Hydrolysis of phosphoranes containing a perfluoropinacolyl ring

Hydrolysis of phosphoranes containing a perfluoropinacolyl ring1 ALEXANDER F. JANZEN, ALBERTA E. L E M I I ~RONALD E, K I R KMARAT,A N D ALANQUEEN Dc,prlrrrrlolr r!/' Clzollisrr:\.. U r ~ i ~ ~ e r ~o?/'MrrrliroOrr, .sir~ Wirlrripeg, Mrrrr.. Crrrrrrrlrr K i 7 ' 2 N 2

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Kcccivccl January 26. I983

The liyclrolysis of phosphorancs PhzP(pfp)CI, Pli,P(pfp)OR, PhP(pfp)(Oli),. ancl (liO)iP(pfp), containing a pcsfluosopinacolyl (pfp) ring, occurs under neutral, basic. or acidic conciitions anel lcacls to thc formation ol' acyclic pcrfluoropinacolyl products, i.c. R,P(0)OC(CFi),C(CF3)OH. as clctcrminccl by 'H. '"F, ' P . I3C nmr and ir studies. In tlic prcscncc of a basc, all trifluoromctliyl groups in Ph2P(0)OC(CF3)2C(CFi)20H arc cquilibratcd, as vcrificcl by '"F and '.'C nmr: anel from the vxiablc tcrnpcraturc '"F nmr linc-shapc analysis tlic activation parameters for this exchange proccss wcrc found to bc: E,, = 36.0 + 3 kJ rnol ' , A H = 33.5 2 3 kJ mol- I, AS' = -86 + 8 J K ' rnol '. AG&, = 59.0 2 0.6 kJ mol ' . The rcnctions of phosphorancs with HF wcrc brictly invcstizatcd. ALEXANDERF. J A N Z E N , ALBERTAE. LEMIRE, RONALDK I R K M I \ R A ~ct' ALANQUEEN. Can. J . Chc~n.61. 2264 (1983). L'hydrolysc dcs phosphorancs: Ph,P(pfp)Cl. PlizP(pfp)OR,P l i P ( p f ~ ) ( 0 Rct) ~(RO)3P(pfIp)contcnant Ic cyclc pcrfluoropinacolylc (pfp) sc produit cn milicu ncutrc, acidc ~ L basiquc I et conduit i la formation dc pcrtl~~oropinacolylcs acycliqucs i.c. R2P(0)OC(CF,)zC(CF3)zOH commc I'indiqucnt la rmn du 'H. du '"F, clu "P. clu "C ct Ics Ct~lclcsen is. La rmn du "'F ct du I3 C rdvblc qu'cn milicu basiquc tous lcs groupcs trifluorornCthylc clu PliLP(O)OC(CFi),C(CF3)~OH sont CquilibrCs. L'analysc dc In formc dcs saics dc la rmn clu '"F i ternpdraturc variable donnc Ics parametres d'activation suivants: E;, = 36.0 + 0.3 kJ mol ' , A H F = 33.5 k 3 kJ mol ', A S = = -86.8 J K ' rnol '. A G = = 59.0 + 0,6 kJ rnol ' . On discutc britvement des rkactions dcs pliosphorancs avcc Ic HF. [Tratluit par lc j o ~ ~ s n a ] ]

The hydrolysis of phosphoranes containing a five-membered ring may produce either an acyclic phosphoryl compound A or a cyclic hydroxyphosphorane B. In some instances, an equilibrium between isomers A and B has been observed and the equilibrium may be shifted by varying the temperature or the

base concentration (1 - 5 ) . Such equilibria, of interest because of their role in the hydrolysis of phosphorus esters in organic and biological systems, may be studied by "F nmr since the trifluoromethyl groups are often equivalent in cyclic perfluoropinacolyl phosphoranes, but non-equivalent in acyclic analogues. Recent studies with Me2P(0)OC(CF,),C(CF,)zOH have demonstrated that I9F nmr can be used to monitor an equilibrium between cyclic and acyclic phosphorus compounds

Although 1 gave a single fluorine nmr peak in the pfp region (pfp = -OC(CF3)2C(CF3)20-) and ir bands characteristic of a cyclic pfp derivative, our evidence did not disting~~ish between an acyclic A or cyclic B structure for the hydrolysis product. More recently, as part of a kinetic study of the hydrolysis of PhP(pfp)[OCH(CF,)z]2(8), we required structural information about the hydrolysis procluct; consequently, we have re-examined the synthesis and hydrolysis of 1. and also studied the hydrolysis of a variety of other perfluoropinacolyl phosphoranes using "F, j1P, and 13C nlnr to distinguish between acyclic and cyclic products and using dynamic nmr techniques to investigate the equilibrium between acyclic A and cyclic B products. Phosphoranes 1-8 were prepared froin hexatluoroacetone and the corresponding phosphines and identified by a combination of nmr, ir, ms, and elemental analysis. The chlorophosphorane 1 , although stable in a sealed tube, hydrolyzed rapidly and was conveniently identified by its reaction with (CF,)?CHOH to give 2.

