Phosphinous Acid Platinum Complex as Robust Catalyst for Oxidation: Comparison with Palladium and Mechanistic Investigations

Article abstract of DOI:10.1002/ejoc.201801040

Secondary phosphine oxides proved to be effective preligands to stabilise a hydroxy-platinum based catalyst that allows the aerobic/anaerobic oxidation of challenging substrates. Kinetic comparisons showed that this system is more efficient and stable than previously reported similar palladium-based catalysts. A neutral platinum dimer bearing bridging hydroxy ligands has been isolated and fully characterised by X-ray diffraction and its involvement in the mechanism has been evidenced by mechanistic studies.

Full text of DOI:10.1002/ejoc.201801040

DOI: 10.1002/ejoc.201801040
Full Paper
Platinum vs. Palladium
Phosphinous Acid Platinum Complex as Robust Catalyst for
Oxidation: Comparison with Palladium and Mechanistic
Investigations
Romain Membrat,[a] Alexandre Vasseur,[a] Alexandre Martinez,*[a] Laurent Giordano,*[a] and
Didier Nuel*[a]
Abstract: Secondary phosphine oxides proved to be effective based catalysts. A neutral platinum dimer bearing bridging
preligands to stabilise a hydroxy-platinum based catalyst that hydroxy ligands has been isolated and fully characterised by
allows the aerobic/anaerobic oxidation of challenging sub- X-ray diffraction and its involvement in the mechanism has
strates. Kinetic comparisons showed that this system is more been evidenced by mechanistic studies.
efficient and stable than previously reported similar palladium-
Introduction
tion is still a challenging research objective. In this respect, the
chemoselective oxidation of alcohols under mild conditions
The design of robust metal complexes able to give raise to long arouse a great interest in the chemists community involved in
life catalysts efficient in mild conditions and avoiding degrada- organometallic chemistry[1,2] enegised by the difficulty
Scheme 1. Alcohols oxidation: proposed strategy to retard black Pd deposit.
[a] Aix-Marseille Univ, Centrale Marseille, CNRS,
of such a study goal. The use of palladium(II)-based catalysts[3]
13397, Marseille, CNRS, iSm2 UMR 7313,
France.
in combination with oxygen[4–6] has driven the design of well-
E-mail: didier.nuel@centrale-marseille.fr
defined complexes for achieving highly selective oxidation of
alcohols.[7–9] Although remarkable achievements have been
alexandre.martinez@centrale-marseille.fr
laurent.giordano@centrale-marseille.fr
http://ism2.univ-amu.fr/fr/annuaire/chirosciences/nueldidier
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201801040.
witnessed, the main issue in these reactions still remains the
catalyst degradation. In a recent work, we demonstrated that a
way to tackle this problem lies in the use of secondary phos-
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phine oxides (SPOs) as efficient pre-ligands to stabilise palla- by the following reasons (i) 1a was previously highlighted as a
dium(II) species thus retarding the black palladium deposit precursor of hydroxy-palladium species efficient for the oxid-
(Scheme 1).[10] This particular stability can be attributed to ation of alcohols[10] (ii) Compared to 1a or 1b, 1c should exhibit
(a) the use of air and water insensitive PA ligands, (b) the a slightly better oxophilicity,[22] and could therefore be promis-
hydrogen interaction bonding the two PA ligands leading to a ing. The results are presented in Table 1.
bidentate like that provides a chelate effect, (c) the excellent σ-
Table 1. Comparison of catalysts activity.[a]
electron-donating properties of this particular type of phos-
phine that greatly stabilise the PdII species and finally (d) the
formation of a palladium hydride that contains no other X li-
gand, which thus prevents the reductive elimination and there-
fore the palladium black formation. Although providing some-
what ground-breaking results, our palladium-based system re-
quired relatively elevated temperature and was not compatible
with peculiar substrates such as 1,2-diols, 1,3-diols[1,11] and
amino alcohols. In this context, recently, the use of additives
such as styrene has been proved beneficial for these pur-
poses.[12] Having in mind that platinum hydride are easily
formed and stable complexes,[13,14] we anticipated that a plati-
num based catalytic system similar to our palladium one could
be more efficient and would possibly allow to work at lower
temperature.
