Synthesis, characterization and Pd(II)-coordination chemistry of the ligand tris(quinolin-8-yl)phosphite. Application in the catalytic aerobic oxidation of amines

Article abstract of DOI:10.1039/c7dt04000h

The synthesis and characterization of the ligand tris(quinolin-8-yl)phosphite, (P(Oquin)₃), are described and its coordination chemistry toward the metal precursor [Pd(COD)Cl₂] (COD = 1,5-cyclooctadiene, C₈H₁₂) is reported. A new Pd(ii)-P(Oquin)₃ metal complex was isolated and fully characterized ([Pd{P(Oquin)₃}Cl₂]), and its X-ray diffraction analysis shows that the phosphite ligand coordinates as a bidentate P-N chelate. This complex is an efficient catalyst for the solvent-free mild oxidative coupling of primary amines to imines using air as an oxidant, obtaining moderate to good yields (up to 99%) and turnover numbers (TONs up to 230). This catalyst can be recovered from the reaction mixture and reused in a subsequent run without a significant loss of activity. Kinetic measurements of the oxidation of benzylamine suggest that the rate law is r = k_app[BnNH₂][cat] ([BnNH₂] = molar concentration of benzylamine, [cat] = molar concentration of [Pd{P(Oquin)₃}Cl₂], k_app = k[O₂]^c = 0.756 L mol^-1 h^-1 = apparent rate constant). In situ NMR tests were performed to gain some insight into the reactivity of the Pd(ii)-P(Oquin)₃ complex toward benzylamine.

Full text of DOI:10.1039/c7dt04000h

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DOI: 10.1039/C7DT04000H.
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Synthesis, Characterization and Pd (II)-Coordination Chemistry of
the Ligand Tris(quinolin-8-yl)phosphite. Application in Catalytic
Aerobic Oxidation of Amines †
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
R. E. Rodríguez-Lugo,*, a,b M. A. Chacón-Terán,a S. De León,a M. Vogt,c A. J. Rosenthal,d and V. R.
Landaeta.*,a
www.rsc.org/
Abstract: The synthesis and characterization of the ligand tris(quinolin-8-yl)phosphite, (P(Oquin)3), are described and its
coordination chemistry toward the metal precursor [Pd(COD)Cl2] (COD = 1,5-cyclooctadiene, C8H12) is reported. A new Pd
(II) - P(Oquin)3 metal complex was isolated and fully characterized ([Pd{P(Oquin)3}Cl2]), and its X-ray diffraction analysis
shows that the phosphite ligand coordinates as a bidentate P-N chelate. The complex is an efficient catalyst for the
solvent-free mild oxidative coupling of primary amines to imines using air as oxidant, obtaining moderate to good yields
(up to 99%) and turnover numbers (TONs up to 230). The catalyst can be recovered from the reaction mixture and reused
in a subsequent run without significant loss of activity. Kinetic measurements of the oxidation of benzylamine suggest that
the rate law is r = kapp[BnNH2][cat] ([BnNH2] = molar concentration of benzylamine, [cat] = molar concentration of
[Pd{P(Oquin)3}Cl2], kapp = k[O2]c = 0.756 L mol-1 h-1 = apparent rate constant). In situ NMR tests were performed to gain
some insights on the reactivity of the Pd (II) - P(Oquin)3 complex toward benzylamine.
three equivalents of the corresponding alcohol. The great
variety of suitable alcohols allows for an easy tuning of the
steric and electronic properties of the phosphite ligand.2,7 In
Introduction
Phosphites, of general formula P(OR)3, are trivalent σ3λ3
phosphorus compounds similar to phosphines1 which have
attracted considerable attention lately, both in coordination
chemistry and homogeneous catalysis.2 The importance of
phosphites as steering ligands in catalytic reactions was
underlined in 1971 when the nickel-catalyzed hydrocyanation
of butadiene was commercialized.3,4 Unlike the first
hydrocyanation catalysts based on cobalt carbonyl complexes,5
the use of the Ni-phosphite system yields a selective process
toward anti-Markonikov products with terminal alkenes.6 One
of the main advantages of phosphites is that they can be
readily prepared by treating phosphorus trichloride (PCl3) with
general, phosphites are less prone to oxidation than the
corresponding phosphines. However they can decompose by
hydrolysis, alcoholysis and the Arbuzov reaction.2c
Nevertheless, it is well known that the rate of hydrolysis is
often low for bulky phosphites.2a These ligands are strong π-
acceptors and are commonly used to stabilize electron-rich
transition metals,7 although phosphite-containing complexes
have been reported with a wide variety of metal centers.2,7
The P(OR)3 function is present in many ligands combined with
2c, 8-11
other coordinating heteroatoms such as nitrogen.2a,
A
wide range of phosphite ligands with P-N coordination modes
using oxazoline,9 imine10 and amine11 units has been
developed, which have been used to prepare Ir,9d,e Rh,10a Ni11c
a.Universidad Simón Bolívar, Departamento de Química,
Valle de Sartenejas, Baruta. Caracas 1020-A, Venezuela
E-mail: rerodriguez@usb.ve, rrodriguez@ivic.gob.ve, vlandaeta@usb.ve.
b.Laboratorio de Química Bioinorgánica, Centro de Química
Instituto Venezolano de Investigaciones Científicas (IVIC)
Carretera Panamericana Km. 11, Altos de Pipe. Caracas 1020-A, Venezuela.
c. Institut für Anorganische Chemie und Kristallographie, Universität Bremen
Leobener Straße 7, C2060, NW2, 28359 Bremen, Germany.
d.Department of Chemistry and Applied Biosciences
9a-c,f, 11a,b
and Pd3,
complexes. Phosphite ligands, and their
corresponding metal complexes, bearing the quinoline moiety
are known (Figure 1, compounds
).12 However, to the best
A-O
of our knowledge, no reports are found in the literature for the
compound tris(quinolin-8-yl)phosphite, (P(Oquin)3).
Imines and their derivatives are important building blocks
for the synthesis of heterocyclic compounds and fine
chemicals of pharmaceutical and biological relevance.13 These
are usually prepared through effective Lewis acid-catalyzed
condensation reactions between amines and carbonyl
compounds in presence of dehydrating agents,14 but of other
protocols have attracted considerable attention.15 Among
Eidgenössische Technische Hochschule Zürich
Vladimir-Prelog-Weg 1-5/10, 8093 Zürich, Switzerland
e.
† This contribution is in memory of Dr. Roberto A. Sánchez-Delgado (1950-2015). A
lifetime dedicated to organometallic chemistry and catalysis (RIP).
