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
D. A. Evans. B. Breit. B. Dunn Electrocyclic Reactions: FMO Analysis Chem 206 FMO Treatment of Electrocyclic reactions. Photochemical Activation. When light is used to initiate an electrocyclic reaction, an electron is I Examine the interactions that occur in the HOMO as the reaction excited from Y2 to Y3. Treating y3 as the HOMo now shows that I If the overlap is constructive (i.e. of the same phase)then the disrotatory closure is allowed and conrotatory closure is forbidden reaction is"allowed I If the overlap is destructive(i.e of different phases) then the 4 reaction is"forbidden Thermal Activation H HAMe Y2f Conrotatory Closure: (Allowed and observed) y2( HOMO 中3 Constructive ovena Disrotatory Closure: (Allowed and observed Me Me 00 00 e p2(diene HOMO) I3(new HOMO Constructive Disrotatory Closure: (Forbidden and not observed) Conrotatory Closure: (Forbidden and not observed Me y2(diene HOMO) Destructive HOMe B2Re-25 Y3(new HOMO overlap A similar analysis for the hexatriene system proves that under We have so far proven which ring closures are allowed and which are thermal conditions disrotation is allowed and conrotation is forbidden. Do we now have to go back and examine all the ring forbidden NO! The principle of microscopic reversibility says that if the reaction is allowed in one direction it must be allowed in the other direction
Y4 Y3 Me H H Me hn Me H H Me H H Me Me H Me Me H Me H H Me Me H H Me Me H H Me Y1 Y2 Me H H Me Me H H Me Me H H Me Me H H Me Y2 (HOMO) Me H H Me Y3 (HOMO) H H Me Me H Me Me H Y1 Y4 Y3 Y2 D. A. Evans, B. Breit, T. B. Dunn Chem 206 ■ FMO Treatment of Electrocyclic reactions. ■ Examine the interactions that occur in the HOMO as the reaction proceeds. ■ If the overlap is constructive (i.e. of the same phase) then the reaction is "allowed." ■ If the overlap is destructive (i.e. of different phases) then the reaction is "forbidden." Destructive overlap Constructive overlap Y2 (diene HOMO) Y2 (diene HOMO) Conrotatory Closure: (Allowed and observed) Disrotatory Closure: (Forbidden and not observed) Thermal Activation: A similar analysis for the hexatriene system proves that under thermal conditions, disrotation is allowed and conrotation is forbidden. Photochemical Activation: When light is used to initiate an electrocyclic reaction, an electron is excited from Y2 to Y3. Treating Y3 as the HOMO now shows that disrotatory closure is allowed and conrotatory closure is forbidden. Photon absorption Destructive overlap Constructive overlap Y3 (new HOMO) Conrotatory Closure: (Forbidden and not observed) Disrotatory Closure: (Allowed and observed) Y3 (new HOMO) We have so far proven which ring closures are allowed and which are forbidden. Do we now have to go back and examine all the ring openings? NO! The principle of microscopic reversiblity says that if the reaction is allowed in one direction, it must be allowed in the other direction. Electrocyclic Reactions: FMO Analysis
D. A. Evans. B. Breit. B. Dunn Electrocyclic Reactions: Dewar-Zimmerman Chem 206 The Dewar- Zimmerman analysis is based on identifying transition states Connect as aromatic or antiaromatic. We will not go into the theory behind why Orbitals this treatment works, but it will give the same predictions as FMO or Orbital Symmetry treatments, and is fundamentally equivalent to them Using the Dewar-Zimmerman model Disrotatory y Closure Closure a Choose a basis set of 2p atomic orbitals for all atoms involved(1s for I Assign phases to the orbitals. Any phases will suffice. It is not important to identify this basis set with any molecular orbital I Connect the orbitals that interact in the starting material, before the eaction begins zero phase inversions One Phase Inversion Huckel Topology Mobius Topology L Allow the reaction to proceed according to the geometry 4 electrons in system 4 electrons in system postulated. Connect those lobes that begin to interact that were not Antiaromatic and Aromatic and interacting in the starting materials Allowed a Count the number of phase inversions that occur as the electrons Note that I can change the phase of an abitrary orbital and the analysis flow around the circuit. Note that a phase inversion within an orbital is is still valid! not counted Connect Orbitals a Based on the phase inversions, identify the topology of the system Odd number of phase inversi Mobius topology Even number of phase invers Disrotatory Closure Assign the transition state as aromatic or antiaromatic, based on the number of electrons present Aromat Antiaromatic 4q Two Phase Inversions Three phase Inversions If the transition state is aromatic then the reaction will be allowed thermally. If the transition state is antiaromatic, then the reaction will Huckel Topology Mobius Topology be allowed photochemically electrons in system 4 electrons in syster Antiaromatic and Aromatic and Forbidden
