Tub and boat-chair forms in a 1,3,2-dioxaphosphocin ring: do C-H center dot center dot center dot 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 analys is, whereas a tub conformation is found for the eight-membered 1,3,2-phosph ocin ring in the phosphite [CH2{6-t-Bu-Me-C6H2O}(2)]P(O)H (1). Compound I e xists as a weakly bonded dimer through C-H . . .O hydrogen bonding in the s olid state. No discernible changes were observed in the H-1 NMR spectra in solution (C6D6) for the two compounds in the temperature range 25-70 degree sC, suggesting that the structures are fairly rigid in solution. Optimisati on of the structures using AM1 calculations suggests that a boat-chair conf ormation should be favoured for 1 in the absence of intermolecular interact ions. (C) 2001 Elsevier Science Ltd. All rights reserved.