Gas-Phase Base-Induced 1,4-Eliminations: Occurrence of Single-, Double-, and Triple-Well E1cb Mechanisms

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_N2) and allylic (Sn2’) substitution. Overall bimolecular rate constants for baseinduced reactions of 1 range from 6 x 10^-10 (F^- + 1) to 66 x 10^-10 (OH^- + 1) cm³ 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 β,γ-double bond of the substrate has no detectable influence on the course of base-induced 1,4-eliminations. Deuterium labeling experiments with 2 reveal 6-proton transfer only. The absence of product ions from a-proton transfer is ascribed to a facile electron detachment from the α-allyl anions. The base-induced 1,4-eliminations studied proceed via an Elcb mechanism, as indicated by experiment and shown by theory. This mechanism exists in various modifications amongst which are single-, double-, and triple-well Elcb elimination. To our knowledge, the single-well Elcb mechanism is conceptually unprecedented.