Chemistry 202 - Organic Reaction Mechanisms II

Introduction

David Van Vranken University of California, Irvine About the Mech II

Instructor: David L. Van Vranken Office Hours: Mon 5-6 pm? TA: Stan Hiew Office Hours: TBA

Books: Ian Fleming Molecular Orbitals and Organic Chemical Reactions. F. A . C a re y a n d R . J. Sundberg Advanced Organic , Part A: Structure and Mechanisms. 4th or 5th Edition

GRADING: Midterm 1 20% Fri, Feb 2, 11:00-11:50 am (subject to change) Midterm 2 20% Mon, Feb 26, 11:00-11:50 am (subject to change) Problem Sets 10% weekly D.S. Participation 10% weekly Final Exam 40% Wed, Mar 18, 8:00-10:00am Chem 201 (Fall 2017)

I. Arrow-Pushing and Molecular Orbitals A. Rules for arrow-pushing B. Molecular Orbital Theory II. Energy and Selectivity III. Proton Transfers and Acid-Base Equilibrium IV. Orbitals and Arrow-Pushing A. Addition to Empty p Orbitals B. Addition to Empty pi* Orbitals Chem C. Addition to Empty sigma* Orbitals 201 D. Non-Bonding Orbitals as Nucleophiles E. Pi Orbitals as Nucleophiles F. Sigma Orbitals as Nucleophiles V. Chemistry of Third Row A. Chemistry of Phosphorus Functional Groups Chem 202 (Winter 2018) V. Chemistry of Third Row Atoms, contd. B. Sulfur Chemistry VI. Conformations A. Conformation and Stereoelectronics B. Rings and Cyclizations VII. Pericyclic Reactions A. reactions B. Ene reactions C. Electrocyclic reactions D. Sigmatropic reactions VIII. Kinetics A. Concentration dependence and rate equations B. Selectivity and Free Energy C. Transition State Theory, Temperature and Half -Lives IX. Reactions X. Electron Transfer Reductions XI. Photochemistry XII. Modeling chemical reactions and reactivity Arrow-Pushing Rules: Same as Chem 201

1. DRAW CORRECT LEWIS STRUCTURES • Formal charges • Draw lone pairs • No arrow-pushing for dative bonds or hydrogen bonds 2. MAKE ARROWS START WITH BONDS OR LONE PAIRS 3. MAKE ARROWS END ON ATOMS OR BONDS 4. OBEY THE THREE-ARROW RULE 5. DON'T DRAW TERMOLECULAR ELEMENTARY REACTIONS 6. TRANSFER PROTONS FROM ACIDS TO BASES • Use H-A/A- and H-B+/B • Don’t transfer protons through 4-membered T.S.s 7. H IS ALWAYS ATTACHED TO SOMETHING • No such thing as “H+”, ”H-” or “H•” in solution-phase reactions. No Agreement on the Meaning of a

■ What is the mechanism for this interesting reaction? Loh, T.-P.; et al. Chem. Comm. 2008, 1353.

O 20 mol% InBr3 Me Me O 37% H CH2Cl2 • 3 new rings O 20 °C, 1 h HO H • 5 stereogenic centers Me Me

■ To a kineticist, “the mechanism” is an expression of in terms of reactant concentrations. If you want to be careful, then distinguish a “kinetic mechanism” from an “arrow-pushing mechanism”.

mechanism: k1 k2 reaction rate = [InBr3][epoxide] k-1

O k1 O k2 + InBr3 product R.D.S. O k-1 +O - InBr3

■ Steps that occur after the rate-determining step don’t affect the rate; so a kinetic mechanism sometimes ignores the most interesting parts of the reaction. What is an Arrow-Pushing Mechanism? ■ An arrow-pushing mechanism breaks the reaction down into a series of elementary reaction steps, each with a single transition state. By every elementary step of a mechanism, we gain insight into useful chemical processes.

+ concerted? O O O H H Br3In :A- Br3InO :O + O - H

Br3In - or stepwise? O O O + H + Br InO Br InO Br InO 3 - + 3 - H 3 - H

■ In Chem 202 we will think about in terms of orbital interactions AND the concentration-dependence on competing processes.