Research Article pubs.acs.org/acscatalysis
Two-Silicon Cycle for Carbonyl Hydrosilylation with Nikonov’s Cationic Ruthenium(II) Catalyst Julien Fuchs, Hendrik F. T. Klare, and Martin Oestreich* Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany
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S Supporting Information *
ABSTRACT: An experimental analysis proves that Nikonov’s carbonyl hydrosilylation proceeds through a two-silicon cycle rather than the originally proposed one-silicon cycle. The intermediate ruthenium(II) monohydride is not sufficiently hydridic to transfer its hydride onto the silylcarboxonium ion. However, that hydricity is enhanced by oxidative addition of another hydrosilane molecule to afford the corresponding ruthenium(IV) silyl dihydride as the actual hydride donor. The present study also demonstrates that the acetonitrile ligands in Nikonov’s ruthenium(II) catalyst are not innocent. That complex is able to hydrosilylate its own ligand(s), and the resulting N,N-disilylated amine base accounts for competing dehydrogenative silylation of enolizable carbonyl compounds, explaining the formation of a silyl enol ether in substantial quantities next to the expected silyl ether. Both findings lead to a revised mechanistic picture that provides the basis for the development of more efficient and chemoselective catalysts. KEYWORDS: homogeneous catalysis, hydrosilanes, ionic hydrosilylation, Lewis-acid catalysis, reaction mechanism, ruthenium, Si−H bond activation
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INTRODUCTION Ionic outer-sphere mechanisms have been documented for both main-group- and transition-metal-catalyzed carbonyl hydrosilylation with hydrosilanes.1 Prominent examples include contributions by Piers2,3 and Brookhart4,5 employing either B(C6F5)3 (1) or [(POCOP)Ir(H)(acetone)]+[B(C6F5)4]− (2+[B(C6F5)4]−) as catalysts (Figure 1) [POCOP = 2,6-
Scheme 1. Ionic Outer-Sphere Carbonyl Hydrosilylation Catalyzed by Main-Group and Transition-Metal Lewis Acids: One-Silicon versus Two-Silicon Cycle (Si = R3Si)
Figure 1. Lewis-acid catalysts 1 (Piers),2,3 2+[B(C6F5)4]− (Brookhart),4,5 and 3+[B(C6F5)4]− (Nikonov)8−10 for ionic outer-sphere carbonyl hydrosilylation.
together with the borohydride and is eventually reduced by [HB(C6F5)3]−.3 In the other case with cationic pincer complex 2+, the resulting neutral iridium(III) dihydride does not react with the silylcarboxonium ion. Another molecule of the hydrosilane is essential to turn on its ability to release one of its hydrides either by formation of the corresponding hydrosilane−dihydride Lewis adduct or by oxidative addition to the iridium(V) silyl trihydride.5b This conforms with a two-
bis(di-tert-butylphosphinito)phenyl].6 These processes share electrophilic Si−H bond activation7 through η1- or η2-σ coordination of the Si−H bond by the neutral or cationic Lewis-acidic catalyst (LA) followed by silyl transfer from the activated hydrosilane to the carbonyl oxygen atom (Scheme 1). Their difference lies in the actual reduction step, i.e., delivery of the hydride to the silylcarboxonium ion (gray box). In the case of neutral 1, the silylcarboxonium ion arises as an ion pair © 2017 American Chemical Society
Received: September 28, 2017 Revised: October 26, 2017 Published: October 26, 2017 8338
DOI: 10.1021/acscatal.7b03336 ACS Catal. 2017, 7, 8338−8342
Research Article
ACS Catalysis silicon cycle while the main-group catalysis is a one-silicon cycle. Armed with this insight, we asked ourselves whether neutral transition-metal hydrides generally require the assistance of a hydrosilane to close the catalytic cycle. We now report here the reinvestigation of Nikonov’s carbonyl hydrosilylation promoted by [Cp(Ph3P)Ru(acetonitrile)2]+[B(C6F5)4]− (3+[B(C6F5)4]−, Figure 1).8−10 Our experimental analysis again supports a twosilicon cycle instead of the postulated one-silicon cycle (Scheme 2).8 The catalytically active 16-electron complex 4+ is formed Scheme 2. Proposed One-Silicon Cycle of Nikonov’s Catalytic Carbonyl Hydrosilylationa
Figure 2. Selected segment of the 1H/1H NOESY NMR spectrum (500/500 MHz, CD2Cl2, 300 K, Tm = 600 ms) of hydrosilane− ruthenium(II) adduct 6a+[B(C6F5)4]− (Si = Me2PhSi).
