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Theoretical mechanism for selective catalysis of double hydrophosphination of terminal arylacetylenes by an iron complex† Mingshu Liu, Chuanzhi Sun,* Fang Hang, Nan Sun and Dezhan Chen* The detailed mechanism of the double hydrophosphination of terminal arylacetylenes catalyzed by an iron complex was studied by density functional theory. The calculated results suggest that the reaction
Received 18th October 2013, Accepted 4th December 2013 DOI: 10.1039/c3dt52941j www.rsc.org/dalton
1.
proceeds in three steps: active species generation, single hydrophosphination reaction (Cycle 1), double hydrophosphination reaction, viz., active species regeneration (Cycle 2). The results uncovered the selectivity of the iron complex for double hydrophosphination of terminal arylacetylenes. The symmetry of frontier molecular orbitals determines the effectiveness of the catalyst. We also discuss the formation mechanism of the single hydrophosphination product with Z configuration.
Introduction
Phosphines have received much attention because of their essential role in various fields of chemistry. For instance, they can be used as synthetic reagents, ligands in transition-metal complexes, biologically active molecules, polymers and building blocks of supramolecular assemblies.1–3 In 2001, Knowles obtained the Nobel Prize for the synthesis of L-Dopa catalyzed by an optically pure DIPAMP–rhodium complex, which indicates the importance of chiral phosphines.4–6 More and more scientists have made efforts to develop synthesis methods of phosphines. Among the large number of methods, the transition metal complex catalyzed addition of a P–H bond to an unsaturated carbon–carbon multiple bonds opens a unique opportunity to combine high-atom efficiency and exceptional selectivity.7 In recent years, both the P(ш)–H and P(v)–H substrates were successfully utilized in the addition reactions catalyzed by transition metal complexes (Scheme 1).8,9 However, hydrophosphination by metal complex catalysis also encounters some difficulties. For example, lanthanide complexes catalyzed hydrophosphination shows superb efficiency, but the catalytic system requires the organic chemists to prepare intricate and highly air- and moisture-sensitive metal complexes.10 Palladium and nickel complexes catalyzed reactions have also been reported. The scope of the alkynes available for use is limited.11–13 In addition, double
College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, P. R. China. E-mail: [email protected], [email protected]; Fax: +86 531 86180304; Tel: +86 531 86180304 † Electronic supplementary information (ESI) available. See 10.1039/c3dt52941j
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DOI:
Scheme 1
P–H bond addition to alkyne.
hydrophosphination of an alkyne catalyzed by transition metal complexes is especially difficult. Because the product serves as a bidentate ligand, and it binds to the transition metal complex catalyst more strongly than a monodentate one due to the chelate effect.14 Consequently, the transition metal catalyst can form a coordinatively saturated complex, which results in a decrease in or complete loss of catalytic activity. There are only a few examples of double hydrophosphination of alkynes which are successfully promoted by stoichiometric quantities of transition metal complexes.15 Recently, of special interest to us is the experimental work of Kamitani and Itazaki, who reported the first example of catalytic double hydrophosphination of various alkynes promoted by an iron catalyst.16 Iron is an inexpensive, ubiquitous, and environmentally friendly transition metal. So the creation of catalytic activity of iron complexes instead of rare and expensive metal complexes is very significant. They discovered experimentally
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that the single hydrophosphination product with Z configuration increased for 12 h after the reaction started, and decreased gradually after that. A little amount of the single hydrophosphination product with E configuration was detected. This means that the intermediate with Z configuration is the major single hydrophosphination product. Besides, this iron–metal catalytic system is only effective for terminal arylacetylenes but not for alkylacetylenes and internal alkynes. Why the single hydrophosphination product with Z configuration is the major intermediate? Why it is only effective for terminal arylacetylenes? In order to answer the above questions, we carry out a computational study based on density functional theory (DFT). The important elementary steps, intermediates and transition states are also displayed in detail. We hope this work presents a detailed understanding of the catalytic mechanism and provides further guide for experimentalists.
2. Computational details Geometry optimizations were carried out by using the B3LYP density functional method as implemented in the Gaussian 09 program package.17–19 The standard 6-31G(d,p) basis set was used for H, C, O, and P atoms, and the effective core potentials (ECPs) of Hay and Wadt are combined with double-ζ valence basis sets (LanL2DZ) for Fe.20–22 Frequencies were analyzed at the same level to characterize the nature of stationary points (energy minima or first order saddle points) and to provide thermodynamic quantities. The intrinsic reaction coordinate (IRC) paths were also traced to verify the profiles that connect each transition state to correct associated local minima.23 The solvent effect was calculated by using the self-consistent reaction field (SCRF) method based on the integral equation formalism polarizable continuum model (IEFPCM) model at M06/[LANL2DZ+6-311++G(d,p)] level of theory.24 Phenylacetylene was chosen as the solvent. For all cited energies, ZPE corrections were taken into consideration. The energies showed in the whole text were all calculated at this level. In addition, natural bond orbital (NBO) analysis was performed to investigate the electronic properties and the contribution of atom orbitals to the frontier molecular orbital.25
3. Results and discussion In order to understand the formation processes of single hydrophosphination product with E and Z configurations, two reaction pathways, named as Path A and Path B, were designed (Scheme 2). The whole mechanism for the double hydrophosphination was divided into three steps: active species generation, single hydrophosphination reaction (Cycle 1), double hydrophosphination reaction, viz., active species regeneration and target product generation (Cycle 2). The pathways of the reaction were optimized and the energies for all stationary points were calculated.
