Acetylene-bridged P,C,P'-ligands and corresponding cyclopalladated compounds

Article abstract of DOI:10.1016/S0040-4039(99)02181-4

Bis-'pincer'-cyclopalladates 1 and 2 containing an ethynediyl- or a butadiynediyl-bridge have been synthesized, characterized and used as precatalysts in the Heck reaction. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(99)02181-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1075–1079
Acetylene-bridged P,C,P0-ligands and corresponding
cyclopalladated compounds
Irina P. Beletskaya,a,∗ Alexey V. Chuchurjukin,a Harm P. Dijkstra,b Gerard P. M. van Klink b
and Gerard van Koten ∗
aLaboratory of Organoelement Compounds, Department of Chemistry, M.V. Lomonosov Moscow State University,
119899, GSP-3 Moscow, Russia
bDepartment of Metal-Mediated Synthesis, Debye Institute, Utrecht University, Padualaan 8, 3584 CH Utrecht,
The Netherlands
Received 4 October 1999; accepted 24 November 1999
Abstract
Bis-‘pincer’-cyclopalladates 1 and 2 containing an ethynediyl- or a butadiynediyl-bridge have been synthesized,
characterized and used as precatalysts in the Heck reaction. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: alkynes; coupling reactions; phosphines; palladium compounds.
Metallocycles containing a tridentate P,C,P0-coordination mode have been investigated extensively
during the last decade.1 It was found that these systems can perform some interesting new types of
transformations.2
We have synthesized binuclear palladium complexes of a new type, in which two cyclopalladated
‘pincer’ groups are connected via an ethynediyl- (1) or a butadiynediyl-bridge (2) (Fig. 1). Such
polynuclear conjugated organometallic compounds have attracted considerable attention due to their
possible applications as building blocks for new conjugated organometallic oligomers and polymers,
new types of catalysts and materials with magnetic, nonlinear optical or liquid crystalline properties.3
Fig. 1.
∗
Corresponding authors.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02181-4
tetl 16109

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Multidentate tetraphosphine ligand 7 was prepared in two different ways, as shown in Scheme 1. First,
3,5-bis(bromomethyl)iodobenzene4 was converted into phosphine oxide 3 with ethyl diphenylphosphi-
nite using Arbuzov reaction conditions.5 When compound 3 was used in a cross-coupling reaction with
acetylene gas using the Sonogashira reaction conditions, tetraphosphine oxide 4 was obtained in 92%
yield (Method A, Scheme 1).6 Second, compound 4 could also be obtained by cross-coupling of 3 with
(trimethylsilyl)acetylene (Sonogashira conditions),7 followed by the basic removal of the trimethylsilyl
substituent8 and subsequent cross-coupling of the obtained monoarylacetylene 6 with another equivalent
of 3;9 overall yield 88% (Method B, Scheme 1). In the last step, 4 was reduced with trichlorosilane to the
desired 3,30,5,50-tetra[(diphenylphosphino)methyl]tolane (7) in quantitative yield.10
Scheme 1.
3,30,5,50-Tetra[(diphenylphosphinoxide)methyl]diphenylbutadiyne (8) was prepared in 90% yield
from 6 via an oxidative coupling reaction using iodine (Scheme 2).11 Reduction of 8 using trichlorosilane,
afforded the desired tetraphosphine 9 in quantitative yield.10
Scheme 2.
Cyclopalladated products 1a and 2a were obtained via a direct electrophilic palladation with
[Pd(NCMe)4](BF4)2 in refluxing acetonitrile in 51 and 21% yield, respectively (Scheme 3).12,13
When these ionic complexes were treated with NaCl in a CH2Cl2/H2O mixture, the corresponding
neutral palladium chloride complexes 1b and 2b were obtained in quantitative yields (Scheme 3).14,15

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Scheme 3.
The somewhat low yields of 1a and 2a were probably caused by competitive palladium-catalyzed
polymerization of the triple bonds during the aromatic palladation reaction. The side products were highly
soluble in acetonitrile and formed dark-colored solutions whereas 1a and 2a were white crystals, poorly
soluble in cold acetonitrile. The nature of these side products is currently under investigation.
All new compounds were characterized by 1H, 13C and 31P NMR spectroscopy, and elemental analysis.
