Development of highly electron-deficient and less sterically-hindered phosphine ligands possessing 1,3,5-triazinyl groups

Article abstract of DOI:10.1016/j.mcat.2017.11.008

Highly electron-deficient and less sterically-hindered phosphine ligands with two or three 1,3,5-triazinyl groups on the phosphorus atoms have been synthesized and examined in transition metal-catalyzed reactions for the first time. Due to the lack of any hydrogens or substituents at ortho-positions of the 1,3,5-triazine towards the phosphorous atom, it is considered that the steric hindrance of the tris(triazinyl)phosphine ligand to a metal center is least among triarylphosphine ligands. In the Stille coupling of aryl iodides, these electron-poor phosphine ligands provided good product yields compared to hitherto well-known phosphine ligands.

Full text of DOI:10.1016/j.mcat.2017.11.008

Molecular Catalysis 445 (2018) 87–93
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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Research Paper
Development of highly electron-deficient and less sterically-hindered
phosphine ligands possessing 1,3,5-triazinyl groups
Kazumi Abe, Masanori Kitamura, Hikaru Fujita, Munetaka Kunishima∗
Faculty of Pharmaceutical Sciences, Institute of Medical, Pharmaceutical, and Health Sciences, Kanazawa University, Kakuma-machi, Kanazawa 920-1192,
Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly electron-deficient and less sterically-hindered phosphine ligands with two or three 1,3,5-triazinyl
groups on the phosphorus atoms have been synthesized and examined in transition metal-catalyzed
reactions for the first time. Due to the lack of any hydrogens or substituents at ortho-positions of
the 1,3,5-triazine towards the phosphorous atom, it is considered that the steric hindrance of the
tris(triazinyl)phosphine ligand to a metal center is least among triarylphosphine ligands. In the Stille
coupling of aryl iodides, these electron-poor phosphine ligands provided good product yields compared
to hitherto well-known phosphine ligands.
Received 6 September 2017
Received in revised form 24 October 2017
Accepted 3 November 2017
Available online 16 November 2017
Keywords:
Phosphine ligand
1,3,5-Triazine
© 2017 Elsevier B.V. All rights reserved.
Homogeneous catalysis
Stille coupling
␲-Interactions
1. Introduction
In our laboratory, we investigated a variety of synthetic reagents
that are based on 1,3,5-triazine for the dehydrative condensing
Electron-rich phosphines have been widely used as ligands
in transition metal catalysts for controlling the catalytic activ-
ity because they promote the oxidative addition of metals. In
contrast, electron-poor ligands are believed to accelerate the reduc-
tive elimination, transmetalation, and insertion reactions [1]. In
fact, several types of electron-deficient P-containing ligands that
exhibit prominent ligand effects have been developed. For example,
phosphinite-, phosphonite-, and phosphite-containing electroneg-
ative O-substituents have been used [2]. However, the P O bonds of
these compounds occasionally undergo hydrolysis [2e,2f]. Conse-
quently, electron-poor phosphine ligands having stable P C bonds,
such as fluoroarylphosphines [1,3] and ␣-cationic phosphines [4],
have also been studied. One example of this is commercially
available tris(pentafluorophenyl)phosphine [P(C6F5)3, Fig. 1]; how-
ever, its ortho-fluorine atoms destabilize the metal–phosphorus
bond and can occasionally cause the decomposition of the metal
complex due to the steric hindrance [1,5]. To overcome this
issue, Korenaga and Sakai prepared electron-poor phosphines with
2,6-bis(trifluoromethyl)-4-pyridyl (BFPy) groups whose steric hin-
drance at the ortho-positions (hydrogens) was less than that of the
pentafluorophenyl group (Fig. 1) [1].