Electronic Supplementary Information (ESI) available: Experimental details,
spectral and kinetic data and CIF files for ligand
(CCDC 1525821). See DOI: 10.1039/x0xx00000x
2 (CCDC 1547722) and complex 3
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them, one-pot oxidative processes are interesting alternatives obtained via slow evaporation of a THF solution of 2. The molecular
since these reduce energy consumption, waste emissions, and structure of 2 is shown in Figure 2. Two independent molecules are
the number of operating stages and purification steps observed in the asymmetric unit (Figure 2), with a distorted
compared to traditional methodologies.16 Three kinds of trigonal-pyramidal geometry around the
P atom (pseudo-
oxidative processes for imine formation have been tetrahedral taking the phosphorus lone-pair into account). The
described:17 1) (dehydrogenative) oxidative couplings of heterocyclic rings are oriented in a way that the O—C bonds are
alcohols with amines;18 2) self-coupling of primary amines;19 directed toward the electron lone pair of the P atom. In this
and 3) oxidative dehydrogenation of secondary amines,20 scenario, two nitrogen centers (N3/N2 and N5/N6) are pointing
based on metal,21 metal-free,22 photo-,20b, 23 and bioinspired24 toward the vertex of the pyramid (P1 and P2, correspondingly)
catalysts. The atmospheric oxidation of amines mediated by while the third nitrogen is directed away of the phosphorus center
transition metals has been described.21-35 Homogeneous (N1 and N4, correspondingly). Indeed, each of the nitrogen atoms
systems based on Cu,24b, 25-29 V,19b, 30, 31 Zn,32 Co,33 Fe,34 Ag,35 N2 and N6 shows a short contact with the oxygen O1 and O4,
Pd,36, 37 Au,38 Ir39 and Os23b coordination compounds have been correspondingly. The N–O interatomic distance is in average 3.03
used as catalysts for the aerobic oxidation of amines to imines. (4) Å. An interesting feature in the crystal packing of 2 is the
existence of π-stacking between proximal N-heterocyclic rings in
each one of the independent molecules (see Supporting
Information). The centroid-centroid distance is 3.765 Å. This value is
within the equilibrium distances calculated by Sherrill40 for π-
stacking, i.e. 3.45-3.95 Ǻ.
Scheme 1. Synthesis of 2, tris(quinolin-8-yl)phosphite (P(Oquin)3).
Fig. 1. Reported phosphorus compounds bearing the 8-oxy-quinoline unit.
Here the synthesis and characterization of the ligand P(Oquin)3
and its corresponding Pd (II) complex [Pd{P(Oquin)3}Cl2] is
described. The application of this coordination compound as
homogeneous catalyst for the oxidative coupling of primary amines
to yield imines is also discussed, along with kinetic measurements.
To the best of our knowledge, no examples of Pd complexes bearing
phosphite ligands have been reported as catalysts for this type of
reaction.
Fig. 2. ORTEP drawing of the molecular structure of the phosphite ligand tris(quinolin-
8-yl)phosphite, P(Oquin)3 2. Thermal ellipsoids are drawn at 50% probability level.
Hydrogen atoms have been neglected for clarity. Two independent molecules per
Results and discussion
asymmetric unit were found. The values reported for selected bond distances and
Synthesis of the ligand P(Oquin)3.
angles correspond to the average of both independent molecules. Selected bond
distances [Å] and angles [°]: P1—O1/P2—O4 1.6406 (11), P1—O2/P2—O5 1.6558 (12),
Upon reaction of PCl3 with a slight excess of 8-hydroxy-
P1—O3/P2—O6 1.6526 (12), O1—C8/O4—C35 1.3752 (18), O2—C17/O5—C44 1.3799
quinoline (1), in presence of triethylamine, the new phosphite
(18), O3—C26/O6—C53 1.3767 (19); O1—P1—O2/O4—P2—O5 96.06 (6), O2—P1—
ligand tris(quinolin-8-yl)phosphite, (P(Oquin)3 2), was obtained
O3/O5—P2—O6 88.34 (6), O3—P1—O1/O6—P2—O4 96.23 (6).
(Scheme 1). The product was isolated as a white microcrystalline
solid in high yield (75%) and can be prepared in a multigram scale
(10 g). P(Oquin)3 is very sensitive to air and moisture but can be
Several contacts are present in the asymmetric unit of 2: P···H,
stored at room temperature for months in a glovebox under inert
O···H, N···H and C—H··· C—H interactions. The average of the
atmosphere. The 1H and 31P{1H} NMR spectra of 2 in THF-d8, as well
interatomic distances and angles around the phosphorus atoms are
as the IR and the HRMS spectra, are provided in the Supporting
listed in Table 1. The P—O bond lengths are significantly longer in 2
Information. Crystals suitable for X-ray diffraction analysis were
than those in the related compound tris(1-naphthyl) phosphite,41 as
2 | J. Name., 2012, 00, 1-3
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shown in Table 1. The C—O distances and the O—P—O angles are The other two quinoline units are oriented in such a way that a
significantly shorter in 2 than the analogous distances for tris(1- short contact is established between the N atoms (N2 and N3) and
naphthyl) phosphite.41 As a matter of fact, to the best of our the O center (O1) of the chelating quinoline fragment. The N—O
knowledge, in average the O—P—O angles (93.5 degrees) in 2 are distance for this interaction is, in average, 3.01 (6) Å. The complex
shorter than the values previously obtained for any other phosphite exhibits a slightly distorted square planar geometry, since the
ligand, cyclic or acyclic. The shortest average value for this kind of chelate (plane P1/Pd1/N1) is not co-planar with the chlorine atoms
angle, 95.6 degrees, was the one reported for tris((2,6- (plane Cl1/Pd1/Cl2). The deviation from co-planarity between the
diphenyl)phenyl)phosphite.42 Such small bond angles are attributed planes P1/Pd1/N1 and Cl1/Pd1/Cl2 is only 5.06 degrees. The two
to the steric hindrance of the substituents surrounding the P-atom. aromatic units of the coordinated quinoline moiety are twisted, as
In average, the O—P—O angles of phosphite ligands are in the indicated by the C19-C7-N1-C11 and C19-C7-C17-C25 torsion angles
range of 97-98 degrees.
(Table 2, numbering scheme in Figure 3). This is similarly observed
in related compounds, L’-O’.12s,t By taking the bond distances ± 3σ,
the lengths Pd—P, P—O and Pd—Cl (trans to the P center) in 3 are
significantly shorter than in the related compounds L’-O’. The Pd—
Cl bond length (trans to the N atom) is similar to the one observed
in O’,12t while the interatomic distance Pd—N is similar to that
observed in L’.12s
Table 1. Selected geometric parameters (Å, °) for 2 and the related compound tris(1-
naphthyl) phosphite, P. 41
2[a]
P
Lengths (Å)
1.6406 (11)
1.6526 (12)
1.6558 (12)
1.3752(18)
1.3767 (19)
1.3799 (18)
1.6086 (11)
1.6376 (11)
1.6425 (11)
1.3889 (17)
1.3906 (18)
1.4026 (17)
Scheme 2. Synthesis of the Palladium (II) complex 3, [Pd{P(Oquin)3}Cl2].
