SUBJECTS forms alkalis 559 from ammonia

TTHE REACTION OF benzene or one of its derivatives with an alkali metal in liquid ammonia containing an alcohol cosolvent is known as the Birch reduction (1-6). As expected with anionic intermediates, 1-substituted-1,4-dihydro products result from electron-withdrawing groups, and 2,5-dihydro products result from electron-donating groups, as shown in Scheme I. Application of these conditions to polynuclear aromatic compounds produces complications because the products formed initially are subject to reduction under the reaction conditions (3). Although methods such as the use of iron salts minimize side reactions, more recently developed procedures avoid... 

In some cases it is observed that, under the experimental conditions used (mobile phase composition, ionization and API interface parameters), more than one ionized form of the intact analyte molecule is observed, i.e. adduct ions of various kinds (see Section 5.3.3 and Table 5.2). An example is shown in Figure 9.6, in which a well known anticancer drug (paclitaxel, Figure 9.6(a)) was analyzed by positive ion ESI-MS (infusion of a clean solution). The first spectrum (Figure 9.6(b)) shows four different adducts (with H+, NH, Na+ and K+). Adjustment of the cone (skimmer) potential (Section 5.3.3a), to lower values in this case, enabled production of the ammonium ion adduct to dominate the MS spectrum (Figure 9.6(c)) in a robust fashion, and this ion yielded a useful product ion spectrum (that appeared to proceed via a first loss of ammonia to give the protonated molecule) which was exploited to develop an MRM method that was successfully validated and used. It is advisable to avoid use of analyte adducts with alkali metal ions (commonly Na+ and to some extent K+) since, when subjected to colli-sional activation, these adducts frequently yield the metal ion as the dominant product ion with only a few low abundance product ions derived from the analyte molecule. However, when feasible, both the ammonium adduct and protonated molecule should be investigated as potential precursor ions at least until it becomes clear that one will provide superior performance (sensitivity/selectivity compromise) than the other. 

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