reactions [6] or O-alkylations [7]. Some important characteristics
of 1,3,5-triazine are its highly ␲-electron-deficient property [8]
and the lack of substituents on the three nitrogens of the tri-
azine ring. The steric hindrance around the nitrogen atoms should
be minimal [8,9][8b,9]; thus, we expected that 1,3,5-triazin-2-yl
phosphines could be useful electron-deficient and less sterically-
hindered ligands for metal catalysts. In addition, 1,3,5-triazine have
great potential as a supramolecular component owing to their
non-covalent bonds, which involve either its nitrogen lone pairs
(coordination to metals and hydrogen bonds) or its heteroaromatic
␲-electrons (“cation–␲ interactions,” “anion–␲ interactions,” and
“␲–␲ stacking interactions”) [10]. Therefore, it was anticipated that
these non-covalent bonds could afford novel reactivity for metal
catalysts. However, to the best of our knowledge, there have been
a few reports on the reactions using triazinylphosphines as metal
ligands, all of which were limited to mono(triazinyl)phosphines
[11], possibly because of the lack of facile triazinylphosphine
synthesis methods. In the reported synthesis procedures, tri-
azinylphosphines were prepared by the reaction of nucleophilic
phosphorus reagents, such as R2PSiMe3 and R2PM (M = Li, Na,
and K), with electrophilic triazine compounds, such as chlorotri-
azine derivatives [11,12]. Because chlorotriazines are known to
readily undergo nucleophilic aromatic substitution, the synthetic
methods for the preparation of the triazinylphosphines reported
to date typically employ nucleophilic phosphorous compounds.
∗
Corresponding author.
E-mail address: kunisima@p.kanazawa-u.ac.jp (M. Kunishima).
https://doi.org/10.1016/j.mcat.2017.11.008
2468-8231/© 2017 Elsevier B.V. All rights reserved.

88
K. Abe et al. / Molecular Catalysis 445 (2018) 87–93
Scheme 1. Synthesis of mono-, bis-, and tris(triazinyl)phosphines.
2.2. Experimental procedures and characterization data for the
phosphine ligands
2.2.1. Diphenyl(4,6-diphenyl-1,3,5-triazin-2-yl)phosphine (2a)
To
a
solution of 2-iodo-4,6-diphenyl-1,3,5-triazine [15]
(860.0 mg, 2.39 mmol) in THF (8.0 mL), n-butylmagnesium chlo-
ride (1.45 M solution in THF, 2.0 mL, 2.90 mmol) was added
dropwise at −78 ◦C. After the reaction mixture was stirred for 1 h
at −78 ◦C, distilled chlorodiphenylphosphine (0.53 mL, 2.87 mmol)
was added dropwise. The reaction mixture was allowed to warm
to room temperature and stirred for 2 h, followed by quenching
with sat. aq. NH4Cl (0.4 mL). Subsequently, Et2O (30 mL) was
added. The resulting mixture was filtered through a Celite pad and
the filtrate was evaporated. The residue was purified by column
chromatography (hexane:toluene = 75:25) to afford 2a (839.3 mg,
84%) as a white solid.
Fig. 1. Chemical structures of previously reported phosphine ligands.
However, the execution of such procedures for synthesizing bis-
and tris(triazinyl)phosphines is difficult because the nucleophilic-
ity of phosphines will decrease with an increase in the number
of electron-withdrawing triazinyl groups on phosphorous atoms
[13]. Herein, we report the development of a practical method for
the synthesis of mono-, bis-, and tris(triazinyl)phosphines (2a–2c
in Scheme 1) and the use of these phosphines as ligands in Stille
coupling.
Mp: 132–133 ◦C; 1H NMR (400 MHz, CDCl3): ı 8.52–8.47 (m,
4H), 7.70–7.62 (m, 4H), 7.54 (t, J = 7.3 Hz, 2H), 7.50–7.39 (m, 10H);
13C NMR (100 MHz, CDCl3): ı 187.0 (d, JC−P = 6.7 Hz), 169.7 (d,
JC−P = 4.8 Hz), 136.0, 135.0 (d, JC−P = 19.2 Hz), 134.4 (d, JC−P = 6.7 Hz),
132.7, 129.5, 129.1, 128.7, 128.5 (d, JC−P = 7.7 Hz); 31P NMR
(162 MHz, CDCl3): ı 1.49; HRMS (EI) Calcd for C27H20N3P ([M]+):
417.1395, Found: 417.1400; Anal. Calcd for C27H20N3P: C, 77.68; H,
4.83; N, 10.07. Found: C, 77.38; H, 4.71; N, 10.00; IR (KBr): 3053,
1599, 1585, 1516, 1495, 1437, 1354, 1254, 1173, 1024, 833, 756,
692, 644 cm−1
.
