Mass spectrometry, reactive intermediate

Mass spectrometry is a useful tool to detect the existence of reactive iron-imido intermediates. In intramolecular aromatic aminations, Que and coworkers used electrospray ionization mass spectrometry to show the presence of a molecular ion at m/z 590.3 and 621.2, which could be attributed to the formation of [(6-(o-TsN-C6H4)-TPA)Fe ]+ and [(6-(o-TsN-C6H4)-TPA)Fe° OMe)]+. With the isoto-... 

A second role for mass spectrometry in the investigation of reactive intermediates involves the nse of spectroscopy. Althongh an important nse of ion spectroscopy is the determination of thermochemical properties, including ionization energies (addition or removal of an electron), as in photoelectron or photodetachment spectroscopy, and bond dissociation energies in ions, as in photodissociation methods, additional spectroscopic data can also often be obtained, inclnding structural parameters such as frequencies and geometries. 

For further information regarding the investigation of reactive intermediate thermochemistry using mass spectrometry, the reader should consider ... 

We must note that we are dealing here not with static molecules, as no molecule is stationary even at the absolute zero of temperature, but rather with non-reacting molecules. This will be extended, however, to include mass spectrometry and the reactions which proceed within the mass spectrometry tube, as these are used to define the structure of the parent molecule. Obviously, though, such reactions have an importance of their own which is not neglected. Details of species involved as reactive intermediates, which may exist long enough for definition by physical techniques, will also be considered. For example, the section on ESR (Section 2.04.3.7) necessarily looks at unpaired electron species such as neutral or charged radicals, while that on UV spectroscopy (Section 2.04.3.3) considers the structure of electronically excited heterocyclic molecules. 

Closed-shell ions are among the most important intermediates in solution chemistry, and no treatise on reactive intermediates (including the present one) would be complete without extensive sections on carbocations and carbanions, if not also on heteroanalogues of the above species. Nevertheless, closed-shell ions are conspicuously absent from matrix isolation studies, apart from a few cases where such species were coincidentally formed in discharges, or where charged species were deliberately isolated by mass spectrometry (cf. Section 6.4). The reason for... 

The development of mass spectrometric ionization methods at atmospheric pressures (API), such as the atmospheric pressure chemical ionization (APCI)99 and the electrospray ionization mass spectrometry (ESI-MS)100 has made it possible to study liquid-phase solutions by mass spectrometry. Electrospray ionization mass spectrometry coupled to a micro-reactor was used to investigate radical cation chain reaction is solution101. The tris (p-bromophenyl)aminium hexachloro antimonate mediated [2 + 2] cycloaddition of trans-anethole to give l,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane was investigated and the transient intermediates 9 + and 10 + were detected and characterized directly in the reacting solution. However, steady state conditions are necessary for the detection of reactive intermediates and therefore it is crucial that the reaction must not be complete at the moment of electrospray ionization to be able to detect the intermediates. 

As future direction, and the fact that mass spectrometry has evolved very rapidly in the last decade, other mass spectrometry based technologies such as ion mobility (Thalassinos et al., 2004 Clemmer et al., 2005) may play an important role in the screening of reactive metabolites. This separation stage is orthogonal to the LC and mass spectrometric separations and occurs on an intermediate timescale... 

King, R., Dieckhaus, C., Nitkowski, N., and Gamache, P. H. (2004). On-line electrochemical oxidation used with HPLC-MS for the study of reactive drug intermediates. In Proceedings of the 52nd ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN. 

Keywords. Neutralization-reionization mass spectrometry, Ion chemistry, Collisional electron transfer, Franck-Condon effects, Reactive intermediates... 

Most of the mechanistic work on this reaction has been devoted to determining the role of the base. Its most obvious function would be to complex the Lewis-acidic boron reagent, rendering it nucleophihc and thus activating it toward transmetallation. However, Miyaura, Suzuki, and coworkers noted that an electron-rich tetracoordinate boronate complex was less reactive than a bivalent boronic ester. From this, they surmised that the role of the base was not to activate the boron toward transmetallation, but rather to transform the palladium halide intermediate to the hydroxide or alkoxide species, which would then be more reactive toward boron. However, in a mass spectrometry study of a reaction between a pyridyl halide substrate and an aryl boroiuc acid, Aliprantis and Canary saw no evidence of palladium hydroxide or alkoxide intermediates, despite observing signals in the mass spectra assignable to every other palladium intermediate of the proposed catalytic cycle. ... 

Phosphitenylium cations have been postulated as intermediates in numerous reactions of 1-chlorophosphirenes including phosphirene complexes. Recently, the uncomplexed cation has been generated in the gas phase from 4 upon 70 eV electron ionization and its reactivity toward representative nucleophiles and dienes has been studied by pentaquadrupole mass spectrometry <20030BC395>. The first complex of a phosphirenylium cation 9 was prepared from 8 according to Scheme 5. The phosphirene complex 8 is rather unstable in neat or in solution and decomposes with the formation of the bisphosphirenyl ether 10 <19990M817>. 

Flash-vacuum thermolysis of thietane 1-oxide affords the reactive intermediate, sulfine, CH2=S=0. Field-ionization mass spectrometry of the thermolysis products also indicates the formation of thietane, propenal, ethylene, CaHsO, CaHg, and hydrogen sulfide. A 1,2-oxathiolane intermediate was suggested. The exo sulfoxide 118 is thermally stable, but the endo derivative 115 decomposes around 200°C, probably because of the ease of -elimination ... 

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