Communication pubs.acs.org/JACS
Low-Valent Lead Hydride and Its Extreme Low-Field 1H NMR Chemical Shift Julia Schneider, Christian P. Sindlinger, Klaus Eichele, Hartmut Schubert, and Lars Wesemann* Institut für Anorganische Chemie der Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany S Supporting Information *
phosphine Lewis pair 1 (Scheme 1), we also speculated on the intermediate formation of an aryllead hydride.27 Recently, J.
ABSTRACT: Although hydrides of the group 14 elements are well-known as versatile starting materials in many chemical transformations, a hydride of lead in oxidation state II is so far unknown. In this work, we finally complete the jigsaw puzzle by reporting the isolation of the first low valent organolead hydride. The thermolabile dimeric organolead hydride was synthesized at low temperature and features a hydride 1H NMR signal (in solution 35.61 ppm; in the solid state 31.1 ppm) at the lowest field observed so far for a diamagnetic compound in agreement with quantum chemical predictions.
Scheme 1. Preparation of the Lead Hydride 2a
I
n the series of group 14 hydrides MH4 (M = Si, Ge, Sn, Pb), the endothermic character of these volatile compounds is increasing with the increasing atomic number of the central atom.1 Silane, germane and stannane are well characterized substances, which are highly reactive in contact with air but under the exclusion of air and moisture are thermostable up to 300 °C in the case of silane (GeH4 285 °C; SnH4 150 °C).1,2 Due to the fast decomposition of PbH4 even at low temperatures, lead tetrahydride was characterized by means of matrix experiments using IR spectroscopy at 3.5 K or by gasphase IR spectroscopy.3 Organic derivatives of Pb(IV) hydrides like Me3PbH or Et2PbH2, which are reported to be stable only below −20 °C, were synthesized in situ and used for hydroplumbation reactions.4,5 Characterization of Me3PbH by means of solution NMR spectroscopy was carried out at −50 °C to give a 1H NMR signal for the Pb−H hydride at 7.68 ppm with a 1J207PbH coupling of 2379 Hz.6 In the case of the lower oxidation state M(II) (M = Si−Pb), the dihydrides MH2 were also studied by matrix experiments.3,7,8 Furthermore, silicon, germanium and tin dihydrides were isolated and structurally characterized in the vicinity of an electron acceptor and donor.9−12 Syntheses of divalent group 14 hydrides stabilized by bulky organic substituents or chelating ligands were reported in the last 15 years for silicon, germanium and tin.12−24 Their spectroscopic and structural characterization were followed by an increasing number of recent reactivity studies in which for example low valent tin hydrides act as catalysts in hydroboration or dehydrocoupling reactions.20,25 In contrast to these elaborate studies on group 14 hydrides, an organolead(II) hydride was so far not isolated and characterized. Power’s group postulated an organolead(II) hydride as a possible intermediate in the formation of the diplumbyne from the reaction of aryllead bromide with lithium aluminum hydride.26 In the course of our hydroboration studies with plumbylene © 2017 American Chemical Society
a
ORTEP of the molecular structure of 2 with thermal ellipsoids at 50% level, isopropyl groups and hydrogen atoms except hydrides have been omitted for clarity. [Trip = C6H2-2,4,6-iPr3].53
Vı ́cha, M. Straka and co-worker presented a study on density functional calculations of 1H NMR chemical shifts of Sn(II) and Pb(II) hydrides.28 In this publication, 1H NMR chemical shift calculations of different isomers of the aryl lead hydride (Ar′PbH)2 were presented and the hydride resonances were predicted in these calculations at very low field: Ar′Pb(μH)2PbAr′ 30.8 ppm; Ar′Pb−PbH2Ar′ 40.6 ppm (Ar′ = C6H32,6-Mes2; Mes = C6H2-2,4,6-Me3), outside the established chemical shift range from +20 to −60 ppm.29 The authors concluded from relativistic DFT calculations that the large spin−orbit effects induced by heavy atoms are responsible for the high frequency signals.28,30−36 However, to the best of our knowledge these lead hydrides have neither been isolated nor has their intermediate formation been spectroscopically proven