Cation adducts

A completely different method of synthesis of azo compounds from diazonium salts involving radical intermediates was found by Citterio et al. (1980, 1982 c), Cit-terio and Minisci (1982), and Fontana et al. (1988). It is a new general synthesis of arylazoalkanes based on the addition of an alkyl radical to an arenediazonium ion followed by reduction of the intermediate azo radical cation adduct by a metal salt (Scheme 12-80). The preferred source for the alkyl radical R in this reaction is an alkyl iodide, which gives rise to alkyl radicals cleanly in the presence of an arenediazonium salt and a Ti3+ or Fe2+ salt as in Scheme 12-81. The overall stoichiometric equation is therefore as given in Scheme 12-82. The yields vary between 36% and 79% (with respect to alkyl iodide). 

Triphenylphosphonium ylide reacts with the silylene complex 93 which has a highly electrophilic silicon center, to give the corresponding cationic adduct 94 [115]. The lengthening of the PC bond indicates a loss of the double bond character of the ylide and corresponds to the formation of a tetrahedral silicon center with four covalent bonds (Scheme 28). 

Numbers used in the cycle AG° for addition of hydroxide, assumed to be the same as for dimethyl sulfate AG° for ionization of the nentral adduct, based on a estimated by the method of Branch and Calvin AG° for ionization of the cationic adduct, based on a estimated by the method of Branch and Calvin.)... 

The first one regards the addition of a nitrogen atom of PTAD (177) to BCP (3), giving an aziridinium cation adduct, which is only postulated as intermedi-... 

Extraction with a solution of methyltricaprylammonium chloride in chloroform results in nearly quantitative isolation of pertechnetate from aqueous media, ranging from 4 M sulfuric acid or 9 M hydrochlorid acid to pH 13 . A 1 1 pertechne-tate-organic cation adduct seems to be formed at any pH an excess of the organic reagent is only necessary if extraneous anions can compete with pertechnetate. 

For ESP ionization, the analytes must be ionic, or have an ionizable functional group, or be able to form an ionic adduct in solution the analytes are commonly detected as deprotonated species or as cation adducts of a proton or an alkali metal ion. When using positive ion ESP ionization, use of ammonium acetate as a mobile-phase modifier is generally unsuitable. Instead, organic modi-... 

Trimesitylphosphine has been described as the bulkiest known phosphine.184 When this was treated with silver(I) hexafluorophosphate in CH2CI2 in the dark, a 1 2 adduct was obtained in 70% yield as a white crystalline solid. The crystal structure of this, the first 1 2 cationic adduct with phosphines, has been determined.185 In the cations, the Ag—P distance was 246.1pm, a little larger than expected, possibly due to the intermolecular overcrowding between mesityl groups on opposing ligands. The P—Ag—P moiety was essentially linear (179.4°). 

Dihydroxacephenone (DHAP) Mixed with diammonium hydrogen citrate to limit cation adduction, offers improved ionization of sialylated glycopeptides with limited in-source fragmentation compared to DHB and HCCA36... 

Trihydroxyacetophenone (THAP) THAP can be used for the analysis of oligosaccharides and can offer improved sensitivity for detection of acidic glycopeptides over HCCA.37 Ionization with THAP can be improved with the addition of di-ammonium hydrogencitrate to limit cation adduction... 

The aminophosphoranide molybdenum adduct 6 was obtained in 66% yield as yellow crystals by allowing LiMe to react with the cationic adduct 15 in a thf/ether solution (3 1) at -20°C. Infrared monitoring of the reaction showed that the v(C0) vibrations of at 1850 and 1978 cm- had completely disappeared after 30 mn, while two new absorptions had developed at 1855 and 1945 cm-. ... 

Starting azinium cation Adduct The dihydropyrazine ring ... 

DCI spectrum of mannitol, a non-volatile compound, with H20 as an reagent gas. Note that water yields radical cation adducts (M +HiO) +. 

