Note pubs.acs.org/Organometallics
Role of Steric Strain in the Chemistry of Phosphiranes Feny Ho, Yongxin Li, and François Mathey* Division of Chemistry & Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371 S Supporting Information *
ABSTRACT: The reaction of transient [MeP-W(CO)5] with norbornene gives the two corresponding exo phosphiranes with W syn or anti to the norbornane bridge. The syn complex is selectively decomplexed by carbon monoxide at 120 °C under 50 bar. Both complexes are easily cleaved by Pd(OAc)2 at room temperature to give [MeP(OAc)2)]W(CO)5.
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INTRODUCTION The chemistry of phosphiranes is known to be highly sensitive to external perturbations. As a first illustration, substituents such as amino or alkoxy polarize the P−C ring bonds and favor their heterolytic cleavage.1 In a different vein, steric bulk heavily stabilizes the ring against ring-opening, as shown by the isolation of the first phosphirane oxide with a supermesityl P-substituent.2 Whereas 1-phenyl phosphirane is quaternized by methyl triflate, the polycyclic BABAR-Phos is not.3 With such a background, it was tempting to study the potential role of steric strain on the chemistry of phosphiranes. In a preceding work, Lammertsma4 studied the condensation of terminal phosphinidene complexes with norbornene and norbornadiene and showed that the condensation exclusively takes place on the exo side of the bicyclic system. This stereochemistry is ideal to study the effect of the steric strain exerted by the norbornane bridge on the phosphirane ring. This is the subject of the present report.
Figure 1. Computed structure of syn phosphirane (1) Main distances (Å) and angles (deg): P−Me 1.862, P−C14 1.867, C12−C14 1.513; Me−P−C14 98.6, C12−P−C14 47.8, C3−C14−P 119.3, C14−C3− C9 102.6.
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RESULTS AND DISCUSSION In order to have an idea of what could be expected, we first decided to compute the structures of 1 (lone pair syn to the bridge) and 2 by DFT at the B3PW91/6-311+G(d,p) level.5 The structures are shown in Figures 1 and 2. The only significant differences between the two structures concern the repulsion between the norbornane bridge and the phosphirane ring. Whereas the Me-P/phosphirane plane angle is 93.9° in the syn isomer, it increases up to 100.4° in the anti isomer. Symmetrically, the norbornane bridge is repelled by the P-Me group in the anti isomer, as shown by the bridge plane/junction plane angle, which increases from 127.5° (syn) to 130.0°. Surprisingly, the HOMO corresponding to the lone pair is not affected by this dramatic distortion at −6.50 eV in the syn and −6.48 eV in the anti isomers. On the contrary, the LUMO of the syn isomer at +0.30 eV is significantly higher in energy than the LUMO of the anti isomer at +0.11 eV. We thus expect that 2 will be a better acceptor than 1. But the key point is that the steric repulsion between the bridge and the P-Me substituent destabilizes 2 by as much as 7.7 kcal mol−1 in comparison with 1. The structures of the corresponding P-W(CO)5 complexes 3 and 4 were also computed at the B3PW91/6-31G(d)-Lanl2dz (W) level. The agreement with the X-ray structures (see later) © XXXX American Chemical Society
Figure 2. Computed structure of anti phosphirane (2) Main distances (Å) and angles (deg): P−Me 1.868, P−C14 1.863, C12−C14 1.521; Me−P-C14 112.1, C12−P−C14 48.1, C3−C14−P 128.6, C14−C3− C9 104.1.
is quite good. For example, the computed W- - -C(bridge) distance in 3 is 4.148 vs 4.121 Å for the experimental value. The key point is that 3 is more stable than 4 by only 4.4 kcal mol−1. In other words, if we admit that the destabilization by steric Received: August 13, 2012
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strain is identical for 2 and 4, then 3 is destabilized by 3.3 kcal mol−1. The disappearance of this strain could facilitate the decomplexation of 3. Complexes 3 and 4 were prepared by the usual route involving the reaction of the transient terminal phosphinidene complex [MeP-W(CO)5] with norbornene. As shown by Lammertsma,4 the condensation exclusively takes place on the exo side for steric reasons (eq 1). Figure 4. X-ray crystal structure of anti phosphirane complex 4. Main distances (Å) and angles (deg): P−W 2.4983(14), P−Me 1.835(7), P−C7 1.834(8), C7−C8 1.538(11); Me−P-C7 116.9(11), C7−P−C8 49.8(4), C11−C7−P 127.4(18), C10−C11−C7 103.2(15).
