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European Journal of Organic Chemistry, no.24, pp.5031-5039, 2004 (SCI-Expanded)
The dissociation constants of the conjugate acids of N-tritylacetamide (1h; pKBH+ = 3.81) and N-benzyl-N-methyl-4,4′, 4″- trimethoxytritylamine (4i; pKBH+ = 9.86) have been measured in aqueous acetonitrile at 25 °C and at other temperatures to determine the enthalpies and entropies of reaction. For 1h, ΔH⊖ = 40.7 kJ·mol-1 and ΔS⊖ = 64 J·K -1·mol-1, and for 4i ΔH⊖ = 9.1 kJ·mol-1 and ΔS⊖ = -159 J·K-1·mol-1. In addition, gas-phase base strengths at 25 °C (GB values in kJ·mol-1) of TrNH 2 (1a; 902.1), TrNHPh (1c; 926.3), TrNHAc (1h; 929.7), TrNHC 6H4(o-NO2) (1i; 895.0), DMTrNH2 (3a; 921.3), DMTrNHCH2CO2Me (3b; 879.1), and DMTrNH(p-NO 2Bn) (3d; 886.6) have been determined by ICR measurements. The GB of TrNHAc corresponds to protonation at oxygen and B3LYP/6-31G* calculations indicate that the N-protonated isomer is 46.4 kJ·mol-1 less stable, i.e. the GB value for N-protonation is 883.3 kJ·mol-1. Correspondingly, the literature GB value of 857.6 5 kJ·mol-1 for N-methylacetamide corresponds to protonation at oxygen, and B3LYP/6-31G* calculations indicate that the N-protonated isomer is 58.1 kJ·mol-1 less stable, i.e. the GB value for N-protonation of MeNHAc is 799.5 kJ·mol-1. The GB of PhNH(tBu) (5; 920.1 kJ·mol-1) has been measured and compared with values for other N-alkylanilines, PhNHR, including PhNHTr; the results indicate that the increasing GB values as R increases in size are due solely to the increasing polarisability of R. This indicates that the increasing solution base strength of PhNHR as R increases in size is a solvation effect and is not due to decreasing resonance interactions between the nitrogen lone-pair and the phenyl ring. Similarly, the base-strengthening effect in solution of the (substituted) trityl in TrNHZ, where Z is an alkyl with an electron-withdrawing group, is shown to be due to solvation phenomena as it is absent in the gas phase; for one such compound, TMTrNHCH2CO2Me (4b; pKBH+ = 9-30), ΔH⊖ = 17.9 kJ·mol-1 and ΔS⊖ = -118 J·K-1·mol -1. In contrast, the difference in solution base strengths between MeNHAc (pKBH+ = -0.56) and TrNHAc (pKBH+ = 3.81) is attributed, at least in part, to a reduced base-weakening resonance interaction between the lone pair on N and the acetyl group in TrNHAc, as the effect is also evident in the gas phase. The GB value for tribenzylamine (6) has also been measured (965.2 kJ·mol-1) and is unexceptional; this indicates that the low base strength of 6 in aqueous solution (pKBH+ = 4.90 at 25 °C) is a solvation effect which is expressed mainly through an abnormally large positive entropy of reaction (ΔS⊖ = 76 J·K-1·mol-1), the value of ΔH ⊖ (50.5 kJ·mol-1) being only slightly larger than normal for tertiary amines. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.