Compound 1a was also characterized by crystal structure determination.
Furthermore, initial experiments have shown that 1a and 2a are highly active catalyst precursors for
the Heck reaction of iodobenzene with styrene or methyl acrylate.
Acknowledgements
We thank the EU COST-D12 Program on ‘Selective Transformations and Catalysis’ for financial
support.
References
1. (a) Lee, D. W.; Kaska, W. C.; Jensen, C. M. Organometallics 1998, 17, 1–3; (b) Cross, R. J.; Kennedy, A. R.; Muir, K. W.
J. Organomet. Chem. 1995, 487, 227–233; (c) Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C. M. Chem.
Commun. 1996, 2083–2084; (d) Jia, G.; Lee, H. M.; Williams, I. D. J. Organomet. Chem. 1997, 534, 173–180.
2. Van der Boom, M. E.; Liou, S.-Y.; Ben-David, Y.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 1998, 120, 6531–6541,
and references cited therein.
3. (a) Steenwinkel, P.; Kolmschot, S.; Gossage, R. A.; Dani, P.; Veldman, N.; Spek, A. L.; van Koten, G. Eur. J. Inorg.
Chem. 1998, 477–483; (b) Steenwinkel, P.; Kooijman, H.; Smeets, W. J. J.; Spek, A. L.; Grove, D. M.; van Koten, G.
Organometallics 1998, 17, 5411–5426; (c) Steenwinkel, P.; Grove, D. M.; Veldman, N.; Spek, A. L.; van Koten, G.
Organometallics 1998, 17, 5647–5655; (d) Onitsuka, K.; Yamamoto, S.; Takahashi, S. Angew. Chem. Int. Ed. 1999, 38,
174–176; (e) Ward, M. D. Chem. Soc. Rev. 1995, 121–134; (f) Keene, F. R. Chem. Soc. Rev. 1998, 27, 185–193; (g) Hong,
B.; Ortega, J. V. Angew. Chem. Int. Ed. 1998, 37, 2131–2134; (h) Ohshiro, N.; Takei, F.; Onitsuka, K.; Takahashi, S. J.
Organomet. Chem. 1998, 569, 195–202; (i) Tzalis, D.; Tor, Y. Chem. Commun. 1996, 1043–1044; (j) Hissler, M.; El-
Ghayoury, A.; Harriman, A.; Ziessel, R. Angew. Chem. Int. Ed. 1998, 37, 1717–1720, and references cited therein; (k) Khan,
M. S.; Kakkar, A. K.; Ingham, S. L.; Raithby, P. R.; Lewis, J.; Spencer, B.; Wittmann, F.; Friend, R. H. J. Organomet. Chem.
1994, 472, 247–255; (l) Collin, J. P.; Gavina, P.; Heitz, V.; Sauvage, J. P. Eur. J. Inorg. Chem. 1998, 1–14.
4. Duchêne, K. H.; Vögtle, F. Synthesis 1986, 659–661.
5. A modification of a literature procedure3a was used: 3,5-Bis(bromomethyl)iodobenzene (14.0 g, 35.9 mmol) and ethyl
diphenylphosphinite (16.6 g, 72 mmol) were mixed in m-xylene (20 mL) and stirred at 130°C for 1 h. The reaction mixture

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was cooled to room temperature and concentrated in vacuo. Benzene (70 mL) was added to the residue and the solution was
heated, filtered and cooled to room temperature. Upon cooling, 3·benzene was obtained as white crystals. The product was
washed with C6H6 and dried, giving 25 g (98%) of 3, m.p. 91°C. Anal. calcd for C38H33P2O2I: C, 64.21; H, 4.73; P, 8.71.
Found: C, 64.14; H, 4.75; P, 8.80. Compound 3 can be obtained as a white amorphous powder by removal of the solvent in
vacuo. 1H NMR (200 MHz, CDCl3): δ 3.43 (d, 4H, J=13.8 Hz), 6.95 (t, 1H, J=1.8 Hz), 7.15, (d, 2H, J=1.8 Hz), 7.3–7.7 (m,
20H).