Entry Cat.
Additive
Temp [°C]
Time
Conv. [%]
1
2
3
4
5
6
7
8
1a
1b
1c
1a
1b
1c
1a
1b
1c
1a
1b
1c
1a
1b
1c
–
–
–
–
–
–
20
20
20
70
70
70
20
20
20
20
20
20
70
70
70
72 h
72 h
72 h
16 h
16 h
16 h
3 h
–
–
15
41
64
100
11
3
73
36
17
91
58
52
100
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
3 h
3 h
9
Thus, we report herein a platinum-based catalytic system
both highly efficient for the oxidation of notoriously difficult
substrate classes and operating under mild conditions. Further-
more, we provide mechanistic insights and striking evidence of
the involvement of hydroxy-platinum complex in the catalytic
process as well as the full characterisation of its dimer.
10
11
12
13
14
15
16 h
16 h
16 h
40 min
40 min
40 min
[a] Conditions: 2a (0.6 mmol), Methyl Vinyl Ketone (MVK) (1.2 mmol), 1
(5 mol-%), when used: 0.1 -NaOH (10 mol-%), toluene (6 mL).
M
Results and Discussion
We compared the catalytic activities of phosphinous acid (PA)
coordinated platinum(II) complexes [(PA)-PtII] with palladium
analogues [(PA)-PdII] in oxidation reactions of alcohols. In order
to assess the potential of such a species, we turned our atten-
tion towards a class of compounds particularly resistant to oxid-
ation as target substrates, namely N-alkyl-2,2,6,6-tetramethyl-
piperidin-4-ols which were weakly reactive in our palladium ca-
talysed system. Moreover, they are of particular interest as their
corresponding ketones are added-value compounds. Indeed,
they are largely used for their antioxidant and stabilizing prop-
erties in polymer chemistry[15–18] and are also useful in carbon
dioxide removal from gas steaming in industrial chemical proc-
esses.[19–21]
Although all the tested catalysts appeared to be efficient in
neutral medium at 70 °C at first sight, only 1c furnished a partial
conversion at room temperature and a total conversion in 16 h
(Table 1 entries 1–6). It is worth noticing that the addition of
sodium hydroxide (10 mol-% relative to substrate) proved to be
highly beneficial in all cases (Table 1, entries 7–15). But here
again, only 1c led to a satisfactory conversion at room tempera-
ture irrespective of the considered reaction time (Table 1, entry
7–12) and a complete conversion in 40 minutes at 70 °C
(Table 1, entry 13–15). It should be stressed that adding NaOH
allowed reaching a nearly total conversion at room temperature
when 1c was used (Table 1, entry 12), the total conversion be-
ing attained in 24 h. While 1a and 1b were inactive in the
absence of NaOH,[23] the use of 1c revealed a 30 min-induction
period which could be attributed to the time required for the
conversion of the inactive precatalyst 1c into an active species.
This induction period can be drastically reduced either by in-
creasing the temperature or by adding sodium hydroxide or
both, in which case the induction period nearly disappears irre-
spective of the used precatalyst 1. As 1a and 1b were poorly
efficient at room temperature, kinetic studies were performed
at 70 °C. The estimated kinetic constant ratios (Figure 1 and
Figure in part 5.1 of the SI) showed that 1c was eight times
faster than 1a (k1c/k1a = 7.38) in the presence of NaOH at that
temperature. Furthermore, it was also four times faster than 1b
(K1c/k1b = 4.05) under the same conditions. Eventually, the
k1c/k1a ratio was evaluated at 7.73 while the K1c/k1b ratio was
2.82 in the absence of NaOH.