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3.1
Active species generation
For the active species generation process, the relevant structures together with the partial optimized structural parameters were shown in Fig. 1. The process starts with CO ligand insertion into the Fe–Me bond in the precursor Cat-pre forming complex CpFe(CO){C(vO)}Me (Fe-Cat) (Cp stands for η5-C5H5). This insertion step proceeds via transition state TS with the free energy barrier of 15.3 kcal mol−1, and it can create a vacant coordination site on Fe to allow the PPh2H coordination. After then, Fe-Cat coordinates with PPh2H giving Int1, which is about 18.9 kcal mol−1 lower in energy than the sum of separate Fe-Cat and PPh2H. It is obvious from this result that the coordination of PPh2H to Fe-Cat makes the complex Int1 more stable. Via a transition state TS1 with an energy barrier of 30.2 kcal mol−1, an intermediate Int2 forms. The transition vector of TS1 with a single imaginary frequency of 191i cm−1 corresponds to H transfer from P to Fe. As shown in Fig. 1, most of the structural variables of the Int2 are no significantly different from Int1, except that Fe–H distance becomes 1.410 Å shorter than Int1. It means that conversion of Int1 into Int2 occurs via P–H oxidative addition to the iron– metal. Then with an activation barrier 4.0 kcal mol−1 via TS2, an elimination reaction of Int2 occurs giving HC(O)Me and Int3. The transition vector of TS2 corresponds to H transfer from Fe to C4. In this process, one problem should be noted that why the conversion of Int1 to Int3 can not occur directly via HC(O)Me elimination from the coordinated PPh2H and C(O)Me ligand. In order to resolve the problem, the NBO charges were analyzed for Int1 and Int2 using Natural Bond Orbital Theory (NBO). The NBO charges of C4, O, and H in Int1 are 0.566, −0.586, and 0.034, respectively. In Int2, they are 0.557, −0.575, and 0.019. From the calculated results, it can be known that the charges of H and C4 in Int2 are more negative than Int1, and the repulsion force between H and C4 in Int2 is smaller than in Int1. So the H–C4 bond can form more easily in Int2 than that in Int1. Consequently, the HC(O)Me elimination from the coordinated H and C(O)Me ligand in Int2 can occur easily. The intermediate Int3 is very important, and it does not satisfy the 18-electron rule. Thus, it can participate in the following reactions as an active species. 3.2
Single hydrophosphination reaction
The relative Gibbs free energy profile for this process was displayed in Scheme 3. PhCuCH coordinates to the iron centre of Int3 in an η2-fashion. When the phosphination occurs on C(H) of the PhCuCH, Int4 is obtained. From Scheme 3, it can be found that the free energy of π-complex Int4 is 0.1 kcal mol−1 higher than the sum of separate Int3 and PhCuCH. The Fe– C1, Fe–C2 distances are 2.044 Å and 2.095 Å, respectively (Fig. 2). No transition state is located, which suggests that it is a one step process. Zhou’s group also reported that the coordination of alkynes to transition metal complex was a one step process, which testifies the validity of our conclusion.26 Int4 undergoes an insertion of the coordinated phenylacetylene into the Fe–P bond providing Int5 (Int4→Int5). One of the π
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Scheme 2
Proposed catalytic cycle for the double hydrophosphination of terminal arylacetylenes catalyzed by an iron complex.
bonds of phenylacetylene breaks accompanied by the formation of the Fe–C1 and P–C2 bond. The transition state TS3 for this process is located.27 The phosphination may also occur on C(Ph) of the PhCuCH forming Int4′. Transition state TS3′ can be located in this process. However, the activation barrier of Int4′ to TS3′ is 22.2 kcal mol−1, and it is 13.0 kcal mol−1 higher than that of Int4 to TS3. The calculation results suggest the phosphination will preferentially occur on C(H) of the PhCuCH (Relative optimized structures were presented in the ESI†). Then, the formation processes of single hydrophosphination product with E and Z configuration were discussed respectively. Int5 has a vacant coordination site on Fe to allow the second PPh2H coordination forming Int7′ (Path A). Through TS4′, an elimination reaction occurs yielding the corresponding single hydrophosphination product PhHCvCH(PPh2) with E configuration (2-E) and Int3. Int5 can also readily isomerize into Int6 (Path B). Because the two large
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groups in Int5 are in the same side of the carbon–carbon double bond, and the strong repulsion force between the two large groups results in the instability of Int5. Besides, the free energy of Int6 is only 1.2 kcal mol−1 higher than Int5. Therefore, this isomerization can occur easily. Int6 has a vacant coordination site on Fe to permit the second PPh2H coordination forming Int7. The free energy of Int7 is 17.2 kcal mol−1 lower than the sum of separate Int6 and PPh2H, it reveals that the coordination of PPh2H to Int6 makes intermediate Int7 more stable. Then via TS4, this reaction yields the corresponding single hydrophosphination product PhHCvCH(PPh2) with Z configuration (2-Z) and Int3 to complete Cycle 1. The free energy of Int7′ is 4.6 kcal mol−1 higher than Int7, and TS4′ is 4.8 kcal mol−1 higher in energy than TS4. These results suggest that Path B is the preferred energetically pathway. Therefore, it can be concluded that 2-Z is the major single hydrophosphination product, and this result is well in agreement with the observation of the experimental work of
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Fig. 1
Optimized structures of Cat-pre, TS, Fe-Cat, Int1, TS1, Int2, TS2, Int3. Bond lengths are in angstroms.
