Brønsted Basicities of Diamines in the Gas Phase, Acetonitrile, and Tetrahydrofuran

Abstract

A comprehensive basicity study of alpha,omega-alkanediamines and related bases has been carried out. Basicities in acetonitrile (AN, pK(a) values), tetrahydrofuran (THF, pK(alpha) values), and gas phase (GP, GB values), were measured for 16, 14, and 9 diamine bases and for several related monoamines. In addition the gas-phase basicities and equilibrium geometries were computed for 19 diamino bases and several related monoamines at the DFT B3LYP 6-311+G** level. The effects of the different factors (intrinsic basicity of the amino groups, formation of intramolecular hydrogen bonds, and molecular strain) determining the diamine basicities were estimated by using the method of isodesmic reactions. The results are discussed in terms of molecular structure and solvation effects. The GP basicity is determined by the molecular size and polarizability, the extent of alkylation, and the energy effect of intramolecular hydrogen bond formation in the protonated base. The basicity trends in the solvents differ very much from those in GP: 1) The solvents severely compress the basicity range of the bases studied (3.5 times for the 1,3-propanediamine family in AN, and 7 times in THF), and 2) while stepwise alkylation of the basicity center leads to a steady basicity increase in the gas phase, the picture is complex in the solvents. Significant differences are also evident between THF and AN. The high hydrogen bond acceptor strength of THF leads to this solvent favoring the bases with "naked" protonation centers. In particular, the basicity order of N-methylated 1,3-propanediamines is practically inverse to that in the gas phase. The picture in AN is intermediate between that of GP and THF.

Conformational study of charged cyclohexyldiamines and their gas phase acid–base properties

“…Intramolecular hydrogen bonding has been also found to play an important role in the conformational preferences of monocharged a,x-alkanediamines [10,12,13]. The H-bond parameters and energies are identical in the a,xalkanediamines and CHDAs except when the amino groups occupy the 1,4-positions.…”

“…Regarding charged diamines, some works have been performed in a,x-alkanediamines [10][11][12][13][14][15], but none in cyclic aliphatic diamines.…”

Gas-phase proton affinities of guanidines with heteroalkyl side chains

“…It is well known that the unusually high basicity of proton sponges is significantly influenced by the formation of intramolecular hydrogen bonds [13]. The tendency of diamines and diols to form IMHBs has been recognized for quite some time [14][15][16][17][18], but only recently, several papers of Raczynska et al [19][20][21][22][23], Koppel and coworkers [24] and theoretical investigations of Maksić and coworkers [25][26][27] have shown that the existence of flexible intramolecular hydrogen bonds plays a significant role in the tailoring of organic superbases. A representative example is provided by a guanidine bearing three 3-(N,N-dimethylamino)propyl groups, for which a gas-phase basicity comparable to those of phosphazenes has been predicted [28].…”

Piperazine-Containing Polymer Brush Layer as Supported Base Catalyst in a Glass Microreactor

“…A second-order rate constant of 0.20±0.01/(s M) was calculated. TBD (pK a in acetonitrile=26.03) [32] is a stronger basic organocatalyst than piperazine (pK a in acetonitrile=18.69) [33]. Consequently, when the same reaction was carried out at 65°C and under ambient pressure conditions as reported for the TBD wall-coated glass microreactor [15], the reaction proceeded about ten times slower with a second-order rate constant of 1.18±0.01× 10 −2 /(s M) against 0.10±0.01/(s M), reported in the literature for the TBD-catalyzed reaction [15].…”