Chelate Palladium(II) Complexes with Saturated N-Phosphanyl-N-Heterocyclic Carbene Ligands: Synthesis and Catalysis

Article abstract of DOI:10.1021/acs.organomet.5b01019

N-Phosphanyl-N-heterocyclic carbenes (NHCPs) featuring a saturated imidazolin-2-ylidene or tetrahydropyrimid-2-ylidene ring have been synthesized and characterized. The free carbenes exhibit good stability and can be stored in the solid state for months at ambient temperature without decomposition. Contrary to imidazoline-based NHCPs, which decompose by ring opening, N-phosphanyltetrahydropyrimid-2-ylidenes isomerize to 2-phosphanyl tetrahydropyrimidines upon heating. The free carbenes are capable of acting as chelating ligands toward palladium(II), forming very stable mononuclear complexes that have been structurally characterized. The catalytic potential of the complexes has been preliminarily assessed in cross-coupling reactions, most notably in the Suzuki coupling of aryl chlorides, where these complexes display promising activity, and in the copper- and amine-free Sonogashira coupling of aryl bromides.

Full text of DOI:10.1021/acs.organomet.5b01019

Article
pubs.acs.org/Organometallics
Chelate Palladium(II) Complexes with Saturated N‑Phosphanyl-N-
Heterocyclic Carbene Ligands: Synthesis and Catalysis
†
†
†
†
,†
Anatoliy Marchenko, Georgyi Koidan, Anastasiia N. Hurieva, Yurii Vlasenko, Aleksandr Kostyuk,*
,
‡
and Andrea Biffis*
†Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Murmanska 5, Kyiv-94, 02660, Ukraine
‡Dipartimento di Scienze Chimiche, Universita
di Padova, Via Marzolo 1, 35131 Padova, Italy
S Supporting Information
̀
*
ABSTRACT: N-Phosphanyl-N-heterocyclic carbenes (NHCPs)
featuring a saturated imidazolin-2-ylidene or tetrahydropyrimid-
2-ylidene ring have been synthesized and characterized. The free
carbenes exhibit good stability and can be stored in the solid state
for months at ambient temperature without decomposition.
Contrary to imidazoline-based NHCPs, which decompose by
ring opening, N-phosphanyltetrahydropyrimid-2-ylidenes isomer-
ize to 2-phosphanyl tetrahydropyrimidines upon heating. The
free carbenes are capable of acting as chelating ligands toward palladium(II), forming very stable mononuclear complexes that
have been structurally characterized. The catalytic potential of the complexes has been preliminarily assessed in cross-coupling
reactions, most notably in the Suzuki coupling of aryl chlorides, where these complexes display promising activity, and in the
copper- and amine-free Sonogashira coupling of aryl bromides.
INTRODUCTION
Scheme 1. Typical Structures of NHCP and ADCP Ditopic
Ligands
■
Chelating bidentate ligands are ubiquitous in coordination
chemistry, where the chelate effect greatly helps in stabilizing
ligand−metal interactions. Particularly interesting are chelating
1
bidentate ligands featuring coordinating moieties of different
nature (heteroditopic ligands) since in their complexes the
different nature of the groups interacting with the metal center
allows tuning the stability of the complex, its stereochemical
properties, and its reactivity at the coordination sites trans to
the bidentate ligand; furthermore, if the strength of the
interaction of the two coordinating groups with the metal
center is widely different, one of the coordinating moieties can,
under proper conditions, reversibly detach from the metal
center, thus opening a coordination site on the metal and giving
concern the properties of these complexes, such as their
7e,8
catalytic properties.
In this contribution, we would like to
expand this chemistry, and in particular we report on the
preparation of chelate palladium(II) complexes with novel,
saturated N-phosphanyl NHCs, as well as on the catalytic
performance of these complexes in C−C coupling reactions.
2
rise to the so-called “hemilabile” ligands.
In the course of the past decade, several novel heteroditopic
ligands have been prepared that feature N-heterocyclic carbenes
RESULTS AND DISCUSSION
3
,4
■
(NHCs) as one of the coordinating moieties, as well as
NHCPs 3a−c were synthesized following the procedure
previously developed by us for N-phosphanylimidazolin-2-
another coordinating group connected to the NHC through a
linker, and their coordination chemistry has been thoroughly
6c
5
ylidenes (Scheme 2). The parent N-aryl heterocycles 1a−c
were treated with di(tert-butyl)chlorophosphane and sodium
triflate in THF, affording the target salts 2a−c in high yield.
The salts are crystalline solids, highly sensitive to hydrolysis. As
expected, the quaternization of the nitrogen atom causes in all
studied. In this context, we and others have recently developed
6
,7
N-phosphanyl-N-heterocyclic carbenes (NHCPs), as well as
8
N-phosphanyl acyclic diaminocarbenes (ADCPs) as a novel
class of bidentate ligands at carbon and phosphorus (Scheme
1
).
1
13
cases a significant downfield shift of the H and C NMR
signals of the CH group in the 2-position, with respect to the
parent heterocycle. Interestingly, while the H and C NMR
The coordination chemistry of these ligands has been,
however, mainly investigated with group 11 metal centers,
which typically form dinuclear complexes in which the NHCP
acts as a bridging ligand. Much more scarce are the reports
concerning complexes in which these molecules act as chelating
1
13
Received: January 6, 2016
7a,e,8
ligands toward a single metal center,
and even fewer studies
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.5b01019
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Scheme 2. Synthesis of the Saturated NHCP Ligands
spectra of all compounds display one set of signals, in the 31P
NMR spectra of 2a,b two signals are recorded, at δ 140.2, 124.4
Figure 1. Ball-and-stick view of 3b. Selected bond distances (Å) and
bond angles (deg) are as follows: P(1)−N(1) 1.736(2), N(1)−C(1)
1
.360(3), N(2)−C(1) 1.340(3), N(2)−C(2) 1.436(3); P(1)N(1)-
(
7:1) and 139.3, 123.0 (10:1) ppm, respectively, which i9s
C(1) 122.0(1), N(1)C(1)N(2) 115.4(2), C(1)N(2)C(2) 118.6(2).
probably due to hindered rotation about the P−N bond.
Subsequent deprotonation by potassium hexamethyldisilazide
allowed preparing in good yield the saturated NHCP ligands
similarly to previously reported N-phosphanyl imidazol-,
6a,10,11
benzimidazol-, and triazol-2-ylidenes.
3
a−c. A bulky strong base has to be employed in the final
deprotonation step; otherwise the base can behave as a
nucleophile, attacking the heterocycle at the 2-position. This
reactivity has been previously observed with N-phosphanyl-
imidazolinium salts and characterizes also the N-phosphanyl-
tetrahydropyrimidinium salts: this has been verified with
substrate 2a, which upon reaction with methyllithium
Scheme 4. Thermal Rearrangement of Carbenes 3a,b to
Phosphanes 5a,b
6
c
(nucleophilic attack by methyl) or tert-butyllithium (hydride
abstraction from tert-butyl) yields the corresponding N-
phosphanylhexahydropyrimidines, which can be isolated and
characterized as the selenides 4a,b (Scheme 3).
Scheme 3. Alternative Reactions of Salt 2a with Lithium
Alkyls
Group 16 elements (selenium in particular) usually react
with NHCs affording the corresponding carbene−selenium
12
adducts (selones), and previously investigated unsaturated
6a,11
NHCPs are no exception,
although ADCPs behave instead
differently and can provide either the product of Se attack at the
13
carbene carbon or, with more sterically encumbered carbenes,
9a
the product of Se attack at the phosphorus atom. In the case
of saturated NHCPs, whereas five-membered 3c furnished the
6c
expected selone upon attack of Se to the carbene carbon,
addition of Se to carbenes 3a,b led to a mixture of compounds,
among which the expected selones were not detected. The
reaction went to completion upon addition of 2 equiv of
selenium, and products 6−8 (Scheme 5) could be separated
The novel free carbenes 3a,b were thoroughly characterized
by NMR. The spectral features of the carbenes are comparable
to those of the previously characterized, five-membered NHCP
6c
13
3
c, particularly concerning the C NMR signal of the carbene
31
carbon and the P NMR signal of the phosphanyl moiety.
Furthermore, the structure of compound 3b was confirmed by
an X-ray study (Figure 1). It was found that there are two
symmetrically independent molecules of 3b in the unit cell. The
only difference between them is the location of the C(17) atom
in the N(1)C(1)N(2)C(16−18) six-membered cycle. The
N(1)C(1)N(2)C(16)C(18) fragment is planar within 0.032 Å
in both molecules, and the atom C(17) is located above this
plane in one molecule and under the plane in another one. The
aromatic ring is orthogonal to the plane of the N(1)C(1)-
N(2)C(16)C(18) fragment; the dihedral angle between them is
Scheme 5. Reaction of Carbenes 3a,b with Selenium
8
6° for both independent molecules of 3b. N(1) and N(2)
and isolated as individual compounds by silica gel chromatog-
raphy, thus proving that Se reacted at both carbon and
phosphorus atoms. We are currently evaluating whether this
different reactivity is dictated by steric or electronic reasons.
