Lecture 20 Organic Chemistry 1
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1 CEM 232 rganic Chemistry I at Chicago Lecture 20 rganic Chemistry 1 Professor Duncan Wardrop March 18,
2 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 construct the terminal alkyne 3? Squirellock says... A I + C C C C B C C + C 3 Br 3 several steps C I + C C C3 D Ts + C C at Chicago CEM 232, Spring 2010 capnellene (4) Slide 2 2
3 Self-Test Question Which set of reagents would most likely cause the transformation below?? A NaC C 3. LDA 4. methyl bromide C 1. B /Na 3. Ms, pyrindine 4. NaC C 5. NaN 2, N 3 6. ethanol B 1. 2 S 4, 2 2. Ts, pyridine 3. NaC C 4. NaN 2, N 3 5. iodopropane D 1. Br, R 2. NaC C 3. NaN2, N3 4. iodoethane University of Illinois at Chicago CEM 232, Spring 2010 Slide 3 3
4 Preparation of Alkynes There are two main methods for the preparation of alkynes: 1. Alkylation of acetylide anions (C-C bond formation) C 3 C 3 C CNa 3 C C 3 Br 3 C C 3 C C 2. Functional group transformations (Today s topic) at Chicago CEM 232, Spring 2010 Slide 4 4
5 Another Retrosynthesis Example C-C bond formation alkylation C C-C bond formation + Br functional group transformation Br alkylation + C C at Chicago CEM 232, Spring 2010 Slide 5 5
6 CEM 232 rganic Chemistry I at Chicago Chapter 9 Preparation of Alkynes Double Dehydrohalogenation Sections
7 Dihalides in Double Dehyrohalogenation geminal and vicinal dihalides are most frequently used in the preparation of terminal alkynes X C C X geminal dihalide X X C C vicinal dihalide geminal: separated by two bonds (same carbon) vicinal: separated by three bonds (adjacent carbons) at Chicago CEM 232, Spring 2010 Slide 7 7
8 Double Dehydrohalogenation Geminal Dihalide N 2 E2 + + N 2 E2 C C at Chicago CEM 232, Spring 2010 Slide 8 8
9 Double Dehydrohalogenation Vicinal Dihalide N 2 E2 + + N 2 E2 C C at Chicago CEM 232, Spring 2010 Slide 9 9
10 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. C C 3 allene (minor) 3 C C 3 + NaN 2 3 C C 3 alkyne (major) 3 C C C 3 allene (minor) at Chicago CEM 232, Spring 2010 Slide 10 10
11 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. C C 1. halogen (X2) addition 2. double dehydrohalogenation C 2 C at Chicago CEM 232, Spring 2010 Slide 11 11
12 Self-Test Question Predict the Product. 1. Br 2, hv 2. NaC 2 C 3 A B 1 Br NaN 2, N ,5 C D E University of Illinois at Chicago CEM 232, Spring 2010 Slide 12 12
13 CEM 232 rganic Chemistry I at Chicago Functional Group Interconversions (FGI) of Alkynes Sections: You are responsible for section 9.13 &
14 Reactions of Alkynes Acid/base reactions Alkylation ydrogenation Metal-Ammonia Reduction Addition of ydrogen alides ydration Addition of alogens zonolysis at Chicago CEM 232, Spring 2010 Functional Group Transformations Alkynes ther FGs Slide 14 14
15 Review: ydrogenation of Alkenes Pd/C, C3C2 requires transition metal catalyst (fine powder) solvent is typically an alcohol (e.g. ethanol, C3C2) metals are insoluble (heterogeneous mixture) exothermic reaction (- º) heat of hydrogenation ( hydrog) = - º at Chicago CEM 232, Spring 2010 Slide 15 15
16 Review: ydrogenation 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) at Chicago CEM 232, Spring 2010 Slide 16 16
