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 th ree 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-io n in the crystalline state and has pI = 6.4 in aqueous acetonitrile. Base s trengths of a range of tritylamines have been measured in aqueous acetonitr ile. The pK(BH+) values (pK(a) values of the corresponding tritylammonium i ons), including ones for N-tritylglycine methyl ester and a range of N-trit ylanilines, are remarkable for their similarity at pK(BH+) = ca. 9, i.e. ch aracteristic of values for simple alkylamines. It is proposed that the (sub stituted) trityl group sterically inhibits solvation of the protonated trit ylglycine ester cation selectively, and prevents significant resonance inte raction between the arene ring and the amino group in the anilines.