Cutest Benzene PDF

Cutest Benzene PDF - Benzyne The Adventures of a Reactive...

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Pamela Tadross Stoltz Group Literature Presentation Noyes 147 8 PM, May 15, 2006 Benzyne The Adventures of a Reactive Intermediate
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Cl NaNH 2 NH 3 (l) NH 2 NH 2 + Nearly 1:1 ratio! History and Structure Nu - Nu E E + Polar Reactions Pericyclic Reactions O + O N Ph N N Methods of Generation + N 2 - O 2 C TMS OTf Cl Overview Applications to Natural Product Synthesis PhN 3
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Cl NaNH 2 NH 3 (l), -38 °C NH 2 OCH 3 Cl OCH 3 NEt 2 Cl LiNEt 2 NEt 2 NEt 2 + Unusual Rearrangements of Substituted Benzenes Gilman, Wittig, and Roberts observed strange behavior in reactions of halobenzenes in the 1940s and 1950s F C 6 H 5 Li Li G. Wittig, Naturwissenschafter 1942 , 30 , 696. LiNEt 2 J. D. Roberts, H. E. Simmons Jr., L. A. Carlsmith, C. W. Vaughan, J. Am. Chem. Soc. 1953 , 75 , 3290. H. Gilman, S. Avakian, J. Am. Chem. Soc. 1945 , 67 , 349. F. W. Bergstrom, C. H. Horning, J. Org. Chem. 1946 , 11 , 334. NH 3 (l), -38 °C NH 3 (l), -38 °C THF, -78 °C
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Wittig's Explanation Close but no cigar. .. F C 6 H 5 Li F Li zwitterionic intermediate C 6 H 5 Li Li G. Wittig, Naturwissenschafter 1942 , 30 , 696. - LiF THF, -78 °C
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John D. Roberts: The Right Place at the Right Time Cl LiNEt 2 NEt 2 NEt 2 + NH 3 (l), -38 °C Reactions are very rapid even at -38 °C No reaction occurs when there is no hydrogen adjacent to the leaving halogen The entering amino group is never more than one carbon away from the position occupied by the leaving halogen no p-aminotoluene formed The starting halides and the resulting anilines are not isomerized under reaction conditions Br LiNEt 2 NH 3 (l), -38 °C No Reaction J. D. Roberts, H. E. Simmons Jr., L. A. Carlsmith, C. W. Vaughan, J. Am. Chem. Soc. 1953 , 75 , 3290. Cl NH 3 (l), -38 °C Cl NEt 2 NH 3 (l), -38 °C NEt 2
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Further Mechanistic Evidence. .. NH 2 NH 2 NH 2 Cl Cl Cl % % % 45 40 none 55 none 52 62 8 38 substrate product J. D. Roberts, C. W. Vaughan, L. A. Carlsmith, D. A. Semenow, J. Am. Chem. Soc . 1956 , 78 , 611-14.
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The Classic 14 C Labeling Experiment Cl NaNH 2 NH 3 (l) NH 2 The Hypothesis: If one started with chlorobenzene-1- 14 C, equal amounts of aminobenzenes with the 14 C at C-1 and C-2 would be formed. NH 2 + Nearly 1:1 ratio! Roberts' Proposed Intermediate: BENZYNE! "These facts as well as the orientation data for various substituents can be accomodated by an elimination-addition mechanism involving at least transitory existence of an electrically neutral "benzyne" intermediate." J. D. Roberts, H. E. Simmons Jr., L. A. Carlsmith, C. W. Vaughan, J. Am. Chem. Soc. 1953 , 75 , 3290.
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Further Support from Wittig and Huisgen O + O Wittig finds that benzyne can participate in Diels-Alder reactions as a dienophile: Huisgen reports that arynes generated from different precursors have identical reactivity: F Li F MgBr N 2 + CO 2 - S O 2 N N O O k 1 k 2 k 1 /k 2 = constant G. Wittig, L. Pohmer, Angew. Chem. 1955 , 67(13) , 348. R. Huisgen, R. Knorr, Tetrahedron Lett. 1963 , 1017.
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Cl NaNH 2 NH 3 (l) NH 2 NH 2 + Nearly 1:1 ratio! History and Structure Nu - Nu E E + Polar Reactions Pericyclic Reactions O + O N Ph N N Methods of Generation + N 2 - O 2 C TMS OTf Cl Overview Applications to Natural Product Synthesis PhN 3
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Generation of Benzyne Earlier basic and harsh methods give way to recent neutral and mild methods of aryne generation F Li F MgBr N 2 + CO 2 - S O 2 N N X X 1 X 2 N N N NH 2 TMS
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Cutest Benzene PDF - Benzyne The Adventures of a Reactive...

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