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0Chem 360-Jasperse Chapter 22 (Enolate Chemistry) Reaction Summary

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0Chem 360-Jasperse Chapter 22 (Enolate Chemistry) Reaction Summary Chem 360 Jasperse Ch. 22 Notes. Enolate Chemistry 1 0Chem 360-Jasperse Chapter 22 (Enolate Chemistry) Reaction Summary PROTON as ELECTROPHILE O OH base, ROH 1. Ph Ph -Base-catalyzed keto-enol equilibrium -know mech (either direction) -know impact of substituents on enol concentration O O O base, ROH 2. Ph * Ph + Ph H H CH CH3 CH3 3 H optically active racemic -Racemization of α-chiral optically active carbonyls -Mech HALOGEN as ELECTROPHILE O O excess Br (Cl ) 3. 2 2 Ph base Ph Br Br -Base catalyzed halogenation -with excess halogen, all α-hydrogens get replaced -Mech O O O 1. 3 I2, 3 NaOH, H2O . 4 + CHI3 + CHI3 Ph 2. H+ Ph ONa Ph OH after basic step one after step two acidification -Iodoform reaction. -chemical test for methyl ketones Chem 360 Jasperse Ch. 22 Notes. Enolate Chemistry 2 ALKYL HALIDE as ELECTROPHILE O O 1. LDA 6. R Z 2. R-X Z -Enolate alkylation -strong LDA base required to completely deprotonate carbonyl -Mech -Ketones, Esters, Amides, Aldehydes: doesn’t matter which kind of carbonyl -unsymmetrical ketones give isomer problems -SN2 alkylation restricts R-X to active ones O O O O 1. NaOR + O H3O , heat 7. R OR 2. R-X OR R -Enolate alkylation of 1,3-ketoester -alkoxide base strong enough to completely generate enolate -Mech for alkylation -SN2 alkylation restricts R-X -position of alkylation is unambiguous -acid-catalyzed hydrolysis/decarboxylation O O O O 1. NaOR + O H3O , heat 8. 2. R-X R RO OR RO OR HO R -Enolate alkylation of 1,3-diester -alkoxide base strong enough to completely generate enolate -Mech for alkylation -SN2 alkylation restricts R-X -acid catalyzed hydrolysis/decarboxylation -Final product is an ACID (Diester Acid) O O O H H + O O O H3O , heat 9. -CO2 Z OH Z -CO2 Z O Z R R R R -decarboxylation of a 1,3-carbonyl acid -”Z” can be anything so that you end with a ketone, aldehyde, or acid at the end -know the mechanism for the decarboxylation, and acid-catalyzed enol to carbonyl isomerization -rate will be impacted by stability of the enol intermediate Chem 360 Jasperse Ch. 22 Notes. Enolate Chemistry 3 ALDEHYDE/KETONE as ELECTROPHILE O OH O base 10. R R H ROH H R -Aldol Reaction -Mech O Z OH O base, ROH Z O base heat 11. R R R Z H (or acid) Z R R -Aldol Condensation -Ketones as well as Aldehydes can be used -In ketone case, unfavorable aldol equilibrium is still drawn off to enone -In Aldehyde case, can stop at aldol if you don’t heat -Mech Z OH O base, ROH Z O heat 12. R R H (or acid) Z R R -Aldol dehydration -Mech under basic conditions O OH O O base, ROH O base heat 13. + R R' H R' Z Z (or acid) R' Z R R -Crossed Aldol (2 different carbonyls) -Many variations, but there must be some differentiation so that one acts selectively as the enolate and the other as the electrophile -Mech O O O O base, ROH base heat 14. H (or acid) HO -Intramolecular aldol -Mech -many variations -Normally only good for 5, 6-membered rings Chem 360 Jasperse Ch. 22 Notes. Enolate Chemistry 4 ESTER as ELECTROPHILE O O O base 15. R R OR ROH OR R -Claisen Reaction -Mech -Produces 1,3-ketoester O O O O base 16. + R R' OR Z ROH R' Z ketone or ester R -Crossed Claisen -May include cyclic Claisen reactions -If the “enolate” carbonyl is a ketone, get a 1,3-diketone -If the “enolate” carbonyl is an ester, get a 1,3-ketoester -Mech WITTIG REACTION O X A X 19. + PPh3 A B Y B Y -Mech Br PPh3+ 1. Ph3P 20. - R R1 2. BuLi (or some R R other base) 1 -Mech Chem 360 Jasperse Ch. 22 Notes. Enolate Chemistry 5 Chem 360-Jasperse Chapter 22 (Enolate Chemistry) Reaction Mechanisms Summary • Note: in many of these reactions, I simply write in “base”. But for specific reactions, you need to recognize and specify the actual base that does the work. PROTON as ELECTROPHILE Ketone to Enol H-OR O base, ROH O OH 1. H Ph Ph Ph O Ph Enol Back to Ketone: H O H-OR O O base, ROH 1.