Letter pubs.acs.org/acscatalysis
Bench-Stable, Substrate-Activated Cobalt Carboxylate Pre-Catalysts for Alkene Hydrosilylation with Tertiary Silanes Christopher H. Schuster, Tianning Diao, Iraklis Pappas, and Paul J. Chirik* Department of Chemistry, Frick Laboratory, Princeton University, Princeton, New Jersey 08544, United States S Supporting Information *
ABSTRACT: High-spin pyridine diimine cobalt(II) bis(carboxylate) complexes have been synthesized and exhibit high activity for the hydrosilylation of a range of commercially relevant alkenes and tertiary silanes. Previously observed dehydrogenative silylation is suppressed with the use of sterically unencumbered ligands, affording exclusive hydrosilylation with up to 4000 TON. The cobalt precatalysts were readily prepared and handled on the benchtop and underwent substrate activation, obviating the need for external reductants. The cobalt catalysts are tolerant of epoxide, amino, carbonyl, and alkyl halide functional groups, broadening the scope of alkene hydrosilylation with earth-abundant metal catalysts. KEYWORDS: cobalt, hydrosilylation, alkenes, tertiary silanes, redox active, silicone
T
catalyst precursors are challenges for their widespread implementation. In situ activation of more air-stable pyridine diimine iron halides has been explored to overcome this limitation; however, these methods often rely on strong reductants such as Grignard reagents or NaBEt3H and result in decreased activity and selectivity.10 This approach, while offering improvements in a laboratory setting, is ultimately not viable on scale as such additives are known to promote alkoxide exchange in alkoxysilanes which can lead to impure product formation, and in the case of trialkoxysilanes, formation of pyrophoric silane gas.11 Here we describe a new substrate activation approach that relies on readily prepared and benchstable pyridine diimine cobalt bis(carboxylate) complexes. During the preparation of this manuscript, Nagashima and co-workers described a similar approach using a mixture of cobalt carboxylates and isocyanides.12 For the pyridine diimine catalysts reported here, the alkene hydrosilylation activity and anti-Markovnikov selectivity with a range of commercially relevant alkenes and tertiary silanes is among the highest reported with earth-abundant transition metal catalysts. Previously, our group reported the use of aryl substituted pyridine diimine cobalt alkyl complexes for the dehydrogenative silylation of terminal alkenes.13 Mechanistic studies revealed rapid β-hydride elimination prior to turnover-limiting reaction with silane as the origin of product selectivity. We
he platinum-catalyzed hydrosilylation of alkenes is one of the largest applications of homogeneous catalysis in industry owing to its widespread use in the preparation of a range of consumer goods.1 The high-activity, anti-Markovnikov selectivity and robustness of platinum catalysts such as Speier’s2 (H2PtCl6·(H2O)2) and Karstedt’s3 catalysts (Pt2[(Me2SiCH CH2)2O]3) has made these compounds the industry standard for the past several decades. The relatively high and volatile price of platinum, coupled with the environmental footprint associated with its terrestrial extraction,4 inspires the discovery of alternative catalysts that rely on earth-abundant transition metals. Recently, there has been considerable effort from our laboratory and others exploring both iron and cobalt catalysts for alkene hydrosilylation.5−9 Central to the development of new catalyst systems is the importance of utilizing proper olefin and silane combinations to reflect commercial product profiles. Among recently reported base metal catalysts, pyridine diimine iron dinitrogen complexes are notable due to their extremely high activity and anti-Markovnikov selectivity for the hydrosilylation of relevant terminal alkenes.5 For instance, the high selectivity for the monohydrosilylation of the 4-position of 1,2,4-trivinylcyclohexane, an important component for lowrolling resistance tires, exceeds the state-of-the-art precious metal system.6 Importantly, this family of catalysts is reactive with commercially relevant tertiary silanes, a distinguishing feature from most other classes of alkene hydrosilylation catalysts that rely on earth-abundant transition metals.7−9 Unfortunately, the extreme air- and moisture-sensitivity of the © XXXX American Chemical Society
Received: January 29, 2016 Revised: March 12, 2016