6. Compound 3·benzene (11.2 g, 15.6 mmol), PdCl2(PPh3)2 (100 mg, 0.14 mmol) and CuI (30 mg, 0.18 mmol) were dissolved
in degassed piperidine (60 mL) and heated to 80°C. Acetylene gas was then bubbled slowly through this mixture at the same
temperature for 3 h. The solvent was removed in vacuo and the residue was washed with benzene, dissolved in CH2Cl2 (150
mL). The organic layer was washed with water (3×150 mL) and dried (MgSO4). Next, the solvent was evaporated and the
crude product was suspended in benzene, boiled for 30 min, cooled to room temperature, collected and dried in vacuo at
100°C, giving 7.5 g (92%) of 4. 1H NMR (200 MHz, CDCl3): δ 3.47 (d, 8H, J=13.6 Hz), 7.00 (s, 6H), 7.30–7.70 (m, 40H).
7. Compound 3·benzene (11.2 g, 15.6 mmol), (trimethylsilyl)acetylene (1.8 g, 19.0 mmol), PdCl2(PPh3)2 (100 mg, 0.14 mmol)
and CuI (30 mg, 0.18 mmol) were dissolved in degassed piperidine (60 mL) and heated to 80°C for 3 h. The solvent was
removed in vacuo and the residue was dissolved in CH2Cl2 (150 mL). The organic layer was washed with water (5×200
mL) and dried (MgSO4). The solvent was evaporated and the product was crystallized from benzene, giving 9.0 g (95%) of
1
5. H NMR (200 MHz, CDCl3): δ 0.12 (s, 9H), 3.48 (d, 4H, J=13.2 Hz), 6.94 (t, 1H, J=1.4 Hz), 7.04 (d, 2H, J=1.4 Hz),
7.30–7.70 (m, 20H).
8. To a solution of 5 (9.0 g, 15.0 mmol) in methanol (100 mL) was added 5 M NaOH-solution in water (3.5 mL) and this mixture
was stirred at room temperature for 2 h. The solution was diluted with water (100 mL) and the product was extracted into
CH2Cl2 (100 mL). The organic layer was collected, washed with water (2×200 mL) and dried (MgSO4). After removal of
the solvent, white crystals of 6·benzene were obtained by crystallization from benzene. Yield 8.9 g (97%). M.p. 110°C.
Anal. calcd for C40H34P2O2: C, 78.93; H, 5.63; P, 10.18. Found: C, 78.85; H, 5.75; P, 10.16. Compound 6 could be obtained
as a white amorphous solid by removal of the solvent in vacuo at 110°C. 1H NMR (200 MHz, CDCl3): δ 2.89 (s, 1H), 3.48
(d, 4H, J=13.6 Hz), 7.00 (s, 3H) 7.30–7.70 (m, 20H).
9. Reaction conditions as mentioned in Ref. 7. Same work-up as mentioned in Ref. 6.
10. The procedure described in the literature3a was used. Compound 7: m.p. 187°C. 1H NMR (200 MHz, CDCl3): δ 3.34 (s,
4H), 6.86 (t, 2H, J=1.2 Hz), 7.05 (d, 4H, J=1.2 Hz), 7.30–7.50 (m, 40H). Compound 9: m.p. 195°C. 1H NMR (200 MHz,
CDCl3): δ 3.32 (s, 8H), 6.93 (t, 2H, J=1.5 Hz), 7.03 (d, 4H, J=1.5 Hz), 7.30–7.50 (m, 40H).
11. Lio, Q.; Burton, D. J. Tetrahedron Lett. 1997, 38, 4371–4374. A modification of a literature procedure was used: 6·benzene
(1.15 g, 1.90 mmol), iodine (0.30 g, 1.20 mmol), PdCl2(PPh3)2 (13 mg, 18 µmol) and CuI (3 mg, 16 µmol) were dissolved in
piperidine (12 mL) and stirred at 80°C for 4 h. The solvent was then removed in vacuo, the residue washed with benzene and
dissolved in CH2Cl2 (20 mL). The organic layer was washed with water (3×20 mL) and dried (MgSO4). After evaporation of
the solvent, the crude product was suspended in benzene (15 mL), heated to reflux for 30 min, cooled to room temperature,
collected and dried in vacuo at 100°C, yielding 0.90 g (90%) of 8, m.p. 287°C. 1H NMR (200 MHz, CDCl3): δ 3.46 (d, 8H,
J=13.6 Hz), 6.97 (d, 4H, J=1.8 Hz), 7.00 (t, 2H, J=1.8 Hz), 7.30–7.70 (m, 40H).
