Note pubs.acs.org/Organometallics
Revisiting the Chemistry of Phosphinidene Sulfides Lili Wang,† Rakesh Ganguly,‡ and François Mathey*,†,‡ †
College of Chemistry and Molecular Engineering, International Phosphorus Laboratory, Joint Research Laboratory for Functional Organophosphorus Materials of Henan Province, Zhengzhou University, Zhengzhou 450001, P. R. China ‡ Nanyang Technological University, Division of Chemistry & Biological Chemistry, 21 Nanyang Link, Singapore 637371 S Supporting Information *
ABSTRACT: The reaction of triethylamine with the [4 + 2] cycloadducts of phosphole sulfides and 3-bromo-N-phenylmaleimide provides a convenient access to phosphinidene sulfides [RPS]. These transient species are trapped by 2,3-dimethyl-1,3butadiene to give the previously unknown trivalent [4 + 2] cycloadducts. One of these (R = Ph) has been characterized as its P-W(CO)5 complex by X-ray crystal structure analysis. With cyclopentadiene, the subsequent insertion of a second molecule of [RPS] leads to a new type of bicyclic product containing a thiadiphospholane ring.
A
DFT at the B3LYP/6-311+G(d,p) level.10 The computed bond order of PS is 1.93, thus showing a genuine double bond character. The frontier orbitals of MePS are shown in Figure 1.
mong the various dicoordinate phosphorus species, phosphinidene sulfides (also called thioxophosphines) are certainly, together with phosphinidene oxides, the least investigated members of the P(II) family. Several precursors for the generation of phosphinidene sulfides and a few of their trapping reactions have been described in the literature.1−8 However, some discrepancies can be noticed in the published data. If we consider the reaction between [PhPS] and 2,3-dimethyl-1,3-butadiene, it is stated that it yields either the phospholene sulfide 1,8 the tetrahydrothiaphosphinine sulfide 2, or the corresponding oxide 3.2−4 The expected trivalent tetrahydrothiaphosphinine 4a has never been detected.
Figure 1. Frontier orbitals (Kohn−Sham) of MePS.
It is immediately clear that the electronic structure of MePS is different from the structure of a typical phosphaalkene that is characterized by a π HOMO and a π* LUMO. Here, the HOMO is an in-plane orbital combining the σ(P-C), the P lone pair, and an in-plane p orbital of sulfur. The situation is somewhat similar to that of iminophosphine in which a σ HOMO is intercalated between a π (HOMO-1) and a π* orbital (LUMO). A carbenelike behavior is not excluded in some cases for MePS since the HOMO contains the P-lone pair and the LUMO the pz empty orbital at P, and thus, the species can be considered as isolobal with a singlet carbene as discussed by Gaspar et al.4 We wished to devise a precursor of phosphinidene sulfides that could work at room temperature in order to avoid the possible degradation of their reaction products. Some time ago, Kashman et al. observed that the reaction of 1-phenyl-3,4-dimethylphosphole
When looking at the other fundamental dicoordinate phosphorus species, it must be stressed that they can behave either as alkenes (e.g., the phosphaalkenes) or as carbenes (e.g., in some cases, the iminophosphines9). Apparently, the behavior of phosphinidene sulfides is erratic since they behave either as carbenes, leading to products like 1, or as alkenes, leading to degradation products like 2 or 3. Willing to shed some light on this question, we decided to build new precursors of phosphinidene sulfides working under very mild conditions and to reinvestigate their chemistry.
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RESULTS AND DISCUSSION In order to have an idea of what could be expected, we first decided to investigate the electronic structure of Me-PS by © 2014 American Chemical Society
Received: August 14, 2014 Published: September 25, 2014 5614
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sulfide with dimethyl acetylenedicarboxylate at room temperature directly yields the phthalate, meaning that the intermediate 7-phosphanorbornadiene sufide collapses immediately to release PhPS.11 On this basis, we decided to study the reaction of 3-bromo-N-phenylmaleimide12 with a series of phosphole sulfides, as shown in eq 1.
