Catalytic Phosphite Hydrolysis under Neutral Reaction Conditions

6 days ago - Synopsis. Platinum(II) bis(aqua)diphosphanemetallocene complexes were used to catalyze the hydrolysis of P(OR)3 (R = phenyl and alkyl) un...
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Catalytic Phosphite Hydrolysis under Neutral Reaction Conditions Werner Oberhauser* and Gabriele Manca Istituto di Chimica dei Composti Organometallici (CNR-ICCOM), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy S Supporting Information *

chloride precursor 1a/1b (Scheme 1) by Ag(Otf) (Otf = trifluoromethylsulfonate) in CH2Cl2 at room temperature.

ABSTRACT: Cationic phosphametallocene-based platinum(II) aqua complexes were used as efficient precatalysts for the hydrolysis of aromatic and aliphatic tertiary phosphites under neutral reaction conditions at room temperature, leading to the selective cleavage of one P−O bond of the phosphite. NMR labeling experiments combined with stoichiometric model reactions and theoretical density functional theory calculations, performed with the appropriate model compounds, shed light on the operative catalytic cycle, which comprises intramolecular water molecule transfer to the cis-coordinated phosphite molecule.

Scheme 1. Synthesis of 2a/2b

The coordination of two water molecules in 2a/2b was confirmed by 1JPtP = 4330 and 4326 Hz, respectively, and in the case of 2a by single-crystal structure analysis (see the Supporting Information, SI). The P1−Pt−P2 bite angle of 2a is larger compared to that of [Pt(H2O)2(dppf)](Otf)2 (2a′)15 [i.e. 102.73(5)° vs 97.37(7)°], while the O1−Pt−O2 angle is comparable [i.e., 85.09(13)° vs 86.0(2)°]. Notably, the Pt−O distances in 2a are significantly different from each other [i.e. 2.114(3) and 2.165(3) Å], which contrast with that found for the dppf counterpart [i.e. 2.122(4) and 2.103(5) Å].15 2a/2b were tested as precatalysts in the hydrolysis reaction of monophosphites of the type P(OR)3 [R = phenyl (Ph), n-butyl, ethyl, methyl (Me), and isopropyl] in THF/water mixtures at room temperature under a nitrogen atmosphere (Table 1). The catalytic conversion and products’ distribution reported for the hydrolysis products A−C (Scheme 2) were determined by 31P NMR spectroscopy (Figures S4 and S5), based on 1JPH with authentic samples (R = Ph) and on the multiplicity of the 31P signal in the case of R = alkyl. After the catalytic reaction occurred, PPh3 was added to the catalytic solution and the products’ distribution was determined by integration of the corresponding 31P{1H} NMR signals. Hydrolysis of P(OPh)3, applying a 1:3 molar ratio between phosphite and water, occurred very slowly at room temperature (Table 1, entry 1). The addition of 2a/2b under otherwise the same experimental conditions gave a significant increase of the P−O hydrolysis activity, which was higher for 2b than for 2a (entries 2−5). With a change in the phosphite/water molar ratio to 1:6, almost identical catalytic activity was observed (entry 6 vs 5), whereas an 1:1 molar ratio halved the catalytic activity (entry 7 vs 5). The catalytic reaction was not selective for product A (i.e., first hydrolysis product). Instead we observed a timedependent progressive conversion of P(OPh)3 to B and C, which

