The oxidation of the methylenediphosphanes 3a,b with HF or Cl 2 to the ylidic bonds. The chloro derivative 14b, and its analogue 17 were used for the synthesis of phosphonium hexafluoroacetone does not lead to the expected dioxaphospholane heterocycles, but yields quantitatively the substituted carbenes. carbodiphosphoranes 5a,b. Compounds 5a,b easily add HCl, It is well-known that the reaction of phosphanes with phanes 3a,b, the reaction mixture was found to consist of starting material 3 and reaction product 5 in equal pro-two equivalents of hexafluoroacetone (HFA) leads to the formation of phosphoranes in which the phosphorus atom portions. The reaction proceeds smoothly at room temperature by slow bubbling of gaseous HFA through a solution is included in dioxaphospholane heterocycles. [1Ϫ4] It turns out that this is not the only possible pathway for this reac-of 3a or 3b in hexane. The 31 P{ 1 H} NMR spectra of the reaction mixtures show the presence of only one compound tion. In one of our previous papers we described the formation of the phosphorus ylide 2 from the reaction of the 5a or 5b, respectively (one singlet at δ ϭ 33 Ϫ 37). The location of the two equivalent protons in the α-positions to methylenephosphinophosphorane 1 with one equivalent of HFA. However, we considered this to be an exception to the CF 3 groups is confirmed by the 19 F-NMR spectra: all fluorine atoms are equivalent and appear as a doublet at the rule as 1 possesses unique chemical properties due to the ability of the λ 5 -phosphorus atom to form zwitterionic δ ϭ Ϫ73.0 with a 3 J HF coupling constant of 6.8 Hz. The HC(CF 3 ) 2 protons display the same coupling constant in structures with tetra-and hexacoordinate phosphorus atoms bearing opposite charges. According to the X-ray the 1 H-NMR spectra, showing a characteristic low field multiplet at δ ϭ 6.2. NMR spectroscopic studies revealed data 2 may indeed have the zwitterionic structure 2Ј. It appears, however, that this reaction is not an exception, an interesting phenomenon: this multiplet of nine slightly broadened lines is a combination of a septet, caused by but has a more common nature. In our preliminary communication we reported an unusual reaction of the 3 J FH coupling, and a triplet caused by 3 J PH coupling, with approximately equal coupling constants. The two marginal bis[bis(dialkylamino)phosphinyl]methanes 3a,b with two equivalents of hexafluoroacetone, which quantitatively lines are of very low intensity and can only be distinguished at high signal-to-noise ratio and amplification. This unex-yields the carbodiphosphoranes 5a,b. The addition of HFA to 3a,b is accompanied by the migration of two protons pected 3 J PH triplet was also found in the 31 P-NMR spectrum of 5a or 5b when measured with the selective decoup-from the PϪCH 2 ϪP unit to the carbon atoms of HFA. Thus the presence of a methylene group between the two ling of the protons of the NR 2 group. This means that the protons of the two HC(CF 3 ) 2 units are equivalent for each phosphorus atoms is a necessary condition for this reaction.
Formally, the mechanism of the formation of the carbo-of the two phosphorus atoms and vice versa. This unusual PϪH coupling is probably accounted for by a rapid ex-diphosphoranes 5a,b can be considered to be an addition of the tautomeric PϪH ylide form 3Јa,b to the carbonyl change between the two protons of the HC(CF 3 ) 2 groups, as in 5Јa,b. Derivatives of 5a,b Ϫ the hydrochloride salts function of HFA. However, it is more likely that the migration of the proton from the PϪCH 2 ϪP unit follows an 7a,b, the tetrafluoroborate 16 and the dichloro-and tetrachloro derivatives 13, 14 Ϫ exhibit the same spectroscopic electrophilic attack by HFA on the phosphorus. This reaction probably consists of two steps with the intermediate phenomenon (see below). It should be noted that the dimethylamino groups of 5a appear in the 1 H-NMR spec-formation of the monoylides 4a or 4b. However, we could not detect these compounds by NMR spectroscopy. If only trum not as a doublet, but as a pseudo-triplet, at δ ϭ 2.59, with a broadened central line which is consistent with the one equivalent of HFA was added to methylenediphos-NMR spectroscopic data of known compounds. The analogous carbodiphosphorane, which has chlorine atoms in-