D. A. Evans, B. Breit, T. B. Dunn Electrocyclic Reactions: Dewar-Zimmerman Chem 206 Connect Orbitals Disrotatory Closure Conrotatory Closure Zero Phase Inversions \Hückel Topology 4 electrons in system \ Antiaromatic and Forbidden One Phase Inversion \Möbius Topology 4 electrons in system \ Aromatic and Allowed Note that I can change the phase of an abitrary orbital and the analysis is still valid! Connect Orbitals Disrotatory Closure Conrotatory Closure Two Phase Inversions \Hückel Topology 4 electrons in system \ Antiaromatic and Forbidden Three Phase Inversions \Möbius Topology 4 electrons in system \ Aromatic and Allowed The Dewar-Zimmerman analysis is based on identifying transition states as aromatic or antiaromatic. We will not go into the theory behind why this treatment works, but it will give the same predictions as FMO or Orbital Symmetry treatments, and is fundamentally equivalent to them. Using the Dewar-Zimmerman model: ■ Choose a basis set of 2p atomic orbitals for all atoms involved (1s for hydrogen atoms). ■ Assign phases to the orbitals. Any phases will suffice. It is not important to identify this basis set with any molecular orbital. ■ Connect the orbitals that interact in the starting material, before the reaction begins. ■ Allow the reaction to proceed according to the geometry postulated. Connect those lobes that begin to interact that were not interacting in the starting materials. ■ Count the number of phase inversions that occur as the electrons flow around the circuit. Note that a phase inversion within an orbital is not counted. ■ Based on the phase inversions, identify the topology of the system. Odd number of phase inversions: Möbius topology Even number of phase inversions: Hückel topology ■ Assign the transition state as aromatic or antiaromatic, based on the number of electrons present. System Aromatic Antiaromatic Hückel 4q + 2 4q Möbius 4q 4q + 2 ■ If the transition state is aromatic, then the reaction will be allowed thermally. If the transition state is antiaromatic, then the reaction will be allowed photochemically
D. A. Evans. B. Breit. T.B. Dunn [1, 3-Sigmatropic Rearrangements: FMO Analysis Chem 206 The stereochemical issues The migrating group can migrate across the conjugated pi system in a[1, 3]Sigmatropic Rearrangements(H migration one of two ways. If the group migrates on the same side of the system, Construct TS by considering an ally anion and the proton (or radical it is said to migrate suprafacially with respect to that system. If the 丰 group migrates from one side of the pi system to the other, it is said to migrate antarafacially with respect to that system Suprafacial migration: The group moves across the same face A..B 8 xr Antarafacial migration: The group moves from one face to the other Proton 1S(LUMO) ￥Y ￥Y p2(allyl anion HOMO bondi Sigmatropic Rearrangements: FMO Analysis Suprafacial Geometry Antarafacial Geometry IImagine the two pieces fragmenting into a cation/anion pair, (or a a The analysis works if you consider the other ionic reaction, or pair of radicals)and examine the HOMO/LUMO interaction consider a radical reaction. In each case it is the same pair of orbitals a If the overlap is constructive at both termini then the reaction is interacting allowed. If the overlap is destructive at either terminus then the reaction is forbidden a The suprafacial migration is forbidden and the bridging distance too great for the antarafacial migration. Hence, [1, 3] hydrogen migrations a If the migrating atom is carbon then we can also entertain the are not obseryed under thermal conditions possiblity of the alkyl group migrating with inversion of configuration (antarafacial on the single atom a Under photochemical conditions, the[1, 3] rearrangement is allowed suprafacially. How would you predict this using FMO? L If the migrating atom is hydrogen, then it cannot migrate wit nversion