The reaction is sluggish but affords the expected [4 + 2] adducts in acceptable yields. The structure of 5b has been confirmed by X-ray analysis (Figure 2). The cycloaddition takes
Figure 3. X-ray crystal structure of (6a). Main distances (Å) and angles (deg): P1−W1 2.4960(6), P1−S1 2.0974(8), P1−C6 1.853(3), P1−C12 1.827(3), S1−C9 1.856(3), C6−C7 1.513(3), C7−C10 1.350(3), C10−C9 1.506(4); C6−P1−S1 99.94(8), C6−P1−C12 101.02(12), S1−P1−C12 106.88(8), W1−P1−S1 112.27(3), W1− P1−C6 115.81(8), W1−P1−C12 118.73(8).
Figure 2. X-ray crystal structure of (5b). Main distances (Å) and angles (deg): P1−S1 1.9412(9), P1−C1 1.883(3), P1−C5 1.871(3), P1−C10 1.857(3), C5−C12 1.557(4), C10−C11 1.560(4), C12−Br1 1.954(3); C5−P1−C10 81.23(12), S1−P1C1 114.44(9); S1−P1−C5 118.30(9), S1−P1−C10 115.35(9), Br1−C12−C5 114.12(17), H− C11−C10 111.7, P1−C5−C12 101.33(17), P1−C10−C11 99.17(16).
thermal fragmentation of 1-arylphosphiranes that were supposed to generate arylphosphinidenes.13 We noticed that 4a is not very stable on standing. In fact, it evolves to give 214 in 1 h at 90 °C in toluene, thus explaining the detection of 2 in the former experiments on the generation of [PhPS]. We were never able to detect the dihydrophosphole that could be the byproduct of this transformation. Apparently, the presence of triethylamine stabilizes adducts like 4. While screening the reactions of various dienes with [PhPS], we found that the reaction with cyclopentadiene gives abnormal products containing two phosphorus atoms (eq 3).
place on the side of the PS bond as expected. There is clearly some repulsion between the negatively charged bromine and sulfur atoms (Br···S separation 3.619 Å). This can be seen from the fact that S−P−C5 is larger than S−P−C10 (118.30° vs 115.35(9)°). This observation explains why the cycloaddition proceeds slowly and gives modest yields. These cycloadducts proved to be efficient precursors of phosphinidene sulfides upon reaction with triethylamine even at room temperature. As expected, the intermediate 7-phosphanorbornadiene sulfides immediately collapse to release the two-coordinate species. Running this dehydrobromination in the presence of 2,3-dimethyl-1,3-butadiene allowed us to get for the first time the trivalent [4 + 2] cycloadducts 4 (eq 2). The tetrahydrothiaphosphinines 4 were detected by their 31P NMR resonances, and their structures were established by NMR and HRMS of their P-W(CO5) complexes. The structure of 6a was confirmed by X-ray crystal analysis (Figure 3). We also generated [PhPS] in toluene at 110 °C in the presence of dimethylbutadiene and observed exclusively the formation of 4a. No phospholene sulfide 1 was ever detected. We suspect that the formation of 1 or analogues from some potential precursors of phosphinidene sulfides proceeds via a mechanism that does not involve free [RPS]. In this vein, it is interesting to read the study of Borden, Gaspar, and co-workers on the