P—O
C—O
N
CH2Cl2
N
O
N
N
O
O N
-75 °C to 25 °C
O P
O
P
Pd
Cl
Cl
3
[Pd(COD)Cl2]
-COD
O
N
2
Angles (Å)
88.34 (6)
96.06 (6)
96.23 (6)
94.73 (5)
98.02 (5)
101.91 (6)
O—P—O
[a] Average of the two independent molecules in the unit cell.
Synthesis and Characterization of the Pd (II) complex.
The reaction of 2 with the metal precursor [Pd(COD)Cl2] (COD =
1,5-cyclooctadiene,
C8H12)
was
studied.
The
complex
[Pd{P(Oquin)3}Cl2], 3, is accessible by treating such metal precursor
in dichloromethane with a solution of the ligand in the same solvent
at -75 °C (Scheme 2). When the reaction mixture was allowed to
reach room temperature, the coordination compound precipitated
as a pale yellow solid. The 1H and 31P{1H} NMR spectra of 3 in CDCl3,
as well as the IR and UV-Vis spectra, are provided in the Supporting
Information. The resonance of the 31P nucleus in the free ligand is
strongly shifted to lower frequencies upon coordination to the
palladium center (129 ppm for 2 vs. 32 ppm for 3). The opposite
effect is observed for the resonances of the protons in ortho and
para position with respect to the N atom. Upon coordination, such
protons are significantly shifted to higher frequencies compared
with those of the free ligand.
Fig. 3. ORTEP drawing of the molecular structure of the complex [Pd{P(Oquin)3}Cl2], 3.
Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms and
a
dichloromethane solvate molecule are neglected for clarity. Selected bond distances
[Å] and angles [°]: Pd1—P1 2.1513 (6), Pd1—N1 2.1275 (19), Pd1—Cl1 2.3027 (6),
Pd1—Cl2 2.3756 (6), P1—O1 1.5773(16), P1—O2 1.5962(15), P1—O3 1.5846(17); P1—
Pd1—N1 91.07(5), Cl1—Pd—Cl2 88.67(2), P1—Pd1—Cl1 85.00(2), N1—Pd1—Cl2 95.43
(5), P1—Pd1—Cl2 172.91(2), N1—Pd—Cl1 174.91(5).
Crystals suitable for X-ray diffraction analysis were obtained
from dichloromethane. The solid state structure of 3 reveals that
ligand 2 binds the metal center as a bidentate chelate through the P
atom and one of the N donors of the heterocycles (N1 in Figure 3).
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Table 2. Selected geometric parameters (Å, °) for 3 and the related compounds
PdL(allyl) (L’), PdMCl2 (M’), PdNCl2 (N’) and PdOCl2 (O’).12s,t
Table 3. Solvent free oxidative coupling of benzylamine to N-
benzylidenebenzylamine. Screening of catalysts.[a]
3
L’
M’
N’[a]
O’
Lengths (Å)
Yield[b] (%)
TON/TOF (h-1)
2.1513
(6)
2.2554
(14)
2.1765
(6)
2.1835
(6)
2.1804
(7)
Entry
Catalyst [cat]
Pd—P
Pd—N
1
2
3
4
5
6
7
[Pd{(P(Oquin)3)}Cl2]
[Pd(COD)Cl2]
>99
5
100/5.6
5/0.3
4/0.2
52/3.0
11/0.6
90/5.0
-
2.1275
(19)
2.137
(5)
2.0803
(18)
2.0615
(18)
2.072
(2)
[PdCl2]
4
2.3027
(6)
2.2880
(7)
2.2982
(6)
2.3000
(7)
Pd—Cl[b]
Pd—Cl[c]
-
-
[Pd(PPh3)2Cl2]
[Pd(P(OPh)3)2Cl2]
[Pd{(P(Oquin)3)}Cl2]/Hg(l)
None
52
11
90
2
2.3756
(6)
2.3809
(6)
2.3993
(6)
2.4130
(7)
1.5773
(16)
1.632
(4)
1.6176
(16)
1.6236
(16)
1.6244
(18)
[a] Reaction conditions: Benzylamine (154.3 mg, 1.44 mmol), [cat] (14 μmol, 1 mol%),
open to atmosphere, 80 °C, 18 h. [b] Yield determined by 1H NMR using 1, 3, 5-
trimethoxybenzene (121.1 mg, 0.72 mmol) as internal standard.
1.5846
(17)
P—O
-
-
-
-
-
-
-
1.5962
(15)
-
Table 4. Palladium catalyzed oxidative coupling of benzylamine to N-
benzylidenebenzylamine. Screening of reaction conditions: solvent.[a]
Torsion angles (°)
-4.76
-10.62,
11.21[d]
C19-C7-N1-C11
C19-C7-C17-C25
9.05
7.17
-6.72
-4.38
5.73
6.86
Entry
Solvent
None
Yield[b] (%)
TON/TOF (h-1)
100/5.6
87/4.8
1
2
3
4
>99
87
18
8
-10.56,
9.27[d]
-1.58
Toluene
2-Propanol
Acetonitrile
18/1.0
8/0.4
[a] Reaction conditions: Benzylamine (154.3 mg, 1.44 mmol), [Pd]
=
[a] Average of the two independent molecules in the unit cell. [b] Trans to N. [c]
Trans to P. [d] For each one of the independent molecules.
[Pd{(P(Oquin)3)}Cl2] (9.2 mg, 14 μmol, 1 mol%), solvent (1.4 mL, 1 mol/L amine), open
to atmosphere, 80 °C, 18 h. [b] Yield determined by 1H NMR using 1, 3, 5-
trimethoxybenzene (121.1 mg, 0.72 mmol) as internal standard.
Catalytic oxidative coupling of primary amines to imines.
Different catalyst loadings were evaluated (Table 5). Under the
established reaction conditions, the yield drops significantly for
catalyst loadings below 0.5 mol% (entries 4-6 vs. entries 1-3). The
model reaction was carried out using different oxidants (Table 6).
Oxygen, pure or present in air, is a suitable oxidant (entries 1-4),
while nitrous oxide or organic oxidants (quinones) are not
appropriate (entries 5-7). Under inert atmosphere (N2, entry 8) the
benzylamine homo-coupling does not occur, thus demonstrating
the need for oxidizing conditions. Entries 1 and 2 formally
correspond to experiments under different oxidant (O2) pressure,
but the long reaction time (18 h) masked its effect on the reaction
rate. The same experiments were conducted shortening the
reaction time (entries 3 and 4) to address the role of the oxygen
pressure. The yield drops significantly when the reaction is carried
out for 4 h open to atmosphere (61%) compared with the almost
quantitative oxidation of the amine under 50 psi air (ca. 10 psi O2)
during the same period of time.
Compound 3 was tested as catalyst precursor for the oxidative
homo-coupling of benzylamine to N-benzylidenebenzylamine, as a
model reaction (Table 3). Other Pd sources, such as [Pd(COD)Cl2] or
PdCl2, were not efficient catalyst precursors for this reaction under
the studied conditions (entries
2 and 3). Moreover, other
phosphorus-containing compounds, e.g. [Pd(PPh3)2Cl2] and
[Pd(P(OPh)3)2Cl2], were not as efficient as 3 for the oxidative homo-
coupling of benzylamine (entries 4 and 5). The mercury test
suggests that metallic palladium or nanoparticles are not likely to
be responsible for the activity observed (entry 6). In fact, no
palladium black is formed. In absence of catalyst (blank run) a very
poor yield was achieved (entry 7) confirming the catalytic nature of
the transformation.
