GAS-PHASE BASE-INDUCED 1,4-ELIMINATIONS - OCCURRENCE OF SINGLE-, DOUBLE-, AND TRIPLE-WELL E1CB MECHANISMS [Review]

摘要

The reactions of the allylic ethers CH3-CH=CH-CH2-OEt (1), CH3-CH=CH-CH2-OMe (2), and CH2=CH-CH2-OEt (3) with a variety of anionic first-row (carbon, nitrogen, oxygen, and fluorine) bases have been investigated with use of FT-ICR mass spectrometry and density-functional theory (DFT). Base-induced 1,4-elimination is an extremely facile process which competes effectively with simple proton transfer 1,2-elimination, and vinylic 1,2-elimination as well as aliphatic (S(N)2) and allylic (S(N)2') substitution. Overall bimolecular rate constants for base-induced reactions of 1 range from 6 x 10(-10) (F- + 1) to 66 x 10(-10) (OH- + 1) cm(3) molecule(-1) s(-1). Oxygen bases are the most reactive amongst the employed bases. The ionic products of base-induced 1,4-elimination are either the bare leaving group, RO(-), or the leaving group solvated by the conjugate acid of the base, [BH, RO(-)]. The former reaction channel prevails for strong bases (e.g., NH2-). The latter pathway becomes dominant for weaker bases (e.g., F-), because the complexation energy compensates for the reduced exothermicity. This makes the reaction an efficient tool for the preparation of solvated anions under low-pressure conditions. The stereochemistry (i.e., E or Z) around the beta,gamma-double bond of the substrate has no detectable influence on the course of base-induced 1,4-eliminations. Deuterium labeling experiments with 2 reveal delta-proton transfer only. The absence of product ions from alpha-proton transfer is ascribed to a facile electron detachment from the alpha-allyl anions. The base-induced 1,4-eliminations studied proceed via an E1cb mechanism, as indicated by experiment and shown by theory. This mechanism exists in various modifications amongst which are single-, double-, and triple-well E1cb elimination. To our knowledge, the single-well E1cb mechanism is conceptually unprecedented. [References: 127]