a
Our earlier mechanistic analyses of the carbonyl hydrosilylations catalyzed by B(C6F5)3 (1)3b and Brookhart’s complex 2+[B(C6F5)4]−5b had showcased the use of siliconstereogenic hydrosilanes as a tool to probe reaction mechanisms by tracking the stereochemical course at the silicon atom.13 Inversion is seen with 13b,13c and stoichiometrydependent racemization (equimolar hydrosilane) or partial inversion (excess hydrosilane) are obtained with 2+[B(C6F5)4]−.5b Racemization at low hydrosilane concentration relative to acetophenone is explained by the intermediacy of configurationally labile pentacoordinate silicon compounds competing with the hydride transfer at the stage of the silylcarboxonium ion.5b For the stereochemical analysis of Nikonov’s carbonyl hydrosilylation catalyzed by 3+[B(C6F5)4]−, we employed enantiomerically enriched acyclic hydrosilane (SiS)-5b13b (e.r. = 95:5, Scheme 3, top). To our surprise, we obtained the expected silyl ether 10b as the minor (!) component of a mixture with the silyl enol ether 11b (see below). Racemization at the silicon atom was found for that silyl enol ether 11b regardless of the amount of (SiS)-5b. To
[B(C6F5)4]− as a counteranion is omitted for clarity.
by reversible dissociation of acetonitrile from 3+. The Si−H bond of the hydrosilane 5 then coordinates to 4+ in η2-σ fashion to form 6+.11 The subsequent silyl transfer from 6+ to the carbonyl oxygen atom of 7 was calculated to be rate determining. The step in question, that is the hydride transfer from the neutral ruthenium(II) monohydride 8 to the silylcarboxonium ion 9+ (gray box), is believed to proceed over a low barrier to regenerate 4+ along with the silyl ether 10.
Scheme 3. Stereochemical Analysis with Silicon-Stereogenic Hydrosilane
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RESULTS AND DISCUSSION Our mechanistic investigation began with an NMR spectroscopic analysis of the Si−H bond activation by complex 3+. In a stoichiometric reaction using hydrosilane Me2PhSiH (5a), the short-lived adduct 6a+ previously described by Nikonov and coworkers8 was studied in detail. Reversibility of the Si−H bond coordination was verified by chemical exchange between the hydride of the coordinated hydrosilane at δ −9.0 ppm and the hydride of the free hydrosilane at δ 4.4 ppm in the 1H NMR spectrum (Figure 2). The 1JH,Si coupling constant of 56.3 Hz of the coordinated hydrosilane in 6a+ suggests a nonclassical σ interaction between the hydrosilane and the ruthenium center.7b,8 We note here that the generation of 6a+ is accompanied by the formation of doubly hydrosilylated acetonitrile, thereby showing that 3+ is able to hydrosilylate its own ligand(s).12
a
Diastereomeric ratios and ratios of silyl ether to silyl enol ether are determined by GLC analysis. bEnantiomeric ratios are determined by HPLC analysis using chiral stationary phases. 8339
DOI: 10.1021/acscatal.7b03336 ACS Catal. 2017, 7, 8338−8342
Research Article
ACS Catalysis check whether catalyst 3+ is able to erode the enantioenrichment of (SiS)-5b, we treated (SiS)-5b with 2.7 mol % of 3+; no loss of enantiomeric purity was detected after 24 h (Scheme 3, bottom). These results suggest the same scenario for Nikonov’s ruthenium(II) complex 3+[B(C6F5)4]− as for Brookhart’s iridium(III) complex 2+[B(C6F5)4]−.5b The fact that the silyl enol ether 11b is formed as the major product (with liberation of dihydrogen) raises the question of whether the desired silyl ether 10b traces back to hydrogenation of 11b catalyzed by 3+. To verify this, we used deuterium-labeled acetophenone-d3 (7-d3) in the hydrosilylation with Me2PhSiH (5a) (Scheme 4, top). The NMR
tion of 9a+-d3 and shifts the ratio from 30:70 to 60:40 in favor of the silyl ether. With sterically more demanding (SiS)-5b, there was no dependence on the hydrosilane-to-acetophenone ratio (cf. Scheme 3, top). We interpret this as additional evidence of the involvement of two silicon compounds in the reduction step, either preventing the anticipated activation of the neutral ruthenium(II) monohydride 8 or the hydride delivery to the silylcarboxonium ion for steric reasons. To identify the actual hydride donor in Nikonov’s carbonyl hydrosilylation, we initially targeted the preparation of the postulated neutral ruthenium(II) monohydride 8 (Scheme 6, Scheme 6. Synthesis of Ruthenium(IV) Silyl Dihydride 13a
