D. A Evans.TB. Dunn Pericyclic Reactions: Part-1 Chem 206 Other Reading material: http://www.courses.fasharvardedu/-chem206/ aWoodward-Hoffmann Theory R. B. Woodward and r. hoffmann the conservation of orbital Symmetry, Verlag Chemie, Weinheim, 1970 Chemistry 206 I Frontier Molecular Orbital Theory Fleming, Frontier Orbitals and Organic Chemical Reactions Advanced Organic Chemistry John-wiley and sons, New York, 1976 IDewar-Zimmerman Theory T. H Lowry and K.S. Richardson, Mechanism and Theory in Lecture number 11 Organic Chemistry, 3rd Ed, Harper Row, New York, 1987 ■ Genera| Referenc R E Lehr and A P Marchand, Orbital Symmetry. A Problem Pericyclic Reactions-1 Solving Approach, Academic Press, New York, 1972 Introduction to Pericyclic Reactions Electrocyclic Reactions Sigmatropic Reactions ■ Problems of the Day Cycloaddition Reactions Predict the stereochemical outcome of this reaction Reading Assignment for week berg: Part A; Chapter 11 Concerted Pericyclic Reactions Fleming: Chapter 4 Thermal Pericyclic Reactions Huisgen, TL, 1964, 3381 Ph Suggest a mechanism for the following reaction heat Matthew d shair October 11. 2002
http://www.courses.fas.harvard.edu/~chem206/ Ph Ph O O O CO2Me CO2Me CO2Me MeO2C H H O O O Ph Ph D. A. Evans, T. B. Dunn Chem 206 Matthew D. Shair Friday, October 11, 2002 ■ Reading Assignment for week: Carey & Sundberg: Part A; Chapter 11 Concerted Pericyclic Reactions Pericyclic Reactions: Part–1 Chemistry 206 Advanced Organic Chemistry Lecture Number 11 Pericyclic Reactions–1 ■ Introduction to Pericyclic Reactions ■ Electrocyclic Reactions ■ Sigmatropic Reactions ■ Cycloaddition Reactions ■ Other Reading Material: Fleming: Chapter 4 Thermal Pericyclic Reactions ■ Woodward-Hoffmann Theory R. B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry, Verlag Chemie, Weinheim, 1970. ■ Frontier Molecular Orbital Theory I. Fleming, Frontier Orbitals and Organic Chemical Reactions, John-Wiley and Sons, New York, 1976. ■ Dewar-Zimmerman Theory T. H. Lowry and K. S. Richardson, Mechanism and Theory in Organic Chemistry, 3rd Ed., Harper & Row, New York, 1987. ■ General Reference R. E. Lehr and A. P. Marchand, Orbital Symmetry: A Problem Solving Approach, Academic Press, New York, 1972. ■ Problems of the Day: Huisgen, TL, 1964, 3381. Predict the stereochemical outcome of this reaction. ❉ heat ❉ Suggest a mechanism for the following reaction. heat Bloomfield, TL, 1969, 3719
D. A. Evans. B. Breit. B. Dunn Pericyclic Reactions: Introduction Chem 206 Pericyclic Reactions-Introduction/Definitions The theories Three theories are commonly used to explain and predict pericyclic A pericyclic reaction is characterized as a change in bonding relationships that reactions. We will only concern ourselves with two of these theories takes place as a continuous, concerted reorganization of electrons 1)Fukui: Frontier Molecular Orbital Interactions The term"concerted specifies that there is one single transition state and therefore no intermediates are involved in the process. To maintain a Much easier to use than the original orbital symmetry argument continuous electron flow, pericyclic reactions occur through cyclic HOMO/LUMO interactions transition states More precisely: The cyclic transition state must correspond to an arrangem of the participating orbitals which has to maintain a bonding interaction 2)Dewar-Zimmerman: Aromatic Transition States between the reaction components throughout the course of the reaction Some factors to consider in our analysis siest to apply for all reaction types, but it is not as easy to tand why itit valid The number of electrons involved has a profound influence on reactivity. Aromatic or antiaromatic transition states oodward-Hoffmann: Conservation of Orbital Symmetry 日=N②=0 First theory to explain and predict the outcome of many reactions ■ Correlation diagrams On the three methods "There are several ways of applying the orbital-symmetry principle to cycloaddition 4 electrons 6 electrons reactions, three of which are used more frequently than others. Of these three, we will discuss two: the frontier-orbital method and the mobius-Hockel method. The third called the correlation diagram method, is less convenient to apply than the other two Pericyclic reactions are stereospecific rry March in"Advanced Organic Chemistry" A The Five Major Categories of Pericyclic Reactions (1)ELECTROCYCLIC RING CLOSURE/RING OPENING An electrocyclic ring closure is the creation of a new sigma bond at the expense of the terminal p orbitals of a conjugated pi system. There is a correspondi reorganization of the conjugated pi system. We usually classify the reaction Reactions behave differently depending on the conditions used according to the number of electrons involved (i.e. thermal versus photochemical conditions) Examples A 4 e electrocyclic reaction A 6e electrocyclic react △orh A or h Cyclobutene 1.3.5-Hexatriene 1, 3-Cyclohexadiene
