Use of electrospray ionization mass spectrometry for the investigation of radical cation chain reactions in solution: detection of transient radical cations

Abstract

Electrospray ionization mass spectrometry (ESI-MS) coupled to a microreactor is an excellent tool for the investigation of reactions in solution. Here, we report the first results of our investigations into preparatively interesting electron-transfer-initiated chain reactions in solution which proceed via radical cations as reactive intermediates. The tris(p-bromophenyl)aminium hexachloroantimonate (1)-mediated [2+2] cycloaddition of trans-anethole (2) to give 1,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane (3) was investigated. The reaction proceeds as a radical cation chain reaction via transient intermediates 2*+ and 3*+ that could be detected and characterized unambiguously directly in the reacting solution by ESI-MS/MS. The identity of the intermediates was confirmed by comparison with authentic MS/MS spectra of 2*+ and 3*+ obtained by atmospheric pressure chemical ionization mass spectrometry (APCI-MS). In addition, substrate and product can be monitored easily in the reacting solution by APCI-MS.

Online Monitoring of Polymerizations: Current Status

“…Mostly the mixing tee is connected to the ionization probe with a fused-silica capillary providing residence times from 0.7 to 28 s in a continuous flow mode by varying the length of the transfer capillary. [66][67][68][69][70][71][72] Continuous online MS monitoring of polymerization reactions was first made available by Santos and Metzger. They investigated the mechanism of the homogeneously catalyzed Ziegler-Natta polymerization of ethane and Brookhart polymerization of alkenes in early stages of the polymerization.…”

Radical Cations in Electrospray Mass Spectrometry: Formation of Open‐Shell Species, Examination of the Fragmentation Behaviour in ESI‐MSn and Reaction Mechanism Studies by Detection of Transient Radical Cations

“…[37][38][39][40][41][42] For these studies a microreactor was coupled directly to a mass spectrometer fitted either with an ES or with an atmospheric pressure chemical ion source. [37][38][39][40][41][42] To investigate the electron-transfer-initiated reaction in solution, which proceeds via radical cations as reactive intermediates, catalytic amounts of tris(p-bromophenyl)aminium hexachloroantimonate (6) were used as radical mediator to initiate the stereoselective [2 + 2] cycloaddition of trans-anethole (7) to give 1,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane (8; see Scheme 4). [38,40] Kinetic studies regarding the stereo-and regioselective dimerization process for this conversion suggested a radical chain reaction mechanism.…”

“…[36,37] This article highlights contributions in which transient odd-electron-number (OE) reaction intermediates were successfully detected and reliably identified. [38][39][40][41][42][43][44][45] In particular, for solution-phase mechanistic studies of chemical reactions it is very important to verify unambiguously that potentially indicative species detected in the gas phase by ESI-MS are definitely not a result of ESI red/ox ionization processes. Furthermore, it must be established that the ions of interest are indeed reactive intermediates of a predicted reaction path.…”

“…Furthermore, it must be established that the ions of interest are indeed reactive intermediates of a predicted reaction path. [38] A straightforward MS analysis of reaction mechanism studies based on ESI therefore requires an exclusive, efficient and soft phase transfer of solution-phase species (i.e., radical reaction intermediates) into the gas phase, without any change in the ions or additional formation of ESI artefacts.…”

Extracellular proteasome in the human alveolar space: a new housekeeping enzyme?

“…To analyze proteins in the peak Suc-LLVY-AMC eluate activity of the BAL supernatant (fractions 22-26) (as described above) and the cell pellet lysate (fractions 21-24) gel spots in a molecular range (above 20 kDa) likely to contain proteasomal proteins, if present, were excised from the SDS-PAGE of one individual and subjected to electrospray ionization mass spectrometry measurements (44). This technique provides a plot of detector signal intensity (abundance of ions) vs. the mass-to-charge ratio (of ions produced in the instrument; m/z).…”