Communication pubs.acs.org/Organometallics
Formation of Iridium(III) Allene Complexes via Isomerization of Internal Alkynes Neha Phadke and Michael Findlater* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States. S Supporting Information *
ABSTRACT: Syntheses of the alkyne complexes (POCOP)Ir(η2-RCCR) (1-DPA, R = Ph; 1-Oct, R = C3H7; 1-Hex, R = C2H5; POCOP = 2,6-bis(di-tert-butylphosphonito)benzene) are reported. 1-DPA is stable in both the solid state and as a benzene solution; however, both 1-Oct and 1-Hex slowly isomerize to afford the corresponding allene complexes 2-Oct and 2-Hex, respectively. A single-crystal X-ray structure determination of 2-Oct confirmed the assignment of an iridium-bound allene isomerization product. The rates of isomerization were measured using NMR techniques over a range of temperatures to allow determination of thermodynamic parameters. Finally, in contrast to prior reports, initial mechanistic studies have indicated the isomerization process is not catalyzed by the presence of an acid.
I
equiv of alkyne; however, only in the case of diphenylacetylene were we able to cleanly generate the desired η2 species (Scheme 1). The 31P{1H} NMR spectrum of 1-DPA displays a singlet at
somerization of organic compounds catalyzed by transitionmetal complexes provides a mild and efficient route to allenes that cannot be easily accessed under thermal conditions.1 The isomerization of alkynes to allenes is particularly valuable due to their use as precursors for organic synthesis,2 and because the transformation itself may play a role in organometallic synthesis.3 The repulsive interaction of an alkyne π orbital with a filled metal d orbital in LnM(η2-RC CH) renders such complexes less stable relative to the corresponding vinylidene complexes LnMCCHR,4 and numerous reports have appeared concerning the isomerization of terminal alkynes to vinylidenes. 5 Analogous orbital interactions between metal fragments and internal alkynes would be anticipated to give rise to a similar destabilization. Indeed, although rare,6 internal alkynes can undergo isomerization to afford disubstituted vinylidenes. However, only a few reports on metal-mediated isomerization of internal alkynes to allenes have appeared. Both ReCl(N2)(dppe)27 and RhCl(C2H4)(As(i-Pr)3)28 have been shown to form allenes upon reaction with internal alkynes. The isomerization of preformed metal−alkyne complexes to allenes can be promoted using acid,9 alumina,10 or silica.11 In elegant mechanistic studies, Casey and co-workers described acid-promoted isomerization of alkyne complexes to allene complexes. Both 1-metallacyclopropene8 and η1-vinyl species3c were suggested as key intermediates in the isomerization mechanism. These insights prompted further work, which explored the interaction of metal hydrides with internal alkynes.12 Given the general paucity of reports detailing the isomerization of internal alkynes to allenes and the ongoing interest in the interaction of unsaturated species at late-transition-metal centers,13 we chose to study the chemistry of an iridium pincer complex with hex-3-yne, oct-4-yne, and diphenylacetylene. Treatment of (POCOP)IrH2 in benzene solution with ca. 1 equiv of TBE (tert-butylethylene) affords the coordinatively unsaturated (not isolated) 14-electron species “(POCOP)Ir”.14 We attempted to trap the η2-alkyne adduct by addition of 1 © 2013 American Chemical Society
Scheme 1. Alkyne Binding at a 14-Electron Iridium Center
166 ppm that is shifted significantly upfield relative to that for 1-H2. The corresponding 1H NMR spectrum exhibits features consistent with assignment of 1-DPA as a η2-bound species.15 Corresponding treatment of “(POCOP)Ir” with either oct-4yne or hex-3-yne under ambient conditions in benzene solution gives one major product, which we assign as the η2-alkyne complex, as determined by 31P{1H} NMR spectroscopy. Over the course of several days the singlet assigned to the η2-alkyne complex (ca. 166 ppm for both 1-Oct and 1-Hex) decays and is replaced by resonances corresponding to two new species. In the 31P{1H} NMR spectrum of 2-Oct both of the new species display two doublets and are present in an ∼1:1 ratio. Both species display a JA‑B coupling constant of 356 Hz arising from strong 31P−31P coupling of inequivalent phosphorus nuclei. A similar NMR spectrum is obtained in the case of 2-Hex. On the basis of the side-to-side inequivalence present in the ligand framework, we tentatively assigned these new peaks as arising from two iridium−allene complexes. Gratifyingly, we were able to grow crystals suitable for study by X-ray diffraction experiments from cold (−36 °C) solutions of the complex in Received: November 11, 2013 Published: December 24, 2013 16
dx.doi.org/10.1021/om4010934 | Organometallics 2014, 33, 16−18
Organometallics
Communication
methylene chloride. The solid-state structure of one stereoisomer of 2-Oct is shown in Figure 1.