The 31P NMR data (7a δ 31P 100.3 and −15.8, JP−P = 277 Hz; 7b δ 31P 94.9 and 12.6, JP−P = 271 Hz) suggest that these products are two diastereomers with a P−P bond between a pentavalent and a trivalent phosphorus atom. We were able to isolate 7a in the pure state and to establish its structure by X-ray crystal analysis (Figure 4). The trivalent phosphorus is highly pyramidal (∑ angles 297.9°), its phenyl substituent sits above the cyclopentene ring, the two phenyls are in a trans disposition 5615
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129.30 (s, N-Ph CH), 129.44 (s, N-Ph CH), 131.27 (s, N-Ph C), 131.52 (d, 4JC−P = 2.9 Hz, P-Ph CH), 132.06 (d, 1JC−P = 86.2 Hz, P-C), 134.14 (d, 2JC−P = 6.2 Hz, C-CH3), 135.32 (d, 2JC−P = 8.5 Hz, C-CH3), 171.25 (d, 3JC−P = 11.3 Hz, CO), 172.78 (d, 3JC−P = 16.3 Hz, CO). HRMS: Calcd for C22H20BrNO2PS: 472.0136, found 472.0102. Synthesis of 5b. A solution of 3,4-dimethyl-1-t-butylphosphole sulfide (0.55 g, 2.7 mmol) and 3-bromo-N-phenylmaleimide (0.76 g, 3.0 mmol) in 10 mL of xylene was stirred for 11 h at 140 °C in a sealed tube. The mixture was chromatographed on silica gel with 7:1 hexane/ethyl acetate and then 3:1 hexane/ethyl acetate mixtures as the eluent to give 5b as a white solid (0.21 g, 17% yield). 31P NMR (CDCl3): δ 127.7. 1H NMR (CDCl3): δ 1.36 (d, 3JH−P = 16.4 Hz, 9H, t-Bu), 1.77 (s, 3H, CH3), 1.81 (s, 3H, CH3), 3.39 (m, 2H, CH), 4.64− 4.65 (m, 1H, CH), 7.12−7.14, 7.40−7.48 (m, 5H, Ph CH). 13C NMR (CDCl3): δ 15.34 (d, 3JC−P = 4.0 Hz, CH3), 15.48 (d, 3JC−P = 3.8 Hz, CH3), 29.29 (d, 2JC−P = 1.5 Hz, 3CH3), 37.21 (d, 1JC−P = 38.0 Hz, PC-t-Bu), 53.81 (d, 1JC−P = 42.3 Hz, P-CH), 55.14 (d, 2JC−P = 13.1 Hz, CH), 56.20 (d, 2JC−P = 17.1 Hz, C-Br), 56.26 (d, 1JC−P = 41.5 Hz, P-CH), 126.33 (s, Ph CH), 129.21 (s, Ph CH), 129.40 (s, Ph CH), 131.30 (s, Ph C), 133.34 (d, 2JC−P = 5.5 Hz, C-CH3), 133.98 (d, 2JC−P = 7.2 Hz, C-CH3), 171.34 (d, 3JC−P = 10.0 Hz, CO), 172.93 (d, 3JC−P = 14.6 Hz, CO). HRMS: Calcd for C20H24BrNO2PS: 452.0449, found 452.0424. Synthesis of 5c. A solution of 1,3,4-trimethylphosphole sulfide (0.87 g, 5.5 mmol) and 3-bromo-N-phenylmaleimide (1.39 g, 5.5 mmol) in 25 mL of xylene was stirred for 11 h at 130 °C. The mixture was chromatographed on silica gel with 7:1 hexane/ethyl acetate and then 3:1 hexane/ethyl acetate mixtures as the eluent to give 5c as a white solid (0.51 g, 22% yield). 31P NMR (CDCl3): δ 103.3. 1H NMR (CDCl3): δ 1.79 (s, 3H, CH3), 1.82 (s, 3H, CH3), 1.90 (d, 2JH−P = 12.4 Hz, 3H, CH3), 3.33−3.38 (m, 2H, CH), 4.53−4.55 (m, 1H, CH), 7.12−7.14, 7.41−7.50 (m, 5H, Ph CH). 13C NMR (CDCl3): δ 14.92 (d, 1JC−P = 56.5 Hz, P-CH3), 15.32 (d, 3JC−P = 3.2 Hz, CH3), 15.39 (d, 3JC−P = 3.4 Hz, CH3), 54.50 (d, 2JC−P = 16.1 Hz, P-CH), 55.00 (d, 1JC−P = 48.0 Hz, CH), 55.01 (d, 2JC−P = 20.0 Hz, C-Br), 58.16 (d, 1JC−P = 47.9 Hz, P-CH), 126.32 (s, Ph CH), 129.28 (s, Ph CH), 129.43 (s, Ph CH), 131.21 (s, Ph C), 133.71 (d, 2JC−P = 5.9 Hz, C-CH3), 134.90 (d, 2JC−P = 7.9 Hz, C-CH3), 171.29 (d, 3JC−P = 11.1 Hz, CO), 172.90 (d, 3JC−P = 16.3 Hz, CO). HRMS: Calcd for C17H18NO2PSBr: 409.9979, found 409.9994. Synthesis of 6a. Precursor 5a (0.24 g, 0.5 mmol) and an excess of 2,3-dimethyl-1,3-butadiene (0.57 mL, 5 mmol) were dissolved in 5 mL of dichloromethane, and then Et3N (0.14 mL, 1 mmol) was added. The mixture was stirred at room temperature for 41 h, and the reaction was monitored by 31P NMR (δ 31P 4a −1.9 ppm). Then, the solution of W(CO)5(CH3CN) (prepared from 0.5 mmol of W(CO)6 and 0.5 mmol of Me3NO·2H2O) in THF was added, and the mixture was stirred for 3 h at room temperature. The mixture was chromatographed on silica gel with hexane as the eluent and then with a 20:1 hexane/CH2Cl2 mixture to give 6a as a blue solid (0.12 g, 44% yield). 31P NMR (CDCl3): δ 15.5 (JP−W = 247 Hz). 1H NMR (CDCl3): δ 1.57 (s, 3H, CH3), 1.68 (s, 3H, CH3), 2.90−3.13 (m, 4H, 2CH2), 7.34−7.37, 7.41−7.44, 7.53−7.57 (m, 5H, Ph CH). 13C NMR (CDCl3): δ 20.02 (s, CH3), 21.82 (d, 3JC−P = 4.6 Hz, CH3), 32.98 (d, 2JC−P = 2.8 Hz, CH2), 38.68 (d, 1JC−P = 17.5 Hz, CH2), 125.81 (d, 3JC−P = 9.2 Hz, C), 127.05 (d, 2JC−P = 10.4 Hz, C), 128.50 (d, 3 JC−P = 9.3 Hz, Ph CH), 129.52 (d, 2JC−P = 11.9 Hz, Ph CH), 129.97 (d, 4JC−P = 1.9 Hz, Ph CH), 136.44 (d, 1JC−P = 27.4 Hz, P-C), 196.37 (d, JC−P = 6.7 Hz, cis-CO), 199.65 (d, JC−P = 25.2 Hz, trans-CO). HRMS: Calcd for C17H15O5PSW: 545.9887, found 545.9884. Synthesis of 6b. Et3N (0.10 mL, 0.74 mmol) was dropped into a solution of 5b (0.17 g, 0.37 mmol) and an excess of 2,3-dimethyl-1,3butadiene (0.42 mL, 3.7 mmol) in 5 mL of toluene at 110 °C. The reaction was finished in 1.5 h as monitored by 31P NMR (δ 31P 4b 25.8 ppm). Then, the solution of W(CO)5(CH3CN) (prepared from 0.5 mmol of W(CO)6 and 0.5 mmol of Me3NO·2H2O) in THF was added, and the mixture was stirred for 15 min at 90 °C. The mixture was chromatographed on silica gel with hexane as the eluent and then a 20:1 hexane/CH2Cl2 mixture to give 6b as a blue solid (21 mg, 11% yield). 31 P NMR (CDCl3): δ 55.2 (JP−W = 246 Hz). 1H NMR (CDCl3): δ 1.26
Figure 4. X-ray crystal structure of (7a). Main distances (Å) and angles (deg): P1−P2 2.2137(16), P2−S1 2.0852(16), S1−C2 1.888(4), C2−C1 1.540(6), C1−P1 1.838(4), P1−S2 1.9431(16), P1−C6 1.822(4), P2−C12 1.834(4), C1−C2 1.540(6), C2−C3 1.491(6), C3−C4 1.327(6), C4−C5 1.498(7), C5−C1 1.553(6); C1−P1−P2 101.08(15), P1−P2−S1 90.67(6), P2−S1−C2 104.47(15), S1−C2−C1 114.1(3), C2−C1−P1 113.1(3).
(Ph−P−P−Ph dihedral angle 178.3°), and the C1−P1 and C2− S1 form a dihedral angle of 37°. We propose the mechanism depicted in eq 4 for the formation of 7.
The normal [4 + 2] adduct 8 is strained as a result of its bicyclic structure. Another molecule of [PhPS] inserts into the P−C intracyclic bond to give 9. Here, [PhPS] works as a carbene. A sulfur [1,3] shift completes the process. It is clear that the reaction of triethylamine with 5 provides a convenient access to phosphinidene sulfides at various temperatures according to the best choice for the subsequent trapping reaction. This approach allows the synthesis of the genuine [4 + 2] cycloadducts with conjugated dienes for the first time. In the case of cyclopentadiene, the insertion of a second molecule of phosphinidene sulfide generates a new type of bicyclic compound.
