Synthesis of a Copper Silyl Complex by Disilane Activation

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Synthesis of a Copper Silyl Complex by Disilane Activation Bethany J. McCarty,† Brogan M. Thomas,† Matthias Zeller,‡ and Ryan Van Hoveln*,† †

Department of Chemistry and Physics, Indiana State University, Terre Haute, Indiana 47809, United States Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States

‡

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S Supporting Information *

ABSTRACT: Silylations remain a longstanding challenge in organic synthesis. While much recent work has focused on the use of first-row transition metals to perform silylations, catalysts based on platinum and palladium remain the catalysts of choice. One of the challenges of copper-catalyzed silylations is the use of specialized silicon reagents, such as silylboranes, that are required to create the intermediate copper silyl complex. While silylboranes have enjoyed considerable success in copper catalysis, they are air and water sensitive, making them difficult to work with, and they are difficult to synthesize or expensive to buy. Herein, we report the synthesis of a copper silyl complex from an easy to handle and relatively inexpensive disilane. The structure of the complex was confirmed by X-ray crystallography, and the mechanism has been investigated computationally.

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ilanes are immensely important functional groups and are used in a variety of applications.1−5 However, silane synthesis remains challenging, and as such, transition-metalcatalyzed silylations remain the focus of much recent work.1−4 There are a few examples of first-row transition metals, such as iron, cobalt, nickel, and copper, performing silylations. In spite of their development, precious metals such as platinum and palladium are still the workhorses for silylation chemistry. Catalysts based on platinum or palladium tend to be more robust than their first-row counterparts, and the silicon sources for silylation tend to be simple silanes. Conversely, copper silylations, for instance, require the use of inconvenient silicon reagents because of the difficulty associated with forming the necessary copper silyl complex.6,8,10 In most cases, the copper silyl complex is formed from silylboranes (Figure 1) and a copper alkoxide such as 1.6 Since the borane is more Lewis acidic than the silicon, the alkoxide attacks the boron, breaks the Si−B bond, and makes a thermodynamically stable B−O bond.7 Silylboranes are most often used as the silicon source in copper-catalyzed silylations6b,8 in spite of the fact that the reagents are expensive, are difficult to synthesize, and are air and water sensitive.9 Disilylzinc reagents have also been used in catalysis, but these have all of the same problems as a silylborane except they are not commercially available at all and they must be stored as solutions.10 Copper silyl complexes have also been synthesized from silyl potassium reagents. However, this has only been for the sake of characterization and a silyl potassium has not been used as a precursor for catalysis (Figure 1, top).11 Central to further development of copper-catalyzed silylations is a better way to make a copper silyl complex that involves using a reagent that is less costly, functional group tolerant, and easy to handle. © XXXX American Chemical Society

Figure 1. Previous and current work.

A simpler method of generating a copper silyl complex could be based on a disilane instead of a silylborane (Figure 1, bottom). The thermodynamic driving force in this case would be the formation of a Si−O bond, which is almost as strong as a B−O bond.12 Disilanes have only been used in copper catalysis as a silylating reagent in a handful of reports.13 All of these reports propose a copper silyl intermediate; however, none of them provide any evidence that a copper silyl complex Received: July 3, 2018

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DOI: 10.1021/acs.organomet.8b00455 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

reported copper silyl complexes, which generally bear alkyl, aryl, or silyl groups on the silicon rather than alkoxy.6,11 Furthermore, disilane 6h has rarely been used as a reagent for synthesis.16 When the copper alkoxide was replaced with the corresponding copper fluoride, the reaction with disilane 6h resulted in full consumption of the starting material; however, no copper silyl complex was detected (entry 9). Instead, a complex mixture of products formed. The disilane likely reduced the copper, since the reaction mixture turned dark, indicating the formation of copper nanoparticles. Nevertheless, we could reliably produce a copper silyl complex in good yield. The copper silyl complex was then crystallized to obtain an isolated yield as well as an X-ray crystal structure (Figure 2).

