THE TRIS(2-THIENYL)METHYL CATION PROBLEM - A NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPIC STUDY

A series of homologues of the trityl cation obtained by total or partial substitution of the phenyl rings for 2- or 3-thienyl groups has been studied by H-1 and C-13 NMR. In general, the spectral parameters found follow the expected trends. However, a most unexpected effect is found in the transformation of tris(2-thienyl)carbinol (1d) into tris(2-thienyl)methyl cation (2d), since the increase of the total chemical shift (DELTA-SIGMA-delta) associated with the ionization is as low as 105 ppm. To account for this observation, several hypotheses are discussed including the possibility of through-space charge delocalization by the three sulfur atoms and the interaction of the thiophene rings in an alternate conformation.