Ion‐Pair SN2 Reaction of OH− and CH3Cl: Activation Strain Analyses of Counterion and Solvent Effects

We have theoretically studied the non‐identity S_N2 reactions of M_nOH^(n?1)+CH₃Cl (M⁺=Li⁺, Na⁺, K⁺, and MgCl⁺; n=0, 1) in the gas phase and in THF solution at the OLYP/6‐31++G(d,p) level using polarizable continuum model (PCM) implicit solvation. We want to explore and understand the effect of the metal counterion M⁺ and solvation on the reaction profile and the stereoselectivity of these processes. To this end, we have explored the potential energy surfaces of the backside (S_N2‐b) and frontside (S_N2‐f) pathways. To explain the computed trends, we have carried out analyses with an extended activation strain model (ASM) of chemical reactivity that includes the treatment of solvation effects.     

The α‐effect exhibited in gas‐phase SN2@N and SN2@C reactions

In order to explore the existence of α‐effect in gas‐phase S_N2@N reactions, and to compare its similarity and difference with its counterpart in S_N2@C reactions, we have carried out a theoretical study on the reactivity of six α‐oxy‐Nus (FO^?, ClO^?, BrO^?, HOO^?, HSO^?, H₂NO^?) in the S_N2 reactions toward NR₂Cl (R = H, Me) and RCl (R = Me, i‐Pr) using the G2(+)_M theory. An enhanced reactivity induced by the α‐atom is found in all examined systems. The magnitude of the α‐effect in the reactions of NR₂Cl (R = H, Me) is generally smaller than that in the corresponding S_N2 reaction, but their variation trend with the identity of α‐atom is very similar. The origin of the α‐effect of the S_N2@N reactions is discussed in terms of activation strain analysis and thermodynamic analysis, indicating that the α‐effect in the S_N2@N reactions largely arises from transition state stabilization, and the “hyper‐reactivity” of these α‐Nus is also accompanied by an enhanced thermodynamic stability of products from the n_(N) → σ*_(O?Y) negative hyperconjugation. Meanwhile, it is found that the reactivity of oxy‐Nus in the S_N2 reactions toward NMe₂Cl is lower than toward i‐PrCl, which is different from previous experiments, that is, the S_N2 reactions of NH₂Cl is more facile than MeCl. © 2013 Wiley Periodicals, Inc.     

Activation Strain Analyses of Counterion and Solvent Effects on the Ion-Pair SN2 Reaction of and CH3Cl

We have computationally studied the bimolecular nucleophilic substitution (S_N2) reactions of M_nNH₂⁽ⁿ⁻¹⁾ + CH₃Cl (M⁺ = Li⁺, Na⁺, K⁺, and MgCl⁺; n = 0, 1) in the gas phase and in tetrahydrofuran solution at OLYP/6-31++G(d,p) using polarizable continuum model implicit solvation. We wish to explore and understand the effect of the metal counterion M⁺ and of solvation on the reaction profile and the stereochemical preference, that is, backside (S_N2-b) versus frontside attack (S_N2-f). The results were compared to the corresponding ion-pair S_N2 reactions involving F⁻ and OH⁻ nucleophiles. Our analyses with an extended activation strain model of chemical reactivity uncover and explain various trends in S_N2 reactivity along the nucleophiles F⁻, OH⁻, and , including solvent and counterion effects. © 2019 Wiley Periodicals, Inc.     

SN1‐SN2 and SN2‐SN3 mechanistic changes revealed by transition states of the hydrolyses of benzyl chlorides and benzenesulfonyl chlorides

Hydrolysis reactions of benzyl chlorides and benzenesulfonyl chlorides were theoretically investigated with the density functional theory method, where the water molecules are explicitly considered. For the hydrolysis of benzyl chlorides (para‐Z? C₆H₄? CH₂? Cl), the number of water molecules (n) slightly influences the transition‐state (TS) structure. However, the para‐substituent (Z) of the phenyl group significantly changes the reaction process from the stepwise (S_N1) to the concerted (S_N2) pathway when it changes from the typical electron‐donating group (EDG) to the typical electron‐withdrawing one (EWG). The EDG stabilizes the carbocation (MeO? C₆H₄? CH₂⁺), which in turn makes the S_N1 mechanism more favorable and vice versa. For the hydrolysis of benzenesulfonyl chlorides (para‐Z? C₆H₄? SO₂? Cl), both the Z group and n influence the TS structure. For the combination of the large n value (n > 9) and EDG, the S_N2 mechanism was preferred. Conversely, for the combination of the small n value and EWG, the S_N3 one was more favorable. © 2014 Wiley Periodicals, Inc.     

