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WH RulesWH Rules 2WH Rules 3WH Rules p1.pdfWH Rules.pdfWH Rules 2.pdfWH Rules 3.pdfCh242bScott VirgilThe Woodward-Hoffmann Rulesand the Conservation of Orbital SymmetryThe Woodward-Hoffmann rules encompass the realm of pericyclic reactions:electrocyclizations cycloadditions sigmatropic rearrangementsH Hene reactionsPericyclic reactions are prevalent in synthetic organic chemistry as well as in biosynthetic processes. This can be seen in the case of the endiandric acids, biosynthetically synthesized by Nicolaou:COOCH3PhPhHHHHendiandric acid BThe principle of conservation of orbital symmetry applies ONLY to concerted pericyclic reactions. In these cases it serves as a powerful predictive tool.Electrocyclic Reactions"We define as electrocyclic transformations the formation of a single bond between the termini of a linear system containing k π-electrons and the converse process.""In such changes, fixed geometrical isomerism imposed upon the open-chain system is related to rigid tetrahedral isomerism in the cyclic array. A priori, this relationship might be disrotatory or conrotatory."CBADACBDCONROTATORYDBACACBDDISROTATORYWoodward, R. B.; Hoffmann, R. J. Am. Chem. Soc. 1965, 87(2), 395-397.COOCH3Nicolaou, K. C. J. Am. Chem. Soc. 1982, 104, 5555; J. Am. Chem. Soc. 1982, 104, 5557; J. Am. Chem. Soc. 1982, 104, 5558; J. Am. Chem. Soc. 1982, 104, 5560.Electrocyclic Reactions of 1,3-Butadiene:HOMOLUMOButadiene Molecular Orbitals Under ThermalConditionsThermal Cyclization of ButadieneABCDACDBCBADACBDCONROTATORY"Thus, in an ope an open chai open chain system chain system containing 4nπ-electrons,the symmetry of of the highest the highest occupie highest occupied ground-stais such th that a bondi a bonding inte bonding interaction bemustinvolve overlap between orbial envelopes on opposite faces of the sy system, andthis can only only be achie be achieved in aconrotatory process."HOMOLUMOButadiene Molecular Orbitals Under PhotochemicalConditionsPhotochemical Cyclization of ButadieneABDCACDBDBACACBDDISROTATORY"On the oth promotion of an electr an electron to excited state leads t leads to a reversal to a reversal of the t a reversal of the termin reversal of the terminal symmetryrelationships in the orbitals the orbitals mainly orbitals mainly involved in bredistribution, with t the conseq consequence that a sysundergoes a thermally thermally induced dis induced disrotatory electrocytransformation in the ground sta the ground state shou ground state should follow aconrotatory course when photochemically excited, and vice versa."Analogous analyses of larger π-systems will allow determination of the stereochemical course of electrocyclic ring closings and openings under thermal and photochemical conditions.Thermal and Photochemical Cyclizations of 1,3,5-Hexatrienes:CBADThermal HOMODISROTATORYCBDACBADPhotochemical HOMOCONROTATORYDBCA"Conversely, in open systems containing 4n + 2 π-electrons, terminal bonding interaction within ground-state molecules requires overlap of orbital envelopes on the same face of the system, attainable only by disrotatory displacements."Based on these these observat observations for electrocyc for electrocyclic reactions of electrocyclic reactions of conjugated polyeneWoodward-Hoffmannelectrocyclic reactions can be summarized in terms of the number of electron pairs involved in the cyclization or ring opening: Number of electron pairs Δ hνOddEvenDisrotatoryConrotatoryConrotatoryDisrotatoryOdd-electron systems follow same stereochemical course as the even system containing one further electron.Charged systems should behave in the same manner as neutral systems containing the same number of electrons.However, there are a few caveats that deserve mention!"It should be emphasized that our hypothesis specifies in any case which of two types of geometrical displacements will represent a favored process, but does not exclude the operation of the other under very energetic conditions."ΔXConrotatory(ThermallyAllowed)Disrotatory(Thermally"Disallowed")ΔHHHHHHXConrotatory(ThermallyAllowed)HHHHHHDisrotatory(Thermally"Disallowed")ΔConrotatory(ThermallyAllowed)Disrotatory(Thermally"Disallowed")XWhen these traditionally "disallowed" processes are observed they are usually occurring by non-concerted pathways, such as diradical pathways.In the ca case of cyc of cyclobutene cyclobutene electrocyclic electrocyclic ring opening, you may have ring opening, you may have noticed that there opening, you may have noticed that there are two possi you may have noticed that there are two possible conrotatoprocesses that give different isomeric products. The principle of torquoselectivity guides us in deciding which conrotatory process will be favored for a given electrocyclic ring opening.HHCH3CH3HCH3H3CHHHHCH3CH3HHCH3CH3XHHHHHHHHHHHXHHH3CCH3Examination of the two pos the two possible t two possible transitio possible transition states for transition states for the ring opening of trans-dimethyl-cyclobutenenegative steric interaction in transition state A that is not seen in transition state B. Thus, only product B is observed. The preference of transition state A over transition state B is called torquoselectivity.For the ele electrocyclic ring op ring opening of the [4.2.0 opening of the [4.2.0] fused bicyc of the [4.2.0] fused bicycle below, t the [4.2.0] fused bicycle below, torquose [4.2.0] fused bicycle below, torquoselectivity pr fused bicycle below, torquoselectivity predicts that ri bicycle below, torquoselectivity predicts that ring opening leathe cyclooctatriene with two trans olefins (B) will be highly disfavored compared to the cyclooctatriene containing only cis olefins (A).TS ATS BABA BTorquoselectivity in Electrocyclic Reactions:Electrocyclic Reactions in Nature and Synthetic Chemistry: The Endiandric AcidsCO2RPhΔPhHHH HHHCO2Rendiandric acid BCO2RPhHHHCO2Rendiandric acid F8π ThermalConrotatoryElectrocyclization6π ThermalDisrotatoryElectrocyclization[4+2] ThermalCycloaddition(Diels-Alder)PhNicolaou, K. C. J. Am. Chem. Soc. 1982, 104, 5555; J. Am. Chem. Soc. 1982, 104, 5557; J. Am. Chem. Soc. 1982, 104, 5558; J. Am. Chem. Soc. 1982, 104, 5560.Cycloaddition ReactionsHoffmann, R.; Woodward, R. B. J. Am. Chem. Soc. 1965, 87(9), 2046-2048.Whereas electrocyclic reactions involve the net intramolecular interconversion of one σ-bond and one π-bond, cycloaddition reactions consist of the net intermolecular conversion of k π-bonds to k


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CALTECH CH 242B - Lecture notes

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