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due to the insolubility of the complex in the reaction medium. Performing the catalytic hydrosilylation at 80 °C resulted in complete conversion of 1-octene, albeit with accompanying dehydrogenative silylation, generating allylsilane as a minor (13%) byproduct (entry 3). The poor solubility of (MeAPDI)Co(OAc)2 motivated exploration of other more soluble cobalt(II) bis(carboxylate) sources. (MeAPDI)Co(2-EH)2 (EH = ethylhexanoate) was prepared in a similar manner on the benchtop in air and isolated as a brown powder in 94% yield. The more soluble precatalyst proved to be highly active, exhibiting high anti-Markovnikov selectivity and reaching complete conversion in less than an hour at 23 °C with no detectable evidence for dehydrogenative silylation (entry 4). Importantly, (MeAPDI)Co(2-EH)2 exhibited no erosion in catalytic hydrosilylation performance after exposure of the precatalyst to air for 18 h (see Supporting Information for details). The corresponding cobalt complexes of PyBox (entry 5) and its carbon analogue TFAPDI15 (entry 6) were also found to give efficient hydrosilylation, with slightly reduced selectivity with the use of PyBox. The high activity observed with (MeAPDI)Co(2-EH)2 and TF ( APDI)Co(2-EH)2 prompted further evaluation for the hydrosilylation of 1-octene with a variety of tertiary silanes (Table 2). Although both catalysts proved to be highly effective
reasoned that the use of ligands bearing smaller imine substituents would accelerate the bimolecular reaction of the silane with a cobalt alkyl relative to the unimolecular βhydrogen elimination pathway. If successful, rational catalyst modification could switch selectivity between dehydrogenative silylation and hydrosilylation. The N-methyl imine-substituted pyridine diimine cobalt alkyl complex (( Me APDI)Co(CH2SiMe3)) was prepared using standard methods and evaluated for alkene hydrosilylation activity (Table 1). The Table 1. Evaluation of Substrate-Activated Pyridine Diimine Cobalt Precatalysts for the Hydrosilylation of 1-Octene with (EtO)3SiHa
Table 2. Hydrosilylation of 1-Octene with Tertiary Silanesa
a
All reactions were performed on 0.89 mmol scale using a 1:1 mixture of olefin to silane, HS = hydrosilylation, DHS = dehydrogenative silylation, EH = ethylhexanoate. bDetermined by 1H NMR analysis of the crude reaction mixture. cYield of isolated material. dDetermined by 1 H NMR analysis of isolated material. eReaction performed at 80 °C.
addition of triethoxysilane to 1-octene (neat) was chosen as a model reaction as the resulting noctylsilane product is manufactured annually on >6000 ton scale and finds commercial applications in coatings such as weatherproofing for masonry products, metal oxides, and glass.1 Notably, this precatalyst resulted in high (>98:2) hydrosilylation selectivity and afforded nOctSi(OEt)3 in 95% yield (entry 1). Having identified a successful ligand to promote selective hydrosilylation, attention was devoted to improving both the bench stability and cost profile of the cobalt precatalysts. Cobalt(II) carboxylates are perhaps the most attractive because they are among the most inexpensive sources of cobalt, are bench stable, and offer structural diversity by variation of the carboxylate substituent. Our laboratory previously demonstrated that treatment of phosphine-Co(OAc)2 mixtures with pincolborane generated active catalysts for the hydroboration of alkenes,14 suggesting that silane activation may also be plausible. The desired pyridine diimine cobalt acetate complex, (MeAPDI)Co(OAc)2 was isolated in 67% yield as a brown powder following straightforward addition of the free ligand to a THF slurry of anhydrous Co(OAc)2. Although (MeAPDI)Co(OAc)2 exhibited excellent bench stability, it proved essentially inactive at ambient temperature (entry 2), likely
a
All reactions were performed on 1.00 mmol scale with a 1:1 mixture of olefin and silane. bYield of isolated material. cDetermined from the 1 H NMR spectrum of isolated material. d0.1 mol % catalyst loading. e Reaction performed at 80 °C.
with alkoxysilanes (entries 1−4), (TFAPDI)Co(2-EH)2 exhibited superior performance with siloxanes (entries 5−8) and triethylsilane (entries 9,10). High selectivity for hydrosilylation over dehydrogenative silylation was observed in all cases, with the exception of triethylsilane, where 56% of the product distribution was dehydrogenative silylation. Magnetic measurements and X-band EPR spectra of all prepared (MeAPDI)Co(O2CR)2 and (TFAPDI)Co(O2CR)2 support S = 3/2, high-spin Co(II) complexes. Each compound exhibited sharp but paramagnetically shifted resonances in the CDCl3 1H NMR spectrum that proved useful for routine characterization. The more crystalline derivatives (MeAPDI)Co(OPiv)2 and (TFAPDI)Co(OPiv)2 were prepared and characterized by X-ray diffraction (Figure 1). Six-coordinate cobalt complexes were observed with the carboxylate ligands adopting both κ2 and κ1 hapticity similar to the coordination geometry reported by Constable and co-workers with a related 2633
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Table 3. Hydrosilylation of Functionalized Terminal Alkenes with (EtO)3SiHa
Figure 1. Representations of the solid state molecular structures of (MeAPDI)Co(OPiv)2 (left) and (TFAPDI)Co(OPiv)2 (right) at 30% probability ellipsoids. Hydrogen atoms are omitted for clarity.
