columbia大学有机化学课件10_12_14 html

1、10.12The Diels-Alder ReactionSynthetic method for preparingcompounds containing a cyclohexene ring+conjugateddienealkene(dienophile)cyclohexeneIn general.transition stateviaMechanistic featuresconcerted mechanism cycloaddition pericyclic reactiona concerted reaction that proceedsthrough a cyclic tra

2、nsition state+conjugateddienealkene(dienophile)cyclohexeneThe equation as written is somewhat misleading because ethylene is a relatively unreactive dienophile.Recall the general reaction.What makes a reactive dienophile?The most reactive dienophiles have an electron-withdrawing group (EWG) directly

3、 attached to the double bond.Typical EWGsEWGCOCCCNO+H2CH2CCHCHCH2CHCHbenzene100COCH(100%)ExampleO+H2CH2CCHCHCH2CHCHbenzene100COCH(100%)via:OCHExampleOH2CCHCCH2+CH3benzene100C OOH3CO(100%)OOExampleOH2CCHCCH2+CH3benzene100C Ovia:OOH3CH C3OO(100%)OOOExampleOO+H2CCHCHCH2CH3CH2OCCCCOCH2CH3benzene100COCOC

4、H2CH3(98%)COCH2CH3OAcetylenic Dienophilesyn addition to alkenecis-trans relationship of substituents on alkene retained in cyclohexene product*A stereospecific reaction is one in whichstereoisomeric starting materials givestereoisomeric products;characterized byterms like syn addition, anti eliminat

5、ion,inversion of configuration, etc.Diels-Alder Reaction is Stereospecific*OCOHC6H5+H2CCHCHCH2CCHHHC6H5Honly productHOCOExampleOCOHH+H2CCHCHCH2CCHC6H5C6H5HHonly productHOCOExampleCyclic dienes yield bridged bicyclic Diels-Alder adducts.OHCOCH3+CCHCH3OCOHOCH3HCOCH3OCOOOHCH3COCH3is thesame asHCOCH3COC

6、H3OOHHCO10.13The p Molecular Orbitals ofEthylene and 1,3-ButadieneOrbitals and Chemical ReactionsA deeper understanding of chemical reactivity can be gained by focusing on the frontier orbitals of the reactants.Electrons flow from the highest occupied molecular orbital (HOMO) of one reactant to the

7、lowest unoccupied molecular orbital (LUMO) of the other.Orbitals and Chemical ReactionsWe can illustrate HOMO-LUMO interactions by way of the Diels-Alder reaction between ethylene and 1,3-butadiene.We need only consider only the p electrons ofethylene and 1,3-butadiene.We can ignorethe framework of

8、s bonds in each molecule.red and blue colors distinguish sign of wave functionbonding p MO is antisymmetric with respect to plane of moleculeBonding p orbital of ethylene; two electrons in this orbitalThe p MOs of EthyleneAntibonding p orbital of ethylene; no electrons in this orbitalLUMOHOMOBonding

9、 p orbital of ethylene; two electrons in this orbitalThe p MOs of EthyleneFour p orbitals contribute to the p system of 1,3- butadiene;therefore, there are four p molecular orbitals.Two of these orbitals are bonding; antibonding.two areThe p MOs of 1,3-ButadieneHOMO4 p electrons; each orbital2 inLow

10、est energy orbitalThe Two Bondingp MOs of 1,3-ButadieneHighest energy orbitalLUMOBoth antibondingorbitals are vacantThe Two Antibondingp MOs of 1,3-Butadiene10.14A p Molecular Orbital Analysis of theDiels-Alder ReactionInasmuch as electron-withdrawing groups increase the reactivity of a dienophile,

11、we assume electrons flow from the HOMO of the diene to the LUMO of the dienophile.MO Analysis of Diels-Alder ReactionHOMO of 1,3-butadieneHOMO of 1,3-butadiene and LUMO of ethylene are in phase with one anotherallows s bond formation between the alkene and the dieneLUMO of ethylene (dienophile)MO Analysis of Diels-Alder ReactionHOMO of 1,3-butadieneLUMO of ethylene (dienophile)MO Analysis of Diels-Alder ReactionH2CCH2+H2CCH2The dimerization of ethylene to give cyclobutane does not occur under conditions of typicalDiels-Alder reactions.Why not?A forbidden react