Carbenes 3a−c readily reacted with bis(benzonitrile)-
palladium(II) chloride to give chelating complexes 9a−c
(Scheme 6).
atoms have trigonal-planar bond orientation (sum of the bonds
is 360.0(3)° and 360.0(4)°, respectively, for both molecules).
Contrary to their five-membered congener 3c, which
decomposes with ring opening upon heating, tetrahydropyr-
imid-2-ylidenes 3a,b cleanly undergo 1,2-P shift at 125 °C to
give 2-phosphanyl tetrahydropyrimidines 5a,b (Scheme 4),
6c
B
DOI: 10.1021/acs.organomet.5b01019
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Article
Scheme 6. Synthesis of the Palladium(II) Complexes 9a−c
Figure 3. Perspective view of the molecular structure of 9b, together
with the atomic numbering scheme. Selected bond distances (Å) and
angles (deg): Pd(1)−Cl(1) 2.3634(9), Pd(1)−Cl(2) 2.3509(8),
Pd(1)−P(1) 2.2180(9), Pd(1)−C(1) 2.012(3), P(1)−N(1)
1.707(3), N(1)−C(1) 1.353(4), N(2)−C(1) 1.320(4), N(2)−C(2)
1.448(4); P(1)Pd(1)C(1) 68.07(9), Pd(1)P(1)N(1) 85.8(1), N(1)-
C(1)Pd(1) 104.9(2), P(1)N(1)C(1) 101.1(2), N(1)C(1)N(2)
The complexes are light yellow solids that are well soluble in
several polar organic solvents; they feature good stability and
can be handled without special precautions both in the solid
state and in solution. They were thoroughly characterized by
1
13
31
H, C, and P NMR spectroscopy, where they exhibited the
1
19.4(3).
expected resonances. With respect to the free carbenes,
coordination to palladium(II) invariably results in a consid-
1
3
erable upfield shift of the C NMR signal of the carbene
carbon, as commonly observed in the formation of metal
6
−8
complexes of NHC ligands, including N-phosphanyl ones.
In contrast, no significant shift of the phosphorus signal in the
3
1
P NMR spectrum is observed upon coordination of 3c to Pd
to form 9c (δ 87.7 to 90.1 ppm), whereas a very remarkable
upfield shift (δ ca. 112 to ca. 59 ppm) was recorded upon
formation of 9a,b. The negligible variation of chemical shift
recorded in the formation of 3c resembles the one previously
7b
reported with another five-membered NHCP ligand, whereas
the large variations observed with the six-membered NHCPs
are comparable to the ones previously observed with ADCP
8
Figure 4. Perspective view of the molecular structure of 9c, together
with the atomic numbering scheme. Selected bond distances (Å) and
angles (deg): Pd(1)−Cl(1) 2.3532(9), Pd(1)−Cl(2) 2.3435(8),
Pd(1)−P(1) 2.2363(8), Pd(1)−C(1) 1.981(3), P(1)−N(1)
ligands.
The structure of compounds 9a−c was determined by X-ray
diffraction methods (Figures 2−4). All complexes display a
1
.708(3), N(1)−C(1) 1.357(4), N(2)−C(1) 1.307(4), N(2)−C(2)
1
.435(4); P(1)Pd(1)C(1) 68.55(9), Pd(1)P(1)N(1) 84.53(9), N(1)-
C(1)Pd(1) 105.4(2), P(1)N(1)C(1) 101.5(2), N(1)C(1)N(2)
10.1(3).
1
14
electron donation to the metal center by the former. The
most notable difference between the complexes with five- and
six-membered NHCPs can be appreciated upon looking at the
15
so-called “yaw distortion angle”, namely, the angle between
the actual Pd−C bond axis and the “ideal” bond axis that passes
through the carbene carbon and lies on the symmetry plane of
the heterocyclic ring. This angle is easily estimated as the
semidifference between the two Pd−C−N angles (if there is no
significant distortion from planarity of the Pd−NCN moiety, as
in the present case) and is much greater for 9c (19.51°) than
for 9a,b (around 15°), indicating greater distortions in the
structure of 9c, which should imply a lower stability of this
complex. Finally, the Pd−C and Pd−P distances do not vary
very significantly between the various complexes and are in the
Figure 2. Perspective view of the molecular structure of 9a, together
with the atomic numbering scheme. Selected bond distances (Å) and
angles (deg): Pd(1)−Cl(1) 2.361(2), Pd(1)−Cl(2) 2.369(2), Pd(1)−
P(1) 2.211(2), Pd(1)−C(1) 2.008(6), P(1)−N(1) 1.710(5), N(1)−
C(1) 1.368(9), N(2)−C(1) 1.320(9), N(2)−C(2) 1.429(8); P(1)-
Pd(1)C(1) 68.2(2), Pd(1)P(1)N(1) 86.1(2), N(1)C(1)Pd(1)
1
04.7(4), P(1)N(1)C(1) 100.2(4), N(1)C(1)N(2) 118.8(5).
7b,8
distorted square planar coordination geometry with a very small
CPdP angle (around 68°) imposed by the ligand geometry.
The structure of the ligands is also strongly distorted compared
to the free state; in particular, the CNP angle decreases from
range of similar complexes reported in the literature.
We then set out to examine the potential of the Pd
complexes as catalysts. Among the plethora of reactions that are
known to be catalyzed by Pd species, we chose cross-couplings
for these first studies, a reaction class for which very numerous
catalytic systems based on Pd complexes with NHC ligands
have been proposed and extensive literature surveys are
1
22° to 101° in the case of 9b, whereas in the case of 9c the
decrease is even more pronounced (from 126.9° to 101.55°).
The NCN angle is as expected much wider for the complexes
with the six-membered NHCPs (around 119°) than for the
five-membered NHCP (110.17°), which should imply stronger
16
available, thus facilitating a critical evaluation of the catalytic
performance of the novel complexes.
C
DOI: 10.1021/acs.organomet.5b01019
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We considered initially the Suzuki coupling of aryl bromides
and chlorides with phenylboronic acid, a reaction for which
NHC−Pd complexes are among the most efficient catalysts
known to date. We started by working with an unactivated
aryl bromide substrate such as 4-bromoanisole in order to
optimize the reaction conditions and in particular the solvent
and the temperature. We screened the most commonly
employed media for this reaction and found that ethanol
provided the best catalytic performance (Table 1, entries 1−3).
chlorides at room temperature with down to 0.05 mol %
catalyst ), are encouraging for further investigation on NHCP
9
ligands and in particular compare favorably with those
previously obtained by us with acyclic N-phosphanyl
diaminocarbene ligands, where the lower stability of the Pd
complexes was found to negatively affect the catalytic
1
6
8
performance.
We then turned our attention to another reaction, namely,
the Hartwig−Buchwald coupling of aryl chlorides with
substrates containing an N−H function to yield the
corresponding N-aryl derivatives. This reaction is efficiently
Table 1. Suzuki Reactions Catalyzed by Complexes 9a−c
1
6
catalyzed by several palladium−NHC complexes, and
chelating ligands bearing an NHC and a phosphane moiety
have also been employed with success, enabling efficient
a
b
18
R
X
cat solvent
T (°C) /time (h) yield (%)
catalysis with 1−2 mol % Pd already at room temperature.
1
2
3
4
5
6
7
8
9
1
1
1
OCH3
OCH3
OCH3
OCH3
OCH3
COCH3
COCH3
CH3
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
9c
9c
9c
9a
9c
9c
9a
9a
9a
9a
9a
9b
DME
DMF
80/17
80/17
80/5
80/5
80/2
80/5
80/5
80/5
80/5
60/5
28/5
28/5
46
94
We took into consideration the reaction of 4-chloroacetophe-
none or 4-chlorotoluene with model substrates such as N-
methylaniline or morpholine under a variety of conditions by
changing in particular solvent (ethanol, dimethoxyethane, 1,4-
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
>99
>99
65
c
t
dioxane, N,N-dimethylformamide), base (KO Bu, K CO ), and
2
3
75
temperature (room temperature to 80 °C). Unfortunately, in
no case were we able to detect production of the expected N-
arylated product using complexes 9a−c as catalysts. We
currently tend to attribute this lack of activity to the fact that
NHCP palladium(II) complexes fail to efficiently effect the final
reductive elimination to form the new C−N bond. This
reaction step is rate-determining in Hartwig−Buchwald
>99
>99
89
OCH3
OCH3
OCH3
OCH3
0
1
2
93
42
8
19
a
b
amination, and the NHCP ligands apparently fail to promote
it, since the small bite angle furnishes smaller steric pressure
toward reductive elimination and also disfavors formation of a
chelate NHCP-palladium(0) complex, which would require an
even broader bite angle.
