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

Abstract

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. © 2001 Elsevier Science Ltd. All rights reserved.