Cyclic–acyclic equilibrium and selective P—O

bond cleavage in 2-hydroxyphenyldiphenyl-

phosphinate, Ph2P(O)OC6H4OH

Edward Paul Segstro, Kerry Davie, Xiaoling Huang, and Alexander Frank Janzen

Abstract: The equilibrium in the Ph2P(O)OC6H4OH–base system may be shifted by varying the base concentration, where

base = imidazole, triethylamine, dimethyl sulfoxide, pyridine, and 1,4-dioxane. This equilibrium was studied by 31P and 1H

NMR, and information about the symmetry of phosphorus-containing intermediates is provided by the 1H NMR spectrum of

the catecholyl ring, with its ABCD spin system. The equilibrium is also affected by trifluoroacetic acid. A mechanism is

proposed that involves protonation–deprotonation, cyclic–acyclic equilibria, and selective P—O bond cleavage, with all steps

occurring rapidly on the NMR time scale.

Key words: Ph2P(O)OC6H4OH–base system, cyclic–acyclic equilibrium, and selective P—O bond cleavage.

Résumé: On peut déplacer la position de l’équilibre du système Ph2P(O)OC6H4OH–base en faisant varier la concentration de

base (base = imidazole, triéthylamine, diméthylsulfoxyde, pyridine, 1,4-dioxane). On a étudié cet équilibre par RMN du 31P et

du 1H et l’information relative à la symétrie des intermédiaires contenant du phosphore découle du spectre RMN du 1H du

noyau catécholyle comportant un système de spin ABCD. L’équilibre est aussi affecté par l’acide trifluoroacétique. On

propose un mécanisme pour cet équilibre qui implique une protonation–déprotonation, un équilibre cyclique–acyclique et la

rupture sélective de la liaison P—O; toutes les étapes se produiraient rapidement à l’échelle de temps de la RMN.

Mots clés : système Ph2P(O)OC6H4OH–base, équilibre cyclique–acyclique, rupture sélective de la liaison P—O.

[Traduit par la rédaction]

Introduction

Five-membered rings, cyclic–acyclic equilibria, and protonation–deprotonation are common features of the hydrolysis, and en-zymatic hydrolysis, of phosphorus esters (see examples inref. 1). Previously, we studied the base-initiated cyclic–acyclicequilibrium of a perfluoropinacolyl phosphorus compound (2).The perfluoropinacolyl septets in the 19F NMR spectrum col-lapsed to a broad peak with increasing base concentration ortemperature, which is consistent with the cyclic–acyclic equi-librium of Scheme 1, but line broadening made it impossibleto distinguish between trigonal bipyramidal or square pyrami-dal intermediates; both structural types have been establishedby X-ray crystallography for phosphorus compounds that con-tain five-membered rings (3).

In this study, perfluoropinacol was replaced by catechol,and the equilibrium of 2-hydroxyphenyldiphenylphosphinate,Ph2P(O)OC6H4OH 1, in the presence of base was monitoredby 31P and 1H NMR. Analysis of the 1H NMR spectrum of the

planar catecholyl ring, with its ABCD spin system, was carriedout in an attempt to obtain information about the symmetry ofphosphorus-containing intermediates.

Results and discussion

In a series of experiments, the 31P chemical shift of 1 wasmeasured in CH2Cl2 solution as a function of base concentra-tion. As seen in Fig. 1, there is an upfield shift with increasingbase concentration, i.e., towards five-coordinate phosphorus(4), and the relative shift is in the order: imidazole ~ Et3N >DMSO > pyridine > 1,4-dioxane. Only limited data are avail-able for imidazole and triethylamine because these bases bringabout decomposition of 1, but solutions of 1 in excess DMSO,pyridine, or 1,4-dioxane are stable in CH2Cl2, as judged by 31Pand 1H NMR.