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.     

Computational study of SN2 reactions promoted by crown ether: Contact ion pair versus solvent‐separated ion pair mechanism

We examined by quantum chemical methods the mechanism of S_N2 reaction using metal bromide MBr (M = Na, K, Cs) and KX (X= F, Cl) in CH₃CN promoted by crown ether (18‐crown‐6). We focus on whether the metal salts react as a contact ion pair (CIP; M⁺ and X^– in close contact) or as a solvent‐separated ion pair (SSIP; M⁺ and X^– at large distance). In SSIP mechanism, X^– is removed far enough from M⁺ for the metal salt to be considered as “separated” by the effects of the crown ether and the solvent. In the CIP picture, conversely, the coordination of 18‐crown‐6 to M⁺ is not sufficient to overcome the powerful Coulombic interactions between M⁺ and X^–. We find that the CIP route is favored for S_N2 bromination processes using MBr (M = Na, K, Cs). For S_N2 reaction using KF, the feasibility of the two pathways is essentially equal, whereas for S_N2 chlorination by KCl the SSIP route is predicted to be favored.     

Probing the reactivity of microhydrated α‐nucleophile in the anionic gas‐phase SN2 reaction

To probe the kinetic performance of microsolvated α‐nucleophile, the G2(+)_M calculations were carried out for the gas‐phase S_N2 reactions of monohydrated and dihydrated α‐oxy‐nucleophiles XO^?(H₂O)_{n = 1,2} (X = HO, CH₃O, F, Cl, Br), and α‐sulfur‐nucleophile, HSS^?(H₂O)_{n = 1,2}, toward CH₃Cl. We compared the reactivities of hydrated α‐nucleophiles to those of hydrated normal nucleophiles. Our calculations show that the α‐effect of monohydrated and dihydrated α‐oxy‐nucleophiles will become weaker than those of unhydrated ones if we apply a plot of activation barrier as a function of anion basicity. Whereas the enhanced reactivity of monohydrated and dihydrated ROO^? (R = H, Me) could be observed if compared them with the specific normal nucleophiles, RO^? (R = H, Me). This phenomena can not be seen in the comparisons of XO^?(H₂O)_{n = 1,2} (X = F, Cl, Br) with ClC₂H₄O^?(H₂O)_{n = 1,2}, a normal nucleophile with similar gas basicity to XO^?(H₂O)_{n = 1,2}. These results have been carefully analyzed by natural bond orbital theory and activation strain model. Meanwhile, the relationships between activation barriers with reaction energies and the ionization energies of α‐nucleophile are also discussed. © 2015 Wiley Periodicals, Inc.     

On the Gas‐Phase Reactivity of Complexed OH+ with Halogenated Alkanes

OH⁺ is an extraordinarily strong oxidant. Complexed forms (L? OH⁺), such as H₂OOH⁺, H₃NOH⁺, or iron–porphyrin‐OH⁺ are the anticipated oxidants in many chemical reactions. While these molecules are typically not stable in solution, their isolation can be achieved in the gas phase. We report a systematic survey of the influence on L on the reactivity of L? OH⁺ towards alkanes and halogenated alkanes, showing the tremendous influence of L on the reactivity of L? OH⁺. With the help of with quantum chemical calculations, detailed mechanistic insights on these very general reactions are gained. The gas‐phase pseudo‐first‐order reaction rates of H₂OOH⁺, H₃NOH⁺, and protonated 4‐picoline‐N‐oxide towards isobutane and different halogenated alkanes C_nH_2n+1Cl (n=1–4), HCF₃, CF₄, and CF₂Cl₂ have been determined by means of Fourier transform ion cyclotron resonance meaurements. Reaction rates for H₂OOH⁺ are generally fast (7.2×10^?10–3.0×10^?9 cm³ mol^?1 s^?1) and only in the cases HCF₃ and CF₄ no reactivity is observed. In contrast to this H₃NOH⁺ only reacts with tC₄H₉Cl (k_obs=9.2×10^?10), while 4‐CH₃‐C₅H₄N‐OH⁺ is completely unreactive. While H₂OOH⁺ oxidizes alkanes by an initial hydride abstraction upon formation of a carbocation, it reacts with halogenated alkanes at the chlorine atom. Two mechanistic scenarios, namely oxidation at the halogen atom or proton transfer are found. Accurate proton affinities for HOOH, NH₂OH, a series of alkanes C_nH_2n+2 (n=1–4), and halogenated alkanes C_nH_2n+1Cl (n=1–4), HCF₃, CF₄, and CF₂Cl₂, were calculated by using the G3 method and are in excellent agreement with experimental values, where available. The G3 enthalpies of reaction are also consistent with the observed products. The tendency for oxidation of alkanes by hydride abstraction is expressed in terms of G3 hydride affinities of the corresponding cationic products C_nH_2n+1⁺ (n=1–4) and C_nH_2nCl⁺ (n=1–4). The hypersurface for the reaction of H₂OOH⁺ with CH₃Cl and C₂H₅Cl was calculated at the B3 LYP, MP2, and G3_m* level, underlining the three mechanistic scenarios in which the reaction is either induced by oxidation at the hydrogen or the halogen atom, or by proton transfer.     

