Reagent ion capture

In positive ion Cl, the two commonest types of ionizing reaction are (a) proton transfer and (b) reagent ion capture... 

Reagent Ion Capture The capture of negative reagent gas ions was described in the early 1970s by Manfred von Ardenne and coworkers for the analysis of long-chain aliphatic hydrocarbons with hydroxyl ions (Ardenne etal., 1971). 

Synthetic applications of the /1-pathway include conversions of the TIPS allylic azides to enones with tetrabutylammonium fluoride, ionization of the C-N3 bond with alkylaluminum reagents and capture of the TIPS enonium ions with carbon nucleophiles, and development of a procedure for y-lactamization [15-17]. Allylic azidonations of TIPS enol ethers have also been incorporated into syntheses of (+)-pancratistatin [18,19] and the core structure of lycorane [20]. 

An intermediate that cannot be observed spectroscopically might be trapped by added reagents. In Chapter 5 we discussed the use of spin trap reagents to capture transient radicals for analysis by EPR spectrometry. Another example can be seen in the studies that led to the bromonium ion mechanism for the addition of bromine to frans-2-butene (14) to produce meso-2,3-dibromobutane (15, equation 6.7). Adding a nucleophile such as methanol to the reaction mixture led to a product incorporating the nucleophile (16) as shown in equation 6.8, which suggested that a cation may be an intermediate in the reaction. Additional evidence is necessary to determine the structure of the intermediate (i.e., bromonium ion or bromine-substituted carbocation), but that is a matter of detail once the existence of an intermediate of some kind is established. 

In negative ion mode, thermal electrons are presumed to generate mostly O2"" ions from air that serve as reagent ions. Of course, direct electron capture by the analyte as well as dissociative electron capture, deprotonation, or anion attachment are also feasible [58]. Thus, it has been inferred from the close similarity of the negative-ion products of DART and APPI that the same group of mechanisms of ion formation are effective. 

A technique known as selective electron capti sensitization has been used to increase the response of the BCD weakly electron-capturing compounds [117]. In this mode a standard electron-capture detector is used with a supply of makeup gas doped with a specific sensitizing reagent such as oxygen nitrous oxide. In this way the BCD functions as an ion-aoleculSj... 

The addition reactions discussed in Sections 4.1.1 and 4.1.2 are initiated by the interaction of a proton with the alkene. Electron density is drawn toward the proton and this causes nucleophilic attack on the double bond. The role of the electrophile can also be played by metal cations, and the mercuric ion is the electrophile in several synthetically valuable procedures.13 The most commonly used reagent is mercuric acetate, but the trifluoroacetate, trifluoromethanesulfonate, or nitrate salts are more reactive and preferable in some applications. A general mechanism depicts a mercurinium ion as an intermediate.14 Such species can be detected by physical measurements when alkenes react with mercuric ions in nonnucleophilic solvents.15 The cation may be predominantly bridged or open, depending on the structure of the particular alkene. The addition is completed by attack of a nucleophile at the more-substituted carbon. The nucleophilic capture is usually the rate- and product-controlling step.13,16... 

Recently, reports have appeared on a class of ILs known as task specific ionic liquids (TSIL). The term was introduced by J. H. Davis, Jr s group to refer to those ILs which have functional groups attached to them so as to give specific properties and functionalities. Thus, they not only perform specific functions like metal ion extraction,catalysisand capture of but also maintain the desired physical characteristics such as physical state, non-volatility, viscosity, etc. The implementation of TSILs further enhances the versatility of classical ILs where both reagent and medium are coupled. The union of reagent with medium has been... 

Bifunctional reagents have recently been used to facilitate separations in the Mitsunobu reaction.39 Mitsunobu products are often hard to separate from excess reagents and byproducts, including phosphines and phosphine oxides. The tagged phosphine 21 and azodicarboxylate 22 and the byproducts formed from these are converted to the carboxylic acid forms by treatment with trifluoroacetic acid (TFA) at the end of the reaction. The excess reagents and byproducts could then be captured on an ion exchange resin for convenient removal. 

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