DME = dimethoxyethane; DMF = N,N-dimethylformamide. Yields
determined by H NMR using 1,4-bis(trimethylsilyl)benzene as an
internal standard. Reaction performed with 0.02 mol % catalyst.
1
c
In this solvent, the reaction proved to be very efficient with all
employed catalysts and could be efficiently performed also with
very low catalyst loadings. For example, the reaction using 0.02
mol % 9c allowed reaching a 65% yield within 2 h, with an
Finally, we evaluated our complexes as catalysts in the
copper- and amine-free Sonogashira coupling of aryl halides
20
and terminal alkynes to yield arylalkynes. Copper salts are
generally required as cocatalysts for this reaction, since they
react with the alkyne, forming intermediate Cu−alkynyl species,
which then transmetalate to Pd; however, the use of copper
species also makes the process air sensitive and promotes side
3
−1
average TOF of 1.6 × 10 h (Table 1, entry 5).
Subsequently, we turned our attention to more challenging
substrates for the Suzuki reaction and selected some model aryl
chlorides for this purpose. Indeed, while it is known that even
simple Pd salts catalyze the Suzuki reaction of aryl iodides or
2
1
reactions such as the Glaser−Hay coupling of the alkyne.
1
7
bromides, the decreased reactivity of aryl chlorides makes the
catalytic interference of Pd centers deriving from the potential
decomposition of the complexes less likely and also allows a
better discrimination between the catalytic activity of the
various Pd complexes. Indeed, a significantly better perform-
ance in the conversion of 4-chloroacetophenone was recorded
for complex 9a, bearing the six-membered NHCP, compared to
complex 9c, with the five-membered one (Table 1, entries 6
and 7). This difference could be the consequence of the higher
electron-donating capability of the six-membered NHC moiety
compared to the five-membered one and/or of the increased
stability of the complex due to lower strain of the chelating
ligand in 9a compared to 9c. Complex 9a was a very efficient
catalyst also in the conversion of less activated or even
deactivated aryl chlorides, which allowed, in the case of
deactivated 4-chloroanisole, performing the reaction in high
yields at 60 °C and in moderate yields even at room
temperature (Table 1, entries 10 and 11). In contrast, the
more sterically encumbered complex 9b exhibited poor activity
with 4-chloroanisole at room temperature (Table 1, entry 12),
possibly due to steric reasons. These results, although not the
best ever reported for NHC−palladium(II) complex catalysts
Therefore, reaction protocols avoiding the use of copper
cocatalysts, upon addition of a large excess of amine base or by
other means, are currently being intensively investigated. In
this connection, several NHC−palladium(II) complexes have
recently been proposed as catalysts for copper-free Sonogashira
reactions, although these complexes have not displayed up to
now a very high reactivity, being able to efficiently convert only
reactive aryl iodides and, to some extent, aryl bromides.
20
22
Preliminary results obtained by employing complexes 9a−c
as catalysts for this reaction are reported in Table 2. All
complexes were found to be able to efficiently convert in 2 h an
activated aryl bromide, such as 4-bromoacetophenone, and
phenylacetylene into the Sonogashira product with good
selectivity (Table 2, entries 1−4). The reaction could be
efficiently run also at lower temperature, albeit by employing
longer reaction times (Table 2, entry 2). In contrast, the
situation was different with a deactivated aryl bromide such as
4-bromoanisole. In this case, complex 9a turned out to be
inactive for the reaction after 2 h, whereas complexes 9b and 9c
displayed a moderate activity (Table 2, entries 5−7); the
catalytic performance was slightly better for the former
complex, reaching 54% yield after 24 h (Table 2, entry 8). It
is interesting to remark that the catalytic inactivity observed
(
which currently enable high conversion of deactivated aryl
D
DOI: 10.1021/acs.organomet.5b01019
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Organometallics
Article
Table 2. Sonogashira Reactions Catalyzed by Complexes
diaminocarbene and a phosphane group. The lack of catalytic
activity recorded in Hartwig−Buchwald couplings (and to some
extent also in Sonogashira couplings) has been tentatively
attributed to the fact that due to their small bite angle NHCPs
as chelating ligands disfavor reductive elimination processes
leading to palladium(0) species. Consequently, further
investigation on the catalytic potential of these complexes will
be extended to include also redox-neutral reactions, in which no
reductive elimination processes are involved.
9
a−c
T (°C) /time
conversion
yield
b
a
R
X
cat
(h)
(%)
(%)
1
2
3
4
5
6
7
8
9
COCH3
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
9a
9a
9b
9c
9a
9b
9c
9b
9a
9b
9a
100/2
80/16
100/2
100/2
100/2
100/2
100/2
100/24
100/24
100/24
80/21
89
90
92
98
0
81
85
82
85
0
COCH3
COCH3
COCH3
EXPERIMENTAL SECTION
■
OCH3
All manipulations of air- and moisture-sensitive compounds were
carried out using standard Schlenk techniques under an atmosphere of
argon or dinitrogen. The reagents were purchased from Aldrich as
high-purity products and generally used as received. All solvents were
purified and dried by standard methods. Tetrahydropyrimidines 1a,b
OCH3
27
15
54
37
0
27
15
54
37
0
OCH3
OCH3
OCH3
2
4
1
1
0
1
COCH3
COCH3
were prepared according to literature procedures. NMR spectra were
0
0
recorded on a Bruker Avance spectrometer working at 500 MHz
1
13
31
a
b
(500.13 MHz for H, 125.75 MHz for C, 202.43 MHz for P), 300
Conversions based on the aryl halide. Conversions and yields
1
13
19
1
MHz (300.1 MHz for H, 75.5 MHz for C, 282.2 MHz for F, and
determined by H NMR using 1,4-bis(trimethylsilyl)benzene as an
1
21.5 MHz for 31P), and 200 MHz (80.89 MHz for P); chemical
31
internal standard.
shifts (δ) are reported in units of ppm relative to the residual solvent
31
signals and to external 85% H PO (for P). ESI-MS analyses were
3
4
with 9a and 4-bromoanisole appears to be due to the reluctance
of the complex to undergo reduction under catalytic conditions
to form the corresponding palladium(0) species from which the
catalytic cycle starts with the oxidative addition of the aryl
halide. Indeed, by prolonging the reaction time to 24 h a 37%
yield in coupling product is recorded (Table 2, entry 9). Finally,
all complexes proved completely inactive with an aryl chloride
substrate such as 4-chloroacetophenone (Table 2, entries 10
and 11), which is a feature shared by all NHC-based Pd
catalysts reported to date for this reaction. Summarizing these
results, the reactivity of the complexes in these preliminary
Sonogashira coupling tests seems at present comparable to or
higher than that of previously reported mono- or di-NHC−
performed using an LCQ-Duo (Thermo-Finnigan) operating in
positive ion mode. Instrumental parameters: capillary voltage 10 V;
spray voltage 4.5 kV; capillary temperature 200 °C; mass scan range
from 150 to 2000 amu; N was used as sheath gas; the He pressure
2
inside the trap was kept constant. The pressure directly read by an ion
−5
gauge (in the absence of the N stream) was 1.33 × 10 Torr. Sample
2
solutions were prepared by dissolving the compounds in acetonitrile.
Sample solutions were directly infused into the ESI source by a syringe
pump at an 8 μL/min flow rate. Elemental analyses were carried out
with a Fisons EA 1108 CHNS-O apparatus or with a Carlo Erba
analyzer.
3
-(Di-tert-butylphosphanyl)-1-phenyl-3,4,5,6-tetrahydro-
pyrimidin-1-ium Trifluoromethanesulfonate, 2a. To a solution
of 1-phenyl-1,4,5,6-tetrahydropyrimidine (1a; 3.2 g, 20 mmol) and
CF SO Na (3.9 g, 23 mmol) in THF (25 mL) was added di(tert-
22
3
3
palladium(II) complexes and markedly higher than that of
previously reported Pd complexes with chelating ligands
featuring a diaminocarbene and a phosphane group, which
butyl)chlorophosphane (3.9 g, 22 mmol). Within 2 h the solvent was
removed in vacuo, the solid residue was treated with CH Cl (60 mL),
2
2
the insoluble part was filtered off under argon and washed with
CH Cl (2 × 10 mL), and the filtrate was concentrated in vacuo. The
2
3
were moderately active only with aryl iodides. More
investigations will be necessary in order to fine-tune the
characteristics of the ligand for this peculiar application.