Characterization of N‐Boc/Fmoc/Z‐N′‐formyl‐gem‐diaminoalkyl derivatives using electrospray ionization multi‐stage mass spectrometry

N‐Boc/Fmoc/Z‐N′‐formyl‐gem‐diaminoalkyl derivatives, intermediates particularly useful in the synthesis of partially modified retro‐inverso peptides, have been characterized by both positive and negative ion electrospray ionization (ESI) ion‐trap multi‐stage mass spectrometry (MSⁿ). The MS² collision induced dissociation (CID) spectra of the sodium adduct of the formamides derived from the corresponding N‐Fmoc/Z‐amino acids, dipeptide and tripeptide acids show the [M + Na‐NH₂CHO]⁺ ion, arising from the loss of formamide, as the base peak. Differently, the MS² CID spectra of [M + Na]⁺ ion of all the N‐Boc derivatives yield the abundant [M + Na‐C₄H₈]⁺ and [M + Na‐Boc + H]⁺ ions because of the loss of isobutylene and CO₂ from the Boc protecting function. Useful information on the type of amino acids and their sequence in the N‐protected dipeptidyl and tripeptidyl‐N′‐formamides is provided by MS² and subsequent MSⁿ experiments on the respective precursor ions. The negative ion ESI mass spectra of these oligomers show, in addition to [M‐H]^?, [M + HCOO]^? and [M + Cl]^? ions, the presence of in‐source CID fragment ions deriving from the involvement of the N‐protecting group. Furthermore, MSⁿ spectra of [M + Cl]^? ion of N‐protected dipeptide and tripeptide derivatives show characteristic fragmentations that are useful for determining the nature of the C‐terminal gem‐diamino residue. The present paper represents an initial attempt to study the ESI‐MS behavior of these important intermediates and lays the groundwork for structural‐based studies on more complex partially modified retro‐inverso peptides. Copyright © 2013 John Wiley & Sons, Ltd.     

3‐Rhoda‐1,2‐diazacyclopentanes: A Series of Novel Metallacycle Complexes Derived From CN Functionalization of Ethylene

Rh‐containing metallacycles, [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NR)₂‐]Cl; TPA=N,N,N,N‐tris(2‐pyridylmethyl)amine have been accessed through treatment of the Rh^I ethylene complex, [(TPA)Rh(η²‐CH₂CH₂)]Cl ([ 1 ]Cl) with substituted diazenes. We show this methodology to be tolerant of electron‐deficient azo compounds including azo diesters (RCO₂N?NCO₂R; R=Et [ 3 ]Cl, R=iPr [ 4 ]Cl, R=tBu [ 5 ]Cl, and R=Bn [ 6 ]Cl) and a cyclic azo diamide: 4‐phenyl‐1,2,4‐triazole‐3,5‐dione (PTAD), [ 7 ]Cl. The latter complex features two ortho‐fused ring systems and constitutes the first 3‐rhoda‐1,2‐diazabicyclo[3.3.0]octane. Preliminary evidence suggests that these complexes result from N–N coordination followed by insertion of ethylene into a [Rh]?N bond. In terms of reactivity, [ 3 ]Cl and [ 4 ]Cl successfully undergo ring‐opening using p‐toluenesulfonic acid, affording the Rh chlorides, [(TPA)Rh^III(Cl)(κ¹‐(C)‐CH₂CH₂(NCO₂R)(NHCO₂R)]OTs; [ 13 ]OTs and [ 14 ]OTs. Deprotection of [ 5 ]Cl using trifluoroacetic acid was also found to give an ethyl substituted, end‐on coordinated diazene [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NH)₂‐]⁺ [ 16 ]Cl, a hitherto unreported motif. Treatment of [ 16 ]Cl with acetyl chloride resulted in the bisacetylated adduct [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NAc)₂‐]⁺, [ 17 ]Cl. Treatment of [ 1 ]Cl with AcN?NAc did not give the Rh?N insertion product, but instead the N,O‐chelated complex [(TPA)Rh^I ( κ²‐(O,N)‐CH₃(CO)(NH)(N?C(CH₃)(OCH?CH₂))]Cl [ 23 ]Cl, presumably through insertion of ethylene into a [Rh]?O bond.     

