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-CyclohexadieneD. 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