so far. We are studying the chemistry of intramolecular Lewis pairs between low valent group 14 fragments and phosphine moieties.27,37−41 The reactivities of these ylenes of germanium, tin and lead have been studied with respect to coordination properties and catalytic hydroboration of aldehydes and ketones.27,39−41 During the course of the hydroboration studies with plumbylene [Ar*PbCHPh(PPh2)] 1 (Ar* = C6H3-2,6Trip2; Trip = C6H2-2,4,6-iPr3), we found in reaction with alkoxyboranes the formation of diplumbyne [(Ar*Pb)2] 4 and evolution of hydrogen.26,27 We speculate that the Pb−C bond in compound 1 between the phosphinobenzyl unit and lead was Received: February 27, 2017 Published: April 25, 2017 6542
DOI: 10.1021/jacs.7b01856 J. Am. Chem. Soc. 2017, 139, 6542−6545
Communication
Journal of the American Chemical Society hydroborated to give the isolated boryl-phosphinylmethane [catBCHPh(PPh2)] 3 (cat = o-C6H4O2), and, along the lines of Power, lead hydride [(Ar*PbH)2] 2 as a putative intermediate on the way to the diplumbyne 4 (Schemes 1 and 2).26 This Scheme 2. Derivatization of 2a,b
Figure 1. (a) 1H NMR (500.13 MHz) spectra of 2 in solution, top 298 K, 1J207PbH = 734 Hz (35.61 ppm); (bottom) 193 K, 1J207PbH = 725 Hz (35.83 ppm), 1J207PbH = 649 Hz (31.43 ppm); (b) 1H solid state MAS NMR (500.13 MHz) spectra of 2, Pb−H: δiso = 31.1 ppm; (bottom) simulated spectrum. Asterisks denote spinning side bands, spinning frequency: 25 kHz.
the major low temperature isomer at 3736 ppm showing the hydride signal at 31.43 ppm. In the case of the minor isomer showing the hydride signal at 35.83 ppm, we are only able to give the range (4400−4500 ppm) for the expected 207Pb signal, based on a selectively 207Pb decoupled 1H NMR experiment. 207 Pb chemical shifts cover a broad range of approximately 17 000 ppm, from +11000 to −6000. Nonstabilized monomeric plumbylenes carrying alkyl, aryl or silyl substituents show signals in the 207Pb NMR spectrum at low field beyond 3800 ppm.43−46 In the case of a cyclotriplumbane as well as stabilized plumbylenes carrying amido substitutents or a further Lewis base donor the resonance in the 207Pb NMR spectrum is shifted to higher field (+5400 to −1500 ppm).46−51 The two shifts observed for the two isomers of 2 lying in a narrow region around 4000 ppm corroborate the triply coordinated hydrogen bridged plumbylene. The close proximity of both 207Pb chemical shifts supports the assumption of two isomers of similar coordination environments around Pb (vide infra).52 Due to the thermolability of 2 and similar solubilities of 2 and 3 their separation by fractional crystallization is challenging. A sample of 12 mg of the crystalline hydride 2, obtained from toluene/hexane by slow diffusion at −40 °C (sample was slightly contaminated with 3), was also characterized by solid state magic angle spinning (MAS) 1H NMR spectroscopy. Due to the substantial low field shift of the hydride protons of 2 we were able to identify this signal also in the solid state NMR spectrum (Figure 1, right). The 1H MAS NMR signal for the lead hydride protons was found at 31.1 ppm, which is close to the chemical shift of the major low temperature isomer (the line width of 800 Hz prevents the detection of the 207Pb satellites). The crystal structure analysis of 2 reveals (vide infra) the typical trans-arrangement of the aryl substituents. We conclude that this isomer corresponds to the 1H MAS NMR signal detected in the solid state sample (31.1 ppm) and the signal of the major isomer at 31.43 ppm in solution at low temperature. Furthermore, the found geometry is in good agreement with results of quantum chemical calculations of RPbH-dimers {R = H, Me, Ph, [C6H3-2,6-(C6H2-2,4,6-Me3)2]} exhibiting the μ2-hydrogen-bridged dimer with trans-position of the substituents R to be the most stable Pb(II) isomer in all
a
[Trip = C6H2-2,4,6-iPr3];53 reaction of the hydride 2 with MeNHC; ORTEP of the molecular structure of 5 with thermal ellipsoids at 50% level, isopropyl groups and hydrogen atoms except the hydride atom have been omitted for clarity. bThermal decomposition of 2 to 4 studied by NMR spectroscopy, at 10 °C t0.5 ≈ 7.6h.