A complete decomposition was observed. We were more lucky when using carbon monoxide under pressure (eq 2). The overall yield is up to 65%, and the anti product is the major isomer. The two complexes were separated by chromatography, and their structures established by X-ray crystal structure analysis (Figures 3 and 4). Overall, the trends observed
In line with the theoretical predictions, a selective decomplexation of 3 was observed. The good thermal stability of 1 is noteworthy. We also found that 1 is a poor ligand. Complete decomposition was observed when 1 was allowed to react in toluene with Pd(OAc)2, [Rh(CO)2(acac)], and RuCl3 at room temperature for 3 h. No reaction was observed with Pd2dba3. Ordinary phosphiranes are known to give stable complexes with these metals.6 While studying the chemistry of 3 and 4, we discovered another reaction that can be linked to the steric strain existing in these species. It is well known that Pd(0) readily inserts into the P−C ring bonds of phosphirane P-W(CO)5 complexes via a η2 transition state,7 but this kind of chemistry is not known with Pd(II) and, anyhow, does not lead to the complete disruption of the ring. In our case, we noted that palladium acetate performs the stoichiometric oxidative cleavage of 3 and 4 at room temperature (eq 3).
Figure 3. X-ray crystal structure of syn phosphirane complex 3. Main distances (Å) and angles (deg): P−W 2.5224(9), P−Me 1.823(3), P−C7 1.831(3), C7−C8 1.562(5); Me−P−C7 101.69(16), C7−P−C8 50.46(15), C12−C7−P 122.1(2), C13−C12−C7 103.2(3).
when comparing the structures of 3 and 4 are similar to those observed when comparing 1 and 2. The key point is that the folding angle between the phosphirane and the norbornane junction planes (126.3° in 2 vs 125.3° in 4) is not modified by the complexation in the anti isomer. On the contrary some steric influence of the bridge can be noted when comparing 1 with its complex 3. An increase of the phosphirane/junction interplane angle from 116.0° to 120.2° is observed. From another standpoint, the P−W bond lengths (2.5224(9) Å for the syn complex and 2.4983(14) Å for the anti complex) seem to indicate a weaker interaction between tungsten and phosphirane in the syn isomer that is coherent with the theoretical prediction that 1 is a weaker acceptor than 2 for low-valent metals. We attempted to recover 1 and 2 from their complexes 3 and 4 using the reaction with Ph2PCH2CH2PPh2 at 80 °C.
It is clear that this reaction involves an initial insertion of palladium into the P−C ring bond, which is favored by the additional strain induced in the phosphirane ring by the norbornane annellation. We propose the following mechanism (eq 4).
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P NMR (CDCl3) δ 165.8 (1JPW = 338.1 Hz); 1H NMR (CDCl3) δ 2.23 (d, 6H, 4JPH = 1.37 Hz, CH3), 2.32 (d, 3H, 2JPH = 3.2 Hz, CH3); 13 C NMR (CDCl3) δ 22.4 (s, OCH3), 28.2 (d, 1JCP = 26.9, CH3), 167.5 (d, 2JCP = 6.7 Hz, CO), 195.1 (d, 2JCP = 8.6 Hz, cis CO), 198.2 (d, 2JCP =37.3 Hz, trans CO); MS (m/z, relative intensity) 487.9496 (calcd for C13H13O5PW 487.9494). 31
The initial insertion would be followed by the intramolecular nucleophilic attack of the acetate ion onto the electrophilic phosphorus. The exo low-coordinate Pd would then insert into the exo norbornanyl−P bond destabilized by the steric repulsion of the bridge. An alternative mechanism involving the generation of [MeP-W(CO)5] from 3 and 4, followed by a redox reaction with Pd(OAc)2, is excluded since the reaction proceeds at rt, far below the temperature needed for the cycloreversion. Also of interest is the fact that other phosphirane complexes such as those derived from cyclohexene do not yield 5 in any significant amount when reacting with Pd(OAc)2 under the same reaction conditions.
