NOTE pubs.acs.org/Organometallics
Methyl Ester Function: An Intramolecular Electrophilic Trap for the Isolation of Aryltellurenyl Hydroxide and Diorganotellurium Dihydroxide K. Selvakumar,† Harkesh B. Singh,*,† Nidhi Goel,‡ and Udai P. Singh‡ † ‡
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247667, India
bS Supporting Information ABSTRACT: The formation of an aryltellurenyl hydroxide analogue proposed in the telluroxide elimination reaction has been studied. The reaction of the monotelluride dimethyl 5-tertbutyl-2-(butyltellanyl)isophthalate (6) with H2O2 afforded the corresponding aryldiacyloxy-n-butyltellurane (8). The reaction of 6 with oxygen in the presence of moisture resulted in the formation of 8 as the major product along with traces of dimethyl 5-tert-butyl-2-(butyltellurinyl)isophthalate (7) and methyl 5-tert-butyl-3-oxo-3H-benzo[c][1,2]oxa tellurole-7-carboxylate (9). The telluroxide intermediate 7 readily undergoes a facile elimination reaction to give the elusive aryltellurenyl hydroxide, which is immediately trapped by the adjacent methyl ester functionality to give the cyclic tellurenate ester 9. Similarly, the hydrolysis of 7 with H2O affords the corresponding dihydroxytellurane 10, which is immediately converted to tellurane 8. Both compounds 8 and 9 have been characterized by single-crystal X-ray crystallographic studies.
’ INTRODUCTION Organotellurium reagents have received considerable interest in organic transformations due to their facile incorporation into organic molecules via both electrophilic or nucleophilic pathways and further functional group manipulation.1 The important organic transformations include halogenation of olefins,2 oxidation of alcohols,3 and dehydration of nucleosides to 20 ,30 -didehydro-20 ,30 -dideoxynucleosides.4 They have also attracted wide interest in biology due to their interesting redox properties.5 Organotellurium compounds such as ditellurides and monotellurides have been used as glutathione peroxidase (GPx) mimics.6,7 Glutathione peroxidase is a selenoenzyme that catalyzes the reduction of hyroperoxides by using glutathione (GSH). The diorganyl telluroxide 6,60 -telluroxybis(6-deoxy-β-cyclodextrin) (6-TeOdiCD), obtained from 6-TediCD, has been found to show hydrolase mimetic activity.8 Therefore, a detailed understanding of the mechanism of tellurium-centered redox reactions would lead to the design of potential reagents and redox catalysts. The elimination reaction of terminal β-oxy selenoxides is commonplace in olefin synthesis.9 This reaction occurs in a syn stereospecific manner to give the corresponding selenenic acid and olefin. Similarly, β-elimination reaction of unsymmetrical diorganyl telluroxides having sec-alkyl and aryl groups has been considered to be an important step in olefin synthesis1 and bioactivation of Te-phenyl-L-tellurocysteine.10 Lee and Cava, for the first time, studied the nature of the telluroxide elimination reaction using dodecyl-(4-methoxyphenyl) telluroxide.11 The formation of aryltellurenyl hydroxide 1 as an intermediate was r 2011 American Chemical Society
proposed on the basis of the observation of the disproportionation product, diaryl ditelluride.11
Uemura and Fukuzawa described the synthesis of some olefins, allylic alcohols, and allylic ethers via facile elimination reaction of sec-alkyl phenyl telluroxide.12 They proposed that CTe bond fission could probably occur via an intermediate state, 2.