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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, an AV 400 MHz, or an AV 500 MHz 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. Synthesis of 5a. A solution of 3,4-dimethyl-1-phenylphosphole sulfide (0.66 g, 3.0 mmol) and 3-bromo-N-phenylmaleimide (0.83g, 3.3 mmol) in 12 mL of xylene was stirred for 6 h at 140 °C in a sealed tube. The mixture was chromatographed on silica gel with 7:1 hexane/ ethyl acetate and then 3:1 hexane/ethyl acetate mixtures as the eluent to give 5a as a white solid (0.30 g, 21% yield). 31P NMR (CDCl3): δ 106.0. 1H NMR (CDCl3): δ 1.559 (s, 3H, CH3), 1.562 (s, 3H, CH3), 3.67−3.73 (m, 2H, CH), 4.66−4.67 (m, 1H, CH), 7.12, 7.14, 7.39− 7.55 (m, 10H, Ph-CH). 13C NMR (CDCl3): δ 15.31 (d, 3JC−P = 3.0 Hz, CH3), 15.43 (d, 3JC−P = 3.1 Hz, CH3), 54.42 (d, 1JC−P = 48.7 Hz, P-CH), 54.51 (d, 2JC−P = 17.3 Hz, CH), 55.10 (d, 2JC−P = 21.5 Hz, C-Br), 57.54 (d, 1JC−P = 49.0 Hz, P-CH), 126.33 (s, N-Ph CH), 129.12 (d, 2JC−P = 11.7 Hz, P-Ph CH), 129.11 (d, 3JC−P = 10.6 Hz, P-Ph CH), 5616
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(d, 3JH−P = 16.0 Hz, 9H, 3CH3), 1.82 (s, 3H, CH3), 2.00 (s, 3H, CH3), 2.89−3.21 (m, 4H, 2CH2). 13C NMR (CDCl3): δ 19.09 (d, 4JC−P = 2.3 Hz, CH3), 21.22 (d, 3JC−P = 3.3 Hz, CH3), 25.68 (d, 2JC−P = 6.6 Hz, 3CH3), 33.13 (d, 1JC−P = 5.9 Hz, CH2), 34.56 (d, 2JC−P = 4.5 Hz, CH2), 37.52 (d, 1JC−P = 7.5 Hz, P-C), 126.34 (d, 3JC−P = 7.1 Hz, C), 132.99 (d, 2JC−P = 8.0 Hz, C), 197.29 (d, JC−P = 6.7 Hz, cis-CO), 199.45 (d, JC−P = 26.1 Hz, trans-CO). HRMS: Calcd for C15H19O5PSW: 526.0200, found 526.0191. Synthesis of 6c. The same procedure as for 6b from 5c (0.21 g, 0.5 mmol). Intermediate formation of 4c (δ 31P −9.0 ppm). 6c was obtained as a blue solid (83 mg, 34% yield). 31P NMR (CDCl3): δ 4.87 (JP−W = 246 Hz). 1H NMR (CDCl3): δ 1.80 (d, 2JH−P = 5.6 Hz, 3H, CH3), 1.86 (s, 3H, CH3), 1.89 (s, 3H, CH3), 2.70 (m, 2H, CH2), 3.18 (m, 2H, CH2). 13C NMR (CDCl3): δ 19.85 (d, 4JC−P = 1.2 Hz CH3), 22.21 (d, 3JC−P = 4.2 Hz, CH3), 22.40 (d, 1JC−P = 17.5 Hz, CH3), 32.45 (d, 2JC−P = 3.4 Hz, CH2), 39.10 (d, 1JC−P = 17.4 Hz, CH2), 125.67 (d, 3 JC−P = 9.4 Hz, C), 127.36 (d, 2JC−P = 10.8 Hz, C), 196.33 (d, JC−P = 6.9 Hz, cis-CO), 199.53 (d, JC−P = 24.1 Hz, trans-CO). HRMS: Calcd for C12H13O5PSW: 483.9731, found 483.9739. Synthesis of 7a. Et3N (0.28 mL, 2 mmol) was dropped into a solution of 5a (0.47 g, 1.0 mmol) and an excess of cyclopentadiene (0.82 mL, 10 mmol) in 6 mL of dichloromethane at room temperature. The reaction was finished in 11 h as monitored by 31P NMR. The mixture was chromatographed on silica gel with 2:1 hexane/ CH2Cl2 as the eluent to give 7a as a white solid (30 mg, 17% yield). 31 P NMR (CDCl3): δ 100.3 (d, 1JP−P = 277 Hz, PS), −15.8 (d, P− S). 