D. A. Evans, B. Breit, T. B. Dunn [1,3]-Sigmatropic Rearrangements: FMO Analysis Chem 206 The Stereochemical issues: The migrating group can migrate across the conjugated pi system in one of two ways. If the group migrates on the same side of the system, it is said to migrate suprafacially with respect to that system. If the group migrates from one side of the pi system to the other, it is said to migrate antarafacially with respect to that system. A B A B A B A B A B A B Suprafacial migration: The group moves across the same face. A B A B A B A B A B A B Antarafacial migration: The group moves from one face to the other. ■ If the overlap is constructive at both termini then the reaction is allowed. If the overlap is destructive at either terminus then the reaction is forbidden. ■ Imagine the two pieces fragmenting into a cation/anion pair, (or a pair of radicals) and examine the HOMO/LUMO interaction. ■ If the migrating atom is carbon, then we can also entertain the possiblity of the alkyl group migrating with inversion of configuration (antarafacial on the single atom). ■ The suprafacial migration is forbidden and the bridging distance too great for the antarafacial migration. Hence, [1,3] hydrogen migrations are not observed under thermal conditions. Suprafacial Geometry Antarafacial Geometry bonding Y2 (allyl anion HOMO) bonding antibonding bonding ■ Construct TS by considering an allyl anion and the proton (or radical pair): ■ [1,3] Sigmatropic Rearrangements (H migration) Y X Y X ■ Under photochemical conditions, the [1,3] rearrangement is allowed suprafacially. How would you predict this using FMO? H ■ The analysis works if you consider the other ionic reaction, or consider a radical reaction. In each case it is the same pair of orbitals interacting. Proton 1S (LUMO) X X H H Y Y X Y •• ■ If the migrating atom is hydrogen, then it cannot migrate with inversion. H X X H H Y Y X Y ■ Sigmatropic Rearrangements: FMO Analysis H H
D. A Evans. B. Breit.T B. Dunn [1, 3-Sigmatropic Rearrangements Chem 206 [1, 3] Sigmatropic Rearrangements(C migration) Sigmatropic Rearrangements: Dewar-Zimmerman Dewar-Zimmerman also predicts the [1, 3] suprafacial migration to be H3 forbidden The basis set of s and p orbitals with arbitrary phase Two Phase Inversions a Construct TS by considering an allyl anion and the methyl cation Huckel Topology Four Electrons Forbidden therma Retention at carbon Inversion at carbon 2p on Carbon H Orbital interactions in the Completing the circuit antibonding bondi ing parent syster Y2(allyl anion HOMO) Suprafacial on allyl fragment Suprafacial on allyl fragment The [1.5 shit of a hydrogen atom across a diene u The analysis works if you consider the other ionic reaction, or consider a radical reaction. In each case it is the same pair of orbitals interacting I Under photochemical conditions, the [1, 3] rearrangement is allowed Zero phase Inversions suprafacially with retention of stereochemistry Huckel Topology Six Electrons a The stereochemical constraints on the migration of carbon Allowed thermally with inversion of configuration is highly disfavored on the basis of strain uch rearrangements are rare and usually only occur in highly strained Orbital interactions in the Completing the circuit systems parent system across the bottom face Using a similar analysis, one can prove that [1, 5] hydrogen and alk shifts should be allowed when suprafacial on the pi component and proceeding with retention. Please refer to Fleming for more applications of FMo theory to [1, n] sigmatropic shifts
D. A. Evans, B. Breit, T. B. Dunn [1,3]-Sigmatropic Rearrangements Chem 206 Y2 (allyl anion HOMO) ■ Construct TS by considering an allyl anion and the methyl cation: ■ Under photochemical conditions, the [1,3] rearrangement is allowed suprafacially with retention of stereochemistry. ■ The stereochemical constraints on the migration of carbon with inversion of configuration is highly disfavored on the basis of strain. Such rearrangements are rare and usually only occur in highly strained systems. bonding bonding Inversion at carbon Suprafacial on allyl fragment Retention at carbon bonding antibonding Suprafacial on allyl fragment C H H X H C H H Y X H Y H H H H 2p on Carbon ■ The analysis works if you consider the other ionic reaction, or consider a radical reaction. In each case it is the same pair of orbitals interacting. Using a similar analysis, one can prove that [1,5] hydrogen and alkyl shifts should be allowed when suprafacial on the pi component and proceeding with retention. Please refer to Fleming for more applications of FMO theory to [1,n] sigmatropic shifts. ■ Sigmatropic Rearrangements: Dewar-Zimmerman The [1,5] shift of a hydrogen atom across a diene. H H Orbital interactions in the parent system The basis set of s and p orbitals with arbitrary phase: Completing the circuit across the bottom face Zero Phase Inversions Hückel Topology Six Electrons Allowed thermally Dewar-Zimmerman also predicts the [1,3] suprafacial migration to be forbidden. Orbital interactions in the parent system Completing the circuit across the bottom face Two Phase Inversions Hückel Topology Four Electrons Forbidden thermally CH3 X X CH3 CH3 Y Y X Y •• ■ [1,3] Sigmatropic Rearrangements (C migration) H H