2. Experimental section
2.2.2. Bis(4,6-diphenyl-1,3,5-triazin-2-yl)phenylphosphine (2b)
Compound 2b was prepared in manner same as that
2.1. General information
a
of 2a using 2-iodo-4,6-diphenyl-1,3,5-triazine [15] (1.98 g,
5.51 mmol), n-butylmagnesium chloride (1.45 M solution in THF,
3.6 mL, 5.22 mmol), distilled dichlorophenylphosphine (0.33 mL,
2.43 mmol), and THF (18.4 mL). The crude product was purified
by column chromatography (hexane:toluene = 60:40) to yield 2b
(1.02 g, 73%) as a white solid.
Nuclear magnetic resonance spectra were determined on a
JEOL JNM-ECS400 spectrometer [1H NMR (400 MHz), 13C NMR
(100 MHz), 31P NMR (162 MHz)]. Chemical shifts for 1H NMR are
reported as ı values relative to tetramethylsilane as an internal
standard and coupling constants are in hertz (Hz). The following
abbreviations are used for spin multiplicity: s = singlet, d = doublet,
t = triplet, q = quartet, m = multiplet, br = broad. Chemical shifts for
13C NMR were reported in ppm relative to the center line of a
triplet at 77.16 ppm for deuteriochloroform. Chemical shifts for
31P NMR are reported as ı values relative to 85% H3PO4 as an
external standard. Mass spectra were measured on a JMS-T100TD
AccuTOF TLC (DART-MS, ESI–MS), JMS-SX102A (FAB-MS, EI-MS).
Infrared (IR) spectra were recorded on a HORIBA FT-720 spec-
trometer and are reported in wavenumbers (cm−1). GC analysis
was carried out using SHIMADZU GC-17A equipped with AGILENT
TECHNOLOGIES DB-5 capillary column (length 30 m, 0.32 mm I.D.).
Analytical TLC was performed on Merck precoated analytical plates,
0.25 mm thick, silica gel 60 F254. Flash chromatography separations
were performed on SiO2 [KANTO CHEMICAL Silica Gel 60 N (spheri-
cal, neutral, 40–100 mesh)] or SiO2/K2CO3 (9:1, w/w) [14]. Reagents
were commercial grades and were used without any purification
unless otherwise noted. Tetrahydrofuran (THF) was distilled over
sodium/benzophenone ketyl before use. All reactions sensitive to
oxygen or moisture were conducted under a N2 atmosphere.
Mp: 212–214 ◦C; 1H NMR (400 MHz, CDCl3): ı 8.54–8.50 (m,
8H), 7.95–7.89 (m, 2H), 7.59–7.48 (m, 7H), 7.43 (t, J = 7.6 Hz,
8H); 13C NMR (100 MHz, CDCl3): ı 184.6 (d, JC−P = 12.5 Hz), 170.1
(d, JC−P = 5.8 Hz), 137.1 (d, JC−P = 22.0 Hz), 135.8, 132.7, 131.4 (d,
JC−P = 3.8 Hz), 130.7, 129.2, 128.7, 128.5 (d, JC−P = 9.6 Hz); 31P NMR
(162 MHz, CDCl3): ı 4.62; HRMS (EI) Calcd for C36H25N6P ([M]+):
572.1878, Found: 572.1873; Anal. Calcd for C36H25N6P: C, 75.51;
H, 4.40; N, 14.68. Found: C, 75.54; H, 4.51; N, 14.35; IR (KBr): 3059,
1599, 1585, 1502, 1439, 1390, 1356, 1319, 1255, 1173, 1026, 837,
756, 690, 644 cm−1
.