To our knowledge, there has been no report on the isolation and characterization of the aryltellurenyl hydroxide intermediates formed during the β-elimination reaction of diorganyltelluroxides. In a different context, McWinnie and co-workers, for the first time, Received: February 18, 2011 Published: June 22, 2011 3892
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Organometallics Scheme 1. Bertozzi’s Version of the Staudinger Reaction
isolated and characterized an acyclic tellurenate ester, 2-(phenylazo)phenyl-C,N0 )tellurium acetate, using intramolecular Te 3 3 3 N interaction.13 It was obtained by the metathesis of the corresponding aryltellurenyl chloride with sodium acetate. This tellurenate ester could be considered as a protected form of the aryltellurenyl hydroxide 2-(phenylazo)phenyl-C,N0 )tellurium hydroxide. Minkin and co-workers have also reported the synthesis of such stable tellurenate esters and aryltellurenyl hydroxide anhydrides using intramolecular secondary bonding Te 3 3 3 N interaction.14 Very recently, our group has succeeded in the isolation and characterization of an anhydride, bis[(2-(phenylazo)phenyl-C,N0 )tellurium] oxide, which is also stabilized by Te 3 3 3 N secondary bonding interaction.15 This results from the hydrolysis of 2-(phenylazo)phenyl-C,N0 )tellurium chloride followed by the condensation of the resultant (2-(phenylazo)phenyl-C,N0 )tellurenyl hydroxide. Bertozzi’s versions of the Staudinger reaction illustrate the intramolecular aza-ylide transfer as an effective tool for the amide bond formation reaction in aqueous environment (Scheme 1).16 In the presence of water, the aza-ylide undergoes facile hydrolysis to form an amine and the phosphine oxide, which is known as the Staudinger reaction. Bertozzi and co-workers used a suitably placed methyl ester functionality as an electrophilic trap to capture the aza-ylide to form an amide bond. We envisaged that an appropriately positioned electrophilic trap, such as a methyl ester function, around the tellurium center would capture the nucleophilic aryltellurenyl hydroxide by an intramolecular cyclization. At the same time, the unstable cyclic tellurenate ester would be stabilized by an additional functional group that is capable of providing a Te 3 3 3 O secondary bonding interaction. Recently, we have succeeded in the isolation of unstable selenenic acid as cyclic selenenate esters 317 and 418 using the formyl and ester groups respectively as an electrophilic trap and an intramolecular donor group. These compounds (3 and 4) are highly stable under ambient conditions due to the presence of strong secondary bonding Se 3 3 3 O intramolecular interactions. This prompted us to test this hypothesis to isolate the unstable organotellurium analogues, such as aryltellurenyl hydroxide and diorganotellurium dihydroxide. Herein, we describe the isolation and characterization of an aryltellurenyl hydroxide, the presumed intermediate in the telluroxide elimination reaction, as cyclic tellurenate ester. In addition, the trapping of the diorganotellurium dihydroxide as a hypervalent diorganyldiacyloxytellurane is also described.
NOTE
’ RESULTS AND DISCUSSION Synthesis. The SNAr reaction of aryl bromide 5 with n-BuTeLi afforded the monotelluride 6 (Scheme 2). Attempts to purify 6 were unsuccessful due to its facile oxidation, especially under ambient conditions. This compound is reasonably stable under nitrogen in a Schlenk flask at 22 °C. The oxidation of 6 (leads to the formation of 7 and 8, vide infra) in open air is clearly evident from its 1H NMR spectrum. The three expected singlets at 7.74, 3.91, and 1.30 ppm (see Supporting Information, Figure S2) were broadened. The aerial oxidation of telluride 6 is not surprising, and similar aerial oxidations of diaryltelluride have been known in the literature.19 The oxidation process was also revealed from the mass spectral studies. The oxidized species [7 + H]+ (m/z 453.09) and its corresponding tellurone [tellurone Te(VI) + H]+ (m/z 469.09) have been observed in the HRMS(ES) spectrum of 6 ([6 + H]+, m/z 437.10). The relative abundance of the m/z corresponding to the tellurone is ∼35%. The m/z peak for 7 was observed as a base peak with 100% relative abundance. However, the formation of the tellurone was not observed under ambient conditions. Generally, tellurones are prepared under relatively stronger oxidizing conditions using NaIO4 as an oxidizing agent in place of peroxides/molecular oxygen20 or photochemical conditions using singlet oxygen as an oxidizing source.21 The reaction of monotelluride 6 with H2O2 did not afford the expected telluroxide 7 (vide infra). Instead, it resulted in the formation of the hypervalent