Kamitani and Itazaki.16 Additionally, 2-E has two larger groups which are in both side of carbon–carbon double bond. Due to the steric hindrance of the two large groups, the coordination of 2-E to Int3 in Cycle 2 is difficult. Herein, 2-Z is only discussed in the following steps. In addition, the phosphorus atom in Int5 may coordinate to the iron centre to give Int5′ with Fe–P bond length of 2.277 Å. Int5′ (−30.9 kcal mol−1) is quite stable compared with Int5 (−12.8 kcal mol−1), so it is difficult to coordinate with the second PPh2H. However, the
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energy of Int5′ is higher than the target product 1a. Thus, Int5′ can not be the final product from the viewpoint of thermodynamics. In experiment, no Int5′ is detected which supports our calculated results. 3.3
Active species regeneration
Int3 can react with 2-Z produced in Cycle 1 leading the reaction to Cycle 2. The relative optimized structures of this process were shown in Fig. 3. The single hydrophosphination
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Scheme 3
Relative Gibbs free energy profile of catalytic cycle for double hydrophosphination.
product (2-Z) first coordinates to the iron centre of Int3 in an η2-fashion to form the complex Int8. Then the olefin (2-Z) inserts into the Fe–P bond via TS5 forming Int9 (Int9 is in equilibrium with catalytically inactive Int9′). Int9′ (−34.1 kcal mol−1) is more stable than Int9 (−12.9 kcal mol−1), which counts against the generation of target product. The metastable state Int9 is beneficial to the reaction proceeding. Subsequent coordination of PPh2H to Int9 provides intermediate Int10. Then an elimination reaction occurs via TS6 producing the active species Int3 and target product 1a. As can be seen from Scheme 3, the activation barrier of this elimination reaction is 38.6 kcal mol−1 and it is the rate-determining process of the whole reaction. The C1–C2 bond length in the product 1a is 1.544 Å (Fig. 3), showing that it is a single bond, viz., the two sp carbons in the starting alkyne are changed to sp3 carbons. This calculated result is also consistent with the experimental work of Kamitani and Itazaki.16 3.4
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Frontier molecular orbital analysis
In order to understand the role of catalyst in this double hydrophosphination reaction, a frontier molecular orbital (FMO) analysis was made for Ph2PH, Fe-Cat, PhCuCH and the intermediate Int3 (Scheme 4).28 As is well known, the smaller the HOMO–LUMO energy gap is, the more reactive the reaction will be.29,30 HOMO–LUMO energy gaps of non-catalyzed and iron-catalyzed hydrophosphination were calculated respectively to compare the reaction origin. It can be seen that the large value of HOMO (Ph2PH)–LUMO (PhCuCH) energy gaps (Egap) (5.34 eV) may account for the difficulty of direct electrophilic addition of Ph2PH to PhCuCH. In the presence of Fe-Cat, the Egap of HOMO (Ph2PH)–LUMO (Fe-Cat) (4.60 eV) is smaller than that of HOMO (PhCuCH)–LUMO (Fe-Cat) (4.76 eV). This may be the reason why the substrate Ph2PH is first activated by Fe-Cat. The visual orbitals of HOMO and LUMO of the catalyst and substrates were exhibited in Fig. 4. As can be seen, the
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LUMO of cat are mainly distributed at Fe position which can accept electrons from the HOMO of Ph2PH. Besides, the Egap between HOMO of PhCuCH and LUMO of Int3 is reduced to 4.32 eV. It is 0.44 eV smaller than previous HOMO (PhCuCH)– LUMO (Fe-Cat) (4.76 eV). The visual orbital shows that the LUMO of Int3 is mainly distributed at Fe position. Thus, the coordination of Int3 and PhCuCH will proceed via HOMO (PhCuCH)–LUMO (Int3) interaction. From the above analysis, it can be concluded that the double hydrophosphination will take place easily with the help of Fe-Cat. 3.5 Why the iron complex catalyst is only effective to terminal arylacetylenes More additional terminal arylacetylenes, alkylacetylenes and internal alkynes were studied in order to explore why this iron complex is only effective for terminal arylacetylenes. As mentioned above, the coordination of Int3 and alkynes will proceed via HOMO (PhCuCH)–LUMO (Int3) interaction. We herein attempted to solve the problem from the viewpoint of HOMO (alkynes)–LUMO (Int3) interaction. The matching of frontier molecular orbitals between the HOMO of alkynes and LUMO of Int3 was investigated based on the NBO theory. The contribution from the C orbital of the triple bond in the alkynes to the HOMO and Fe in Int3 to the LUMO were calculated. As shown in Tables 1–3, we discovered that the lowest unoccupied molecular orbital (LUMO) of Int3 is mainly contributed by Fe-dxz. The highest occupied molecular orbital (HOMO) of the terminal arylacetylenes is mainly contributed by C1-pz and C2-pz. The HOMO of alkylacetylenes and internal alkynes is mainly contributed by C1-py and C2-py. If two molecular can react with each other, their frontier molecular orbitals should be symmetry-adapted.31 The atom orbital Fe-dxz of Int3 and C1-pz, C2-pz of terminal arylacetylenes are symmetryadapted (Fig. 5), so Int3 can coordinate with terminal arylacetylenes. However, the atom orbital Fe-dxz of Int3 and C1-py,
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Fig. 2
Optimized structures of Int4, TS3, Int5, Int5’, Int6, Int7, Int7’, TS4, TS4’, 2-Z, 2-E. Bond lengths are in angstroms.