(6). In some early work, we prepared the perfluoropinacolyl derivative 1 from chlorodiphenylphosphine and hexafluoroacetone and observed its rapid hydrolysis to Ph,P(pfp)OH (7).

'Presented at the 63rd C.I.C. Conference, Ottawa, Ont., Canada, June 8- 11. 1980.

Those ir bands previously found to be characteristic of cyclic pfp derivatives (9) are present in phosphoranes 1-8 (Table 1 ). The fluorine nmr spectra of 1-8 gave single peaks in the pfp region, thus showing that the trifluoromethyl groups are equivis also evialent in the cyclic phosphoranes. This eq~~ivalence dent in the carbon ninr spectra of 2 and 6 (Table 2) which show only one CF3 and one 0-C peak assigned to the pfp ring. Other perfluoropinacolyl phosphoranes also show this equivalence in their carbon nmr spectra (10). The hydrolysis of cyclic phosphoranes 1, 3-8 was carried out under neutral, basic, or acidic conditions to give products 9-11. In some instances, further reaction occurred and free

2265

ET AL.

TABLE1 . Infrared bands ( c m ' ) chasacteristic of cyclic pfp derivatives

1nfl.arcd band

Compound

1 2 3

6

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7 8

1095 1109 1108 1120 1120 1124

998 1001 1001 1005 1020 998

954 963 966 968 958 965

878 888 888 888 882 880

743 745 736 743 750 745

perfluoropinacol was liberated but, as described in the Experimental section, products 9-11 could be isolated in 10-75% yield. Hydrolysis of 1 occurred on exposure to air or in solvents containing trace amounts of water or in aqueous acetonitrile. Hydrolysis of 3 - 5 was carried out in 99% ethanol or in dilute NaOH solution. Hydrolysis of 6-9 was accomplished with concentrated H2S0, at 25°C; at higher temperatures, pertluoropinacol is liberated (1 1).

show equivalent CF3 groups and equivalent ring carbons in the pfp region (Table 2). ( c ) A comparison of the phosphorus chemical shifts of the cyclic phosphoranes 1 - 8 with the hydrolysis products 9- 11 shows a significant downfielcl shift for 9- 11 (Table 2). For example, on hydrolysis of 1 to 9 there is a shift from -5.4 to 41.3 pprn: from 3 to 1 0 there is a shift from -38.6 to 12.2 ppm; and from 7 to 11 there is a shift from -53.2 (13) to -8.6 pprn. The phosphorus cheriiical shift of Ph?P(O)OCH(CF,)?(12) 39.2 ppm (14), however, is very similar to that of Ph,P(0)OC(CF,)2C(CF3)20H(9) 41.3 ppm, as expected if 9 has an acyclic structure. Compound 9 was used to study the equilibrium between acyclic A and cyclic B isomers. The doublet of septets in the nmr spectrum of 9 collapsed to a single peak as the rate of exchange of trifluoro~nethylgroups increased. In solvents such as acetone, dioxane, dichloromethane, or toluene, exchange is evident above 60°C. At 25°C exchange is promoted by bases such as pyridine, imidazole, quinoline, and excess dirnethylsulfoxide. Typical pyridine-promoted te~iiperature-dependent I9 F nrnr spectra are shown in Fig. 1. A line-shape analysis of the variable temperature spectra gave the following activation parameters: E,, = 36.0 f 3 kJ rnol-I, A H ; . = 33.5 2 3 kJ mol-I, AS+ = -86 k 8 J K-' mol-I, AG&, = 59.0 ? 0.6 kJ mol-'. Deprotonation. following by ring-closure, adequately accounts for the nrnr results. An exchange mechanism involving base

. base Hf 3 R = OCH(CF& 4 R = OPh 5 R = OMe

11

Ph2POC(CF3)2C(CF3)20-

Ph\!OC(CF3)2C(CF3)20H HO/

R 6 R = OPh 7 R = OEt 8 R = OBu"

