Wittig Olefination, Background: • in the Formation of Z-Alkenes, an Early, Four-Centered Transition State Is Proposed
Myers Stereoselective Olefination Reactions: The Wittig Reaction Chem 115 Reviews: Vedejs, E.; Peterson, M. J. In Topics in Stereochemistry; Eliel, E. L. and Wilen, S. H. Ed.; John • Phosphonium ylides react with aldehydes to produce oxaphosphetane 1Z or 1E, which Wiley & Sons: New York, 1994, Vol. 21, pp. 1–158. decomposes by a syn-cycloreversion process to the alkene. Maryanoff, B. E.; Reitz, A. B. Chem. Rev. 1989, 89, 863-927. Wittig Olefination, Background: • In the formation of Z-alkenes, an early, four-centered transition state is proposed. TSZ is believed to be kinetically favored over TS because it minimizes 1,2 interactions between R and R in the • Olefin synthesis employing phosphonium ylides was introduced in 1953 by Wittig and Geissler: E 1 2 forming C–C bond. • The reaction of non-stabilized phosphonium ylides with aldehydes favors (Z)-alkene products. O PhLi CH2 Ph3P CH3 Ph Ph Ph Ph Br Et2O, 84% Wittig, G.; Geissler G. Liebigs Ann. 1953, 580, 44-57. Non-stabilized Ylides: Ar3P R = simple alkyl R • Terminology introduced by Professor E. J. Corey in Chem 115 to help students conduct retrosynthetic analysis of trisubstituted olefins: T-branch RT L-branch H (trans) RL (lone) Rc O – NaHMDS C-branch N Cl N CCl3 (cis) O + O Ph P THF, –40 ºC 3 CCl 3 59% Mechanism: H O CCl3 Ar Ar P O Ar H 3 P H H Ar O R H 2 R1 R2 R1 R2 R1 1Z (Z)-alkene Ar3P TSZ R1 + Karatholuvhu, M. S.; Sinclair, A.; Newton, A. F.; Alcaraz, M.-L.; Stockman, R.
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