The effect of the solvents on the oxidative coupling of
benzylamine to N-benzylidenebenzylamine catalyzed by 3 was
studied (Table 4). Under solvent-free conditions and in toluene the
yield is high, > 99% and 87% respectively (entries 1 y 2). Acetonitrile
and 2-propanol were not efficient as reaction media for this system,
evidenced by the diminished catalytic activity (entries 3 and 4).
These experiments were carried out open to atmosphere and
identical flasks were used in order to prevent problems of air
(oxygen) diffusion into the reaction mixture.
4 | J. Name., 2012, 00, 1-3
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(6 h), a high yield was obtained (entry 8). The need of longer
reaction times and higher temperatures when the reaction is
carried out at atmospheric pressure was confirmed by performing
the reaction at 60 °C and only 6h, which gave the product in
moderate yields (entry 9). At 60 °C, the reaction time cannot be
further shortened to 4 h without significantly affecting the activity
(entries 6 and 10). In conclusion, these experiments indicate that
the optimal reaction conditions are: no solvent, 0.5-1 mol %
[Pd{(P(Oquin)3)}Cl2] as catalyst precursor, at 60 °C for 6 h and using
air as terminal oxidant (30 psi, in 98 mL autoclave, ca. 1.72 mmol
O2, 1.2 equivalents). Under these conditions, the amine was cleanly
converted into the imine and neither benzaldehyde nor benzonitrile
were detected as side-products. Ammonia formation was
qualitatively proved by bubbling the gas phase of the reaction
through a basic mixture of KI and HgI2 (Nessler's reagent), upon
which a brown precipitate immediately formed. Finally, the
oxidative coupling of benzylamine to N-benzylidenebenzylamine
was also tested under more drastic conditions to aim at higher TON
and TOF (i.e., higher pressure and temperature, shorter reaction
time and lower catalyst loading). Although only a 23% of 5a was
achieved, a TON of 230 was reached and almost 60 molecules of
substrate were converted per molecule of catalyst in one hour
(Table 7, entry 11).
Table 5. Solvent free oxidative coupling of benzylamine to N-
benzylidenebenzylamine. Catalyst loading.[a]
Yield[b] (%)
TON/TOF (h-1)
100/5.6
137/7.6
120/6.7
113/6.3
86/4.8
Entry
[Pd] = [Pd{(P(Oquin)3)}Cl2] (mol%)
1
2
3
4
5
6
1.00
0.75
0.60
0.50
0.25
0.100
>99
>99
70
55[c]
21
9
93/5.1
[a] Reaction conditions: Benzylamine (154.3 mg, 1.44 mmol), open to atmosphere, 80
°C, 18 h. [b] Yield determined by 1H NMR using 1, 3, 5-trimethoxybenzene (121.1 mg,
0.72 mmol) as internal standard. [c] Average of two experiments (55 ±4%)
Table 6. Palladium catalyzed oxidative coupling of benzylamine to N-
benzylidenebenzylamine. Screening of reaction conditions: oxidant (ox).[a]
1 mol% [Pd]
NH2
N
2
+ NH3(g) + H2ox
18h, 80 °C
oxidant (ox)
4a
5a
Entry
Oxidant (ox)
Yield[b] (%)
TON/TOF (h-1)
1
2
3
4
5
6
7
8
Oxygen
Air
>99[c]
>99
61[d]
>99[e]
<1
100/5.6
100/5.6
Table 7. Palladium catalyzed oxidative coupling of benzylamine to N-
benzylidenebenzylamine. Screening of reaction conditions: pressure, temperature
and time.[a]
Air
61/15.3
Air
100/5.6
N2O
DDQ
-
-
-
-
-
Naphthoquinone
-
Yield[b] (%)
TON/TOF (h-1)
100/5.6
Entry
Pressure (psi), temperature (°C), time (h)
None (N2 atmosphere)
<1
1
2
Atmospheric, 80, 18
Atmospheric, 25, 18
30[c], 80, 12
>99
4
[a] Reaction conditions: Benzylamine (154.3 mg, 1.44 mmol), [Pd]
=
4/0.2
[Pd{(P(Oquin)3)}Cl2] (9.2 mg, 14 μmol, 1 mol%), open to atmosphere, 80 °C, 18 h. H2ox
is the reduced form of the oxidant ox. [b] Yield determined by 1H NMR using 1, 3, 5-
trimethoxybenzene (121.1 mg, 0.72 mmol) as internal standard. [c] 10 psi O2 in 98 mL
autoclave, ca. 2.73 mmol O2. [d] Open to atmosphere, 80 °C, 4 h. [e] 50 psi air (ca. 10
psi O2) in 98 mL autoclave, ca. 2.73 mmol O2, 80 °C, 4 h.
3
>99
>99
>99[d]
>99[d]
36[d]
>99[d]
68
100/8.3
4
30[c], 80, 6
100/16.7
200/33.3
200/50.0
72/36.0
5
30[c], 80, 6
6
30[c], 80, 4
7
30[c], 80, 2
8
30[c], 60, 6
200/33.3
68/11.3
Once the appropriate oxidant, catalyst precursor, and reaction
media (solvent-free) were identified, the effect of other variables
on the model reaction was studied (Table 7). Under atmospheric
pressure, the yield drops significantly if the reaction is performed at
room temperature (entries 1 vs. 2). In an autoclave (30 psi air, ca.
1.72 mmol O2) the reaction time can be shortened up to one third if
the catalytic reaction is carried out at 80 °C (entries 1, 3 and 4).
Under these conditions (30 psi air, 80 °C and 6 h) the catalyst
loading can be lowered to 0.5 mol% without significant loss of
activity (entries 4 vs. 5). Likewise, with this catalyst loading the
reaction time was shortened to 4 h and the yield remained very
high (entry 6), but the attempts to additionally reduce the reaction
time (entry 7) caused a significant drop in product formation. When
the temperature was reduced to 60 °C while the catalyst loading
remained low (0.5 mol%) and the reaction time was relatively short
9
Atmospheric, 60, 6
30[c], 60, 4
10
11
50[d]
23[e]
100/25.0
230/57.5
300, 80, 4
[a] Reaction conditions: Benzylamine (154.3 mg, 1.44 mmol), [Pd]
=
[Pd{(P(Oquin)3)}Cl2] (9.2 mg, 14 μmol, 1 mol%), open to atmosphere, 80 °C, 18 h. [b]
Yield determined by 1H NMR using 1, 3, 5-trimethoxybenzene (121.1 mg, 0.72 mmol)
as internal standard. [c] 30 psi air, 21 % O2, 98 mL autoclave, ca. 1.72 mmol O2. [d]
Catalyst loading: 0.5 mol%. [e] Catalyst loading: 0.1 mol%, 300 psi air, ca. 17.2 mmol
O2.