-reverse H Ph Ph Ph O Ph Deprotonation/Reprotonation to Racemize an optically active α-chiral center O O O base, ROH 2. * H-OR Ph Ph Ph H CH3 CH3 CH3 H optically active racemic enantiomer or original. Protonation from the front gives enantiomer, from the back gives the original. The result is a 50/50 racemic mixture. HALOGEN as ELECTROPHILE Base catalyzed halogenation. Sequential deprotonation/halogenation until all the α-hydrogens are replaced. • Note: addition of an electronegative, electron-withdrawing halogen stabilizes subsequent anion formation. As a result, the bromoketone formed after the first substitution is actually more acidic and therefore more reactive than the original ketone. For this reason you can’t just stop with a single halogenation under base conditions. (But you can under acid conditions, via an enol rather than enolate mechanism.) O O O O Br-Br base Br-Br O base 3. Ph Ph Ph Ph Ph Br Br H H excess H Br Br2 H Br Chem 360 Jasperse Ch. 22 Notes. Enolate Chemistry 6 ALKYL HALIDE as ELECTROPHILE With Strong LDA as Base, using a Monocarbonyl O N(iPr)2 O R-Br O 6. H R Z Z Z 1. Z can be anything: works for ketones, esters, aldehydes, esters,… 2. “LDA” is lithium diisopropylamine, provides the nitrogen anion shown 3. strong LDA base required to completely deprotonate carbonyl. The base strength enables the enolate to form completely, no equilibrium or reversibility issues. 4. unsymmetrical ketones give isomer problems. If there are α-hydrogens on both left and right side of ketone, which will get deprotonated selectively? 5. SN2 alkylation restricts R-X to active ones (ideally primary or allylic/benzylic…) 6. Sequencing: the LDA must be added first, allowing the enolate to form completely; then the alkyl halide is added subsequently. If you add the halide at the beginning, it reacts with LDA 7. LDA deprotonates the carbonyl rather than adding to the carbonyl carbon for steric reasons Using 1,3-Dicarbonyls, Such that Weaker Oxygen Bases are Strong Enough Strong LDA as Base, using a Monocarbonyl O O O O O O R-Br OR 7. R OR R OR R OR H H H H R Test Responsible + H , H2O OH + O O O O H , H2O O heat heat R OR R OH R R several R enol R ketone R R steps keto-ester keto-acid ester hydrolysis decarboxylation acid catalyzed enol hydrolysis (ch. 18) Not Test Responsible -alkoxide base strong enough to completely generate enolate -SN2 alkylation restricts R-X -acid-catalyzed hydrolysis/decarboxylation -not test responsible for the acid/catalyzed ester hydrolysis or the keto-acid decarboxylation mechanisms -you are responsible for the acid-catalysis enol hydrolysis (not detailed here, but was in Ch. 18) Chem 360 Jasperse Ch. 22 Notes. Enolate Chemistry 7 O O O O O O R-Br OR 8. RO OR RO OR RO OR H H H H R Test Responsible + H , H2O OH + O O O O H , H2O O heat heat RO OR HO OH HO HO several R enol R R R steps acid ester-ester acid-acid ester hydrolysis decarboxylation acid catalyzed enol hydrolysis (ch. 18) Not Test Responsible -alkoxide base strong enough to completely generate enolate -SN2 alkylation restricts R-X -acid-catalyzed hydrolysis/decarboxylation -not test responsible for the acid/catalyzed ester hydrolysis or the keto-acid decarboxylation mechanisms -you are responsible for the acid-catalysis enol hydrolysis (not detailed here, but was in Ch. 18) H H + O O bond O O heat O H , H2O O rotation -CO2 9. Z OH Z O Z Z slow (ch. 18 R R enol R mech) ketone R step or acid decarboxylation Not Fully Test Responsible. But must know that ENOL is key intermediate that forms in the slow step. What is good for the enol (and it's alkene) accelerates the decarboxylation -decarboxylation of a 1,3-carbonyl acid -”Z” can be anything so that you end with a ketone, aldehyde, or acid at the end -rate will be impacted by stability of the enol intermediate (more highly substituted enol alkene is better; conjugated enol alkene will form faster….) -since the mechanism depends on the conversion of the left carbonyl into an enol, decarboxylations are limited to 1,3-carbonyl acids. If you have a 1,2-carbonyl acid or a 1,4- carbonyl acid (etc), the formation of an enol will not be possible and the decarboxylation will not occur Chem 360 Jasperse Ch. 22 Notes. Enolate Chemistry 8 ALDEHYDE/KETONE as ELECTROPHILE Simple Aldol Reaction, giving a β-hydroxy-carbonyl. In which the same carbonyl functions as both enolate precursor and electrophile. O O O O O OH O R H-OR 10. H OR R H H H R H H H R H R R R -Deprotonate-react-protonate -Notice in this case that it’s the same carbonyl that functions as both the enolate precursor but also as the electrophile. Aldol Condensation, giving an enone. In which the initial aldol product undergoes dehydration. O base Z OH O Z OH O Z O OR 11. R R R R Z Z Z Z R H Aldol R R formation (see #10) -The aldol product is formed as shown in mechanism 10.
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