Preparations, X-ray crystal structure determinations, and base strength measurements of substituted tritylamines
A range of tritylamines TrNRR′, and 4-methoxy-, 4,4′-dimethoxy-, and 4,4′,4′′-trimethoxy-substituted analogues, have been prepared from (substituted) trityl chloride, bromide, or tetrafluoroborate with ammonia or with amines HNRR′ where R and R′ are hydrogen, alkyl, or aryl. Crystal structures of 4,4′,4′′-trimethoxytritylamine, N-tritylglycine methyl ester, tritylammonium chloride, and N-tritylglycine have been determined. The central C–N bond of tritylamine is not significantly affected by the introduction of p-methoxy substituents into the trityl group, or by N-alkylation, but is lengthened upon protonation of the amino group. Some degree of planarisation of the three C–C bonds to the central carbon of the trityl group is also associated with this C–N bond lengthening. N-Tritylglycine is shown to be a zwitter-ion in the crystalline state and has pI = 6.4 in aqueous acetonitrile. Base strengths of a range of tritylamines have been measured in aqueous acetonitrile. The pKBH+ values (pKa values of the corresponding tritylammonium ions), including ones for N-tritylglycine methyl ester and a range of N-tritylanilines, are remarkable for their similarity at pKBH+ = ca. 9, i.e. characteristic of values for simple alkylamines. It is proposed that the (substituted) trityl group sterically inhibits solvation of the protonated tritylglycine ester cation selectively, and prevents significant resonance interaction between the arene ring and the amino group in the anilines.