Bridging versus Hydride Shift in Gaseous Cations:  Hydroxy as a Vicinal Substituent

Abstract

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.

Synthesis of B- and P-Heterocycles by Reaction of Cyclic Acetals and Ketals with Borinium and Phosphonium Ions

“…Note that C 3 H 7 O + could be generated from the m / z 117 ion by either intramolecular S N 2 to give a methylated oxirane structure or by 1,2-hydride shift to give the structure CH 3 CHOCH 3 + . According to a recent study of unimolecular dissociation of β-hydroxylated cations by Morton and co-workers, C 3 H 7 O + is most likely to be CH 3 CHOCH 3 + . Interestingly, reactions of CH 3 OB + OCH 3 with acetaldehyde and methyl ketones occur via two 1,2-hydride shifts in the adducts and give an isomeric product ion of m / z 117, i.e., (CH 3 O) 2 B−O + CHCH 3 .…”

Comment on: “Reactions of Phenyl Cations with Methanol and Methyl Fluoride”

“…Concerning the excitation energy imparted to the nucleogenic phenylium ion 1 P by the β-transition in the 1,4-T 2 -benzene precursor, one should consider that the tritium nucleus transmutes into 3 He following emission of an antineutrino and of a β - particle, with a mean energy of 5.6 keV and a maximum energy of 18.6 keV . As pointed out above, nucleogenic phenylium ion 1 P can be left behind with some degree of vibrational excitation ( E def < 2 eV) . Further excitation may arise from the collision of the ejected particles (the β - particle or the 3 He atom) with the nucleogenic ion ( E coll ).…”

“…Further excitation may arise from the collision of the ejected particles (the β - particle or the 3 He atom) with the nucleogenic ion ( E coll ). Morton et al pointed out that this latter mechanism may produce ions with a vibrational energy > 2 eV (superexcited ions) . In the decay of tritiated ethanol, they estimated as large as ca.…”

“…14 As pointed out above, nucleogenic phenylium ion 1 P can be left behind with some degree of vibrational excitation (E def < 2 eV). 15 Further excitation may arise from the collision of the ejected particles (the βparticle or the 3 He atom) with the nucleogenic ion (E coll ). Morton et al pointed out that this latter mechanism may produce ions with a vibrational energy > 2 eV (superexcited ions).…”

“…Morton et al pointed out that this latter mechanism may produce ions with a vibrational energy > 2 eV (superexcited ions). 15 In the decay of tritiated ethanol, they estimated as large as ca. 50% the maximum net probability that one of the ejected particles collides with the nucleogenic ions from which it is departing.…”