To eliminate the possibility that changes in δ31P are due tosolvent or concentration effects, two compounds were chosenthat lack a hydroxyl substituent, namely, (MeO)3PO andPh2P(O)OC6H4OSiMe3. Addition of pyridine to these com-pounds produced only minor changes in δ31P. For example,δ31P of (MeO)3PO (0.071 M in CH2Cl2 at 27°C) shifts from1.39 to 1.10 ppm as the pyridine concentration increases from

Can. J. Chem. 76: 518–521 (1998)

Received November 3, 1997.

E.P. Segstro.Department of Chemistry, University ofWinnipeg, Winnipeg, MB R3B 2E9, Canada.X. Huang. Department of Chemistry, Wuhan University,Wuhan 430072, China.K. Davie and A.F. Janzen.1 Department of Chemistry,University of Manitoba, Winnipeg, MB R3T 2N2, Canada.

1 Author to whom correspondence may be addressed.Telephone: (204) 474-9731. Fax: (204) 474-7608. E-mail:ajanzen@cc.umanitoba.ca

1

44'

56

32

13' 2'

1'Oa

HOb

O

P

518

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0 to 12.4 M. Similarly, δ31P of Ph2P(O)OC6H4OSiMe3 inCH2Cl2 at 27°C changes from 29.83 to 29.04 ppm as the pyri-dine concentration increases from 0 to 9.3 M.

A mechanism is proposed in Scheme 2 to account for thebehaviour of the 1-base system. This mechanism is written insufficient detail so that the connectivity of atoms along thereaction pathway is evident, and ring-opening and rapid ring-closing of five-membered rings is denoted as –C5-center and+C5-center, respectively (5). The overall shift towards five-coordinate phosphorane is rather modest, for example, δ31P of1 (37.80 ppm in CH2Cl2) shifts to 29.85 ppm in excess pyridine(Fig. 1). Much greater changes of 63–79 ppm have been foundin related compounds as they are converted to authentic five-coordinate phosphoranes (4). If a chemical shift of about –32ppm is assumed for 6, i.e., 70 ppm upfield from that of 1, thenan equilibrium constant can be crudely estimated for Scheme 2by assuming an equilibrium between two species only,

namely, 1 and 6. In that case, from the weighted average ofδ13P, and standard equations (6), equilibrium constants are es-timated as ~0.09 for imidazole and triethylamine, ~0.04 forDMSO and pyridine, and ~0.01 for 1,4-dioxane. Thus the con-centration of five-coordinate phosphorane depends on the na-ture and concentration of base, but the equilibrium inScheme 2 favours phosphinate 1 in all experiments.

All samples of 1 and base (Fig. 1) give only single 31P peaksat 27°C, and individual species cannot be observed by NMRat ambient temperature. On lowering the temperature of a sam-ple of 1 (0.031 M) and pyridine (9.31 M) in CH2Cl2, δ31P1shifts from 31.07 ppm at 27°C to 29.62 ppm at –63°C, in thedirection of five-coordinate phosphorane. However, no appre-ciable line-broadening was evident, confirming that all stepsof Scheme 2 are rapid on the NMR time scale, even at –63°C.It follows from this result that only very weak bonds arecleaved, and these are identified in Scheme 2 as bridging

30

32

34

38

42

δ31P

442 4 6 8 10 12 14

concentration (M)

imidazole

dmso

pyridine

1,4-dioxane

triethylamine

(ppm)

0

CF3COOH

36

Fig. 1. Plot of δ31P of Ph2P(O)OC6H4OH 1 as a function of base

concentration, and of CF3COOH concentration. Temperature is

27°C, solvent is CH2Cl2, and [1] = 0.032 M in all experiments.

base

baseH+Ph2POC(CF3)2C(CF3)2OH

O

C(CF3)2

C(CF3)2O

OPPh

O

Ph

Ph2POC(CF3)2C(CF3)2O

O _

_

C(CF3)2

C(CF3)2O

OPPhO

Ph

_

or

+C5-center-C5-center

Scheme 1.Cyclic–acyclic equilibrium in a perfluoropinacolyl

phosphorus compound.

py

-py1

Ph

O

Ph P O

OHpy

2

-pyH+

pyH+_Ph

O

Ph P O

O

3

-C5-center

+C5-center

_

PPh

O

PhO

O

4

py

--H--pypyH+

-pyH+

-py

OPPh

O

PhO

OPPh

OH

PhO

5 6

py

etc.