actually forms. Ball and co-workers have shown that a copper alkoxide or fluoride can activate a C−Si bond.14 Thus, activation of a Si−Si bond with a copper alkoxide should be possible. Herein, we report (1) the first synthesis of a copper silyl complex from a disilane, (2) characterization of the copper silyl complex by X-ray crystallography, and (3) an energy profile for the mechanism of activation. Our efforts to synthesize a copper silyl complex from a disilane began by reacting a series of disilanes with IPrCuOtBu. This ligand architecture for the copper alkoxide was chosen because of its success in activating SiX bonds and because it is amenable to characterization by X-ray crystallography.6,15 Alkyl- and aryl-substituted disilanes exhibited no reactivity (Table 1, entries 1−4). Interestingly, two of these disilanes Table 1. Reactions of Disilanes with Copper Complexes

Figure 2. X-ray crystal structure of IPrCuSi(OMe)3. Hydrogen atoms are omitted for clarity. Selected bond lengths and angles: Cu−Si 2.280 Å, C−Cu 1.937 Å, C−Cu−Si 168.26°.

The X-ray structure showed Cu−Si bond lengths and C−Cu− Si bond angles that were in good agreement with those of recently published (NHC)CuSiR3 complexes.6,11 Typical Cu− Si bond lengths are 2.26−2.29 Å. However, the C−Cu−Si bond angle was slightly smaller than what is typically reported (170−178°). This can be attributed to a greater amount of π back-bonding, which is not surprising, as the Si−O antibonding orbital should be able to accept electron density to a greater extent than the Si−alkyl or Si−aryl antibonding orbitals in the analogous complexes previously reported.17 Next, the copper silyl complex was reacted with a variety of electrophiles (Table 2). The silyl group was sufficiently nucleophilic to displace a leaving group on both a carbonyl and an activated sp3-hybridized carbon (entries 1 and 3) in good yield. However, when the copper silyl complex was added to a Michael acceptor, the two-step yield was more modest (entry 2). The reduced yield is likely due to the nucleophilic copper enolate 8 adding to the residual methyl vinyl ketone. Overall, this demonstrates that this copper silyl complex reacts similarly to what is reported in the literature despite different electronics at the silicon.8,10,13 Finally, the reaction pathway was investigated computationally. The optimal pathway begins with the alkoxide approaching one of the silicons of the disilane and the formation of the slightly endothermic silicate intermediate INT-2 (Figure 3). Then, the complex proceeds through a lowenergy transition state in which the Si−Si bond breaks with concomitant formation of a Cu−Si bond. This results in the

a

NMR yield determind using mesitylene as an internal standard. Isolated yields shown in parentheses. bHexaphenyldisilane is insoluble in toluene.

(6b,c) are known to react with a copper catalyst to perform silylations.13 However, these reports did not confirm the formation of a copper silyl complex. Next, adding better electron-withdrawing groups, such as dimethylamino and methoxy groups, resulted in minimal reactivity (entries 5−7). Longer reaction times or heating did not improve the yield but resulted in the decomposition of the copper silyl complex. Finally, replacing all groups on the disilane with methoxy groups resulted in a good yield of the desired copper silyl complex (entry 8). Notably, this is a novel copper complex that bears a silane which is electronically different from previously B

DOI: 10.1021/acs.organomet.8b00455 Organometallics XXXX, XXX, XXX−XXX

Organometallics

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Table 2. Reactions of IPrCuSi(OMe)3 with Different Electrophiles

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AUTHOR INFORMATION

Corresponding Author

*E-mail for R.V.H.: [email protected]. ORCID

Matthias Zeller: 0000-0002-3305-852X Ryan Van Hoveln: 0000-0002-0291-4493 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by Indiana State University, including student scholarship support from the Center for Student Research and Creativity and the Summer Undergraduate Research Experiences (SURE) program. B.J.M. acknowledges stipend support from Eli Lilly and Company. Instrumentation was supported by the National Science Foundation, CHE-0521075 (MRI funding for the 400 MHz NMR) and CHE-1531972 (MRI funding for the MS). X-ray crystallography was carried out at Purdue University with support by the National Science Foundation, CHE-1625543 (MRI funding for the single-crystal X-ray diffractometer). The authors also thank Prof. Rick Fitch for numerous, helpful conversations pertaining to this paper.

a

NMR yield determined using mesitylene as an internal standard. Two-step yields from IPrCuOtBu.

