olefin - C=C BOND FORMATION 103 C=C Bond Formation C&S...

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Unformatted text preview: C=C BOND FORMATION 103 C=C Bond Formation C&S Chapt. 2 # 5,6,8,9,12 1. Aldol Condensation 2. Wittig Reaction (Smith, Ch. 8.8.A) 3. Peterson Olefination 4. Julia-Lythgoe Olefination 5. Carbonyl Coupling Reactions (McMurry Reaction) (Smith Ch. 13.7.F) 6. Tebbe Reagent 7. Shapiro and Related Reaction 8. β− Elimination and Dehydration 9. From Diols and Epoxides 10 From Acetylenes 11. From Other Alkenes-Transition Metal Catalyzed Cross-Coupling and Olefin Metathesis Aldol Condensation-Aldol condensation initially give β-hydroxy ketones which under certain conditions readily eliminated to give α , β-unsaturated carbonyls. OMe OR OMe O CHO O O LDA, THF, -78°C-78C → RT OMe OR OMe O O O Tetrahedron 198 4 , 40 , 4741 Robinson Annulation : Sequential Michael addition/aldol condensation between a ketone enolate and an alkyl vinyl ketone (i.e. MVK) to give a cyclohex-2-en-1-one JOC 1984 , 49 , 3685 Synthesis 1976 , 777. O O O L-proline O O Weiland-Mischeler Ketone (chiral starting material) Mannich Reaction- α , β-unsaturated carbonyls ( α-methylene carbonyls) O H 2 C N Me Me + Cl-Mannich Reaction H 2 C=O, Me 2 NH•HCl O C=C BOND FORMATION 104 Wittig Reaction review: Chem. Rev. 1989 , 89 , 863. mechanism and stereochemistry: Topic in Stereochemistry 1994 , 21 , 1- reaction of phosphonium ylide with aldehydes, ketones and lactols to give olefins R PPh 3 strong base + R PPh 3 + _ X -R PPh 3 ylide phosphorane R X Ph 3 P R 1 R 2 O R R 2 R 1 R R 2 R 1 O -Ph 3 P R R 2 R 1 O Ph 3 P + betaine oxaphosphetane- Ph 3 P=O- Olefin Geometry S L S O L Ph 3 P L L S O S Ph 3 P Z- olefin E- Olefin- With "non-stabilized" ylides the Wittig Reaction gives predominantly Z-olefins. Seebach et al JACS L S O L S PPh 3 + S PPh 3 L S L O +-O L S PPh 3 L S + S L S O L Ph 3 P L PPh 3 S O S L- Ph 3 P=O L L S S Z-olefins- "Stabilized ylides" give predominantly E-olefins Ph 3 P CO 2 Me + L S O L CO 2 Me S _ C=C BOND FORMATION 105- Betaine formation is reversible and the reaction becomes under thermodynamic control to give the most stable product.- There is NO evidence for a betaine intermediate.- Vedejs Model: P O Ph Ph Ph R H H R Cis O P Ph Ph Ph H H R R Trans Pukered (cis) oxaphosphetane (kinetically favored) Planar (trans) oxaphosphetane (thermodynamically favored) Phosphonate Modification (Horner-Wadsworth-Emmons) R X (EtO) 3 P R P(OEt) 2 O Arbazov Reaction R P(OEt) 2 O _- R is usually restricted to EWG such as CO 2 H, CO 2 Me, CN, SO 2 Ph etc. and the olefin geometry is usually E.- Still Modification TL 1983 , 24 , 4405. CO 2 Me (CF 3 CH 2 O) 2 P O- CF 3 CH 2 O- groups make the betaine less stable, giving more Z-olefin. Me 3 Si CHO CO 2 Me (CF 3 CH 2 O) 2 P O (Me 3 Si) 2 N- K + , THF, 18-C-6 Me 3 Si CO 2 Me Z:E = 25:1 JACS 1988 , 110 , 2248 Peterson Olefination review: Synthesis 1984 , 384 Organic Reactions 1990 , 38 1....
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olefin - C=C BOND FORMATION 103 C=C Bond Formation C&S...

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