lec_29_30_31_32 - Massachusetts Institute of Technology...

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Unformatted text preview: Massachusetts Institute of Technology 5.13: Organic Chemistry II Outline & Study Guide for Unit VII. Carbocations “Every generation of scientific men (i.e. scientists) starts where the previous generation left off; and the most advanced discoveries of one age constitute elementary axioms of the next.” –Aldous Huxley Carbocationic species are intermediates in many chemical reactions carried out in the laboratory, as well as numerous reactions that take place in nature. Carbocations are so important, in fact, that Professor George A. Olah of The University of Southern California was awarded the 1994 Nobel Prize in Chemistry for “his contribution to CH3 carbocation chemistry.” H3C One famous example of a carbocationic cascade reaction CH3 is the biosynthesis of cholesterol from squalene oxide. Conversion of the linear polyene to the four-ring steroid nucleus happens through a series of carbocation rearrangements. Elucidation of this extraordinary biosynthetic pathway, one of the most complex known, was accomplished by Konrad Bloch, Feodor Lynen, John Cornforth, and George Popjak in the late 1950s. Interestingly, Bloch and Lynen were awarded the 1994 Nobel Prize in Physiology or Medicine for their work. In the course of the next few lectures, you will learn how you can harness the power of the carbocation in a number of useful synthetic transformations. Steroid Biosynthesis: The Carbocation's Wild Ride CH3 H3C O H A CH3 CH3 CH3 HO CH 3 CH3 C H3 CH3 CH3 C H3 CH 3 CH3 Fall 2006 Squalene Oxide CH3 CH3 CH3 CH3 HO H3C CH3 CH3 CH3 CH3 C H3 HO H3 C CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 HO H3 C CH3 CH3 CH3 CH3 CH3 C H3 HO H3C CH3 CH3 CH 3 CH3 CH3 H CH3 CH3 HO H3C CH3 H CH3 CH3 CH3 CH2 CH3 H C H3 HO H H3 C CH3 CH3 CH 3 H3C CH3 Cholesterol Protosterol VIII. Carbocations A. Introduction 1. Structure 2. Stabilization a. Alkyl groups b. Hybridization c. Aromaticity d. Resonance i. p bond ii. a-heteroatom B. Generation of Carbocations 1. Ionization of C-X 2. Lone pair bonds with Lewis acid 3. Addition of E+ to p bond C. Reactions of Carbocations 1. Elimination 2. Combination with Nucleophile a. n b. aromatic ring c. p bond 3. Rearrangements and Fragmentations a. 1,2-Migration i. How ii. Why b. Solvent effects c. Reaction Examples i. Dienone-Phenol Rearrangement ii. Epoxides to Aldehydes iii. Pinacol Rearrangement iv. Tiffeneau–Demjanov Rearrangement v. Baeyer–Villiger Oxidation vi. Beckmann Rearrangement 4. Neighboring Group Participation (Anchimeric Assistance) ...
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