2.2.3. Tris(4,6-diphenyl-1,3,5-triazin-2-yl)phosphine (2c)
To a solution of 2-iodo-4,6-diphenyl-1,3,5-triazine [15] (2.95 g,
8.21 mmol) in THF (27.0 mL), n-butylmagnesium chloride (1.45 M
solution in THF, 5.5 mL, 7.98 mmol) was added dropwise at −78 ◦C.
After the mixture was stirred for 1 h at −78 ◦C, distilled phosphorus
trichloride (0.21 mL, 2.40 mmol) was added dropwise. The reaction
mixture was allowed to warm to room temperature and stirred for

K. Abe et al. / Molecular Catalysis 445 (2018) 87–93
89
Table 1
Evaluation of the electronic properties of triazinylphosphines (2a–c) using 31P NMR spectra, IR spectra, and X-ray single-crystal structure analysis.
3 (PR3)
ꢀCO (cm−1
)
JRh−P (Hz) [ı (ppm)]b
Rh−P bond length (Å)c
a
3c (2c)
3b (2b)
3a (2a)
3d [P(2-furyl)3]
3e (PPh3)
2006
1996
1986
1998
1979
145.2 [44.0]
140.9 [42.7]
130.0 [34.3]
130.0 [−23.9]
127.9 [29.6]
2.295d
2.292d
2.323d
2.303d,e
2.325e,f
aIR spectra in CH2Cl2.
b31P NMR spectra in CDCl3.
cFrom X-ray single-crystal structure analysis.
dObtained from this study (refer to the Supplementary data).
eThe mean value of two Rh P bonds.
fFrom reference [20].
Table 2
Stille coupling of 5a with 6a by using various phosphine ligands.
Entry
[PdCl(C3H5)]2 (mol%)
PR3
P:Pd
Yield of 7aa (%)a
1
2
3
4
5
6
7
8
9
2.5
2.5
2.5
2.5
2.5
2.5
1.0
1.0
1.0
2a
2b
2c
2:1
2:1
2:1
2:1
2:1
–
2:1
1:1
3:1
21
79
92
80
35
28
91 (86)
90
83
P(2-furyl)3
PPh3
–
2c
2c
2c
Yields were determined by 1H NMR; isolated yield is shown in parentheses.
a
3 h, followed by quenching with sat. aq. NH4Cl (0.8 mL). The pre-
cipitate was collected using a centrifuge, successively washed with
THF, water, MeOH, and THF, and dried in vacuo to yield 2c (1.10 g,
63%) as a white solid. 13C NMR was measured at 50 ◦C because of its
low solubility. For HRMS measurement, [PdCl(C3H5)]2 was added
into the sample solution to dissolve 2c.
2.4. General experimental procedure for the Stille coupling
reactions
To the mixture of
a phosphine ligand (8.8 ␮mol) and
[PdCl(C3H5)]2 (0.8 mg, 2.2 ␮mol) in the appropriate solvent
(2.5 mL), a hydrocarbon (0.22 mmol, an internal standard for GC
analysis), aryl halide (0.22 mmol), and organostannane (0.24 mmol)
were added. The reaction mixture was stirred at the appropriate
temperature until GC or TLC analysis indicated the consumption
of the starting materials. Subsequently, the mixture was diluted
with Et2O and washed with H2O and brine. The organic extracts
were dried over anhydrous sodium sulfate and filtered. The solvent
was removed by rotary evaporation, and the residue was purified
by column chromatography (SiO2/K2CO3 (9:1, w/w)) to afford the
coupling product 7.
