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Phosphonic Acid Anhydrides [R P02]„: Oligomerization and Structure

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Phosphonic Acid Anhydrides [R P02]„: Oligomerization and Structure Phosphonic Acid Anhydrides [RP02]„: Oligomerization and Structure Stefan Fuchs, Hubert Schmidbaur* Anorganisch-chemisches Institut der Technischen Universität München, Lichtenbergstraße 4, D-85747 Garching, Germany Z. Naturforsch. 50b, 855-858 (1995); received November 11, 1994 Organophosphonic Acid Anhydrides, Phosphonic Acid Anhydrides, Stereochemistry, NMR Spectra The phosphonic acid anhydrides of the general formula [RP02]„ have been prepared with R = Me, Et, z-Pr, r-Bu, and Ph from the corresponding phosphonic acids and their chlorides and esters. Mass spectrometric data indicate that the trimers are the dominant oligomers for all five systems. According to their NMR spectra, the methyl and r-butyl compounds have a symmetrical (C3v) structure with equivalent RP groups, while the ethyl, /-propyl and phenyl homologues have the Cs structure with non-equivalent PR groups in the ratio 1:2. Introduction reference treatises on Organophosphorus Chemis­ Phosphonic acid anhydrides have been known try [3-5], for at least a century. In 1892 A. Michaelis re­ ported the synthesis of “Phosphenylsäure”, which Preparation and Properties of the Compounds we now classify as phenylphosphonic acid PhP02H, as the hydrolysis product of “Phosphe- Conventional methods for anhydride prepara­ nylchlorid” (dichlorophenylphosphine and/or di- tion were used for the synthesis of the title com­ chlorophenylphosphine oxide PhP(0)Cl2). Con­ pounds. These included the condensation of phos­ densation of these two components gave phonic acid dichloride and phosphonic acid “Phosphinobenzol”, most probably phenylphos­ dimethyl ester (R = Me), hydrolysis of phosphonic phonic acid anhydride [1]. This product remained acid dichlorides (R = Et), and reaction of a phos­ poorly characterized even up to the present time, phonic acid with its dichloride (R = /-Pr, r-Bu and and neither the degree of oligomerisation nor the Ph). The yields were generally quite acceptable, structure have been determined. Most surprisingly and elemental analyses gave satisfactory data. this is also true for the alkyl homologues, in spite However, according to spectroscopic data the pu­ of their wide-spread usage as condensation and rity of the products is poor in most cases (for de­ dehydration reagents [2]. We have therefore initi­ tails see the Exp. Part below and references ated a study of the title compounds in order to [6-8]). elucidate further their structure and properties. (M eP02)„ is a colourless solid, m.p. 168-175 °C, Contrary to expectations, this proved to be an which is insoluble in most standard organic extremely difficult task. None of the members of solvents, with the exception of dimethylformamide a series of five compounds could be obtained in and dimethylsulfoxide. All attempts to purify the satisfactory purity, none of them could be crystal­ compound by crystallization or distillation were lized, and analytical and spectroscopic data gave unsuccessful. ambiguous results in more than one case. Our re­ (E tP 0 2)„ is obtained as a yellowish, resinous sults are summarized in this Note, because even material after distillation (with decomposition) at the new limited data are meaning a small progress 210 °C/0.1 Torr, which is soluble in chloroform and leading out of the complete lack of information, dichloromethane, but could not be crystallized. as one would describe the present situation in all The purity is unsatisfactory as checked by NMR spectroscopy (below). (/-PrP02)„ is a colourless, resinous solid, b.p. 205 °C/0.1 Torr, readily soluble in benzene and * Reprint requests to Prof. Dr. H. Schmidbaur. chloromethane solvents. 0932-0776/95/0600-0855 $ 06.00 © 1995 Verlag der Zeitschrift für Naturforschung. All rights reserved. 856 St. Fuchs-H . Schmidbaur • Phosphonic Acid Anhydrides (f-BuPO?),, is a white powder (from toluene/ pentane), m.p. 194-195 °C, soluble in benzene and chloromethane solvents. (PhP02)„ is a white powder with a broad melt­ ing range (120-180 °C), soluble in dimethylsulfox- ide. The literature melting point (209-212 °C) could not be reproduced. Mass Spectrometric Data Mass spectra with chemical ionization were ob­ tained for all five compounds. In each case the B(CJ mono-ions of the trimers have been detected as the parent peaks in high relative intensity. These experiments thus confirm assumptions made in most previous papers considering phosphonic acid structure. The resonances of the proton-coupled anhydrides. Surprisingly, the mass of the hexamers spectra show the usual second order splittings ow ­ also showed up in low relative intensity (ca. 1%) ing to coupling between magnetically non-equiva­ in the spectra of the compounds. It is not clear if lent nuclei in (A„X)V spin systems. Representative this result indicates some “secondary” association examples