Cationic formation

Successive introduction of two methyl groups at ring carbon increases the hydrolysis rate by a factor of 10 in each step, indicating cation formation in the transition state as in acetal hydrolysis. Equilibrium protonation before hydrolysis becomes evident from an increasing rate of hydrolysis with a decreasing pH value (Table 3). Below pH 3 no further increase of rate is observed, so that protonation is assumed to be complete. 

Increased sensitivity towards acid is observed when protonation occurs on a functional group outside the diazirine ring, giving rise to electron dilution at the carbon atom adjacent to the diazirine carbon. The products isolated are in accord with the proposal (79AHC(24)63) that cation formation at this carbon atom leads to nitrogen extrusion, probably with formation of a vinyl cation. Thus protonated hydroxydiazirine (209) yields acetone, and methylvinyldiazirine (199) on treatment with acids yields butanone (67CB2093). 

The hydrochloride of (3) holds water rather tenaciously, and the infrared spectrum indicates that the water is covalently bound. Mild oxidation of the cation (3) gives 4-hydroxyquinazoline in high yield and ring-chain tautomerism is excluded on the grounds that quinazo-line does not give a positive aldehyde test in acid solution, 2-Methyl-quinazoline also has an anomalous cationic spectrum and a high basic strength (see Table I), but 2,4-dimethylquinazoline is normal in both these respects, which supports the view that abnormal cation formation entails attack on an unsubstituted 4-position. ... 

The obviation of side reactions is essential to the success of ADMET, and this can be realized if the proper catalyst is chosen. Catalyst choice must avoid the possibility of cation formation,13 vinyl addition, and/or formation of multiple catalytic species, all of which are detrimental to clean metathesis chemistry. Over the past 10 years, our group has utilized a variety of different catalysts, several of which are illustrated in Fig. 8.4. 

That cation formation by proton transfer to the pyrrolidine nitrogen atom is required for strong inhibition was demonstrated by the conversion of 9... 

Rosenblatt etal have examined the effect of structure and isotopic substitution upon the permanganate oxidation of some alky famines (Table 4). The isotope effect of 1.84 is considered to be sufficiently low to be compatible with aminium radical-cation formation, and it is felt that, while C-H cleavage is significant for oxidation of primary amines, the dominant mode of oxidation of tertiary amines is electron-transfer, e.g. 

The following outer-sphere oxidation mechanism is favoured over the radical-cation formation which is a feature of similar oxidations by Mn(III) acetate (p. 375). With toluene... 

These relatively facile oxidations may involve hydroxyl transfer, in preference to radical-cation formation, viz. 

Cation formation gets trickier for atoms with higher atomic numbers. Cadmium, for instance, lies between the noble gases krypton and xenon ... 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

For protonation-dehydration processes, such as trityl cation formation from triphenylcarbinols, equation (24), the water activity has to be included if the formulation of the activity coefficient ratio term is to be the same as that in equation (7), which it should be if linearity in X is to be expected see equation (25). The excess acidity expression in this case becomes equation (26) this is a slightly different formulation from that used previously for these processes,36 the one given here being more rigorous. Molarity-based water activities must be used, or else the standard states for all the species in equation (26) will not be the same, see above. For consistency this means that all values of p/fR listed in the literature will have to have 1.743 added to them, since at present the custom... 

A major concern of this review is the tailoring of the redox behaviour of organic compounds, i.e. the optimization of such systems for electron storage and electron hopping. While the emphasis is on reduction and thus on anion formation, it has been shown on many occasions that oxidative cation formation leads to analogous conclusions (Meerholz and Heinze, 1990 Lewis and Singer, 1965). The structure of this text is thus obvious. 

Bobrowski and Das33 studied the transient absorption phenomena observed in pulse radiolysis of several retinyl polyenes at submillimolar concentrations in acetone, n -hexane and 1,2-dichloroethane under conditions favourable for radical cation formation. The polyene radical cations are unreactive toward oxygen and are characterized by intense absorption with maxima at 575-635 nm. The peak of the absorption band was found to be almost independent of the functional group (aldehyde, alcohol, Schiff base ester, carboxylic acid). In acetone, the cations decay predominantly by first-order kinetics with half life times of 4-11 ps. The bimolecular rate constant for quenching of the radical cations by water, triethylamine and bromide ion in acetone are in the ranges (0.8-2) x 105, (0.3-2) x 108 and (3 — 5) x 1010 M 1 s 1, respectively. 

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