Theoretical investigation on identical anionic halide‐exchange SN2 reaction processes on N‐haloammonium cation NH3X+ (X = F,Cl, Br,and I)

CCSD(T) calculations have been used for identically nucleophilic substitution reactions on N‐haloammonium cation, X^? + NH₃X⁺ (X = F, Cl, Br, and I), with comparison of classic anionic S_N2 reactions, X^? + CH₃X. The described S_N2 reactions are characterized to a double curve potential, and separated charged reactants proceed to form transition state through a stronger complexation and a charge neutralization process. For title reactions X^? + NH₃X⁺, charge distributions, geometries, energy barriers, and their correlations have been investigated. Central barriers ΔE for X^? + NH₃X⁺ are found to be lower and lie within a relatively narrow range, decreasing in the following order: Cl (21.1 kJ/mol) > F (19.7 kJ/mol) > Br (10.9 kJ/mol) > I (9.1 kJ/mol). The overall barriers ΔE relative to the reactants are negative for all halogens: ?626.0 kJ/mol (F), ?494.1 kJ/mol (Cl), ?484.9 kJ/mol (Br), and ?458.5 kJ/mol (I). Stability energies of the ion–ion complexes ΔE_comp decrease in the order F (645.6 kJ/mol) > Cl (515.2 kJ/mol) > Br (495.8 kJ/mol) > I (467.6 kJ/mol), and are found to correlate well with halogen Mulliken electronegativities (R² = 0.972) and proton affinity of halogen anions X^? (R² = 0.996). Based on polarizable continuum model, solvent effects have investigated, which indicates solvents, especially polar and protic solvents lower the complexation energy dramatically, due to dually solvated reactant ions, and even character of double well potential in reactions X^? + CH₃X has disappeared. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Silicon‐ and Tin‐Containing Open‐Chain and Eight‐Membered‐Ring Compounds as Bicentric Lewis Acids toward Anions

Herein, we report the syntheses of silicon‐ and tin‐containing open‐chain and eight‐membered‐ring compounds Me₂Si(CH₂SnMe₂X)₂ ( 2 , X=Me; 3 , X=Cl; 4 , X=F), CH₂(SnMe₂CH₂I)₂ ( 7 ), CH₂(SnMe₂CH₂Cl)₂ ( 8 ), cyclo‐Me₂Sn(CH₂SnMe₂CH₂)₂SiMe₂ ( 6 ), cyclo‐(Me₂SnCH₂)₄ ( 9 ), cyclo‐Me_(2?n)X_nSn(CH₂SiMe₂CH₂)₂SnX_nMe_(2?n) ( 5 , n=0; 10 , n = 1, X= Cl; 11 , n=1, X= F; 12 , n=2, X= Cl), and the chloride and fluoride complexes NEt₄[cyclo‐ Me(Cl)Sn(CH₂SiMe₂CH₂)₂Sn(Cl)Me?F] ( 13 ), PPh₄[cyclo‐Me(Cl)Sn(CH₂SiMe₂CH₂)₂Sn(Cl)Me?Cl] ( 14 ), NEt₄[cyclo‐Me(F)Sn(CH₂SiMe₂CH₂)₂Sn(F)Me?F] ( 15 ), [NEt₄]₂[cyclo‐Cl₂Sn(CH₂SiMe₂CH₂)₂SnCl₂?2 Cl] ( 16 ), M[Me₂Si(CH₂Sn(Cl)Me₂)₂?Cl] ( 17 a , M=PPh₄; 17 b , M=NEt₄), NEt₄[Me₂Si(CH₂Sn(Cl)Me₂)₂?F] ( 18 ), NEt₄[Me₂Si(CH₂Sn(F)Me₂)₂?F] ( 19 ), and PPh₄[Me₂Si(CH₂Sn(Cl)Me₂)₂?Br] ( 20 ). The compounds were characterised by electrospray mass‐spectrometric, IR and ¹H, ¹³C, ¹⁹F, ²⁹Si, and ¹¹⁹Sn NMR spectroscopic analysis, and, except for 15 and 18 , single‐crystal X‐ray diffraction studies.     