Figure 2. Eyring plot for conversion of 1-Oct to 2-Oct. Figure 1. ORTEP diagram of 2-Oct with 40% probability thermal ellipsoids. Key bond lengths (Å) and bond angles (deg): C3−C4 = 1.387(8), C4−C5 = 1.307(8), Ir−C3 = 2.180(6), Ir−C4 = 2.054(5); C3−C4−C5 = 148.6(6), P1−Ir−P2 = 155.98(5).
Table 1. Temperature Dependence of Rate Constants for Conversion of 1-Oct to 2-Octa k (s−1)
In the solid state, the octa-3,4-diene ligand of 2-Oct is bound unsymmetrically to iridium through the ethyl-substituted C3 C4 π bond with a shorter Ir−C4 and a longer Ir−C3 interaction (Δd = 0.126 Å). The complex 2-Oct adopts a distorted-trigonal-bipyramidal conformation with a P−Ir−P pincer bite angle of 156° presumably due to steric interactions emanating from the bulky tert-butyl phosphine substituents. Within the allene ligand, the coordinated C3C4 bond is elongated by 0.08 Å relative to the uncomplexed C4C5 bond, and the allene unit is bent with a C3−C4−C5 angle of 149°. This unsymmetrical binding motif and allene distortion is in keeping with previously published metal−allene complexes.7,8,12,16 Variable-temperature NMR experiments confirmed that the two species present in solution do not interconvert even at elevated temperatures (80 °C).15
5.0 4.0 7.0 3.0 3.0
× × × × ×
T (K)
10−4 10−4 10−5 10−5 10−6
333.0 328.0 318.0 308.0 295.0
Rate constants measured for ∼0.020 M benzene-d6 solutions of 1Oct. a
solution.17 Mindful of this study and the earlier work of Casey,3c,9 which determined the crucial role acids could play in catalyzing the isomerization of alkynes to allenes, we probed the effects of added base on the observed rate of isomerization. To a benzene-d6 solution of “(POCOP)Ir” was added sequentially ca. 1.5 equiv of oct-4-yne and 2 equiv of either 2,6di-tert-butylpyridine or triethylamine. The rate of isomerization was monitored using 31P{1H} NMR spectroscopy. In both cases, in the presence of an amine, which would be expected to scavenge protons from solution and thus inhibit catalysis, no change in the observed rate constant could be detected. Werner and co-workers invoked a transient zwitterionic intermediate to account for their observed internal alkyne → allene isomerization.8 This transient species resembles that of a transition state postulated by Hoffmann in the concerted rearrangement of 1-alkynes to vinylidenes.18 However, very recent studies concerning the iridium-catalyzed isomerization of alkenes has led us to propose an alternate mechanistic pathway (Scheme 2).19 Thus, the initially formed π-bound alkyne (A) undergoes oxidative addition of a Cα−H bond to the iridium center, resulting in an Ir(III) alkyl hydride possibly stabilized by π donation from the alkyne (B). Evidence supporting the formation of an intermediate of structural type B is found in the crystallographically characterized species [Os(η3-PhC3CHPh)(PMe3)4]+.20 Subsequently, the alkyl hydride slips to an allyl hydride (C), which can easily undergo rotation followed by a hydride migration to give the η2-allene product D. The absence of a Cα−H bond in diphenylacetylene may explain the lack of observed isomerization behavior in 1-DPA. In summary, we have prepared a series of internal η2-alkyne complexes (POCOP)Ir(η2-RCCR). When R = Ph, the η2alkyne was found to be stable in solution and the solid state. Both alkyl-substituted alkynes (R = C3H7, C2H5) were found to undergo an isomerization to afford the corresponding η2-allene complexes. The rates of isomerization were measured over a
