Facile synthesis of Pd(ii) and Ni(ii) pincer carbene complexes by the double C-H bond activation of a new hexahydropyrimidine-based bis(phosphine): Catalysis of C-N couplings

Article abstract of DOI:10.1039/c8dt03413c

Hexahydropyrimidine-based bis(phosphine), a pro-NHC ligand, was synthesized in one step and excellent yield. It underwent spontaneous double C-H bond activation to give cationic pincer NHC complexes of the type [(PCP)MCl]X (M = Pd, Ni and X = Cl, BF₄) in the absence of any external reagents. Their structures were determined by X-ray diffraction methods and the mechanism of formation of palladium carbene complexes as analyzed by DFT calculations showed two transition states. The Pd(ii) carbene complex effectively catalyzes a few C-N cross coupling reactions.

Full text of DOI:10.1039/c8dt03413c

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Facile synthesis of Pd(II) and Ni(II) pincer carbene
complexes by the double C–H bond activation of
a new hexahydropyrimidine-based bis(phosphine):
catalysis of C–N couplings†
Cite this: DOI: 10.1039/c8dt03413c
Vasudevan Subramaniyan,
Ganesan Mani
Bidisa Dutta, Anbarasu Govindaraj and
*
Hexahydropyrimidine-based bis(phosphine), a pro-NHC ligand, was synthesized in one step and excellent
yield. It underwent spontaneous double C–H bond activation to give cationic pincer NHC complexes of
the type [(PCP)MCl]X (M = Pd, Ni and X = Cl, BF4) in the absence of any external reagents. Their structures
were determined by X-ray diffraction methods and the mechanism of formation of palladium carbene
complexes as analyzed by DFT calculations showed two transition states. The Pd(II) carbene complex
effectively catalyzes a few C–N cross coupling reactions.
Received 21st August 2018,
Accepted 9th January 2019
DOI: 10.1039/c8dt03413c
rsc.li/dalton
complex formations most often involve treatment of the cat-
ionic azolium salts with strong bases, reflux conditions or
Introduction
The first pincer metal complex was synthesized by the chelate- transmetalation reactions using a silver or mercury complex.
assisted C–H activation reported by Moulton and Shaw in In 2012, Hill and co-workers first reported the one-step syn-
1976.1 Since then the chelate-assisted C–H activation in metal thesis of N,N-bis(phosphinomethyl)dihydroperimidines, and
complex formation is a well-known method, and has been their Rh(I) and Ir(III) carbene complexes formed via facile
used most commonly for synthesizing PCP pincer complexes double C–H bond activation in the absence of any base or
containing the M–C σ-bond.2 Conversely, the double C–H heating.12 In addition, the versatile nature of the ligand was
bond activation of the methylene group, first reported by Shaw shown by Ru(II),13 Ni(0), Au(I)11f and Ir(I)11j complexes, in which
and co-workers in 1977,3 has led to pincer carbene complexes it is coordinated as a neutral ligand. Following this, Gade and
containing the M–C double bond.4 Alternatively, the central co-workers have reported diperimidine derivatives containing
anchoring carbene carbon in pincer complexes has readily the perylene backbone and studied their Rh(I) complexes.11i To
been installed using N-heterocycle carbene (NHC) precursors.5
the best of our knowledge, there is no report of a spontaneous
Since the first report of the isolable stable NHC by double CH activation without a base or heating to give a Pd(II)
Arduengo’s group in 1991,6 great progress has been made in or Ni(II) complex containing the hexahydropyrimidine ring-
NHC organometallic chemistry.7 These NHC complexes have based NHC ligand. Herein, we report the facile synthesis and
been shown to be excellent catalysts for numerous organic structural characterization of Pd(II) and Ni(II) NHC complexes
transformation reactions.8 Among NHCs, the five-membered formed by the double C–H bond activation of the new pro-NHC
imidazolylidene-based NHCs are very common.9 However, in ligand based on the hexahydropyrimidine ring along with DFT
recent years, a large number of metal complexes containing calculations and the catalysis of C–N cross couplings.