Nonlinear Brønsted and Hammett Correlations Associated with Reactions of 4‐Chloro‐7‐nitrobenzofurazan with Anilines in Dimethyl Sulfoxide Solution

The kinetics and mechanism of nucleophilic aromatic substitution reactions of 4‐chloro‐7‐nitrobenzofurazan 1 with 4‐X‐substituted anilines 2a–g (X = OH, OCH₃, CH₃, H, I, Cl, and CN) are investigated in a dimethyl sulfoxide (Me₂SO) solution at 25°C. The Hammett plot of log k₁ versus σ is nonlinear for all the anilines studied due to positive deviations of the electron‐donating substituents. However, the corresponding Yukawa–Tsuno plot resulted in a good linear correlation with σ+r (σ⁺?σ). The corresponding Brønsted‐type plot is also nonlinear, i.e., the slope (β_nuc) changes from 1.60 to 0.56 as the basicity of anilines decreases. These results indicate a change in a mechanism from a polar S_NAr process for less basic nucleophiles (X = I, Cl, and CN) to a single electron transfer for more basic nucleophiles (X = OH, OCH₃, and CH₃). The satisfactory log k₁ versus E^o correlation obtained for the reactions of 1 with anilines 2a–d in the present system is consistent with the proposed mechanism. Interestingly, the β_nuc = 1.60 value measured for 1 in Me₂SO reflects one of the highest coefficients Brønsted ever observed for S_NAr reactions. © 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 152–160, 2013     

Impact of the anion and chalcogen on the crystal structure and properties of 4,6‐dimethyl‐2‐pyrimido(thio)nium halides

By the reaction of urea or thiourea, acetylacetone and hydrogen halide (HF, HBr or HI), we have obtained seven new 4,6‐dimethyl‐2‐pyrimido(thio)nium salts, which were characterized by single‐crystal X‐ray diffraction, namely, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium bifluoride, C₆H₉N₂O⁺·HF₂^? or (dmpH)F₂H, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium bromide, C₆H₉N₂O⁺·Br^? or (dmpH)Br, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium iodide, C₆H₉N₂O⁺·I^? or (dmpH)I, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium iodide–urea (1/1), C₆H₉N₂O⁺·I^?·CH₄N₂O or (dmpH)I·ur, 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium bifluoride–thiourea (1/1), C₆H₉N₂S⁺·HF₂^?·CH₄N₂S or (dmptH)F₂H·tu, 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium bromide, C₆H₉N₂S⁺·Br^? or (dmptH)Br, and 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium iodide, C₆H₉N₂S⁺·I^? or (dmptH)I. Three HCl derivatives were described previously in the literature, namely, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium chloride, (dmpH)Cl, 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium chloride monohydrate, (dmptH)Cl·H₂O, and 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium chloride–thiourea (1/1), (dmptH)Cl·tu. Structural analysis shows that in 9 out of 10 of these compounds, the ions form one‐dimensional chains or ribbons stabilized by hydrogen bonds. Only in one compound are parallel planes present. In all the structures, there are charge‐assisted N⁺—H…X^? hydrogen bonds, as well as weaker C_Ar⁺—H…X^? and π⁺…X^? interactions. The structures can be divided into five types according to their hydrogen‐bond patterns. All the compounds undergo thermal decomposition at relatively high temperatures (150–300 °C) without melting. Four oxopyrimidinium salts containing a π⁺…X^?…π⁺ sandwich‐like structural motif exhibit luminescent properties.     