17 ydrogenation of Alkynes Alkynes are reduced completely to alkanes under standard conditions (metal catalyst + 2) Alkene is an intermediate, but is subsequently also reduced. at Chicago CEM 232, Spring 2010 Slide 17 17
18 ydrogenation of Alkynes Alkynes are reduced completely to alkanes under standard conditions (metal catalyst + 2) Alkene is an intermediate, but is subsequently also reduced. at Chicago CEM 232, Spring 2010 Slide 18 18
19 Lindlar s Catalyst Lindlar s catalyst (Lindlar Pd) Pd/CaC3 lead (IV) acetate quinoline N quinoline lead (Pb) and quinoline poison the Pd catalyst = reduces reactivity of catalyst alkynes are more reactive than alkenes Lindlar Pd only active enough to reduce alkyne at Chicago CEM 232, Spring 2010 Slide 19 19
20 Metal-Ammonia Reduction also known as dissolving metal reduction requires group I metal (Li, Na, K) provides trans-alkenes solvent = ammonia (N3) at Chicago CEM 232, Spring 2010 Slide 20 20
21 Metal-Ammonia Reduction C C C 3 Na N 3 C 3 C 3 C 3 (Z)-alkenyl radical (less stable) (E)-alkenyl radical (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. at Chicago CEM 232, Spring 2010 Slide 21 21
22 Useful Synthetic Tools for the Stereoselective Preparation of Alkenes Both reactions are stereoselective C C C 3 Na N 3 C 3 metal-ammonia reduction (trans-alkenes) Lindlar Pd, 2 C C C 3 C 3 hydrogenation with Lindlar Pd (cis-alkenes) Neither of these reactions is stereospecific. Why? at Chicago CEM 232, Spring 2010 Slide 22 22
23 Self-Test Question What organic molecules could be used to construct the alkene below? A 3 C metal-n 3 [R] B 3 C Br C C Br 2 alkylations C Ts C C University of Br C 3 C + + C Br Illinois at Chicago CEM 232, Spring 2010 D Slide 23 23
24 Reactions of Alkynes Acid/base reactions Alkylation ydrogenation Metal-Ammonia Reduction Addition of ydrogen alides ydration Addition of alogens zonolysis at Chicago CEM 232, Spring 2010 Functional Group Transformations Alkynes ther FGs Slide 24 24
25 Review: Addition to Alkenes Alkynes undergo addition reactions similar to alkenes 2 S 4 / 2 hydration C C C C X hydrogen halide addition X C C X 2 halogen addition X X C C at Chicago CEM 232, Spring 2010 Slide 25 25
26 Self-Test Question Presuming alkynes react similarly to alkenes, chose the column containing the correct set of products. A B C D Actual! 3 C 2 S 4 / 2 3 C 3 C 3 C 3 C C 3 3 C Br 3 C Br Br 3 C Br 3 C Br Br Br 3 C C 3 3 C 2 3 C 3 C 3 C 3 C University of Illinois at Chicago CEM 232, Spring 2010 Slide 26 26
27 Markovnikov Addition of X to Alkynes Unlikely Mechanism alkenyl cation = sp hybridized = more electronegative = high energy experimental evidence: 3rd order reaction at Chicago CEM 232, Spring 2010 Slide 27 27
28 Markovnikov Addition of X to Alkynes (vinyl halide) Better Mechanism 3rd oder (termolecular): rate=k[alkyne][x] 2 transition state = no δ+ on alkyne carbons at Chicago CEM 232, Spring 2010 Slide 28 28
29 Markovnikov Addition of X to Alkynes When excess X is added, two equivalents are added across the alkyne to provide the geminal dihalide F faster F resonance stabilization F F F major product F F slower F F F at Chicago CEM 232, Spring 2010 Slide 29 29