Mp: >300 ◦C; 1H NMR (400 MHz, CDCl3): ı 8.55–8.50 (m, 12H),
7.53–7.48 (m, 6H), 7.38 (t, J = 7.6 Hz, 12H); 13C NMR (100 MHz,
CDCl3, 50 ◦C): ı 182.1 (d, JC−P = 15.3 Hz), 170.5 (d, JC−P = 5.8 Hz),
135.9, 132.8, 129.3, 128.7; 31P NMR (162 MHz, CDCl3): ı 9.01; HRMS
(FAB) Calcd for C45H31N9P ([M+H]+): 728.2440, Found: 728.2438;
Anal. Calcd for C45H30N9P·3H2O: C, 69.13; H, 4.64; N, 16.12. Found:
C, 69.25; H, 4.36; N, 16.11 (In the IR spectrum, a signal for water
was observed.); IR (KBr): 3402, 3068, 1601, 1585, 1504, 1439, 1394,
1358, 1317, 1254, 1173, 1024, 839, 758, 692, 644 cm−1
.
2.3. X-ray single-crystal structure analysis of the rhodium
complexes (3a–d) and the palladium complex (4c)
3. Results and discussion
Because of the aforementioned reasons, we chose to react elec-
trophilic phosphines, such as phosphorus halides, with nucleophilic
1,3,5-triazinylmagnesium reagents. Although various functional
groups can be generally introduced into the 4,6-positions of the
1,3,5-triazin-2-yl group, e.g., the amino and alkoxy groups, these
types of functional groups are not suitable because they may
be replaced by strongly nucleophilic Grignard reagents. Thus, we
employed a triazinylmagnesium reagent (1) with a carbon sub-
stituent, i.e., a phenyl group, at the 4,6-positions [15]. From the
reaction of 1 with chlorophosphines possessing different numbers
of chloro substituents [ClxPPh3–x (X = 1, 2, or 3)], we successfully
synthesized mono-, bis-, and tris(diphenyltriazinyl)phosphines
(2a–c) in 84%, 73%, and 63% yields, respectively (Scheme 1). All
compounds were stable solids. Compounds 2a and 2b were solu-
The rhodium complexes (3a–e) and the palladium complex
(4c) were prepared from [RhCl(CO)2]2 or [PdCl(C3H5)]2 and cor-
responding phosphine ligands (refer to the Supplementary data for
details). Measurements for X-ray single-crystal structure analysis
were made on a Rigaku R-AXIS RAPID diffractometer using graphite
monochromated Mo-Kɑ radiation at −170 ◦C. The structures were
solved by direct methods [16] and refined by SHELXL [17]. For 3b,
3c, and 4c, PLATON SQUEEZE [18] was used for disordered solvents.
CCDC 1552090 (3c), CCDC 1552091 (3b), CCDC 1552092 (3a), CCDC
1552093 (3d), and CCDC 1552094 (4c) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.

Table 3
Stille coupling reactions of various starting materials.
Entry
Ar − X
Bu3Sn − R
Solvent
Temp. (◦C)
Time (h)
Yield of 7 (%)a
5
6
2c
2b
2a
P(2-furyl)3
PPh3
1
2
5b
6a
THF
THF
50
50
3
91 (89)
91 (88)
60
74
13
66
67
42
11
39
5b
5b
6b
6c
10
3
toluene
toluene
90
90
1.5
9
99 (quant.)
91 (89)
42
74
15
20
50
50
8
4b
5b
6d
11
5c
6d
7
5b
5b
6e
6f
toluene
toluene
THF
90
7
82 (78)
46
87
56
95
90
84
39
75
48
90
73
Reflux
RT
21
24
81 (78)
64
5c
6a
6a
93 (90)
8
9
5d
THF
THF
50
50
3
6
83e (83)
67 (66)
59e
68
12e
59f
58e
63
8e
5e
6a
20f
10
11
12
13
5f
6a
6a
6a
6a
THF
50
90
90
90
5
52 (43)
30
51
12
50
14
5g
toluene
toluene
toluene
1.5
1
69
90 (83)
75e (83)
(56)
52
96
5h
6e
30e
(48)
20e
(36)
72e
(67)
5i
24
(40)
14
5j
6a
toluene
90
2.5
40
65
82 (76)
84
72
Yields were determined by 1H NMR unless otherwise noted; isolated yields are shown in parentheses.
