Comparative spectroscopic and theoretical studies on the conformation of some α-diethoxyphosphoryl carbonyl compounds and their α-ethylsulfonyl analogues

Abstract

Comparative ν_CO IR analysis of β-carbonylphosphonates [XC(O)CH₂P(O)(OR)₂: X = Me 1, Ph 2, OEt 3, NEt₂4 and SEt 5; R = Et] (series I) and β-carbonylsulfones [XC(O)CH₂SO₂R: X = Me 6, Ph 7, OEt 8, NEt₂9 and SEt 10; R = Et] (series II) along with ab initio 6-31G** calculations on 1a and 6a (R = Me) suggest the existence of only a single gauche conformer for series I. The negative carbonyl frequency shifts for both series follow approximately the electron-affinities of the π*_CO orbital of the parent compounds MeC(O)X 11–15. The less positive asymmetric sulfonyl frequency shifts (Δν_SO2) for II in relation to the phosphoryl frequency shifts (Δν_PO) for I and the larger negative carbonyl frequency shifts for II with respect to the corresponding values for I are in line with the upfield ¹³C NMR chemical shifts of the carbonyl carbon for II compared to I. These trends agree with the shorter O_(SO2)⋯C_(CO) contact in comparison with the O_(PO)⋯C_(CO) one and are discussed in terms of O_lp→π*_CO charge transfer and electrostatic interactions, which are stronger for series II than for I, indicating that the sulfonyl oxygen atom is a better electron donor than the phosphoryl oxygen atom. Intrinsic geometrical parameters of OS–CH₂ and OP–CH₂ moieties seem to be responsible for this behaviour as indicated by X-ray study and ab initio calculations of dialkyl (methylsulfonyl)methylphosphonate MeSO₂CH₂P(O)(OR)₂ (R = Et 18, Me 18a).