Structural effects responsible for stability and solvolytic reactivity of sulfonium ions

The solvolysis rates of X‐substituted benzhydryltetrahydrothiophenium ions ( 1 ) in pure and aqueous alcohols were determined at 25 °C and compared with the rates of the corresponding benzhydryldimethylsulfonium ions ( 2 ). The linear free energy relationship equation log k = s_f(E_f + N_f) has been used to relate quantitatively the leaving group abilities of tetrahydrothiophene (THT) and dimethyl sulfide (Me₂S). It has been demonstrated that although generating a stronger base by heterolysis, substrates 1 solvolyze over lower barriers than 2 . Steric and electronic influences that determine the relative reactivities of sulfonium salts have been examined computationally at B3LYP level of theory by calculating the energy of exchange of electrofuges with different substituents between THT and dimethyl sulfide. Because of more efficiently delocalized positive charge in THT moiety, tetrahydrothiophenium ions are more stable than the corresponding dimethylsulfonium ions, regardless of an electrofuge. The Hammond–Leffler coefficient is negative (α < 0) for the rate determining heterolysis of sulfonium salts 1 and 2 . Copyright © 2011 John Wiley & Sons, Ltd.     

Solvolytic reactivity of pyridinium ions

The leaving group abilities of pyridine, 4‐methylpyridine, and 4‐chloropyridine in S_N1 solvolytic reactions have been determined by analyzing the rate constants of X,Y‐substituted benzhydrylpyridinium salts obtained in various solvents. By applying the linear free energy relationship equation, log k = s_f (E_f + N_f), the nucleofuge specific parameters of 4‐substituted pyridine have been extracted. Because of solvation in the reactant ground state, the reactivity (nucleofugality, N_f) of a given pyridine decreases as the polarity of the solvent increases. High slope parameters (s_f > 1) may be due to the spread of the energy levels of the benzhydrylium ion/pyridine pair intermediates in comparison to benzhydrylium ion/chloride pairs (s_f ≈ 1). Because of slow heterolysis step of pyridinium salts in various solvents, some are stable under normal conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

Alkali‐Metal Ion Catalysis and Inhibition in SNAr Displacement: Relative Stabilization of Ground State and Transition State Determines Catalysis and Inhibition in SNAr Reactivity

We report here the first observation of alkali‐metal ion catalysis and inhibition in S_NAr reactions. The plot of k_obsd versus [alkali‐metal ethoxide] exhibits downward curvature for the reactions of 1‐(4‐nitrophenoxy)‐2,4‐dinitrobenzene with EtOLi, EtONa, and EtOK, but upward curvature for the corresponding reaction with EtOK in the presence of 18‐crown‐6‐ether (18C6). Dissection of k_obsd into the second‐order rate constants for the reactions with the dissociated EtO^? and the ion‐paired EtOM (i.e., k and k_EtOM, respectively) has revealed that the reactivity increases in the order EtOLi<EtONa<EtOK<EtO^?<EtOK/18C6. This indicates that the reaction is inhibited by Li⁺, Na⁺, and K⁺ ions but is catalyzed by 18C6 K⁺ ion. The reactions of 1‐(Y‐substituted‐phenoxy)‐2,4‐dinitrobenzenes have been proposed to proceed through a stepwise mechanism, in which expulsion of the leaving group occurs after the rate‐determining step based on the kinetic result that σ^o constants exhibit a much better Hammett correlation than σ^? constants. Alkali‐metal ion catalysis or inhibition has been discussed in terms of differential stabilization of ground‐state and transition‐state complexes through a qualitative energy profile. A π‐complexed transition‐state structure is proposed to account for the kinetic results.     

Reactions between Diazo Compounds and Hypervalent Iodine(III) Reagents

Site‐selective “cut and sew” transformations employing diazo compounds and hypervalent iodine(III) compounds involve the departure of leaving groups, a “cut” process, followed by a reorganization of the fragments by bond formation, a “sew” process. Bearing controllable cleavage sites, diazo compounds and hypervalent iodine(III) compounds play a critical role as versatile reagents in a wide range of organic transformations because their excellent nucleofugality allows for a large number of unusual reactions to occur. In recent years, the combination of diazo compounds and hypervalent iodine(III) reagents has emerged as a promising tool for developing new and valuable approaches, and has met considerable success. In this Minireview, this combination is systematically illustrated with recent advances in the field, with the aim of elaborating the synthetic utility and potential of this concept as a powerful strategy in organic synthesis.