310-13 - Organic Lecture Series CH 310 N LECTURE 13...

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Organic Lecture Series 1 CH 310 N LECTURE 13 Textbook Assignment: Chap 16 CONT’D Homework (for credit): POW 4 posted (Due Feb 21) Today’s Topics: Wittig reaction; hydrates & hemiacetals Notice & Announcements: Organic Lecture Series 2 O Ketone Can Be Cyclic C Alkenes or Olefins R' R Overall Synthetic Transformation of Wittig Reagents & Its Variations
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Organic Lecture Series 3 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: Organic Lecture Series 4 Phosphonium ylides are formed in two steps: Step 1: nucleophilic displacement of iodine by triphenylphosphine Step 2: treatment of the phosphonium salt with a very strong base, most commonly BuLi, NaH, or NaNH 2 3 H 3 -I 3 P-CH 3 I Triphenylphosphine + + S N 2 Methyltriphenylphosphonium iodide (an alkyltriphenylphosphine salt) 3 2 2 2 - Li + H-CH 2 -PPh 3 I - - 2 3 3 2 2 3 LiI Butane Butyllithium + + + + + Phosphonium ylides ylides Not exam material
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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 2-Methyl-2-heptene + Acetone O H 3 PP h P h 3 Phenyl-
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This note was uploaded on 04/24/2011 for the course CH 56255 taught by Professor Colapret during the Spring '11 term at University of Texas at Austin.

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310-13 - Organic Lecture Series CH 310 N LECTURE 13...

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