type of Pb−C hydroborolysis is known for Sn−N and Sn−O bonds from the groups of Jones and Power as a method to synthesize tin(II) hydrides.20,42 To isolate the putative lead(II) hydride, we reinvestigated the reaction mixture between 1 and catecholborane below −20 °C to circumvent decomposition of a possibly formed lead hydride (Scheme 1). On the basis of NMR spectroscopic investigations, we can conclude that the lead hydride 2 is formed quantitatively. The reaction product was isolated as yellow crystals after careful repetitive crystallization at −40 °C from a toluene solution of the mixture between hydride 2 and phosphine 3 by slow diffusion of hexane in an overall yield of 55%. In the room temperature 1 H NMR spectrum of isolated crystals of 2, the signal of the lead hydride was found at very low field at +35.61 ppm exhibiting 207Pb satellites with a coupling constant 1J207PbH of 734 Hz (Figure 1a, top). The integration ratios of these satellites is indicative of coupling of the hydride with two equivalent lead atoms confirming a symmetric dimeric structure in solution. The coupling constant of 1J207PbH of 734 Hz in 2 is considerably smaller than the Pb−H coupling found in Me3Pb−H of 2379 Hz.6 At −80 °C, the hydride 1H NMR resonance splits into two signals at 35.83 and 31.43 ppm (ratio 1:4), which belong to isomers of the dimeric lead hydride (Figure 1a, bottom). Both signals show satellites due to coupling with two equivalent lead atoms and coupling constants (35.83 ppm: 725 Hz, 31.43 ppm: 649 Hz), which are closely related to the room temperature value. On the basis of 1H EXSY NMR experiments, these isomers show slow exchange with each other. To characterize both isomers of 2 also by their respective signal in the 207Pb NMR spectrum in solution, we recorded the 207Pb NMR spectrum of the hydride at −80 °C. On the basis of a 1H-207Pb HMQC NMR experiment, we were able to detect the 207Pb NMR signal for 6543
DOI: 10.1021/jacs.7b01856 J. Am. Chem. Soc. 2017, 139, 6542−6545
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Journal of the American Chemical Society cases.28,54 In agreement to these findings, all other isomers probed computationally in this study (R = Ar*, see SI) are higher in energy and are unlikely to cause the second 1H NMR signal observed in solution at low temperatures. Moreover the lower symmetry of these species in terms of their Pb2H2 moiety does not coincide with the symmetry indicated by the isotopomeric distribution of this 1H NMR signal. We suggest rotamers, with respect to terphenyl rotation around the Pb−C bonds enclosing the preserved Pb(μ-H)2Pb moiety to cause the observed second signal set. Please note that along the lines of computational energy approximations the dimerization is strongly favored (ΔE ≈ − 40 kcal mol−1). No indications for the monomeric species have been observed spectroscopically. Furthermore, we were also able to carry out a single crystal X-ray diffraction structure analysis of 2 featuring a dinuclear lead core with a Pb−Pb distance of 3.2646(2) Å and the Ar* substituent exhibiting C−Pb−Pb angles of 92.4(1)° (Scheme 1). The structure of the diplumbyne [Ar*Pb]2 is closely related to the hydride structure of 2 showing a Pb−Pb distance of 3.1881(1) Å and a C−Pb−Pb angle of 94.26(4)°.26 In the final Fourier map, we found electron density in bridging position between the lead atoms that could be interpreted as the position of the hydride atoms. This electron