are shown in Figures 1 and 2. of the compound in the condensed phase, or if the The ethyl, /-propyl, and phenyl compounds ex­ conditions prevailing in the mass spectrometer in­ hibit NMR spectra readily assigned to structures duce and favour short-lived associations. Minor with non-equivalent PRO? units in the relative amounts of a tetramer have been observed only in abundance of 2:1, as expected for a Cs-configura- the mass spectra of the ethyl and isopropyl com­ tion. The {‘H p'P NMR spectra show AB2 spin sys­ pound. There was no evidence for these higher tems with similar 2/(P -P ) coupling constants for oligomers in the NMR spectra of solutions of all three examples. The proton and carbon spectra these compounds, however, and it therefore ap­ have the corresponding multiplet features, details pears that only small non-equilibrium amounts are of which are given in the Experimental section. present in the solid materials obtained directly A few experimental observations are note­ from the reaction mixtures, which are difficult to worthy. The primary product of the phenylphos- purify. phonic acid anhydride initially showed a singlet With the trimers as the dominant species, the phosphorus resonance, which slowly transformed mass spectral data suggest six-membered ring into the A?B doublet/triplet pattern upon storage structures, which can appear in two different con­ of the samples or their solutions at ambient tem- figurations, for each of which two different chair conformations are possible (A,B). The first struc­ ture has threefold symmetry in both conforma­ tions (point group C3v), while the other has a mir­ ror plane for both conformations (point group Cs). Boat or twist conformations are not considered here, because they are likely to be high energy components of the ring inversion equilibria in solution. NMR Spectroscopic Data The methyl and the f-butyl compound have been found to show only a single set of resonances in their {'Hp'P, 'H and {1H}13C NMR spectra indica­ Fig. 1. {'H}13C NMR spectrum of (MePO->)3 in DMSO- tive of equivalent R P02 units in the rings of a C3v d6 at 25 °C. St. Fuchs-H. Schmidbaur • Phosphonic Acid Anhydrides 857 LiAlH4, molecular sieves), distilled, and stored under nitrogen. All glassware was heated to 150 °C, evacuated and filled with nitrogen. - NMR: JEOL GX 400. References: Tetramethylsi- lane for ‘H and 13C spectra, external H3P 04 (85% in H20) for 31 P. The spectra were recorded at 25 °C. - MS: Varian M AT90 (CI, Isobutane), Var- ian MAT311 A (El, 70 eV). - Elemental analysis: Microanalytical laboratory of this Institute (M. Barth). - Melting points: Electrothermal IA 9200 (not corrected). Reagents were commercially available or prepared according to the cited 4 3 2 ppm literature. Fig. 2. {1 H}31P NMR spectrum of (P hP 02)3 in CDC13 Preparation o f the compounds at 25 °C. (MeP02)n [6]: Dimethyl methylphosphonate was slowly added to equivalent quantities of methylphosphonic dichloride (up to 5% excess) at perature. This result may indicate that the more 70 °C. Stirring was continued for 4 h and excess symmetrical structure may be kinetically con­ phosphonic chloride removed under reduced pres­ trolled and may not represent the ground state sure. A colourless, microcrystalline solid was ob­ structure. Such phenomena may also explain the tained after crystallization from dichloromethane/ difficulties experienced in attempts to purify the hexane. - Yield 68%, m.p. 168-175 °C. 'H NMR (d6-DMSO): ö = 1.49 (m, A 3XX '2A '6 or (A 3X)3). materials by standard techniques. The presence of 13C NMR (d6-DMSO): (3 = 13.95 (m, AXX'2, small amounts of other oligomers (above) is an­ '/(PC ) = 147.5, 3/(PC ) = 3.2 Hz). ^P^H } NMR other source of ambiguities regarding the physical (d6-DMSO): (3 = 18.8 (s). 31P NMR (d6-DMSO): properties of the systems. (3 = 18.8 (m, X-part of (A3X)3). CI-MS: m/z (%): 234.9 (100) [M+H+], 219.9 (1.5) [M+H+-C H 3j. Conclusions C3H90 6P3 (234.02) Although the present results have delineated Calcd C 15.40 H 3.88 P 39.71%, the basic configurations of a few phosphonic acid Found C 15.53 H 4.07 P 39.21%. anhydrides, they do not allow any assignments of (EtP02)n: Water was carefully added to ethyl- the analytical and spectral data to specific confor­ phosphonic dichloride (molar ratio 1:1) with vig­ mations (chair/boat/twist rings; axial/equatorial orous stirring in such a way that the internal tem­ groups R). Based on steric arguments we propose, perature did not exceed 50 °C. Stirring at ambient however, that the t-butyl compound should have a temp, was continued for 1 h and excess phospho­ symmetrical structure A with three equatorial t- nic chloride removed under reduced pessure. - Yield 100%, yellowish resin, b.p. 210°C/0.1 Torr butyl groups. For the i-propyl, ethyl and phenyl (with dec.). *H NMR (CDC13): (3 = 1.05-1.35 (m. compound mirror symmetry structures B appear A3 of A3B?XY2, 9H), 1.75-2.35 (m, B2 of to be valid, with rapidly interconverting chair con­ A3B,XY2, 6H).
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