NHCs based on hexahydropyrimidine and dihydroperimidine,
which are better σ-donors than the five-membered imidazolyli-
dene NHCs,10 have been reported.11 These NHC carbene
Results and discussion
Synthesis of the ligand and metalation
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur,
India 721 302. E-mail: gmani@chem.iitkgp.ac.in; Fax: +91 3222 282252;
Tel: +91 3222 282320
The one-pot reaction between 1,3-diaminopropane, diphenyl-
phosphine and paraformaldehyde in a 1 : 2 : 3 ratio, respect-
ively, under reflux conditions, readily afforded 1,3-bis(diphe-
nylphosphanylmethyl)hexahydropyrimidine 1 as an oily com-
pound in an excellent yield (91%) (Scheme 1). This method of
synthesis is similar to that of N,N-bis(phosphinomethyl)dihy-
†Electronic supplementary information (ESI) available: NMR, IR, crystal struc-
tures, crystallographic data (CIF), optimized geometries with coordinates. CCDC
1862877–1862880 for 2a–3b. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c8dt03413c
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The structures of 2a and 2b were determined by single
crystal X-ray diffraction studies. An ORTEP diagram of 2a
along with selected bond distances and angles is shown in
Fig. 1. The structure of 2b is provided in the ESI.† Complexes
2a and 2b crystallize in the monoclinic C2/c and the triclinic
ˉ
P1 space groups, respectively, and their asymmetric units con-
stitute the whole molecule together with the solvent of crystal-
lization. The structure contains two fused five-membered rings
around the palladium atom formed by the two diphenylpho-
sphinomethyl arms. This results in the formation of trans dis-
position of the two phosphorus atoms, with the carbene
carbon and chlorine atoms being trans to each other. The geo-
metry around the palladium atom is distorted square planar,
in which the fused ring angles (P–Pd–Ccarbene = 82.15(9)° and
80.67(9)°) are smaller than the other two angles (P–Pd–Cl =
95.97(3)° and 101.09(3)°). As a result, the palladium square
plane and the mean plane formed by C18, N2, C17, N1 and
C13 atoms are twisted. The twist angle is 19.6° for 2a and
15.1° for 2b. The Pd–Ccarbene distance of 2.011(3) Å in 2a and
1.997(4) Å in 2b is in the range reported for several palladium
complexes containing the hexahydropyrimidine-based NHC
ligands14,11h and others.5c,h,15 The sum of the angles around
the hexahydropyrimidine nitrogens in 2 is close to 360° (for
example, 359.9(2)° and 359.4(2)° in 2a), suggesting the planar-
ity around these nitrogen atoms. The carbene carbon atoms in
both the structures are completely planarized with the sum of
the angles being equal to 360(2)° (2a).
In an analogous manner, the cationic nickel(II) complexes
3a and 3b were synthesized by treating 1 with [NiCl2(DME)]/
NaBF4 in CH3CN at room temperature. Like the palladium
complexes, these complexes are also formed by facile double
C–H bond activation. The 31P NMR spectrum 3a or 3b dis-
played a singlet at δ = 24.8 ppm or δ = 25.4 ppm, respectively,
which is close to the palladium complex signals, suggesting
similar structures. Furthermore, the 13C NMR spectrum of 3a
showed a broad singlet at δ = 186.7 ppm and 3b displayed a
triplet at δ = 190.0 ppm, supporting the carbene-bound nickel(II)
Scheme 1 Synthesis of N,N’-bis(diphenylphosphanylmethyl)hexahy-
dropyrimidine 1.
droperimidine.12 1 is air-sensitive and well soluble in common
1
organic solvents. The H NMR spectrum of 1 in CDCl3 showed
broad signals for all the hexahydropyrimidine methylene
protons, indicating that the structure is in a dynamic process.
However, the P-methylene protons showed a doublet owing to
2
the proton–phosphorus coupling J(P,H) = 4 Hz. The 31P NMR
spectrum of 1 in CDCl3 featured a single resonance at δ =
−26.3 ppm. This remains close to the chemical shift value of
−26.0 ppm reported for 1,3-bis(diphenylphosphinomethyl)-
2,3-dihydroperimidine,12 indicating that both ligands have
similar electronic environments. However, 1 possesses a flex-
ible saturated hexahydropyrimidine ring and was synthesized
from the cheap starting compound 1,3-diaminopropane.