An acyclic A or cyclic B structure was considered for the hydrolysis products 9-11. Although a cyclic structure (HO),P(pfp) has been proposed for the hydrolysis product of (Et0)3P(pfp), 7 (12), the following spectral evidence is entirely consistent with an acyclic phosphoryl structure A for 9-11: ( a ) The "F nmr spectra of 9- 11 show, in each case, two septets with 'JFF= 10 HZ (Table 2). A typical spectrum is shown in Fig. 1. A cyclic structure would be expected to give only a single perfluoropinacolyl peak, as found for the cyclic phosphoranes 1-8. ( 0 ) The "C nrnr spectra of 9-11 show nonequivalent CF3 groups as well as non-equivalent ring carbons, whereas the I3C nrnr spectra of cyclic phosphoranes 2 and 6

complete loss of the perfluoropinacolyl ligand was eliminated by adding perfluoropinacol to a rapidly exchanging system and observing separate IyF nrnr peaks for bound and free perfluoropinacol. That exchange in 9 can occur by a ring-closing and ring-opening process, with rates that are rapid on the nmr time scale, is of interest in view of alternative mechanisms of exchange in cyclic phosphoranes such as Berry pseudorotation or turnstile rotation (15). Attempts to observe exchange of the 0CH(CF3)? group in PhP(pfp)[OCH(CF3)2]2(3) with free HOCH(CF3)? in solvent dimethoxyethane, or with HOCH(CF,)? in the presence of pyridine, or with NaOCH(CF3), in dimethylformamide, all at 25"C, were unsuccessful; in each case separate fluorine nmr signals were observed for free and bound OCH(CF,)? groups. Finally, as part of our interest in reactions of I-IF, we briefly investigated the reactions of phosphoranes with anhydrous HF. With 1 rnol HF, PhP(pfp)[OCH(CF3)2]2(3) gave a mixture of PhPF(pfp)OCH(CF3), and PhPF?(pfp) while adding 2-4 mol H F gave mainly PhPF2(pfp), but on standing at 25"C, perfluoropinacol was liberated. After 6 days, PhP(pfp)(OPh)?and 2 rnol H F showed mainly PhPFJpfp) and perfluoropinacol. Reaction of (PhO)5P with 1 or 2 mol H F gave (PhO),PF and (PhO)3PF2but on standing at 25°C or on increasing the ratio of HF, the formation of (PhO),PiPF; (16) was favoured. PhP(OMe),(pfp) (5) with 1 rnol H F liberated perfluoropinacol.

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CAN. .I. Cl-IElvl. V O L . h l . 1'183

TABLE 2. Nmr spectral pararnctcrs of some pcrfluoropinacolyl phosphorus con~pounds" Compound

61, -5.4 -18.7 -38.6 -31.2 41.3

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12.2 -8.6

Sf:

6c (CFI)

Sc. (-OC)

-67.4 - 67 -72 - 67 - 72 -68 - 68 -68.9 -67.3 ( i ~ ~ l ~ l O ) -69.4 -67.3 ( ' ~ 1 - 1 . 1 0) -69.4 - 67 ('J ':I. I 0) - 68

"Chemical shifts in ppm are positive to low field of TMS, CFCI,, and 85% H,PO,. Coupling constants in Hz ref. 13: Ph,P(O)OCH(CF,), (12), ref. 22; PhP[OCH(CF,),l,, ref. 20; (Ph0)5P, ref. 21; and pcrfluoropinacol, ref. I I . Othcr compounds wcrc commercial samplcs. HF solutions wcrc made by condensing anhydrous HF into acctonitrilc to give approximately 0.7 M solutions. Solvcnts wcrc dricd and rcdistillcd prior to usc. Thc following compounds wcrc not isolated but idcntificd by thcir published spcctra: (PhO)4P'PF, (16). (PhO)3PFZ(23), and PhPF,(pfp) (24). PIl?P(/di))Cl (1) Ph,PCI (2.54 g. 11.5 mmol) and (CF,),CO (4.5 g, 27 mmol) were scalcd in a tube ant1 kept at 25°C for I wcck. Exccss (CF3),C0 was rcmovcd and an oil rcmaincd which dccomposcd in air. It was idcntificd as Ph,P(pfp)CI ( I ) by is (Tablc I ) , "F and "P nmr (Tablc 2). and by its rcaction with an cquimolar amount of HOCH(CF,), in tolucnc-C