Scheme 2.Proposed protonation–deprotonation and cyclic–acyclic

equilibrium in the Ph2P(O)OC6H4OH–base system.

_

_

4a 4b

OaPh

O

PhP

Ob

A

B

CD

OaPh

O

PhP

Ob

A

B

CD

1

.

Ph

O

Ph P O

HO

H OOCCF3

7

Ph

OH

Ph P O

HO

+

8

- CF3COO-

CF3COO-

CF3COOH

- CF3COOH -C5-center

+C5-center

9 10

6 etc.OPPh

OH

PhO

H +

- CF3COO-

CF3COO-OP

Ph

OH

PhO

H

CF3COOH

- CF3COOH

CF3COO--

Scheme 3.Proposed protonation–deprotonation and cyclic–acyclic

equilibrium in the Ph2P(O)OC6H4OH–acid system.

Segstro et al. 519

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N--H--O bonds in 2 and 5, and a P—Ob bond in 4. The latterbond is trans to an oxo substituent, and contributing structures4a and 4b may account for the weakness of the trans P—Ob

bond.Trifluoroacetic acid was also added to 1, and the chemical

shift of 1 moves downfield, as seen in Fig. 1, presumably as aresult of protonation at the phosphoryl bond. Authentic phos-phonium ions have downfield 31P chemical shifts (7), and amechanism that accounts for the behaviour of 1 in the presenceof acid is shown in Scheme 3.

According to the mechanism of Scheme 2, the axial P—Ob

bond in 4 is cleaved, but not the equatorial P—Oa bond. Evi-dence that the P—Oa bond remains intact is based on the 1HNMR spectrum of the catecholyl ring. Its ABCD spectrum con-tains information about the symmetry of the intermediates andthe stereospecificity of the overall reaction, and the questionis whether this ABCD spin system is maintained at all stagesof the equilibrium, as required by Scheme 2, or whether it isconverted into an A2B2 (or AA′BB′) spin system.

The 1H NMR spectra of 1 were recorded at 300 MHz, anda complete assignment was carried out for the catecholyl andphenyl regions at various pyridine concentrations. Hydrogensare assigned by comparison with 2-methoxyphenol (8) andrelated compounds (9). The downfield shift of H3 with increas-ing pyridine concentration is in agreement with the effect ofbase on the chemical shift of ortho hydrogens in phenol (10),but no argument that depends on the ABCD spin pattern ischanged if the assignment of hydrogens is interchanged.

The variation in δH as a function of pyridine concentration

is shown in Fig. 2, and the corresponding changes in J(H,P)and J(H,H) are given in Table 1. As seen in Fig. 2 and Table 1,the NMR parameters depend on pyridine concentration, butH3 and H6 never become equivalent, nor H4 and H5, andtherefore the ABCD spin system is preserved. Selective P—Ob

bond cleavage in 4, and trigonal bipyramidal geometry of 4–6,as postulated in Scheme 2, is entirely compatible with this“ABCD criterion.”

Our results do not allow us to eliminate square pyramidalintermediate 11 from consideration, however, square pyrami-dal 12 is incompatible with an ABCD spin system, moreover,any polytopal rearrangement such as Berry pseudorotation orturnstile rotation that generates 12 is eliminated. Bond break-ing of the P—Oa bond is not allowed by the ABCD criterion,nor complete loss of catechol, and there cannot be any path-ways that interchange Oa and Ob substituents.

