Adrian Weisz

Stereospecificity and Concertedness of Retro-Diels-Alder Fragmentation in Some Diester Systems Upon Chemical Ionization

Retro-Diels–Alder (RDA) fragmentation of cis- and trans-2,3-diethoxycarbonyl-5,6,7,8-dibenzo-bicyclo[2.2.2]octanes under isobutane and methane chemical ionization conditions is highly stereospecific, giving rise to protonated diethyl maleate and fumarate, respectively. This behaviour indicates a single-step concerted mechanism, analogous to the ground-state RDA process that occurs in neutral molecules in the condensed phase. The analogous dissociation is partially stereospecific in stereoisomeric endo-,exo- and trans-2,3-diethoxycarbonylbicyclo[2.2.1]heptanes and non-stereospecific in endo-,exo- and trans-2,3-diethoxycarbonyl-5,6-benzobicyclo[2.2.2]octanes, indicating the involvement of a stepwise mechanism in the latter two systems. The different behaviour of the above systems is explained in terms of the energy of the RDA fragmentation. The differentiation and quantitative estimates of protonated diethyl maleate and fumarate were obtained from collision-induced dissociation measurements.

Site specific gas-phase protonation of 2-t-butylmaleates and 2-t-butylsuccinates upon chemical ionization: stereochemical effects and kinetic control

An intense effects of steric hindrance on gas-phase protonation under chemical ionization (Cl) conditions, reflected by a very high site specificity in chemical ionization mass spectrometry (CIMS) of isomeric ethyl methyl esters of 2-t-butylmaleic and 2-t-butylsuccinic acids, provides new direct evidence of kinetic control in the protonation process.

Photoisomerisation of the adduct of bicyclohept-1-en-1-yl and 2,5-dimethyl-p-benzoquinone

The adduct (4) of bicyclohept-1-en-1-yl and 2,5-dimethyl-p-benzoquinone on exposure to light isomerizes to the pentacyclic product (3) by a long range hydrogen migration which has been previously observed in analogous systems under electron impact.

Observation and interpretation of an apparent inverse secondary isotope effect on the electron impact-induced loss of a methyl radical from methyl isobutyrate

The loss of CD3· from the molecular ion of methyl [2H3]isobutyrate (4) is 2·2 times faster than the loss of CH3· at 70 eV; the unusual direction and magnitude of the isotope effect are explained by a two-step mechanism, in which a rate-determining hydrogen or deuterium atom transfer precedes the loss of CD3· or CH3·.