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Figure 3. Proposed mechanism of disilane activation. Free energies are shown in kcal/mol relative to INT-1 computed at the M06/6311G(2d,p):LANL2TZ(f) (Cu) level of theory.

energetically favorable copper silyl complex and the byproduct, t BuOSi(OMe)3. This proposed mechanism is in good agreement with the analogous mechanism for the activation of B−B bonds with copper alkoxides.18 For additional discussion, see the Supporting Information. In summary, we have reported a new means of generating a copper silyl complex from readily accessible and easy to handle starting materials. The structure of the copper silyl complex was verified by X-ray crystallography, and the mechanism of activation was investigated computationally. Future work will focus on developing copper-catalyzed silylations using a disilane as the silicon source.

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REFERENCES

(1) Marciniec, B.; Maciejewski, H.; Pietraszuk, C.; Pawluc, P.; In Hydrosilylation: A Comprehensive Review on Recent Advances, Marciniec, B., Matisons, J., Eds.; Springer: 2009. (2) Troegel, D.; Stohrer, J. Recent advances and actual challenges in late transition metal catalyzed hydrosilylation of olefins from an industrial point of view. Coord. Chem. Rev. 2011, 255, 1440−1459. (3) Du, X.; Huang, Z. Advances in Base-Metal-Catalyzed Alkene Hydrosilylation. ACS Catal. 2017, 7, 1227−1243. (4) Roy, A. K. A. Review of Recent Progress in Catalyzed Homogeneous Hydrosilation (Hydrosilylation). Adv. Organomet. Chem. 2007, 55, 1−59. (5) (a) Fleming, I.; Dunogues, J.; Smithers, R. The Electrophilic Substitution of Allylsilanes and Vinylsilanes. Org. React. 1989, 37, 57− 575. (b) Jones, G. R.; Landais, Y. The oxidation of the carbon-silicon bond. Tetrahedron 1996, 52, 7599−7662. (c) Nakao, Y.; Hiyama, T. Silicon-based cross-coupling reaction: an environmentally benign version. Chem. Soc. Rev. 2011, 40, 4893−4901. (d) Gurung, S. K.; Thapa, S.; Vangala, A. S.; Giri, R. Copper-Catalyzed Hiyama Coupling of (Hetero)aryltriethoxysilanes with (Hetero)aryl Iodides. Org. Lett. 2013, 15, 5378−5381. (e) Gurung, S. K.; Thapa, S.; Shrestha, B.; Giri, R. Copper-Catalyzed Cross-Coupling of Aryl- and Heteroaryltriethoxysilanes with Aryl and Heteroaryl Iodides and Bromides. Synthesis 2014, 46, 1933−1937. (6) (a) Plotzitzka, J.; Kleeberg, C. [(NHC)CuI−ER3] Complexes (ER3 = SiMe2Ph, SiPh3, SnMe3): From Linear, Mononuclear Complexes to Polynuclear Complexes with Ultrashort CuI···CuI Distances. Inorg. Chem. 2016, 55, 4813−4823. (b) Kleeberg, C.; Cheung, M. S.; Lin, Z.; Marder, T. B. Copper-Mediated Reduction of CO2 with pinB-SiMe2Ph via CO2 Insertion into a Copper−Silicon Bond. J. Am. Chem. Soc. 2011, 133, 19060−19063. (7) The bond dissociation energy for a B−O bond is 806 kJ/mol: Dean, J. A. Lange’s Handbook of Chemistry, 15th ed.; McGraw-Hill: 1999; p 4.42. (8) For selected silylations using a silylborane, see: (a) Tani, Y.; Fufihara, T.; Terao, J.; Tsuji, Y. Copper-Catalyzed Regiodivergent Silacarboxylation of Allenes with Carbon Dioxide and a Silylborane. J. Am. Chem. Soc. 2014, 136, 17706−17709. (b) Hensel, A.; Nagura, K.; Delvos, L. B.; Oestreich, M. Enantioselective Addition of Silicon Nucleophiles to Aldimines Using a Preformed NHC−Copper(I) Complex as the Catalyst. Angew. Chem., Int. Ed. 2014, 53, 4964−4967.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00455. NMR spectra, optimized geometries, and crystallographic procedures and data (PDF) Cartesian coordinates for the calculated structures (XYZ) Accession Codes