A new class of Xe-N bonded compounds is obtained by the interaction of the electron lone pair of a nitrile with the Lewis acid XeF+. The cationic adducts RC =N-XeF+ with R = H, Me, CH2F, Et, C2F5, C3F7, and Ph have been isolated by the reaction of the appropriate nitrile with XeF+AsFg or Xe2F3+AsF6 in AHF at low temperatures. 

The second example27 belongs to the category of addition via onium-type intermediates . It exemplifies how the choice of the electrophile may affect the stereochemical course of the cyclization. When the hydroxy cnones rac-4a, b were treated with mercury(II) trifluoroacetate, both (E)- and (Z)-enone afforded basically the same yield and distribution of the diastereomer-ic cyclization products rac-5a, b. However, when phenylselenenyl chloride was used as electrophile, the (Z/Zj-isomers rac-4a, b produced quite contrasting ratios of the products rac-5 a, b. This result was interpreted in terms of EjZ equilibration in a single-bonded cationic adduct of the mercuric ions vs. nucleophilic attack on a rigid epi-selenonium ion27. 

For fluorine binding energies the predictions of the simple equivalent cores approximation disagree with the measured core-electron binding energy shifts by several electron volts in some cases. Shaw and Thomas have suggested that these discrepancies arise because of Jolly s assumption that the heats of formation of all neon-cation adducts (relative to free neon plus the... 

A ring carbon can also be involved, however, as in the reaction of the thianthrene and phenothiazine radical cations in neat pyridine or with pyridine in an anhydrous solvent. In this reaction the 1-pyridinium group is inserted on to the benzo ring (43), apparently via nucleophilic attack on di-cations 42, in turn resulting from oxidation of the initially formed radical cation adducts (Scheme 27). In the presence of moisture the sulfoxides are again formed [84]. 

APCI has been shown to be more readily influenced by the presence of negative or positive species from salts or buffers which will prevent protonation or deprotonation by competition with the analyte molecules [25,38-39]. Salts are sometimes deliberately added to samples to be assayed by electrospray in order to increase analyte ionization through cation adduct formation ((M + Na)+ or (M +... 

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. 

Separation selectivity can also be altered by the type of ion that is formed for the compound of interest. Cation or anion adducts can have a significant effect on the separation. In the positive ion mode, most ions formed from electrospray or any chemical ionization methods are protonated cations. However, if Na+ is added to the electrospray solution, sodiated adducts may be formed as the primary response ion. For example, the protonated ions of two isomers, methyl-P-glucopyranoside and methyl-a-glucopyranoside, cannot be baseline separated in DTIMS, but the sodiated adducts of the same isomers can be baseline separated. Figure 8.9 demonstrates separation selectivity induced by cation adduction. These IMS spectra are of the methyl-p-D-galactopyranoside and the methyl-a-D-galactopyranoside isomers adducted with cobalt acetate, silver, and lead acetate. The cobalt acetate adducts had a separation factor of 1.02, the silver adducts had a separation factor of 1.05, and the lead acetate adducts had a separation factor of 1.07. 

FIGURE 8.9 Selectivity induced by cation adduction. Ion mobility separation of methyl-p-D-galactopyranoside from its isomer methyl-a-D-galactopyranoside using different cation adducts. (From Dwivedi et al., Rapid resolution of carbohydrate isomers by electrospray ionization ambient pressure ion mobility spectrometry-time-of-flight mass spectrometry (ESI-APIMS-TOFMS), J. Am. Soc. Mass Spectrom. 2007, 18, 1163-1175. With permission.)... 

FIGU RE 9.12 Drift time ion mobility (ion trap) mass spectrometric data illustrating the necessity of isomer separation prior to MS analysis. (From Glowers and Hill, Influence of cation adduction on the separation characteristics of flavonoid diglycosides isomers using dual-gate ion mobility-quadrupole-ion trap mass spectrometry, 7. Mass Spectrom. 2006,41, 339-351. With permission.)... 

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