The Nature of Cationic Intermediates Derived from α-Thiophosphoryl and α-Thiocarbonyl Mesylates. Neighboring Thiophosphoryl and Thiocarbonyl Participation

Article abstract of DOI:10.1021/jo00351a002

A series of mesylate derivatives of α-hydroxy thiophosphonathes, RCH(OMs)PS(OEt)2 (8), have been prepared.These mesylates can react by k_c processes under solvolytic conditions if cation stabilizing groups such as p-anisyl or p-thioanisyl groups are present.However, under conditions of appropriate electron demand, mesylates 8 can react by k_Δ processes involving thiophosphoryl participation.The cyclic intermediate ions capture acetic acid at phosphorus and lead ultimately to α-thio acetate derivatives of phosphonates.This overall transformation converts the P=S function to the P=O group.Rates of acetolyses of 8 can exceed those of the O-phosphoryl analogues by large factors when k_Δ processes are involved (as expected for anchimerically assisted processes).Mesylate derivatives of α-hydroxy thio esters, R2C(OMs)CSOCH3 (9), can also react by k_Δ processes as shown by enhanced rates relative to simple ester analogues.The substrate (R)-(-)-PhCH(OMs)CSOCH3 (38) gives an acetolysis product that is largely retained, while 9 (R = Me) gave a large fraction of a rearranged product.These data all argue in favor of neighboring thiocarbonyl participation in 8, giving cyclized intermediate cations.The behavior of mesylates 8 and 9 can therefore be quite different from that of the corresponding O-phosphoryl and carbonyl analogues.Neighboring group participation in 8 and 9 accounts for the differences.The contrasting behavior of 8 and 9 also argues against neighboring phosphoryl or carbonyl participation in solvolyses of mesylate derivatives of α-hydroxy phosphonates or α-keto mesylates.