SN2’ versus SN2 Reactivity: Control of Regioselectivity in Conversions of Baylis–Hillman Adducts |
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Authors: | Mahiuddin Baidya Grygoriy Y. Remennikov Dr. Peter Mayer Dr. Herbert Mayr Prof. Dr. |
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Affiliation: | Department Chemie und Biochemie, Ludwig‐Maximilians‐Universit?t München, Butenandtstrasse 5–13, 81377 München (Germany), Fax: (+49)?89‐2180‐77717 |
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Abstract: | TiCl4‐induced Baylis–Hillman reactions of α,β‐unsaturated carbonyl compounds with aldehydes yield the (Z)‐2‐(chloromethyl)vinyl carbonyl compounds 5 , which react with 1,4‐diazabicyclo[2.2.2]octane (DABCO), quinuclidine, and pyridines to give the allylammonium ions 6 . Their combination with less than one equivalent of the potassium salts of stabilized carbanions (e.g. malonate) yields methylene derivatives 8 under kinetically controlled conditions (SN2’ reactions). When more than one equivalent of the carbanions is used, a second SN2’ reaction converts 8 into their thermodynamically more stable allyl isomers 9 . The second‐order rate constants for the reactions of 6 with carbanions have been determined photometrically in DMSO. With these rate constants and the previously reported nucleophile‐specific parameters N and s for the stabilized carbanions, the correlation log k (20 °C)=s(N + E) allowed us to calculate the electrophilicity parameters E for the allylammonium ions 6 (?19<E 18). The kinetic data indicate the SN2’ reactions to proceed via an addition–elimination mechanism with a rate‐determining addition step. |
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Keywords: | allylic substitution kinetic control linear free‐energy relationships regioselectivity thermodynamics |
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