density was refined isotropically as the positions of the hydride atoms [Pb−H 1.98(4), Pb′−H 2.03(4) Å]. Details of the structure analysis are presented in the SI. By comparison of the IR spectrum of a solid sample of the hydride 2 and the deuteride 2-D with results of quantum chemical calculation, we were able to identify the Pb−H vibrations in the range from 900 to 1200 cm−1 (spectrum see SI). Andrews has reported bridging Pb−H vibrations of the matrix isolated dimeric lead dihydride [HPb(μ-H)] 2 around 1000 cm −1 .3 Comparable Sn−H vibrations were also found in the homologous tin hydrides.55 We also investigated the reaction mixture between 2 and boryl phosphine 3. No interaction of the nucleophilic phosphine unit with the lead hydride was found in this mixture: we have detected the same hydride resonance in the mixture between 2 and 3 as for the phosphine free sample of 2. In an attempt to cleave the dimeric hydride by a stronger nucleophile we reacted dimer 2 with an N-heterocyclic carbene (MeNHC = 1,3,4,5-tetramethylimidazol-2-ylidene) (Scheme 2).56 In reaction with an excess (>1.5 equiv) of NHC donor, we were able to convert reproducibly the dimeric hydride 2 completely into the NHC adduct 5 [Ar*PbH(MeNHC)]. In reaction of 2 with one equivalent NHC, we found formation of an equilibrium mixture between 2 and 5. Adduct 5 features a diagnostic signal in the 1H NMR spectrum at low field: + 23.81 ppm exhibiting satellites due to coupling with one lead nucleus (1J207PbH = 955 Hz, spectrum see SI). The respective lead signal was found at +834 ppm in the range known for Lewis base adducts of plumbylenes.46 So far, isolation of compound 5 from the mixture was unsuccessful making further analytics unfeasible. However, a very small amount of single crystals of 5 could be separated and the result of the single crystal structure determination was placed in the SI and the molecular structure is shown in Scheme 2. The distances Pb−C1 2.332(2) Å and Pb−C36 2.411(2) Å can be compared with the bond lengths found for the iPrNHC adduct of Pb(Trip)2.57 The sum of angles around the lead atom of 279° is indicative for the trigonal pyramidal coordination at lead. In conclusion, we present the first low valent organolead hydride, which was prepared following a challenging protocol at temperatures around −40 °C, thereby filling in one of the few
last empty blanks within the main group element hydrides. We can also experimentally confirm the assumption that lead(II) hydrides are intermediates in the diplumbyne formation following Power’s hydride protocol. Furthermore, the lead hydride features hydride 1H NMR signals in solution as well as in the solid state at the lowest field detected so far for diamagnetic compounds.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01856. Experimental procedures, characterizations and analytical data, subsequent reaction of 2 at 10 °C, crystal structure analysis of 2 and 5 (CCDC reference number: 1531247, 1541061), IR spectra of 2 vs [Ar*PbD]2 2-D, computational details: structure optimization, frequency calculation and thermochemical approximations (PDF) Data for C39H57Pb (CIF) Data for C43H62N2Pb (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Lars Wesemann: 0000-0003-4701-4410 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.S. is grateful to the Landesgraduiertenförderung of BadenWürttemberg for a fellowship. Computational resources were provided by JUSTUS hpc BW facilities in Ulm.
