Arnold Jarczewski

A review of proton transfer reactions between various carbon-acids and amine bases in aprotic solvents

Abstract The subject of proton transfer between carbon acids and nitrogen bases in aprotic solvents is reviewed. Equilibrium and rate constants that characterize such reactions are most often determined utilizing UV–visible spectrophotometry. At ambient temperature reaction rates are sufficiently rapid that fast reaction methods, for example, the stopped-flow and temperature-jump techniques are required in many cases. Variation of the properties of the donor and acceptor reaction pairs enables electronic and steric effects upon thermodynamic and kinetic parameters of proton transfer to be assessed. Determination of the kinetic isotope effect (KIE), i.e. k (protium)/ k (deuterium) led to the conclusion that, under certain circumstances and when the KIE is greater than seven, the proton undergoes reaction with a significant degree of quantum mechanical tunneling, consistent with a theoretical prediction advanced several decades earlier. In fact this aspect may be one of the most significant outgrowths of these studies. Many reactions have been characterized (by tunneling) but rarely are the reacting systems experimentally amenable to obtaining all the experimental criteria that support tunneling. Controversy that has arisen regarding treatment of experimental data and resulting conclusions from them is visited in this review. The structural nature of the product state of reaction is formulated based on spectroscopic evidence, in favorable cases, and probable structures of the transition state can be inferred.

FTIR, NMR and kinetic studies of proton transfer reactions from nitro-substituted diarylmethanes to N-bases with guanidine character

Abstract Deprotonation of bis(2,4-dinitrophenyl)methane (C-acid 1) and 2,4-dinitrophenyl-2,4,6-trinitrophenylmethane (C-acid 2) by 7-methyl-1,5,7-triazabicyclo[4,4,0]dec-5-ene (MTBD) and 1,5,7-triazabicyclo[4,4,0]dec-5-ene (TBD) in acetonitrile has been studied using 1 H and 13 C NMR as well as FT-IR spectroscopy. The 1:1 mixtures of N-bases with C-acids form protonated N-bases and products, for which only one respective structure was found. In the case of deprotonated C-acid 1 a symmetrical charge distribution in the phenyl rings was found, whereas for deprotonated C-acid 2 the charge distribution was asymmetrical. The spectral feature was explained as a strong electrostatic interaction between the product and the protonated N-bases. The kinetics of the proton transfer reaction from C-acid 1 and C-acid 2 to MTBD and TBD in acetonitrile are reported. The activation parameters are discussed in two aspects: the steric effect of the basic centre and the delocalization of charge for the carbanion products.

Homoconjugated hydrogen bonds with amidine and guanidine bases Osmometric, potentiometric and FTIR studies

Five very strong N bases, 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), pK a =23.4; 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),pK a =23.9; tetramethylguanidine (TMG), pK a =23.3; 2-phenyl-tetramethylguanidine (PhTMG), pK a =20.6; and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), pK a =24.97; have been studied by osmometric measurements which showed that they are monomeric in acetonitrile solutions. The constants of the formation of homoconjugated complexes were determined by potentiometric measurements. In the IR spectra of the semi-protonated complexes of DBN, DBU and TMG, the homoconjugated N + –H···NN ···H– + N hydrogen bonds cause broad band complexes in the region 3200–2500 cm -1 instead of the expected continua. This spectral peculiarity is discussed.

Kinetics, isotope effects, and mechanism of the reaction of 1-nitro-1(4-nitrophenyl)alkanes with DBU in acetonitrile

The kinetics of the reaction of O2NC6H4C(L)(R)NO2(R = Me, Et, Pri, NNPE, NNPP, or MNNPP, respectively; L = H, D) with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in acetonitrile (MeCN) are reported. The nature of the product indicates that substantial dissociation into free ions occurs. The usefulness of Benesi–Hildebrand relationship for distinguishing between ion pairs and ions of the product is discussed in detail.The reaction shows low activation enthalpy value ΔH‡= 15.4, 17.8, and 19.9 kJ mol–1 and large negative entropies of activation ΔS‡=–131, –134, and –147 J mol–1 K–1 for NNPE, NNPP, and MNNPP respectively.The kinetic isotope effects kH/kD(12.5, 12.4, and 12.3) are large, showing no variation, the more sterically hindered the substrate. The values of the isotope effects exerted on the activation parameters indicate the contribution of a tunnelling effect QH/QD= 1.35 at 25 °C.The influence of water on the kinetics is also examined and discussed with respect to reliability of kinetic measurements of such reactions systems.

