浙江大学马成有机化学乙课件Chapter7 Conjugation in.doc

2009/9/131Chapter 7: Conjugation in Alkadienes and Allylic Systems2009/9/132SummaryA adjacent systemin the form of a double (or triple) bond can alter the characteristic chemistry of the functional group, due to the effects of resonance within a conjugated systemThis system does not have to be all C as in a C=C or CC but can also contain heteroatoms, e.g. C=O or CN1.Functional groups (independent units, conjugation units)2.Allylic System 3.Dienes4.Conjugation addition5.Diels-Alder Reaction (M.O.)2009/9/1337.1 Conjugation1.Some Dienes and Polyenes2009/9/1342.ConjugationIn organic chemistry, Conjugation is used to describe: systems p-systems-systemsCharacter:Lowered Molecule Energy1)The action not decreasewith the distance in the conjugation system2)Charge changes one by one (electronegativety, electronic charge)Comared to inductive effect !The fundamental -Orbital involved in the bonding within the system1)A continuous array of p orbitalsthat can align to produce a bonding overlapalong the whole system. (parallel and co-plane)2)In -systems,with orbitals, Pseudo-conjugation or Super-conjugation The Effect of three Conjugation: p - -2009/9/135 CCCCCCC NCCCOHCCC CCC2009/9/136The native conformation of cyclooctatetraene. Adjacent double bonds are not coplanar, so there is not strong conjugation between them 2009/9/137p ClCCRCOOHCH3CHCHCH22009/9/138- and -pOOCH2CHCH3CCCHHHHHCH2CHCHHHCCHHH2CCHHHCH2HHCH3CCHHH2CCHHHHHCH2CH32009/9/139The strength of conjugationElectronegativity and Major quantum number (-C)1)2)CCCCCCC NCCCOCNR2009/9/1310p (+C)ENMajor quantum number ()NR2CCOHCCCCCCCCFCC2009/9/1311Dienea)Heat of Hydrogenation2009/9/1312Another stereoisomeric factor associated with conjugated dienes exist:Rotation about the single bond joining the two double bonds (colored blue) converts a trans-like s-transconformation to its s-cisform. The energy barrier to this conformational isomerisation is normally low, and the s-trans conformer is often more stable than the s-cis conformer2009/9/1313b) Ioslated and Conjugated dienec) Cumulated Diene (allene)2009/9/13142009/9/1315Bond Energy and Bond Length of Single Bond2009/9/1316Other Conjugation systems2009/9/13172.Resonance (electron delocation)systems and lone pair electronsDelocalisation increases the stablility of systems (esp. for charged systems)Functional groups next to systems (i.e.conjugated functional groups) have some reactivity trends that are modified compared to those of the non-conjugated system. Here are a few simple examples:1)Allyl chloride : very reactive in nucleophilic substitution reactions 2)1,3-butadiene : can undergo addition via two different modes3)Propenal : nucleophiles can add to the C=C due to the presence of the conjugated C=O2009/9/13187.2 Allylic Systems The positions adjacent to C=C often show enhanced reactivity compared to simple alkanes due to the proximity of the adjacent system.Such positions are referred as allylic.1.Allylic carbocations ( delocalization more stable)2009/9/13192.Allylic radicals3.ReactivityAllylic bonds are often weaker and are easily broken, for example compare the bond dissociation energies: 2009/9/1320The stability of the allylic radical can be utilised in the preparation of allylic halides (esp. -Cl and -Br) Allylic halides readily undergo substitution reactions via either SN1 or SN2 pathways 4.Reaction of Allylic Systems Radical Halogenation of Allylic systemsN-bromosuccinimide (NBS) can be used as an alternative source of Br2,NCS2009/9/1321Substitution Reactions of Allyl HalidesThe process can be complicated by the allylic rearrangement where the nucleophile can attack either of the deficient sites. Exp: (SN1) Pay attention to the reaction condition!Problem:Why the 3oalcohol is the major product ? What does the fact that both the reactions shown give the same product mixture suggest ? 2009/9/13227.3 Conjugated DienesConjugated Dienes2009/9/13231.Preparation of Conjugated DienesDienes can be prepared by elimination reactions of unsaturated alcohols and alkyl halides.The formation of the conjugated diene is usually favaoured over the isolated diene unless the structure prevents the formation of the conjugated system.2.Reactions of Dienes in generalIn general terms, dienes undergo electrophilic addition reactions in a similar fashion to alkenes2009/9/1324Conjugated dienesundergo addition but the proximity of the conjugated C=C influences the reactionsIsolated dienes react just like alkenesCumulated dienes react more like alkynes (after all, both have sp hybridized C atoms)3.Reaction of conjugated dienes:Overall reaction :Electrophilic AdditionAddition of hydrogen halides Addition of halogens Diels-Alder reaction2009/9/1325a.Addition of Hydrogen Halides to DienesDirect H-Xadds