Insight into the role of the counteranion of an imidazolium salt in organocatalysis: a combined experimental and computational study

N‐Heterocyclic carbenes (NHCs) can serve as very reactive nucleophilic catalysts and exhibit strong basicity. Herein, we initiate a combined experimental and computational investigation of the NHC‐catalyzed ring‐closing reactions of 4‐(2‐formylphenoxy)but‐2‐enoate derivatives 1 to uncover the relationship between the counteranion of an azolium salt, the nucleophilicity and basicity of the carbene species, and the catalytic performance of the carbene species by taking imidazolium salts IPr ? HX (X=counteranion, IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) as the representative precatalysts. The plausible mechanisms of IPr‐mediated ring‐closing reactions have been investigated by using DFT calculations. The hydrogen‐accepting ability, assigned as the basicity of the counteranion of IPr ? HX and evaluated by DFT calculations, is correlated with the rate of deprotonation of C2 in IPr ? HX, which could be monitored by the capture of the free carbene formed in situ with elemental sulfur. The deprotonation of C2 in IPr ? HX with a more basic anion gives rise to a higher concentration of the free carbene and vice versa. At a relatively low concentration, IPr prefers to show a nucleophilic character to induce the intramolecular Stetter reaction. At a relatively high concentration, IPr primarily acts as a base to afford benzofuran derivatives. These data comprehensively disclose, for the first time, that the counteranions of azolium salts significantly influence not only the catalytic activity, but also possibly the reaction mechanism.     

Facile Synthesis of Unsolvated Alkali Metal Octahydrotriborate Salts MB3H8 (M=K,Rb, and Cs), Mechanisms of Formation,and the Crystal Structure of KB3H8

A facile synthesis of heavy alkali metal octahydrotriborates (MB₃H₈; M=K, Rb, and Cs) has been developed. It is simply based on reactions of the pure alkali metals with THF?BH₃, does not require the use of electron carriers or the addition of other reaction media such as mercury, silica gel, or inert salts as for previous procedures, and delivers the desired products at room temperature in very high yields. However, no reactions were observed when pure Li or Na was used. The reaction mechanisms for the heavy alkali metals were investigated both experimentally and computationally. The low sublimation energies of K, Rb, and Cs were found to be key for initiation of the reactions. The syntheses can be carried out at room temperature because all of the elementary reaction steps have low energy barriers, whereas reactions of LiBH₄/NaBH₄ with THF?BH₃ have to be carried out under reflux. The high stability and solubility of KB₃H₈ were examined, and a crystal structure thereof was obtained for the first time.     

Fine‐Tuning the Nucleophilic Reactivities of Boron Ate Complexes Derived from Aryl and Heteroaryl Boronic Esters

Boron ate complexes derived from thienyl and furyl boronic esters and aryllithium compounds have been isolated and characterized by X‐ray crystallography. Products and mechanisms of their reactions with carbenium and iminium ions have been analyzed. Kinetics of these reactions were monitored by UV/Vis spectroscopy, and the influence of the aryl substituents, the diol ligands (pinacol, ethylene glycol, neopentyl glycol, catechol), and the counterions on the nucleophilic reactivity of the boron ate complexes were examined. A Hammett correlation confirmed the polar nature of their reactions with benzhydrylium ions, and the correlation lg k(20 °C)=s_N(E+N) was employed to determine the nucleophilicities of the boron ate complexes and to compare them with those of other borates and boronates. The neopentyl and ethylene glycol derivatives were found to be 10⁴ times more reactive than the pinacol and catechol derivatives.     

Nucleophilicity of Alkyl Zirconocene and Titanocene Precatalysts,and Kinetics of Activation by Carbenium Ions and by B(C6F5)3

Kinetics of activation of methyl and benzyl metallocene precatalysts by benzhydrylium ions, tritylium ions, and triarylborane B(C₆F₅)₃ were measured spectrophotometrically. The rate constants correlate linearly with the electrophilicity parameter E of the benzhydrylium and tritylium ions employed, allowing us to determine the σ‐nucleophilicities of the metal–carbon bond of several zirconocenes and titanocenes. Bridging, substitution, metal, and ligand effects on the rates of metal–alkyl bond cleavage (M=Zr, Ti) were studied and structure–reactivity correlations were used to predict the kinetics of generation of metallocenium ions pairs, which are active catalysts in polymerization reactions and are highly electrophilic Lewis acids in frustrated Lewis pair catalysis.     

Reactivities of carbocations and monomers in carbocationic polymerization and copolymerization

In carbocationic polymerization and copolymerization, a recent publication concluded that the substituent effect on carbocation reactivity is much larger than its effect on monomer reactivity, and this by a factor 10⁶ in the case of the rate constant k_12capp for p‐methylstyrene addition (monomer M₂) on, respectively, poly(p‐methoxystyrene)^± or poly(p‐methylstyrene)^± (M). This conclusion is disputed, as well as the assumption that the rate constants of capping (k_12capp) obtained in deactivation reactions of poly(p‐methoxystyrene)^± are identical with cross propagation rate constants in copolymerization (k_12copol). It is shown that the large calculated k_12capp are based on propagation constant values for p‐methylstyrene (k ≈ 10⁹) obtained by the diffusion‐clock method. They are 10⁴ times smaller as found for all styrenes, that is, between 10⁴ and 10⁵ when they are based on the ionic species concentrations. In such a case, the available data are still in agreement with an approximate compensation between the reactivities of a monomer and of the corresponding carbocation. It is also shown that copolymerization data for styrenes are not compatible with k values near to diffusion control, and that variations of log k_12capp and log k_12copol with the nucleophilicity parameter N of the monomers indicate a much lower selectivity of the monomers in the case of copolymerization. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2666–2680, 2010     

A Unified Framework for Understanding Nucleophilicity and Protophilicity in the SN2/E2 Competition

The concepts of nucleophilicity and protophilicity are fundamental and ubiquitous in chemistry. A case in point is bimolecular nucleophilic substitution (S_N2) and base-induced elimination (E2). A Lewis base acting as a strong nucleophile is needed for S_N2 reactions, whereas a Lewis base acting as a strong protophile (i.e., base) is required for E2 reactions. A complicating factor is, however, the fact that a good nucleophile is often a strong protophile. Nevertheless, a sound, physical model that explains, in a transparent manner, when an electron-rich Lewis base acts as a protophile or a nucleophile, which is not just phenomenological, is currently lacking in the literature. To address this fundamental question, the potential energy surfaces of the S_N2 and E2 reactions of X⁻+C₂H₅Y model systems with X, Y = F, Cl, Br, I, and At, are explored by using relativistic density functional theory at ZORA-OLYP/TZ2P. These explorations have yielded a consistent overview of reactivity trends over a wide range in reactivity and pathways. Activation strain analyses of these reactions reveal the factors that determine the shape of the potential energy surfaces and hence govern the propensity of the Lewis base to act as a nucleophile or protophile. The concepts of “characteristic distortivity” and “transition state acidity” of a reaction are introduced, which have the potential to enable chemists to better understand and design reactions for synthesis.