2
2
residue was washed with Et O (3 × 30 mL) to give the target salt as a
2
1
white powder. Yield: 8.2 g (91%). Mp: 175−176 °C (THF). H NMR
(
CDCl ): δ = 1.33 (d, J = 13.5 Hz, 18H, CH ), 2.33 (m, 2H, CH ),
3
3
2
4
5
.02 (br s, 2H, CH ), 4.10 (t, J = 5.4 Hz, 2H, CH ), 7.41−7.49 (m,
13
CONCLUSIONS
In this work, we have disclosed novel N-phosphanyl-N-
heterocyclic carbenes featuring a saturated tetrahydropyrimid-
-ylidene ring. The carbenes exhibit good stability: they can be
2
2
■
H, CH), 8.22 (d, J = 6.3 Hz, 1H, CH). C NMR (CDCl ): δ = 19.3
3
(s, CH ), 28.8 (d, J = 16 Hz, CH ), 35.5 (d, J = 31 Hz, C), 44.9 (s,
2
3
CH ), 46.8 (s, CH ), 120.5 (q, J = 321 Hz, CF ), 123.0 (s, CH), 129.2
(s, CH), 130.0 (s, CH), 141.1 (s, iC), 157.8 (d, J = 60 Hz, CH).
2
2
3
3
1
2
P
stored in the solid state for months at ambient temperature
without decomposition and isomerize to the corresponding 2-
phosphanyl tetrahydropyrimidines only upon heating at 125
NMR (CDCl ): δ = 140.2, 124.4 (7:1). Anal. Calcd (%) for
3
C
H
F
N
O
PS (454.5): C, 50.21; H, 6.65; N, 6.16. Found: C,
19
30
3
2
3
4
9.87; H, 6.41; N, 5.91.
3
-(Di-tert-butylphosphanyl)-1-(2,4,6-trimethylphenyl)-
°C. The carbenes, together with a related imidazoline-based
3
,4,5,6-tetrahydropyrimidin-1-ium Trifluoromethanesulfonate
b. The synthesis was performed following the same procedure
NHCP, act as chelating ligands toward palladium(II), forming
in high yield complexes that are indefinitely stable in the solid
state even without strict exclusion of air and moisture. The
resulting palladium(II) complexes have been employed as
precatalysts in cross-coupling reactions, most notably in Suzuki
couplings of aryl chlorides, where the complexes display
promising catalytic activity, in Hartwig−Buchwald couplings of
aryl chlorides, in which the complexes are instead catalytically
inactive, and finally in copper- and amine-free Sonogashira
couplings of aryl bromides, where the catalytic performance of
the complexes is comparable to that of previously reported
NHC−palladium(II) species but superior to that of related
palladium(II) complexes with chelating ligands featuring a
2
employed for the preparation of 2a, starting from 2.8 g (14 mmol) of
1
1b. The reaction time was 1 h. Yield: 2.88 g (84%). H NMR
(CDCl ): δ = 1.31 (d, J = 13.2 Hz, 18H, CH ), 2.19 (s, 6H, 2 CH ),
3
3
3
2.27 (s, 3H, CH
), 2.35 (m, 2H, CH
), 3.77 (m, 2H, CH ), 4.07 (br s,
3
2
2
1
3
2
H, CH ), 6.95 (s, 2H, CH), 7.92 (d, J = 6.9 Hz, 1H, CH). C NMR
2
(
CDCl ): δ = 16.9 (CH), 19.4 (CH ), 20.6 (CH), 28.7 (d, J = 16 Hz,
3
2
CH), 35.3 (d, J = 30 Hz, C), 45.0 (d, CH , J = 6.3 Hz), 46.5 (CH ),
2
2
1
20.5 (q, J = 321 Hz, CF ), 129.8 (CH), 133.6 (C), 136.1 (C), 140.3
3
31
(
(
3
C), 159.5 (d, J = 58 Hz, CH). P NMR (CDCl ): δ = 139.3, 123.0
10:1). ESI-MS (positive ions, DMSO): m/z 347.26 [M] (5.9%);
3
+
+
+
63.0 [M + O] (51.5%); 203 [M − (tBu P)] (28%). Anal. Calcd
2
(%) for C H F N O PS (496.58): C, 53.21; H, 7.31; N, 5.64. Found:
22
36
3
2
3
C, 53.03; H, 7.43; N, 5.34.
E
DOI: 10.1021/acs.organomet.5b01019
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
-(Di-tert-butylphosphanyl)-1-phenyl- 3,4,5,6-tetrahydro-
The solvents were removed in vacuo. The residue was extracted with
hot hexane (3 × 20 mL); the extract was concentrated in vacuo to 15
mL. Upon cooling, a precipitate was formed, collected, and dried in
vacuo to give 4b as white crystals. Yield: 1.09 g (86%, oil). Mp: 115−
pyrimidin-2-ylidene, 3a. To a stirred solution of tetrahydropyr-
imidinium salt 2a (2.34 g, 5.15 mmol) in anhydrous and degassed
THF (15 mL) cooled to −90 °C was added via syringe a 1 M solution
of potassium hexamethyldisilazide (5.15 mL, 5.15 mmol) in THF. The
reaction temperature was increased over 30 min to 17 °C. The solvent
was removed in vacuo (15 Torr), and the residue was kept in vacuo
until dry. Then pentane (20 mL) was added, the precipitate was
filtered under argon and washed with pentane (3 × 10 mL), and the
extracts were combined and concentrated in vacuo to 18 mL. The
crystals formed at −18 °C were collected and dried in vacuo to give
1
117 °C (hexane). H NMR (300 MHz, C D ): δ = 1.42 (d, J = 15.6
6
6
Hz, 18H, CH ), 1.77 (br m, 2H, CH ), 3.39 (t, J = 5.4 Hz, 2H, CH ),
3
2
2
3.66 (q, J = 5.4 Hz, 2H, CH ), 4.99 (d, J = 7.8 Hz, 2H, CH ), 6.83 (t, J
2
2
= 7.2 Hz, 1H, CH), 7.08 (d, J = 8.1 Hz, 2H, CH), 7.26 (t, J = 8.4 Hz,
2H, CH). 13C NMR (125.7 MHz, C D ): δ = 24.6 (s, CH ), 28.4 (s,
6
6
2
CH ), 42.0 (d, J = 42 Hz, C), 48.2 (s, CH ), 49.4 (s, CH ), 69.3 (d, J =
3
2
2
4 Hz, CH ), 116.3 (s, CH), 119.3 (s, CH), 128.7 (s, CH), 148.4 (s,
2
1
31
white crystals. Yield: 1.45 g (93%). H NMR (500 MHz, C D ): δ =
.35 (d, J = 12.0 Hz, 20 H, CH +CH ), 2.80 (br s, 2H, CH ), 2.94 (br
s, 2H, CH ), 6.95 (dd, J = 7.0 Hz, 1H, CH), 7.19 (dd, J = 6.5; 8.0 Hz,
H, CH), 7.66 (d, J = 8.0 Hz, 2H, CH). C NMR (125.7 MHz,
C D ): δ = 21.2 (d, J = 10 Hz, CH ), 28.6 (d, J = 16.3 Hz, CH ), 34.9
d, J = 26 Hz, C), 40.3 (s, CH ), 47.5 (d, J = 44 Hz, CH ), 118.1 (s,
CH), 122.5 (s, CH), 127.9 (s, CH), 150.0 (s, C), 256.0 (s, C).
iC). P NMR (202.4 MHz, C D ): δ = 110.9 (J = 737 Hz). ESI-MS
6
6
6
6
PSe
+
1
(positive ions, DMSO): m/z 387.0 [M + 2] (29.8%); 375.0 [M −
CH + 4] (70.2%). Anal. Calcd (%) for C H N PSe (385.39): C,
3
2
2
+
2
2
18 31
2
13
2
56.10; H, 8.11; N, 7.27; P, 8.04. Found: C, 55.78; H, 7.89; N, 7.56; P,
6
6
2
3
7.96.
(
2-(Di-tert-butylphosphanyl)-1-phenyl-1,4,5,6-tetrahydro-
pyrimidine, 5a. A solution of 3a (164 mg, 0.54 mmol) in benzene-d
2
2
31
P
6
NMR (81 MHz, C D ): δ = 113.5. Anal. Calcd (%) for C H N P
(100 mg) was heated in the presence of tris(dimethylamino)-
phosphane selenide (26 mg, 0.11 mmol, 20 mol %) as catalyst at
125 °C for 45 min in a sealed tube. The solvent was removed in vacuo,
and the solid residue was recrystallized from pentane. The crystals
formed at −18 °C were collected and dried in vacuo to give 5a as a
6
6
18 29
2
(
304.42): C, 71.02; H, 9.60; N, 9.20. Found: C, 71.24; H, 9.71; N,
9
.03.
