Conformational analysis of six-membered ring dioxaphosphinanes. Part 1: Anancomeric thiophosphates

A study of the conformation of a series of anancomeric axial and equatorial 2-aryloxy-2-thio-1,3,2λ5-dioxaphosphinanes in solution and solid state is reported. In accord to the stereoelectronic theory, aryl thiophosphates substituted with electron-withdrawing (EW) groups will tend to occupy axial positions in chair ring conformations due to the stabilizing endo-anomeric (nπO–σP–X*) hyperconjugative interaction. The antiperiplanar orientation of the orbitals involved in the stereoelectronic interaction is a requirement that is fulfiled in the axial series of compounds when the ring adopts a chair conformation. Therefore, in the equatorial series of thiophosphates, the axial seeking characteristics of aryloxy-EW groups might render the molecule with distortion of the chair conformation. An opposite trend is anticipated for the less axial seeking aryl thiophosphates substituted with electron releasing (ER) groups. A detailed analysis of the 3JHH, 3JPH and 3JCP coupling constants allowed us to conclude that there is no contribution of high energy twist-boat conformations in the equatorial thiophosphates substituted with aryl-EW groups in solution. In consequence, single chair conformations were found in solid state for aryl thiophosphates in both configurations. X-ray geometrical analysis of bond distances and bond angles supports clearly the participation of hyperconjugative endo-anomeric (nπO–σP–OAr*) effect in the stabilization of axial series of compounds and the participation of endo-anomeric (nπO–σPdouble bond; length as m-dashS*) effect in the stabilization of the equatorial thiophosphates in chair conformations.