Synthesis of unsymmetrically substituted TTF derivatives via the 'phosphonate way'. Limitations due to the instability of some metalated 1,3-dithiol-2-yl-phosphonates

The poor results in the syntheses of unsymmetrically substituted TTF derivatives carried out with mono or unsubstituted diethyl 1,3-dithiol-2-yl-phosphonates is here explained by the existence of an equilibrium between the cyclic and open forms of their corresponding carbanionic salt, with possible dimerisation of the open form. The monomeric cyclic form can react with a high reactive iminium salt, affording then the corresponding TTF derivative in moderate yield.