Study of ion/molecule reactions of ion derived from tert-butyl methyl ether and trimethylsilyl methyl ether

Abstract

The ion/molecule reactions of the ions (CH 3) 2C +–OCH 3 ( 1a) and (CH 3) 2Si +–OCH 3 ( 2a), which are the main fragment ions in the electron ionization mass spectrum of tert-butyl methyl ether (MTBE, 1) and trimethylsilyl methyl ether (MTSE, 2), respectively, and of the protonated MTBE ( 1H +) and protonated MTSE ( 2H +), generated by chemical ionization, with MTBE or MTSE, with acetone and with ammonia have been studied by Fourier transform ion cyclotron resonance (FTICR) spectrometry. The carbenium ion 1a is practically unreactive toward these substrates whereas the silicenium ion 2a efficiently forms adduct ions with all three reactants. Clearly 1a is a very stable tertiary carbenium ion due to the resonance stabilization of the methoxy group so that it lacks significant electrophilic character, whereas 2a does not gain much stabilization by this substituent. This interpretation is supported by the results of ab initio calculations at the restricted Hartree-Fock/6-31G( d) level of the structures and stabilities of these ions, of the tert-butyl cation and of the trimethylsilicenium ion and of their adducts with H 2O, (CH 3) 2O, and NH 3. The reactions of the protonated silyl ether 2H + are straightforward and correspond to transfer of the trimethylsilyl group to the reactants, as expected for a silylation reagent. Silyl group transfer competes effectively with proton transfer even in the case of NH 3 as reactant. The typical reaction of the protonated tert-butyl ether 1H + with the three substrates is an efficient elimination of C 4H 8 to generate a proton bound heterodimer of the attacking reactant with methanol as the primary product ion. This reaction also takes place when NH 3 is the substrate, although in this case proton transfer is the main process. In view of the easy interconversion of the protonated ether 1H + with its isomeric ion/neutral complexes consisting of proton bound isobutene and methanol (β complex), as proven by Audier et al. [J. Orig. Chem. 60 (1995) 7198], it is suggested that this fast elimination process is in fact a ligand switching process of the β complex which is either present as a stable species in the gas phase of the FTICR cell or is generated from 1H + by electrostatic activation in the encounter complex of the reactants.