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REFERENCES
(1) Holleman, A.; Wiberg, E. Lehrbuch für Anorganische Chemie, 102nd ed.; de Gruyter, 2007; pp 918−1041. (2) Rivard, E. Chem. Soc. Rev. 2016, 45, 989−1003. (3) Wang, X.; Andrews, L. J. Am. Chem. Soc. 2003, 125, 6581−6587. (4) Juenge, E. C.; Hawkes, S. J.; Snider, T. E. J. Organomet. Chem. 1973, 51, 189−195. (5) Neumann, W. P.; Kühlein, K. Angew. Chem., Int. Ed. Engl. 1965, 4, 784−785. (6) Flitcroft, N.; Kaesz, H. D. J. Am. Chem. Soc. 1963, 85, 1377− 1380. (7) Wang, X.; Andrews, L.; Chertihin, G. V.; Souter, P. F. J. Phys. Chem. A 2002, 106, 6302−6308. (8) Yamada, C.; Kanamori, H.; Hirota, E.; Nishiwaki, N.; Itabashi, N.; Kato, K.; Goto, T. J. Chem. Phys. 1989, 91, 4582−4586. (9) Al-Rafia, S. M. I.; Malcolm, A. C.; Liew, S. K.; Ferguson, M. J.; Rivard, E. J. Am. Chem. Soc. 2011, 133, 777−779. (10) Bharatam, P. V.; Moudgil, R.; Kaur, D. Organometallics 2002, 21, 3683−3690. (11) Swarnakar, A. K.; McDonald, S. M.; Deutsch, K. C.; Choi, P.; Ferguson, M. J.; McDonald, R.; Rivard, E. Inorg. Chem. 2014, 53, 8662−8671. (12) Al-Rafia, S. M. I.; Malcolm, A. C.; McDonald, R.; Ferguson, M. J.; Rivard, E. Chem. Commun. 2012, 48, 1308−1310. (13) Eichler, B. E.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 8785− 8786. (14) Richards, A. F.; Phillips, A. D.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 3204−3205. (15) Peng, Y.; Brynda, M.; Ellis, B. D.; Fettinger, J. C.; Rivard, E.; Power, P. P. Chem. Commun. 2008, 6042−6044. 6544
DOI: 10.1021/jacs.7b01856 J. Am. Chem. Soc. 2017, 139, 6542−6545
Communication
Journal of the American Chemical Society (16) Ding, Y.; Hao, H.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.G. Organometallics 2001, 20, 4806−4811. (17) Pineda, L. W.; Jancik, V.; Starke, K.; Oswald, R. B.; Roesky, H. W. Angew. Chem., Int. Ed. 2006, 45, 2602−2605. (18) Jana, A.; Roesky, H. W.; Schulzke, C.; Döring, A. Angew. Chem., Int. Ed. 2009, 48, 1106−1109. (19) Hadlington, T. J.; Hermann, M.; Li, J.; Frenking, G.; Jones, C. Angew. Chem., Int. Ed. 2013, 52, 10199−10203. (20) Hadlington, T. J.; Hermann, M.; Frenking, G.; Jones, C. J. Am. Chem. Soc. 2014, 136, 3028−3031. (21) Jana, A.; Leusser, D.; Objartel, I.; Roesky, H. W.; Stalke, D. Dalton Trans. 2011, 40, 5458−5463. (22) Jungton, A.-K.; Meltzer, A.; Prasang, C.; Braun, T.; Driess, M.; Penner, A. Dalton Trans. 2010, 39, 5436−5438. (23) Abraham, M. Y.; Wang, Y.; Xie, Y.; Wei, P.; Schaefer, H. F.; Schleyer, P. v. R.; Robinson, G. H. J. Am. Chem. Soc. 2011, 133, 8874− 8876. (24) Rodriguez, R.; Gau, D.; Contie, Y.; Kato, T.; Saffon-Merceron, N.; Baceiredo, A. Angew. Chem. 2011, 123, 11694−11697. (25) Erickson, J. D.; Lai, T. Y.; Liptrot, D. J.; Olmstead, M. M.; Power, P. P. Chem. Commun. 2016, 52, 13656−13659. (26) Pu, L.; Twamley, B.