terpyridine example.16 Notably, for both (MeAPDI)Co(OPiv)2 and (TFAPDI)Co(OPiv)2, a single tert-butyl resonance was observed by 1 H NMR spectroscopy, suggesting rapid equilibration of carboxylate coordination modes on the NMR time scale. Significant differences in the pyridine nitrogen− cobalt bond lengths (2.0442(8)Å and 2.112(4)Å for MeAPDI and TFAPDI, respectively) as well as the chelate bite angle (151° versus 144°) support a more open metal center in the TF APDI example, consistent with the improved catalytic performance noted in Table 2. All of the ligand bond distances, a diagnostic for redox activity, support neutral pyridine diimines, and hence, the compounds are best described as high-spin, Co(II). The discovery of highly active, selective, and bench-stable cobalt precatalysts enabled exploration of the scope of the alkene substrates in the hydrosilylation reaction. Trialkoxysilanes with functionalized alkyl groups are of interest due to the improved adhesion properties between materials in siliconebased caulks and sealants as well as imparting other beneficial properties to coatings.1 Certain basic functional groups such as amines are known to inhibit or poison platinum-based catalysts. As presented in Table 3, (TFAPDI)Co(2-EH)2 was effective for the hydrosilylation of a range of terminal alkenes bearing functional groups using (EtO)3SiH. High to excellent isolated yields were obtained after 1 h of reaction time at 23 °C using 0.25 mol % of the cobalt precatalyst. Notable findings include tolerance of amides as well as tertiary and secondary amines. Styrene, a notoriously challenging substrate for regioselective hydrosilylation,17 yielded exclusively the anti-Markovnikov product. Oxygen-based functional groups such as esters, ethers, epoxides, and ketones were also well-tolerated by the cobalt catalyst. Notably, the hydrosilylation of vinylacetate proceeded efficiently in contrast to reactivity observed with (iPrPDI)Fe(N2)2, where rapid deactivation arising from C−O bond cleavage was observed.18 (TFAPDI)Co(2-EH)2 also proved tolerant of remote bromide functionality and large alkyl groups, albeit with reduced efficiency. Unfortunately, allyl chloride proved to be a catalyst poison. Among functionalized terminal alkenes, there is considerable interest in the hydrosilylation of allyl glycidyl ether using earthabundant transition metal catalysts. Trialkoxysilanes derived from this substrate have found widespread application in surface coatings as well as bonding agents in a wide range of products from caulks to sealants to binders used by the fiberglass industry.19 These products offer advantages to amine and sulfur-based alternatives which can lead to discoloration
a
All reactions performed on 2.00 mmol scale with a 1:1 mixture of olefin and silane unless otherwise noted; products were obtained with >98:2, HS:DHS selectivity. b0.5 mol % catalyst loading. c1.0 mol % catalyst loading for 5 h, 1.00 mmol scale. d5 h reaction time. e24 h reaction time. f0.1 mol % catalyst loading, isolated as a mixture of 59:41, anti-Markovnikov to Markovnikov products.
and odorous vapors, respectively. Reduced pyridine diimine iron catalysts have been ineffective for this transformation due to competing ring opening of the epoxide. To fully demonstrate the performance of (TFAPDI)Co(2-EH)2, the hydrosilylation of allyl glycidyl ether with triethoxysilane was conducted on 10 g scale (Scheme 1). Rapid and exothermic reactions were observed at catalyst loading as low as 0.025 mol %, resulting in complete conversion in under 40 min with high antiMarkovnikov selectivity and no evidence for epoxide ring opening. This reactivity compares favorably with both heterogeneous20 and homogeneous21 platinum catalysts, which often promote competing alkene isomerization and Markovnikov addition, leading to side products that must be removed by distillation. The cross-linking of silicone fluids by platinum-catalyzed hydrosilylation produces silicone products that find application as release coatings in everything from tapes to stamps and labels. Due to the viscous morphology of the product, the platinum catalyst becomes trapped in the final product and is not recovered, accounting for approximately 30% of the cost of the final silicone. Commercial standards mandate that this curing process proceed rapidly with low catalyst loadings and without leaving the product colored in any way. The crosslinking activity of (TFAPDI)Co(2-EH)2 was examined at four different catalyst loadings (Scheme 1). Rapid gel formation was observed in each case, and effective silicon cross-linking was observed as low as 1 ppm (based on Co metal, wt/wt). The benefits of lower catalyst loading on the discoloration of the product are illustrated in Scheme 1, where the 1−2 ppm levels yielded colorless product. In summary, a new class of highly active and selective alkene hydrosilylation catalysts based on the earth-abundant metal cobalt has been discovered. Use of inexpensive cobalt(II) 2634
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ACKNOWLEDGMENTS We thank Momentive Performance Materials Inc. for financial support. We also acknowledge Drs. Kenrick Lewis (MPM), Aroop Roy (MPM), Keith Weller (MPM), Julie Boyer (MPM), and Jos Delis (MPM) for helpful discussions.
