecture Number 1 Introduction to FMO Theory General Bonding Considerations The H 2 Molecule Revisited (Again!) Donor & Acceptor Properties of Bonding & Antibonding States Hyperconjugation and "Negative" Hyperconjugation Anomeric and Related Effects An Introduction to Frontier Molecular Orbital Theory-1 Problems of the Day The molecule illustrated below can react through either Path A or Path B to form salt 1 or salt 2 . In both instances the carbonyl oxygen functions as the nucleophile in an intramolecular alkylation. What is the preferred reaction path for the transformation in question? + + Br Br 1 2 Path A Path B Br N H O Br O O Br O N H O O N H Br This is a "thought" question posed to me by Prof. Duilo Arigoni at the ETH in Zuerich some years ago http://evans.harvard.edu/problems/ O P O OMe O P O OMe O P O O A B C (RO) 3 P + (First hr exam, 1999) The three phosphites illustrated below exhibit a 750fold span in reactivity with a test electrophile (eq 1) (Gorenstein, JACS 1984 , 106 , 7831). Rank the phosphites from the least to the most nucleophilic and provide a concise explanation for your predicted reactivity order. El(+) (RO) 3 PEl (1) + 1-01-Cover Page 9/15/03 8:56 AM Chem 206 D. A. Evans An Introduction to Frontier Molecular Orbital Theory-1 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): C Br Me R R C R R Me Nu RO H S N 2 O Me 2 CuLi RO H O H Me RO H O Me H Inductive Effects: Through-bond polarization Field Effects: Through-space polarization "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: The effect of bond and through-space polarization by heteroatom substituents on reaction rates and selectivities + Br: + S N 1 rate decreases as R becomes more electronegative C R R Me Br C Me R R "Organic chemists are generally unaware of the impact of electronic effects on the stereochemical outcome of reactions." "The distinction between electronic and stereoelectronic effects is not clear-cut." General Reaction Types Radical Reactions (~10%): A...
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