1.1 Catalysts Comparison
We used as a bench marking reaction the oxidation of the chal-
lenging substrate 2a.[11] We compared the activities of the two
palladium-based catalysts [(PA)-PdII] 1a–b and the platinum-
based catalyst 1c (Scheme 2). This selection can be rationalized
Scheme 2. Studied catalysts precursors and model substrate.
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Scheme 3. Scope of the alcohol oxidation using 1c as catalyst precursor.
Conditions: alcohol (0.6 mmol), methyl vinyl ketone (MVK) (1.2 mmol), 1c
(5 mol-%), 0.1 M-NaOH (10 mol-%), toluene (6 mL), room temp., 16 h.
Figure 1. Compared kinetic profiles of the catalytic systems at 70 °C without
(top) or with (bottom) addition of NaOH.[a] Conversions were measured by
1H NMR spectroscopy. Profiles obtained by the arithmetic mean of 3 experi-
ments.
our efforts on this last challenge. Although some efficient and
selective catalytic systems have been proposed to this
end,[12,27,28] it should be noted that most of them are based
on the use of palladium in combination with nitrogen li-
gands,[29–31,12,32,33] rather than oxidation-sensitive phosphine-
type ligand. To our delight, our method allowed the totally se-
lective oxidation of such substrates in good to excellent yields
as evidenced by the compounds 3i (89 %) and 3j (68 %).
Interestingly, our platinum-based conditions proved to be
compatible with hydroxyl ethers where similar chelation prop-
erties could be expected (see 3k–l). Finally, compounds pos-
sessing base-sensitive functional groups like esters were also
reactive (see 3m).
1.2 Scope of the Reaction
We then examined the scope of our optimised catalytic system.
Oxidation reactions of N-Alkyl-2,2,6,6-tetramethyl-piperidin-4-ol
derivatives with different substituents at the nitrogen atom,
which are not prone to be oxidised through palladium catalysis
methods,[1,11] were conducted at room temp. with 1c as precat-
alyst and sodium hydroxide as additive (10 mol-% relative to
substrate). The results are summarised in Scheme 3.
Notwithstanding the difficulty of such a challenge,[11] N-Alkyl
2,2,6,6-tetramethylpiperidin-4-ols could be oxidised smoothly in
our conditions as shown in Scheme 3 (see products 3a–f, 67–
99 % yield). It is worth noticing that the reaction can be run
with a smaller amount of catalyst although with a slight loss of
efficiency. For example, 3a was obtained in 75 % yield and 3f
in 50 % yield when using 1 mol-% of 1c in the same conditions.
Whereas N-methyl 2,2,6,6-tetramethylpiperidinol was converted
into 3g in an excellent yield (95 %), 3h was obtained in a disap-
pointing 14 %. This result clearly highlights the crucial role of
all-carbon quaternary centres being positioned adjacent to the
nitrogen. This can be attributed to the highly sterically crowded
environment of the metal and the restricted number of degrees
of freedom associated[24] that prevent the poisoning of the cat-
alyst through coordination of the nitrogen atom to the metal.
1.3 Mechanistic Investigations
To learn more about the nature of the active catalyst, we pur-
sued our mechanistic investigations. In view of the above
kinetic studies (Figure 1) and the literature data,[10,11,34,35] we
reckoned that the noticed induction period could be ascribed
to the time required for the conversion of the inactive 1c into
the dimeric hydroxy-platinum catalyst 4 from which the active
monomeric specie A would be formed (Scheme 4). We then
suggested that the reaction of A with an alcohol derivative
would lead to the corresponding ketone and the hydrido-metal
species B, either through the formation of an alkoxy-platinum
intermediate followed by a ꢀ-H elimination or a metal pro-
Since the oxidation of 1,2- or 1,3- diols into α- or ꢀ-hydroxy moted direct H-abstraction .[36] Coordination of MVK to the
ketones respectively is particularly challenging owing to poten- metal centre followed by Insertion of its double bond into the
tial metal chelation or competitive reaction,[25,26] we focused Pt–H bond would provide C, in tautomeric equilibrium with the
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Pt-enolate D. Eventually, hydrolysis step of the O–Pt bond of D approach adapted from the one proposed by Sodeoka's group
would deliver the hydrogenated acceptor H2MVK and the active for the synthesis of a dihydroxypalladium dimer,[54] namely us-
catalyst species A. it should be noted that no PA ligand de- ing silver hexafluorophosphate as the chloride abstractor to
coordination was observed during the process.[37]
avoid any hydrogen abstraction (Scheme 5).
