Stabilities of Trityl‐Protected Substrates: The Wide Mechanistic Spectrum of Trityl Ester Hydrolyses

Abstract

Ionization rates of para-substituted triphenylmethyl (trityl) acetates, benzoates, and para-nitrobenzoates have been determined in aqueous acetonitrile and aqueous acetone at 25 degrees C. Conventional and stopped-flow techniques have been used to evaluate rate constants ranging from 1.38 x 10(-5) to 2.15 x 10(2) s(-1) by conductimetry and photospectrometry methods. The varying stabilities of the differently substituted tritylium ions account for a gradual change of reaction mechanism. Poorly stabilized carbocations are generated slowly by the ionization of their covalent precursors and trapped fast by water. Better stabilized carbocations are generated more rapidly and accumulate, so that ionization and trapping by water can be observed as separate steps in a single experiment. Finally, highly stabilized tritylium ions do not react with water, and only the rates of their formation could be measured. The ionization rate constants correlate linearly with Winstein's ionizing powers Y; the low slopes (0.17<m<0.58) indicate non-carbocation-like transition states. While the correlation between the ionization rates and Hammett-Brown's sigma(+) parameters is excellent for symmetrically substituted tritylium derivatives, deviations for unsymmetrically substituted systems are observed. The failing rate-equilibrium relationship between the rates of ionizations (log k(ion)) and the stabilities of the carbocations in aqueous solution (pK(R(+))) may be explained by the late development of resonance between a p-amino group and the carbocationic center of the tritylium ion during the ionization process.

Direct Organocatalytic Oxo‐Metathesis, a trans‐Selective Carbocation‐Catalyzed Olefination of Aldehydes

“…[12,19] As expected, the oxo-metathesis became slower as the electron density of one of the aromatic rings increased (i.e., decreasing the Lewis acidity of the carbocation) on going from trityl to a mono-methyl-(4) to mono-methoxy-substituted (5) trityl cation ( Table 2, entries 9-12). Unfortunately, decreasing the Lewis acidity of the trityl ion did not prevent any side-reactions and had a negative effect on the yield.…”

“…Both 2-methyl-2-hexene (2b) and citronellyl acetate (2c) turned out to be substantially less reactive than amylene (2a), and no reaction was observed at -78°C. After extensive optimization the best reaction conditions for olefins 2b and 2c were found to be -20°C for 135 h. Under these conditions the corresponding trans olefins were obtained in yields of 36-51 % (Table 3, entries [15][16][17][18][19][20].…”

Electrochemical, Spectroelectrochemical, Mößbauer, and EPR Spectroscopic Studies on Ferrocenyl‐Substituted Tritylium Dyes

“…Increasing electron richness of the other cyclopentadienyl ring attenuates this effect, as shown by the comparison of FcOMe + and Fc Me5 OMe + . The 13 C NMR shift of the methylium carbon atom is particularly sensitive to the electronic properties of the 4‐substituents at the aryl rings . The methylium resonance of the present monoferrocenyl tritylium ions falls in between those of the bis‐ or tris(4‐anisyl)methylium cations ( δ =201.0 and 194.0 ppm) and crystal violet ( δ =179.0 ppm), and its shift increases in the order FcNMe 2 + < Fc Me5 OMe + < FcOMe + < FcMeOMe + .…”

A comprehensive view on stabilities and reactivities of triarylmethyl cations (tritylium ions)

“…As only few investigations of trityl solvolyses had been reported in the past, we have recently performed a systematic investigation of solvolysis reactions of trityl halides and carboxylates . By using stopped‐flow techniques, a method that had not commonly been employed for studying ionization rates, we were able to significantly extend the accessible reactivity range into the millisecond time scale and gain important new insights in rate–equilibrium relationships and the interpretation of linear free energy relationships in general .…”