Mechanistic Origin of Antagonist Effects of Usual Anionic Bases (OH−, CO32−) as Modulated by their Countercations (Na+, Cs+, K+) in Palladium‐Catalyzed Suzuki–Miyaura Reactions

The mechanism of the reaction of trans‐ArPdBrL₂ (Ar=p‐Z‐C₆H₄, Z=CN, H; L=PPh₃) with Ar′B(OH)₂ (Ar′=p‐Z′‐C₆H₄, Z′=H, CN, MeO), which is a key step in the Suzuki–Miyaura process, has been established in N,N‐dimethylformamide (DMF) with two bases, acetate (nBu₄NOAc) or carbonate (Cs₂CO₃) and compared with that of hydroxide (nBu₄NOH), reported in our previous work. As anionic bases are inevitably introduced with a countercation M⁺ (e.g., M⁺OH^?), the role of cations in the transmetalation/reductive elimination has been first investigated. Cations M⁺ (Na⁺, Cs⁺, K⁺) are not innocent since they induce an unexpected decelerating effect in the transmetalation via their complexation to the OH ligand in the reactive ArPd(OH)L₂, partly inhibiting its transmetalation with Ar′B(OH)₂. A decreasing reactivity order is observed when M⁺ is associated with OH^?: nBu₄N⁺> K⁺> Cs⁺> Na⁺. Acetates lead to the formation of trans‐ArPd(OAc)L₂, which does not undergo transmetalation with Ar′B(OH)₂. This explains why acetates are not used as bases in Suzuki–Miyaura reactions that involve Ar′B(OH)₂. Carbonates (Cs₂CO₃) give rise to slower reactions than those performed from nBu₄NOH at the same concentration, even if the reactions are accelerated in the presence of water due to the generation of OH^?. The mechanism of the reaction with carbonates is then similar to that established for nBu₄NOH, involving ArPd(OH)L₂ in the transmetalation with Ar′B(OH)₂. Due to the low concentration of OH^? generated from CO₃^2? in water, both transmetalation and reductive elimination result slower than those performed from nBu₄NOH at equal concentrations as Cs₂CO₃. Therefore, the overall reactivity is finely tuned by the concentration of the common base OH^? and the ratio [OH^?]/[Ar′B(OH)₂]. Hence, the anionic base (pure OH^? or OH^? generated from CO₃^2?) associated with its countercation (Na⁺, Cs⁺, K⁺) plays four antagonist kinetic roles: acceleration of the transmetalation by formation of the reactive ArPd(OH)L₂, acceleration of the reductive elimination, deceleration of the transmetalation by formation of unreactive Ar′B(OH)₃^? and by complexation of ArPd(OH)L₂ by M⁺.     

Bis‐tert‐Alcohol‐Functionalized Crown‐6‐Calix[4]arene: An Organic Promoter for Nucleophilic Fluorination

A bis‐tert‐alcohol‐functionalized crown‐6‐calix[4]arene (BACCA) was designed and prepared as a multifunctional organic promoter for nucleophilic fluorinations with CsF. By formation of a CsF/BACCA complex, BACCA could release a significantly active and selective fluoride source for S_N2 fluorination reactions. The origin of the promoting effects of BACCA was studied by quantum chemical methods. The role of BACCA was revealed to be separation of the metal fluoride to a large distance (>8 Å), thereby producing an essentially “free” F^?. The synergistic actions of the crown‐6‐calix[4]arene subunit (whose O atoms coordinate the counter‐cation Cs⁺) and the terminal tert‐alcohol OH groups (forming controlled hydrogen bonds with F^?) of BACCA led to tremendous efficiency in S_N2 fluorination of base‐sensitive substrates.     

Studies on the effect of the cation nature on the kinetics of the isotopic exchange reaction between chloride ion and O,O-diphenylphosphorochloridothionate in acetonitrile

Rate constants and activation parameters for the isotopic exchange reactions between (PhO)₂PSCl and M³⁶Cl (M = Me₄N⁺, Et₄N⁺, n-Bu₄N⁺, Et₃HN⁺, EtH₃N⁺, Li⁺) in acetonitrile were measured in order to find the effect of the cation nature onthe kinetics of the reaction. The rate constants measured for a range of concentrations of Et₃HN³⁶Cl, EtH₃N³⁶Cl, and Li³⁶Cl were analyzed using the Acree equation. The equivalent conductance of LiCl in acetonitrile was determined. The nature of the cation has no effect on the mechanism of the reaction. The cation changes only the experimental rate constant proportionally to the dissociation degree of the salt. Smaller values of the rate constant and smaller activation parameters ΔH? and ΔS? for the reaction with Li³⁶Cl indicate the existenceof the intermolecular interaction between lithium ions and O,O-diphenylphosphorochloridothionate.     