diorganyldiacyloxytellurane 8 (85%). Tellurane 8 was characterized by routine spectroscopic techniques (1H, 13C, and 125Te NMR). Monotelluride 6, when kept for one week in open air, afforded 8 as the major product along with a trace amount of the telluroxide 7 and the tellurenate ester 9 (7 and 9 were identified by TLC) (Scheme 3). The reaction of 6 with oxygen at 80 °C in the absence of moisture allowed the isolation and purification of the telluroxide 7 and the tellurenate ester 9 by column chromatography. At 110 °C, the reaction afforded the cyclic tellurenate ester 11 in addition to 9. Compound 11 could probably result from the telluroxide 7 via the intermediate 10. The 125Te NMR spectrum of compounds 7 and 9 showed peaks at 1207 and 2280 ppm, respectively. The chemical shift of 7 is slightly shifted upfield as compared to that of the related telluroxide bis[2-(phenylazo)phenyl-C,N0 ]tellurium(IV) oxide (1228 ppm).15 However, other spectral data for 7 could not be obtained due to its strong propensity to form 8 in open air. Isolation of 7 is not reproducible all the time. Available literature data suggest that the aryltellroxides with reactive functional groups such as amide and carboxylic acid around the tellurium center are unstable and undergo rapid hydrolysis followed by intramolecular cyclization to afford the corresponding telluranes.19,22 Compound 11 was characterized by 1 H, 13C, and 125Te NMR and IR spectroscopic techniques. In the 125 Te NMR spectrum, the resonance signal was observed at 2263 ppm, which is in agreement with the chemical shift observed for the tellurenate ester 9 (2280 ppm). The molecular ion ([M + H]+, m/z 407.05) peak of 11 was found as a base peak in its HRMS spectrum. This observation leads us to propose the following mechanism (Scheme 4) for the formation of 8 and 9. Unlike sulfoxides and selenoxides, the telluroxides formed during oxidation are seldom isolated.21 They exist usually in the hydrated form.23 Hence, telluroxide 7 formed during oxidation has the capacity to associate with water molecules,24 leading to the formation of diorganotellurium dihydroxide 12. Tellurane 12 undergoes an intramolecular cyclization to form the diorganyldiacyloxytellurane 8. This clearly indicates that path A is more favored than path B in the presence of moisture. Strict exclusion of moisture diverts the reaction into a different path 3893
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NOTE
Scheme 2. Synthesis of 8a
a
Reagents and conditions: (i) n-BuTeLi, THF, rt, 3 h; (ii) aqueous H2O2, MeOH, 5 h.
Scheme 3. Formation of 9 in Open Air and Aerobic Conditionsa
a
Reagents and conditions: (i) open air, rt, 1 week; (ii) toluene, O2, 80 °C, 24 h; (iii) toluene, O2, 110 °C, 24 h.
(path B), in which the telluroxide undergoes β-elimination to form aryltellurenyl hydroxide 13, which in turn cyclizes to form the lowvalent tellurenate ester 9. The formation of 9 under moisture-free conditions suggests that the CTe bond fission does not require a diorganotellurium dihydroxide intermediate such as that proposed for the β-elimination reaction (2).12 Although β-elimination is one of the reaction paths (path B) for the formation of 9 from 7, the possibility of 1,2-shift of the n-butyl group from tellurium to oxygen cannot be ignored (path C). This reaction would lead to the formation of 10. It is speculated that the n-butanol liberated from 10 upon its exposure to moisture may produce 13, which undergoes facile intramolecular cyclization to form 9. This corroborates the observation of Uemura et al., who have shown the formation of small amounts of 2-phenylethanol from phenylethyl phenyl telluride upon its oxidation with m-chloroperbenzoic acid.25 Compound 9 probably undergoes tellurium-assisted transesterification reaction with n-butanol to form 11. At present we do not know the detailed mechanism of this transestrification process. The butene formed from 7 could not be detected due to its low boiling point and its high volatility. To figure out the formation of olefin, the mononotelluride 14 was synthesized (Scheme 5). The reaction of sodium 2-phenylethanetellurolate with 5 afforded 14. Aerobic oxidation of 14 under anhydrous conditions at 80 °C led to the formation of 9 and styrene. Styrene could be detected in the GC-MS (m/z 104.0, M+; see Supporting Information, p S38). Compound 14 was characterized by routine spectroscopic techniques. HRMS studies on 14 are of particular interest. All the reaction products, styrene ([M + H]+, m/z 105.07), cyclic tellurenate
ester 9 ([M + H]+, m/z 365.10), and hypervalent tellurane 15 ([M + H]+, m/z 455.06), and the telluroxide 16 ([M + H]+, m/z 499.11) intermediate could be identified in the HRMS spectrum.