Compound 1 could be the precursor of a new PCP pincer
carbene ligand because of the presence of methylene protons
flanked by two nitrogens. Hence, 1 was first treated with
[PdCl2(COD)] in CH3CN at room temperature to give the cat-
ionic Pd(II) carbene complex 2a containing the chloride coun-
teranion. The same reaction in the presence of NaBF4 afforded
−
complex 2b with the BF4 ion (Scheme 2). Both complexes are
formed by the spontaneous double dehydrogenation of the
NCH2N methylene protons facilitated by two chelate rings
formed around the palladium atom and are air stable. The pres-
ence of a Pd–C bond in the structure indirectly suggested by the
number of methylene signals in their 1H NMR spectra displayed
three instead of four signals. Their 31P NMR spectra showed
singlets (δ = 26.3 ppm for 2a and δ = 25.9 ppm for 2b) and dis-
played a significant coordination-caused chemical shift change
Δδ of ∼52 ppm with respect to the free ligand value. In addition,
their 13C NMR spectra displayed triplet patterns owing to the
phosphorus coupling (δ = 189.9 ppm for 2a and 186.7 ppm for
2b) for the palladium-bound carbene carbon atoms.
Fig. 1 Crystal structure of 2a (50% displacement ellipsoids). Hydrogen
atoms, Cl− ion and solvent molecules are omitted for clarity. Selected
bond lengths (Å) and bond angles (°): P1–Pd1 2.2912(8), P2–Pd1 2.2726
(8), Cl1–Pd1 2.3415(8), C17–Pd1 2.011(3), N1–C17 1.334(4), N2–C17
1.338(4), C17–Pd1–P2 82.15(9), C17–Pd1–P1 80.67(9), P2–Pd1–P1
162.47(3), C17–Pd1–Cl1 177.49(9), P2–Pd1–Cl1 95.97(3), P1–Pd1–Cl1
101.09(3).
Scheme 2 Synthesis of the NHC pincer Pd(II) and Ni(II) complexes 2
and 3.
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Fig. 2 Crystal structure of 3a (50% displacement ellipsoids). Hydrogen
atoms, Cl− ion and solvent molecules are omitted for clarity. Selected
bond lengths (Å) and bond angles (°): P1–Ni1 2.1638(7), P2–Ni1 2.1661
(7), Cl1–Ni1 2.1850(7), C17–Ni1 1.900(3), N2–C17 1.350(3), C17–Ni1–P2
86.29(8), C17–Ni1–P1 86.17(8), P2–Ni1–P1 170.13(3), C17–Ni1–Cl1
176.96(8), P2–Ni1–Cl1 93.78(3), P1–Ni1–Cl1 94.09(3).
Scheme 3 The proposed mechanism for the formation of pincer
carbene complexes through the transition states B and D found in DFT
calculations.
complexes. In addition, the carbon atom in the NCH2P and
hexahydropyrimidine ring N–CH2 groups, and the phenyl ipso,
ortho and meta carbons displayed triplets owing to the phos-
phorus coupling.
As expected, the NBO analysis showed second-order pertur-
bation stabilization energies of 131.05 and 136.42 kcal mol−1
for the N–Ccarbene π-interactions in 2a and 2b, respectively.
Similar values were obtained for the nickel complexes 3a and
3b (132.35 and 130.91 kcal mol−1, respectively). These values
suggest that, in addition to the chelate effect imposed by the
diphosphine, carbene formation by double C–H bond acti-
vation is driven by the π electron delocalization from the flank-
ing nitrogens. As a result, the average carbene C–N bond
length in 2a, 2b, 3a and 3b is 1.336, 1.341, 1.349 and 1.347 Å,
respectively. The calculated average carbene C–N bond order
in 2 or 3 is 1.31, which is higher than the calculated average
carbene C–N bond order (1.27) with a distance of 1.375 Å
found in the energy-minimized model complex [PdCl{C
(NCH2PPh2)2C10H6}]+ containing the dihydroperimidine ring.
