Catalytic Phosphite Hydrolysis under Neutral Reaction Conditions

Article abstract of DOI:10.1021/acs.inorgchem.8b00546

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.

Full text of DOI:10.1021/acs.inorgchem.8b00546

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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
Scheme 1. Synthesis of 2a/2b
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, per-
formed with the appropriate model compounds, shed light
on the operative catalytic cycle, which comprises intra-
molecular water molecule transfer to the cis-coordinated
phosphite molecule.
The coordination of two water molecules in 2a/2b was
confirmed by 1JPtP = 4330 and 4326 Hz, respectively, and in the
hosphites are important ligands to stabilize rhodium(I) in
case of 2a by single-crystal structure analysis (see the Supporting
Pthe course of olefin hydroformylation reactions, which are Information, SI).
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 phospho-
nates 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 bis-
cationic platinum(II) complexes of the formulas [Pt-
(H2O)2(dppomf)](Otf)2 (2a; dppomf = 1,1′-bis-
(diphenylphosphanyl)octamethylferrocene)13 and [Pt-
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
1
NMR spectroscopy (Figures S4 and S5), based on JPH 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 time-
dependent progressive conversion of P(OPh)3 to B and C, which
( H 2 O ) 2 ( d p p r ) ] ( O t f ) 2 ( 2 b ; d p p r
= 1 , 1 ′ - b i s -
(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
Received: March 2, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00546
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Table 1. Catalytic Phosphite Hydrolysis in THF
b
products’ distribution (%)
a
entry
precatalyst
phosphite (R)
t (min)
conv (%)/TOF (h−1
)
A
B
C
1
2
3
4
5
Ph
480
180
480
180
480
480
480
180
480
5
4.0
45.0
6.0
53.0
93.0
88.0
91.0
50.0
34.0
91.0
2.0
1.0
2a
2a
2b
2b
2b
2b
Ph
4.0/n.d.
9.0/n.d.
30.0/16
75.0/15
77.0/n.d.
31.0/6
Ph
3.0
9.0
Ph
6.0
3.0
Ph
4.0
46.0
64.0
1.0
c
6
Ph
2.0
d
7
Ph
8.0
8
9
n-butyl
n-butyl
n-butyl
n-butyl
ethyl
methyl
isopropyl
41.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
e
10
2a
2b
2a
2a
2a
81.0/15647
79.0/7598
90.0/17386
99.0/19124
89.0/17193
e
11
10
e
12
5
e
13
5
e
14
5
a
b
Catalytic conditions: precatalyst (2.370 μmol), substrate (0.380 mmol), THF (1.0 mL), water (1.140 mmol), T = 20 °C. Products’ distribution
c
d
e
(%) defined as mmol of hydrolysis product/∑mmol(A + B + C) × 100. Water (2.280 mmol). Water (0.380 mmol). Precatalyst (0.237 μmol).
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/2b-
catalytzed 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 phos-
phites 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).
THF-d8 solutions of 3a/4a reacted with water (3.0 equiv),
yielding 5a and 6a, respectively, as the only phosphorus-
containing 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.
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-cyclo-
metalation product16 (see the SI) during catalysis, which is
catalytically in-active. Interestingly, analogous hydrolysis reac-
1
The H 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 phosphorus-
containing 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
15
tions of aliphatic phosphites using Pt(H2O)2(dppf)](Otf)2
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
B
DOI: 10.1021/acs.inorgchem.8b00546
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Communication
(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(O-
spectroscopy. On the other hand, in the presence of water, M3a
leads to M4, which shows a weak intramolecular hydrogen-
bonding 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 tautomeriza-
tion 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).
4
n-butyl)2 (1JPH= 691.1 Hz and JPH= 8.8 Hz) and DPO(O-n-
butyl)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
ASSOCIATED CONTENT
* Supporting Information
Scheme 4. Proposed Catalytic Cycle for the Phosphite
Hydrolysis Reaction
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.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_
request@ccdc.cam.ac.uk, 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: werner.oberhauser@iccom.cnr.it.
■
ORCID
Werner Oberhauser: 0000-0002-9800-1700
Notes
The authors declare no competing financial interest.
coordination of P(OMe)3 to M1a, giving M2a, 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).
ACKNOWLEDGMENTS
G.M. thanks HPC-CINECA HP1-CFMSCC for computational
resources.
■
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