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Chem 232 Lecture 20 CHEM 232 University of Illinois Organic Chemistry I at Chicago UIC Lecture 20 Organic Chemistry 1 Professor Duncan Wardrop March 18, 2010 1 Self-Test Question Capnellene (4) is a marine natural product that was isolated from coral reef. It has been shown to be cytotoxic to various tumor cell lines. What starting organic fragments (1 and 2) could be used to Squirellockconstruct says. the terminal alkyne 3? O O O O I 1 + 2 A + C CH C CH 3 O O + CH Br several B C 3 C steps O O C H H C I + C CH3 H O O capnellene (4) D OTs + C CH University of Illinois Slide 2 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 2 Self-Test Question Which set of reagents would most likely cause the transformation below? ? 1. B2H6 1. HCl 2. H2O2/NaOH 2. NaC CH 3. MsCl, pyrindine A 3. LDA C 4. NaC CH 4. methyl bromide 5. NaNH2, NH3 6. ethanol 1. H2SO4, H2O 2. TsCl, pyridine 1. HBr, ROOH B 3. NaC CH D 2. NaC CH 4. NaNH2, NH3 3. NaNH2, NH3 5. iodopropane 4. iodoethane University of Slide 3 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 20: March 18 3 Preparation of Alkynes There are two main methods for the preparation of alkynes: 1. Alkylation of acetylide anions (C-C bond formation) OCH3 OCH3 H C CNa H CO Br H CO C 3 3 C CH3 CH3 H 2. Functional group transformations (Today’s topic) University of Illinois Slide 4 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 4 Another Retrosynthesis Example CH + C-C bond Br formation alkylation C-C bond formation alkylation functional group transformation Br HC + CH University of Illinois Slide 5 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 5 CHEM 232 University of Illinois Organic Chemistry I at Chicago UIC Chapter 9 Preparation of Alkynes Double Dehydrohalogenation Sections 9.1 - 9.6 6 Dihalides in “Double Dehyrohalogenation” geminal and vicinal dihalides are most frequently used in the preparation of terminal alkynes H X X X C C C C H X H H geminal dihalide vicinal dihalide geminal: separated by two bonds (same carbon) vicinal: separated by three bonds (adjacent carbons) University of Illinois Slide 7 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 7 Double Dehydrohalogenation Geminal Dihalide Cl Cl E2 E2 H NH2 + + NH2 Cl H C H H C H H University of Illinois Slide 8 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 8 Double Dehydrohalogenation Vicinal Dihalide H Cl E2 E2 Cl NH2 + + NH2 H H C H H C Cl H University of Illinois Slide 9 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 9 Double Dehydrohalogenation Allenes are formed as minor products from double dehydrohalogenation when there is more than one set of β-hydrogen atoms. Since allenes are less stable (higher energy) than alkynes, they are only minor. H CH H C 3 allene H H (minor) H Cl H H H H H H3C CH3 + NaNH2 CH3 alkyne Cl H (major) H3C H H C allene H3C CH3 (minor) University of Illinois Slide 10 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 10 Double Dehydrohalogenation in Synthesis Addition of halogens across double bonds followed by double dehydrohalogenation is a convenient method for preparing terminal alkynes from terminal alkenes. 1. halogen (X2) addition 2. double dehydrohalogenation CH2 C C H C H University of Illinois Slide 11 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 11 Self-Test Question A Predict the Product. B 1. Br2, hv 2. NaOCH2CH3 3. Cl2 C 4. NaNH2, NH3 H 5. H2O 1 4,5 H Br Cl D 2 3 Cl Cl E Cl University of Slide CHEM 232, Spring 2010 12 Illinois at Chicago UIC Lecture 20: March 18 12 CHEM 232 University of Illinois Organic Chemistry I at Chicago UIC Functional Group Interconversions (FGI) of Alkynes Sections: 9.8-9.13 You are responsible for section 9.13 & 9.14. 13 Reactions of Alkynes Acid/base reactions Alkylation Hydrogenation Metal-Ammonia Reduction Functional Group Transformations Addition of Hydrogen Halides Hydration Alkynes Addition of Halogens ↓ Other FGs Ozonolysis University of Illinois Slide 14 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 14 Review: Hydrogenation of Alkenes Pd/C, CH3CH2OH • requires transition metal catalyst (fine powder) • solvent is typically an alcohol (e.g. ethanol, CH3CH2OH) • metals are insoluble (heterogeneous mixture) • exothermic reaction (-∆Hº) • heat of hydrogenation (∆Hhydrog) = -∆Hº University of Illinois Slide 15 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 15 Review: Hydrogenation of Alkenes as a consequence of mechanism, both hydrogens are added to the same face of the π-bond: syn addition no anti-addition prodcuts are formed (addition of hydrogen to opposite faces) University of Illinois Slide 16 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 16 Hydrogenation of Alkynes • Alkynes are reduced completely to alkanes under