Notes for Lecture #17: Sigmatropic Rearrangments

Massachusetts Institute of Technology 17 Organic Chemistry 5.13 October 20, 2003 Prof. Timothy F. Jamison Notes for Lecture #17 Sigmatropic Rearrangments Woodward-Hoffmann Rules for Sigmatropic Rearrangments # Electrons Stereochemical Course Thermal Mode Photochemical Mode 4n + 2 [s,s] [s,a] 4n [s,a] [s,s] Continued on next page... Summary of Unit 3 Molecular Orbital Theory, Pericyclic Reactions A. Molecular Orbital Theory 1. Ethylene 2. Allyl cation, radical, anion 3. 1,3-Butadiene 4. Benzene 5. Cyclobutadiene – Frost’s Circle (Polygon Rule) B. Cycloaddition Reactions 1. Diels-Alder Reaction a. Basics – diene + dienophile (electron-poor) b. Only s-cis conformation of diene reacts. c. Cantharidin example d. Stereochemical course – part 1: Always suprafacial with respect to dienophile – stereospecific. e. Diels-Alder is a concerted reaction f. Frontier Molecular Orbital analysis of Diels-Alder reaction. HOMO of diene interacts with LUMO of dienophile. “HOMO/LUMO analysis”. g. Stereochemical course – part 2: Always suprafacial with respect to diene. Disrotatory motion of orbitals. h. Woodward-Hoffman rules correctly predict the stereochemical course of the Diels-Alder reaction (and all other cycloaddition reactions): π4s + π2s. i. Regioselectivity – “ortho-para rule”. Not governed by W-H rules, but a general trend. j. Diastereoselectivity – Alder endo rule. k. MO explanations: Coefficients of wavefunctions that represent orbitals vary with substitution. Secondary orbital interactions between dienophile and carbon(s) 2 and/or 3 of diene favor endo approach of dienophile. 2. Photochemical [2+2] Cycloaddition a. Review of 4 requirements of cycloaddition (concerted, pericyclic; cyclic T.S.; 2 separate arrays of orbitals; form 2 sigma bonds) and what W-H predicts for photo [2+2] (π2s + π2s) b . Explanation – HOMO/LUMO analysis. Excite electron to higher energy orbital, which becomes HOMO. c. Examples: cis-2-butene + maleic anhydride, thymidine dimmers 3. Thermal [2+2] Cycloaddition – Ketenes + Alkenes a. MO analysis and W-H rules predict π2s + π2a b. Side-on approach of diene and ketene (“+” sign) 4. Dipolar [4+2] Cycloaddition, Thermal a. Diene vs. Dipole – same # π electrons (4), different # atoms (4 vs. 3) b. Examples – azides, diazocompounds, nitrile oxides 5. Ozonolysis – Most important dipolar cycloaddition a. Mechanism – [4+2], retro-[4+2], [4+2]. (Then reduction or oxidation with additional reagent b. Zn or (CH3)2S gives carbonyls in oxidation state as is (after hydrolysis) c. NaBH4 reduces aldehydes, ketones to alcohols d. H2CrO4 (Jones) oxidizes aldehyde to acid (no effect on ketone) C. Electrocyclic Reactions 1. Introductory Examples - (E,Z)-2,4-hexadiene and (E,E)-2,4-hexadiene 2. Definitions a. Concerted pericyclic process b. Cyclic transition state c. Intramolecular, i.e. 1 overlapping array of orbitals d. Formation/Breaking of 1 sigma bond 3. FMO Analysis of Stereochemical Results (see examples in A, above) a. (Thermal) (E,Z)-2,4-hexadiene –> cis–3,4-dimethylcyclobutene (conrotatory) and (E,E)-2,4-hexadiene, thermal –> trans–3,4dimethylcyclobutene (conrotatory). b. (Photochemical) (E,Z)-2,4-hexadiene, –> trans–3,4-dimethylcyclobutene electron promoted to higher energy orbital; thus HOMO is different than in thermal reactions. (disrotatory) and (E,E)-2,4-hexadiene –> cis–3,4dimethylcyclobutene - electron promoted to higher energy orbital; thus HOMO is different than in thermal reactions. (disrotatory) 4. Electrocyclic Ring Opening – W-H rules still apply a. trans–3,4-dimethylcyclobutene, thermal -> (E,E) -2,4-hexadiene (conrotatory) b. cis–3,4-dimethylcyclobutene, thermal -> (E,Z) -2,4-hexadiene (conrotatory) 5. Electrocyclic Reactions Involving 6π Electrons a. (E,Z,E)-2,4,6-octatriene, thermal -> cis–5,6-dimethyl-1,3-cyclohexadiene (disrotatory) b. (E,Z,E)-2,4,6-octatriene, photochemical -> trans–5,6-dimethyl-1,3cyclohexadiene - electron promoted to higher energy orbital; thus HOMO is different than in thermal reactions. (conrotatory) c. Found in Vitamin D synthesis D. Sigmatropic Rearrangements 1. Definitions a. Concerted pericyclic process b. Cyclic transition state c. Intramolecular, i.e. 1 overlapping orbital array 2. 3. 4. 5. 6. 7. d. One sigma bond formed, one broken e. [i,j] – i + j = number of atoms in orbital array i, j = number of atoms on either side of sigma bond broken (or formed) Cope Rearrangement, Thermal [3,3] a. Thermodynamically driven i. Alkene substitution ii. Ring strain Claisen Rearrangement, Thermal [3,3] Woodward-Hoffmann Rules Thermal [1,5] H-Shift Thermal [1,3] H-Shift Thermal [1,7] H-Shift Good Luck on the Exam! – Tim Jamison –