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 3524−3525. (27) Schneider, J.; Sindlinger, C. P.; Freitag, S. M.; Schubert, H.; Wesemann, L. Angew. Chem., Int. Ed. 2017, 56, 333−337. (28) Vícha, J.; Marek, R.; Straka, M. Inorg. Chem. 2016, 55, 10302− 10309. (29) Mason, J. Multinuclear NMR.; Plenum Press: New York, NY, 1987. (30) Maldonado, A. F.; Aucar, G. A. Phys. Chem. Chem. Phys. 2009, 11, 5615−5627. (31) Greif, A. H.; Hrobárik, P.; Hrobáriková, V.; Arbuznikov, A. V.; Autschbach, J.; Kaupp, M. Inorg. Chem. 2015, 54, 7199−7208. (32) Hrobárik, P.; Hrobáriková, V.; Meier, F.; Repiský, M.; Komorovský, S.; Kaupp, M. J. Phys. Chem. A 2011, 115, 5654−5659. (33) Kaupp, M.; Malkina, O. L.; Malkin, V. G.; Pyykkö, P. Chem. Eur. J. 1998, 4, 118−126. (34) Vícha, J.; Marek, R.; Straka, M. Inorg. Chem. 2016, 55, 1770− 1781. (35) Vícha, J.; Foroutan-Nejad, C.; Pawlak, T.; Munzarová, M. L.; Straka, M.; Marek, R. J. Chem. Theory Comput. 2015, 11, 1509−1517. (36) Vícha, J.; Straka, M.; Munzarová, M. L.; Marek, R. J. Chem. Theory Comput. 2014, 10, 1489−1499. (37) Freitag, S.; Henning, J.; Schubert, H.; Wesemann, L. Angew. Chem., Int. Ed. 2013, 52, 5640−5643. (38) Freitag, S.; Krebs, K. M.; Henning, J.; Hirdler, J.; Schubert, H.; Wesemann, L. Organometallics 2013, 32, 6785−6791. (39) Schneider, J.; Krebs, K. M.; Freitag, S.; Eichele, K.; Schubert, H.; Wesemann, L. Chem. - Eur. J. 2016, 22, 9812−9826. (40) Krebs, K. M.; Freitag, S.; Schubert, H.; Gerke, B.; Pöttgen, R.; Wesemann, L. Chem. - Eur. J. 2015, 21, 4628−4638. (41) Krebs, K. M.; Maudrich, J. J.; Wesemann, L. Dalton Trans. 2016, 45, 8081−8088. (42) Rivard, E.; Steiner, J.; Fettinger, J. C.; Giuliani, J. R.; Augustine, M. P.; Power, P. P. Chem. Commun. 2007, 4919−4921. (43) Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P. Organometallics 1997, 16, 1920−1925. (44) Wilfling, P.; Schittelkopf, K.; Flock, M.; Herber, R. H.; Power, P. P.; Fischer, R. C. Organometallics 2015, 34, 2222−2232. (45) Stürmann, M.; Weidenbruch, M.; Klinkhammer, K. W.; Lissner, F.; Marsmann, H. Organometallics 1998, 17, 4425−4428. (46) Arp, H.; Baumgartner, J.; Marschner, C.; Zark, P.; Müller, T. J. Am. Chem. Soc. 2012, 134, 6409−6415. (47) Stabenow, F.; Saak, W.; Marsmann, H.; Weidenbruch, M. J. Am. Chem. Soc. 2003, 125, 10172−10173. (48) Charmant, J. P. H.; Haddow, M. F.; Hahn, F. E.; Heitmann, D.; Fröhlich, R.; Mansell, S. M.; Russell, C. A.; Wass, D. F. Dalton Trans. 2008, 6055−6059.
(49) Tam, E. C. Y.; Maynard, N. A.; Apperley, D. C.; Smith, J. D.; Coles, M. P.; Fulton, J. R. Inorg. Chem. 2012, 51, 9403−9415. (50) Yao, S.; Block, S.; Brym, M.; Driess, M. Chem. Commun. 2007, 3844−3846. (51) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Dalton Trans. 2000, 3094−3099. (52) Wrackmeyer, B.; Horchler, K. 207Pb-NMR Parameters. In Annual Reports on NMR Spectroscopy; Webb, G. A., Ed.; Academic Press, 1990; Vol. 22, pp 249−306. (53) Schiemenz, B.; Power, P. P. Organometallics 1996, 15, 958−964. (54) Trinquier, G. J. Am. Chem. Soc. 1990, 112, 2130−2137. (55) Rivard, E.; Fischer, R. C.; Wolf, R.; Peng, Y.; Merrill, W. A.; Schley, N. D.; Zhu, Z.; Pu, L.; Fettinger, J. C.; Teat, S. J.; Nowik, I.; Herber, R. H.; Takagi, N.; Nagase, S.; Power, P. P. J. Am. Chem. Soc. 2007, 129, 16197−16208. (56) Kuhn, N.; Kratz, T. Synthesis 1993, 1993, 561−562. (57) Stabenow, F.; Saak, W.; Weidenbruch, M. Chem. Commun. 1999, 1131−1132.
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