Synthesis of Tetrathia[7]helicene Mono and Diphosphanes

Tetrathia[7]helicenes (7-TH) are polyconjugated π-systems in which four thiophene rings are orthofused to alternating arene rings to generate a non planar, chiral, stable helix which allows the existence of M and P enantiomers. The 7-TH systems are very interesting structures1 even because they can be easily and selectively functionalized in the alpha positions of the terminal thiophene rings, making it possible the introduction of appropriate substituents.2 In the course of our studies on the synthesis of phosphane derivatives of 7-TH as potential innovative chiral ligands in asymmetric organometallic catalysis3 and organocatalysis, novel 7-TH mono- and diphosphanes 3 have been selectively synthesized (Scheme 1). Scheme 1 Since the poor stability of the 7-TH phosphanes toward air, we have developed an efficient one-pot procedure to obtain air-stable borane-complexes 2 directly from helicene 1 avoiding the isolation of the corresponding free phosphines 3, which can be easily regenerated on heating with alcohol. The new borane-complexes 2 and the corresponding free 7-TH phosphanes 3 have been fully characterized through the standard analytical and spectroscopic methods, and X-ray analysis are in progress.