12. A solution of [Pd(NCMe)4](BF4)2 (1.83 g, 4.12 mmol) in MeCN (150 mL) was added to 5 (2.0 g, 2.06 mmol) and heated
to reflux for 2 h. The reaction mixture was cooled to room temperature and the solvent evaporated in vacuo. The crude
product was recrystallized from MeCN giving 1.5 g (51%) of white crystals. Compound 1a: 1H NMR (200 MHz, CD3CN):
δ 4.21 (t, 8H, J=4.8 Hz, CH2), 7.34 (s, 4H, ArH), 7.6–8.0 (m, 40H, PPh2). 13C NMR (50.3 MHz, CD3CN): δ 40.5 (t, J=15.2
Hz), 132.2, 133.2 (t, J=6.9 Hz), 148.7 (t, J=10.4 Hz), 156.9 Hz. 31P NMR (80.96 MHz, CD3CN): δ 48.03. Anal. calcd for
C70H58P4N2B2F8Pd2: C, 58.48; H, 4.07; N, 1.95. Found: C, 58.31; H, 4.01; N, 1.81.
13. A solution of [Pd(NCMe)4](BF4)2 (1.15 g, 2.59 mmol) in MeCN (100 mL) was added to 9 (1.3 g, 1.31 mmol) and heated to
reflux for 1 h. The reaction mixture was cooled to room temperature and the solvent evaporated in vacuo. The crude product
was recrystallized from MeCN giving 0.4 g (21%) of white crystals. Compound 2a: 1H NMR (400 MHz, CD3CN): δ 4.21
(t, 8H, J=4.8 Hz, CH2), 7.38 (s, 4H, ArH), 7.6–7.9 (m, 40H, PPh2). 31P NMR (161.9 MHz, CD3CN): δ 46.8. Anal. calcd
for C72H58P4N2B2F8Pd2: C, 59.17; H, 4.00; N, 1.92. Found: C, 59.32; H, 4.08; N, 1.83.
14. To a suspension of 1a (0.35 g, 0.24 mmol) in CH2Cl2 (40 mL) was added brine (40 mL) and this mixture was stirred
overnight. The organic layer was separated, dried (MgSO4), filtered and evaporated to dryness. Compound 1b: yield 0.3 g
(98%). 1H NMR (200 MHz, CDCl3): δ 3.92 (t, 8H, J=4.6 Hz, CH2), 7.26 (s, 4H, ArH), 7.3–8.0 (m, 40 H, PPh2). 13C NMR
(50.3 MHz, CDCl3): δ 42.7 (t, J=14.5 Hz), 89.8, 120.8, 126.1 (t, J=11.2 Hz), 129.1 (t, J=5.3 Hz), 130.9, 131.9 (t, J=21.7 Hz),
133.2 (t, J=6.7 Hz), 148.3 (t, J=10.8 Hz), 161.6. 31P NMR (80.96 MHz, CDCl3): δ 34.3. Anal. calcd for C66H52P4Cl2Pd2:
C, 63.28; H, 4.18. Found: C, 63.10; H, 4.05.

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15. To a suspension of 2a (0.20 g, 0.14 mmol) in CH2Cl2 (20 mL) was added brine (20 mL) and this mixture was stirred
1
overnight. The organic layer was separated, dried (MgSO4), filtered and evaporated to dryness. 2b: yield 0.2 g (97%). H
NMR (400 MHz, CDCl3): δ 3.96 (t, 8H, J=4.2 Hz, CH2), 7.29 (s, 4H, ArH), 7.3–8.0 (m, 40 H, PPh2). 13C NMR (100.6
MHz, CDCl3): δ 42.40 (t, J=14.5 Hz), 73.95, 81.89, 118.94, 126.64 (t, J=11.4 Hz), 128.81 (t, J=5.4 Hz), 130.71, 131.50 (t,
J=21.4 Hz), 132.91 (t, J=6.8 Hz), 148.17 (t, J=11.2 Hz), 163.05. 31P NMR (161.9 MHz, CDCl3): δ 32.8. Anal. calcd for
C68H52P4Cl2Pd2: C, 63.97; H, 4.10. Found: C, 63.78; H, 4.01.