English

• 1.   Introduction

  Alkenes can provide two π electrons to interact with an empty orbital of metal, forming a σ bond, while the empty π* orbital can cocurrently receive electrons from the central metal to form a π-back-donation bond. The unique nature makes them excellent ligands in coordination chemistry.^[1] Many metal complexes coordinated with alkenes such as Ni(COD)₂, Pd₂(dba)₃ and Zeise salt are reported. These complexes are widely applied in organic synthesis and catalysis. During the transition metal-cata- lyzed synthesis and conversion of alkenes, the interaction between alkenes and catalyst would also lead to in-situ generation of alkene-metal complexes, which might play a positive or negative role in these reactions.^[1]

  Alkenylphosphoryl compounds are a kind of important building blocks in organic synthesis.^[2] Some of them are also bioactive.^[3] However, traditional methods for their synthesis are limited.^[4] In 1996, we disclosed the palladium-catalyzed hydrophosphorylation of alkynes with (EtO)₂P(O)H, generating the corresponding alkenylpho-sphoryl compounds.^[5] Since then, the field has grown rapidly, and a variety of such compounds have been synthesized through transition metal-mediated hydrophosphorylation.^[6] Today, some of them can even be prepared on industrial scales.^[7] Structurally speaking, alkenylpho-sphoryl compounds commonly have an alkene unit and an electron-withdrawing phosphoryl group, and would strongly interact with transition metals through σ and π bonds formation. Recently, we reported that the vinyl-Pd-phosphoryl complexes upon heating could undergo reductive elimination to give the alkene-palladium complexes.^[8] Herein, we report a more facile synthesis of such complexes by simply mixing alkenylphosphoryl compounds with Pd(PEt₃)₄. Similar nickel and platinum complexes were also synthesized by the method. Their reactivity is also preliminarily investigated. The present work can not only enrich the coordination chemistry of alkenes, but also provide more mechanistic insights into the palladium-catalyzed hydrophosphorylation of alkynes with P(O)-H compounds.

  2.   Results and discussion

  When an equivalent alkenylphosphoryl compound 1a was added to a solution of Pd(PEt₃)₄ in C₆D₆ at room temperature (Table 1, Entry 1), the color of the solution turned from brown to colorless immediately. As followed by ¹H NMR and ³¹P NMR spectroscopies, the reaction took place very fast since the starting martials disappeared completely in half an hour. Besides that of PEt₃, three new phosphorus signals were clearly observed at δ 30.6 (dd, J₁＝J₂＝11.0 Hz, 1P), 15.9 (dd, J₁＝J₂＝11.0 Hz, 1P), 11.1 (dd, J₁＝ J₂＝11.0 Hz, 1P), indicative of the formation of a new metal complex. After removal of the volatiles in vacuum, 3a was obtained as a white solid. By recrystallization of the solid at －30 ℃ in toluene/hexane, the single crystal suitable for X-ray analysis was obtained. The molecular structure clearly showed that the palladium atom ligated to two PEt₃ coordinated with the alkene unit (Figure 1). Interestingly, it seems that the palladium atom did not interact with the phosphoryl group in the static state, despite the fact that the oxygen atom in the phosphoryl group also bears lone pairs. The result might be mainly ascribed to the formation of strong π-back-donation bond between the alkene unit and the central palladium atom. The low chemical valence of palladium (the value is zero) and the strong electron-donating ligands (PEt₃) might also play an important role.

  Table 1

  

  Table 1.  Synthesis of group 10 metal complexes ligated to alkenylphosphoryl compounds

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  Entry	1	2	3	Isolated yield/%
  1		Pd(PEt3)4	3a	91
  2			3b	96
  3			3c	95
  4			3d	90
  5			3e	89
  6	1e	Ni(PEt3)4	3f	83
  7	1e	Pt(PEt3)4	3g	80

  Figure 1

  

  Figure 1.  ORTEP drawing of palladium complex 3a

  Thermal ellipsoids are drawn at 50% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): C(1)—C(2)＝1.435(3), C(1)—Pd＝2.106(2), C(2)—Pd＝2.146(2), P(12)—Pd＝2.3279(6), P(2)—Pd＝2.3169(7), P(3)—C(1)＝1.775(2); C(1)—Pd—C(2)＝39.44(9), C(2)—C(1)—Pd＝71.81(13), C(1)—Pd—P(1)＝104.30(7), C(2)—Pd—P(2)＝104.03(3), P(1)—Pd—P(2)＝112.13(3)

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  This reaction seems to be a general method for synthesizing such kind of metal complexes. As shown in Table 1, alkenylphosphoryl compounds 1b~1e all could rapidly replace two PEt₃ ligand in Pd(PEt₃)₄ at room temperature to produce the corresponding palladium complexes in high yields (Table 1, Entries 2~4). By using similar strategy, the nickel and platinum complexes 3f and 3g were also generated in 83% and 80% isolated yields, respectively (Table 1, Entries 5, 6).