1H NMR (CDCl3): δ 2.55−2.62 (m, 2H, CH2), 3.49−3.56 (m, 1H, P-CH), 4.92−4.98 (m, 1H, S-CH), 5.44−5.45 (m, 1H, CH), 5.89− 5.90 (m, 1H, CH), 7.30−7.37, 7.48−7.56, 7.73−7.77, 8.06−8.11 (m, 10H, Ph CH). 13C NMR (CDCl3): δ 36.72 (d, 2JC−P = 1.8 Hz, CH2), 47.76 (dd, 1JC−P = 37.6 Hz, 2JC−P = 5.3 Hz P-CH), 63.48 (dd, 2 JC−P = 8.3 Hz, 2JC−P = 6.0 Hz, S-CH), 127.39 (dd, JC−P = 5.7 Hz, JC−P = 2.6 Hz, Ph CH), 128.76 (d, JC−P = 11.7 Hz, Ph CH), 128.80 (dd, 1JC−P = 43.3 Hz, JC−P = 2.4 Hz, P−C), 129.43 (d, JC−P = 3.1 Hz, Ph CH), 130.44 (d, JC−P = 7.0 Hz, CH), 131.02 (dd, JC−P = 9.4 Hz, JC−P = 3.9 Hz, Ph CH), 131.74−131.84 (d + dd, CH + Ph CH), 132.54 (dd, JC−P = 16.9 and 3.3 Hz, Ph CH), 134.21 (dd, 1JC−P = 69.8 Hz, JC−P = 18.0 Hz, P-C). HRMS: Calcd for C17H17P2S2: 347.0247, found 347.0247.
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(5) Tomioka, H.; Nakamura, S.; Ohi, T.; Izawa, Y. J. Chem. Soc., Perkin Trans. 2 1980, 1017. (6) Holand, S.; Mathey, F. J. Org. Chem. 1981, 46, 4386. (7) Tomioka, H.; Miura, S.; Izawa, Y. Tetrahedron Lett. 1983, 24, 3353. (8) Santini, C. C.; Fischer, J.; Mathey, F.; Mitschler, A. J. Am. Chem. Soc. 1980, 102, 5809. (9) This question is discussed by: Schoeller, W. W. In Multiple Bonds and Low Coordination in Phosphorus Chemistry; Regitz, M., Scherer, O. J., Eds.; Thieme: Stuttgart, 1990; pp 5−32. (10) 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.; Al-Laham, M. A., Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003. (11) Kashman, Y.; Wagenstein, I.; Rudi, A. Tetrahedron 1976, 32, 2427. (12) Sahoo, M. K.; Mhaske, S. B.; Argade, N. P. Synthesis 2003, 346. (13) Lam, W. H.; Gaspar, P. P.; Hrovat, D. A.; Trieber, D. A.; Davidson, E. R.; Borden, W. T. J. Am. Chem. Soc. 2005, 127, 9886. (14) The complete characterization of 2 is reported in: Compain, C.; Donnadieu, B.; Mathey, F. Organometallics 2005, 24, 1762.
ASSOCIATED CONTENT
S Supporting Information *
X-ray crystal structure analyses of 5b, 6a, and 7, and NMR spectra of compounds 5, 6, and 7. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (F.M.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS L.W. thanks Henan Science and Technology Department (No. 144300510011) and Zhengzhou University in China, and R.G. and F.M. thank Nanyang Technological University in Singapore for the financial support of this work.
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REFERENCES
(1) Review: Yoshifuji, M. Sci. Synth. 2009, 42, 39−44. (2) Nakayama, S.; Yoshifuji, M.; Okazaki, R.; Inamoto, N. J. Chem. Soc. D 1971, 1186. (3) Nakayama, S.; Yoshifuji, M.; Okazaki, R.; Inamoto, N. Bull. Chem. Soc. Jpn. 1975, 48, 546. (4) Gaspar, P. P.; Qian, H.; Beatty, A. M.; d’Avignon, D. A.; Kao, J. L.-F.; Watt, J. C.; Rath, N. P. Tetrahedron 2000, 56, 105. 5617
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