Bài giảng Organic Chemistry - Chapter 13: Alkynes: The C = C Triple Bond

Like alkenes, but ending -ene turns into –yne.HC CHEthyne543211-PentyneBr1234565-Bromo-2-hexyne-ol -yne>OH1232-Propyn-1-olChapter 13: Alkynes: The C C Triple BondNamingPriority:When the alkyne contains also double bonds, it is called an enyne. However, despite being an “yne”, numbering begins closest to either group: 1234563-Hexen-1-yne1-Penten-4-yneWhen double and triple bond are equidistant from each terminus: ene first (alphabetical)12345671-Hepten-4-yne12345Substituents:Ethynyl2-Propynyl (or propargyl)Smaller R is a substituent to larger R3-Cyclobutyl-1-hexyneEthynylcyclohexane but122-Propenyl (or allyl)12Remember:Two perpendicular π bonds; sp hybrids R C C REthyneStructureThe Triple Bond Is EnergeticHeat of hydrogenation: more than two alkene  bonds (which would be ~ -60 kcal mol-1)Acetylene torch: ~3,500 CInternal triple bond is more stable:: RC C H + B R C C + HB Hydrogens get more acidic (blue)--pKa ~ 25!Why? 50% s-characterKAlkynes Are Relatively Acidic-+LiLiLi+++CH3MgBrMgBr+CH4pKa 25pKa 50H H + Na NH2+::HNaC C::--NH3+NaNH2Na++pKa 331 equiv:-Synthetic Use of Acidity-Preparation Of AlkynesElimination E2 of dihaloalkanesCCHHXXCCCCBrBrNaB:-HXB:-NaNH2 excessNH3 liq.H+, H2O work up75%Application in synthesis:RCH CHR R C C RBrBrBrBrBr2NaNH2NH3 liq1,5-Hexadiyne2. Alkylation of Alkynyl Metals SN2 rules applyLiTHFLiI∆90%Best: RI, THF, ∆. RBr or RCl need “coordinating” additives: e.g. ; or HMPA solvent. Remember: Grignards don’t work for coupling with RX (but O.K. for or carbonyls).H2NNH2O+ CH3MgBrMgBrCH2 OOH+ LiNH2 (1 equiv)LiOOH+ 2 CH3MgBrMgBrBrMgHOOHLi+LiOCH3OHCH3CHOReactions Of Alkynes1. Reductionsa. Complete hydrogenationH2, Pt100%b. Partial hydrogenation with “poisoned” Lindlar’s catalyst: syn addition gives cis product:Pd-CaCO3, Pb(OCCH3)2, quinolineONH2, Lindlar100%HHcis-3-Heptenec. Na reduction: anti addition gives trans product:+ NaNH3 liq.86%HHtrans-3-HepteneMetalLindlar’s catalyst:Via stepwise 2e transferMechanism Of The Na Reduction Of AlkynesEquilibrates between cis and trans (more stable)LipHolidayNa dissolves in liquid ammonia, makes “solvated” electronsMarkovnikov2. Electrophilic additionsa. HX:RRH+ + CCRRH+X-Anti to H; pushes R transCCRHRXH+CRXRCH2X-RCH2CX2RGeminal dihalideCHRCHXCCXRHHHXRCX2CH3Markovnikov twicesp+sp 2Internal alkynesResonance with XTerminal alkyneslinearExamples:HBrBrBrHIIIII+HClClClSynthetic application:Br2BrBrNaNH2NH3HBrBrBr Vicinal Geminal b. X2: Anti addition, as for alkenesCH3Br2BrCH3BrBr2BrBrBrBrc. Cat. HgSO4, H2O hydration, MarkovnikovCRRCCat. HgSO4H2OCCHROHRTautomerizationOH+ or OH catalyzed-UnstableRC‒CRNo NaBH4 neededHHH+OMechanism Of TautomerizationORH+ :COHRCH2+RCRCH2RCH2CRO-H+OH :RCH2CRO+H+CCHRORCCHRR---CCHROHRH+ or OH catalyzed-HBr-BrHBrROORCHRCHgSO4H2Ocat.CHORCH2RCCH3OA methyl ketoned. Radical HBr: Anti-Markovnikov additionBrBrBrHHMixtures+Examples:BrBR2e. Hydroboration-Oxidation Use R2BH (R = bulky group: cyclohexyl) to ensure single hydroborationCHRC+B‒H2CCRHHH2O2, -OHOHCCRHHTautomerizationRCH2CHOAldehyde !Steric controlAn anti-Markovnikov hydration1. R2BH 2. H2O2,-OH1. HBR2 2. H2O2,-OHTherefore:HOBut:HgSO4, H2Ocat.OR’RRCCH2R’ +OMixturesRCH2CR’OAnd:No SN2:No SN1:HaloalkenesBut organolithium and Grignards reagents:Alkenyl halides can be coupled with alkenes to give 1,3-dienesThe Heck ReactionRichard F. Heck (b. 1931) Nobel Prize 2010Examples:Mechanism Of The Heck ReactionPd catalyst is typically Pd(OCCH3)2OAdditional ligands around Pd are omitted