Base‐induced 1,4‐elimination:Insights from theory and mass spectrometry*

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Abstract
I. Introduction 347 II. Gas‐Phase Ion–Molecule Reactions 348 III. Scope and Applications of Gas‐Phase Base‐Induced 1,4‐Elimination 349 A. Anionic and Cationic 1,4‐Elimination 349 B. Anionic 1,4‐Elimination in an FT‐ICR Cell 350 C. Anionic 1,4‐Elimination in an FA Flow Tube 351 IV. Stereochemistry of 1,4‐Elimination 352 A. Syn Versus Anti Stereochemistry 352 B. Effect of E Versus Z Stereochemistry of the Substrate 353 V. Mechanism of 1,4‐Elimination: E1cb Versus E2 353 A. Base‐Induced 1,4‐Elimination of 1‐Alkoxy‐2‐Butenes 353 B. Base‐Induced 1,4‐Elimination of 3‐Alkoxycyclohexenes 354 C. New DFT Computations on Base‐Induced 1,4‐Eliminations 354 VI. Conclusions and Outlook 358 Acknowledgment 359 References 359 Experimental and theoretical studies on gas‐phase base‐induced 1,4‐elimination reactions are summarized and discussed. The emphasis is on the synergy that is achieved by combining the complementary data from mass spectrometry and theoretical chemistry. The scope and applications of 1,4‐eliminations are discussed and compared with other elementary reactions; e.g., 1,2‐elimination and aliphatic (SN2) and allylic (SN2′) nucleophilic substitution. Furthermore, the syn versus anti stereochemistry of 1,4‐elimination reactions and the effect of E versus Z stereochemistry of the substrate are examined. Particular attention is paid to the mechanistic nature of 1,4‐elimination, i.e., E2 or E1cb, as well as special features such as the single‐well E2 and E1cb mechanism. Also, new results from density functional theory computations (BP86/TZ2P) are presented. © 2002 Wiley Periodicals, Inc., Mass Spec Rev 20:347–361, 2001; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mas.10007

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