directly across the ends of a C=CConjugate H-Xadds across the ends of the conjugated systemThe numbers 1,2-and 1,4-denote the relative positions of Hand Xin the products 2009/9/1326Why? The Stability of Product and Other?For the two reactions above, we should resolve two problem:The reaction mechanismThe difference of the ratios of 1,2-addition and 1,4-addition products2009/9/1327Mechanism for the reaction 0f 1,3-butadiene with HBr2009/9/1328The potential outcome of a reaction is usually influenced by two factors: pThe realtive stability of the products(i.e. thermodynamic factors)pThe rate of product formation(i.e. kinetic factors)2009/9/1329Potential Energy diagram2009/9/1330Summary for Kinetic and Thermodynamic Control : At low temperature, the reaction is under kinetic control(rate, irreversible conditions) and the major product is that from fastest reaction. At high temperature, the reaction is underthermodynamiccontrol (equilibrium, reversible conditions) and the major product is the more stable system Other exp.2009/9/1331b)Addition of Halogens to DienesLike the addition of hydrogen halides to conjugated dienes, halogens add to dienes via direct and conjugate addition pathways:The major products are usually the more stable, conjugate addition products with the more stable E configuration of C=C. 2009/9/1332Nobel Prizes in Chemistry 1950 2009/9/1333c)Diels-Alder Reaction (Nobel Prize in 1950) Classic 4+2The Diels-Alder reaction is a conjugate addition reaction of a conjugated dieneto an alkene or alkyne (the dienophile) to produce a cyclohexene or cyclohexadieneConcerted mechanism viaa cyclic transition state“Cycloaddition“Regio-and stereoselectivity; usually thermodynamically favorable2009/9/1334For example:2009/9/1335Stereochemistry in DA Reaction (Stereoselectivity)Addition is synon both components (bonds form same species at the same time) cis principleunderstood that relative configuration of substituent on dienophile2009/9/1336Stereospecificity2009/9/1337RegioselectivityIn fact, DA is electrophilic addition2009/9/1338Favored by:Electron withdrawinggroups on the ElectrophilicdienophileElectron donatinggroups on the Nucleophilic diene2009/9/1339These resolved theRegioselectivityand Favoured reagentsDiene structure a.s-cis and s-transUnfavourable steric interactionof substituents at C1 and C4 12 kJ/mol (2.8 kcal/mol)2009/9/1340b.Cyclic dienesConcert mechanism made the s-trans no DA reactionWhats more?Cyclic dienescan give stereoisomeric products depending on whether the dienophile lies underor away fromthe diene in the transition state.The endo product is usually the major product (due to kinetic control)2009/9/1341Exo (apart from) and Endo (lies under)2009/9/1342Steric interaction made the exo-product more stable than endo-form (thermodynamic control)However, sometimes, secondary interactionsmade the active energyof endo-form lower than exo-form (kinetic control)2009/9/1343Exp.2009/9/1344Asymmetric Diels-Alder reactionsMacMillan catalystNew Strategies for Organic Catalysis: The First Highly Enantioselective Organocatalytic Diels-Alder ReactionAhrendt, K. A.; Borths, C. J.; MacMillan, D. W. C. J. Am. Chem. Soc.; (Communication); 2000; 122(17); 4243-4244. doi:10.1021/ja000092s2009/9/13451. aza-Diels-Alder Reactions Tetrahedron2005, 61, 11837. 2009/9/1346Angew. Chem. Int. Ed.2005, 44, 4877. 2009/9/13472. oxo-Diels-Alder Reactions Tetrahedron2005, 61, 9465. 2009/9/13483. Other Hetero Diels-Alder Reactions Tetrahedron2005, 61, 10521 2009/9/1349The essential characteristics of the Diels-Alder cycloaddition reaction may be summarized as follows:(i) The reaction always creates a new six-memberedring. When intramolecular, another ring may also be formed.(ii) The diene component must be able to assume a s-cisconformation.(iii) Electron withdrawing groups on the dienophile facilitate reaction.(iv) Electron donating groups on the diene facilitate reaction.(v) Steric hindrance at the bonding sites may inhibit or prevent reaction.(vi) The reaction is stereospecificwith respect to substituent configuration in both the dienophile and the diene.2009/9/1350Nomenclature of polycycle compounds2009/9/13512009/9/13527.4 Molecular OrbitalsMolecular Orbitals TheoryThe workings of organic reactions based on how orbitals interactto control the outcome of reactionsMolecular Orbitalswere developed by Fukui, Hoffmann and Woodward, leading to a Nobel Prize for Fukui and Hoffmann in 1981Applications Diels-Alder reactionDimerization of ethene to cyclobutaneKEY:“Number of molecular orbital” equal to “Number of atomic orbital combined to make them”2009/9/1353Nobel Prize in Chemistry 19812009/9/13542009/9/13551.IntroductionFrontier (前线) orbitals : HOMO and LUMOLUMOLowest unoccupied molecular orbitalLUMO receives electrons lowest en