Synthesis, Characterisation, and Organocatalytic Activity of Chiral Tetrathiahelicene Diphosphine Oxides

Two new chiral tetrathia[7]helicene (7-TH) based tertiary phosphine oxides (±)-2b and (±)-2c, bearing two substituents on the phosphorus atoms with different steric and electronic properties, have been synthesised and fully characterised by means of analytical and spectroscopic techniques. The resolution of (±)-2a–c into their antipodes was accomplished by HPLC separation on a chiral stationary phase, and their chiroptical properties were investigated by CD spectroscopy. The behaviour of 2a–c as organocatalysts was assessed in representative reactions mediated by tri- or tetrachlorosilane. 7-TH-based phosphine oxides 2a and 2b promoted both ketoimine reduction and stereoselective carbon–carbon bond formation in good chemical yields and diastereoselectivity, albeit with low enantioselectivity, thus opening the way to the development of a novel class of enantiopure helical-based phosphorus organocatalysts.