310_7 - Organic Lecture Series CH 310 N LECTURE 7 Textbook...

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Organic Lecture Series 1 CH 310 N LECTURE 7 Textbook Assignment: Chapter 16 Aldehydes & Ketones Homework (for credit): Problem set 2 posted Today’s Topics: Wittig & related Rxns; Hemi- & Acetal Rxns Notice & Announcements: EXAM 1: Monday, 9/19; 7-9 PM A-L:WEL 1.308 M-Z: WEL 1.316 Organic Lecture Series 2 Aldehydes Aldehydes And And Ketones Ketones
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Organic Lecture Series 3 O Ketone Can Be Cyclic C Alkenes or Olefins R' R Overall Synthetic Transformation of Wittig Reagents & Its Variations Organic Lecture Series 4 The Wittig reaction is a very versatile synthetic method for the synthesis of alkenes (olefins) from aldehydes and ketones Triphenyl- phosphine oxide Methylene- cyclohexane A phosphonium ylide + + - + CH 2 Ph 3 P= O 3 P- 2 Cyclohexanone O Wittig Reaction Wittig Reaction Ph 3 PC H 2 Ph 3 P CH 2 Ylides are reagents (or reactive intermediates) which have adjacent charges:
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Organic Lecture Series 5 Phosphonium ylides react with the C=O group of an aldehyde or ketone to give an alkene Step 1: nucleophilic addition of the ylide to the electrophilic carbonyl carbon Step 2: decomposition of the oxaphosphatane CR 2 O CH 2 Ph 3 P 2 - :O 2 3 P OC R 2 3 PC H 2 - + An oxaphosphetane + A betaine 2 R 2 3 P 3 P= O R 2 C=CH 2 An alkene + Triphenylphosphine oxide Wittig Reaction Wittig Reaction Organic Lecture Series 6 O 3 P + 3 P=O 2-Methyl-2-heptene + Acetone O H 3 PP h P h 3 Phenyl- acetaldehyde ++ + ( Z )-1-Phenyl-2- butene (87%) ( E )-1-Phenyl-2- (13%) O H OEt 3 P O O 3 Eth yl ( E )-4-phenyl-2-butenoate (only the E isomer is formed) + + Wittig Reaction Resonance stabilized Wittig reagent:
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Organic Lecture Series 7 – some Wittig reactions are Z selective, others are E selective – Wittig reagents with an anion-stabilizing group, such as a carbonyl group, adjacent to the negative charge are generally E selective – Wittig reagents without an anion-stabilizing group are generally Z selective OEt Ph 3 P O 3 P O Resonance contributing structures for an ylide stabilized by an adjacent carbonyl group Wittig Reaction Wittig Reaction Organic Lecture Series 8 Resonance Stabilization: Resonance Stabilization:
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Organic Lecture Series 9 1. treatment of a phosphonoester with a strong base followed by an aldehyde or ketone gives an alkene 2. a particular value of using a phosphonoester-stabilized anion is that
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This note was uploaded on 11/21/2011 for the course CH 51455 taught by Professor Colapret during the Spring '11 term at University of Texas at Austin.

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310_7 - Organic Lecture Series CH 310 N LECTURE 7 Textbook...

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