Scheme 5. Synthesis of the hydroxyl platinum dimer 4.
Thus, the reaction of 1c with AgPF6 was carried out in wet
dichloromethane in the presence of molecular sieves. The cat-
ionic intermediate 5 was thereafter obtained after filtration
through celite© followed by purification. Subsequent treatment
of the latter by diluted sodium hydroxide (0.1
M) led to the
formation of 4 as the major product in 56 % isolated yield after
crystallisation. 1H NMR analysis showed a resonance at δ =
–0.8 ppm corresponding to the proton of the OH group. The
31P NMR spectrum of compound 4 exhibited a resonance peak
at δ = 61.6 ppm in agreement with the chemical shifts observed
for similar complexes[55] and the large Pt-P coupling constant
of 5063 Hz is consistent with the phosphorus atoms in cis posi-
tion.[56,57] This also confirmed that the major phosphorus con-
taining product in our reaction is the expected hydroxyl plati-
num compound. The absence of any signal for the PF6 anion
(δ = 146 ppm) confirmed that complex 4 is neutral and it is, to
the best of our knowledge, the first neutral complex of this
type. This is due to the particular bidentate-like form of the
phosphinous acid ligands that provided an intramolecular an-
ion. Careful crystallization in hexane/DCM mixture (2 layers)
gave crystals of 4 suitable for X-ray diffraction analysis (Fig-
ure 2).[58] The two platinum centres present a square-planar
geometry although the dihedral angles Pt-O–Pt-P exhibit a very
slight tetrahedral distortion [maximum deviation from the
mean plane of 0.02 Å]. The distance between the two metal
centres (3.328 Å), the average Pt–O bond length of 2.130 Å as
well as O–Pt–O angles are similar to those observed in hydroxyl-
bridged dinuclear complexes without metal–metal interaction
(Table 2).[39,40,47,49]
Scheme 4. Proposed mechanism for the platinum-catalysed oxidation of alco-
hols.
Although seldom demonstrated, hydroxy-metal species are
often suspected to be active in such a transformation when an
aqueous medium is used.[11] As a consequence, it appeared
clear to us that synthesising, characterising and testing this up
till now putative complex 4 was essential. It should be
noted that quite a few hydroxy-platinum dimers bearing
mono-[38–45] or di-phosphine[46–49] ligands have been already
reported. Nevertheless, to the best of our knowledge none has
been made with SPOs as preligands. The reaction of platinum
chloride with SPOs in our conditions led to the formation of a
mixture of products (see 31P NMR spectra in SI) containing one
major compound (δ = 61.5 ppm in 31P NMR). Unfortunately, we
were unable to isolate it. As we suspected it may be the
hydroxyl derivative, we sought for an alternative method which
would allow us to synthesise and characterise this compound
and confirm (or not) this hypothesis.
Direct treatment of 1c by sodium hydroxide or NBu4OH in
Likewise, P–Pt–P angles values are consistent with those re-
dichloromethane at reflux gave only decomposition of the ported for similar complexes bearing either PAs[59–62] or bident-
starting material whereas no reaction occurred at room temper- ate phosphines[4,12–14] as ligands. Finally, the short distances be-
ature. Trivial treatment of 1c with silver or sodium tetrafluoro- tween the two oxygen atoms of the PA ligands [O(1) O(2):
borate should be proscribed as this might result in the replace- 2.389 Å, O(5) O(6): 2.398 Å] suggest a strong H–O–H bonding.[63]
ment of the proton linking the PA ligands by a BF2 unit.[50–53] Accordingly, all P–O bond lengths are of similar values [in the
Using AgNO3 to remove the chloride ligands of 1c resulted in range 1.5224(10)–1.553(10) Å][64] and are significantly longer
a complex mixture of compounds. We then turned toward an than P=O distances (1.48–1.50 Å).[65,66]
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Figure 3. Compared kinetic profiles of the reaction at room temperature using
1c or 4 in early times of the reaction – conversions were measured by 1H
NMR spectroscopy. Profiles obtained by the arithmetic mean of 3 experi-
ments.