  Subsequently, the reactivity of these complexes was preliminarily studied as exemplified by complex 3e. It was found that 3e could react with acetic acid to release the alkenylphosphoryl compound 1e gradually (Eq. 1). The reaction proceeded very slowly at room temperature. As indicated from ¹H NMR and ³¹P NMR spectroscopies, the ratio of 3e to 1e was 1:0.2 at 0.5 h, 1:0.26 at 4 h, and 1:0.6 at 20 h. Interestingly, under similar reaction conditions, a new product phosphonium 4 was deposited as a crystal (the lattice contains one molecule of HOAc and C₆D₆) by further addition of 2 equiv. of PEt₃ (Eq. 2). Phosphonium 4 was considered to be generated by the reaction with HOAc, PEt₃, and compound 1e released in situ. The hypothesis was further confirmed by a separated experiment: by mixing compound 1e, 2 equiv. of HOAc and 2 equiv. of PEt₃ in C₆D₆ at room temperature, phosphonium 4 was generated in a quantitative yield (Eq. 3). The structure of phosphonium 4 was unambiguously determined by X-ray diffraction analysis (Figure 2). Worth noting is that the formation of phosphoniums from alkenes usually required transition metals or strong acids, ^[9] while HOAc, a weak acid, served well in the present reaction. The result would be ascribed to the special phosphoryl group in alkenes. This finding also provides a solid evidence for the phosphine-catalyzed transformations of alkenes.^[10]

  Figure 2

  

  Figure 2.  ORTEP drawing of phosphonium 4

  Thermal ellipsoids are drawn at 50% probability. H atoms, OAc^－, HOAc and solvent C₆D₆ are all omitted for clarity. Selected bond lengths (Å) and angles (°): C(2)—P(2)＝1.807(3), C(1)—C(2)＝1.550(3), C(1)—P(1)＝1.828(3), C(4)—P(2)＝1.799(3), C(1)—C(3)＝1.535(4), P(1)—O＝1.489(2); P(1)—C(1)—C(2)＝108.12(18), C(3)—C(1)—C(2)＝113.8(2), C(1)—C(2)—P(2)＝115.61(19), C(2)—P(2)—C(4)＝107.26(13), O—P(1)—C(1)＝113.69(12)

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  Being similar to Pd(PEt₃)₄, complex 3d could also undergo hydropalladation with the combination of phenylacetylene and (EtO)₂P(O)H, regioselectively producing the corresponding vinyl-Pd-phosphoryl complex 5 (Eq. 4). However, the reaction rate was rather slow. As reported, the hydropalladation with Pd(PEt₃)₄ would be complete in ca. 30 h, ^{[8, 11]} while the yield of the current reaction was only ca. 45% in 50 h. The results would be attributed to the strong coordination ability of alkenylphosphoryl compound, which hindered the interaction of phenylacetylene with palladium atom and thus decreased the rate of hydropalladation. Along this line, it would also be rational to deduce that alkenylphosphoryl compounds from the palladium-catalyzed hydrophosphorylation of alkynes with H-phosphonates might play a negative role in the hydropalladation step of the catalytic cycle.^{[5, 6, 8]}

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  3.   Conclusions

  A facile synthesis of group 10 metal complexes with alkenylphosphoryl compounds was reported. The alkene unit could be released by the aid of acetic acid. The preliminary studies on their reactivity also revealed that alkenylphosphoryl compounds might hinder the hydropalladation step in the palladium-catalyzed hydrophosphorylation of alkynes with H-phosphonates. The current work not only enriched the coordination chemistry of alkenes, but also promoted understanding to the related chemical reactions.

