Comparative spectroscopic and theoretical studies on the conformation of some α-diethoxyphosphoryl carbonyl compounds and their α-ethylsulfonyl analogues 1

Comparative νCO 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 (ΔνSO2) for II in relation to the phosphoryl frequency shifts (ΔνPO) 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 Olp→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).