P

hosphites are important ligands to stabilize rhodium(I) in the course of olefin hydroformylation reactions, which are industrially relevant chemical processes.1 The main drawback of this type of ligand is hydrolysis of the P−O bonds, leading to P(OH)(OR)2 or its tautomer HPO(OR)2.2 The former has been applied as the ligand for metal-catalyzed hydroformylation3 and C−C coupling reactions,4 while HP(O)(OR)2 has been used for the following: (i) phosphonation reactions,5 giving phosphonates that find widespread application as flame-retardants6 and analogues of natural phosphates;7 (ii) the synthesis of phosphoramidates, phosphate esters,8 and phosphanes.9 The synthesis of HP(O)(OR)2 upon hydrolysis of uncoordinated P(OR)3 was described to occur by either a Michaelis−Arbuzov or an organic ester analogue mechanism.10 17O NMR and IR spectroscopic studies proved that a nucleophilic attack of the phosphorus atom by a water molecule is the dominant reaction mechanism, causing the scission of a P−O bond instead of a C− O bond.10 Unlike the metal-based phosphate hydrolysis,11 which is well studied and mainly based on the intermolecular nucleophilic attack of a metal-coordinated hydroxide ion on the phosphate ester, hydrolysis of metal-coordinated phosphites has been much less studied.2,12 To the best of our knowledge, no metal-catalyzed P−O hydrolysis of phosphites under neutral reaction conditions (i.e., the absence of acid and base) was described in the literature. In this context, we found that biscationic platinum(II) complexes of the formulas [Pt(H 2 O) 2 (dppomf)](Otf) 2 (2a; dppomf = 1,1′-bis(diphenylphosphanyl)octamethylferrocene) 13 and [Pt(H2O)2(dppr)](Otf)2 (2b; dppr = 1,1′-bis(diphenylphosphanyl)ruthenocene)14 are suitable precatalysts for hydrolysis of P(OR)3 (R = aromatic and aliphatic substituents) in tetrahydrofuran (THF)/water reaction mixtures at room temperature. Both complexes were straightforwardly obtained upon chloride abstraction of the corresponding © XXXX American Chemical Society

Received: March 2, 2018

A

DOI: 10.1021/acs.inorgchem.8b00546 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Table 1. Catalytic Phosphite Hydrolysis in THF

products’ distribution (%)b entry 1 2 3 4 5 6c 7d 8 9 10e 11e 12e 13e 14e

a

precatalyst 2a 2a 2b 2b 2b 2b

2a 2b 2a 2a 2a

−1

phosphite (R)

t (min)

conv (%)/TOF (h )

A

B

C

Ph Ph Ph Ph Ph Ph Ph n-butyl n-butyl n-butyl n-butyl ethyl methyl isopropyl

480 180 480 180 480 480 480 180 480 5 10 5 5 5

4.0 4.0/n.d. 9.0/n.d. 30.0/16 75.0/15 77.0/n.d. 31.0/6 41.0 100.0 81.0/15647 79.0/7598 90.0/17386 99.0/19124 89.0/17193

45.0 6.0 3.0 6.0 4.0 2.0 8.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

53.0 93.0 88.0 91.0 50.0 34.0 91.0

2.0 1.0 9.0 3.0 46.0 64.0 1.0

Catalytic conditions: precatalyst (2.370 μmol), substrate (0.380 mmol), THF (1.0 mL), water (1.140 mmol), T = 20 °C. bProducts’ distribution (%) defined as mmol of hydrolysis product/∑mmol(A + B + C) × 100. cWater (2.280 mmol). dWater (0.380 mmol). ePrecatalyst (0.237 μmol).