CCDC 1851814 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. C

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Organometallics (c) Hazra, C. K.; Irran, E.; Oestreich, M. Regio- and Diastereoselective Copper(I)-Catalyzed Allylic Substitution of δ-Hydroxy Allylic Chlorides by a Silicon Nucleophile. Eur. J. Org. Chem. 2013, 2013, 4903−4908. (d) Fujihara, T.; Tani, Y.; Semba, K.; Terao; Tsuji, Y. Copper-Catalyzed Silacarboxylation of Internal Alkynes by Employing Carbon Dioxide and Silylboranes. Angew. Chem., Int. Ed. 2012, 51, 11487−11490. (e) Cirriez, V.; Rasson, C.; Riant, O. Synthesis of Acylsilanes by Copper(I)-Catalyzed Addition of Silicon Nucleophiles onto Acid Derivatives. Adv. Synth. Catal. 2013, 355, 3137−3140. (9) Yamamoto, E.; Shishido, R.; Seki, T.; Ito, H. Tris(trimethylsilyl)silylboronate Esters: Novel Bulky, Air- and Moisture-Stable Silylboronate Ester Reagents for Boryl Substitution and Silaboration Reactions. Organometallics 2017, 36, 3019−3022. (10) (a) Hazra, C. K.; Oestreich, M. Copper(I)-Catalyzed Regioand Chemoselective Single and Double Addition of Nucleophilic Silicon to Propargylic Chlorides and Phosphates. Org. Lett. 2012, 14, 4010−4013. (b) Vyas, D. J.; Oestreich, M. Expedient access to branched allylic silanes by copper-catalysed allylic substitution of linear allylic halides. Chem. Commun. 2010, 46, 568−570. (11) Sgro, M. J.; Piers, W. E.; Romero, P. E. Synthesis, structural characterization and thermal properties of copper and silver silyl complexes. Dalton Trans 2015, 44, 3817−3828. (12) The bond dissociation energy for a Si−O bond is 798 kJ/mol; see ref 7, p 4.50. (13) (a) Ito, H.; Horita, Y.; Sawamura, M. Copper(I)-Catalyzed Allylic Substitution of Silyl Nucleophiles through Si−−Si Bond Activation. Adv. Synth. Catal. 2012, 354, 813−817. (b) Iannazzo, L.; Molander, G. A. Activation of Si−Si Bonds for Copper(I)-Catalyzed Conjugate Silylation. Eur. J. Org. Chem. 2012, 2012, 4923−4926. (c) Ito, H.; Ishizuka, T.; Tateiwa, J.-I.; Sonoda, M.; Hosomi, A. New Method for Introduction of a Silyl Group into α,β-Enones Using a Disilane Catalyzed by a Copper(I) Salt. J. Am. Chem. Soc. 1998, 120, 11196−11197. (d) Clark, C. T.; Lake, J. F.; Scheidt, K. A. Copper(I)Catalyzed Disilylation of Alkylidene Malonates Employing a Lewis Base Activation Strategy. J. Am. Chem. Soc. 2004, 126, 84−85. (e) Minami, Y.; Shimizu, K.; Tsuruoka, C.; Komiyama, T.; Hiyama, T. Synthesis of HOMSi Reagents by Pd/Cu-Catalyzed Silylation of Bromoarenes with Disilanes. Chem. Lett. 2014, 43, 