X-Ray Crystallographic Study. Molecular Structure of 8. The structure of compound 8 was determined by X-ray crystallographic analysis (Figure 1). It crystallizes in a monoclinic crystal system. The n-Bu and t-Bu groups are disordered due to the availability of the rotational degree of freedom around the CC bonds. The molecular structure of tellurane 8 is similar to that of the spirotellurane 1719 and has a pseudotrigonal-bipyramidal geometry about the central tellurium atom with the more electronegative acyloxy ligands in the axial positions and the two aryl carbons and the lone electron pair in the equatorial plane.
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NOTE
Scheme 4. Plausible Mechanism for the Formation of 8, 9, and 11
Scheme 5. Synthesis of 14 and Its Aerobic Oxidationa
a Reagents and conditions: (i) PhCH2CH2TeNa+, THF, rt, 6 h; (ii) toluene, O2, 80 °C, 24 h.
Figure 2. Molecular structure of 9. Thermal ellipsoid are drawn at 50% probability. Selected bond lengths (Å) and angles (deg): Te(1)C(2) 2.045(4), Te(1)O(2) 2.094(5), Te(1)O(1) 2.504(4), C(2)Te(1)O(2) 78.64(16), C(2)Te(1)O(1) 71.17(16), O(2)Te(1) O(1) 149.81(12).
Figure 1. Molecular structure of 8. Thermal ellipsoids are drawn at 50% and angles (deg): TeC(1) 2.047(2), probability. Selected bond lengths (Å) TeC(13B) 2.059(10), TeO(1) 2.133(2), TeO(3) 2.173(2), Te C(13A) 2.204(9), C(1)TeC(13B) 94.3(2), C(1)TeO(1) 77.05(8), C(13B)TeO(1) 91.4(3), C(1)TeO(3) 76.40(8), C(13B)TeO(3) 85.7(3), O(1)TeO(3) 153.00(6), C(1)Te C(13A) 97.3(2), C(13B) TeC(13A) 8.8(4), O(1)TeC(13A) 84.1(2), O(3)TeC(13A) 94.4(2).
The Te(1)O(1) and Te(1)O(3) bond lengths are 2.133(2) and 2.173(2) Å, respectively. These distances are slightly longer than the sum of the tellurium and oxygen covalent radii (2.04 Å)26 and indicate the hypervalent nature of the TeO axial bonds. The nature of the carboxylate group 8 was studied by FT-IR spectroscopy. The stretching frequencies for 8 were observed at 1664 (νas (CO)) and 1330 (νs (CO)) cm1 and could be assigned as the absorptions resulting from the CO bonds in the carboxylate groups. The difference between νas and νs is 334 cm1. It is in agreement with the IR stretching frequencies (1640 (νas (CO)) and 1298 3895
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Organometallics (νs (CO)) cm1) reported for 2-(phenylazo)phenyl-C,N0 )tellurium acetate, (C12H9N2)Te(O2CMe), which has a difference of 342 cm1.13 The antisymmetric and symmetric stretches for the related triaryltelluronium carboxylate 18 were observed at 1564 (νas (CO)) and 1379 (νs (CO)) cm1, respectively, with a deference of 185 cm1.27 They indicate that acyloxy groups in 8 and 2-(phenylazo)phenyl-C,N0 )tellurium acetate have an ester character and the acyloxy group in 18 has a carboxylate character.28 The O(1)TeO(3) bond angle (153.00(6)°) in compound 8 is considerably compressed in comparison to that in spirotellurane 17, in which it was found to be 161.3°.19 The presence of a compressed O(1)TeO(3) angle in 8 is presumably due to the geometrical constraints. Molecular Structure of 9. The molecular structure of cyclic tellurenate ester 9 was also confirmed by single-crystal X-ray crystallography (Figure 2). It crystallizes in an orthorhombic crystal system. The molecular geometry around the tellurium atom is distorted T-shaped. The distance between Te(1) and O(1) is 2.094(5) Å. The presence of a T(1) 3 3 3 O(1) interaction affects the stretching frequency of the interacting carbonyl group. The IR bond stretching frequency for the carbonyl group coordinated to Te was observed at 1624 cm1, which is lower than the corresponding stretching frequency observed for the selenenate ester 4 (1642 cm1).18 This indicates that the Te 3 3 3 O (9) interaction is stronger than the Se 3 3 3 O interaction (4).
