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

Ionization rates of para‐substituted triphenylmethyl (trityl) acetates, benzoates, and para‐nitrobenzoates have been determined in aqueous acetonitrile and aqueous acetone at 25 °C. Conventional and stopped‐flow techniques have been used to evaluate rate constants ranging from 1.38×10^?5 to 2.15×10² 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 σ⁺ 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) 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.     

The reactivity-selectivity principle: an imperishable myth in organic chemistry

The reactivity-selectivity principle (RSP), once a tenet of organic chemistry, eroded during the 1970s and was more or less abandoned by 1980. Although it has been clear for more than 25 years that a decrease in selectivity with increasing reactivity can only be expected with certainty if diffusion control is approached, the RSP has survived as an intuitively appealing rule. This Minireview shows why selectivity cannot generally decrease with increasing reactivity and highlights the weaknesses of the theoretical foundations of the RSP.     

Farewell to the HSAB treatment of ambident reactivity

The concept of hard and soft acids and bases (HSAB) proved to be useful for rationalizing stability constants of metal complexes. Its application to organic reactions, particularly ambident reactivity, has led to exotic blossoms. By attempting to rationalize all the observed regioselectivities by favorable soft-soft and hard-hard as well as unfavorable hard-soft interactions, older treatments of ambident reactivity, which correctly differentiated between thermodynamic and kinetic control as well as between different coordination states of ionic substrates, have been replaced. By ignoring conflicting experimental results and even referring to untraceable experimental data, the HSAB treatment of ambident reactivity has gained undeserved popularity. In this Review we demonstrate that the HSAB as well as the related Klopman-Salem model do not even correctly predict the behavior of the prototypes of ambident nucleophiles and, therefore, are rather misleading instead of useful guides. An alternative treatment of ambident reactivity based on Marcus theory will be presented.     

Hydrocarbation of CC Bonds: Quantification of the Nucleophilic Reactivity of Ynamides

Donor‐substituted diarylcarbenium ions Ar₂CH⁺ react with ynamides to give 1‐amido‐substituted allyl cations (α,β‐unsaturated iminium ions). Kinetic studies show that these adducts, which correspond to the addition of a C? H bond across the C?C bond, are formed stepwise with initial formation of keteniminium ions and subsequent 1,3‐hydride shifts. The linear correlations between the second‐order rate constants (lg k₂, 20 °C) with the electrophilicity parameters E of the diarylcarbenium ions allow us to include ynamides in our comprehensive nucleophilicity scale and thus predict potential electrophilic reaction partners.     

Determination of the electrophilicity parameters of diethyl benzylidenemalonates in dimethyl sulfoxide: reference electrophiles for characterizing strong nucleophiles

The second-order rate constants of the reactions of nine substituted diethyl benzylidenemalonates 1 a-i with the carbanions 2 a-e have been determined spectrophotometrically in dimethyl sulfoxide (DMSO). Product studies show that the nucleophiles attack regioselectively at the electrophilic C==C double bond of the Michael acceptors to form the carbanionic adducts 4. The correlation log k(20 degrees C)=s(N+E) allows the determination of the electrophilicity parameters E for the electrophiles 1 a-i from the rate constants determined in this work and the previously published N and s parameters for the nucleophiles 2 a-e. The electrophilicities E for compounds 1 a-i cover a range of six units (-17.7>E>-23.8) and correlate excellently with Hammett's substituent constants sigma(p). The title compounds are roughly ten orders of magnitude less reactive than analogously substituted benzylidene Meldrum's acids, their cyclic analogues. Due to their low reactivities, compounds 1 a-i are suitable reference electrophiles for determining the reactivities of highly reactive nucleophiles, such as carbanions with 16相似文献   

Quantification of Ion‐Pairing Effects on the Nucleophilic Reactivities of Benzoyl‐ and Phenyl‐Substituted Carbanions in Dimethylsulfoxide

Second‐order rate constants for the reactions of acceptor‐substituted phenacyl (PhCO?CH^??Acc) and benzyl anions (Ph?CH^??Acc) with diarylcarbenium ions and quinone methides (reference electrophiles) have been determined in dimethylsulfoxide (DMSO) solution at 20 °C. By studying the kinetics in the presence of variable concentrations of potassium, sodium and lithium salts (up to 10^?2 mol L^?1), the influence of ion‐pairing on the reaction rates was examined. As the concentration of K⁺ did not have any influence on the rate constants at carbanion concentrations in the range of 10^?4–10^?3 mol L^?1, the acquired rate constants could be assigned to the reactivities of the free carbanions. The counter ion effects increase, however, in the series K⁺<Na⁺<Li⁺, and the sensitivity of the carbanion reactivities toward variation of the counter ion strongly depends on the structure of the carbanions. The reactivity parameters N and s_N of the free carbanions were derived from the linear plots of log k₂ against the electrophilicity parameters E of the reference electrophiles, according to the linear‐free energy relationship log k₂(20 °C)=s_N(N+E). These reactivity parameters can be used to predict absolute rate constants for the reactions of these carbanions with other electrophiles of known E parameters.     