3
-(Di-tert-butylphosphanyl)-1-(2,4,6-trimethylphenyl)-
3
,4,5,6-tetrahydropyrimidin-2-ylidene, 3b. The synthesis was
1
performed following the same procedure employed for the preparation
white powder. Yield: 153 mg (93%). Mp: 102−103 °C. H NMR (500
of 3b, starting from 1.5 g (3.0 mmol) of 2b. Yield: 690 mg (67%). Mp:
MHz, C D ): δ = 1.32 (d, J = 11.5 Hz, 18H, CH ), 1.51 (m, 2H, CH ),
6
6
3
2
1
1
1
2
13−114 °C. H NMR (300 MHz, C D ): δ = 1.37 (d, J = 11.7 Hz,
3.13 (t, J = 5.5 Hz, 2H, CH ), 3.46 (dd, J = 5.5 and 6.0 Hz, 2H, CH ),
6
6
2
2
8H, CH ), 1.60 (m, 2H, CH ), 2.12 (s, 3H, CH ), 2.22 (s, 6H, CH ),
.63 (t, J = 3.0 Hz, 2H, CH ), 3.04−3.07 (m, 2H, CH ), 6.76 (s, 2H,
CH). C NMR (125.7 MHz, C D ): δ = 17.3 (s, CH ), 20.2 (s, CH ),
6.90 (dd, J = 7.0 and 7.5 Hz, 1H, CH), 6.97 (d, J = 8.0 Hz, 2H, CH)
7.08 (dd, J = 7.5 and 8.0 Hz, 2H, CH). 13C NMR (125.7 MHz, C D ):
3
2
3
3
2
2
6
6
13
6
6
3
3
δ = 22.7 (s, CH ), 29.8 (d, J = 14 Hz, CH ), 32.95 (d, J = 24 Hz, C),
2
3
2
1.8 (d, J = 10.0 Hz, CH ), 28.8 (d, J = 16.3 Hz, CH ), 34.8 (d, J = 25
45.5 (s, CH ), 49.7 (d, J = 2.5 Hz, CH ), 124.3 (CH), 127.2 (CH),
2 2
2
3
3
1
Hz, C), 42.5 (s, CH ), 47.2 (d, J = 45 Hz, CH ), 128.7 (s, CH), 133.9
128.0 (CH), 147.6 (d, J = 4 Hz, C), 160.7 (d, J = 18 Hz, CP).
P
2
2
31
(
s, C),134.8 (s, C), 144.9 (s, C), 254.1 (s, C). P NMR (81 MHz,
NMR (81 MHz, C D ): δ = 27.7. Anal. Calcd (%) for C H N P
6
6
18 29
2
C D ): δ = 110.0. Anal. Calcd (%) for C H N P (346.50): C, 72.79;
(304.42): C, 71.02; H, 9.60; N, 9.20. Found: C, 70.78; H, 9.42, N,
9.15.
2-(Di-tert-butylphosphanyl)-1-(2,4,6-trimethylphenyl)-
1,4,5,6-tetrahydropyrimidine, 5b. A solution of 3b (150 mg, 0.4
6
6
21 35
2
H, 10.18; N, 8.08. Found: C, 72.91; H, 10.02; N, 8.19.
-(Di-tert-butylphosphoroselenoyl)-2-methyl-3-phenyl-
1
hexahydropyrimidine, 4a. To a stirred and cooled to −90 °C
suspension of tetrahydropyrimidinium salt 2a (1.44 g, 3.2 mmol) in
THF (10 mL) was added via a syringe a 1.6 N solution of
methyllithium (2.0 mL, 3.2 mmol) in ether. The reaction mixture was
stirred at −90 °C for 30 min. Then, the reaction temperature was
slowly increased to +16 °C, and finely ground selenium (250 mg, 3.2
mmol) was added. The reaction mixture was stirred at 16 °C for 1 h.
The solvents were removed in vacuo. The residue was extracted with
hot hexane (3 × 20 mL); the extract was concentrated in vacuo to 18
mL. Upon cooling, the precipitate was formed, then collected and
dried in vacuo to give 4a as white crystals. The mother liquor was
concentrated, and residual matter was purified by chromatography on
a SiO plate using CH Cl as an eluent, R 0.6−1.0, and recrystallized.
mmol) in benzene-d (800 mg) was heated at 125 °C for 30 min in a
6
sealed tube. The solvent was removed in vacuo, and the solid residue
was recrystallized from pentane. The crystals formed at −18 °C were
collected and dried in vacuo to give 5b as a white powder. Yield: 80
1
mg (53%). Mp: 98−99 °C. H NMR (300 MHz, C D ): δ = 1.30 (d, J
6
6
= 11.5 Hz, 18H, CH ), 1.59 (m, 2H, CH ), 2.06 (s, 3H, CH ), 2.23 (s,
3
2
3
6H, CH ), 2.93 (t, J = 5.0 Hz, 2H, CH ), 3.50 (t, J = 5.0 Hz, 2H,
3
2
13
CH ), 6.69 (s, 2H, CH). C NMR (125.7 MHz, C D ): δ = 18.9 (d, J
2
6
6
= 5 Hz, CH ), 20.2 (s, CH ), 22.2 (s, CH ), 30.1 (d, J = 15 Hz, CH ),
3
3
2
3
32.8 (d, J = 24 Hz, C), 45.0 (s, CH ), 48.2 (d, J = 2.5 Hz, CH ), 128.8
2
2
(s, CH), 135.8 (s, C), 136.4 (d, J = 1.3 Hz, C), 140.4 (d, J = 2.5 Hz,
C), 160.1 (d, J = 18 Hz, CP). 31P NMR (81 MHz, C D ): δ = 34.5.
2
2
2
f
6
6
1
Combined yield: 0.99 g (79%). Mp: 144−145 °C (hexane). H NMR
CDCl ): δ = 1.23 (d, J = 6.5 Hz, 3H, CH ), 1.41 (d, J = 16.0 Hz, 9H,
Anal. Calcd (%) for C H N P (346.50): C, 72.79; H, 10.18; N, 8.08.
21 35 2
(
Found: C, 72.92; H, 10.02; N, 7.87.
3
3
CH ), 1.49 (d, J = 16 Hz, 9H, CH ), 1.67 (d, J = 14.0 Hz, 1H, CH ),
Reaction of Carbene 3a with Selenium. To a stirring
suspension of salt 2a (2.27 g, 5.0 mmol) in THF (20 mL), cooled
to −90 °C, was added a solution of lithium hexamethyldisilazide (880
mg, 5.2 mmol) in THF (10 mL) in one portion. The reaction mixture
was kept stirring at −90 °C for 1 h; then the temperature was raised to
−70 °C, and finely ground selenium (0.99 g, 12 mmol) was added.
The reaction mixture was allowed to warm to 16 °C and kept stirring
for 3 days. The reaction mixture was concentrated in vacuo. The
residue was extracted with hexane (3 × 20 mL), and the solution was
treated with activated charcoal and filtered. The solvents were
evaporated, and the residue was separated by plate chromatography
on silica gel using CH Cl as an eluent. The crop R 0.8−1.0 gave a
3
3
2
2
.03 (m, 1H, CH ), 3.21−3.28 (m, 1H, CH ), 3.41−3.43 (m, 1H,
2
2
CH ), 3.49−3.55 (m, 1H, CH ), 3.65−3.68 (m, 1H, CH ), 6.63 (q, J =
2
2
2
6
.0 Hz, 1H, CH), 6.79 (dd, J = 7.0; 7.5 Hz, 1H, CH), 6.96 (d, J = 8.0
13
Hz, 2H, CH), 7.24 (dd, J = 7.5; 8.5 Hz, 2H, CH). C NMR (CDCl ):
3
δ = 11.3 (s, CH ), 25.0 (s, CH ), 27.9 (s, CH ), 30.0 (s, CH ), 39.4 (s,
3
2
3
3
CH ), 41.0 (s, CH ), 41.2 (d, J = 43 Hz, C), 42.5 (d, J = 43 Hz, C),
2
2
6
1
8.9 (d, J = 6.3 Hz, CH), 115.8 (s, CH), 118.6 (s, CH), 128.8 (s, CH),
48.9 (s, iC). 31P NMR (CDCl ): δ = 111.5 (J = 727 Hz). ESI-MS
3
PSe
+
(
positive ions, DMSO): m/z 401.2 [M + 2] (97.9%). Anal. Calcd (%)
for C H N PSe (399.42): C, 57.14; H, 8.33; N, 7.01; P, 7.75. Found:
19
33
2
C, 57.02; H, 7.93; N, 7.22; P, 7.63.