30 ydration of Alkynes ydration of alkynes proceeds through a two-step process: Markovnikov addition to provide an enol followed by tautormerization Markovnikov addition Tautomerization 3 C 3 C 2 S 4 / 2 ( 3 + ) gs 4 3 C 3 C C 2 3 C 3 C C 3 enol ketone tautomers: rapidly equilibrating constitutional isomers carbonyl/enol: equilibrium lies toward carbonyl at Chicago CEM 232, Spring 2010 Slide 30 30
31 Why is g 2+ Catalyst Needed? g 2+ forms a more stable (low energy) mercuronium intermediate Without g 2+, the alkyne would have to be protonated to give a 2º vinylic carbocation. Unstabilized vinylic carbocations are high energy species and thus unlikely intermediates. g 2+ 3 C 3 C + 2-methyl-3-butyn-2-ol (1) gs 4 2 S 4, 2 no g 2+ = no reaction X 2 S 4, 2 3 C 3 C g 2+ cyclic mercuronium ion intermediate (2) 3 C 3 C 2º vinylic carbocation less stable more stable 3 C 3 C g 1+ 2º vinylic carbocation at Chicago CEM 232, Spring 2010 Slide 31 31
32 Complete Mechanism g 2+ 3 C 3 C g 2+ g 1+ 3 C 3 C 3 C 3 C mercuronium ion 2 3 C 3 C g C C 2 3 C enol 3 C C 3 3 C ketone Note: water adds to the most substituted carbon of mercuronium ion since it is most partially positively charged = nly ketones are possible products for alkyne hydration (except acetylene) at Chicago CEM 232, Spring 2010 Slide 32 32
33 Summary of Reactions of Alkynes Acid/base reactions Alkylation ydrogenation Metal-Ammonia Reduction Addition of ydrogen alides ydration Addition of alogens zonolysis at Chicago CEM 232, Spring 2010 Products C-C bond formation alkane or cis-alkene trans-alkene vinyl halide or vicinal dihalide ketone tetrahaloalkane carboxylic acid Slide 33 33
34 i>icker Question A Predict the product. B 1. 2, C 3 2. NaC 2 C 3, DMS 3. 2 S 4 / 2, gs 4 C C 3 1 D 2 3 E University of Illinois at Chicago CEM 232, Spring 2010 Slide 34 34
35 Alkyne ydration of Terminal Alkynes Alkyne hydration is best for the preparation of methyl ketones from terminal alkynes. ydration of internal alkynes gives mixtures of products. 3 C C 3 2 S 4 / 2, gs 4 alkyne hydration 3 C C C C 3 3 C 2 S 4 / 2, gs 4 alkyne hydration 3 C C 3 at Chicago CEM 232, Spring 2010 Slide 35 35
36 More Synthetic Strategies... You now know two methods for preparing ketones: ozonolysis of alkenes and hydration of terminal alkynes C C 3 g cat. ydration C C C C 3 zonolysis R C R C C 3 at Chicago CEM 232, Spring 2010 Slide 36 36
37 Self-Test Question? Synthesis: Which compounds could be used to prepare the ketone below? int: Perform a retrosynthetic analysis. C 3 A B C 3 C Br C C Br NaC C C C D E C C C C University of Illinois at Chicago CEM 232, Spring 2010 Slide 37 37
38 Self-Test Question Aldehydes can be made from terminal alkynes by forming the least substituted enol instead. This can be accomplished using a method we ve already encountered for the preparation of the least subsituted alcohols from alkenes. What are the conditions? C C? C C A. Br, peroxides B. 2, hv B 2 6 tautomerization C. B26 then Na/22 B 2 C C Na/ 2 2 C C D. NaC2C3 E. Br University of Illinois at Chicago CEM 232, Spring 2010 Slide 38 38
39 CEM 232 rganic Chemistry I at Chicago Next Lecture... After Spring Break! Chapter 9: Sections st Quiz after S.B... Synthesis Problems 39
sp 2 geometry tetrahedral trigonal planar linear ΔH C-C ΔH C-H % s character pk a 464 KJ/mol 33% 44
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