This is attributed to the extent of π-interactions in the carbene
C–N bonds which is higher when the ring is saturated in
which the nitrogen lone pair is fully available to stabilize the
carbene carbon, leading to shorter bonds. In the dihydroperi-
midine ring-based NHC complex, the nitrogen lone pairs are
involved in extended conjugation with the naphthalene ring.
This observation is in line with the reported palladium com-
plexes containing the saturated and unsaturated NHCs.18
While the saturated NHC palladium complex exhibits a slightly
shorter average carbene C–N distance of 1.332(4) Å, the unsatu-
rated one shows a slightly longer average distance of 1.345(4)
Å. Furthermore, the NBO analysis showed a metal to carbene
carbon back donation (M ⇒ Ccarbene) energy of 16.85 kcal
mol−1 in 2 and 12.23 kcal mol−1 in 3. The Cl ⇒ M
π-interaction energy of 8.08 kcal mol−1 in 2 and 11.12 kcal
mol−1 in 3 was also estimated.
The X-ray structure of 3a with selected bond distances and
angles is shown in Fig. 2. The structure of 3b is provided in
the ESI.† While the chloride counteranion complex 3a crystal-
−
lizes in the monoclinic C2/c space group, the BF4 anion
ˉ
complex 3b crystallizes in the triclinic P1 space group. The geo-
metry around the nickel atom in each complex is distorted
square planar. As found in the palladium complexes, the P1–
Ni–Cl and P2–Ni–Cl angles are larger than the P1–Ni–C and
P2–Ni–C angles. The distance between the carbene carbon and
nickel atoms is 1.900(3) Å in 3a and 1.905(6) Å in 3b, which are
close to that found (1.911(2) Å) in the dichlorobis(1,3-dicyclo-
hexylimidazol-2-ylidene) nickel(II) complex,16 but are longer
than that found in another nickel carbene complex, [(PCP)
NiH]PF6 (PCP
=
o-C6H4(PPri2)(NC3H5N)(PPri2)o-C6H4).17
Furthermore, the Ni–P distances in 3a or 3b are slightly longer
than those (2.1080(8) Å and 2.1129(9) Å) found in the reported
complex, [(PCP)NiH]PF6. The hexahydropyrimidine nitrogens
and the carbene carbon are planar, indicating the donation of
a lone pair of electrons from nitrogens to the carbon atom.
DFT calculations
To understand the double C–H bond activation and the elec-
tronic structure of complexes 2 and 3, DFT calculations were
performed, which supported two transition states for the for-
mation of the carbene complexes 2, as shown in Scheme 3.
The initial reaction between [PdCl2(COD)] and 1 gives the
chelate complex A in which the methylene protons are in close
proximity to the palladium atom. As a result, the tetrahydro-
pyrimidinium cationic palladium hydride intermediate C is
formed via the transition state B which subsequently abstracts
the remaining methylene hydrogen as a proton to give 2a with
Catalysis of C–N couplings
H2 formation through the transition state D showing the con- The catalytic application of the palladium complexes 2a and
certed fashion of proton abstraction.
2b was examined with the Buchwald–Hartwig coupling. A
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Table 1 The Buchwald–Hartwig coupling between bromobenzene and
morpholine catalyzed by 2a and 2b in the presence of different bases
and solventsa
Conclusions
The new pro-NHC pincer ligand was synthesized conveniently
in a single step in an excellent yield. The NHC pincer palla-
dium(II) and nickel(II) complexes were formed readily at room
temperature without a base or transmetalation reaction, repre-
senting their facile formation via the double C–H bond acti-
vation of the hexahydropyrimidine-based NHC precursor 1.
The X-ray structures and DFT calculations showed the nitrogen
π-electron delocalization over the carbene carbon atom, which
makes it a better σ-donor. Furthermore, the DFT calculations
showed that the double C–H bond activation occurred in a
stepwise fashion. The palladium carbene complex is proposed
to be formed via two intermediates and two transition states in
which the methylene hydrogens are abstracted successively in
a concerted fashion with the formation of H2. Furthermore,
the preliminary catalytic study using palladium complexes was
demonstrated with Buchwald–Hartwig C–N couplings in which
cross-coupled products were formed in very high yields, indi-
cating that a highly active metal center is generated in situ
during the catalysis processes with the support of the hexahy-
dropyrimidine ring-based NHC ligand. Syntheses of other
metal complexes and catalysis studies are ongoing research in
our laboratory.