-(Di-tert-butylphosphoroselenoyl)-3-phenylhexahydro-
2
2
f
1
mixture of 6a and 8, the crop with R 0.7−0.8 gave 480 mg (1.25
f
pyrimidine, 4b. To a stirred and cooled to −100 °C suspension of
tetrahydropyrimidinium salt 2a (1.60 g, 3.5 mmol) in THF (12 mL)
was added dropwise over 3 min a 1.7 M solution of tert-butyllithium
mmol) of 7a (17%). The first crop was separated a second time using
as an eluent CH Cl /hexane (2:1) to give 0.92 g (2 mmol) of 6a
2
2
(27%), R 0.6−0.75, and 0.55 g (1.23 mmol) of 8 (33%), R 0.75−0.9.
f
f
(
2.1 mL, 3.5 mmol) in pentane. The reaction mixture was stirred at
Di-tert-butyl(1-phenyl-1,4,5,6-tetrahydropyrimidin-2-ylselanyl)-
1
−
60 °C for 2 h to give a clear solution. The reaction temperature was
slowly increased to +16 °C, and finely ground selenium (280 mg, 3.5
mmol) was added. The reaction mixture was stirred at 16 °C for 2 h.
phosphaneselenide, 6a. Mp: 107−108 °C. H NMR (300 MHz,
CDCl ): δ = 1.31 (d, J = 17.7 Hz, 18H, CH ), 2.03 (m, 2H, CH ), 3.61
3
3
2
(m, 2H, CH ), 3.76 (t, J = 6.0 Hz, 2H, CH ), 7.13−7.17 (m, 3H, CH),
2
2
F
DOI: 10.1021/acs.organomet.5b01019
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
3
C). 31P NMR (81 MHz, DMSO-d ): δ = 59.5. Anal. Calcd (%) for
7
.24−7.30 (m, 2H, CH). C NMR (125.7 MHz, CDCl ): δ = 22.9 (s,
3
6
CH ), 28.0 (s, CH ), 43.4 (d, J = 23 Hz, C), 47.9 (s, CH ), 50.7 (s,
C H Cl N PPd (481.73): C, 44.88; H, 6.07; N, 5.82. Found: C,
2
3
2
18 29
2
2
CH ), 125.1 (s, CH), 126.5 (s, CH), 128.1 (s, CH), 145.8 (s, iC),
44.51; H, 5.71; N, 5.69.
2
31
1
46.9 (d, J = 6.3 Hz, CSe). P NMR (202 MHz, CDCl ): δ = 118.3
Palladium Complex 9b. To a stirred solution of salt 2b (1.34 g,
2.7 mmol) in THF (15 mL), cooled to −90 °C, was added a solution
of lithium hexamethyldisilazide (460 mg, 2.7 mmol) in THF in one
portion. The reaction mixture was allowed to warm to 20 °C, then
cooled to −70 °C, and a solution of bis(benzonitrile)palladium(II)
chloride (960 mg, 2.5 mmol) in THF (20 mL) was added. After 15 h
at 20 °C a very fine-grained precipitate was filtered off, washed with
THF (2 × 20 mL), and dried in vacuo. The solid was extracted with
dichloromethane (3 × 15 mL); undissolved solid was filtered off and
recrystallized from acetonitrile. Dichloromethane was evaporated in
vacuo. The residual solid was combined with the crystals that were
3
(
JPSe = 364 and 751 Hz). ESI-MS (positive ions, DMSO): m/z 462.9
+
[
M + 1] (100%). Anal. Calcd (%) for C H N PSe (462.34): C,
18 29 2 2
4
6.76; H, 6.32; N, 6.06. Found: C, 46.29; H, 6.01; N, 6.97.
-(Di-tert-butylphosphoroselenoyl)-1-phenyl-1,4,5,6-tetrahydro-
2
1
pyrimidine, 7a. Mp: 126−127 °C. H NMR (300 MHz, CDCl ): δ =
1
Hz, 2H, CH ), 3.74 (m, 2H, CH ), 7.05 (d, J = 7.2 Hz, 2H, CH), 7.19
3
.45 (d, J = 15.6 Hz, 18H, CH ), 1.87 (m, 2H, CH ), 3.44 (t, J = 5.4
3
2
2
2
13
(
dd, J = 7.2; 7.5 Hz, 1H, CH), 7.25−7.30 (m, 2H, CH). C NMR
(
125.7 MHz, CDCl ): δ = 21.4 (s, CH ), 28.3 (s, CH ), 39.5 (d, J = 31
3
2
3
Hz, C), 46.3 (d, J = 14 Hz, CH ), 52.3 (d, J = 4 Hz, CH ), 125.2 (s,
2
2
CH), 126.8 (s, CH), 128.2 (s, CH), 147.3 (s, iC), 154.4 (d, J = 101
collected from acetonitrile to give 9b as a yellow solid. Yield: 0.77 g
Hz, CP). 31P NMR (202 MHz, CDCl ): δ = 73.0 ppm (J = 723 Hz).
1
3
PSe
(58%). Mp: 300 °C (dec). H NMR (500 MHz, DMSO-d ): δ = 1.59
6
+
ESI-MS (positive ions, DMSO): m/z 385.0 [M + 2] (100%). Anal.
(d, J = 17.0 Hz, 18H, tBu), 2.13 (br s, 2H, CH ), 2.17 (s, 6H, Me),
2
Calcd (%) for C H N PSe (383.38): C, 56.39; H, 7.62; N, 7.31.
2.22 (s, 3H, Me), 3.41 (br s, 2H, CH ), 3.61 (br s, 2H, CH ), 6.83 (s,
18
29
2
2
2
2H, CH). 13C NMR (125.7 MHz, DMSO-d ): δ = 17.3 (s, CH ), 20.4
Found: C, 55.93, H 7.38, N 7.45.
6
3
1
Compound 8. Mp: 106−107 °C. H NMR (300 MHz, CDCl ): δ =
(s, CH ), 20.7 (s, CH ), 28.2 (s, CH ), 43.9 (s, CH ), 49.8 (s, CH ),
2 3 3 2 2
3
13
31
1
.51 (d, J = 17.7 Hz, CH ). C NMR (125.7 MHz, CDCl ): δ = 28.4
128.8 (s, CH), 133.5 (s, C), 186.5 (s, C). P NMR (81 MHz, DMSO-
3
3
3
1
(
CH ), 43.7 (d, J = 19 Hz, C). P NMR (202 MHz, CDCl ): δ =
d ): δ = 58.0. Anal. Calcd (%) for C H Cl N PPd (523.81): C,
3
3
6
21 35
2
2
1
17.6 ppm (JPSe = 737 and 391 Hz).
Reaction of Carbene 3b with Selenium. The same procedure
48.15; H, 6.74; N, 5.35. Found: C, 47.71; H, 6.49; N, 5.51.
Palladium Complex 9c. To a stirred solution of salt 2c (1.32 g,
2.7 mmol) in THF (15 mL), cooled to −40 °C, was added a solution
of lithium hexamethyldisilazide (0.46 g, 2.7 mmol). The reaction
mixture was allowed to warm to 20 °C, then cooled to 0 °C, and a
solution of bis(benzonitrile)palladium(II) chloride (2.7 mmol) in
THF (10 mL) was added. After 15 h at 20 °C a very fine-grained
precipitate settled. THF was evaporated to dryness, and the residue
was washed with 3 × 10 mL of diethyl ether. The solid was dried and
then dissolved in methylene chloride (15 mL). Insoluble LiOTf was
filtered and washed with methylene chloride (2 × 5 mL). The volume
of methylene chloride was reduced to one-fifth, and the solution was
was applied as for 3a. After evaporation of the solvents the residue was
separated by plate chromatography on silica gel using as an eluent
CH Cl . The crop R 0.85−1.0 gave a mixture of 6b and 8, and the
2
2
f
crop with R 0.7−0.85 gave 107 mg (0.25 mmol) 7b (18%). The first
f
crop was separated a second time using as an eluent CH Cl /hexane
2
2
(
2:1) to give 219 mg (0.43 mmol) of 6b (31%), R 0.75−0.9, and 38
f
mg (0.08 mmol) of 8 (12%), R 0.75−0.9.
f
Di-tert-butyl(1-(2,4,6-trimethylphenyl)-1,4,5,6-tetrahydro-
1
pyrimidin-2-ylselanyl)phosphaneselenide, 6b. Mp: 149−150 °C. H
NMR (300 MHz, CDCl ): δ = 1.51 (d, J = 18.3 Hz, 18H, CH ), 2.01
3
3
(
m, 2H, CH ), 2.26 (s, 3H, CH ), 2.28 (s, 6H, CH ), 3.33 (t, J = 5.4
left in a freezer at −10 °C to give 9c as yellow crystals.Yield: 0.77 g
2
3
3
13
1
Hz, 2H, CH ), 3.51 (t, J = 5.4 Hz, 2H, CH ), 6.85 (s, 2H, CH).