Experimental

All solvents were dried and stored over molecular sieve underN2 in a dry box. Diphenylchlorophosphane, Ph2PCl, andbis(pentafluorophenyl)chlorophosphane, (C6F5)2PCl, wereoxidized to Ph2P(O)Cl and (C6F5)2P(O)Cl by bubbling O2

through a benzene solution. Elemental analyses were carriedout by Galbraith Laboratories, Knoxville, Tenn.

The 31P and phosphorus-decoupled 1H NMR spectra wererecorded on a Bruker WH90 spectrometer, and the 13C and 1HNMR spectra were recorded on a Bruker AM300 spectrome-ter. A complete assignment of 1H NMR spectra was carriedout of the catecholyl and phenyl regions of 1 at various pyri-dine concentrations, with the aid of computer program LAME(11).

2-Hydroxyphenyldiphenylphosphinate 1The Ph2P(O)Cl (9.5 g, 40 mmol) in benzene (25 mL) wasadded dropwise over 30–40 min to a stirred solution of 1,2-benzenediol (catechol) (4.4 g, 40 mmol) and pyridine (3.2 g,40 mmol) in benzene (30 mL). At the completion of the reac-tion, solid was filtered off, washed several times with water,dried under vacuum and recrystallized from 1:1 ben-zene:methanol, and identified as 2-hydroxyphenyldiphenyl-phosphinate 1. Anal. calcd. for C18H15O3P: C 69.68, H 4.87;found: C 69.51, H 4.87. The filtrate was concentrated on a

706050403020100

H4H6H3H3'H4'H2' H5

δ1H of 1 (ppm)

8.0 7.8 7.6 7.4 7.2 7.0 6.8 6.6

[pyr][1]

Fig. 2. Plot of δH of catecholyl and phenyl protons of 1 as a

function of pyridine concentration, showing retention of an ABCD

spin system in the catecholyl ring. H3-H6 are catecholyl hydrogens,

and H2′-H4′ are phenyl hydrogens.

[Py]/[1] 5J(P,3) 6J(P,4) 5J(P,5) 4J(P,6) 3J(3,4) 4J(3,5) 5J(3,6) 3J(4,5) 4J(4,6) 3J(5,6)

0 –0.25 1.00 –0.84 1.36 8.13 1.63 0.32 7.37 1.59 8.08

8.1 –0.29 0.97 –0.80 1.37 8.13 1.62 0.32 7.37 1.59 8.09

9.8 –0.32 0.97 –0.77 1.37 8.12 1.60 0.31 7.41 1.59 8.09

17.1 –0.43 0.97 –0.72 1.38 8.12 1.62 0.31 7.38 1.60 8.10

34.7 –0.51 0.93 –0.64 1.38 8.11 1.59 0.30 7.42 1.60 8.09

67.7 –0.64 0.91 –0.55 1.42 8.08 1.62 0.26 7.41 1.58 8.10

a Temperature 27oC; [1] = 0.032 M. Internal reference with respect to CHDCl2, δ H 5.32 ppm.b 1H in CD2Cl2 (without pyridine): δ H3 6.89, δ H4 6.99, δ H5 6.69, δ H6 6.95 ppm.c Coupling constant, Hz.

Table 1.Effect of pyridine on J(P,H) and J(H,H) of the catecholyl ring of 1.a,b,c

11

Oa

Oa

AB

BA

PPh

OH

Ph

Oa

Ob

AB

CD

PHO

Ph

Ph

12

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rotary evaporator and solids filtered off and purified as aboveto give a total yield of 1 of 40%; mp 185°C; MS, m/z: 310 (M)+,233 (M – C6H5)

+, 232 (M – C6H6)+, 201 (Ph2PO)+.