201−203. (14) (a) Herron, J. R.; Ball, Z. T. Synthesis and Reactivity of Functionalized Arylcopper Compounds by Transmetalation of Organosilanes. J. Am. Chem. Soc. 2008, 130, 16486−16487. (b) Herron, J. R.; Russo, V.; Valente, E. J.; Ball, Z. T. Catalytic Organocopper Chemistry from Organosiloxane Reagents. Chem. - Eur. J. 2009, 15, 8713−8716. (c) Russo, V.; Herron, J. R.; Ball, Z. T. Allylcopper Intermediates with N-Heterocyclic Carbene Ligands: Synthesis, Structure, and Catalysis. Org. Lett. 2010, 12, 220−223. (15) For selected examples where IPr is used as a ligand for copper crystal structures, see: (a) Laitar, D. S.; Müller, P.; Sadighi, J. P. Efficient Homogeneous Catalysis in the Reduction of CO2 to CO. J. Am. Chem. Soc. 2005, 127, 17196−17197. (b) Laitar, D. S.; Tsui, E. Y.; Sadighi, J. P. Copper(I) β-Boroalkyls from Alkene Insertion: Isolation and Rearrangement. Organometallics 2006, 25, 2405−2408. (c) Laitar, D. S.; Tsui, E. Y.; Sadighi, J. P. Catalytic Diboration of Aldehydes via Insertion into the Copper−Boron Bond. J. Am. Chem. Soc. 2006, 128, 11036−11037. (d) Mankad, N. P.; Laitar, D. S.; Sadighi, J. P. Synthesis, Structure, and Alkyne Reactivity of a Dimeric (Carbene)copper(I) Hydride. Organometallics 2004, 23, 3369−3371. (16) Examples where hexamethoxydisilane is used as a reagent include the following: (a) Maier, G.; Reisenauer, H. P.; Schottler, K.; Wessolek-Kraus, U. Hetero-π-Systeme: XVI. Alkoxysilylene (Alkoxysilandiyle). J. Organomet. Chem. 1989, 366, 25−38. (b) Gozin, M.; Alzenberg, M.; Liou, S.-Y.; Welsman, A.; Ben-David, Y.; Milstein, D. Transfer of methylene groups promoted by metal complexation. Nature 1994, 370, 42−44. (c) Laska, J. E.; Kaszynski, P. Synthesis, Strain, Conformational Analysis, and Molecular and Crystal Structures of 1,1,4,4-Tetraphenyl-1,4-disilacyclohexane and 1,1,4,4Tetraphenyl-1,4-disilacyclohexa-2,5-diene. Organometallics 1998, 17, 2018−2026. (d) Mitzenheim, C.; Braun, T. SiSi and SiO Bond

Activation at Platinum: Stepwise Formation of a SiH3 Complex. Angew. Chem., Int. Ed. 2013, 52, 8625−8628. (17) Wolters, L. P.; Bickelhaupt, F. M. Nonlinear d 10-ML2 Transition-Metal Complexes. ChemistryOpen 2013, 2, 106−114. (18) Moon, J. H.; Jung, H.-Y.; Lee, Y. J.; Lee, S. W.; Yun, J.; Lee, J. Y. Origin of Regioselectivity in the Copper-Catalyzed Borylation Reactions of Internal Aryl Alkynes with Bis(pinacolato)diboron. Organometallics 2015, 34, 2151−2159.

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DOI: 10.1021/acs.organomet.8b00455 Organometallics XXXX, XXX, XXX−XXX