Entry
Catalyst
Base
Solvent
Yieldb, %
1
2
3
4
6
7
8
9
2a
2a
2a
2a
2a
2a
2a
2a
2b
KOBut
Toluene
DMF
Toluene
DMF
Toluene
DMF
DMF/THF
Toluene/THF
Toluene/THF
81
69
0
0
59
18
83
KOBut
K2CO3
K2CO3
Cs2CO3
Cs2CO3
NaHMDS
NaHMDS
NaHMDS
91(82)
89
10
a ArX (1.9 mmol), amine (2.28 mmol), base (2.9 mmol), 2a or 2b
(1 mol%), solvent (5 mL), 100 °C, 5 h. b Estimated yields by GC (iso-
lated yields are given in parenthesis).
variety of palladium metal complexes as precatalysts have
been reported for this C–N bond formation reaction.19 Using
morpholine and bromobenzene, different bases and solvents
were screened for the optimum conditions to obtain the
product in a high yield (Table 1). While KOBut gave a com-
petitive yield of 81% in toluene, NaHMDS turned out to be
the best base in toluene/THF yielding 91% of the product
using 1 mol% of complex 2a. A similar yield (89%) was also
obtained with 2b. Using these optimized conditions, other C–
N couplings were carried out. The products were formed in
excellent yields as estimated by GC using diphenylamine as
an external standard, which are close to the isolated yields
(Table 2).
Experimental section
General
All reactions were carried out under a nitrogen atmosphere
using standard Schlenk line techniques or a nitrogen-filled
glove box. Petroleum ether (bp 40–60 °C) and other solvents
were distilled under an N2 atmosphere according to the stan-
dard procedures. [PdCl2(COD)]20, [NiCl2(DME)]21 and Ph2PH22
were synthesized according to the reported procedures. Other
chemicals were obtained from commercial sources and used
as received. 1H NMR (200 or 400 MHz) and 13C NMR
(100.6 MHz) spectra were recorded at room temperature. For
1H NMR spectra, chemical shifts are referenced with respect to
the chemical shift of the residual protons present in the deute-
rated solvents and for 13C NMR spectra, the CDCl3 chemical
shift was used as a reference. Chemical shifts are in parts per
million, and coupling constants are in Hz. FTIR and ATR
spectra were recorded using a PerkinElmer Spectrum Rx
Spectrometer. Elemental analyses were carried out using a
PerkinElmer 2400 CHN analyzer. A Thermo Scientific Trace
1310 GC chromatograph was used for detecting and calculat-
ing yields.
Table 2 Buchwald–Hartwig couplings between bromobenzene and
different amines catalyzed by 2a a
Entry
1
Amine
Product
Yieldb, %
92 (85)
2
3
96 (89)
88 (80)
Synthesis of 1,3-bis(diphenylphosphanylmethyl)
hexahydropyrimidine, 1
A mixture of 1,3-diaminopropane (0.97 mL, 11.621 mmol),
diphenylphosphine (4.0 mL, 23.202 mmol) and paraformalde-
hyde (1.045 g, 34.798 mmol) was refluxed at 115 °C with stir-
ring for 1 h. The reaction mixture was cooled to room tempera-
ture to give a colorless sticky precipitate, which was washed
a ArX (1.9 mmol), amine (2.28 mmol), NaHMDS (2.9 mL, 1 M in THF),
2a (1 mol%), toluene (5 mL), 100 °C, 5 h. b Estimated yields by GC (iso-
lated yields are given in parenthesis).