NMR (125.7 MHz, CDCl ): δ = 17.7 (s, CH ), 20.7 (s, CH ), 22.0 (s,
CH ), 28.6 (s, CH ), 43.2 (d, J = 17.6 Hz, C), 46.2 (s, CH ), 48.3 (s,
CH ), 129.2 (s, CH), 136.8 (s, C), 137.7 (s, C), 138.7 (s, C), 150.4 (d,
J = 8.8 Hz, CSe). P NMR (202 MHz, CDCl ): δ = 111.9 (J = 417
and 696 Hz). ESI-MS (positive ions, DMSO): m/z 505.0 [M + 1]
C
(57%). Mp: 250 °C (dec). H NMR (300 MHz, DMSO-d ): δ = 1.53
2
2
6
3
3
3
(d, JP−H = 17.1 Hz, 18H, tBu), 2.16 (s, 6H, Me), 2.24 (s, 3H, Me), 3.88
(t, J = 9.3 Hz, 2H, CH ), 4.15 (t, J = 9.9 Hz, 2H, CH ), 6.89 (s, 2H,
2
3
2
2
2
CHAr). 13C NMR (125.7 MHz, DMSO-d ): δ = 17.4 (s, CH ), 20.6 (s,
2
6
3
31
3
PSe
CH ), 27.7 (d, J
3
C−P
= 4 Hz, CH ), 48.7 (d, J
3
= 6 Hz, CH ), 52.4 (d,
C−P 2
+
J = 3 Hz, CH ), 128.6 (s, CH), 132.9 (s, C), 134.6 (s, C), 137.5 (s, C),
2
31
(
97%). Anal. Calcd (%) for C H N PSe (504.42): C, 50.00; H,
165.8 (s, C). P NMR (81 MHz, DMSO-d ): δ = 90.1. Anal. Calcd
21
35
2
2
6
6
.99; N, 5.55. Found: C, 49.83; H, 6.59; N, 5.81.
-(Di-tert-butylphosphoroselenoyl)-1-(2,4,6-trimethylphenyl)-
(%) for C H Cl N PPd (509.78): C, 47.12; H, 6.52; N, 5.50. Found:
20
33
2
2
2
C, 46.76; H, 6.28; N, 5.71.
1
1
,4,5,6-tetrahydropyrimidine, 7b. Mp: 178−181 °C. H NMR (300
X-ray Crystal Structure Determination of Compounds 3b
and 9a−c. All crystallographic measurements were performed at
173(1) K (for 3b, 9a, 9c) and 123(1) K (for 9b) on a Bruker Smart
Apex II diffractometer (Mo Kα) operating in the ω and φ scan mode.
Data were corrected for Lorentz and polarization effects. The SADABS
procedure absorption correction was applied. The structures were
solved by direct methods and refined by the full-matrix least-squares
technique in the anisotropic approximation using the SHELXS97 and
MHz, CDCl ): δ = 1.44 (d, J = 15.9 Hz, 18H, CH ), 1.98 (m, 2H,
CH ), 2.27 (s, 3H, CH ), 2.29 (s, 6H, CH ), 3.26 (t, J = 5.4 Hz, 2H,
CH ), 3.67 (br s, 2H, CH ), 6.81 (s, 2H, CH). C NMR (125.7 MHz,
CDCl ): δ = 18.8 (s, CH ), 20.6 (s, CH ), 22.4 (s, CH ), 28.9 (s,
CH ), 40.7 (d, J = 33 Hz, C), 44.9 (d, J = 15 Hz, CH ), 51.2 (d, J = 4
Hz, CH ), 128.7 (s, CH), 135.9 (s, C), 136.4 (s, C), 140.1 (s, C),
3
3
2
3
3
13
2
2
3
3
3
2
3
2
2
31
1
50.9 (d, J = 102 Hz, CP). P NMR (202 MHz, CDCl ): δ = 71.5
3
2
5,26
27
ppm (J = 716 Hz). ESI-MS (positive ions, DMSO): m/z 427.0 [M
+
SHELXL97 programs
and CRYSTALS program package. In the
PSe
+
28
2] (99%). Anal. Calcd (%) for C H N PSe (425.46): C, 59.29; H,
refinement the Chebychev weighting scheme was used. All hydrogen
atoms were located in the difference Fourier maps and refined with
fixed positional and thermal parameters. CCDC 1437123 (3b),
1437124 (9a), 1437122 (9b), and 1437233 (9c) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data_request/cif. Full crystal-
lographic details are also displayed in Table S1 in the Supporting
Information.
21
35
2
8
(
.29; N, 6.58. Found: C, 58.89; H, 7.88; N, 6.96.
Palladium Complex 9a. Carbene 3a was freshly prepared from 2a
2.31 g, 5.1 mmol) and a 1 M solution of potassium
hexamethyldisilazide (5 mL, 5.1 mmol) as outlined above. The
resulting pentane solution of the carbene was evaporated to dryness,
and the residual solid was dissolved in THF (25 mL). Bis-
(
benzonitrile)palladium(II) chloride (1.7 g, 2.4 mmol) was added,
and the mixture was stirred at 20 °C for 15 h and then heated at 90 °C
for 5 min. The precipitate was filtered off, washed with THF (2 × 15
mL), and dried in vacuo. The solid was recrystallized from acetonitrile
Catalytic Tests: Suzuki Reaction. Typical procedure: In a
Schlenk tube equipped with a magnetic stirring bar were placed under
an inert atmosphere 81 mg (0.66 mmol) of phenylboronic acid, 167
mg (1.2 mmol) of anhydrous K CO , and 6 μmol (1 mol %) of
(
(
1
25 mL) to give 9a as yellow crystals. Yield: 0.83 g (71%). Mp: 300 °C
dec). H NMR (400 MHz, DMSO-d ): δ = 1.62 (d, J = 16.8 Hz,
8H, tBu), 2.17 (br s, 2H, CH ), 3.55 (br s, 2H, CH ), 3.72 (br s, 2H,
1
6
PH
2
3
catalyst. The tube was degassed and put under an inert atmosphere. A
0.600 mmol amount of aryl halide and 5 mL of dry solvent were
subsequently added. The flask was immediately placed in an oil bath
preheated at the reaction temperature, and the reaction mixture was
2
2
13
CH ), 7.27−7.32 (m, 5H, CH). C NMR (75.5 MHz, DMSO-d ): δ
=
2
6
20.4 (CH ), 28.4 (d, J = 5 Hz, CH ), 44.0 (s, CH ), 51.7 (s, CH ),
2 3 2 2
1
26.6 (s, CH), 127.8 (s, CH), 128.7 (s, CH), 144.8 (s, C), 189.5 (s,
G
DOI: 10.1021/acs.organomet.5b01019
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
vigorously stirred for 5 h. A 14 mg (0.063 mmol) amount of 1,4-
bis(trimethylsilyl)benzene as an internal standard was then added. The
Danopoulos, A. A. Coord. Chem. Rev. 2007, 251, 610−641.
(k) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239−
2246.
mixture was evaporated to dryness and taken up in CDCl . Yields were
3
1
determined by H NMR.
(6) (a) Marchenko, A. P.; Koidan, H. N.; Zarudnitskii, E. V.; Hurieva,
A. N.; Kirilchuk, A. A.; Yurchenko, A. A.; Biffis, A.; Kostyuk, A. N.
Organometallics 2012, 31, 8257−8264. (b) Marchenko, A. P.; Koidan,
H. N.; Hurieva, A. N.; Gutov, O. V.; Kostyuk, A. N.; Tubaro, C.; Lollo,
S.; Lanza, A.; Nestola, F.; Biffis, A. Organometallics 2013, 32, 718−721.
Catalytic Tests: Sonogashira Reaction. Typical procedure: In a
Schlenk tube equipped with a magnetic stirring bar were placed under
an inert atmosphere 0.600 mmol of aryl halide, 167 mg (1.2 mmol) of
anhydrous K CO , and 6 μmol (1 mol %) of catalyst. The tube was
2
3
degassed and put under an inert atmosphere. A 10 mL amount of dry
DMF was subsequently added, followed by 132 μL (1.2 mmol) of
phenylacetylene The flask was immediately placed in an oil bath
preheated at 100 °C, and the reaction mixture was vigorously stirred
for 2 h. The reaction mixture was then partitioned between 30 mL of
water and 20 mL of diethyl ether, and the organic layer was further
(
c) Marchenko, A.; Koidan, H.; Hurieva, A.; Vlasenko, Y.; Kostyuk, A.;
Tubaro, C.; Lenarda, A.; Biffis, A.; Graiff, C. J. Organomet. Chem. 2014,
71, 14−23.
7) (a) Ku
Hofmann, P. Organometallics 2012, 31, 8000−8011. (b) Nag
Herrlich (nee Blumbach), U.; Rominger, F.; Hofmann, P. Organo-
7
(
̈
hnel, E.; Shishkov, I. V.; Rominger, F.; Oeser, T.;
ele, P.;
́
̈
washed with 2 × 20 mL of water and dried over MgSO . A 14 mg
4
metallics 2013, 32, 181−191. (c) Ai, P.; Danopoulos, A. A.; Braunstein,
P.; Monakhov, K. Y. Chem. Commun. 2014, 50, 103−105. (d) Brown,
C. C.; Plessow, P. N.; Rominger, F.; Limbach, M.; Hofmann, P.
Organometallics 2014, 33, 6754−6759. (e) Ai, P.; Danopoulos, A. A.;
Braunstein, P. Inorg. Chem. 2015, 54, 3722−3724. (f) Ai, P.;
Danopoulos, A. A.; Braunstein, P. Organometallics 2015, 34, 4109−
(
0.063 mmol) amount of 1,4-bis(trimethylsilyl)benzene as an internal
standard was then added, after which the mixture was evaporated to
dryness and taken up in CDCl . Yields were determined by H NMR.