C2-py of alkylacetylenes and internal alkynes are not efficient symmetry match (Fig. 5), so Int3 can not coordinate with alkylacetylenes and internal alkynes well. Besides, it can be seen from Table 1, despite C1-px and C2-px of benzylacetylene and Fe-dxz of Int3 is probably a symmetry match, C1-px and C2-px have little contribution to the HOMO. Consequently, Int3 can not coordinate with benzylacetylene well. Therefore, it can be concluded that the symmetry of atom orbitals mainly contributed to the HOMO and LUMO is the important factor, which determines the reactivity of Int3 with alkynes.
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4.
Conclusions
In summary, a detailed mechanism for the double hydrophosphination of terminal arylacetylenes catalyzed by an iron complex has been systematically investigated by performing DFT calculations. The calculation results suggest that the reaction proceeds in three steps. The first step is the formation of active species Int3 by HC(O)Me elimination from Int2. The second step is a single hydrophosphination reaction (Cycle 1). Single hydrophosphination products
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Fig. 3
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Optimized structures of Int8, TS5, Int9, Int9’, Int10, TS6, 1a. Bond lengths are in angstroms.
Scheme 4
Energies of the HOMOs and LUMOs for Ph2PH, Fe-Cat, PhCuCH and the intermediate Int3.
with E and Z configurations were formed in this step. At the same time, the active species Int3 was regenerated. The calculated results indicate that the reaction pathway forming
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single hydrophosphination product with Z configuration is the preferred one. Regenerated complex Int3 in Cycle 1 can react with 2-Z, which leads the reaction to Cycle 2. The third step is
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Fig. 4
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Energies of the HOMOs and LUMOs for Cat, Ph2PH, Int3, and PhCuCH.
Table 1 Contribution of NAO to HOMO of terminal arylacetylenes and alkylacetylenes
Contribution to HOMO R′C1uC2H
C1
C2
NAO type
C6H4 p-Me-C6H4 p-OMe-C6H4 p-NH2-C6H4 3-Pyridyl 3-Thiophenyl 2-Pyridinyl Benzyl n-Hexyl Cyclohexyl
28.26% 9.59% 9.24% 10.41% 13.56% 9.36% 14.28% 4.78% 29.04% 37.77%
11.78% 25.85% 25.19% 26.77% 29.57% 25.61% 28.42% 3.59% 35.28% 47.89%
pz pz pz pz pz pz pz px py py
Table 2
Contribution of NAO to HOMO of internal alkynes: DMAD
Carbon atom of triple bond contribution to HOMO
DMAD
Table 3
C1
C2
12.62% (py)
12.63% (py)
adapted, Int3 can coordinate with terminal arylacetylenes. However, the atom orbital Fe-dxz of Int3 and C1-py, C2-py of alkylacetylenes and internal alkynes are not an efficient symmetry match, therefore, Int3 can not coordinate with alkylacetylenes and internal alkynes well. The selective catalysis of the iron complex for double hydrophosphination of terminal arylacetylenes plays an important role in the synthesis of the functional compounds.
Contribution of NAO to LUMO of Int3
Contribution to LUMO
Int3
Fig. 5 Sketch map for interaction between HOMO (alkynes)–LUMO (Int3) according to NBO analysis. C5H5 ring and hydrogen atoms are omitted for clarity.
Acknowledgements
Fe
P
38.91% (dxz)
21.19% (pz)
active species regeneration and target product formed (Cycle 2). Int3 is a very important intermediate. When the iron center of Int3 coordinates with terminal arylacetylenes (PhCuCH) in an η2-fashion, the reaction goes into Cycle 1. Alternatively, the iron center of Int3 may coordinate with the single hydrophosphination product with Z configuration produced in Cycle 1, leading the reaction to Cycle 2. Additionally, the reactivity between Int3 and alkynes is determined by the symmetry of the frontier molecular orbitals. The atom orbital Fe-dxz of Int3 and C1-pz, C2-pz of terminal arylacetylenes are symmetry-
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This work was supported by the National Natural Science Foundations of China (no. 21375082) and Advanced School Program of Science and Technology of Shandong Province (no. J12LD09).