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with petroleum ether (2 × 10 mL) and dried under vacuum to Synthesis of [NiCl{C(NCH2PPh2)2(CH2)3-κ3P,C,P}]Cl, 3a
give 1 as an oily compound (5.09 g, 10.548 mmol, 91%). 1H
To a solution of [NiCl2(DME)] (0.10 g, 0.455 mmol) in aceto-
NMR (CDCl3, 400 MHz): δ 7.43 (m, 10H, Ph), 7.30 (m, 10H, Ph),
nitrile (20 mL) was added 1 (0.20 g, 0.414 mmol). The solution
2
3.66 (br s, 2H, NCH2N), 3.27 (d, J(P,H) = 4, 4H, PCH2N), 2.82
was stirred at room temperature for 24 h and then the solvent
(br s, 4H, pyrimidine CH2), 1.68 (br s, 2H, pyrimidine CH2).
was removed under vacuum. The resulting residue was washed
1
13C NMR (CDCl3, 100.6 MHz): δ 138.3 (d, J(C,P) = 26.2), 132.7
with petroleum ether (3 × 10 mL), and dissolved in chloroform,
(d, 2J(C,P) = 36.2), 128.3, 128.1, 76.2, 57.0, 53.3 (d, 1J(C,P) =
which was allowed to evaporate slowly to give green crystals of
17.1), 21.4. 31P{1H} NMR (CDCl3, 161.9 MHz): δ −26.3 (s).
1
3a (0.085 g, 0.1334 mmol, 29%). H NMR (CDCl3, 400 MHz):
FT-IR (ATR, cm−1): ν = 3053 (w), 2937 (w), 2856 (w), 2791 (w),
¯
δ 7.92 (br, 8H, Ph), 7.57–7.47 (m, 12H, Ph), 4.66 (br s, 4H,
NCH2PPh2), 3.58 (br s, 4H, pyrimidine NCH2), 2.1 (br s, 2H,
pyrimidine CH2). 13C NMR (DMSO-d6, 100.6 MHz): δ 186.7,
1585 (w), 1481 (w), 1432 (m), 1381 (w), 1277 (w), 1244 (w), 1183
(w), 1098 (m), 1063 (m), 1027 (w), 1008 (w), 978 (w), 949 (w),
894 (w), 787 (w), 739 (s), 693 (vs), 628 (w), 644 (w), 502 (m).
131.3, 129.8, 129.1, 127.2, 125.4 (t, 1J = 24), 57.6, 43.2, 17.1. 31
P
{1H} NMR (CDCl3, 161.9 MHz): δ 24.8 (s). FT-IR (ATR, cm−1):
Synthesis of [PdCl{C(NCH2PPh2)2(CH2)3-κ3P,C,P}]Cl, 2a
ν = 3402 (br, s), 3051 (w), 2935 (w), 1637 (br, w), 1539 (s), 1483
¯
(m), 1436 (s), 1354 (m), 1318 (m), 1263 (w), 1204 (w), 1187 (m),
1104 (s), 1026 (w), 998 (w), 942 (w), 867 (w), 748 (s), 712 (w),
696 (s), 592 (w), 548 (m), 524 (w), 501 (m), 483 (w), 458 (w).