  4.   Experimental

  4.1   General information

  Reactions for the synthesis of metal intermediates were conducted in a glove box (O₂ < 1.0×10^－7, H₂O < 1.0× 10^－7). Dry solvents were obtained by purification according to standard methods. ¹H NMR and ³¹P NMR spectra were acquired on a JEOL LA-400 spectrometer (400 MHz for ¹H NMR, and 160 MHz for ³¹P NMR spectroscopy) or a JEOL LA-500 instrument (500 MHz for ¹H NMR, and 202 MHz for ³¹P NMR spectroscopy) in deuterated solvents. Chemical shifts for ¹H NMR are referred to internal Me₄Si (δ 0). Data for ³¹P NMR were relative to H₃PO₄ (85% solution in D₂O, δ 0). X-ray analysis was performed by the Analytical Center at the National Institute of Advanced Industrial Science and Technology, Japan. Complexes 3d and 3e are known, please refer to Ref. [8] for their characterized data.

  4.2   Synthesis of complex 3a

  In a glove box, 0.076 mmol of Pd(PEt₃)₄ and 0.076 mmol of PhCHCHP(O)Ph₂ were dissolved in 0.5 mL of dry and degassed C₆D₆ in a NMR tube. The reaction was complete after standing the mixture at room temperature for 0.5 h as indicated from ¹H NMR and ³¹P NMR spectra. Removal of the volatile (solvent, PEt₃) in vacuum afforded a white solid. Recrystallization of the white solid from toluene and hexane at －30 ℃ gave analytically pure product 3a with isolated yield of 91%. Crystals suitable for X-ray crystallography was obtained from recrystallization of the white solid from hexane and toluene at －30 ℃. CCDC 1921746, Unit cell parameters (Å): a＝10.0079(17), b＝15.531(3), c＝23.020(4), P-1. ¹H NMR (500 MHz, C₆D₆) δ: 8.31~8.35 (m, 2H), 7.92~7.96 (m, 2H), 7.18 (d, J＝8.5 Hz, 2H), 7.05~7.08 (m, 8H), 6.92 (t, J＝7.0 Hz, 1H), 3.97~4.06 (m, 1H), 3.79~3.84 (m, 1H), 2.00~2.09 (m, 3H), 1.83~1.92 (m, 3H), 1.14~1.23 (m, 3H), 1.02~1.12 (m, 12H), 0.63 (dt, J_H—H＝7.5 Hz, J_P—H＝15.0 Hz, 9H); ³¹P NMR (202 MHz, C₆D₆) δ: 30.57 (dd, J_P—P＝11.0 Hz, 1P), 15.88 (t, 1P, J_P—P＝11.0 Hz), 11.10 (dd, J_P—P＝11.0 Hz, J_P—P＝11.0 Hz, 1P).

  4.3   Synthesis of complex 3b

  In a glove box, 0.068 mmol of Pd(PEt₃)₄ and 0.068 mmol of CH₂CHP(O)Ph₂ were dissolved in 0.5 mL of dry and degassed C₆D₆ in a NMR tube. The reaction was complete after standing the mixture at room temperature for 0.5 h as indicated from ¹H NMR and ³¹P NMR spectra. Removal of the volatile (solvent, PEt₃) afforded colorless oil 3b with isolated yield of 96%. ¹H NMR (500 MHz, C₆D₆) δ: 8.39~8.43 (m, 2H), 7.84~7.88 (m, 2H), 7.22~7.25 (m, 2H), 7.18 (d, J＝8.0 Hz, 1H), 7.03~7.06 (m, 3H), 3.34~3.42 (m, 1H), 2.66~2.77 (m, 1H), 2.23~2.31 (m, 1H), 1.95~2.04 (m, 3H), 1.81~1.90 (m, 3H), 1.27~1.37 (m, 6H), 1.04 (dt, J＝7.5 Hz, J_P—H＝15.0 Hz, 9H), 0.80 (dt, J＝7.5 Hz, J_P—H＝15.0 Hz, 9H); ³¹P NMR (202 MHz, C₆D₆) δ: 30.88 (dd, 1P, J_P—P＝10.1 Hz, J_P—P＝11.1 Hz), 14.40 (dd, J_P—P＝11.0 Hz, J_P—P＝18.4 Hz, 1P), 14.0 (dd, J_P—P＝11.5 Hz, J_P—P＝18.6 Hz, 1P).