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15 L. Tang, Y. Zhang, D. Luo, Y. Li, K. F. Mok, W. C. Yeo and P. H. Leung, Tetrahedron Lett., 2007, 48, 33; S. A. Pullarkat, Y. Ding, Y. Li, G. K. Tan and P. H. Leung, Inorg. Chem., 2006, 45, 7455; W. C. Yeo, S. Y. Tee, H. B. Tan, G. K. Tan, L. L. Koh and P. H. Leung, Inorg. Chem., 2004, 43, 8102; Y. Zhang, L. Tang, Y. Ding, J. H. Chua, Y. Li, M. Yuan and P. H. Leung, Tetrahedron Lett., 2008, 49, 1762; W. Malisch, F. J. Rehmann, H. Jehle and J. Reising, J. Organomet. Chem., 1998, 570, 107. 16 M. Kamitani, M. Itazaki, C. Tamiya and H. Nakazawa, J. Am. Chem. Soc., 2012, 134, 11932. 17 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785. 18 A. D. Becke, J. Chem. Phys., 1993, 98, 5648; A. D. Becke, Phys. Rev. A: At., Mol., Opt. Phys., 1988, 38, 3098. 19 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford, CT, 2010. 20 P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 270. 21 P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 299. 22 W. R. Wadt and P. J. Hay, J. Chem. Phys., 1985, 82, 284. 23 C. Gonzalez and H. B. Schlegel, J. Phys. Chem., 1990, 94, 5523. 24 Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2007, 120, 215; J. Tomasi, B. Mennucci and R. Cammi, Chem. Rev., 2005, 105, 2999. 25 A. E. Reed, L. A. Curtiss and F. Weinhold, Chem. Rev., 1988, 88, 899. 26 B. Zhou, H. Chen and C. Wang, J. Am. Chem. Soc., 2013, 135, 1264. 27 V. P. Ananikov and I. P. Beletskaya, Chem.–Asian J., 2011, 6, 1423. 28 R. Hoffmann, Rev. Mod. Phys., 1988, 60, 601. 29 N. Lu, L. Meng, D. Z. Chen and G. Q. Zhang, J. Mol. Catal. A: Chem., 2011, 339, 99. 30 N. Lu, L. Meng, D. Z. Chen and G. Q. Zhang, J. Phys. Chem. A, 2012, 116, 670. 31 G. Frenking and N. Fröhlich, Chem. Rev., 2000, 100, 717.
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Theoretical mechanism for selective catalysis of double hydrophosphination of terminal arylacetylenes by an iron complex† Mingshu Liu, Chuanzhi Sun,* Fang Hang, Nan Sun and Dezhan Chen* The detailed mechanism of the double hydrophosphination of terminal arylacetylenes catalyzed by an iron complex was studied by density functional theory. The calculated results suggest that the reaction
Received 18th October 2013, Accepted 4th December 2013 DOI: 10.1039/c3dt52941j www.rsc.org/dalton
1.
proceeds in three steps: active species generation, single hydrophosphination reaction (Cycle 1), double hydrophosphination reaction, viz., active species regeneration (Cycle 2). The results uncovered the selectivity of the iron complex for double hydrophosphination of terminal arylacetylenes. The symmetry of frontier molecular orbitals determines the effectiveness of the catalyst. We also discuss the formation mechanism of the single hydrophosphination product with Z configuration.
Introduction
Phosphines have received much attention because of their essential role in various fields of chemistry. For instance, they can be used as synthetic reagents, ligands in transition-metal complexes, biologically active molecules, polymers and building blocks of supramolecular assemblies.1–3 In 2001, Knowles obtained the Nobel Prize for the synthesis of L-Dopa catalyzed by an optically pure DIPAMP–rhodium complex, which indicates the importance of chiral phosphines.4–6 More and more scientists have made efforts to develop synthesis methods of phosphines. Among the large number of methods, the transition metal complex catalyzed addition of a P–H bond to an unsaturated carbon–carbon multiple bonds opens a unique opportunity to combine high-atom efficiency and exceptional selectivity.7 In recent years, both the P(ш)–H and P(v)–H substrates were successfully utilized in the addition reactions catalyzed by transition metal complexes (Scheme 1).8,9 However, hydrophosphination by metal complex catalysis also encounters some difficulties. For example, lanthanide complexes catalyzed hydrophosphination shows superb efficiency, but the catalytic system requires the organic chemists to prepare intricate and highly air- and moisture-sensitive metal complexes.10 Palladium and nickel complexes catalyzed reactions have also been reported. The scope of the alkynes available for use is limited.11–13 In addition, double
College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, P. R. China. E-mail: [email protected], [email protected]; Fax: +86 531 86180304; Tel: +86 531 86180304 † Electronic supplementary information (ESI) available. See 10.1039/c3dt52941j
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DOI:
Scheme 1
P–H bond addition to alkyne.
hydrophosphination of an alkyne catalyzed by transition metal complexes is especially difficult. Because the product serves as a bidentate ligand, and it binds to the transition metal complex catalyst more strongly than a monodentate one due to the chelate effect.14 Consequently, the transition metal catalyst can form a coordinatively saturated complex, which results in a decrease in or complete loss of catalytic activity. There are only a few examples of double hydrophosphination of alkynes which are successfully promoted by stoichiometric quantities of transition metal complexes.15 Recently, of special interest to us is the experimental work of Kamitani and Itazaki, who reported the first example of catalytic double hydrophosphination of various alkynes promoted by an iron catalyst.16 Iron is an inexpensive, ubiquitous, and environmentally friendly transition metal. So the creation of catalytic activity of iron complexes instead of rare and expensive metal complexes is very significant. They discovered experimentally
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that the single hydrophosphination product with Z configuration increased for 12 h after the reaction started, and decreased gradually after that. A little amount of the single hydrophosphination product with E configuration was detected. This means that the intermediate with Z configuration is the major single hydrophosphination product. Besides, this iron–metal catalytic system is only effective for terminal arylacetylenes but not for alkylacetylenes and internal alkynes. Why the single hydrophosphination product with Z configuration is the major intermediate? Why it is only effective for terminal arylacetylenes? In order to answer the above questions, we carry out a computational study based on density functional theory (DFT). The important elementary steps, intermediates and transition states are also displayed in detail. We hope this work presents a detailed understanding of the catalytic mechanism and provides further guide for experimentalists.