D. A. Evans. B. Breit. T.B. Dunn [3, 3-Sigmatropic Rearrangements Chem 206 [3, 3] Rearrangements ■ The Toggle algorithm A thermally allowed reaction in either of two geometries, the"chair"or he boat geometry. Depicted below is the"chair geometry. You The toggle algorithm is a simple way to take one reaction of each should be able to work out the details of the"boat" geometry yourself class that you remember is allowed (or forbidden)and derive if the reaction is allowed or forbidden under new conditions ■ How does it work? All of the various parameters of the pericyclic reaction are the input variables, the"switches The output is either"allowed"or forbidden X&z=conetc Write out all the relevant parameters of a reaction together with the known result Each time you change a parameter by one incremental value The FMO Analysis: Toggle a switch"), the output will switch This is the prediction of the reaction under the new parameters Bring two Allyl radicals together to access for a possible bonding interaction between termini So it's nothing really new, is it? No, its just a convenient way to rederive predictions without memorizing a table of selection rules An EXample Take the [1, 3] sigmatropic rearrangement of an alkyl group. We know this is forbidden under thermal conditions in a supra-supra The nonbonding bonding manner, and so we make it the first entry in the table allyl MO Rearrangement Conditions Component 1 Component 2: Output 13] Heat Suprafacial Suprafacial i Forbidden The Dewar-Zimmerman Analysis [1.3] Heat- Antarafacial+ Suprafacial Allowed [13] Light* Antarafacial\ Suprafacial Forbidden [15] Heat Suprafacial Suprafacial Two Phase Inversions Allowed Thermally Each incremental change in the "input"registers changes the"output" register by one. Multiple changes simply toggle the output back and forth. What is the prediction in the last line?
X Z X Z Z X D. A. Evans, B. Breit, T. B. Dunn Chem 206 [3,3] Rearrangements: A thermally allowed reaction in either of two geometries, the "chair" or the "boat" geometry. Depicted below is the "chair" geometry. You should be able to work out the details of the "boat" geometry yourself. Two Phase Inversions Hückel Topology Six Electrons Allowed Thermally ‡ X & Z = C, O, N etc The FMO Analysis: Bring two Allyl radicals together to access for a possible bonding interaction between termini. · · ‡ bonding bonding The nonbonding allyl MO The Dewar-Zimmerman Analysis: ■ The Toggle Algorithm: The toggle algorithm is a simple way to take one reaction of each class that you remember is allowed (or forbidden) and derive if the reaction is allowed or forbidden under new conditions. ■ How does it work? All of the various parameters of the pericyclic reaction are the input variables, the "switches." The output is either "allowed" or "forbidden." Write out all the relevant parameters of a reaction together with the known result. Each time you change a parameter by one incremental value ("toggle a switch"), the output will switch. This is the prediction of the reaction under the new parameters. ■ So it's nothing really new, is it? No, its just a convenient way to rederive predictions without memorizing a table of selection rules. An Example: Rearrangement [1,3] [1,3] [1,3] [1,5] Conditions Heat Heat Light Heat Component 1 Suprafacial Antarafacial Antarafacial Suprafacial Component 2 Suprafacial Suprafacial Suprafacial Suprafacial Output Forbidden Allowed Forbidden ? Take the [1,3] sigmatropic rearrangement of an alkyl group. We know this is forbidden under thermal conditions in a supra-supra manner, and so we make it the first entry in the table. Each incremental change in the "input" registers changes the "output" register by one. Multiple changes simply toggle the output back and forth. What is the prediction in the last line? [3,3]-Sigmatropic Rearrangements