Trityl derivatives, solvolysis

The solvolysis of trityl derivatives in strongly acidic aqueous media has also been examined. Rates of hydrolysis were correlated to Hr acidity function and theX excess acidity scale. The solvolysis and substitution reactions of 2-(4-methylphenyl)ethyl tosylate have also been studied. " In trifluoroethanobwater (50 50) solution, the corresponding phenonium ion is generated. The kinetics of azide trapping has been studied and compared with results from an analogous system 2-(4-methoxyphenyl)ethyl tosylate. Owing to electronic effects, the methyl-substituted phenonium ion is a more reactive electrophilic species. 

Finally, the case of benzene derivatives having electron withdrawing substituents will be discussed. Experimental support for mein electron withdrawal by nitro and cyano groups has been found (3, (>, 20) in the photochemical solvolyses of nitrophenyl and cyanophenyl trityl ethers. For example, it is found that in the dark w-nitrophenyl trityl other (XLII) is essentially unreactive in 90% aqueous dioxanc while the para isomer (XLIII) solvolyzes smoothly this is in accord with ground-state, expectation, for p-nitrophenolate is a better departing anion than -nitropheiiolate. In contrast, on irradiation under the same conditions, m-nitrophcnyl trityl other solvolyzes rapidly to m-nitrophenol and tri-phenylcarbinol, as major products, while the para isomer s (XLIII) solvolysis is scarcely enhanced beyond experimental error. V similar... 

Ionization is exothermic and favored at lower temperatures in systems such as trityl, alkoxycarbenium, and benzhydryl derivatives which generate stabilized carbenium ions due to their electron-donating substituents. The exothermicity of ionization of benzhydryl species with BCl3 in CH2CI2 decreases from AH = -62 kJ/mol for di(p-anisyl) to -22 kJ/ mol for less stabilized p-tolyl and phenyl derivatives, and to approximately -8 kJ/mol for unsubstituted benzhydryl chloride [193]. These values, based on the solvolysis rates, extrapolate to AH = -12 kJ/mol for cumyl chloride (HCl adduct of a-methylstyrene) and AH = 0 kJ/mol for 1-phenyl-ethyl chloride (styrene adduct) and ( -butyl chloride (isobutene adduct) [240]. The reported entropies of ionization do not vary significantly and... 

The 9-phenyl-9-fluorenyl (PhFl, 118, Scheme 56) amino-protecting group has been proposed by Christie and RapoportF as it is significantly more stable to solvolysis in acid conditions than the trityl group because of the antiaromatic character of the fluorenyl cation. In fact, solvolysis of Trt-Cl is 6000 times faster than PhFl-Cl.f The AI -PhFl amino acid derivatives are prepared with PhFl-Br (commercially available or prepared from fluorenone and phen-yUithium and subsequent substitution with HBr) and lead nitrate, which is required to trap the bromide (Scheme 56).F f2i] lead-free alternative using nitromethane as solvent has also been reported. ... 

A kinetic expression for the reaction of the trityl cation with added nucleophiles has been derived for the solvolysis of trityl fluoride [77]. The ratio of rates kyjk measures the ability of the added nucleophile Y to compete with water for the cation. The magnitude of the competition factor (kylko) is a property of the particular cation and thus the ky/k(t value for any Y is necessarily independent of the leaving group (in this case fluoride ion) [68]. The competition factors were found to vary... 

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