Chart 1. Proposed Structures of Diastereomers of 2-Oct
The rate of isomerization of 1-Oct or 1-Hex to the allene complex 2-Oct or 2-Hex is slow at room temperature (ca. 50% conversion after 72 h). The activation parameters for the formation of 2-Oct were examined by measuring the observed rate constant for the reaction over a 40 °C temperature range. The effect of temperature on the rate of isomerization of 1-Oct is depicted in Figure 2, and the rate constants are reported in Table 1. From the Eyring plot an activation entropy (ΔS⧧) of 4.1 eu and an activation enthalpy (ΔH⧧) of 26.2 kcal/mol were computed for 1-Oct. The rates of isomerization and activation parameters for 1-Hex are qualitatively similar to those of 1Oct.15 The near zero ΔS⧧ implicates a unimolecular transition structure for the rate-limiting event, which is supported by experiments in which the amount of added octyne appears to have no impact on the observed rate constant. Recently, Brookhart and co-workers disclosed that the hydrogenation of a closely related PONOPIr(CH3) system could be catalyzed by trace quantities of protons present in 17
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(5) For some recent reviews, see: (a) Puerta, M. C.; Valerga, P. Coord. Chem. Rev. 1999, 193−195, 977. (b) Metal Vinylidenes and Allenylidenes in Catalysis: From Reactivity to Applications in Synthesis; Bruneau, C., Dixneuf, P., Eds.; Wiley-VCH: Weinheim, Germany, 2008. (c) Selegue, J. P. Coord. Chem. Rev. 2004, 248, 1543. (6) (a) Ikeda, Y.; Yamaguchi, T.; Kanao, K.; Kimura, K.; Kamimura, S.; Mutoh, Y.; Tanabe, Y.; Ishii, Y. J. Am. Chem. Soc. 2008, 130, 16856. (b) Otsuka, M.; Tsuchida, N.; Ikeda, Y.; Kimura, Y.; Mutoh, Y.; Ishii, Y.; Takano, K. J. Am. Chem. Soc. 2012, 134, 17746. (c) Ikeda, Y.; Mutoh, Y.; Imai, K.; Tsuchida, N.; Takano, K.; Ishii, Y. Organometallics 2013, 32, 4353. (7) Hughes, D. L.; Pombeiro, A. J. L.; Pickett, C. J.; Richards, R. L. J. Chem. Soc. Chem. Commun. 1984, 992. (8) Werner, H.; Schwab, P.; Mahr, N.; Wolf, J. Chem. Ber. 1992, 125, 2641. (9) Casey, C. P.; Brady, J. T. Organometallics 1998, 17, 4620. (10) Frank-Neumann, M.; Brion, F. Angew. Chem., Int. Ed. Engl. 1979, 18, 688. (11) Coughlan, S. C.; Yang, G. K. J. Organomet. Chem. 1993, 450, 151. (12) Wen, T. B.; Zhou, Z. Y.; Lau, C.-P.; Jia, G. Organometallics 2000, 19, 3466. (13) Findlater, M.; Cartwright-Sykes, A.; White, P. S.; Schauer, C. K.; Brookhart, M. J. Am. Chem. Soc. 2011, 133, 12274. (14) For related in situ generation of 14-electron systems see: (a) Gottker-Schnetmann, I.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804. (b) Gottker-Schnetmann, I.; White, P. S.; Brookhart, M. Organometallics 2004, 23, 1766. (c) GottkerSchnetmann, I.