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.