Under the optimized reaction conditions, a screening of several
amines was performed to explore the scope and limitations of the
catalytic system studied (Table 8). As shown before, benzylamine is
converted into N-benzylidenebenzylamine with excellent yield
(entry 1). Under the same conditions, the related 4-methoxy-
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benzylamine gave the corresponding imine with low yield (entry 2). obtained. Imines 7b (entry 2), 7e (entry 6), 7h (entry 9) and 7j (entry
However, at longer reaction time (18 h) the activity increased 11) were not observed. The best results in this screening were
significantly (entry 3). Other primary amines bearing only one alpha obtained for the imines 7c (21%, entry 4), 7f (24%, entry 7) and 7g
hydrogen, 2-propylamine and α-methyl-benzylamine, are (36%, entry 8). In an attempt to improve the ratio 7/5a, a three-fold
oxidatively homo-coupled with moderate yields (entries 4 and 5). excess of p-toluidine was used but no significant improvement in
With the aliphatic primary amine n-propylamine moderate activity the yield of 7c was achieved (entries 3 and 4). Some substrates,
was observed (entry 6). The role of the amine hydrogen atom(s) in such as aniline itself (entry 1), ortho-subtituted anilines (entries 2
alpha position was investigated in two simple experiments using and 6) and those bearing nitro groups (entries 8 and 9), might
N,N-dibenzylamine and t-butylamine as substrates. In both cases, deactivate the catalyst since the conversion of benzylamine drops
no imine formation was detected and only the starting amines were considerably. In a final attempt to obtain cross-coupled products,
observed by 1H NMR or GC-MS. Hence, it can be concluded that this tert-butyl amine was used instead of aniline(s). Unfortunately, once
system of oxidative homo-coupling of amines to imines works best again the reaction only returned the product of benzylamine
for primary amines (entry 7) bearing at least one hydrogen in alpha homocoupling (62%).
position (entry 8). Likewise, functionalized aliphatic primary amines
Table 9. Palladium catalyzed oxidative cross-coupling of benzylamine with anilines to
imines.[a]
(ethanolamine and ethylenediamine), gave only the starting
1
substrate and no imine formation was observed by H NMR or GC-
[Pd]
R
R
NH2
6a-j
Anilines
+
NH2
+
Ph
Ph
N
Ph
Ph
N
MS (entries 9 and 10).
6 h, 60 °C
30 psi air
4a
7a j
-
5a
Table 8. Oxidative coupling of amines to imines. Screening of substrates.[a]
Entry
1
Conversion of 4a[b] (%)
7 (%)[c]
5a (%)[c]
R2 R2
R2
[Pd]
2
+ NH3(g) + H2O(l)
R1
N
R1
R1
NH2
4a i
3
9 (<1)
91
6 h, 60 °C
30 psi air
6a
5a i
-
-
53
-
>99
Entry
1
Amine
Yield[b] (%)
>99 (93)
TON/TOF (h-1)
100/16.7
2
3
6b
6c
6c
>99
>99
>99
19
81
79
85
4a
21 (10)
15 (12)
4[d]
28
28/4.7
5
6
2
4b
4b
6d
6e
16
65
20
-
>99
76
89[c] (82)
59 (44)
60 (56)
89/14.8
59/9.8
60/10
3
4
24 (18)
36 (28)
7
6f
64
4c
8
6g
6h
31
82
-
>99
91
5
9
4d
4e
9 (4)
47 (44)
-
47/7.8
-
6
7
10
11
6i
>99
-
>99
4f
6j
-
-
8
[a] Reaction conditions: benzylamine (1.44 mmol), anilines (1.44 mmol), [Pd]
=
4g
Pd{(P(Oquin)3)}Cl2] (9.2 mg, 14 μmol, 1 mol%), 30 psi air (98 mL autoclave, ca. 1.72
mmol O2.), 60 °C, 6 h. [b] Conversion of 4a to either 7 or 5a determined by 1H NMR
(residual signal of 4a vs. internal standard). [c] Yield determined by 1H NMR using 1,
3, 5-trimethoxybenzene (121.1 mg, 0.72 mmol) as internal standard. Isolated yield in
parenthesis. [d] 3 equivalents of p-toluidine were used.
-
-
-
-
9
4h
4i
10
[a] Reaction conditions: amine (1.44 mmol), [Pd] = [Pd{(P(Oquin)3)}Cl2] (9.2 mg, 14
μmol, 1 mol%), 30 psi air (98 mL autoclave, ca. 1.72 mmol O2), 60 °C, 6 h. [b] Yield
determined by 1H NMR using 1, 3, 5-trimethoxybenzene (121.1 mg, 0.72 mmol) as
internal standard. Isolated yield in parenthesis. [c] 18 h.
Although several homogeneous systems for the aerobic
oxidation of amines to imines have been described, to the best of
our knowledge only two examples of Pd complexes have been
reported as catalysts for this type of reaction.36,37 Both systems
employ Pd compounds with photoactivity and the oxidation of
amines (only secondary amines) takes place under photochemical
conditions. No records were found in the literature for the aerobic
oxidation of amines catalyzed by metal complexes bearing
phosphite ligands. The methodology described here is simple: one
The cross-coupling of benzylamine with aromatic primary
amines (substituted anilines) was also investigated (Table 9).
Anilines were chosen to avoid any additional competitive homo-
coupling and the formation of E/Z isomers. Unfortunately, the
selectivity for the cross-coupled products 7a-j is low and, instead,
the product from benzylamine homo-coupling (5a) is predominantly
6 | J. Name., 2012, 00, 1-3
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pot reaction of the neat amine with air in the presence of the dependence on catalyst concentration is observed by the plot log(r)
catalyst, applying relatively short times, moderate temperatures vs. log([cat]) with slope value of 1.14 (Figure 6).
and moderate pressures. The catalyst can be recovered from the
Although several efforts were made to measure the rate of
reaction mixture by precipitation with hexanes and it can be reused
reaction r at different air pressures, unfortunately the reaction
in a next run without significant loss of the activity (see Supporting
order with respect the oxygen partial pressure could not be
Information).
accurately determined due to technical difficulties. These kinetic
experiments indicate that the rate law is r = kapp[BnNH2][cat], where
kapp = k[O2]c. The apparent rate constant kapp was calculated by
Kinetic measurements.
measuring the rate of reaction as a function of the product
The kinetics for the oxidative coupling of benzylamine (BnNH2) to N-
[BnNH2][cat] = P, i.e., r = kappP (Figure 7). The plot r vs. P depicts a
benzylidenebenzylamine (model reaction) was examined. Although
straight line that almost intercepts the origin (-0.008) and the slope
is the value for k = 0.756 L mol-1 h-1. In consequence, the best
this reaction proceeds well under solvent free conditions, toluene
estimation obtained for the rate law is r = 0.756 L mol-1 h-
was used as solvent to perform the kinetic measurements to study
1[BnNH2][cat] at a constant air pressure (130 psi, 98 mL, ca. 7.45
the effect of changing the amine concentration. The progress of the
1
reaction as a function of time was monitored by H NMR. The T1
mmol O2) and temperature (60 °C). To the best of our knowledge,
relaxation times of benzylamine and N-benzylidenebenzylamine in
no kinetic experiments have been reported for the homo-coupling
toluene were determined to set a relaxation delay (d1 = 15 s) long
of primary amines to yield imines under homogeneous conditions.
