Communication pubs.acs.org/Organometallics
Potassium-Catalyzed Hydrosilylation of Activated Olefins: Evidence for a Silyl Migration Mechanism Valeri Leich, Thomas P. Spaniol, and Jun Okuda* Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany S Supporting Information *
ABSTRACT: The alkali-metal silyl [K(L)SiPh3] (1; L = 18crown-6 ether) catalyzed the hydrosilylation of activated CC double bonds. Isolation and characterization of an addition product is in agreement with the anti-Markovnikov selectivity. Second-order kinetics for the hydrosilylation of 1,1′-diphenylethylene and the kinetic isotope effect of kH/kD = 3.1 indicate that a silyl migration mechanism is operative.
molecular alkali-metal and alkaline-earth-metal silyls.6 We have now examined the mechanism using the structurally wellcharacterized potassium triphenylsilyl [K(L)SiPh3] (1; L = 18crown-6 ether)6a and the substrate scope of this regioselective hydrosilylation reaction. Isolation, characterization, and singlecrystal X-ray diffraction of an intermediate along with kinetic studies suggest the silyl migration mechanism to be operative. Potassium triphenylsilyl [K(L)SiPh3)] (1; L = 18-crown-6 ether) hydrosilylated 1,1′-diphenylethylene (1,1′-DPE) as a model substrate. HSiPh3 led to full conversion within 24 h at 60 °C (Table 1, entry 1).6a The deuterated silane DSiPh3 gave the corresponding anti-Markovnikov product also on a preparative scale, but it took ca. 120 h for completion (Table 1, entry 2). Using 1, styrene and trans,trans-1,4-diphenyl-1,3-butadiene were polymerized within 0.1 and 2 h, respectively; 4-phenyl1-butene and 1,3-cyclohexadiene were not hydrosilylated at 60 °C (Table 1, entries 3−6). 1-Phenylcyclohexene reacted with H3SiPh to give the cis anti-Markovnikov product in 53% yield within 1.5 h (Table 1, entry 7). The reaction time and conversion are much lower than for the reported hydrosilylation of 1-phenylcyclohexadiene with H3SiPh (16 h and 90%).4 Sterically hindered triphenylethylene slowly reacted with HSiPh3 to give the anti-Markovnikov product (Table 1, entry 8). After 24 h, only 10% of the hydrosilylated product was observed in the 1H NMR spectrum.7 With the silanes H2SiPh2 and H3SiPh shorter reaction times of 3 and 1.5 h gave quantitative conversion (Table 1, entries 9 and 10). The hydrosilylation catalyzed by the silyl 1 is limited to activated CC double bonds.
Hydrosilylation of olefins is an efficient method to synthesize and modify organosilicones.1 Catalysts for industrial applications usually are based on late transition metals such as platinum.2 Alternative catalysts could avoid the use of this expensive and toxic metal and lead to different chemo- and regioselectivities. Several catalysts based on inexpensive and earth-abundant main-group metals have been developed over the past decade.3 Using well-defined hydrosilylation catalysts of group 1 and 2 metals (K, Ca, and Sr),4 Harder and co-workers observed both Markovnikov and anti-Markovnikov products.5a The formation of the unusual anti-Markovnikov products was explained by the silyl migration (Scheme 1), for which the formation of a metal silyl is assumed.4 Hydrosilylation of CC double bonds catalyzed by d0 and rare-earth metals occur by the metal hydride mechanism.5b Recently we have obtained exclusively anti-Markovnikov hydrosilylation products using Scheme 1. Silyl Migration Mechanism Proposed by Harder4
Received: February 26, 2016
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.6b00160 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Table 1. Catalytic Hydrosilylation of Olefins Catalyzed by Alkali-Metal Silyl [K(L)SiPh3] (1)
Reaction conditions: cat. 1 (2.5 μmol), olefin (0.1 mmol), silane (0.11 mmol) in THF-d8 (0.6 mL). L = 18-crown-6 ether. bDetermined by 1H NMR spectroscopy. cOne diastereomer, cis product. a