1H NMR of 1 is given in Table 1 and Fig. 2. Variable-temperature 1H NMR of OH in CD2Cl2, δ: 9.02 ppm (+40°C),9.19 ppm (–25°C), 9.35 ppm (–56°C); in DMSO: 8.96 ppm(+39°C), 9.29 ppm (–23°C), 9.57 ppm (–60°C). 1H NMR ofOH as a function of molar ratio of pyridine/1 in CD2Cl2, δ:9.01 ppm (py/1 = 0), 9.10 ppm (py/1 = 1.83), 9.57 ppm (py/1= 14.9), 10.52 ppm (py/1 = 125.4). δ31P of 1 (0.032 M) invarious solvents: (CH2Cl2) δ 37.80, (CDCl3) δ 38.04, (1,4-dioxane) δ 33.75, (pyridine) δ 29.85, (DMSO) δ 29.07,(CF3COOH) δ 43.16.

13C NMR of 1 (CD2Cl2): C1 δ 139.6, 2J(PC) = 9.50 Hz; C2δ 148.6, 3J(PC) = 3.15 Hz; C3 δ 119.9, 4J(PC) = 1.23 Hz; C4δ 126.8, 5J(PC) = 1.27 Hz; C5 δ 120.8, 4J(PC) = 0.68 Hz; C6δ 122.9, 3J(PC) = 4.56 Hz; C1′ δ 129.2, 1J(PC) = 137.1 Hz;C2′ δ 132.2, 2J(PC) = 10.62 Hz; C3′ δ 129.2, 3J(PC) =13.58 Hz; C4′ δ 133.6, 4J(PC) = 2.89 Hz.

A similar reaction of 1,2-benzenediol and (C6F5)2P(O)Clgave 2-HOC6H4OP(O)(C6F5)2; MS, m/z: 490(M)+.

2-Me3SiOC6H4OP(O)Ph2 and 2-MeOC6H4OP(O)Ph2Ph2P(O)OC6H4OH 1 (0.11 g, 0.35 mmol) and Me3SiCl(0.038 g, 0.35 mmol) and 2 drops of Et3N in CH2Cl2 (2 mL)were refluxed for 2 h. The solvent was removed by rotaryevaporator and a white solid remained, which was washed with1:1 benzene:methanol and identified by mass spectrometry asPh2P(O)OC6H4OSiMe3; MS, m/z: 382 (M)+, 367 (M – CH3)

+.This product was also formed if 1 was treated with(Me3Si)2NH or with a mixture of (Me3Si)2NH, Me3SiCl, andpyridine, but a red solid was formed in the reaction of 1 withEt2NSiMe3.

Reaction of 2-MeOC6H4OH (1.4 g, 11 mmol) andPh2P(O)Cl (2.37 g, 10 mmol) in pyridine (13 mL) gave 2-MeOC6H4OP(O)Ph2; MS, m/z: 324 (M)+, 306 (M – 18)+, 293(M – OMe)+. The NMR spectra of these compounds, as wellas of 2-methoxyphenol (8), and others (9), were helpful inassigning the 1H NMR spectrum of 1.

cis-3-Diphenylphosphinatotetrahydro-3,4-furandiolPh2P(O)Cl (8.5 g, 36 mmol) in benzene was added dropwise toa stirred solution of cis-tetrahydro-3,4-furandiol (3.70 g,36 mmol) and pyridine (2.82 g, 36 mmol) in CH2Cl2 (30 mL)over 30 min. The mixture was stirred for 24 h, then n-hexaneadded and the precipitate washed with water, dried under vac-uum, and recrystallized from 1:1 benzene:methanol to givecis-3-diphenylphosphinatotetrahydro-3,4-furandiol (37%).Anal. calcd. for C16H17O4P: C 63.16, H 5.63; found: C 63.35,

H 5.47. MS, m/z: 304 (M)+, 286 (M – H2O)+. 31P NMR(CDCl3), δ: 32.3 ppm. 1H NMR (CDCl3), δ: 3.72–4.78 (ali-phatic), 7.33–7.96 (phenyl), 5.80 (OH). Its complex 1H NMRspectrum made this compound less suitable for NMR studythan 1.

Acknowledgements

This work was supported financially by the Natural Sciencesand Engineering Research Council of Canada. We thank Mr.R. Sebastian for assistance with the calculation of NMR spectra.

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