enough to obtain reliable integrals. The reaction was monitored for
Suib et al. have performed kinetic experiments for the atmospheric
the first 5 hours and was carried out under atmospheric pressure,
oxidation of 4-methoxybenzylamine to the corresponding imine
using a heterogeneous catalyst (meso Cs / MnOx).20a Their results
at 80 °C, with 1 mol% catalyst and 0.5 mol/L of benzylamine. The
representation of ln([BnNH2]/[BnNH2]o) vs. time returns a straight
suggest a first-order rate equatio respect to amine concentration
and a rate constant of 0.42 min-1 (0.007 h-1).
line (Figure 4). Here [BnNH2] is the molar concentration of
benzylamine at any “t” time and [BnNH2]o is the initial molar
concentration of benzylamine. This suggests a first order rate
equation with respect to the amine concentration and a rate
constant of 0.016 h-1. Since very low conversion was achieved under
this condition, which might lead to errors in the conversion
1
determined by H NMR, further kinetic experiments were carried
out under pressure and increasing the amine and catalyst
concentration.
Fig. 5. Representation log([BnNH2]) vs. log(r). Conditions: 0.5, 1.0, 1.5 and 4.8 mol/L
benzylamine (max 4.8 mmol), 0.015 mol/L catalyst (9.6 mg, 15 μmol), 130 psi air (98
mL, ca. 7.45 mmol O2), 60 °C, 2 h, toluene, 1, 3, 5-trimethoxybenzene (84.1 mg, 0.5
mmol in 1.0 mL) as internal standard.
Fig. 4. Ln([BnNH2]/[BnNH2]o) vs. time. Conditions: 0.5 mol/L of benzylamine, 1 mol%
catalyst, atmospheric pressure, 80 °C, toluene, 1, 3, 5-trimethoxybenzene (242.2 mg,
1.4 mmol in 5.6 mL) as internal standard.
In this sense, the rate of reaction (r, mol L-1 h-1) was measured
as a function of the amine concentration, while the concentration
of catalyst, air pressure, temperature and time were kept constant.
Fig. 6. Representation log([cat]) vs. log(r). Conditions: 5 mol/L benzylamine (14 mmol in
The representation of log(r) vs. log([BnNH2]) (Figure 5) gives a
0.28 mL), 0.05, 0.1, 0.15 and 0.2 mol/L catalyst, 130 psi air (98 mL, ca. 7.45 mmol O2),
straight line with a slope of 1.07, thus indicating a first-order
60 °C, 2 h, toluene, 1, 3, 5-trimethoxybenzene (11.8 mg, 0.07 mmol) as internal
dependence on the concentration of benzylamine. Likewise, the
standard.
rate of reaction r was measured as a function of the catalyst
concentration, while the concentration of amine, air pressure,
temperature and time were kept constant.
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A first order
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based on the characteristic resonance of the methylene group (3.84
ppm) and the integral value of aromatic signals. Variable
1
temperature NMR measurements, both for H and 31P{1H}, reveal
the existence of a dynamic process (fluxionality) associated to the
rotation of the Pd-N bond in the coordinated benzylamine. Careful
analysis of the relative ratio of the integrals (aromatics/methylene)
at different temperatures indicates that complex 8/8’ contains only
one coordinated BnNH2. Thus, the formula of 8/8’ would be
[Pd(BnNH2){P(Oquin)3}Cl2]/[Pd(BnNH2){P(Oquin)3}Cl]Cl respectively.
To find out whether the Cl- or the N arm of the chelate P(Oquin)3
dissociate from the Pd(II) center (leading to the cationic complex 8
or the neutral 8’) 8/8’ was reacted with NaPF6. The reaction
proceeds cleanly to a unique species 9 (Scheme 4). The significant
shift in 31P{1H} NMR (ꢀδ = 9 ppm, toward higher frequencies)
suggests that 8’ is more likely to be formed. Given the inherent
trans effect of the phosphorus atom (as evidenced by the different
Pd-Cl distances in the solid state structure of 3) one of the chloride
ligands should be more labile than the other, and the N-arm could
dissociate generating an hemilabile ligand.43
Fig. 7. Representation r vs. [BnNH2][cat] = P. Conditions: 5 mol/L benzylamine (1.4
mmol in 0.28 mL) , 0.05, 0.1, 0.15 and 0.2 mol/L catalyst, 130 psi air (98 mL, ca. 7.45
mmol O2), 60 °C, 2 h, toluene, 1, 3, 5-trimethoxybenzene (11.8 mg, 0.07 mmol) as
internal standard.
In situ NMR experiments: Reactivity of the Pd (II) Complex 3 with
Benzylamine.
The reaction of the metal complex 3 in neat BnNH2 (in absence
and presence of air) and stoichiometric reactions of complex 3 with
two equivalents of benzylamine in CDCl3 (in absence and presence
1
of air) were carried out (monitored by both H and 31P{1H} NMR).
Scheme 3. Reaction of complex 3 with BnNH2. Isolation of 8 or 8’.
The free phosphite ligand, its corresponding oxide or its hydrolytic
product were not detected in any of the 31P{1H} spectra. This might
indicate that ligand dissociation does not take place, although it
N
CH2Cl2
N
O
O
25 °C
8 or 8'
O
P
Cl
does not exclude the possibility of (fast)
(hemilability).43
N dissociation
BnNH2
Pd
Cl
3
N
In neat benzylamine in absence of air, the characteristic 31P{1H}
resonance of complex 3 (δ = 32.0 ppm) was not observed and,
instead, two new resonances (formed in the timescale of the
experiment’s setup) were identified (δ = 48.4 and 44.8 ppm). After
heating 1h (60 °C) five new resonances were detected (δ = 23.1,
19.7, 17.7, 7.5 and 2.7 ppm, singlets). Air (40 psi) was charged into
the reaction. After 1 h at 60 °C, the homo-coupling product 5a was
detected (1H NMR), while in the 31P the resonance at 7.5 ppm was
predominant. Additional heating (overnight) yielded only ca. 3% of
imine 5a (1H NMR) and a featureless 31P{1H}.
where:
N
+ Cl-
N
N
N
O
O
O
N
O
O
P
P
O
Cl
Cl
Cl
or
Pd
Pd
NH2Bn
BnH2N
N
8'
trans)
(or
8
Scheme 4. Reaction of complex 8/8’ with NaPF6. Attempt to elucidate 8’.
