Electronic and ligating properties of carbocyclic carbenes: A theoretical investigation

Carbocyclic carbenes (CCCs) are a class of nucleophilic carbenes which are very similar to N-heterocyclic carbenes (NHCs) in terms of their reactivity, but they do not contain a stabilizing heteroatom in their cyclic ring system. In this study, 17 representative known CCCs and 34 newly designed CCCs are evaluated using quantum chemical methods, and the results are compared in terms of their stability, nucleophilicity, and proton affinity (PA) parameters. The results are divided on the basis of ring size of the known and reported CCCs. The stability, nucleophilicity, PA, complexation energy, and bond strength–related parameters were estimated using M06/6-311++G(d,p) method. The results indicated that the CCCs known in the literature are strong σ-electron donating species and have considerable π-accepting properties. This study led to the design and identification of a few new CCCs with dimethylamine and diaminomethynyl substituents which can be singlet stable and are substantially nucleophilic. © 2018 Wiley Periodicals, Inc.     

Nucleophilic activity of peroxyhydrocarbonate and peroxocarbonate ions relative to 4-nitrophenyl diethyl phosphonate

Peroxocarbonate ions HCO ₄ ^? and CO ₄ ^2? , which are formed in the H₂O₂/NH₄HCO₃/HO^? system, react with 4-nitrophenyl diethyl phosphonate (I) through a nucleophilic mechanism with rate constants \(k_{HCO_4^ - } = 0.008\) and \(k_{CO_4^{2 - } } = 0.13 L/mol \cdot s\). Comparison of these constants with the corresponding constants of other inorganic anions in their reaction with I in the framework of the Brönsted equation indicates that HCO ₄ ^? and CO ₄ ^2? are typical α-nucleophiles.     

Diversity-oriented expedient route for the synthesis of 3-tetrahydropyrimidinyl-coumarins via MCR

A practical, mild, and high-yielding synthetic approach for the synthesis of unknown tetrahydropyrimidinyl substituted 3-coumarins as hybrid scaffolds, potentially useful new chemical entities (NCEs), via metal- and catalyst-free multicomponent cyclization is described. The enhanced nucleophilicity of 3-amino coumarins versus 4-amino coumarin is explained via the difference in ¹³C NMR δ values (Δδ) of vinylic carbons. X-ray crystal analysis defines the structure of a representative set of example.     

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.     

Interface Engineering in Two‐Dimensional Heterostructures: Towards an Advanced Catalyst for Ullmann Couplings

The design of advanced catalysts for organic reactions is of profound significance. During such processes, electrophilicity and nucleophilicity play vital roles in the activation of chemical bonds and ultimately speed up organic reactions. Herein, we demonstrate a new way to regulate the electro‐ and nucleophilicity of catalysts for organic transformations. Interface engineering in two‐dimensional heteronanostructures triggered electron transfer across the interface. The catalyst was thus rendered more electropositive, which led to superior performance in Ullmann reactions. In the presence of the engineered 2D Cu₂S/MoS₂ heteronanostructure, the coupling of iodobenzene and para‐chlorophenol gave the desired product in 92 % yield under mild conditions (100 °C). Furthermore, the catalyst exhibited excellent stability as well as high recyclability with a yield of 89 % after five cycles. We propose that interface engineering could be widely employed for the development of new catalysts for organic reactions.     

Relationship between basicity and nucleophilicity

The empirical concepts of basicity and nucleophilicity are related but not strictly proportional. Hence, the aim of this study is to help in elucidating the range where both concepts are directly proportional. To do this, the relationship between a recently introduced nucleophilicity index and the proton affinity (PA) of several families of bases has been studied. A good correlation between the PA and the nucleophilicity index using HF and HCN as electrophilic partner has been found. Our studies show that the correlation exists only when the interaction is soft–soft in character and for strong bases with weak acids. However, the relationship is not only valid for exothermic reactions as it has been previously postulated but also for endothermic reactions. Copyright © 2007 John Wiley & Sons, Ltd.     

Nucleophilicity of functional surface active substances in the transfer of phosphoryl groups

The reactivity of α-nucleophilic groups in functional detergents is comparable with those for analogous compounds which do not form micelles. Methods are proposed for the modification of surface active compounds to produce supernucleophilic systems for the decomposition of organophosphorus compounds. A new functional detergent — 1-cetyl-3-(2-hydroxyiminoethyl)imidazolium chloride-is the most powerful of the investigated surface active reagents capable of achieving half lives for the decomposition of 4-nitrophenyl diethyl phosphonate and 4-nitrophenyl diethyl phosphate of ≤2 and 14 s respectively. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 5, pp. 288–294, September–October, 2006.     

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.