Scheme 4. Control Experiments Excluding Hydrogenation of the Silyl Enol Ether
top), but its independent synthesis failed. However, we were able to access the ruthenium(IV) silyl dihydride 13a (Scheme 6, bottom)14 which can be regarded as the oxidative-addition complex of hydrosilane 5a added to the coordinatively unsaturated ruthenium(II) monohydride 12; 12, in turn, could be available from 8 by acetonitrile dissociation (Scheme 6, top).15 That silyl dihydride 13a is exactly what, in analogy to Brookhart’s case,5b we imagined to be the hydride source in a two-silicon cycle. Neutral 13a was prepared from cationic complex 3+[PF6]− (Scheme 6, bottom). With silyl dihydride 13a in hand, we performd a series of control experiments to prove or disprove whether 13a is a catalytically competent intermediate in the catalytic cycle (Scheme 7). To substantiate its hydride donor ability, 13a was reacted with freshly prepared silylcarboxonium ion 9c+ in a stoichiometric experiment (Scheme 7A; note that 13a and 9c+ contain different silyl groups). This reaction afforded the silyl ether 10c and ethylbenzene 14 in a complex mixture. This corroborates our hypothesis that the ruthenium(IV) silyl dihydride 13a does indeed release hydride. Conversely, the same complex 13a cannot promote the carbonyl hydrosilylation (Scheme 7B). Hydride abstraction from 13a by a silylcarboxonium ion is a prerequisite to (re)generate Nikonov’s catalytically active intermediates 4+ and 6a+. A clearer picture emerges when a hydride is abstracted from 13a by silylcarboxonium ion 9c+ in the presence of acetophenone (7) prior to the addition of hydrosilane 5a (Scheme 7C). 7 then fully converts into silyl ether 10a (cf. Scheme 7A) along with silyl ether 10a without the formation of either silyl enol ether 11a or 11c. This result leads to the following conclusions: It demonstrates that (1) acetonitrile-free 13a is part of the catalytic cycle and (2) the acetonitrile ligand introduced with catalyst 3+ is involved in the dehydrogenation pathway, i.e., the formation of the silyl enol ether by α deprotonation of the silylcarboxonium ion. To further verify the latter, we repeated control experiment 7C with addition of doubly hydrosilylated acetonitrile 15a (formed in the catalysis with 3+, see above). Indeed, formation of silyl
spectroscopic analysis showed no deuterium incorporation at the carbinol carbon atom in silyl ether 10a-d3, hence excluding hydrogenation of 11a-d2 with generated D−H. The attempted reduction of silyl enol ether 11b with dihydrogen under the catalytic setup did not afford the silyl ether 10b (Scheme 4, bottom). Nikonov and co-workers had not included the formation of silyl enol ethers in their communication.8 We noticed that the hydrosilane-to-acetophenone ratio influences the reaction outcome (Scheme 5). The ratio of 30:70 gradually changed to 60:40 when increasing the amount of Me2PhSiH (5a) from 1.0 to 5.0 equiv. This trend supports the notion that another hydrosilane molecule enhances the hydride transfer to silylcarboxonium ion 9a+ at the cost of its α deprotonation. A distinct isotope effect is seen with acetophenone-d3 (cf. Scheme 4, top); the deuterium-labeling slows down the α deprotonaScheme 5. Ratio of Silyl Ether and Silyl Enol Ether Depending on the Hydrosilane-to-Acetophenone Ratio
a
Ratio of silyl ether to silyl enol ether is determined by 1H NMR analysis. 8340
DOI: 10.1021/acscatal.7b03336 ACS Catal. 2017, 7, 8338−8342
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ACS Catalysis Scheme 7. Control Experiments with Ruthenium(IV) Silyl Dihydride 13a
Scheme 8. Revised Catalytic Cycle for the Hydrosilylation of Acetophenone with Nikonov’s Catalysta
a
a
Si = Me2PhSi, L = acetonitrile or donor.