D. A. Evans, B. Breit, T. B. Dunn Pericyclic Reactions: Introduction Chem 206 The Five Major Categories of Pericyclic Reactions (1) ELECTROCYCLIC RING CLOSURE/RING OPENING: Cyclobutene Butadiene 1,3,5-Hexatriene 1,3-Cyclohexadiene Examples: An electrocyclic ring closure is the creation of a new sigma bond at the expense of the terminal p orbitals of a conjugated pi system. There is a corresponding reorganization of the conjugated pi system. We usually classify the reaction according to the number of electrons involved. Pericyclic Reactions - Introduction/Definitions A pericyclic reaction is characterized as a change in bonding relationships that takes place as a continuous, concerted reorganization of electrons. The term "concerted" specifies that there is one single transition state and therefore no intermediates are involved in the process. To maintain continuous electron flow, pericyclic reactions occur through cyclic transition states. More precisely: The cyclic transition state must correspond to an arrangement of the participating orbitals which has to maintain a bonding interaction between the reaction components throughout the course of the reaction. Pericyclic reactions are stereospecific: A A A A A A Reactions behave differently depending on the conditions used (i.e. thermal versus photochemical conditions): A The number of electrons involved has a profound influence on reactivity: 4 electrons rarely observed 6 electrons often observed Some factors to consider in our analysis: The Theories: Three theories are commonly used to explain and predict pericyclic reactions. We will only concern ourselves with two of these theories. 1) Fukui: Frontier Molecular Orbital Interactions Much easier to use than the original orbital symmetry arguments HOMO/LUMO interactions 2) Dewar-Zimmerman: Aromatic Transition States The easiest to apply for all reaction types, but it is not as easy to understand why it it valid Aromatic or antiaromatic transition states ■ ■ ■ ■ On the three methods: "There are several ways of applying the orbital-symmetry principle to cycloaddition reactions, three of which are used more frequently than others. Of these three, we will discuss two: the frontier-orbital method and the Möbius-Hückel method. The third, called the correlation diagram method, is less convenient to apply than the other two." Jerry March in "Advanced Organic Chemistry" A 4 e- electrocyclic reaction A 6 e- electrocyclic reaction A A A A A A A hn D or hn D or hn First theory to explain and predict the outcome of many reactions Correlation diagrams 3) Woodward-Hoffmann: Conservation of Orbital Symmetry ■ ■ heat heat heat heat heat
D. A. Evans. B. Breit. B. Dunn Pericyclic Reactions: Major classes Chem 206 (2)CYCLOADDITION REACTIONS/CYCLOREVERSION REACTIONS:(4)SIGMATROPIC REARRANGEMENTS A cycloaddition reaction is the A sigmatropic rearrangement is the migration of a sigma bond from one position igma bonds are created at the expense of pi bonds. a cycloaddition can occur in a conjugated system to another position in the system, accompanied cycloaddition reactions are referred to as [m+n] additions when a system of m remains constant. The rearrangement is an(m, n] shift when the sigma tmn in an intramolecular sense, but it must be between two independent pi systems. reorganization of the connecting pi bonds. The number of pi and sigma bond conjugated atoms combines with a system of n conjugated atoms. A migrates across m atoms of one system and n atoms of the second system cycloreversion is simply the reverse of a cycloaddition Examples Examples [1, 3]-shift [1, 5]-shift /(3, 3-shift X=CR2, Cope rearrangement The Diels-Alder reaction (5)GROUP TRANSFER REACTIONS (3)CHELETROPIC REACTIONS In a group transfer reaction one or more groups get transferred to a second reaction partner Cheletropic reactions are a special group of cycloaddition/cycloreversion reactions. Two bonds are formed or broken at a single atom. The nomenclature for cheletropic reactions is the same as for cycloadditions Examples Hydrogen Transfer 4+ N2 =0 Ene reaction: R