soluble in aliphatic hydrocarbons. Elemental analysis and NMR spectroscopic data agree with the proposed formula (see the Supporting Information). A single crystal of 2 was obtained from a concentrated benzene solution at 25 °C. The crystal structure from X-ray diffraction reveals a one-dimensional coordination polymer in the solid state formed by alternating units of [K(L)Ph2CCH2SiPh3] (Figure 1 and the Supporting Information for a graphical representation of the coordination polymer). The potassium atom is coordinated by six oxygen atoms of the 18-crown-6 ether within the plane of the crown ether (C5− K1−Oav = 90.60(4)°). K···CPh interactions are indicated by K− C distances of 3.0817(16)−3.5535 (15) Å, which fall in the range of unsymmetrical K···CPh11 and K−Colefin interactions.12 Ionic interactions of the potassium ion with the phenyl groups of the 1,1′-DPE moiety could stabilize the transition state leading to the anti-Markovnikov product. The 1H NMR spectrum of 2 in THF-d8 agrees with the proposed formula, and the 29Si{1H} NMR spectrum of 2 shows one singlet at δ −17.35 ppm (see the Supporting Information). Next, we investigated the rate-determining C−H bond formation of the addition (“insertion”) product 2 with silanes (Scheme 3). The σ-bond metathesis proceeded more slowly than the addition. Heating a mixture of [K(L)Ph2CCH2SiPh3]
Hydrosilylation catalyzed by alkali or alkaline-earth metals (K, Ca, and Sr) gave both Markovnikov and anti-Markovnikov products, with presumably a metal hydride as the catalytically active species.4,8 Potassium triphenylsilyl 1 led to antiMarkovnikov products selectively with a metal silyl as the catalytically active species. Harder explained the opposite regioselectivity by a mechanism that involves silyl migration,9 a process well established for transition-metal-catalyzed hydrosilylation of CC double bonds.10 To find out if the proposed product B of formal addition is formed, we treated the alkali-metal silyl 1 with 1,1′-DPE in THF at 25 °C. The solution immediately turned red. The product [K(L)Ph2CCH2SiPh3] (2; L = 18-crown-6 ether) was isolated from THF/n-pentane solution after 12 h at −30 °C (Scheme 2). 2 is soluble in THF and benzene but sparingly Scheme 2. Formation of the Addition (“Insertion”) Product [K(L)Ph2CCH2SiPh3] (2)a
a
L = 18-crown-6 ether. B
DOI: 10.1021/acs.organomet.6b00160 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
decomposition, in agreement with the selectivity of this process and the stability of the insertion product 2. We explain this reactivity by a mechanism that involves formal silyl migration. 1,1′-DPE inserts irreversibly into the metal−silyl bond of [K(L)SiPh3] (1). The resonance-stabilized addition (insertion) product [K(L)Ph2CCH2SiPh3] (2) reacts with HSiPh3 under σ-bond metathesis (protonolysis) to regenerate the catalyst [K(L)SiPh3] (1) and to give the antiMarkovnikov product 3. This step is rate-determining with k1 ≫ k2; the C−H bond forming reaction is reversible. A value of K = k2/k−2 = 1.91 was estimated for the reversible σ-bond metathesis step. The hydrosilylation of 1,1′-DPE to give 3 was monitored by 1 H NMR spectroscopy. In THF-d8 at 60 °C, second-order kinetics was found with rate constants between kH = 8.9(5) × 10−4 and [4.44(3)] × 10−3 L mol−1 s−1 (Table 2, entries 1−4).
Figure 1. Molecular structure of 2 in the solid state. Displacement ellipsoids are shown at 50% probability; all hydrogen atoms are omitted for clarity. Selected interatomic distances (Å) and angles (deg): K1−C4 3.5535(15), K1−C5 3.0817(16), K1−C6 3.3400(16), K1−C12 3.3095(16); C4−C5−C6 121.80(15), C5−K1−O av 90.60(4), C1−C2−C3−C8 172.53(12), C1−C2−C9−C10 150.62(13).