a

Scheme 2. Catalytic Hydrolysis of P(OR)3

Scheme 3. Synthesis of 3a/4a and Their Hydrolysis Products 5a/6a

was also confirmed by operando 31P{1H} NMR studies carried out in the presence of 2a/2b (see the SI). In order to verify whether 2a/2b contributes to the conversion of A in B, we carried out analogous catalytic reactions with A as the substrate and found that an uncatalyzed and a 2a/2bcatalytzed A conversion gave the same results (i.e., conversion of 92.0% in a reaction time of 30 min with a selectivity of B and C of 87.0 and 13.0%, respectively). This result might indicate that 2a/ 2b catalyzed only the first hydrolysis step of P(OPh)3 to A. Analogous catalytic hydrolysis reactions with aliphatic phosphites such as P(O-n-butyl)3 showed not only a boost of the catalytic activity with respect to the uncatalyzed case but also complete chemoselectivity for the first hydrolysis product A (entries 8−14). With a substrate-to-precatalyst molar ratio of 1600, we obtained for 2a TOF values between 19000 and 15600 h−1. 2a was much more active than 2b in the case of aliphatic phosphites, which contrasts with the results obtained with P(OPh)3. An operando 31P{1H} NMR study for the 2a-catalyzed P(OPh)3 hydrolysis clearly showed the formation of an ortho-cyclometalation product16 (see the SI) during catalysis, which is catalytically in-active. Interestingly, analogous hydrolysis reactions of aliphatic phosphites using Pt(H2O)2(dppf)](Otf)215 exhibited no catalytic activity. In order to gain mechanistic information concerning the 2a/ 2b-catalyzed phosphite hydrolysis reactions, we carried out stoichiometric phosphite hydrolysis reactions with compounds 3a/4a (Scheme 3), which bear only one molecule of P(OPh)3 and P(O-n-butyl)3, respectively. 3a/4a are stable in the presence of an excess of phosphite. 2b gave under identical reaction conditions analogous compounds, while 2a′ exhibited a different reactivity (see the SI). The 31P{1H} NMR spectra of 3a/4a showed a triplet for the coordinated phosphite, while coordinated dppomf exhibited a doublet. Related compounds have been obtained with other phosphametallocene-based ligands.17

THF-d8 solutions of 3a/4a reacted with water (3.0 equiv), yielding 5a and 6a, respectively, as the only phosphoruscontaining compounds and ROH (Scheme 3). 5a and 6a could not be isolated in pure form. While the coordination geometries of 3a/4a and 5a/6a are comparable (i.e., identical 31P{1H} NMR patterns), the most significant 31P spectroscopic differences are the smaller 1JPtP coupling constant for coordinated P(OH)(OR)2 compared to P(OR)3 [i.e., 6933.0 Hz (5a) and 6492.7 Hz (6a) vs 7281.1 Hz (3a) and 7266.4 Hz (4a)], which is indicative of a weaker coordination of P(OH)(OR)2 compared to P(OR)3,12,18 and a significant 31P NMR chemical shift to lower frequency of coordinated P(OH)(OR)2 compared to coordinated P(OR)3. The 1H NMR spectra of in situ generated 5a/6a showed signals assigned to phenol and n-butanol, respectively, and from the 1H NMR integral ratio of the signals assigned to ROH and coordinated P(OH)(OR)2, we concluded that only one P−O bond in 3a and 4a was subjected to hydrolysis. Since we have no direct spectroscopic evidence for the coordination of P(OH)(OR)2 to platinum(II) in 5a and 6a, because of their in situ generation, we proved the coordination of P(OH)(OR)2 by reacting 2a with 1 equiv of HPO(OPh)2,2 yielding 5a (i.e., proved by 31P NMR spectroscopy) as the only phosphoruscontaining compound. The tautomerization of HPO(OPh)2 to P(OH)(OPh)2 may occur by a described metal-assisted mechanism.19 It is important to mention that 5a/6a did not undergo further hydrolysis reactions even in the presence of a large excess of water (6 equiv), hence confirming the metal-catalyzed hydrolysis of one P−O bond of coordinated P(OR)3. 31P NMR spectra of the hydrolysis products HPO(OR)2 acquired by THF showed, for R = aliphatic, 1JPH/4JPH coupling constants [i.e., 685.8 Hz/8.5 Hz (isopropyl); 689.7 Hz/8.4 Hz (butyl); 689.4 Hz/9.3 Hz B

