Ion–molecule reaction in the gas phase 4—Stereospecific nucleophilic substitution under ammonia chemical ionization conditions from diastereomeric methyl and benzyl ethers of 3‐hydroxy steroids

Abstract

Abstract The ammonia chemical ionization (CI/[NH 4 + ]) mass spectra of a series of diastereomeric methyl and benzyl ethers derived from 3‐hydroxy steroids (unsaturated in position 5 and saturated) have been studied. The adduct ions [M+NH 4 ] + and [MH] + and the substitution product ions [M+NH 4 ROH] + (thereafter called [M s H] + ) are characterized by an inversion in their relative stabilites in relation to their initial configuration. [M+NH 4 ] α + and [MH] α + formed from the α‐Δ 5 ‐steroid isomers are stabilized by the presence of a hydrogen bond which is not possible for the β‐isomers. This stereochemical effect has also been observed in the mass analysed ion kinetic energy (MIKE) spectra of [M+NH 4 ] + and [MH] + . The MIKE spectra of [M s H] + indicate that those issued from the β‐isomers are more stable than the one originating from the α‐isomers. This behavior is also observed in the first field free region (HV scan spectra) for [MH] + , [M s H] + and [M+NH 4 ] + which are precursors of the ethylenic carbocations (base peak in the conventional CI/[NH 4 ] + spectra). Mechanisms, such as S N 1 and S N i, have been ruled out for the formation of [M s H] + , but instead the data support an S N 2 mechanism during the ion‐molecule reaction between [M+NH 4 ] + and NH 3 .