Chapter 13: Alkynes: The C C Triple Bond Naming Like alkenes, but ending -ene turns into –yne. 4 6 1 3 5 4 2 2 HC CH 5 1 Br 3 Ethyne 1-Pentyne 5-Bromo-2-hexyne 1 Priority: 2 2-Propyn-1-ol -ol -yne > 3 OH

When the alkyne contains also double bonds, it is called an enyne When the alkyne contains also double bonds, it is called an enyne. However, despite being an “yne”, numbering begins closest to either group: 2 1 3 4 3 2 6 1 5 4 5 3-Hexen-1-yne 6 1-Hepten-4-yne 7 When double and triple bond are equidistant from each terminus: ene first (alphabetical) 2 1 3 1-Penten-4-yne 4 5

2-Propynyl (or propargyl) 2-Propenyl (or allyl) Ethynyl Substituents: 1 1 2 2 2-Propynyl (or propargyl) Remember: 2-Propenyl (or allyl) Ethynyl Ethynylcyclohexane but 3-Cyclobutyl-1-hexyne Smaller R is a substituent to larger R

Structure Two perpendicular π bonds; sp hybrids R C C R Ethyne

The Triple Bond Is Energetic Acetylene torch: ~3,500 C Heat of hydrogenation: more than two alkene  bonds (which would be ~ -60 kcal mol-1) Internal triple bond is more stable

Alkynes Are Relatively Acidic - - RC C H + B R C C + HB : : pKa ~ 25! Why? 50% s-character Hydrogens get more acidic (blue)

Synthetic Use of Acidity + Li - + + Li Li pKa 25 pKa 50 + CH3MgBr MgBr + CH4 - - + - - + + : NaNH2 H H + Na NH2 : H : Na C C : : Na 1 equiv + NH3 pKa 33

Preparation Of Alkynes Elimination E2 of dihaloalkanes X X - H - B: B: C C C C C C X H H H+, H2O work up NaNH2 excess Br NH3 liq. Br Na 75%

Application in synthesis: RCH CHR R C C R Br Br Br2 Br Br NaNH2 NH3 liq 1,5-Hexadiyne

2. Alkylation of Alkynyl Metals SN2 rules apply Li Li I THF ∆ 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). NH2 H2N O

Li + MgBr + CH3MgBr OH O + LiNH2 (1 equiv) OH HO OH + 2 CH3MgBr BrMg CH2 O + CH3MgBr OH O + LiNH2 (1 equiv) Li OH O HO OH CH3CH + 2 CH3MgBr BrMg MgBr O Li OH CH3 + Li

Reactions Of Alkynes 1. Reductions a. Complete hydrogenation H2, Pt 100% b. Partial hydrogenation with “poisoned” Lindlar’s catalyst: syn addition gives cis product: O Lindlar’s catalyst: Pd-CaCO3, Pb(OCCH3)2, quinoline N H2, Lindlar H H cis-3-Heptene 100% c. Na reduction: anti addition gives trans product: NH3 liq. H + Na trans-3-Heptene 86% H Metal Via stepwise 2e transfer

Mechanism Of The Na Reduction Of Alkynes 9/12/2018 Mechanism Of The Na Reduction Of Alkynes Na dissolves in liquid ammonia, makes “solvated” electrons 1:55 Billy Holiday Swing Brother Swing Equilibrates between cis and trans (more stable) Holiday Lip © Univesity of California

2. Electrophilic additions a. HX: linear Internal alkynes + - R X H+ + R R C C R Anti to H; pushes R trans H sp 2 sp Resonance with X R X X - + X H+ C C RCH2 C RCH2CX2R Markovnikov H R R Geminal dihalide Terminal alkynes X H HX HX RC CH C C RCX2CH3 R H Markovnikov twice

+ Examples: Synthetic application: Vicinal Geminal Br Br HBr I I HI I Cl Cl HCl Synthetic application: Br2 NaNH2 HBr Br Br Br NH3 Br Vicinal Geminal

b. X2: Anti addition, as for alkenes Br CH3 Br2 CH3 Br Br Br Br2 Br Br c. Cat. HgSO4, H2O hydration, Markovnikov H O R Cat. HgSO4 H Tautomerization RC CR RC‒CR C C H2O - H+ or OH catalyzed R OH No NaBH4 needed H Unstable

Mechanism Of Tautomerization - C C H+ or OH catalyzed R OH O R R -H+ + H+ : RCH2CR RCH2 C RCH2 C H + OH O O H R H - R - +H+ OH : C C C C RCH2CR - R O R O

+ d. Radical HBr: Anti-Markovnikov addition O RC CH C CH2 RCCH3 Examples: O cat. HgSO4 HO RC CH C CH2 RCCH3 H2O R A methyl ketone d. Radical HBr: Anti-Markovnikov addition Br HBr ROOR Br Br H Br HBr -Br + Mixtures H

An anti-Markovnikov hydration e. Hydroboration-Oxidation Use R2BH (R = bulky group: cyclohexyl) to ensure single hydroboration R H H2O2, -OH + RC CH B‒H C C 2 H BR2 Steric control O R H Tautomerization C C RCH2CH H OH Aldehyde ! An anti-Markovnikov hydration

Therefore: H O O But: O O And: R R’ RCCH2R’ + RCH2CR’ Mixtures 1. HBR2 2. H2O2,-OH H O cat. HgSO4, H2O O But: 1. R2BH 2. H2O2,-OH O O And: R R’ RCCH2R’ + RCH2CR’ Mixtures

But organolithium and Grignards reagents: Haloalkenes No SN2: No SN1: But organolithium and Grignards reagents:

Richard F. Heck (b. 1931) Nobel Prize 2010 The Heck Reaction Alkenyl halides can be coupled with alkenes to give 1,3-dienes Examples: Richard F. Heck (b. 1931) Nobel Prize 2010

Mechanism Of The Heck Reaction Pd catalyst is typically Pd(OCCH3)2 O Additional ligands around Pd are omitted