ure 4). It appeared that 4 was indeed appreciably active in dry
medium while 1c proved to be totally inactive even though the
former afforded only a 20 % conversion after a 45 h-time reac-
tion. Adding water (1:8 v/v H2O/toluene) proved very slightly
beneficial with both complexes while the addition of sodium
hydroxide allows to recover a good activity. This clearly showed
that 4 is not only the active hydroxyl monomer species but also
the outcome of the reaction of the precatalyst 1c with sodium
hydroxide.
Figure 2. Ellipsoid plot of the X-ray structure of 4 – only polar hydrogen are
shown.
Table 2. Selected bond lengths [Å] and angles [°] for 4. Estimated standard
deviations in parentheses.
Pt(1)–P(1)
Pt(1)–P(2)
Pt(1)–O(3)
Pt(1)–O(4)
Pt(2)–P(3)
2.232(3)
2.227(4)
2.146(10)
2.131(9)
2.234(4)
2.245(3)
2.103(9)
2.140(9)
1.553(10)
1.538(11)
1.550(12)
1.524(10)
91.43(14)
97.9(3)
O(4)–Pt(1)–P(1)
O(4)–Pt(1)–P(2)
O(4)–Pt(1)–O(3)
P(3)–Pt(2)–P(4)
O(3)–Pt(2)–P(3)
O(3)–Pt(2)–P(4)
O(3)–Pt(2)–O(4)
O(4)–Pt(2)–P(3)
O(4)–Pt(2)–P(4)
O(1)–P(1)–Pt(1)
O(2)–P(2)–Pt(1)
O(5)–P(3)–Pt(2)
O(6)–P(4)–Pt(2)
Pt(2)–O(3)–Pt(1)
Pt(1)–O(4)–Pt(2)
174.6(3)
93.9(3)
76.8(4)
91.59(14)
92.8(3)
175.1(3)
77.5(4)
Pt(2)–P(4)
Pt(2)–O(3)
Pt(2)–O(4)
P(1)–O(1)
P(2)–O(2)
P(3)–O(5)
168.9(3)
98.3(3)
116.6(4)
117.7(5)
117.5(5)
116.9(4)
103.2(5)
102.4(4)
P(4)–O(6)
Figure 4. Effects of adding water and then NaOH at room temperature.
P(2)–Pt(1)–P(1)
O(3)–Pt(1)–P(1)
O(3)–Pt(1)–P(2)
169.7(3)
Another evidence of the involvement of 4 in the catalytic
process is given by the 31P NMR spectrum of the crude material
at the end of the reaction.[67] Starting with 1c led to a complex
mixture of phosphorus-containing compounds among which 4
could be detected. In contrast, using well-defined 4 as the cata-
lyst led to a much cleaner reaction and 4 was the major com-
plex detected at the end of the reaction.
Moreover, other information could be extracted from those
studies. Indeed, the regeneration of the active catalytic species
and production of the reduced Michael acceptor could proceed
through two pathways. The first one could involve a hydrolysis
step from D (Scheme 6 path a) while the second one would
In order to demonstrate that 4 is actively involved in the
catalytic cycle, we studied its activity toward our model sub-
strate 2a in our optimised conditions. Firstly, we observed that
4 led to complete conversion in 16 h at room temperature. A
closer look at the results showed that 1c requires a short (< 5
minutes) induction time (Figure 3) while 4 was immediately ac-
tive and more efficient than 1c (k4/k1C = 2) in the early stages
of the reaction. This difference could be due to the time re-
quired to convert the inactive 1c into the active 4 and therefore
to a low concentration of active species.
