Tub and boat–chair forms in a 1,3,2-dioxaphosphocin ring: do C–H⋯O interactions influence the conformations?

A boat–chair conformation is found for the same ring in the thiophosphite [CH2{6-t-Bu-4-Me-C6H2O}2]P(S)H (2), as revealed by X-ray structural analysis, whereas a tub conformation is found for the eight-membered 1,3,2-phosphocin ring in the phosphite [CH2{6-t-Bu-Me-C6H2O}2]P(O)H (1). Compound 1 exists as a weakly bonded dimer through C–H⋯O hydrogen bonding in the solid state. No discernible changes were observed in the 1H NMR spectra in solution (C6D6) for the two compounds in the temperature range 25–70°C, suggesting that the structures are fairly rigid in solution. Optimisation of the structures using AM1 calculations suggests that a boat–chair conformation should be favoured for 1 in the absence of intermolecular interactions.