I-J. Pericyclic ReactionsI. Basic PrinciplesNicolaou, K. C. et al. J. Am. Chem. Soc. 1982, 104, 5560. (Endiandric acids)Dr. P. WipfPage 1 of 602/27/2008Characteristics of Pericyclic Reactions• among the reactants and products usually at least one moleculeis unsaturated• the reactions involve the formation or scission of σ-bonds andthe consumption or generation of π-bonds• the electronic reorganization occurs in some sort of cyclic arrayof the participating atomic centers.Dr. P. WipfPage 2 of 602/27/2008Pericyclic 6-el. Processes:R RR RXRR'RR'6 el‡dyotropic reactions: Xcheletropic reactionsene reactionssigmatropic rearrangementscycloadditions, -reversionselectrocyclic reactionsDr. P. WipfPage 3 of 602/27/2008Pericyclic reactions are apparently concerted: the electronic rearrangements involved inbond-making/bond-breaking proceed simultaneously in a one-step (one TS‡) process.A single kinetic step is a characteristic of a concerted reaction; a concerted reaction in which all changes in bonding occur in comparable extents in the transition state is commonly defined as synchronous. Dr. P. WipfPage 4 of 602/27/2008Woodward-Hoffmann RulesAmong the theories that are commonly used to analyze pericyclicreactions, three are generally considered to be the most useful:1. Fukui’s Frontier MO interactions. This analysis applies HOMO-LUMO interactions as well as orbitals close to HOMO & LUMO,and it is more intuitive that the original orbital symmetryarguments.Closed-Shell Repulsion Coulomb Interaction Frontier Orbital Interaction-unocc.unocc. occ.occ.ssrEr - Es2(!abcracsb"ab)2+k<l#RklQkQlab(qa + qb)"abSab +$E = -r!!!!!!- Coulson, C. A.; Longuet-Higgins, H. C. Proc. Roy. Soc.A 1947, 192, 16.- Klopman & Salem (JACS 1968, 90, 223, 543, 553).- Fukui, K. Acc. Chem. Res. 1971, 4, 57.qa and qb Electron densities in orbitals a and b.! and S Resonance and overlap integral.Qk and QlTotal charges at atoms k and l." Local dielectric constant.RklDistance between atoms k and l.craCoefficient of atomic orbital a at the molecular orbital r.ErEnergy of molecular orbital r.Dr. P. WipfPage 5 of 602/27/2008Rather than considering the relative phases, i.e. symmetry, of allorbitals involved during the transformation of reactants intoproducts, the frontier orbital approach makes predictions for theoutcome of pericyclic reactions based on the highest occupiedmolecular orbital (HOMO) and the lowest unoccupied molecularorbital (LUMO). In this, the electrons in the HOMO of one reactantare looked upon as analogous to the outer (valence) electrons ofan atom, and reaction is then envisaged as involving the overlap ofthis (HOMO) orbital with the LUMO of the other reactant. Where,as in electrocyclic reactions, only one species is involved only theHOMO needs be considered.Dr. P. WipfPage 6 of 602/27/2008OIf, upon examination of the interactions that occur in the frontierorbitals (HOMO, LUMO if applicable, otherwise only HOMO) theinteractions are constructive (of the same phase, the reaction isconsidered “allowed”.If the orbital overlap is destructive (i.e. of different phase), then thereaction is “forbidden”.Dr. P. WipfPage 7 of 602/27/2008Berson, J. A.; Nelson, G. L. J. Am.Chem. Soc. 1967, 89, 5503:HHHDOAcHAcODHDHAcOHOAcDHHDHAcODOAcHHconcertedHOMOLUMO[1,3]sallowed, and observedinversionretentionallowed, but not observed[1,3]aLUMOHOMO[1,3]300 °CDr. P. WipfPage 8 of 602/27/20082. The Dewar-Zimmerman Theory of AromaticTransition States.This analysis is possibly the most general and easiest to apply to a broad range ofpericyclic reactions, but it is also the least anchored in fundamental physicalprinciples.For applying this model, choose a basis set of 2p atomic orbitals for all atomsinvolved (1s for hydrogens).After assigning any phases to these orbitals (no relationship to MO’s is required!),connect the orbitals that interact in the starting materials, before the reactionbegins. Allow the reaction to proceed according to the postulated geometry;connect the lobes that begin to interact and were not connected in the startingmaterials.Count the number of phase inversions that occur - a phase inversion within anorbital is not counted.Based on the number of phase inversion of the orbital perimeter, identify thetopology of the system:Odd # of phase inversions: Möbius topologyEven # of phase inversions: Hückel topologyDr. P. WipfPage 9 of 602/27/2008The transition state can now be assigned as antiaromatic or aromatic,based on the number of electrons in the system:Möbius topology: Aromatic for 4n, Antiaromatic for 4n+2Hückel topology: Aromatic for 4n+2, Antiaromatic for 4nif the transition state is aromatic, then the reaction is thermally allowed.If the transition state is antiaromatic, a photochemical process isrequired.connectorbitalsdisrotatory closureconrotatory closurezero phase inversions - Hückel topology 4 electrons in system - antiaromatic and forbiddenone phase inversion - Möbius topology 4 electrons in system - aromatic and allowedDr. P. WipfPage 10 of 602/27/2008Thermal processes involving 4n+2 electrons will be suprafacial/disrotatory and involve a Hückel-type TS‡, whereas thoseinvolving 4n electrons will be antarafacial/conrotatory and the orbital array will be of the Möbius type.Dr. P. WipfPage 11 of 602/27/20083. The Woodward-Hoffmann Theory. This was the first theory toexplain and predict the outcome of many pericyclic reactions.Therefore, it brought order and understanding to many thermaland photochemical reactions. However, it used correlationdiagrams, which are difficult and time-consuming to apply.For thermally allowed reactions, the occupied orbitals of thestarting materials must correlate with occupied orbitals of theproduct. Correlation of orbitals is determined in terms ofsymmetry relationships.Dr. P. WipfPage 12 of 602/27/2008Sigmatropic RearrangementsDefinition: A sigmatropic rearrangement is defined as an intramolecular rearrangement of a ! bond, adjacent to one or more " systems, to a new position in a molecule, with the " systems becoming reorganized in the process.RRRRHRHRNew position of ! bond! bond that migratesNew position of ! bond! bond that migrates[1,5][3,3]Dr. P. WipfPage 13 of 602/27/2008[2,3]-Sigmatropic Rearrangements WITTIG REARRANGEMENTS - [2,3] Sigmatropic rearrangement of -allyloxy anions ([2,3] Wittig rearrangement): OX-OXΔ- - [1,2] Sigmatropic rearrangement of
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