Kinetic and equilibrium studies of the proton and deuteron transfer reaction between diarylcyanomethanes and 1,2-bis-(dialkylaminomethyl)benzene in acetonitrile

Abstract Equilibrium and rate constants, primary deuterium isotope effects, and activation parameters have been determined for the proton-transfer reactions of 4-nitrophenyl-phenyl-cyanomethane and 4-bromophenyl-4-nitrophenylcyanomethane with 1,2-bis(dimethylamino-methyl)benzene and 1,2-bis(diethylaminomethyl)benzene in acetonitrile. The reaction was followed spectrophotometrically using the stopped-flow technique. Large kinetic isotope effects at 25°C k H / k D = 16.8 − 18.8 were observed for studied reactions. The multistep mechanism of the reactions was proposed.

Proton transfer reactions from dimethyl (4-nitrophenyl)malonate to N-bases in acetonitrile

Abstract Deprotonations of dimethyl (4-nitrophenyl)malonate (C-acid) by 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 1,8-bis(dimethylamino)naphthalene (DMAN) in acetonitrile were studied by kinetic as well as FTIR and 1 H NMR spectroscopic methods. In the 1:1 mixture of C-acid with DMAN no proton transfer was found. The N-bases with guanidine-like character deprotonate C-acid in the acetonitrile solution quantitatively. The mechanisms of proton transfer reactions are discussed.

Factors influencing kinetic isotope effects in some proton-transfer reactions in aprotic solvents

Kinetic studies have been carried out on the proton-transfer reactions of (i) 4-nitrobenzyl cyanide (4-NBC) with tetramethylguanidine (TMG) and (ii) 4-nitrophenylphenylcyanomethane (4-NPPCM) with 1,8-bis(dimethylamino)naphthalene (DMAN) in acetonitrile.The reaction (i) has a moderate enthalpy of activation (ΔH‡= 25 kJ mol–1) and a negative entropy of activation (ΔS‡=–98 J mol–1 K–1). The deuterium kinetic isotope effect is large.The reaction (ii) of the more hindered molecule 4-NPPCM and DMAN has similar activation parameters (ΔH‡= 26 kJ mol–1, ΔS‡=-140 J mol–1 K–1), but the kinetic isotope effect is considerably larger; also ΔHD‡-ΔHH‡ is large (13 kJ mol–1) and AH/AD is small, indicating a considerable tunnelling effect. Evidence that the large isotope effects are not artefacts of hydrogen exchange is presented. The tunnelling factor is estimated.

Proton transfer reactions from NH acid to proton sponges in acetonitrile. part 2

Abstract Dissociation constants in acetonitrile and in mixed acetonitrile:water solvent have been determined for the two NH acids 1,8-bis(4-toluenesulphonamido)-2,4,7-trinitronaphthalene and 1,8-bis(4-toluenesulphonamido)-2,4,5,7-tetranitronaphthalene. Deprotonation of 1,8-bis(4-toluenesulphonamido)-2,4,7-trinitronaphthalene by proton sponges such as DMAN and DMAMN have been studied by kinetic and 1 H NMR methods. The kinetic parameters in acetonitrile were defined for proton transfer reactions yielding cations and anions with [N···H···N] + and [N···H···N] − homoconjugated intramolecular hydrogen bonds, respectively.

Proton transfer reactions from NH acid to various N-bases in acetonitrile

Abstract Deprotonation of 1,8-bis(4-toluenesulphonamido)-2,4,5,7-tetranitronaphthalene (TSATNN) by three proton sponges, three cryptands and pentaisopropylguanidine (PiPG) have been studies by FTIR, 1 H NMR and kinetic methods. In the case of the proton sponges the products of the proton transfer reactions were cations and an anion with [NH⋯N] + and [NH⋯N] − homoconjugated intramolecular hydrogen bonds, respectively. The reaction of the cryptands or PiPG with TSATNN yielded an [NH⋯N] − anion with a homoconjugated intramolecular hydrogen bond and the protonated N-bases. The kinetic parameters of these reactions in acetonitrile were explained by the structure of the transition state.

Kinetic and equilibrium studies of the proton and deuteron transfer reaction between bis (2,4-dinitrophenyl)methane and strong nitrogen bases in acetonitrile

Abstract Deprotonation of bis(2,4-dinitrophenyl)methane (C-acid) by tetramethylguanidine (TMG), 1,2-bis(dimethyl-aminomethyl) benzene (DMAMB) and 1,8-bis(dimethylamino)naphthalene (DMAN) in acetonitrile was studied by visible, 1 HNMR and FT-IR spectroscopy. The mixture of TMG with C-acid forms a protonated base and a product, which was different when compared with the mixtures of DMAMB or DMAN with C-acid. The spectral feature was explained as a strong electrostatic interaction between the product and the protonated TMG. Kinetic and equilibrium measurements of the reactions of C-acid with TMG, DMAMB and DMAN were completed and the kinetic isotope effects k H / k D are reported.