Abstract-The salient features of the l9F and lH NMR spectra of seven steroidal phosphorofluoridates are presented and evidence for restricted rotation, obtained from high temperature studies and deuteration experiments, is discussed.THE l9F NMR spectra of the steroidal phosphorofluoridates1 and phosphoramidofluoridatel shown in Table 1 fall into two categories. Compounds 1 to 4 show four signals for the fluorine nucleus whereas the others 5 to 7 give rise to two only. The four signals in 1 to 4 appear as a pair of doublets, the separation of each arm varying from 0.1 1 to 0.20 ppm (see 6 in Table 1). Typically, compound 4 has spectra displaying the following couplings to fluorine: (i) J(I,,F) 959 Hz; (ii) J(F,NMe2) 1.9 Hz (from IH spectrum).2 As a consequence of coupling between fluorine and the NMe,. protons the signals in the 19F spectrum are broadened slightly.On heating the phosphoramidofluoridate (4) in deuteriobenzene (or in DMSO-d,) from 23 "C to 82 "C the arms of each doublet in the ISF spectrum coalesce. The resulting singlets have the same chemical shift (relative to TFA) as the upfield arm of the room temperature doublet. Identical pairs of doublets are observed on cooling again.It seems most likely that two rotamers are present in compounds 1 to 4. To our knowledge this is the first reported example of restricted rotation in phosphorofluoridates which has been detected by I9F NMR. The chemical shift of the high temperature singlet coincides with one of the room temperature rotamers which suggests that on warming, one rotamer is converted to the other rather than formation of a third (new) rotamer. This explanation for the multiple fluorine signals implies that the proton bearing groups (OCH,, N(CH,),, OCH,CH,) may also be non-equivalent. Such nonequivalence has been discerned in the IH NMR spectra of the closely similar tertiary butylphosphonamidofluoridate,, several phosphine ~x i d e s ,~ phosphonate~~ and phosphonothioates.G However, no such proton non-equivalence is seen in the l H spectrum of 4 either at room temperature or in deuteriobenzene or DMSO-d6 at 8O"C, nor after addition of the chemical shift reagent Eu(fod),. If the assumption is made that effects which change the chemical shifts of fluorine signals produce proportionately the same change in the shifts of proton signals, such changes are more likely to be seen in the 19F spectrum than the IH spectrum owing to the greater frequency range (-150 ppm) over which resonance occurs for fluorine compared to that for protons (-10 ppm). Such an 'amplification factor' would enable the detection of smaller energy differences with fluorine than with proton nuclei.Examination of the l H N M R spectrum of N , Ndimethyl-2,2,4, 4-tetradeuterio-5a-cholestan-3/3-ylphosphoramidofluoridate (4-4) reveals a single coupling of 7 Hz between the 3u proton and phosphorus. If there are