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ASSOCIATED CONTENT
S Supporting Information *
X-ray crystal structure analyses and NMR spectra of compounds 1, 3, 4, and 5. This material is available free of charge via the Internet at http://pubs.acs.org.
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EXPERIMENTAL SECTION
All reactions were carried out with distilled dry solvents. Silica gel (230−400 mesh) was used for the chromatographic separations. NMR spectra were recorded on either a Bruker BBFO 400 MHz or JEOL ECA 400SL spectrometer. All spectra were recorded at 298 K. Proton decoupling was applied for 13C and 31P spectra. HRMS were obtained on a Water Q-Tof Premier MS. X-ray crystallographic analyses were performed on a Bruker X8 APEX CCD diffractometer or a Bruker Kappa CCD diffractometer. Reactions with CO were performed inside a glass liner in a Parr Instrument Co. 4560 mini bench top reactor, using a Parr Instrument Co. 4848 reactor controller. Synthesis of Phosphirane 3 and 4. The 7-phosphanorbornadiene complex (1 g, 1.69 mmol) was stirred with norbornene (0.955 g, 10.14 mmol) in toluene at 120 °C for 24 h. After evaporation of the solvent, the residue was purified by column chromatography on silica gel at −10 °C using hexane as the eluent, yielding a mixture of products 3 and 4. A second chromatography at −10 °C was carried out to separate the two isomers with hexane as the eluent. The syn isomer (31P −194 ppm) was eluted out first from the column (221.7 mg, 28.3%). The anti isomer was then recovered (288.3 mg, 36.74%). Phosphirane 3: 31P NMR (CD2Cl2) δ −194.0 (s, 1JPW = 251.4 Hz); 1 H NMR (CDCl3) δ 0.94 (dt, 1H, 2JHH = 11.0 Hz, 4JHH = 1.4 Hz, CH2(syn)), 1.81 (d, 1H, 2JHH = 11.0 Hz, CH2(anti)), 1.22 (d, 3H, 2 JPH = 5.5 Hz, CH3), 1.31 (s, 2H, PCH), 1.36 (dd, 2H, 2JHH = 7.3 Hz, 4 JHH = 2.3 Hz, CH2(endo)), 1.62 (d, 2JHH = 6.9 Hz, 2H, CH2(exo)), 2.68 (s, 2H, CH); 13C NMR (CD2Cl2) δ 18.6 (d, 1JCP = 21 Hz, CH3), 28.8 (d, 1JCP = 8.6 Hz, CHP), 31.3 (d, 3JCP = 7.6 Hz, CH2CH2), 32.8 (d, 3JCP = 15.3 Hz, CH2 bridge), 37.6 (s, CH), 196.9 (d, 2JCP = 8.6 Hz, cis CO), 199.3 (d, 2JCP = 30.6 Hz, trans CO); MS (m/z, relative intensity) 464.0012 (calcd for C13H13O5PW 464.0010). Phosphirane 4: 31P NMR (CD2Cl2) δ −170.2 (s, 1JPW = 255.8 Hz); 1 H NMR (CD2Cl2) δ 0.97 (d, 1H, 2JHH = 11.3 Hz, CH2(anti)), 1.08 (dt, 1H, 2JHH = 11.3 Hz, 4JHH = 1.8 Hz, CH2(syn)), 1.39 (dd, 2H, 2 JHH = 7.7 Hz, 4JHH = 2.3 Hz, CH2(endo)), 1.48 (d, 2H, 2JHH = 10.9 Hz, PCH), 1.67 (d, 2H, 2JHH = 7.2 Hz, CH2(exo)),1.70 (d, 3H, 2 JPH = 5.0 Hz, CH3), 2.85 (s, 2H, CH); 13C