’ CONCLUSIONS In conclusion, the internal protecting group (COOMe) serves as a good electrophilic trap for the isolation of unstable aryltellurenyl hydroxide and diorganotellurium dihydroxide in their protected form. Tellurenate ester 9 was stabilized by strong intramolecular coordination. Unlike the β-elimination reaction, the formation of 9 does not require diorganotellurium dihydroxide 12 as the intermediate. This study clearly demonstrates that the TeC bond fission can occur via both β-elimination and 1,2-shift reaction. ’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental Section, X-ray crystallographic data for 8 and 9 and the spectroscopic characterization data (1H, 13C, 125Te NMR and MS) for 6, 8, 9, 11, and 14. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT H.B.S. gratefully acknowledges the Department of Science and Technology, New Delhi, for the Ramanna Fellowship. K.S. is thankful to CSIR, New Delhi, for SRF and SAIF, IITB, for spectral data analysis. ’ REFERENCES (1) Petragnani, N.; Stefani, H. A. Tellurium in Organic Synthesis 2nd ed.; Academic Press: London, 2007. (2) (a) Leonard, K. A.; Zhou, F.; Detty, M. R. Organometallics 1996, 15, 4285. (b) Detty, M. R.; Zhou, F.; Friedman, A. E. J. Am. Chem. Soc. 1996, 118, 313. (c) Higgs, D. E.; Nelen, M. I.; Detty, M. R. Org. Lett. 2001, 3, 349. (3) Oba, M.; Endo, M.; Nishiyama, K.; Ouchi, A.; Ando, W. Chem. Commun. 2004, 1672.
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(4) Sheng, J.; Hassan, A. E. A.; Huang, Z. J. Org. Chem. 2008, 73, 3725. (5) (a) Ba, L.; D€oring, M.; Jamier, V.; Jacob, C. Org. Biomol. Chem. 2010, 8, 4203. (b) Cunha, R. L. O. R.; Gouvea, I. E.; Juliano, L. An. Acad. Bras. Cienc. 2009, 81, 393. (c) Jamier, V.; Ba, L. A.; Jacob, C. Chem.—Eur. J. 2010, 16, 10920. (6) (a) Engman, L.; Stern, D.; Cotgreave, I. A.; Andersson, C. M. J. Am. Chem. Soc. 1992, 114, 9737. (b) Mugesh, G.; Panda, A.; Kumar, S.; Apte, S. D.; Singh, H. B.; Butcher, R. J. Organometallics 2002, 21, 884. (c) You, Y.; Ahsan, K.; Detty, M. R. J. Am. Chem. Soc. 2003, 125, 4918. (d) Giles, G. I.; Fry, F. H.; Tasker, K. M.; Holme, A. L.; Peers, C.; Green, K. N.; Klotz, L.-O.; Sies, H.; Jacob., C. Org. Biomol. Chem. 2003, 1, 4317. (e) Ahsan, K.; Drake, M. D.; Higgs, D. E.; Wojciechowski, A. L.; Tse, B. N.; Bateman, M. A.; You, Y.; Detty, M. R. Organometallics 2003, 22, 2833. (7) (a) Nogueira, C. W.; Zeni, G.; Rocha, J. B. T. Chem. Rev. 2004, 104, 6244. (b) Dong, Z.-Y.; Huang, X.; Mao, S.-Z.; Liang, K.; Liu, J.-Q.; Luo., G.-M.; Shen, J.-C. Chem.