In our mechanism suggested in Scheme 4, water would be consist in a ligand exchange between a free OH– and the enol-
produced through the oxidation of the alcohol on one hand, ate ligand in D (Scheme 6 path b). When 4 was used, the addi-
and 4 would be delivered by reaction of 1c with water on the tion of water led to a slight acceleration of the reaction while
other hand. Therefore, if 4 actually resulted from 1c, it would adding NaOH greatly enhanced the rate. As, in this case, the
be active in a dry medium. We thus performed kinetic studies active platinum hydroxyl species is already formed, this result
of the same reaction using dry toluene as the sole solvent (Fig- tends to demonstrate the involvement of the hydroxide ion in
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2.2. Syntheses:
the regeneration step of the active catalytic species and high-
lights the path b as the major route.
General Procedure for N-Alkylation of 2,2,6,6-Tetramethylpiper-
idin-4-ol: In a 30 mL PARR autoclave was introduced the 2,2,6,6-
tetramethylpiperidin-4-ol (2.0 g, 12.7 mmol) and the allyl or benzyl
bromide (0.5 equiv.) dissolved in dry toluene (13 mL). The autoclave
was sealed and the mixture was heated to 130 °C. After 40 h whilst
stirring, the mixture was diluted in AcOEt and water. The aqueous
layer was extracted 3 times with AcOEt (25 mL) and combined or-
ganic layers were washed with water (20 mL), brine (20 mL) and
dry over Na2SO4. After concentration under vacuum, the residue
was purified by flash chromatography on silica gel using a combina-
tion of petroleum ether and ethyl acetate for benzylic compounds
and dichloromethane and methanol for allylic derivatives to yield
pure desired products. All analytical data are available in ESI.
General Procedure for Catalysed Anaerobic Alcohols Oxidation:
In a flame-dried 50 mL Schlenk tube was introduced under argon
atmosphere 1c (17.7 mg, 0.03 mmol) and 2 (0.6 mmol) dissolved in
dry toluene (6 mL). Methyl vinyl ketone (100 μL, 1.2 mmol) and 0.1
M
-NaOH aqueous solution (0.6 μL, 0.06 mmol) were then added to
the reaction mixture. The reaction was heated when necessary and
stirred for the desired time. Organic layer was separated and aque-
ous layer was extracted with ethyl acetate (3 × 5 mL). Combined
organic layers were dried with Na2SO4 and concentrated under vac-
uum. The crude resulting product was purified if necessary by col-
umn chromatography on silica gel using an appropriated combina-
tion of petroleum ether and ethyl acetate to yield pure desired
products. More detailed procedure and analytical data are available
in ESI.
Scheme 6. Possible paths for the regeneration of A.
Conclusions
In conclusion we have developed a new very stable SPO plati-
num based catalyst particularly efficient in the oxidation of
highly challenging substrates working in weakly basic medium
at room temperature. This efficiency clearly demonstrates that
the phosphinous acid ligands greatly stabilise the metal com-
plex, thus retarding the catalyst degradation and the black
metal deposit. We have also isolated and fully characterised the
first hydroxyplatinum dimer bearing phosphinous acid ligands
and obtained strong evidence of its implication in the anaero-
bic oxidation of alcohols. The easy availability of the [Pt](μ-OH)
4 would allow us to explore a new broad field of metal-cata-
lyzed reaction, including asymmetric versions or C–C bonds
forming reactions.
Di-μ-hydroxotetrakis-[(RP)-tert-butylphenylphosphinito-κ-π]di-
platinate (4): In a 50mL flamed dried Schlenk tube under argon
atmosphere,
di-μ-chlorotetrakys-[tert-butylphenylphosphinito-κ-
P]diplatinate(2–) 1c (150 mg, 0.25 mmol) were dissolved in wet di-
chloromethane (20 mL) and molecular sieves 4 Å (20 g) were added.