[4+1] [4+1] [2+1] R R R 1 R 2 O O S O O O CR2 R R C O S O O X R X R R H H R N N H H R R' N2 H H D H H H R H R' R 1 H R 2 D. A. Evans, B. Breit, T. B. Dunn Pericyclic Reactions: Major Classes Chem 206 (2) CYCLOADDITION REACTIONS/CYCLOREVERSION REACTIONS: + [2+2] + [4+2] A cycloaddition reaction is the union of two smaller, independent pi systems. Sigma bonds are created at the expense of pi bonds. A cycloaddition can occur in an intramolecular sense, but it must be between two independent pi systems. Cycloaddition reactions are referred to as [m + n] additions when a system of m conjugated atoms combines with a system of n conjugated atoms. A cycloreversion is simply the reverse of a cycloaddition. Examples: A 2+2 cycloaddition. The Paterno-Büchi reaction. A 4+2 cycloaddition. The Diels-Alder reaction. (3) CHELETROPIC REACTIONS: Cheletropic reactions are a special group of cycloaddition/cycloreversion reactions. Two bonds are formed or broken at a single atom. The nomenclature for cheletropic reactions is the same as for cycloadditions. + + + Examples: (4) SIGMATROPIC REARRANGEMENTS: 1 2 3 1 2 3 [1,3]-shift 1 [1,5]-shift 2 3 4 5 A sigmatropic rearrangement is the migration of a sigma bond from one position in a conjugated system to another position in the system, accompanied by reorganization of the connecting pi bonds. The number of pi and sigma bonds remains constant. The rearrangement is an [m,n] shift when the sigma bond migrates across m atoms of one system and n atoms of the second system. Examples: [3,3]-shift 3 2 1 3' 1' 2' 3 2 1 3' 1' 2' X=CR2, Cope rearrangement X=O, Claisen rearrangement (5) GROUP TRANSFER REACTIONS: In a group transfer reaction one or more groups get transferred to a second reaction partner. Hydrogen Transfer: Ene Reaction: + + + + + hn Examples:
D. A. Evans. B. Breit. B. Dunn Electrocyclic Reactions Chem 206 ELECTROCYCLIC RING CLOSURE/RING OPENING Butadiene to cyclobutene: A 4-electron(4q) system The stereochemical issues: Ring closure can occur in two distinct ways. This has consequences with The orbital lobes that interact heat a The disposition of substituents on the termini Conrotatory Closure: The termini rotate in the same direction Me\Me onorio Hextriene to cyclohexadiene: A 6-electron(4q+2 system AB ATB 83-8 derotation A Disrotatory Closure: The termini rotate in opposite directions derotation It was also noted that changing the "reagent"from heat to light reverse Empirical Observations this reactivity patten. Under photochemical conditions 4 electron systems undergo disrotatory motion, while 6 electron systems undergo conrotatory motion It was noted that butadienes undergo conrotatory closure under thermal conditions, while hexatrienes undergo disrotatory closure under thermal conditions. The microscopic reverse reactions also occur with the same rotational sense (i.e. cyclobutenes open in a conrotatory sense when heated, and cyclohexadienes open in a disrotatory sense when heated h
B A A B A B B A B A A B A B A B A B A B A B A B Me Me Me H H Me Me Me H Me H Me Me Me Me Me D D Me Me Me Me H H Me Me Me H Me H Me Me hn hn Me Me D. A. Evans, B. Breit, T. B. Dunn Electrocyclic Reactions Chem 206 Conrotatory Closure: The termini rotate in the same direction The Stereochemical issues: ELECTROCYCLIC RING CLOSURE/RING OPENING: Ring closure can occur in two distinct ways. This has consequences with regard to: ■ The orbital lobes that interact ■ The disposition of substituents on the termini Disrotatory Closure: The termini rotate in opposite directions Empirical Observations: conrotation conrotation disrotation disrotation Butadiene to cyclobutene: A 4-electron (4q) system Hextriene to cyclohexadiene: A 6-electron (4q+2) system It was also noted that changing the "reagent" from heat to light reversed this reactivity pattern. Under photochemical conditions 4 electron systems undergo disrotatory motion, while 6 electron systems undergo conrotatory motion. disrotation controtation heat heat It was noted that butadienes undergo conrotatory closure under thermal conditions, while hexatrienes undergo disrotatory closure under thermal conditions. The microscopic reverse reactions also occur with the same rotational sense (i.e. cyclobutenes open in a conrotatory sense when heated, and cyclohexadienes open in a disrotatory sense when heated.)
D. A. Evans. T. B. Dunn Conjugated pi systems Chem 206 6 p-orbitals antibond 5 p-orbitals 4 p-orbitals 3 p-orbitals Q00 811 2 p-orbitals 000000 66666 Q00 000000 0000 00000 000 000 00006666 00000( bonding There are no nodal planes in the most stable bonding MO. With each higher MO, one additional nodal plane is adder The more nodes, the higher the orbital energy
C Y1 Y1 Y2 Y3 Y4 Y2 Y3 p p * D. A. Evans, T. B. Dunn Conjugated pi systems Chem 206 There are no nodal planes in the most stable bonding MO. With each higher MO, one additional nodal plane is added. The more nodes, the higher the orbital energy. bonding 2 p-orbitals 3 p-orbitals 4 p-orbitals 5 p-orbitals 6 p-orbitals antibonding nonbonding nonbonding