In the stoichiometric in situ experiment, the signal at 44.1 ppm
was observed upon addition of BnNH2 to a CDCl3 suspension of 3. In
this case, the characteristic 31P{1H} resonance of complex 3 (δ = 32.0
ppm) persisted up to 1h at room temperature and disappeared
after heating (2 h, 60 °C). In contrast to the experiment in neat
benzylamine, a single phosphorus-containing species was detected.
Air (40 psi) was added to the sample and only after 2 h at 60 °C a
new signal (δ = 53.0 ppm) appeared and persisted during 4 h.
Remarkably, after 16 h under these conditions the imine 5a and
complex 3 were quantitatively formed.
PF6
N
N
N
CD2Cl2
25 °C
O
N
O
N
O
O
O
P
O
P
NaPF6
Cl
Cl
Cl
Pd
Pd
NH2Bn
N
BnH2N
8' (or trans)
31P{1H}: 44.2 ppm
9 (or Cl trans to P)
31P{1H}: 53.2 ppm
Based on these results, the isolation of the species observed at
44.1 ppm in the 31P{1H} spectra was attempted. Complex 3 was
treated with 8 equivalents of benzylamine in dichloromethane
under inert atmosphere (Scheme 3). Stirring of the reaction mixture
(RT, 1 h), yields a complete conversion of 3 into a new complex 8/8’
(δ = 44.1 ppm) (evidenced on the 31P{1H} spectrum). Filtration of the
reaction mixture, followed by treatment of the clear solution with
Crystals of 8/8’ suitable for X-ray diffraction analysis could not
be isolated. Attempts to perform EA of this compound have been so
far unsuccessful since the coordinated BnNH2 is eliminated from
8/8’ under vacuum to yield back 3, as confirmed by 31P{1H} NMR
(vacuum was required to remove residual solvents, CH2Cl2 and Et2O
from the synthesis). Two MS analysis of 8/8’ revealed the species
[Pd(NH2){P(Oquin)3}]+
(585.0302
uma,
LIFDI-MS)
and
1
[Pd(ND3){P(Oquin)3}Cl]+ (624.01 uma, HESI-MS, deuterium from
NMR solvents) along with decomposition products from Arbuzov
hexanes, yielded a bright yellow solid. H NMR spectrum suggests
the presence of benzylamine in the compound assigned as 8/8’,
8 | J. Name., 2012, 00, 1-3
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reaction (see Supporting Information). These analyses would cooled to room temperature and the pressure of gas was carefully
indicate that compound 8/8’ decomposes/reacts under the released. The yield was determined by 1H NMR using 1, 3, 5-
conditions of the MS analysis. Thus, the observed species would trimethoxybenzene as internal standard (121.1 mg, 0.72 mmol).
bear either ammonia or amide ligands, confirming the finding of Hexanes were added to the crude of reaction to precipitate the
NH3 evolution in the catalytic reactions. A detailed characterization catalyst and the solution was flash chromatographed through silica.
and study of the behavior in solution of 8/8’ will be discussed in Imines 5a-e were thus obtained upon evaporation of the eluent
future reports. Although these experiments do not provide solid (hexane and ethyl acetate).
evidence of the mechanism that governs the oxidative coupling of
General procedure for catalytic oxidative cross-coupling of amines
primary amines catalyzed by 3, two facts are clear: 1) dissociation of
to imines.
A Parr Reactor (98 mL) was charged with
the phosphite ligand under the studied conditions appears to be
unlikely and 2) amine pre-coordination takes place. Furthermore,
once the amine is coordinated to the Pd center, its transformation
into the corresponding imine only happens in the presence of an
appropriate oxidant (O2).
[Pd{P(OQuin)3}Cl2] (0.01 mmol, 1 mol%) , benzylamine (150 mg, 1.4
mmol), and the corresponding aniline (1.4 mmol) or tert-butyl
amine (307.2 mg, 4.2 mmol). For solid anilines (see Table 9),
toluene was added (1.4 mL, 1 mol/L). The Parr reactor was
pressurized at 30 psi of air and the reaction mixture was stirred at
60 °C for 6h. The system was cooled to room temperature and the
pressure of gas was carefully released. The yield was determined by
1H NMR using 1, 3, 5-trimethoxybenzene as internal standard (121.1
mg, 0.72 mmol). Hexanes were added to the crude of reaction to
precipitate the catalyst and the solution was flash
chromatographed through silica. The imines 7a, 7c, 7d, 7f, 7g and 7i
were thus obtained upon evaporation of the eluent (hexane and
ethyl acetate).
Experimental
Experimental details are provided in the Electronic Supplementary
Information (ESI).
Preparation of tris(quinolin-8-yl)phosphite, 2. To a solution of 8-
hydroxyquinoline (3.28 g, 22.57 mmol) in Et2O (50 mL) a solution of
triethylamine (2.36 g, 23.30 mmol) in Et2O (50 mL) was added
dropwise, while stirring. Then, a solution of phosphorus trichloride
(635 μL, ca. 1g, 7.28 mmol) in Et2O (100 mL) was added dropwise at
5 °C (ice bath) for 1 h. Once this addition was finished, the mixture
was left to warm up to room temperature for 3 h. A colorless solid
was formed (product 2 plus NEt3•HCl) and the mother liquors were
analyzed by 31P{1H} NMR. Typically, PCl3 reacts within 3-5 h.
Depending on the amount of moisture/air present during the
reaction a yellow or reddish solution could be formed. The solid was
filtered off and washed twice with Et2O (30 mL) to eliminate
unreacted 1, hydrolyzed product, NEt3 and the yellow/reddish side
product. Then CH2Cl2 (50 mL) was added two times to extract the
phosphite from the residual ammonium salt. The solvent was
removed under reduced pressure to obtain a white powder (2.85 g,
85% yield). 31P{1H} NMR (121.5 MHz, CD2Cl2, 25ºC): δ = 129.0 ppm
(s).
In situ monitoring of the reaction progress as a function of time. In
a round-bottom flask the complex [Pd{P(OQuin)3}Cl2] (0.03 mmol, 1
mol%) and a solution of benzylamine in toluene (5.6 mL from stock
solution 0.499 mol/L, 300 mg, 2.8 mmol) were placed. The round
flask was connected to a short condenser and the mixture was
heated to 80 °C. 1, 3, 5-trimethoxybenzene was used as internal
standard (242.2 mg, 1.4 mmol). Samples of 0.4 mL were taken every
1
hour during 5 h and analyzed by H NMR. Under these conditions,
the amine was cleanly converted into imine and neither
benzaldehyde nor benzonitrile were detected as side-products. The
progress of reaction, as consumption of benzylamine, was followed
over time. The representation of Ln([BnNH2]/[BnNH2]o) vs. time
returns a straight line. Here [BnNH2] is the molar concentration of
benzylamine at any “t” time and [BnNH2]o is the initial molar
concentration of benzylamine.
