Wlodzimierz Galezowski

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.

The influence of common cation BH+ on the products of reactions between C-acids and strong guanidine bases in acetonitrile solvent

The electronic spectra of the products of reactions between C-acids (4-nitrophenylcyanoalkanes, 4-nitrophenylnitroalkanes and trinitrotoluene) and 1,1,3,3-tetramethylguanidine (TMG), pentamethylguanidine (PMG) and MTBD bases (B) have been recorded in B/BH+ buffers in acetonitrile at 25 °C. Large BH+ concentrations had little or no effect on the spectra of the nitrile or trinitrotoluene anions, while the spectra of nitroalkane anions were shifted to the blue by some 50–75 nm. These shifts were ascribed to the formation of hydrogen-bonded ion pairs. In the case of TMG or PMG reactions two types of ion pairs are formed due to homoconjugation of BH+ cations, which is why an isosbestic point is not observed, while MTBD reactions are free from such complications. The spectra of some ion pair products and ion pair dissociation constants were calculated.

Equilibrium and kinetic study of the proton transfer reactions between nitroalkanes and strong organic bases — phosphazenes in tetrahydrofuran solvent

AbstractProton transfer reactions rates between carbon acids 1-nitro-1-(4-nitrophenyl)ethane (NPNE), 2-methyl-1-nitro-1-(4-nitrophenyl)propane (MNPNP)) and phosphazenes (BEMP, BTPP, P1-t-Oct) in tetrahydrofuran have been measured, and the activation parameters were determined. The results are compared with those previously obtained for P1-t-Bu phosphazene, guanidines and amidines.