Bridging versus hydride shift in gaseous cations: Hydroxy as a vicinal substituent

Primary cations of the form XCH₂CH₂⁺ (1) are unstable with respect to the bridged isomer cyclo-(CH₂)₂X⁺ (2) or the hydride-shift isomer CH₃CHX⁺ (3). Two independent hydrogen isotope experiments for X = OH demonstrate that gaseous HOCH₂CH₂⁺ (1a) gives less of its bridged isomer, protonated oxirane (2a), than of its hydride-shift isomer protonated acetaldehyde (3a). Metastable ion decompositions of ionized HOCH₂CH₂OPh show that the radical cation decomposes via an ion-neutral complex containing 3a and phenoxy radical. Deuterium labeling studies exhibit no detectable interconversion of the two sp³-carbons, implying that complexes containing 2a are not formed to a measurable extent. An independent neutral product study looks at the radioactive oxirane and acetaldehyde produced when tritium on the methyl group of gaseous CH₂TCHTOH undergoes radioactive decay. Loss of a beta-particle and a helium atom forms transient, tritiated 1a. Ions from isomerization of this radiolabeled primary cation were deprotonated by Me₃N and gave a >97% yield of acetaldehyde. Quantitative assessment of possible routes to acetaldehyde (including rearrangement of excited 2a) implies that 1 undergoes hydride shift at least 5 times faster than bridging by neighboring oxygen.