Preparation
of
cis-dichloro-(P,N)-tris(quinolin-8-yl)phosphite
In situ experiment in neat benzylamine. Under inert atmosphere,
the complex [Pd{P(OQuin)3}Cl2] (10.0 mg, 16 μmol), benzylamine
(ca. 0.4 mL) and a sealed inner tube filled with DMSO-d6 (ca. 0.1 mL)
were placed in a young NMR tube. The mixture was immediately
palladium (II), 3. To a solution of [PdCl2(COD)] (242.0 mg, 0.84
mmol) in CH2Cl2 (10 mL) a solution of 2 (397.0 mg, 0.85 mmol) in
CH2Cl2 (10 mL) was added at -75 °C, and the mixture was stirred for
30 min. Then, the mixture was allowed to reach room temperature
and stirred for additional 3 h. All volatile materials were removed
under reduced pressure. The complex was washed twice with dry
Et2O (20 mL) and twice with dried hexane (20 mL). The solid was
dried under vacuum to obtain a pale yellow solid (426.0 mg, 79%
yield). 31P{1H} NMR (121.5 MHz, CDCl3, 25ºC): δ = 31.8 ppm (s).
1
analyzed by H and 31P{1H} NMR and then after 0.5 and 1 h. The
sample was heated to 60 °C and analyzed twice (1 and 2 h). The
tube was pressurized with air (40 psi) and spectra were recorded
every hour for two hours. The system was heated to 60 °C and
analyzed four times (1, 2, 4 and 16 h). Selected 31P{1H} and 1H NMR
spectra are shown in the Supporting Information.
General procedure for catalytic oxidative homo-coupling of
amines to imines. A Glassware flask (5 mL) or Parr Reactor (98 mL)
was charged with the catalyst precursor (Table 3, 0.014 mmol, 1
mol%) and the corresponding amine (Table 2-8, 1.4 mmol). If
applicable, (see Table 4) the corresponding solvent was added (1.4
mL, 1 mol/L). If applicable, an oxidant (Table 6) was added or the
autoclave was pressurized at the specified pressure (see Table 7).
The reaction mixture was stirred at the specified temperatures and
during the specified time (see Table 7). If applicable, the system was
In situ stoichiometric experiment. Under inert atmosphere, the
complex [Pd{P(OQuin)3}Cl2] (10.0 mg, 16 μmol), benzylamine (4.2
mg, 39 μmol, 2.4 equivalents) and CDCl3 (ca. 0.5 mL) were placed in
1
a young NMR tube. The mixture was immediately analyzed by H
and 31P{1H} NMR and then after 0.5 and 1 h. The sample was heated
to 60 °C and analyzed twice (1 and 2 h). The tube was pressurized
with air (40 psi) and spectra were recorded every hour for two
hours. The system was heated to 60 °C and analyzed four times (1,
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2, 4 and 16 h). Selected 31P{1H} and H NMR spectra are shown in anilines) was studied but the selectivity toward the
the Supporting Information.
asymmetric imines was found to be low.
Isolation of the species observed in the stoichiometric
experiments, 8/8’. To a suspension of 3 (50.0 mg, 0.078 mmol) in
CH2Cl2 (3 mL) was added a solution of benzylamine (66.9 mg, 0.62
mmol) in CH2Cl2 (3 mL) and the mixture was stirred for 1 h. The
reaction mixture was filtered-off to remove any unreacted 3. The
clear filtrate was concentrated to ca. 1 mL and Et2O (5 mL) was
added. A bright yellow solid was formed. The product was washed
with Et2O (3 x 5 mL) and was carefully dried under vacuum to obtain
a yellow solid (53.0 mg, 90% yield). BnNH2 can be readily removed
from the Pd coordination sphere by applying vacuum overnight.
31P{1H} NMR (121.5 MHz, CD2Cl2, 25ºC): δ = 44.2 ppm (s).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors thank Professor Dr. H. Grützmacher for allowing
us to use the crystallographic facilities at ETH Zurich. Thanks to
Dr. Juan M. Garcia and Dr. Edgar Ocando-Mavarez for allowing
us to use their glove box. We would like to thank Universidad
Simón Bolívar (Decanato de Investigación y Desarrollo, DID-
USB, projects S1-IC DID-CB-010-15 and S1-AI DID-CB-062-2007)
and Instituto Venezolano de Investigaciones Científicas (IVIC,
Project 1265) for supporting our scientific activities. R. E. R. L
thanks the staff of Laboratorio de Resonancia Magnética
Nuclear (MsC. Ligia Llovera, Lic. Yokoy Leon and Lic. Maria
Nuñez) for their valuable cooperation to perform the NMR
experiments and Pedro Noguera (Laboratorio de Química de
los Metales de Transición) for his cooperation in the kinetic
experiments. We would like to thank Prof. Dr. Robert Wolf for
his valuable help to perform elemental analysis, LIFDI-MS and
FT-IR (ATR) spectra at Regensburg University. M.A.C-T. is
grateful to the Royal Society of Chemistry for funding support
through the Dalton division with the “Travel grant for PhD
Students”.
In situ chloride abstraction from 8/8’. To a solution 8/8’ (20.0 mg,
0.027 mmol) in CD2Cl2 (0.5 mL) was added NaPF6 (4.7 mg, 0.028
mmol) at room temperature and under inert atmosphere. The
reaction takes place within minutes and the precipitate formed
(NaCl) was filtered through a millipore filter using a syringe. The
clear filtrate was analyzed by 1H and 31P{1H} NMR (13C{1H}
1
resonances are poorly resolved). H NMR (300.0 MHz, CD2Cl2,
25ºC): δ = 9.72 (m, 1H), 8.79 (br., 2.09H), 8.20 (d, 3JHH = 8.4 Hz,
3
1.72H), 8.15 (d, JHH = 8.0 Hz, 1.72H), 7.52-7.46 (m, 4.18H),
3
7.36-7.23 (m, 15.63H), 6.92 (d, JHH = 7.7 Hz, 3.29H), 6.72 (d,
3JHH = 7.7 Hz, 1.71H), 5.85 (br., 3.45H), 4.03 (br., 3.02H, CH2).
Ratio CHar / CH2 = 34.80 / 3.02 = 11.52 / 1 = 23.04 / 2 ~ 23 / 2.
31P{1H} NMR (121.5 MHz, CD2Cl2, 25ºC): δ = 53.2 (s, 1P), -143.5
1
(hept, JPF = 711.4 Hz, 1P). Attempts to remove the second Cl-
ligand did not proceed under the specified reaction conditions.
Selected 1H NMR and 31P{1H} spectra are shown in Figures S25-
S26.
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Table of Contents Entry
Synthesis, Characterization and Pd (II)-Coordination Chemistry of the Ligand
Tris(quinolin-8-yl)phosphite. Application in Catalytic Aerobic Oxidation of
Amines †
R. E. Rodríguez-Lugo,*, a,b M. A. Chacón-Terán,a S. De León,a M. Vogt,c A. J. Rosenthal,d and V. R. Landaeta.*,a
Ligand P(Oquin)3 is reported and was coordinated to Pd(II). This complex is a catalyst precursor for the
homocoupling of amines.