2. Computational details Geometry optimizations were carried out by using the B3LYP density functional method as implemented in the Gaussian 09 program package.17–19 The standard 6-31G(d,p) basis set was used for H, C, O, and P atoms, and the effective core potentials (ECPs) of Hay and Wadt are combined with double-ζ valence basis sets (LanL2DZ) for Fe.20–22 Frequencies were analyzed at the same level to characterize the nature of stationary points (energy minima or first order saddle points) and to provide thermodynamic quantities. The intrinsic reaction coordinate (IRC) paths were also traced to verify the profiles that connect each transition state to correct associated local minima.23 The solvent effect was calculated by using the self-consistent reaction field (SCRF) method based on the integral equation formalism polarizable continuum model (IEFPCM) model at M06/[LANL2DZ+6-311++G(d,p)] level of theory.24 Phenylacetylene was chosen as the solvent. For all cited energies, ZPE corrections were taken into consideration. The energies showed in the whole text were all calculated at this level. In addition, natural bond orbital (NBO) analysis was performed to investigate the electronic properties and the contribution of atom orbitals to the frontier molecular orbital.25
3. Results and discussion In order to understand the formation processes of single hydrophosphination product with E and Z configurations, two reaction pathways, named as Path A and Path B, were designed (Scheme 2). The whole mechanism for the double hydrophosphination was divided into three steps: active species generation, single hydrophosphination reaction (Cycle 1), double hydrophosphination reaction, viz., active species regeneration and target product generation (Cycle 2). The pathways of the reaction were optimized and the energies for all stationary points were calculated.
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3.1
Active species generation
For the active species generation process, the relevant structures together with the partial optimized structural parameters were shown in Fig. 1. The process starts with CO ligand insertion into the Fe–Me bond in the precursor Cat-pre forming complex CpFe(CO){C(vO)}Me (Fe-Cat) (Cp stands for η5-C5H5). This insertion step proceeds via transition state TS with the free energy barrier of 15.3 kcal mol−1, and it can create a vacant coordination site on Fe to allow the PPh2H coordination. After then, Fe-Cat coordinates with PPh2H giving Int1, which is about 18.9 kcal mol−1 lower in energy than the sum of separate Fe-Cat and PPh2H. It is obvious from this result that the coordination of PPh2H to Fe-Cat makes the complex Int1 more stable. Via a transition state TS1 with an energy barrier of 30.2 kcal mol−1, an intermediate Int2 forms. The transition vector of TS1 with a single imaginary frequency of 191i cm−1 corresponds to H transfer from P to Fe. As shown in Fig. 1, most of the structural variables of the Int2 are no significantly different from Int1, except that Fe–H distance becomes 1.410 Å shorter than Int1. It means that conversion of Int1 into Int2 occurs via P–H oxidative addition to the iron– metal. Then with an activation barrier 4.0 kcal mol−1 via TS2, an elimination reaction of Int2 occurs giving HC(O)Me and Int3. The transition vector of TS2 corresponds to H transfer from Fe to C4. In this process, one problem should be noted that why the conversion of Int1 to Int3 can not occur directly via HC(O)Me elimination from the coordinated PPh2H and C(O)Me ligand. In order to resolve the problem, the NBO charges were analyzed for Int1 and Int2 using Natural Bond Orbital Theory (NBO). The NBO charges of C4, O, and H in Int1 are 0.566, −0.586, and 0.034, respectively. In Int2, they are 0.557, −0.575, and 0.019. From the calculated results, it can be known that the charges of H and C4 in Int2 are more negative than Int1, and the repulsion force between H and C4 in Int2 is smaller than in Int1. So the H–C4 bond can form more easily in Int2 than that in Int1. Consequently, the HC(O)Me elimination from the coordinated H and C(O)Me ligand in Int2 can occur easily. The intermediate Int3 is very important, and it does not satisfy the 18-electron rule. Thus, it can participate in the following reactions as an active species. 3.2
Single hydrophosphination reaction
The relative Gibbs free energy profile for this process was displayed in Scheme 3. PhCuCH coordinates to the iron centre of Int3 in an η2-fashion. When the phosphination occurs on C(H) of the PhCuCH, Int4 is obtained. From Scheme 3, it can be found that the free energy of π-complex Int4 is 0.1 kcal mol−1 higher than the sum of separate Int3 and PhCuCH. The Fe– C1, Fe–C2 distances are 2.044 Å and 2.095 Å, respectively (Fig. 2). No transition state is located, which suggests that it is a one step process. Zhou’s group also reported that the coordination of alkynes to transition metal complex was a one step process, which testifies the validity of our conclusion.26 Int4 undergoes an insertion of the coordinated phenylacetylene into the Fe–P bond providing Int5 (Int4→Int5). One of the π
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Scheme 2
Proposed catalytic cycle for the double hydrophosphination of terminal arylacetylenes catalyzed by an iron complex.