Anal. calcd for C30H30Cl2N2P2Ni·3(H2O): C, 54.25; H, 5.46; N,
4.22. Found = C, 54.52; H, 5.32; N, 4.31.
To a solution of [PdCl2(COD)] (0.20 g, 0.70 mmol) in aceto-
nitrile (20 mL) was added 1 (0.340 g, 0.704 mmol) and stirred
at room temperature for 24 hours. The solvent was removed
under vacuum and the residue was washed with petroleum
ether (3 × 10 mL). The residue was dissolved in dichloro-
methane and layered with petroleum ether at room tempera-
ture. Orange crystals of complex 2a were formed over a period
of one week, which were separated and dried under vacuum
(0.420 g, 0.565 mmol, 81%). 1H NMR (CDCl3, 400 MHz): δ 7.90
(m, 8H, Ph), 7.55 (m, 12H, Ph), 4.89 (t, 2J(H,P) = 4, 4H, PCH2N),
3.77 (t, 3J(H,H) = 6, 4H, pyrimidine NCH2), 2.20 (t, 3J(H,H) = 6,
2H, pyrimidine CH2). 13C NMR (CD3CN, 100.6 MHz): δ 189.9 (t,
Synthesis of [NiCl{C(NCH2PPh2)2(CH2)3-κ3P,C,P}]BF4, 3b
To a solution of [NiCl2(DME)] (0.100 g, 0.455 mmol) and
NaBF4 (0.050 g, 0.455 mmol) in acetonitrile (20 mL) was added
1 (0.220 g, 0.455 mmol). The solution was stirred at room
temperature for 24 h and then the solvent was removed under
vacuum. The resulting residue was washed with petroleum
ether (3 × 10 mL), dissolved in dichloromethane and then
layered with petroleum ether at room temperature to give crys-
tals of 3b (0.20 g, 0.302 mmol, 66%). 1H NMR (CDCl3,
400 MHz): δ 7.84 (m, 8H, Ph), 7.63–7.49 (m, 12H, Ph), 4.55 (t,
2J(H,P) = 4, 4H, NCH2PPh2), 3.49 (t, 3J(H,H) = 6, 4H, pyrimi-
dine NCH2), 2.07 (m, 2H, pyrimidine CH2). 13C NMR (CD3CN,
2
3
2J(C,P) = 17.6), 133.5 (t, J(C,P) = 6), 132.2, 129.3 (t, J(C,P) =
5), 126.9 (d, 1J(C,P) = 25), 60.6, 45.4, 19.1. 31P{1H} NMR
(CDCl3, 161.9 MHz): δ 26.3 (s). FT-IR (ATR, cm−1): ν = 3365
¯
(w, br), 3053 (w), 2959 (w), 2924 (w), 2865 (w), 1714 (w), 1662
(m), 1549 (w), 1437 (m), 1319 (w), 1248 (m), 1160 (m), 1118 (s),
1105 (s), 1017 (m), 998 (m), 839 (w), 732 (m), 693 (s), 638 (w),
651 (m), 605 (s).
2
2
102.6 MHz): δ 190.0 (t, J(C,P) = 19), 133.6 (t, J(C,P) = 5 Hz),
132.3, 129.5 (t, 3J(C,P) = 5), 127.0 (t, 1J(C,P) = 25), 60.7 (t,
1J(C,P) = 17), 45.5 (t, 3J(C,P) = 5), 19.2. 31P {1H} NMR
Synthesis of [PdCl{C(NCH2PPh2)2(CH2)3-κ3P,C,P}]BF4, 2b
(161.9 MHz, CDCl3): δ 25.4 (s). FT-IR (ATR, cm−1): ν = 3384 (w),
¯
To a solution of [PdCl2(COD)] (0.050 g, 0.175 mmol) and
NaBF4 (0.019 g, 0.173 mmol) in acetonitrile (20 mL) was added
1 (0.085 g, 0.176 mmol). The solution was stirred at room
temperature for 24 h and then the solvent was removed under
vacuum. The resulting residue was washed with petroleum
ether (3 × 10 mL), dissolved in chloroform and layered with
petroleum ether at room temperature to give complex 2b as
3060 (w), 2943 (w), 1650 (m), 1562 (w), 1539 (m), 1484 (w),
1436 (m), 1358 (w), 1319 (w), 1186 (w), 1157 (w), 1053 (s), 1021
(s), 859 (w), 816 (w), 739 (s), 693 (s), 547 (w), 518 (w), 499 (w),
482 (w). Anal. calcd for C30H30BClF4N2P2Ni: C: 54.47 H: 4.57 N:
