School of Chemistry - Theses

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Start date: 2002 × Subject: mass spectrometry × Subject: alkene functionalization ×

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    Gas Phase Chemistry of Iranium Ions with Unsaturated Carbon-Carbon Bonds
    Brydon, Samuel Charles ( 2023-10)
    The study of cyclic iranium ions has developed rapidly over the last few decades as control of stereoselective outcomes during the electrophilic functionalisation of alkenes is mostly determined by the configurational stability of these species. Trace nucleophiles such as solvent, counter-ions, or unreacted alkene may cause decomposition or racemisation of these intermediates as the electrophilicity of both the heteroatom and endocyclic carbons make them susceptible to nucleophilic attack. Mass spectrometry (MS) thus offers an alternative means by which to isolate these charged species and study their bimolecular reactivity in the gas phase. Generation of these ions was achieved by electrospray ionisation of precursors containing a suitably basic group beta to the heteroatom, which upon protonation would fragment either in-source or following collision-induced dissociation of the pseudomolecular ion to give the heterocyclic three-membered ring. The multistage MS capabilities of a modified linear ion trap were utilised to isolate the iranium ion and observe its reactivity with neutral alkenes or alkynes. Changing the chalcogen (Ch) from sulfur to selenium to tellurium had a significant effect on the partitioning between attack at the heteroatom or ring-opening at carbon. Telluriranium ions underwent exclusive pi-ligand exchange with direct transfer of the tellurenium cation to the neutral reagent in a series of identity reactions, whilst all thiiranium ions studied only showed addition products from ring-opening by the neutral species. The reactivity of seleniranium ions towards alkenes partitioned between these two pathways with electron-donating groups on the heteroatom favouring the former, whilst the latter was promoted by electron-withdrawing groups. Computational studies into the pi-ligand exchange reaction revealed a Huckel pseudocoarctate transition state with a disconnection in the orbital array during the bond-breaking and bond-forming step. Extension to the haliranium ions showed kinetics of ion-molecule reactions with both cyclic and linear alkenes proceeding at the collision rate with iodiranium ions reacting dominantly via pi-ligand exchange, but bromiranium ions underwent carbocation-based fragmentation following ring-opening. Conjugation of the double bond to methyl esters suppressed heteroatom attack on iodiranium ions and only gave allylic stabilised oxocarbenium ions. The partitioning between these two reaction channels could be tuned by substituting inductively electron donating methyl groups onto the carbon-carbon double bond or entirely reverted to pi-ligand exchange by disrupting the conjugation with a methylene spacer enabling differentiation between three isomeric unsaturated methyl esters. Stability of the unsaturated irenium ions was examined by natural bond orbital theory to study (anti)aromaticity in these species. This approach revealed the antiaromatic nature of halirenium ions due to repulsion between the lone pairs and filled pi-orbital of the endocyclic double bond, and non-aromatic nature of the chalcogen irenium ions due to introduction of stabilising pi(C=C) - sigma*(Ch-R) hyperconjugative interactions. These species were generated in the gas phase for the first time by ion-molecule reactions of iranium ions with alkynes. The selenirenium ion structure assignment was strongly supported by cross-over experiments showing selenyl transfer to another alkyne, whilst the proposed iodirenium ion showed different reactivity to that of the open beta-iodovinyl cation produced upon reaction with phenylacetylene.