Biaxial Recognition: Towards Chiral Resolution of Allene-Dicarbonyl Compounds by Iodolium-Based Halogen Bond Donors

The coordination of allene dicarbonyl compounds to cyclic iodonium salts is presented as a new biaxial halogen bonding recognition motif. 1H¬ NMR titrations and isothermal titration calorimetry (ITC) of such adducts were performed, resulting in a binding constant as high as 6.9·10^5 M-1 for the complex of dibenzo[b,d]iodolium with an allene diamide. The biaxial binding was confirmed via the comparison of the non-functionalized iodolium with variants in which the halogen bonding axis are blocked by methyl groups, and it was further corroborated by theoretical calculations and a crystal structure analysis. Furthermore, we could experimentally demonstrate the stabilizing effect of the halogen bond donor donor on an allene diamide. In order to differentiate (and eventually separate) the enantiomers of the allene, chiral iodolium diamide derivatives (with one or two chiral side-arms) were synthesized. 1H NMR experiments with these halogen bond donors strongly suggest chiral recognition of allenes.