Group cation formation

Xenon Bonded to Carbon. A number of stmcturally well-characterized compounds containing Xe—C bonds are known. In all cases these occur as colorless salts of xenonium cations, R—Xe" where R is a fluorophenyl or alkynyl group. The formation of the pentafluorophenylxenon(II) cation, CgFgXe+ [121850-39-3] (-30W) and CHgC N (0°C) solutions with the anions B(C3F3)3F [121850-40-6], B(CgFg) 2F- [123168-25-2], and... 

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 second group of reactions is called vicinal difunctionalization. They embrace the C2 and C3 positions of the furan ring simultaneously. Thus, complex 3 (X = O, R = R = R = H) reacts with benzaldehyde dimethyl acetal to give 4H-furanium cation (the product of electrophile addition at C4), which experiences further attack by the methoxide group with formation of the acetal 8 (950M2861). This reaction is possible in the presence of the Lewis acid (BF3—OEt2). Reaction with methyl vinyl ketone in methanol, when run in identical conditions. 

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. 

However, the observations of Ward and Sherman need not rule out triple-bond participation and vinyl cations in the systems studied by Hanack and co-workers (75-79). Presumably, the enol formate 61 itself arises via a transition state involving a rate-determining protonation and vinyl cation 62 (see previous section). A vinyl cation such as 62 with an adjacent phenyl group is considerably more stable and hence more accessible than a vinyl cation such as 63, stabilized only by a neighboring alkyl group. Hence, formation of enol formate 61 and its... 

Scheme 10.1 gives some representative examples of laboratory syntheses involving polyene cyclization. The cyclization in Entry 1 is done in anhydrous formic acid and involves the formation of a symmetric tertiary allylic carbocation. The cyclization forms a six-membered ring by attack at the terminal carbon of the vinyl group. The bicyclic cation is captured as the formate ester. Entry 2 also involves initiation by a symmetric allylic cation. In this case, the triene unit cyclizes to a tricyclic ring system. Entry 3 results in the formation of the steroidal skeleton with termination by capture of the alkynyl group and formation of a ketone. The cyclization in Entry 4 is initiated by epoxide opening. 

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. 

The minor metabolite 11.167 in Fig. 11.20 can be viewed as a cyclic Schiff base formed by reaction of the NH2 and C=0 groups. In contrast, the major and unexpected metabolite 11.166 is also a Schiff base, but, more precisely, it is a permanent iminium cation formed between a secondary amino and a keto group. Presumably, formation of 11.166 is facilitated sterically by the spatial proximity of the two reacting groups. Another factor might well be the stability of the iminium cation, which is expected to be high in acidic media (proton repulsion by the positive charge) and in alkaline media (ab-... 

In the photochemical one-electron oxidation of aromatic sulfides, dimer radical cations were formed in rapid equilibrium with monomeric radical cation (59). The complex formation of a- and tt-types has been shown to be sensitive to the steric and electronic influence of substituent. For the case of jo-(methylthio)anisole the formation of TT-type dimer was shown to be reduced due to steric hindrance of two methyl groups. No formation of dimer radical cation was observed for jo-(methoxy)thioanisole and diphenyl disulfide where the corresponding monomer radical cations are stabilized by the delocalization of positive charge on the sulfur atom. Density-functional calculations supported the experimental results. The intramolecular formation of similar radical... 

Table 2.3 contains the standard Gibbs energies of formation and the standard enthalpies of formation of a selection of main group cations at 25 °C. They refer to the formation of 1 mol dm- solutions of the cations from their elements and are relative to the values for the hydrated proton taken as zero. 

Table 2.3 Standard molar Gibbs energies ol formalion and standard molar enthalpies of formation of some main group cations at 25 "C (in kj mol-1)...

The UV spectra also give clear evidence that protonation of cytosine does not occur at the amino group. There are appreciable differences between the spectra of amino compounds and the parent unsubstituted molecules. The spectra of the amines however, become closely similar to those of the unsubstituted molecules on cation formation at the... 

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