  4.4   Synthesis of complex 3c

  In a glove box, 0.068 mmol of Pd(PEt₃)₄ and 0.068 mmol of CH₂CHP(O)(OEt)₂ were dissolved in 0.5 mL of dry and degassed C₆D₆ in a NMR tube. The reaction was complete after standing the mixture at room temperature for 0.5 h as indicated from ¹H NMR and ³¹P NMR spectra. Removal of the volatile (solvent, PEt₃) afforded colorless oil 3c with isolated yield of 95%. ¹H NMR (500 MHz, C₆D₆) δ: 4.14~4.22 (m, 2H), 4.06~4.11 (m, 2H), 2.97~3.06 (m, 1H), 2.55~2.76 (m, 2H), 1.74~1.91 (m, 6H), 1.36~1.42 (m, 6H), 1.24 (t, J＝7.0 Hz, 3H), 1.18 (t, J＝7.0 Hz, 3H), 1.02 (td, J＝7.5 Hz, J_P—H＝15.0 Hz, 9H), 0.91 (td, J＝7.5 Hz, J_P—H＝15.0 Hz, 9H). ³¹P NMR (202 MHz, C₆D₆) δ: 30.70 (dd, J_P—P＝8.3 Hz, J_P—P＝9.7 Hz, 1P), 15.00 (dd, J_P—P＝7.3 Hz, J_P—P＝18.4 Hz, 1P), 13.25 (dd, J_P—P＝16.6 Hz, J_P—P＝16.2 Hz, 1P).

  4.5   Synthesis of complex 3f

  In a glove box, 0.05 mmol of Ni(COD)₂ and 0.05 mmol of PhC(CH₂)P(O)Ph₂ were dissolved in 0.5 mL of dry and degassed C₆D₆ in a NMR tube, The reaction was complete after standing the mixture at room temperature for 0.5 h as indicated from ¹H NMR and ³¹P NMR spectra. Removal of the volatile (solvent, PEt₃) afforded a red solid. Crystallization of the red solid from the toluene and hexane system at －30 ℃ gave analytically pure product 3f with isolated yield of 83%. ¹H NMR (400 MHz, C₆D₆) δ: 8.32~8.36 (m, 2H), 7.94~7.98 (m, 4H), 7.03~7.12 (m, 8H), 6.83 (t, J＝6.4 Hz, 1H), 2.21~2.31 (m, 1H), 1.87~2.03 (m, 4H), 1.46~1.57 (m, 3H), 1.15~1.30 (m, 6H), 0.81 (dt, J＝7.6 Hz, J_P—H＝13.6 Hz, 9H), 0.70 (dt, J＝7.6 Hz, J_P—H＝14.0 Hz, 9H); ³¹P NMR (162 MHz, C₆D₆) δ: 33.94 (b, 1P), 20.71 (db, J_P—P＝26.2 Hz, 1P), 10.45 (db, J_P—P＝26.2 Hz, 1P).

  4.6   Synthesis of complex 3g

  In a glove box, 0.05 mmol of Pt(PEt₃)₄ and 0.05 mmol of PhC(CH₂)P(O)Ph₂ were dissolved in 0.5 mL of dry and degassed C₆D₆ in a NMR tube, The reaction was complete after standing the mixture at room temperature for 0.5 h as indicated from ¹H NMR and ³¹P NMR spectra. Removal of the volatile (solvent, PEt₃) afforded a white solid. Crystallization of the white solid from the toluene and hexane system at －30 ℃ gave analytically pure product 3g with isolated yield of 80%. ¹H NMR (500 MHz, C₆D₆) δ: 8.37~8.41 (m, 2H), 7.94 (d, J＝7.5 Hz, 2H), 7.85~7.89 (m, 2H), 6.99~7.13 (m, 8H), 6.78 (t, J＝7.0 Hz, 1H), 2.42~2.50 (m, 1H), 2.09~2.19 (m, 3H), 1.91~2.01 (m, 1H), 1.74~1.84 (m, 3H), 1.32~1.47 (m, 6H), 0.77 (td, J_H—H＝7.5 Hz, J_P—H＝15.0 Hz, 9H), 0.68 (td, J_H—H＝7.5 Hz, J_P—H＝15 Hz, 9H); ³¹P NMR (202 MHz, C₆D₆) δ: 33.71 (ddt, J_P—P＝10.9 Hz, J_P—P＝14.7 Hz, J_Pt—P＝58.8 Hz, 1P), 14.74 (ddt, J_P—P＝14.7 Hz, J_P—P＝36.6 Hz, J_Pt—P＝1867.1 Hz, 1P), 12.03 (ddt, J_P—P＝10.9 Hz, J_P—P＝36.8 Hz, J_Pt—P＝1716.2 Hz, 1P).