4.24. Found = C, 54.58; H, 4.42; N, 4.37.
1
General procedure for the Buchwald–Hartwig cross coupling of
aryl halides with amines
2J(H,P) = 6, 4H, NCH2PPh2), 3.47 (t, J(H,H) = 12, 4H, pyrimi- An oven dried Schlenk flask was charged with morpholine
dine NCH2), 2.01 (br s, 2H, pyrimidine CH2). 13C NMR (DMSO- (0.197 mL, 2.284 mmol) and complex 2a (0.014 mg,
orange crystals (0.110 g, 0.1585 mmol, 90%). H NMR (DMSO-
d6, 400 MHz): δ 7.90 (m, 8H, Ph), 7.60 (m, 12H, Ph), 4.98 (t,
3
2
2
d6, 100.6 MHz): δ 186.7 (t, J(C,P) = 5), 133.0 (t, J(C,P) = 14), 0.0188 mmol, 1 mol% with respect to bromobenzene) in 5 mL
132.2, 129.4 (t, 3J(C,P) = 11), 127.4 (t, 1J(C,P) = 50), 60.3 (t, of dry toluene. The mixture was stirred at room temperature
1J(C,P) = 34), 44.8 (t, 3J(C,P) = 10), 18.9. 31P {1H} NMR for 5 minutes under an argon atmosphere and then bromo-
(161.9 MHz, CDCl3): δ = 25.9 (s). FT-IR (ATR, cm−1): ν = 3053 benzene (0.2 mL, 1.911 mmol) and NaHMDS (2.9 mL,
¯
(w), 2930 (w), 2862 (w), 1675 (w), 1556 (m), 1481 (w), 1436 (m), 2.9 mmol, 1 M in THF) were successively added. The reaction
1355 (w), 1319 (w), 1215 (w), 1189 (w), 1050 (vs), 852 (w), 742 mixture was refluxed at 100 °C for 5 h. After cooling to room
(s), 687 (s), 644 (w), 602 (s), 473 (m). Anal. calcd for temperature, the reaction was quenched by adding 20 mL of
C30H30BClF4N2P2Pd: C, 50.81; H, 4.26; N, 3.95. Found = C, distilled water and the reaction mixture was extracted with
50.84; H, 4.33; N, 3.77.
DCM (3 × 10 mL). The combined organic solution was dried
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over anhydrous Na2SO4 and then filtered. 1 μL of this DCM structures were subjected to IRC calculations to further
solution was injected into GC to identify and calculate the confirm that they are in the desired reaction path. The opti-
yield of the coupled product against the external standard, mized model structures were generated using Gaussview 5.0
diphenylamine. DCM was removed under vacuum and the program. Natural bond orbital (NBO) analysis was performed
resulting residue was loaded onto a silica gel column chrom- using the Gaussian NBO Version 3.1,31 integrated into the
atography. Elution using petroleum ether/ethyl acetate (v/v = Gaussian 09 program.
10/1 mL) yielded 4-phenylmorpholine in a pure form after
removing the solvents.
4-Phenylmorpholine. 0.257 g, 1.574 mmol, 82%. 1H NMR Conflicts of interest
(CDCl3, 200 MHz): δ 7.33–7.24 (m, 2H, Ph), 6.95–6.85 (m, 3H,
3
3
There are no conflicts to declare.
Ph), 3.87 (t, J(H,H) = 4.8, 4H, CH2), 3.16 (t, J(H,H) = 4.8, 4H,
CH2). These data match with the reported value.23
1-Phenylpiperidine. 0.263 g, 1.631 mmol, 85%. 1H NMR
(CDCl3, 200 MHz): δ 7.29–7.20 (m, 2H, Ph), 6.95 (d, J(H,H) =
3
Acknowledgements
3
3
8, 2H, Ph), 6.82 (t, J(H,H) = 7.2, 1H, Ph), 3.16 (t, J(H,H) = 5.3,
4H, CH2), 1.77–1.66 (m, 4H, CH2), 1.62–1.52 (m, 2H, CH2).23
1-Phenylpyrrolidine. 0.252 g, 1.711 mmol, 89%. 1H NMR
(CDCl3, 200 MHz): δ 7.28–7.17 (m, 2H, Ph), 6.71–6.57 (m, 3H,
Ph), 3.30 (t, 3J(H,H) = 6.6, 4H, CH2), 2.05–1.98 (m, 4H, CH2).24
Diphenylamine. 0.260 g, 1.536 mmol, 80%. 1H NMR (CDCl3,
We acknowledge the DST, New Delhi, India for financial
support, and the DST FIST Level-II grant (No. SR/FST/CSII-026/
2013) for the 500 MHz NMR machine. We thank Dr S. Mishra,
Department of Chemistry, IIT-Kharagpur for the discussion on
DFT calculations.
3
200 MHz): δ 7.32–7.24 (m, 4H, Ph), 7.09 (d, J(H,H) = 8.6, 4H,
Ph), 6.94 (t, 3J(H,H) = 6.4, 2H, Ph), 5.71 (br s, 1H, NH).
Notes and references
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