Carbanions Stabilized by Phosphoniums (I)

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Presentation transcript:

Carbanions Stabilized by Phosphoniums (I) Organic Chemistry of Fine Chemicals Spring 2006 Carbanions Stabilized by Phosphoniums (I) Wittig reactions R2C=O + Ph3P=CR1R2 R2C= CR1R2 + Ph3P=O ylides & phosphoranes:  85 bottom (5.23a) mechanism addition followed by elimination:  86 top (5.23b) betaine (42a) vs. oxaphosphetane (42 or 42b) OCFC-Chapter 5-3 Chapter 5-3

Carbanions Stabilized by Phosphoniums (II) Organic Chemistry of Fine Chemicals Spring 2006 Carbanions Stabilized by Phosphoniums (II) Non-stabilized ylides: R1 & R2 = alkyl or H, Ph3P=CR1R2 preparation: strong base required,  86 bottom reactive & irreversible: Z-alkenes preferred:  87 top Stabilized ylides: R1 & R2 = -M group, Ph3P=CR1R2 preparation: weak base,  87 middle stable & reversible: E-alkenes preferred OCFC-Chapter 5-3 Chapter 5-3

Stereoselectivity in the Wittig Reactions Organic Chemistry of Fine Chemicals Spring 2006 Stereoselectivity in the Wittig Reactions Unstabilized ylides: kinetic control; Z-alkenes ‘salt-free’ conditions: Na or K vs Li;  87 bottom & 88 top bulky aldehydes: Z-alkenes ;  88 middle mechanism: concerted vs stepwise The Schlosser modification: E-alkenes carbanion formation of the betaine 45a:  89 bottom equilibration to the more stable threo 45b (5.24):  89 bottom OCFC-Chapter 5-3 Chapter 5-3

Mechanisms of the Wittig Reactions (I) Organic Chemistry of Fine Chemicals Spring 2006 Mechanisms of the Wittig Reactions (I) concerted mechanism: unstabilized ylides;  88 bottom reversible mechanism: stabilized ylides;  89 top OCFC-Chapter 5-3 Chapter 5-3

Carbanions Stabilized by Sulfur Cations Organic Chemistry of Fine Chemicals Spring 2006 Carbanions Stabilized by Sulfur Cations Sulfonium & sulfoxonium ylides preparation:  90 top & middle alternative mechanism for sulfoxonium ylides:  90 bottom epoxides (oxiranes) & episulfides (thiiranes): non-conjugated carbonyls & thiocarbonyls;  90 bottommost reactivity difference: sulfonium vs sulfoxonium ylides kinetic vs thermodynamic products OCFC-Chapter 5-3 Chapter 5-3

Nucleophilic Acylation: Umpolung (I) Organic Chemistry of Fine Chemicals Spring 2006 Nucleophilic Acylation: Umpolung (I) Benzoin condensation:  91 middle & 92 top a-hydroxyketones from aldehydes mechanism:  91 bottom catalytic cyanide/thiazolium salts Stetter reaction: conjugate addition of acyl anions;  92 middle OCFC-Chapter 5-3 Chapter 5-3

Nucleophilic Acylation: Umpolung (II) Organic Chemistry of Fine Chemicals Spring 2006 Nucleophilic Acylation: Umpolung (II) Dithianes:  93 top,  93 bottom & 94 top 1,3,5-trithiane: for aldehydes;  94 middle OCFC-Chapter 5-3 Chapter 5-3

Neutral Nucleophiles: (Hetero)Alkenes (I) Organic Chemistry of Fine Chemicals Spring 2006 Neutral Nucleophiles: (Hetero)Alkenes (I) Alkylation: enamines preparation:  95 middle (hygroscopic!!) reactions only with reactive electrophiles useful with unsymmetrical ketones:  95 bottom however, C- vs. N-alkylation; success with aldehydes dihydro-1,3-oxazine: aldehydes;  96 bottom OCFC-Chapter 5-3 Chapter 5-3

Neutral Nucleophiles: (Hetero)Alkenes (II) Organic Chemistry of Fine Chemicals Spring 2006 Neutral Nucleophiles: (Hetero)Alkenes (II) Acylation: arenes: variants of Friedel-Crafts acylation;  97 top ketones (enols):  97 middle enamines:  97 bottom (5.27) &  98 top formylation Vilsmeier reaction:  98 middle (5.28) & bottom & 99 top Reimer-Tiemann reaction:  99 middle (side-reactions) carboxylation: Kolbe-Schmitt reaction;  99 bottom OCFC-Chapter 5-3 Chapter 5-3

Neutral Nucleophiles: (Hetero)Alkenes (III) Organic Chemistry of Fine Chemicals Spring 2006 Neutral Nucleophiles: (Hetero)Alkenes (III) Condensation: in acids;  100 middle (5.29) & bottom Mannich reaction: addition without elimination addition to CH2=N+Me2X-:  101 top (5.30) reactions to give Mannich bases:  101 middle vinyl ketones: Hofmann elimination of the Mannich base b-elimination:  101 (5.31) or (5.32) in situ formation of unstable vinyl ketones:  102 top thermal Michael reaction:  102 middle mechanism:  102 bottom - 103 top OCFC-Chapter 5-3 Chapter 5-3

Organic Chemistry of Fine Chemicals Spring 2006 Review Retrosynthesis:  103 bottom (5.34) & (5.35) Strategy of disconnection:  107 middle target compounds with only C-C single bonds target compounds with only C-C single bonds & two functional groups close together target compounds with a C=C double bond OCFC-Chapter 5-3 Chapter 5-3

Retrosynthetic Analysis: Chapter 5 Organic Chemistry of Fine Chemicals Spring 2006 Retrosynthetic Analysis: Chapter 5 OCFC-Chapter 5-3 Chapter 5-3

Organic Chemistry of Fine Chemicals Spring 2006 Disconnection Points g a b OCFC-Chapter 5-3 Chapter 5-3

Target Compounds: Chapter 5 Organic Chemistry of Fine Chemicals Spring 2006 Target Compounds: Chapter 5 3-Methyloctan-2-one 4-Oxo-4-phenylbutanenitrile Methyl 4-methylpen-2-enoate OCFC-Chapter 5-3 Chapter 5-3

Retrosynthesis (I): 3-Methyloctan-2-one Organic Chemistry of Fine Chemicals Spring 2006 Retrosynthesis (I): 3-Methyloctan-2-one OCFC-Chapter 5-3 Chapter 5-3

Retrosynthesis (II): 4-Oxo-4-phenylbutanenitrile Organic Chemistry of Fine Chemicals Spring 2006 Retrosynthesis (II): 4-Oxo-4-phenylbutanenitrile OCFC-Chapter 5-3 Chapter 5-3

Retrosynthesis (III): Methyl 4-methylpen-2-enoate Organic Chemistry of Fine Chemicals Spring 2006 Retrosynthesis (III): Methyl 4-methylpen-2-enoate OCFC-Chapter 5-3 Chapter 5-3