bonds of phenylacetylene breaks accompanied by the formation of the Fe–C1 and P–C2 bond. The transition state TS3 for this process is located.27 The phosphination may also occur on C(Ph) of the PhCuCH forming Int4′. Transition state TS3′ can be located in this process. However, the activation barrier of Int4′ to TS3′ is 22.2 kcal mol−1, and it is 13.0 kcal mol−1 higher than that of Int4 to TS3. The calculation results suggest the phosphination will preferentially occur on C(H) of the PhCuCH (Relative optimized structures were presented in the ESI†). Then, the formation processes of single hydrophosphination product with E and Z configuration were discussed respectively. Int5 has a vacant coordination site on Fe to allow the second PPh2H coordination forming Int7′ (Path A). Through TS4′, an elimination reaction occurs yielding the corresponding single hydrophosphination product PhHCvCH(PPh2) with E configuration (2-E) and Int3. Int5 can also readily isomerize into Int6 (Path B). Because the two large
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groups in Int5 are in the same side of the carbon–carbon double bond, and the strong repulsion force between the two large groups results in the instability of Int5. Besides, the free energy of Int6 is only 1.2 kcal mol−1 higher than Int5. Therefore, this isomerization can occur easily. Int6 has a vacant coordination site on Fe to permit the second PPh2H coordination forming Int7. The free energy of Int7 is 17.2 kcal mol−1 lower than the sum of separate Int6 and PPh2H, it reveals that the coordination of PPh2H to Int6 makes intermediate Int7 more stable. Then via TS4, this reaction yields the corresponding single hydrophosphination product PhHCvCH(PPh2) with Z configuration (2-Z) and Int3 to complete Cycle 1. The free energy of Int7′ is 4.6 kcal mol−1 higher than Int7, and TS4′ is 4.8 kcal mol−1 higher in energy than TS4. These results suggest that Path B is the preferred energetically pathway. Therefore, it can be concluded that 2-Z is the major single hydrophosphination product, and this result is well in agreement with the observation of the experimental work of
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Fig. 1
Optimized structures of Cat-pre, TS, Fe-Cat, Int1, TS1, Int2, TS2, Int3. Bond lengths are in angstroms.
Kamitani and Itazaki.16 Additionally, 2-E has two larger groups which are in both side of carbon–carbon double bond. Due to the steric hindrance of the two large groups, the coordination of 2-E to Int3 in Cycle 2 is difficult. Herein, 2-Z is only discussed in the following steps. In addition, the phosphorus atom in Int5 may coordinate to the iron centre to give Int5′ with Fe–P bond length of 2.277 Å. Int5′ (−30.9 kcal mol−1) is quite stable compared with Int5 (−12.8 kcal mol−1), so it is difficult to coordinate with the second PPh2H. However, the
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energy of Int5′ is higher than the target product 1a. Thus, Int5′ can not be the final product from the viewpoint of thermodynamics. In experiment, no Int5′ is detected which supports our calculated results. 3.3
Active species regeneration
Int3 can react with 2-Z produced in Cycle 1 leading the reaction to Cycle 2. The relative optimized structures of this process were shown in Fig. 3. The single hydrophosphination
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Scheme 3
Relative Gibbs free energy profile of catalytic cycle for double hydrophosphination.
product (2-Z) first coordinates to the iron centre of Int3 in an η2-fashion to form the complex Int8. Then the olefin (2-Z) inserts into the Fe–P bond via TS5 forming Int9 (Int9 is in equilibrium with catalytically inactive Int9′). Int9′ (−34.1 kcal mol−1) is more stable than Int9 (−12.9 kcal mol−1), which counts against the generation of target product. The metastable state Int9 is beneficial to the reaction proceeding. Subsequent coordination of PPh2H to Int9 provides intermediate Int10. Then an elimination reaction occurs via TS6 producing the active species Int3 and target product 1a. As can be seen from Scheme 3, the activation barrier of this elimination reaction is 38.6 kcal mol−1 and it is the rate-determining process of the whole reaction. The C1–C2 bond length in the product 1a is 1.544 Å (Fig. 3), showing that it is a single bond, viz., the two sp carbons in the starting alkyne are changed to sp3 carbons. This calculated result is also consistent with the experimental work of Kamitani and Itazaki.16 3.4
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Frontier molecular orbital analysis
In order to understand the role of catalyst in this double hydrophosphination reaction, a frontier molecular orbital (FMO) analysis was made for Ph2PH, Fe-Cat, PhCuCH and the intermediate Int3 (Scheme 4).28 As is well known, the smaller the HOMO–LUMO energy gap is, the more reactive the reaction will be.29,30 HOMO–LUMO energy gaps of non-catalyzed and iron-catalyzed hydrophosphination were calculated respectively to compare the reaction origin. It can be seen that the large value of HOMO (Ph2PH)–LUMO (PhCuCH) energy gaps (Egap) (5.34 eV) may account for the difficulty of direct electrophilic addition of Ph2PH to PhCuCH. In the presence of Fe-Cat, the Egap of HOMO (Ph2PH)–LUMO (Fe-Cat) (4.60 eV) is smaller than that of HOMO (PhCuCH)–LUMO (Fe-Cat) (4.76 eV). This may be the reason why the substrate Ph2PH is first activated by Fe-Cat. The visual orbitals of HOMO and LUMO of the catalyst and substrates were exhibited in Fig. 4. As can be seen, the
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LUMO of cat are mainly distributed at Fe position which can accept electrons from the HOMO of Ph2PH. Besides, the Egap between HOMO of PhCuCH and LUMO of Int3 is reduced to 4.32 eV. It is 0.44 eV smaller than previous HOMO (PhCuCH)– LUMO (Fe-Cat) (4.76 eV). The visual orbital shows that the LUMO of Int3 is mainly distributed at Fe position. Thus, the coordination of Int3 and PhCuCH will proceed via HOMO (PhCuCH)–LUMO (Int3) interaction. From the above analysis, it can be concluded that the double hydrophosphination will take place easily with the help of Fe-Cat. 3.5 Why the iron complex catalyst is only effective to terminal arylacetylenes More additional terminal arylacetylenes, alkylacetylenes and internal alkynes were studied in order to explore why this iron complex is only effective for terminal arylacetylenes. As mentioned above, the coordination of Int3 and alkynes will proceed via HOMO (PhCuCH)–LUMO (Int3) interaction. We herein attempted to solve the problem from the viewpoint of HOMO (alkynes)–LUMO (Int3) interaction. The matching of frontier molecular orbitals between the HOMO of alkynes and LUMO of Int3 was investigated based on the NBO theory. The contribution from the C orbital of the triple bond in the alkynes to the HOMO and Fe in Int3 to the LUMO were calculated. As shown in Tables 1–3, we discovered that the lowest unoccupied molecular orbital (LUMO) of Int3 is mainly contributed by Fe-dxz. The highest occupied molecular orbital (HOMO) of the terminal arylacetylenes is mainly contributed by C1-pz and C2-pz. The HOMO of alkylacetylenes and internal alkynes is mainly contributed by C1-py and C2-py. If two molecular can react with each other, their frontier molecular orbitals should be symmetry-adapted.31 The atom orbital Fe-dxz of Int3 and C1-pz, C2-pz of terminal arylacetylenes are symmetryadapted (Fig. 5), so Int3 can coordinate with terminal arylacetylenes. However, the atom orbital Fe-dxz of Int3 and C1-py,
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Fig. 2
Optimized structures of Int4, TS3, Int5, Int5’, Int6, Int7, Int7’, TS4, TS4’, 2-Z, 2-E. Bond lengths are in angstroms.