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 9330. (15) See the Supporting Information for details of the synthesis and characterization of metal complexes and calculations pertaining to rate and thermodynamic parameters. (16) For reports of related transition-metal π-allene complexes, see the following. (a) Au: Brown, T. J.; Sugie, A.; Dickens, M. G.; Widenhoefer, R. A. Organometallics 2010, 29, 4207. (b) Re: Pu, J.; Peng, T.-S.; Arif, A. M.; Gladysz, J. A. Organometallics 1992, 11, 3232. (c) Fe: Omrcen, T.; Conti, N. J.; Jones, W. M. Organometallics 1991, 10, 913. (d) W: Chacon, S. T.; Chisholm, M. C.; Folting, K.; Huffman, J. C.; Hampden-Smith, M. J. Organometallics 1991, 10, 3722. (e) Ir: Lundquist, E. G.; Folting, K.; Streib, W. E.; Huffman, J. C.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1990, 112, 855. (f) Pt: Winchester, W. R.; Jones, W. M. Organometallics 1985, 4, 2228. (g) Pd: Okamoto, K.; Kai, Y.; Yasuoka, N.; Kasai, N. J. Organomet. Chem. 1974, 65, 427. (17) Findlater, M.; Bernskoetter, W. H.; Brookhart, M. J. Am. Chem. Soc. 2010, 132, 4534. (18) Silvestre, A.; Hoffmann, R. Helv. Chim. Acta 1985, 68, 1461. (19) Biswas, S.; Huang, Z.; Choliy, Y.; Wang, D. Y.; Brookhart, M.; Krogh-Jesperson, K.; Goldman, A. S. J. Am. Chem. Soc. 2012, 134, 13276. (20) Gotzig, J.; Otto, H.; Werner, H. J. Organomet. Chem. 1985, 287, 247.
Scheme 2. Proposed Mechanism for Formation of Allene Complexes
broad temperature range, which allowed the thermodynamic parameters to be determined. Addition of external base did not have any impact on the observed rate of isomerization, seeming to preclude a catalytic role being played by trace quantities of acid.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Text, figures, and a CIF file giving X-ray crystallographic data and complete synthetic and characterization details of all metal complexes. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail: michael.fi
[email protected]. Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Dedicated to Professor Maurice Brookhart on the occasion of his 70th birthday. M.F. gratefully acknowledges Texas Tech University for startup funds. The authors thank the National Science Foundation for funding the purchase of NMR instrumentation (Grant No. CHE-1048553) used in this project.
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REFERENCES
(1) For reviews, see (a) Trost, B. M.; Krische, M. J. Synlett 1998, 1. (b) Fairlamb, I. J. S. Angew. Chem., Int. Ed. 2004, 43, 1048. (c) Uma, R.; Crevisy, C.; Gree, R. Chem. Rev. 2003, 103, 27. (d) Hashmi, A.; Stephen, K. In Modern Allene Chemistry; Krause, N., Hashmi, A. S. K., Eds.; Wiley-VCH: Weinheim, Germany, 2004; p 877. (2) (a) Schuster, H. F.; Coppola, G. M. Allenes in Organic Synthesis; Wiley: New York, 1984. (b) Brandsma, L.; Verkruijsse, H. D. Synthesis of Acetylenes, Allenes and Cumulenes, A Laboratory Manual; Elsevier: Amsterdam, 1981. (3) (a) Leeaphon, M.; Ondracek, A. L.; Thomas, R. J.; Fanwick, P. E.; Walton, R. A. J. Am. Chem. Soc. 1995, 117, 9715. (b) Wolf, J.; Werner, H. Organometallics 1987, 6, 1164. (c) Casey, C. P.; Brady, J. T.; Boller, T. M.; Weinhold, F.; Hayashi, R. K. J. Am. Chem. Soc. 1998, 120, 12500. (4) Templeton, J. L. Adv. Organomet. Chem. 1989, 29, 1. 18
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