AgPF6 (63 mg, 0.25 mmol), weighted in a glove box was added
in one portion and the Schlenk tube was protected from light by
aluminium foil. The mixture was stirred at 15 °C (the temperature
must be precisely maintained). After 4 h, the crude mixture was
filtered through a short pad of Celite© and then on a short pad of
no flash silica (elution with Et2O). Volatiles were evaporated and
the resulting yellow solid was introduced in a 50 mL Schlenk tube,
dissolved in wet DCM and treated with 0.1 M-NaOH aqueous solu-
Experimental Section
tion (5 mL). After stirring at room temperature for 1 h, the aqueous
layer was removed and the organic layer was dried with Na2SO4
and filtered through a short pad of non-flash silica (elution with
Et2O) and concentrated under vacuum in order to obtain the de-
sired complex 4 as a crystalline yellow powder in 56 % yield (83 mg)
with a 95:5 meso/dl ratio. Careful recrystallization in hexane/DCM
gave small yellow crystals suitable for X-ray Diffraction. Analytical
data and cif file (CCDC 1582905) are available in ESI.
2.1. General Information: All solvents were purified by standard
procedures or obtained from a Solvent Purification System (Braun
SPS 800). Unless otherwise mentioned all reactions were carried out
under an atmosphere of dry argon. Commercial organic reagents
were purchased from ALDRICH and used without further purifica-
tion. Analytical Thin layer chromatography (TLC) was carried out on
Merck silica gel 60 F254 and product were revealed under ultravio-
let light (254 nm and 366 nm), or stained with dyeing reagent
solutions as 5 % phosphomolybdic acid solution, potassium per-
manganate solution or p-anisaldehyde solution in ethanol followed
by gentle heating. Preparative Flash chromatography was per-
formed with Merck silica gel 60 (230–400Mesh) unless otherwise
mentioned. 1H, 13C and 31P spectra were recorded on Bruker Avance
III nanobay spectrometers operating at 400 and/or 300 MHz. Melt-
ing points (uncorrected) were determined in a capillary tube with
a Mettler LP61 apparatus. High Resolution MS experiments were
performed with a QStar Elite Mass Spectrometer (Applied Biosys-
tems SCIEX, Concord, ON, Canada) equipped with an electrospray
ionization (ESI) source. Complexes 1a,[68] 1b[50] and 1c[59] were syn-
thesized according to reported procedures.
CCDC 1582905 (for 1c) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre.
Acknowledgments
This work was supported by the Ministère de l'Enseignement
Supérieur et de la Recherche (R. Membrat Ph.D. grant), the Cen-
tre National de la Recherche Scientifique (CNRS), AMU, and Cen-
trale Marseille. We thank Dr. Sabine Michaud-Chevallier for her
help in ESI(+)-MS studies, Christophe Chendo and Dr. Valérie
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In order to check that there is no de-coordination of the PA ligands we
performed control experiments (see SI for details). Firstly, we run a reac-
tion between the free OPS and the MVK in our conditions and found no
product (the SPO remains intact). Secondly, complex 1c was stirred with
MVK in the same conditions and 1c was recovered at the end of the
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Full Paper
Platinum vs. Palladium
R. Membrat, A. Vasseur, A. Martinez,*
L. Giordano,* D. Nuel* ...................... 1–9
Phosphinous Acid Platinum Com-
plex as Robust Catalyst for Oxid-
ation: Comparison with Palladium
and Mechanistic Investigations
The use of phosphinous acids as li- is achieved without degradation of the
gands of palladium or platinum com- catalyst. Mechanistic investigations
plexes allows the formation of very give evidence of the implication of a
stable catalytic systems. Efficient anae- platinum hydroxyl species.
robic oxidation of unreactive alcohols
DOI: 10.1002/ejoc.201801040
Eur. J. Org. Chem. 0000, 0–0
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9
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim