FULL PAPER Base Strengths of Substituted Tritylamines, N-Alkylanilines, and Tribenzylamine in Aqueous Solution and the Gas Phase: Steric Effects Upon Solvation and Resonance Interactions Moise ´s Canle L., [a] Ibrahim Demirtas, [b] Antonio Freire, [a] Howard Maskill,* [c] and Masaaki Mishima [d] Keywords: Amines / Basicity / Density functional calculations / Solvent effects / Steric effects The dissociation constants of the conjugate acids of N-trityl- acetamide (1h; pK BH + = 3.81) and N-benzyl-N-methyl- 4,4',4''-trimethoxytritylamine (4i;pK BH + = 9.86) have been measured in aqueous acetonitrile at 25 °C and at other tem- peratures to determine the enthalpies and entropies of reac- tion. 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 6 H 4 (o-NO 2 )(1i; 895.0), DMTrNH 2 (3a; 921.3), DMTrNHCH 2 CO 2 Me (3b; 879.1), and DMTrNH(p- NO 2 Bn) (3d; 886.6) have been determined by ICR measure- ments. The GB of TrNHAc corresponds to protonation at oxy- gen and B3LYP/6-31G* calculations indicate that the N-pro- tonated 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 lit- erature 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 in- creases in size are due solely to the increasing polarisability Introduction The trityl group (triphenylmethyl, Tr) and its methoxy- substituted analogues [4-monomethoxy- (MMTr), 4,4'-di- methoxy- (DMTr), and 4,4',4''-trimethoxytrityl (TMTr); [a] Chemical Reactivity and Photoreactivity Group, Department of Physical Chemistry and Chemical Engineering I, University of A Corun ˜a, 15071 A Corun ˜ a, Galicia, Spain [b] Present address: Department of Chemistry, University of Gaziosmanpasa, 60250, Tokat, Turkey [c] Chemistry Department, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK [d] Institute for Materials Chemistry and Engineering, Kyushu University, Hakozaki, Higashi-ku, Fukuoka, Japan Eur. J. Org. Chem. 2004, 5031-5039 DOI: 10.1002/ejoc.200400497  2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 5031 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 ni- trogen 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, TMTrNHCH 2 CO 2 Me (4b;pK BH + = 9.30), ΔH  = 17.9 kJ·mol -1 and ΔS  = -118 J·K -1 ·mol -1 . In contrast, the difference in so- lution base strengths between MeNHAc (pK BH + = -0.56) and TrNHAc (pK BH + = 3.81) is attributed, at least in part, to a re- duced 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 tribenzyl- amine (6) has also been measured (965.2 kJ·mol -1 ) and is un- exceptional; this indicates that the low base strength of 6 in aqueous solution (pK BH + = 4.90 at 25 °C) is a solvation effect which is expressed mainly through an abnormally large pos- itive 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) Figure 1] have been widely used in synthesis as protecting groups for hydroxyl and amino functions. [1] We have re- ported previously that (substituted) tritylamines under acidic conditions, where they are present as the correspond- ing (substituted) tritylammonium cations, undergo deamin- ation reactions, and that these are specific acid catalysed. [2,3] This was an exceptional finding as the tritylammonium cat- ion has no basic site at the reaction centre to accept a pro- ton, and a mechanism involving intermediate ion-molecule pairs was proposed. Methoxy substituents at the para posi- tions of the phenyl groups are, as expected, rate enhancing, and these rate effects are observed in both the catalysed and uncatalysed reaction channels. Remarkably, however, R substituents on the nitrogen have much more dramatic ef- fects upon the deamination rate constants, and these effects