Bond fission in monocationic frameworks: diverse fragmentation pathways for phosphinophosphonium cations.

A series of phosphinophosphonium cations ([R2PPMe3]+; R = Me, Et, i Pr, t Bu, Cy, Ph and N i Pr2) have been prepared and examined by collision-induced dissociation (CID) to determine the fragmentation pathways accessible to these prototypical catena-phosphorus cations in the gas-phase. Experimental evidence for fission of P-P and P-E (E = P, C) bonds, and ß-hydride elimination has been obtained. Comparison of appearance potentials for the P-P bond dissociation fragments [R2P]+ (P-P heterolysis) and [PMe3]+˙ (P-P homolysis) shows that heterolytic P-P cleavage is more sensitive than P-P homolysis towards changes in substitution at the trivalent phosphorus center. The facility of ß-hydride elimination increases with the steric bulk of R in [R2PPMe3]+. A density functional theory (DFT) study modelling these observed processes in gas-phase, counterion- and solvent-free conditions, to mimic the mass spectrometric environment, was performed for derivatives of [R2PPMe3]+ (R = Me, Et, i Pr, t Bu, Ph and N i Pr2), showing good agreement with experimental trends. The unusual observation of both homolytic and heterolytic cleavage pathways for the P-P and P-C bonds reveals new insight into the fundamental aspects of bonding in monocations and undermines the use of simplistic bonding models.

Practical approaches to the ESI-MS analysis of catalytic reactions.

Electrospray ionization mass spectrometry (ESI-MS) is a soft ionization technique commonly coupled with liquid or gas chromatography for the identification of compounds in a one-time view of a mixture (for example, the resulting mixture generated by a synthesis). Over the past decade, Scott McIndoe and his research group at the University of Victoria have developed various methodologies to enhance the ability of ESI-MS to continuously monitor catalytic reactions as they proceed. The power, sensitivity and large dynamic range of ESI-MS have allowed for the refinement of several homogenous catalytic mechanisms and could potentially be applied to a wide range of reactions (catalytic or otherwise) for the determination of their mechanistic pathways. In this special feature article, some of the key challenges encountered and the adaptations employed to counter them are briefly reviewed.

Substituent-controlled reactivity in the Nazarov cyclisation of allenyl vinyl ketones.

Alkyl substitution α to the ketone of an allenyl vinyl ketone enhances Nazarov reactivity by inhibiting alternative pathways involving the allene moiety and through electron donation and/or steric hindrance. This substitution pattern also accelerates Nazarov cyclisation by increasing the population of the reactive conformer and by stabilising the oxyallyl cation intermediate. Furthermore, α substitution by an alkyl group does not alter the regioselectivity of interrupted Nazarov reactions when the oxyallyl cation intermediate is intercepted by addition of an oxygen nucleophile, or by [4+3] cyclisation with acyclic dienes. The regioselectivity of the Nazarov process for allenyl vinyl ketones was determined to be a result of an electronic bias in the oxyallyl cation intermediate. Computational data are consistent with this observation.