Tributylphenyltin (6d, 1.3 eq.) was used.
Tributyl(2-pyridyl)tin (6e, 1.3 eq.) was used.
Allyltributyltin (6f, 1.3 eq.), [PdCl(C3H5)]2 (2.5 mol%), and PR3 (10 mol%) were used.
Determined by GC analysis.
Reaction time was 24 h.
a
b
c
d
e
f

K. Abe et al. / Molecular Catalysis 445 (2018) 87–93
91
Fig. 2. X-ray crystal structure of complex 4c [Pd(0)(2c)4]. Thermal ellipsoids drawn at 50% probability and hydrogen atoms are omitted for clarity. For details, refer to the
Supplementary data.
ble in common organic solvents, while 2c exhibited low solubility,
which may have been due to the high co-planarity [9] of the
diphenyltriazinyl moiety, which may increase the product’s ability
to crystallize.
In addition, we successfully obtained a crystal structure of the
palladium complex 4c [Pd(0)(2c)4] generated using one equivalent
of [PdCl(C3H5)]2 and four equivalents of 2c (phosphine:palladium
ratio = 2:1; Fig. 2 and Fig. S5 in the Supplementary data). Due to
the minimal steric hindrance of the nitrogen atoms, the diphenyl-
triazinyl moieties were nearly coplanar [9] and the ␲–␲ stacking
interactions were observed between these moieties that are from
two different 2c. Thus, it was expected that these monodentate
2c could function as a bidentate-like ligand through their non-
covalent bonds [22]. As can be observed in the Rh complexes, the
averaged Pd P bond length of 4c was shorter than that of Pd(PPh3)4
[2.305 Å for 4c, 2.450 Å [23] for Pd(PPh3)4].
The ligand effect of the triazinylphosphines (2a–c) in a model
catalytic reaction was examined using the Stille coupling reaction,
the rate-determining step of which is known to be the transmeta-
lation of organostannanes with palladium complexes [3f,24]. Stille
coupling with iodobenzene is more successful when performed
using less ␴-donating ligands, such as P(2-furyl)3 and tripheny-
larsine, because these ligands easily provide a vacant coordination
site for organostannanes.
First, we conducted the Stille coupling of iodobenzene (5a)
with tributyl(phenylethynyl)tin (6a) in the presence of 2.5 mol%
[PdCl(C3H5)]2 and 10 mol% phosphine ligands (Table 2, entries 1–5)
and the time course of product yields of the reactions using each
ligand is shown in Fig. S8 (the supplementary data). In the reactions
using ligands 2a–c, the yields of 7aa and rate of reactions increased
with the number of triazinyl groups (Table 2, entries 1–3 and Fig. S8
in the supplementary data). In particular, 2c, which had the three
triazinyl substituents, resulted in a higher yield compared to P(2-
The electronic properties of these triazinylphosphines (2a–c)
were evaluated by the 31P NMR spectroscopy and the IR
spectroscopy of the corresponding rhodium complexes, i.e., trans-
RhCl(CO)(PR3)2 [19]. Both the JRh–P value of the 31P NMR spectrum
and the ꢀCO value in IR spectrum of the complex generally increase
with an increase in the electron-withdrawing capability of the
phosphine (PR3). As anticipated for the complexes (3a–c) compris-
ing 2a–c, the order of the JRh–P values was 3c >3b >3a (Table 1).
Compared to the well-known phosphines tri(2-furyl)phosphine
[P(2-furyl)3] and PPh3, 3a had a value similar to the values of these
compounds, while 3c and 3b resulted in larger coupling constants.
Similarly, the ꢀCO values also indicated that 3c and 3b possessed a
high electron-withdrawing character.