due to formation of the pentacoordinate intermediate 18a+.5b The actual hydride donor, silyl dihydride 13a, is generated by oxidative addition of another molecule of hydrosilane 5a to ruthenium(II) monohydride 17. The hydride transfer from 13a to silylcarboxonium ion 9a+ closes the catalytic cycle, releasing silyl ether 10a and η2-σ hydrosilane adduct 16a+ with stabilization of L. The dehydrogenation pathway leading to silyl enol ether 11a is also mechanistically understood (Scheme 8, clockwise left). Doubly hydrosilylated acetonitrile 15a, existing in twice the amount of catalyst 3+, acts as the base, converting the silylcarboxonium ion 9a+ into 11a by α deprotonation. The resulting ammonium ion 19a+ then protonates the dormant ruthenium(II) monohydride 17 in the presence of hydrosilane 5a with evolution of dihydrogen to arrive back at the η2-σ hydrosilane adduct 16a+.
Ratios are determined by GLC analysis.
ethers 10a and 10c was now accompanied by the formation of silyl enol ether 11a (Scheme 7D) in the expected ratio (cf. Scheme 5). Notably, the catalysis can also be initiated in the same way from silyl dihydride 13a by using the trityl cation as the hydride acceptor instead of silylcarboxonium ion 9c+ (not shown; see the Supporting Information for details). Finally, we were able to show that 13a does not undergo reductive elimination of hydrosilane 5a. A mixture of 13a and deuteriumlabeled 5a-d1 did not equilibrate to 13a and 13a-d1 (Scheme 7E). Combining the above experimental observations leads to a revised catalytic cycle for Nikonov’s carbonyl hydrosilylation (Scheme 8, counterclockwise right). As catalyst 3+ is able to hydrosilylate its own ligands12 and since the presence of acetonitrile is not essential, η2-σ hydrosilane adduct 16a+ is a reasonable starting point of the catalytic cycle (L = acetonitrile or donor). An SN2-type nucleophilic attack of the carbonyl oxygen atom of acetophenone (7) at the activated hydrosilane in 16a+ yields the silylcarboxonium ion 9a+ and the neutral ruthenium(II) monohydride 17. This step proceeds with inversion of the configuration at the silicon atom but subsequent racemization by pseudorotational processes is facile
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CONCLUSIONS Nikonov’s carbonyl hydrosilylation is now another example of a one-silicon cycle revised to a two-silicon cycle.5b We think that there are several other hydrosilylation reactions where the initially formed (neutral) transition-metal hydride is not sufficiently hydridic to transfer its hydride but where that hydricity is boosted by the aid of another hydrosilane molecule. We are currently in quest of similar cases. 8341
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ACS Catalysis
(9) Yang, Y.-F.; Chung, L. W.; Zhang, X.; Houk, K. N.; Wu, Y.-D. J. Org. Chem. 2014, 79, 8856−8864. (10) [Cp(iPr3P)Ru(acetonitrile)2]+[B(C6F5)4]− was originally used as the hydrosilylation catalyst.8 However, we found that 3+[B(C6F5)4]− with a triphenylphosphine ligand performs equally well in the hydrosilylation of acetophenone. We decided to continue working with 3+ because 1H NMR spectra are better resolved. (11) That η2-σ coordination is supposed not to involve interligand interactions between the hydrosilane and the acetonitrile ligand based on quantum-chemical calculations.8 (12) Unlike [Cp(iPr 3 P)Ru(acetonitrile) 2 ] + [B(C 6 F 5 ) 4 ] − , 3+[B(C6F5)4]− employed by us was shown by Gutsulyak and Nikonov not to promote catalytic hydrosilylation of nitriles with Me2PhSiH (see the Supporting Information for Gutsulyak, D. V.; Nikonov, G. I. Angew. Chem., Int. Ed. 2010, 49, 7553−7556. However, [Cp(Ph3P) Ru(acetonitrile) 2] + [B(C6 F5 ) 4] − did react in our hands in a stoichiometric reaction. (13) For additional examples, see: (a) Rendler, S.; Oestreich, M.; Butts, C. P.; Lloyd-Jones, G. C. J. Am. Chem. Soc. 2007, 129, 502−503. (b) Klare, H. F. T.; Oestreich, M.; Ito, J.-i.; Nishiyama, H.; Ohki, Y.; Tatsumi, K. J. Am. Chem. Soc. 2011, 133, 3312−3315. (c) Fallon, T.; Oestreich, M. Angew. Chem., Int. Ed. 2015, 54, 12488−12491. (14) For the synthesis and crystallographic characterization of related ruthenium(IV) silyl dihydrides, see: (a) Osipov, A. L.; Gerdov, S. M.; Kuzmina, L. G.; Howard, J. A. K.; Nikonov, G. I. Organometallics 2005, 24, 587−602. (b) Gutsulyak, D. V.; Osipov, A. L.; Kuzmina, L. G.; Howard, J. A. K.; Nikonov, G. I. Dalton Trans. 2008, 6843−6850. (15) Campion, B. K.; Heyn, R. H.; Tilley, T. D. J. Chem. Soc., Chem. Commun. 1992, 1201−1203.