—Eur. J. 2006, 12, 3575. (c) Kumar, S.; Johansson, H.; Kanda, T.; Engman, L.; M€uller, T.; Bergenudd, H.; Jonsson, M.; Pedulli, G. F.; Amorati, R.; Valgimigli, L. J. Org. Chem. 2010, 75, 716. (d) Amorati, R.; Pedulli, G. F.; Valgimigli, L.; Johansson, H.; Engman., L. Org. Lett. 2010, 12, 2326, and references therein. (8) Dong, Z.; Li, X.; Liang, K.; Mao, S.; Huang, X.; Yang, B.; Xu, J.; Liu, J.; Luo, G.; Shen, J. J. Org. Chem. 2007, 72, 606. (9) (a) Sharpless, K. B.; Gordon, K. M.; Lauer, R. F.; Patriel, D. W.; Singer, S. P.; Young, M. W. Chem. Scr. 1975, 8A, 9. (b) Sharpless, K. B.; Lauer, R. F. J. Am. Chem. Soc. 1973, 95, 2697. (c) Nicolaou, K. C.; Magolda, R. L.; Sipio, W. J.; Barnette, W. E.; Lysenko, Z.; Joullie, M. M. J. Am. Chem. Soc. 1980, 102, 3784.(d) Paulmier, C. Selenium Reagents and Intermediates in Organic Synthesis; Pergamon Press: Oxford, 1986. (e) Liotta, D. Organoselenium Chemistry; Wiley Interscience: New York, 1987. (f) Tiecco, M.; Testaferri, L; Tingoli, M.; Marini., F. J. Org. Chem. 1993, 58, 1349. (10) Rooseboom, M.; Vermeulen, N. P. E.; Durgut, F.; Commandeur, J. N. M. Chem. Res. Toxicol. 2002, 15, 1610. (11) Lee, H.; Cava, M. P. Chem. Commun. 1981, 277. (12) Uemura, S.; Fukuzawa, S.-I. J. Am. Chem. Soc. 1983, 105, 2748. (13) Ahmed, M. A. K.; McWhinnie, W. R.; Hamor, T. A. J. Organomet. Chem. 1985, 293, 219. (14) Sadekov, I. D.; Maksiinko, A. A.; Maslakov, A. G.; Minkin, V. I. J. Organomet. Chem. 1990, 391, 179. (15) Srivastava, K.; Shah, P.; Singh, H. B.; Butcher, R. J. Organometallics 2011, 30, 534. (16) (a) Lin, F. L.; Hoyt, H. M.; van Halbeek, H.; Bergman, R. G.; Bertozzi, C. R. J. Am. Chem. Soc. 2005, 127, 2686. (b) Saxon, E.; Armstrong, J. I.; Bertozzi, C. R. Org. Lett. 2000, 2, 2141. (c) Saxon, E.; Bertozzi, C. R. Science 2000, 287, 2007, and references therein. (17) Zade, S. S.; Singh, H. B.; Butcher, R. J. Angew. Chem., Int. Ed. 2004, 43, 4513. (18) Selvakumar, K.; Singh, H. B.; Butcher, R. J. Chem.—Eur. J. 2010, 16, 10576. (19) Takaguchi, Y.; Furukawa, N. Heteroat. Chem. 1995, 6, 481. (20) (a) Engman, L.; Cava, M. P. Chem. Commun. 1982, 164. (b) Oba, M.; Okada, Y.; Nishiyama, K.; Shimadab, S.; Ando, W. Chem. Commun. 2008, 5378. (21) Oba, M.; Okada, Y.; Endo, M.; Tanaka, K.; Nishiyama, K.; Shimada, S.; Ando, W. Inorg. Chem. 2010, 49, 10680. (22) Sarma, B. K.; Manna, D.; Minoura, M.; Mugesh, G. J. Am. Chem. Soc. 2010, 132, 5364. (23) (a) Engman, L.; Persson, J.; Merenyi, G.; Lind, J. Organometallics 1995, 14, 3641. (b) Taka, H.; Yamazaki, Y.; Shimizu, T.; Kamigata, N. J. Org. Chem. 2000, 65, 2127. (24) Detty, M. R. J. Org. Chem. 1980, 45, 274. (25) Nishibayashi, Y.; Komatsu, N.; Ohe, K.; Uemura, S. J. Chem. Soc., Perkin Trans. 1 1993, 1133. (26) Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.; Echeverría, J.; Cremades, E.; Barragan, B.; Alvarez, S. Dalton Trans. 2008, 2832. (27) Sato, S.; Kondo, N.; Horn, E.; Furukawa, N. Organometallics 1998, 17, 1897. (28) Nakamoto, K.; Morimoto, Y.; Martell, A. E. J. Am. Chem. Soc. 1961, 83, 4528. 3896
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