DOI: 10.1021/acs.inorgchem.8b00546 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry (ethyl); and 695.5 Hz/12.0 Hz (methyl)], proving that tautomerization of the coordinated P(OH)(OR)2 to HPO(OR)2 occurred (see the SI). A 2a-catalyzed hydrolysis experiment with P(O-n-butyl)3 in the presence of H218O showed the formation of n-C4H916OH and HP18O(16OC4H9)2, which was proven by gass chromatography−mass spectrometry analysis (see the SI). This latter experimental result is in accordance with an attack of a water oxygen atom to the phosphite phosphorus atom with the concomitant release of an unlabeled n-butanol molecule, as observed for the uncatalyzed phosphite hydrolysis.10 An analogous hydrolysis experiment in the presence of a 1:1 (v/v) mixture of H216O and D216O showed the formation of HPO(On-butyl)2 (1JPH= 691.1 Hz and 4JPH= 8.8 Hz) and DPO(O-nbutyl)2 (1JPD= 105.6 Hz and 4JPH= 8.3 Hz) in a 2.2 molar ratio (see the SI). This rather small isotope effect is in contrast to that found for the hydrolysis of uncoordinated phosphite (i.e., KH/ KD= 6.5), where ionization of a water molecule was assumed to be the rate-determining reaction step.10b Density functional theory (DFT) calculations, taking into account dispersion forces and THF solvent effects, with 2b and P(OMe)3 as model compounds, were carried out in order to propose an operative catalytic cycle (Scheme 4). The

spectroscopy. On the other hand, in the presence of water, M3a leads to M4, which shows a weak intramolecular hydrogenbonding interaction between the coordinated water molecule and the P−OH unit of coordinated P(OH)(OMe)2. The presence of this latter hydrogen-bonding interaction, although weak, turns the remaining two P−O bonds of P(OH)(OMe)2 away from the coordinated water molecule, hence precluding further hydrolysis. See SI for details concerning the tautomerization of P(OH)(OR)2. In summary, the selective 2a/2b-catalyzed hydrolysis of P(OR)3 to HPO(OR)2 occurring under neutral reaction conditions is due to (i) the cis coordination of one phosphite molecule and one water molecule at platinum(II) and the successive transfer of the coordinated water molecule to one P− O bond of coordinated P(OR)3 by hydrogen transfer from the water molecule to the oxygen atom of one P−O bond, followed by the nucleophilic attack of coordinated OH− on the phosphorus atom, (ii) the formation of an intramolecular hydrogen-bonding interaction between the coordinated water and coordinated hydrolysis product (P(OH)OR)2, avoiding a further metal-catalyzed hydrolysis step, and (iii) the stronger coordination of P(OR)3 than P(OH)(OR)2 to platinum(II).

Scheme 4. Proposed Catalytic Cycle for the Phosphite Hydrolysis Reaction

* Supporting Information

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ASSOCIATED CONTENT

S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00546. Experimental details, synthesis and characterization, crystallographic data for 2a, computational methods, and coordinates and energies of optimized structures (PDF) Accession Codes

CCDC 1818808 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Werner Oberhauser: 0000-0002-9800-1700 Notes 1a

The authors declare no competing financial interest.

■ ■

2a

coordination of P(OMe)3 to M , giving M , is associated with an energy gain of 29.0 kcal mol−1. M2a is in equilibrium with M2b (a = without Pt−Ru interaction and b = with Pt−Ru interaction). M2b shows a short metal−metal distance of 3.08 Å. The intramolecular hydrogen transfer from the coordinated water molecule in M2a to the oxygen atom of one P−O bond, as shown in the optimized active species M2*, is estimated to be endothermic by 27.0 kcal mol−1. In contrast, the uncatalyzed P(OMe)3 hydrolysis is associated with a much higher activation energy of 43.7 kcal mol−1 (see the SI). The successive migration of coordinated OH− to the phosphorus atom (i.e., in accordance with the 18O NMR labeling experiment) is fast, leading to M3a with a concomitant release of one molecule of MeOH (energy gain of 12.0 kcal mol−1). In the absence of water, M3a converts to M3b, which is the more stable isomer. In fact, 5a and 6a were intercepted by NMR

ACKNOWLEDGMENTS G.M. thanks HPC-CINECA HP1-CFMSCC for computational resources. REFERENCES

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DOI: 10.1021/acs.inorgchem.8b00546 Inorg. Chem. XXXX, XXX, XXX−XXX