  4.7   Synthesis of phosphonium 4

  In a glove box, 0.05 mmol of 3e, 2 equiv. of HOAc and 2 equiv. of PEt₃ were dissolved in 0.5 mL of dry and degassed C₆D₆ in a NMR tube. Phosphonium 4 was deposited gradually as single crystal suitable for X-ray crystallography when standing the mixture at room temperature, colorless solid, isolated yield 85%. CCDC 1921747, Unit cell parameters (Å): a＝9.0172(4), b＝25.1788(11), c＝14.4272(6), P2₁/n. Three A alerts existed in the X-ray analysis data. One alert concerns the short length of one C—C bond in solvent C₆D₆ contained in the lattice, which might be ascribed to the large thermal motion. Other two alerts concern the short distance between two AcO^－ ions, which should be ascribed to the formation of hydrogen bond. These alerts do not affect the confirmation of molecular structure. ¹H NMR (500 MHz, C₆D₆) δ: 16.19 (broad, 1H), 8.86~8.83 (m, 2H), 8.39~8.35 (m, 2H), 8.14 (d, J＝8.0 Hz, 2H), 7.40~7.38 (m, 2H), 7.18~7.13 (m, 2H), 7.06~7.01 (m, 3H), 6.85 (t, J＝8.0 Hz, 2H), 5.70~5.66 (m, 1H), 2.63~2.59 (m, 1H), 2.52~2.46 (m, 1H), 2.28 (s, 6H), 1.95~1.88 (m, 3H), 1.58~1.50 (m, 3H), 0.28~0.21 (m, 9H); ³¹P NMR (202 MHz, C₆D₆) δ: 42.07 (d, J_P—P＝47.9 Hz, 1P), 34.01 (d, J_P—P＝47.9 Hz, 1P).

  Phosphonium 4 could also be prepared by mixing 1e, 2 equiv. of HOAc and 2 equiv. of PEt₃ in C₆D₆ at room temperature. the NMR yield was quantitative.

  Supporting Information Copies of ¹H NMR and ³¹P NMR spectroscopies, and cif files of 3a and 4. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn/.

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• 

  Figure 1  ORTEP drawing of palladium complex 3a

  Thermal ellipsoids are drawn at 50% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): C(1)—C(2)＝1.435(3), C(1)—Pd＝2.106(2), C(2)—Pd＝2.146(2), P(12)—Pd＝2.3279(6), P(2)—Pd＝2.3169(7), P(3)—C(1)＝1.775(2); C(1)—Pd—C(2)＝39.44(9), C(2)—C(1)—Pd＝71.81(13), C(1)—Pd—P(1)＝104.30(7), C(2)—Pd—P(2)＝104.03(3), P(1)—Pd—P(2)＝112.13(3)

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  Figure 2  ORTEP drawing of phosphonium 4

  Thermal ellipsoids are drawn at 50% probability. H atoms, OAc^－, HOAc and solvent C₆D₆ are all omitted for clarity. Selected bond lengths (Å) and angles (°): C(2)—P(2)＝1.807(3), C(1)—C(2)＝1.550(3), C(1)—P(1)＝1.828(3), C(4)—P(2)＝1.799(3), C(1)—C(3)＝1.535(4), P(1)—O＝1.489(2); P(1)—C(1)—C(2)＝108.12(18), C(3)—C(1)—C(2)＝113.8(2), C(1)—C(2)—P(2)＝115.61(19), C(2)—P(2)—C(4)＝107.26(13), O—P(1)—C(1)＝113.69(12)

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  Table 1.  Synthesis of group 10 metal complexes ligated to alkenylphosphoryl compounds

  Entry	1	2	3	Isolated yield/%
  1		Pd(PEt3)4	3a	91
  2			3b	96
  3			3c	95
  4			3d	90
  5			3e	89
  6	1e	Ni(PEt3)4	3f	83
  7	1e	Pt(PEt3)4	3g	80

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