Moreover, we have also revealed the interfering role of the acetonitrile ligand. Its doubly hydrosilylated derivative is an amine base that is responsible for competing dehydrogenative silylation of acetophenone. Nikonov and co-workers had reported 30% yield for the hydrosilylation of acetophenone while not mentioning the formation of the corresponding silyl enol ether (see Table 1 in ref 8). We were able to show that the dehydrogenation pathway can be completely silenced when using an acetonitrile-free catalyst. Hence, this mechanistic insight opens the door to the development of chemoselective catalysts in a targeted way.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b03336. Experimental details, characterization, and spectroscopic data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Julien Fuchs: 0000-0003-0816-6497 Hendrik F. T. Klare: 0000-0003-3748-6609 Martin Oestreich: 0000-0002-1487-9218 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.F. thanks the Fonds der Chemischen Industrie for a predoctoral fellowship (2016−2018), and M.O. is indebted to the Einstein Foundation (Berlin) for an endowed professorship. Additional support by the Cluster of Excellence Unif ying Concepts in Catalysis of the Deutsche Forschungsgemeinschaft (EXC 314/2) is also acknowledged. We thank Dr. Sebastian Kemper (TU Berlin) for expert advice with the NMR measurements.
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REFERENCES
(1) Iglesias, M.; Fernández-Alvarez, F. J.; Oro, L. A. ChemCatChem 2014, 6, 2486−2489. (2) Parks, D. J.; Piers, W. E. J. Am. Chem. Soc. 1996, 118, 9440−9441. (3) (a) Parks, D. J.; Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000, 65, 3090−3098. (b) Rendler, S.; Oestreich, M. Angew. Chem., Int. Ed. 2008, 47, 5997−6000. (c) Sakata, K.; Fujimoto, H. J. Org. Chem. 2013, 78, 12505−12512. (d) Houghton, A. Y.; Hurmalainen, J.; Mansikkamäki, A.; Piers, W. E.; Tuononen, H. M. Nat. Chem. 2014, 6, 983−988. (4) Park, S.; Brookhart, M. Organometallics 2010, 29, 6057−6064. (5) (a) Yang, J.; White, P. S.; Schauer, C. K.; Brookhart, M. Angew. Chem., Int. Ed. 2008, 47, 4141−4143. (b) Metsänen, T. T.; Hrobárik, P.; Klare, H. F. T.; Kaupp, M.; Oestreich, M. J. Am. Chem. Soc. 2014, 136, 6912−6915. (6) Robert, T.; Oestreich, M. Angew. Chem., Int. Ed. 2013, 52, 5216− 5218. (7) For an authoritative review of electrophilic Si−H bond activation, see: (a) Lipke, M. C.; Liberman-Martin, A. L.; Tilley, T. D. Angew. Chem., Int. Ed. 2017, 56, 2260−2294. For a comprehensive overview of Si−H bond activation by transition-metal complexes, see: (b) Corey, J. Y. Chem. Rev. 2011, 111, 863−1071. (8) Gutsulyak, D. V.; Vyboishchikov, S. F.; Nikonov, G. I. J. Am. Chem. Soc. 2010, 132, 5950−5951. 8342
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