Calix[4]pyrrole‐Based Heteroditopic Ion‐Pair Receptor That Displays Anion‐Modulated,Cation‐Binding Behavior

A new ditopic ion‐pair receptor 1 was designed, synthesized, and characterized. Detailed binding studies served to confirm that this receptor binds fluoride and chloride ions (studied as their tetraalkylammonium salts) and forms stable 1:1 complexes in CDCl₃. Treatment of the halide‐ion complexes of 1 with Group I and II metal ions (Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, and Ca²⁺; studied as their perchlorate salts in CD₃CN) revealed unique interactions that were found to depend on both the choice of the added cation and the precomplexed anion. In the case of the fluoride complex [ 1? F]^? (preformed as the tetrabutylammonium (TBA⁺) complex), little evidence of interaction with the K⁺ ion was seen. In contrast, when this same complex (i.e., [ 1? F]^? as the TBA⁺ salt) was treated with the Li⁺ or Na⁺ ions, complete decomplexation of the receptor‐bound fluoride ion was observed. In sharp contrast to what was seen with Li⁺, Na⁺, and K⁺, treating complex [ 1? F]^? with the Cs⁺ ion gave rise to a stable, receptor‐bound ion‐pair complex [Cs ?1? F] that contains the Cs⁺ ion complexed within the cup‐like cavity of the calix[4]pyrrole, which in turn was stabilized in its cone conformation. Different complexation behavior was observed in the case of the chloride complex [ 1? Cl]^?. In this case, no appreciable interaction was observed with Na⁺ or K⁺. In addition, treating [ 1? Cl]^? with Li⁺ produces a tightly hydrated dimeric ion‐pair complex [ 1? LiCl(H₂O)]₂ in which two Li⁺ ions are bound to the crown moiety of the two receptors. In analogy to what was seen in the case of [ 1? F]^?, exposure of [ 1? Cl]^? to the Cs⁺ ion gives rise to an ion‐pair complex [Cs ?1? Cl] in which the cation is bound within the cup of the calix[4]pyrrole. Different complexation modes were also observed when the binding of the fluoride ion was studied by using the tetramethylammonium and tetraethylammonium salts.     

NMR Spectroscopy and X‐Ray Characterisation of Cationic N‐Heteroaryl‐Pyridylamido ZrIV Complexes: A Further Level of Complexity for the Elusive Active Species of Pyridylamido Olefin Polymerisation Catalysts

New [(N^?,N,N^?)ZrR₂] dialkyl complexes (N^?,N,N^?=pyrrolyl‐pyridyl‐amido or indolyl‐pyridyl‐amido; R=Me or CH₂Ph) have been synthesised and tested as pre‐catalysts for ethene and propene polymerisation in combination with different activators, such as B(C₆F₅)₃, [Ph₃C][B(C₆F₅)₄], [HNMe₂Ph][B(C₆F₅)₄] or solid AlMe₃‐depleted methylaluminoxane (DMAO). Polyethylene (M_w>2 MDa and M_w/M_n = 1.3–1.6) has been produced if pre‐catalysts were activated with 1000 equivalents of DMAO (based on Al) [activity >1000 kg_PE (mol_[Zr] h mol atm)^?1] or by using a higher pre‐catalyst concentration and a mixture of [HNPhMe₂][B(C₆F₅)₄] (1 equiv) and AliBu₂H (60 equiv). In the case of propene polymerisation, activity has been observed only if pre‐catalysts were treated with an excess of AliBu₂H prior to addition of DMAO, which led to highly isotactic polypropylene ([mmmm]>95 %). Neutral pre‐catalysts and ion pairs derived from their activation have been characterised in solution by using advanced 1D and 2D NMR spectroscopy experiments. The detection and rationalisation of intercationic NOEs clearly showed the formation of dimeric species in which some pyrrolyl or indolyl π‐electron density of one unit is engaged in stabilising the metal centre of the other unit, which relegates the counterions in the second coordination sphere. The solid‐state structure of the dimeric indolyl‐pyridyl‐amidomethylzirconium derivative, determined by X‐ray diffraction studies, points toward a weak Zr???η³‐indolyl interaction. It can be hypothesised that the formation of dimeric cationic species hampers monomer coordination (especially of less reactive α‐olefins) and that addition of AliBu₂H is crucial to split the homodimers.