standard conditions (metal catalyst + H2) • Alkene is an intermediate, but is subsequently also reduced. University of Illinois Slide 17 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 17 Hydrogenation of Alkynes • Alkynes are reduced completely to alkanes under standard conditions (metal catalyst + H2) • Alkene is an intermediate, but is subsequently also reduced. University of Illinois Slide 18 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 18 Lindlar’s Catalyst Lindlar’s catalyst • lead (Pb) and quinoline “poison” the Pd (Lindlar Pd) catalyst = • reduces reactivity of catalyst Pd/CaCO N 3 alkynes are more reactive than alkenes lead (IV) acetate • quinoline • Lindlar Pd only active enough to quinoline reduce alkyne University of Illinois Slide 19 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 19 Metal-Ammonia Reduction • also known as dissolving metal reduction • requires group I metal (Li, Na, K) • provides trans-alkenes • solvent = ammonia (NH3) University of Illinois Slide 20 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 20 Metal-Ammonia Reduction Na H H CH3 H CH3 C C CH3 NH 3 CH3 H (Z)-alkenyl radical (E)-alkenyl radical (less stable) (more stable) • (Z)-alkenyl radical can interconvert with (E)-alkenyl radical • (E)-alkenyl radical less sterically hindered = more stable • (E)-alkenyl radical undergoes protonation to give a trans alkene • I will not ask you to know the mechanism for this reaction; however, you should at least be able to draw the (E) and (Z)-alkenyl radical intermediates. University of Illinois Slide 21 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 21 Useful Synthetic Tools for the Stereoselective Preparation of Alkenes Both reactions are stereoselective H CH Na 3 metal-ammonia reduction C C CH3 (trans-alkenes) NH3 H Lindlar Pd, H2 H H hydrogenation with C C CH3 Lindlar Pd CH3 (cis-alkenes) Neither of these reactions is stereospecific. Why? University of Illinois Slide 22 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 22 Self-Test Question What organic molecules could be used to construct the alkene below? A H3C metal-NH3 [R] H3C Br HC CH B Br 2 alkylations C OTs HC CH + HC + D H3C Br CH Br University of Slide CHEM 232, Spring 2010 23 Illinois at Chicago UIC Lecture 20: March 18 23 Reactions of Alkynes Acid/base reactions Alkylation Hydrogenation Metal-Ammonia Reduction Functional Group Transformations Addition of Hydrogen Halides Hydration Alkynes Addition of Halogens ↓ Other FGs Ozonolysis University of Illinois Slide 24 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 24 Review: Addition to Alkenes Alkynes undergo addition reactions similar to alkenes H2SO4/H2O H OH C C hydration HX H X C C C C hydrogen halide addition X X X2 C C halogen addition University of Illinois Slide 25 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 25 Self-Test Question Presuming alkynes react similarly to alkenes, chose the column containing the correct set of products. A B C D Actual! OH O H2SO4/H2O H C OH OH H3C H 3 HO H C 3 H C CH H3C 3 3 Br Br Br HBr H C Br Br H C H 3 Br H3C 3 H3C CH3 H3C Cl Cl Cl Cl Cl Cl2 H3C ClCl Cl Cl H C H3C H H C H C 3 3 3 Cl University of Slide CHEM 232, Spring 2010 26 Illinois at Chicago UIC Lecture 20: March 18 26 Markovnikov Addition of HX to Alkynes Unlikely Mechanism • alkenyl cation = sp hybridized = • more electronegative = high energy • experimental evidence: 3rd order reaction University of Illinois Slide 27 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 27 Markovnikov Addition of HX to Alkynes (vinyl halide) Better Mechanism • 3rd oder (termolecular): • rate=k[alkyne][HX]2 • transition state = no δ+ on alkyne carbons University of Illinois Slide 28 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 28 Markovnikov Addition of HX to Alkynes When excess HX is added, two equivalents are added across the alkyne to provide the geminal dihalide H H H H H F F F F F H r faste resonance stabilization major product H F F H F sl ower H F F University of Illinois Slide 29 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 29 Hydration of Alkynes Hydration of alkynes proceeds through a two-step process: Markovnikov addition to provide an enol followed by tautormerization Markovnikov addition Tautomerization H H H2SO4/H2O + O O (H3O ) H3C H3C H3C CH2 CH3 HgSO4 H3C OH H3C OH H3C OH enol ketone tautomers: rapidly equilibrating constitutional isomers carbonyl/enol: equilibrium lies toward carbonyl University of Illinois Slide 30 CHEM 232, Spring 2010 at Chicago UIC Lecture 20: March 18 30 Why is Hg2+ Catalyst Needed? • Hg2+ forms a more stable (low energy) mercuronium intermediate • Without Hg2+, the alkyne would have to be protonated to give a 2º vinylic carbocation.
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