NMR (CD2Cl2) δ 13.3, (d, 1JCP = 4.8 Hz, CH3), 31.1 (d, 3JCP = 9.5 Hz, CH2 bridge), 31.7 (d, 1JCP = 12.4 Hz, CHP), 32.6 (d, 3JCP = 3.9 Hz, CH2CH2), 39.6 (d, 2JCP = 7.6 Hz, CH), 197.2 (d, 2JCP = 8.6 Hz, cis CO), 199.6 (d, 2JCP = 27.7 Hz, trans CO); MS (m/z, relative intensity) 464.0012 (calcd for C13H13O5PW 464.0010). Decomplexation of 3 by CO. Phosphirane 3 (100 mg, 0.216 mmol) was stirred under CO (50 bar) at 120 °C for 16 h in a high-pressure reactor. 31P NMR (CDCl3) δ −240.9; 1H NMR (CDCl3) δ 0.44 (d, 3H, 2JPH = 5.7 Hz, CH3), 0.64 (dm, 1H, 2JHH = 12.0 Hz, CH2(syn)), 1.02 (d, 1H, CH2(anti)), 1.18 (s, 2H, PCH), 1.33 (dd, 2H, 2 JHH = 7.2 Hz, 4JHH = 2.2 Hz, CH2(endo)), 1.51 (d, 2JHH = 7.1 Hz, 2H, CH2(exo)), 2.46 (s, 2H, CH); 13C NMR (CDCl3) δ 8.70 (d, 1JCP = 30.8 Hz, CH3), 30.4 (s, CH2CH2), 30.5 (d, 1JCP = 37.1 Hz, CHP), 31.2 (d, 3JCP = 29.4 Hz, CH2 bridge), 38.3 (d, 2JCP = 7.8 Hz, CH); MS (m/z, relative intensity) 163.0653 (calcd for C8H13PNa 163.0653). Reaction of 3 and 4 with Pd(OAc)2. Phosphiranes 3 and 4 (50 mg, 0.108 mmol) were reacted with Pd(OAc)2 (24 mg, 0.108 mmol) in THF at rt for 3 h. After solvent evaporation, the residue was purified by column chromatography on silica gel at −10 °C using 1:1 hexane/dichloromethane as the eluent, yielding 20 mg (38%) of 5.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Nanyang Technological University in Singapore for the financial support of this work and Dr. Matthew P. Duffy for technical advice.
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REFERENCES
(1) Marinetti, A.; Mathey, F. Organometallics 1984, 3, 456. Marinetti, A.; Mathey, F. Organometallics 1984, 3, 1492. (2) Quin, L. D.; Yao, E.-Y.; Szewczyk, J. Tetrahedron Lett. 1987, 28, 1077. (3) Liedtke, J.; Loss, S.; Alcaraz, G.; Gramlich, V.; Grützmacher, H. Angew. Chem., Int. Ed. 1999, 38, 1623. (4) Hung, J. T.; Chand, P.; Fronczek, F. R.; Watkins, S. F.; Lammertsma, K. Organometallics 1993, 12, 1401. (5) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; M. A. Al-Laham, Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; , and Pople, J. A. Gaussian 03, Revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003. (6) Marinetti, A.; Mathey, F.; Ricard, L. Organometallics 1993, 12, 1207. Maurer, S.; Burkhart, C.; Maas, G. Eur. J. Org. Chem. 2010, 2504. (7) Carmichael, D.; Hitchcock, P. B.; Nixon, J. F.; Mathey, F.; Ricard, L. J. Chem. Soc., Chem. Commun. 1989, 1389. Carmichael, D.; Hitchcock, P. B.; Nixon, J. F.; Mathey, F.; Ricard, L. J. Chem. Soc., Dalton Trans. 1993, 1811.
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