Scheme 1. Hydrosilylation of Allyl Glycidyl Ether and Crosslinking of Silicone Fluidsa
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Top: 10 g scale hydrosilylation of allyl glycidyl ether (AGE) with triethoxysilane (1:1 olefin to silane ratio). Middle: Cure reaction between siloxane polymers to give cross-linked gel. Bottom: Isolated gels from cross-linking reactions with (left to right) 50 ppm (Co metal, wt/wt), 10, 2, and 1 ppm.
bis(carboxylates) imparts bench stability and enables activation of the catalyst by the tertiary silane substrate. Demonstration of efficient catalysis on a multigram scale and the tolerance of a wide range of functionalized alkenes suggests base metals may reach or even surpass the lofty standards set by platinum catalysts decades ago. Mechanistic studies aimed at uncovering this remarkable performance are ongoing.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00304. Experimental procedures and general protocols, as well as characterization of all new compounds (PDF) Crystallographic data (CIF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare the following competing financial interest(s): Two patents related to this work have been filed: (a) Diao, T.; Chirik, P. J.; Roy, A. K.; Lewis, K. M.; Nye, S.; Weller, K. J.; Delis, J. G. P.; Yu, R.: US Patent Publication US2015/0080536 A1 (Filed: November 19, 2014, published: March 19, 2015) (b) Chirik, P. J.; Schuster, C. H.; Boyer, J. L.; Roy, A. K.; Lewis, K. M.; Delis, J. G. P.; U.S. Patent Application filed July 24, 2015 (unpublished). 2635
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ACS Catalysis (10) Chirik, P. J.; Tondreau, A. M.; Delis, J. G. P.; Lewis, K. M.; Weller, K. J.; Nye, S. A. US Patent 8,765,987, 2014. For additional examples of this type of catalyst activation for hydrosilylation reactions, see references 7a−c and 7e. (11) (a) Ryan, J. W. J. Am. Chem. Soc. 1962, 84, 4730. (b) Buchwald, S. L. Chem. Eng. News 1993, 71, 2. (12) Noda, D.; Tahara, A.; Sunada, Y.; Nagashima, H. J. Am. Chem. Soc. 2016, 138, 2480. (13) Atienza, C. C. H.; Diao, T.; Weller, K. J.; Nye, S. A.; Lewis, K. M.; Delis, J. G. P.; Boyer, J. L.; Roy, A. K.; Chirik, P. J. J. Am. Chem. Soc. 2014, 136, 12108. (14) Scheuermann, M. L.; Johnson, E. J.; Chirik, P. J. Org. Lett. 2015, 17, 2716. (15) This ligand can be prepared in one step from inexpensive materials. See: Bernauer, K.; Gretillat, F. Helv. Chim. Acta 1989, 72, 477. See also Supporting Information for details.. (16) Constable, E. C.; Housecroft, C. E.; Jullien, V.; Neuburger, M.; Schaffner, S. Inorg. Chem. Commun. 2006, 9, 504. (17) Sprengers, J. W.; de Greef, M.; Duin, M. A.; Elsevier, C. J. Eur. J. Inorg. Chem. 2003, 2003, 3811. (18) Trovitch, R. J.; Lobkovsky, E.; Bouwkamp, M. W.; Chirik, P. J. Organometallics 2008, 27, 6264. (19) Chernyshev, E. A.; Belyakova, Z. V.; Knyazeva, L. K.; Khromykh, N. N. Russ. J. Gen. Chem. 2007, 77, 55. (20) Monkiewicz, J.; Bade, S.; Schoen, U. U.S. Patent 6,100,408, 2000. (21) Bade, S.; Seliger, B.; Schladerbeck, N.; Sauer, J. U.S. Patent 8,039,646, 2011.
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