Moreover, the Rh P bond lengths determined by single-crystal
X-ray structure analysis also implied similar electronic properties
(Table 1, Fig. S1–S4, S6–S7, and Table S1 in the Supplementary
data). The order of the bond lengths was 3c ≈ 3b < 3a [21], indicating
that the higher electron-withdrawing character of the phosphines
shortened their bond lengths. This tendency in the Rh P bond
lengths has also been observed in other rhodium complexes (Fig.
S6 in the Supplementary data). Furthermore, a linear relationship
between their bond lengths and the ␴ Hammett constants of the
p
substituents on phosphines was observed (Fig. S7 in the Supple-
mentary data).

92
K. Abe et al. / Molecular Catalysis 445 (2018) 87–93
furyl)3 (Table 2, entries 3 vs. 4). Although 2c was sparingly soluble
in THF, a mixture of 2c and [PdCl(C3H5)]2 was more soluble. This
result suggested that 2c coordinates with Pd to form the 2c–Pd
complex, which is soluble in THF. In the absence of the phosphine
ligand, the reaction resulted in a decreased yield of 7aa (Table 2,
entry 6). Both the amount of [PdCl(C3H5)]2 and 2c could be reduced
to 1 mol% and 4 mol%, respectively, without a loss in the product
yield (Table 2, entry 7). The yield of the product was slightly lower
when the phosphine:palladium (P:Pd) ratio was 3:1 rather than
2:1 and 1:1 (Table 2, entries 7 and 8 vs. 9). This result may indicate
that additional 2c interferes with the coordination of the substrate
to Pd. Moreover, the turnover numbers (TON) and turnover fre-
quencies (TOF) were determined for 2c (4400 and 370 h−1) and 2b
(4300 and 360 h−1), respectively (0.005 mol% of [PdCl(C3H5)]2 and
0.04 mol% of the ligands were used and the reaction time was 12 h).
From these findings, we investigated the Stille coupling reaction
of other substrates using 1 mol% [PdCl(C3H5)]2 and 4 mol% of the
triazinylphosphine ligands. The PPh3 and P(2-furyl)3 ligands were
also examined for comparison.
(which comprise three or two triazinyl groups, respectively, on
the phosphorus atoms) provided good results for the Stille cou-
pling reaction of aryl iodides. Ligands 2c and 2b may be effective
for other metal-catalyzed reactions in which transmetalation,
insertion, or reductive elimination is the rate-determining step.
Because various functional aryl groups can be readily introduced
into the 4,6-positions of the triazine ring, the electronic and
steric properties of triazinylphosphine can be finely tuned, pro-
viding phosphine ligands with new functionalities. We believe that
these triazinylphosphine ligands will exhibit different reactivity for
metal-catalyzed reactions due to the non-covalent bonds of 1,3,5-
triazine.
Acknowledgement
Financial support for this research from JSPS KAKENHI Grant
(No. 17H03970 and 16K08161).
Appendix A. Supplementary data
The Stille coupling of several types of organostannanes (6a–f)
with ethyl 4-iodobenzoate (5b) was also investigated (Table 3,
entries 1–6). The order of the transmetalation rate of the organos-
tannanes is known to be alkynyl > alkenyl > aryl > allyl ≈ benzyl »
alkyl [25]. With the most reactive organostannanes (6a and 6b) in
THF at 50 ◦C, the product yields increased with an increase in the
number of triazinyl groups on the phosphines (2a–c), while PPh3
or P(2-furyl)3 produced low-to-moderate product yields (Table 3,
entries 1 and 2). Similarly, ligand 2c afforded the best results for the
reaction with moderately reactive aryltin compounds 6c and 6d at
a higher temperature (90 ◦C) in toluene (Table 3, entries 3 and 4). No
significant difference in the ligand effect (except when using PPh3)
was observed in the reaction employing tributyl(2-pyridyl)tin (6e;
Table 3, entry 5). The reaction of less reactive allyltributyltin (6f)
required harsher conditions (under reflux of toluene for 21 h), and
the ligand effect was reversed compared to the reactions of the
other organostannanes (6a–e), i.e., less electron-deficient PPh3 and
2a afforded good yields (Table 3, entry 6).
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.11.
008.
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