Electrophilic Alkylations of Vinylsilanes: A Comparison of α‐ and β‐Silyl Effects

Kinetics of the reactions of benzhydrylium ions (Aryl₂CH⁺) with the vinylsilanes H₂C?C(CH₃)(SiR₃), H₂C?C(Ph)(SiR₃), and (E)‐PhCH?CHSiMe₃ have been measured photometrically in dichloromethane solution at 20 °C. All reactions follow second‐order kinetics, and the second‐order rate constants correlate linearly with the electrophilicity parameters E of the benzhydrylium ions, thus allowing us to include vinylsilanes in the benzhydrylium‐based nucleophilicity scale. The vinylsilane H₂C?C(CH₃)(SiMe₃), which is attacked by electrophiles at the CH₂ group, reacts one order of magnitude faster than propene, indicating that α‐silyl‐stabilization of the intermediate carbenium ion is significantly weaker than α‐methyl stabilization because H₂C?C(CH₃)₂ is 10³ times more reactive than propene. trans‐β‐(Trimethylsilyl)styrene, which is attacked by electrophiles at the silylated position, is even somewhat less reactive than styrene, showing that the hyperconjugative stabilization of the developing carbocation by the β‐silyl effect is not yet effective in the transition state. As a result, replacement of vinylic hydrogen atoms by SiMe₃ groups affect the nucleophilic reactivities of the corresponding C?C bonds only slightly, and vinylsilanes are significantly less nucleophilic than structurally related allylsilanes.     

Hydride‐Donor Abilities of 1,4‐Dihydropyridines: A Comparison with π Nucleophiles and Borohydride Anions

How are dihydropyridines like indoles? Both groups of compounds have similar nucleophilicity parameters N and are therefore suitable substrates for iminium‐catalyzed reactions of α,β‐unsaturated aldehydes. The N parameters of 1,4‐dihydropyridines were derived from the rates of hydride transfer reactions to benzhydrylium ions (see scheme).

     

SN2’ versus SN2 Reactivity: Control of Regioselectivity in Conversions of Baylis–Hillman Adducts

TiCl₄‐induced Baylis–Hillman reactions of α,β‐unsaturated carbonyl compounds with aldehydes yield the (Z)‐2‐(chloromethyl)vinyl carbonyl compounds 5 , which react with 1,4‐diazabicyclo[2.2.2]octane (DABCO), quinuclidine, and pyridines to give the allylammonium ions 6 . Their combination with less than one equivalent of the potassium salts of stabilized carbanions (e.g. malonate) yields methylene derivatives 8 under kinetically controlled conditions (S_N2’ reactions). When more than one equivalent of the carbanions is used, a second S_N2’ reaction converts 8 into their thermodynamically more stable allyl isomers 9 . The second‐order rate constants for the reactions of 6 with carbanions have been determined photometrically in DMSO. With these rate constants and the previously reported nucleophile‐specific parameters N and s for the stabilized carbanions, the correlation log k (20 °C)=s(N + E) allowed us to calculate the electrophilicity parameters E for the allylammonium ions 6 (?19<E <?18). The kinetic data indicate the S_N2’ reactions to proceed via an addition–elimination mechanism with a rate‐determining addition step.     

Kinetics of Hydride Abstractions from 2‐Arylbenzimidazolines

The rates of the hydride abstractions from the 2‐aryl‐1,3‐dimethyl‐benzimidazolines 1a – f by the benzhydrylium tetrafluoroborates 3a – e were determined photometrically by the stopped‐flow method in acetonitrile at 20 °C. The reactions follow second‐order kinetics, and the corresponding rate constants k₂ obey the linear free energy relationship log k₂(20 °C)= s(N+E), from which the nucleophile‐specific parameters N and s of the 2‐arylbenzimidazolines 1a – c have been derived. With nucleophilicity parameters N around 10, they are among the most reactive neutral C? H hydride donors which have so far been parameterized. The poor correlation between the rates of the hydride transfer reactions and the corresponding hydricities (ΔH⁰) indicates variable intrinsic barriers.