C2-py of alkylacetylenes and internal alkynes are not efficient symmetry match (Fig. 5), so Int3 can not coordinate with alkylacetylenes and internal alkynes well. Besides, it can be seen from Table 1, despite C1-px and C2-px of benzylacetylene and Fe-dxz of Int3 is probably a symmetry match, C1-px and C2-px have little contribution to the HOMO. Consequently, Int3 can not coordinate with benzylacetylene well. Therefore, it can be concluded that the symmetry of atom orbitals mainly contributed to the HOMO and LUMO is the important factor, which determines the reactivity of Int3 with alkynes.
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4.
Conclusions
In summary, a detailed mechanism for the double hydrophosphination of terminal arylacetylenes catalyzed by an iron complex has been systematically investigated by performing DFT calculations. The calculation results suggest that the reaction proceeds in three steps. The first step is the formation of active species Int3 by HC(O)Me elimination from Int2. The second step is a single hydrophosphination reaction (Cycle 1). Single hydrophosphination products
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Fig. 3
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Optimized structures of Int8, TS5, Int9, Int9’, Int10, TS6, 1a. Bond lengths are in angstroms.
Scheme 4
Energies of the HOMOs and LUMOs for Ph2PH, Fe-Cat, PhCuCH and the intermediate Int3.
with E and Z configurations were formed in this step. At the same time, the active species Int3 was regenerated. The calculated results indicate that the reaction pathway forming
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single hydrophosphination product with Z configuration is the preferred one. Regenerated complex Int3 in Cycle 1 can react with 2-Z, which leads the reaction to Cycle 2. The third step is
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Fig. 4
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Energies of the HOMOs and LUMOs for Cat, Ph2PH, Int3, and PhCuCH.
Table 1 Contribution of NAO to HOMO of terminal arylacetylenes and alkylacetylenes
Contribution to HOMO R′C1uC2H
C1
C2
NAO type
C6H4 p-Me-C6H4 p-OMe-C6H4 p-NH2-C6H4 3-Pyridyl 3-Thiophenyl 2-Pyridinyl Benzyl n-Hexyl Cyclohexyl
28.26% 9.59% 9.24% 10.41% 13.56% 9.36% 14.28% 4.78% 29.04% 37.77%
11.78% 25.85% 25.19% 26.77% 29.57% 25.61% 28.42% 3.59% 35.28% 47.89%
pz pz pz pz pz pz pz px py py
Table 2
Contribution of NAO to HOMO of internal alkynes: DMAD
Carbon atom of triple bond contribution to HOMO
DMAD
Table 3
C1
C2
12.62% (py)
12.63% (py)
adapted, Int3 can coordinate with terminal arylacetylenes. However, the atom orbital Fe-dxz of Int3 and C1-py, C2-py of alkylacetylenes and internal alkynes are not an efficient symmetry match, therefore, Int3 can not coordinate with alkylacetylenes and internal alkynes well. The selective catalysis of the iron complex for double hydrophosphination of terminal arylacetylenes plays an important role in the synthesis of the functional compounds.
Contribution of NAO to LUMO of Int3
Contribution to LUMO
Int3
Fig. 5 Sketch map for interaction between HOMO (alkynes)–LUMO (Int3) according to NBO analysis. C5H5 ring and hydrogen atoms are omitted for clarity.
Acknowledgements
Fe
P
38.91% (dxz)
21.19% (pz)
active species regeneration and target product formed (Cycle 2). Int3 is a very important intermediate. When the iron center of Int3 coordinates with terminal arylacetylenes (PhCuCH) in an η2-fashion, the reaction goes into Cycle 1. Alternatively, the iron center of Int3 may coordinate with the single hydrophosphination product with Z configuration produced in Cycle 1, leading the reaction to Cycle 2. Additionally, the reactivity between Int3 and alkynes is determined by the symmetry of the frontier molecular orbitals. The atom orbital Fe-dxz of Int3 and C1-pz, C2-pz of terminal arylacetylenes are symmetry-
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This work was supported by the National Natural Science Foundations of China (no. 21375082) and Advanced School Program of Science and Technology of Shandong Province (no. J12LD09).
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