Basic sites cationic adducts

The general strategy for generating radical adducts to heterocyclic molecules relies on selective protonation of a suitable neutral precursor to prepare a cation of a well-defined structure. The gas-phase acid is chosen so as to attack only the most basic site in the molecule, or alternatively, non-selective protonation can be used to prepare a mixture of ions. For example, protonation of imidazole with NH4+ occurs selectively on the imine nitrogen atom (N-l), which has the highest proton affinity and is the only position that can be protonated by an exothermic reaction (Scheme 22) [239]. 

Although the protonated TMH or TMD complexes as formulated above have so far never been observed directly, and are just postulated intermediates, there exists experimental evidence that the dinitrosyl hydride complexes posses a strong basic site at the atom. For example, we were able to characterize A—ON-ReH(NO)(P Pr3)2 adducts, A representing a Lewis acid like BF [52] or even the cationic metal fragment [Re(NO)2(P Pr3)2] itself [50]. The formation of these adducts not only illustrates the basicity of the nitrosyl group, but also the strong Lewis acidity of the 16e cation. We also found that the bimetallic complex shows a similar reactivity as the free cationic species itself. Further evidence for the proposed heterolytic splitting was obtained when the reaction with H2 or D2 is performed in the presence of an external base (Scheme 5). 

One of the usual requirements for the use of ESI-MS is that the compounds that are transferred to the gas phase are charged. Stable neutral molecules can be detected as well, provided they have a high proton affinity or form stable complexes with ions (the so-called coordination ion-spray MS, CIS-MS). " Formation of cationic adducts with Lewis acids like Na, K, Ag, or protonation at basic sites of the molecule enables the analysis of otherwise undetected neutral species. Also, oxidation of neutral complexes resulting in monocationic species can occur in solution at the capillary tip before droplet formation takes place. 

Using isobutane as reagent gas produces ferf-butyl cations (Equations 3.10 and 3.11), which readily protonate basic sites on the sample molecule (Equation 3.12). Adduct formation is also possible using isobutane in CI-MS (Equation 3.13). 

Substitutions that remove this coincidence may affect the orientation of the cation, as shown in the alkali metal cation adducts of the three isomers of aminopyridines [52] and of methylpyridines [53]. It is noteworthy that the 2-amino substituent acts as a second basic site, enhancing the bonding by a sort of chelate effect. In 2-methylpyridine the cations are slightly attracted toward the methyl group. 

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