Oxidative dimerization of diethyl 3-thienylmalonate by high valent metal salts. Synthesis of benzo[1,2-b:4,5-b']dithiophene derivatives

The oxidation of diethyl 3-thienylmalonate (1) by metal oxidants (Fe(ClO4)3, Mn(OAc)3, MnO2 and CuO) in various solvents at 60 °C affords dimerization products arising from side-chain and nuclear coupling of the intermediate delocalized malonyl radicals 6. Metal to sulphur binding is suggested to play a role in controlling the distribution of dimers 2 – 5. The higher thermodynamic stability of unsymmetric dimer 3, along with its oxidative intramolecular 1,6-cyclization to 4, allows to develop a new simple synthesis of benzo[1,2-b:4,5-b′]dithiophene derivatives 15–18. High valent metal salt oxidation of 1 affords 2, 3, 4 and 5 depending on the metal. The process combined with oxidative homolytic aromatic substitution to 4 represents a new approach to the synthesis of benzo[1,2-b:4,5-b′]dithiophenes 15–18