1
3
ASSOCIATED CONTENT
Supporting Information
■
*
S
4
(
116.
8) Marchenko, A.; Koidan, H.; Hurieva, A.; Vlasenko, Y.; Kostyuk,
A.; Biffis, A. Dalton Trans. 2016, 45, 1967−1975.
9) (a) Marchenko, A.; Koidan, G.; Hurieva, A.; Kurpiieva, O.;
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b01019.
(
NMR spectra of the compounds and additional X-ray
Vlasenko, Y.; Rozhenko, A. B.; Kostyuk, A. Eur. J. Inorg. Chem. 2014,
2014, 3259−3270. (b) Aluri, B. R.; Burck, S.; Gudat, D.; Niemeyer,
M.; Holloczki, O.; Nyulaszi, L.; Jones, P. G.; Heinicke, J. Chem. - Eur. J.
crystallographic details for compounds 3b and 9a−c
(
PDF)
2
009, 15, 12263−12272.
Crystallographic files for compounds 3b and 9a−c (CIF)
(10) (a) Marchenko, A. P.; Koidan, H. N.; Huryeva, A. N.;
Zarudnitskii, E. V.; Yurchenko, A. A.; Kostyuk, A. N. J. Org. Chem.
2010, 75, 7141−7145.
AUTHOR INFORMATION
Corresponding Authors
E-mail: a.kostyuk@yahoo.com (A. Kostyuk).
Tel: +39-049-8275216. Fax: +39-049-8275223. E-mail:
■
(11) Marchenko, A. P.; Koidan, H. N.; Hurieva, A. N.; Pervak, I. I.;
Shishkina, S. V.; Shishkin, O. V.; Kostyuk, A. N. Eur. J. Org. Chem.
2012, 2012, 4018−4033.
*
*
(
12) Bourissou, D.; Guerret, O.; Gabbaı, F. P.; Bertrand, G. Chem.
Rev. 2000, 100, 39−91.
13) Marchenko, A.; Koidan, G.; Hurieva, A.; Savateev, A.; Kostyuk,
A. Tetrahedron Lett. 2013, 54, 5671−5673.
andrea.biffis@unipd.it (A. Biffis).
Notes
(
The authors declare no competing financial interest.
(
14) Nelson, D. J.; Nolan, S. P. Chem. Soc. Rev. 2013, 42, 6723−6753.
REFERENCES
■
(15) Leung, C. H.; Incarvito, C. D.; Crabtree, R. H. Organometallics
(
1) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 6th ed.; Wiley: Hoboken, NJ, 2014.
2) (a) Slone, C. S.; Weinberger, D. A.; Mirkin, C. A. Prog. Inorg.
2006, 25, 6099−6107.
(16) (a) Marion, N.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 1440.
(b) Fortman, G. C.; Nolan, S. P. Chem. Soc. Rev. 2011, 10, 5151−5169.
(c) Valente, C.; Calimsiz, S.; Hoi, K. H.; Mallick, D.; Sayah, M.; Organ,
M. G. Angew. Chem., Int. Ed. 2012, 51, 3314−3332. (d) Ni, C.; Shen,
A.; Cao, Y. C.; Ye, X. F. Youji Huaxue 2014, 34, 278−291.
(
Chem. 1999, 48, 233−350. (b) Braunstein, P.; Naud, F. Angew. Chem.,
Int. Ed. 2001, 40, 680−699. (c) Zhang, W.−H.; Chien, S. W.; Hor, T.
S. A. Coord. Chem. Rev. 2011, 255, 1991−2024.
(
3) General references on NHC ligands: (a) Nolan, S. P., Ed. N-
(
17) Alimardanov, A.; Schmieder-van de Vondervoort, L.; de Vries,
A. H. M.; de Vries, J. G. Adv. Synth. Catal. 2004, 346, 1812−1817.
18) Wheaton, C. A.; Bow, J.-P. J.; Stradiotto, M. Organometallics
Heterocyclic Carbenes, Effective Tools for Organometallic Synthesis; Wiley-
VCH: Weinheim, 2014. (b) Diez-Gonzalez, S., Ed. N-Heterocyclic
Carbenes: From Laboratory Curiosities to Efficient Synthetic Tools; RSC
Catalysis Series, RSC: Cambridge, 2010. (c) Cazin, C. S. J., Ed. N-
Heterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis,
Catalysis by Metal Complexes, Vol. 32; Springer: Heidelberg, 2010.
4) General reviews on N-heterocyclic carbenes: (a) Hopkinson, M.
N.; Richter, C.; Schedler, M.; Glorius, F. Nature 2014, 510, 485−496.
c) Melaimi, M.; Soleilhavoup, M.; Bertrand, G. Angew. Chem., Int. Ed.
010, 49, 8810−8849. (d) Hahn, F. E.; Jahnke, M. C. Angew. Chem.,
Int. Ed. 2008, 47, 3122−3172.
5) (a) Biffis, A.; Baron, M.; Tubaro, C. Adv. Organomet. Chem. 2015,
(
2
(
(
(
013, 32, 6148−6161.
19) Hartwig, J. F. Nature 2008, 455, 314−322.
20) Chinchilla, R.; Najera, C. Chem. Rev. 2007, 107, 874−922.
21) (a) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem.,
(
Int. Ed. 2000, 39, 2632−2657. (b) Hay, A. S. J. Org. Chem. 1962, 27,
320−3321.
22) (a) Batey, R. A.; Shen, M.; Lough, J. Org. Lett. 2002, 4, 1411−
414. (b) Ray, L.; Barman, S.; Shaikh, M. M.; Ghosh, P. Chem. - Eur. J.
3
(
(
2
1
2
2
2
008, 14, 6646−6655. (c) Luo, F.-T.; Lo, H.-K. J. Organomet. Chem.
011, 696, 1262−1265. (e) Huynh, H. V.; Lee, C. S. Dalton Trans.
013, 42, 6803−6809. (f) Yang, L.; Li, Y.; Chen, Q.; Du, Y.; Cao, C.;
(
6
7
7
2
3, 203−288. (b) Gaillard, S.; Renaud, J.-L. Dalton Trans. 2013, 42,
255−7270. (c) Gu, S.; Huang, J.; Chen, W. Youji Huaxue 2013, 33,
15−737. (d) Bierenstiel, M.; Cross, E. D. Coord. Chem. Rev. 2011,
Shi, Y.; Pang, G. Tetrahedron 2013, 69, 5178−5184. (g) Biffis, A.;
Cipani, M.; Bressan, E.; Tubaro, C.; Graiff, C.; Venzo, A.
Organometallics 2014, 33, 2182−2188. (h) Lu, H.; Wang, L.; Yang,
F.; Wu, R.; Shen, W. RSC Adv. 2014, 4, 30447−30452.
(23) Eberhard, M. R.; van Vliet, B.; Duran Pachon, L.; Rothenberg,
G.; Eastham, G.; Kooijman, H.; Speck, A. L.; Elsevier, C. J. Eur. J. Inorg.
Chem. 2009, 2009, 1313−1316.
̈
55, 574−590. (e) Kuhl, O. Functionalised N-Heterocyclic Carbene
Complexes; Wiley: Hoboken, NJ, 2010. (f) Poyatos, M.; Mata, J. A.;
Peris, E. Chem. Rev. 2009, 109, 3677−3707. (g) Normand, A. T.;
Cavell, K. J. Eur. J. Inorg. Chem. 2008, 2008, 2781−2800. (h) Ku
̈
hl, O.
Chem. Soc. Rev. 2007, 36, 592−607. (i) Lee, H. M.; Lee, C.-C.; Cheng,
P.-Y. Curr. Org. Chem. 2007, 11, 1491−1524. (j) Pugh, D.;
H
DOI: 10.1021/acs.organomet.5b01019
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Organometallics
Article
(
2
7
(
24) (a) Alici, B.; C
9−36. (b) Paczal, A.; Ben
1, 5969−5979.
25) Sheldrick, G. M. SHELXS97 Program for the Solution of Crystal
Structure; University of Goettingen: Goettingen, Germany, 1997.
26) Sheldrick, G. M. SHELXL97 Program for the Refinement of
Crystal Structures; University of Goettingen: Goettingen, Germany,
997.
27) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.
̧
etinkaya, E.; C
̧etinkaya, B. Heterocycles 1997, 45,
́
yei, A. C.; Kotschy, A. J. Org. Chem. 2006,
(
1
(
CRYSTALS issue 10; Chemical Crystallography Laboratory, University
of Oxford, 1996.
(
28) Carruthers, J. R.; Watkin, D. J. Acta Crystallogr., Sect. A: Cryst.
Phys., Diffr., Theor. Gen. Crystallogr. 1979, 35, 698−699.
I
DOI: 10.1021/acs.organomet.5b01019
Organometallics XXXX, XXX, XXX−XXX