Table 2. Rate Constants of the Hydrosilylation of 1,1′-DPE with (H/D)SiPh3
Scheme 3. Stoichiometric Reactions Involving C−H Bond Formationa entrya
silane
1 (mol %)
k (L mol−1 s−1)b
1 2 3 4 5
HSiPh3 HSiPh3 HSiPh3 HSiPh3 DSiPh3
4.5 10 19 27 17
8.9(5) × 10−4 1.97(5) × 10−3 3.15(5) × 10−3 4.44(3) × 10−3 8.2(1) × 10−4
Conditions: 100 μmol of 1,1′-DPE, 105 μmol of (H/D)SiPh3, 33.3 μmol of C6Me6 in THF-d8 (total volume 0.7 mL). bDetermined from logarithmic plots (up to 60% conversion). a
a
Results with different catalyst loadings indicate direct involvement of the metal silyl [K(L)SiPh3] (1) by a reaction rate that has a first-order dependence on the concentration of the catalyst (see the Supporting Information). Rate constants between kobs = 0.67 × 10−4 and 3.93 × 10−3 L mol−1 s−1 (T = 0−30 °C) were observed for the hydrosilylation of styrene with HSiMe2Ph catalyzed by a silica-supported platinum catalyst15 and kobs = 83 × 10−3 and 252 × 10−3 L mol−1 s−1 (T = 25 °C) were reported for the hydrosilylation of (allyloxy)benzene with 1,1,1,3,3,5,5-heptamethyltrisiloxane catalyzed by dichloro(dicyclopentadiene)platinum.16 With 1,1′-DPE and 10 equiv of HSiPh3, we observed pseudofirst-order dependence (kobs = 3.69(5) × 10−3 s−1). A rate constant of kobs = 1.7 × 10−3 s−1 (T = −38 °C) was reported for the hydrosilylation of 3,3′-dimethyl-1-butene with HSiEt3 (10− 30 equiv) and a palladium complex.10 Deuterium labeling experiments showed a kinetic isotope effect (KIE). Since deuterium was not incorporated into the phenyl groups, the hydrosilylation of 1,1′-DPE with DSiPh3 is highly selective. A value for KIE of kH/kD = 3.1 (Table 2, entry 5) indicates that a Si−H bond is cleaved during the ratedetermining step, in agreement with the proposed silyl mechanism (Scheme 4). Similar results were found for the hydrosilylation of CC double bonds catalyzed by the zerovalent platinum complex [Pt(PhCHCH2)3] (kH/kD = 3.6)17 and the cationic ruthenium silylene [Cp*(iPr3P)RuH2(SiHMes)] [B(C6F5)4] (kH/kD = 1.5).18
L = 18-crown-6 ether. Legend: (a) reaction of 2 with (H/D)SiPh3 (60 °C, 2 h) and the reverse reaction (25 °C, 10 min) in THF-d8; (b) reaction of 2 with (H/D)SiPh3 in the presence of 1,1′-DPE in THF-d8.
(2) with the silanes (H/D)SiPh3 in THF-d8 for 2 h at 60 °C led to 3 and 3-d1 in yields up to 40% (Scheme 3a). Since prolonged reaction times did not lead to higher conversion, the process appears to be reversible. The alkali-metal silyl [K(L)SiPh3] (1) reacted with the hydrosilylation products 3 and 3-d1 in THF-d8 within 10 min at 25 °C to give the same equilibrium mixture (see the Supporting Information for in situ 1H NMR spectra). The silyl anion (SiPh3 ) −, as a strong Brønsted base (pKa(HSiPh3) = 35.1),13 deprotonates weaker acids such as 3. We estimate a pKa value of 34−35 for 3, as pKa = 33.4 is reported for Ph2CH213 in combination with the acid-weakening effect of the β-silicon atom.14 Similar results were found when arylmethanes were metalated with [KSiPh3].13 Furthermore, stoichiometric reactions revealed that the potassium silyl 1 acts as the active species and the addition product 2 as the resting state of the catalyst. To avoid the backreaction of the metal silyl 1 with the hydrosilylation product 3, we added 1 equiv of 1,1′-DPE to the addition product 2 and (H/D)SiPh3. The metal silyl 1 was expected to quickly react with 1,1′-DPE to regenerate 2. After the mixture was heated in THF-d8 for 2 h at 60 °C, we observed a 1:1 composition of 2 and 3 (or 3-d1) (Scheme 3b). The 1H NMR spectrum does not show signals from other products due to side reactions or C
DOI: 10.1021/acs.organomet.6b00160 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
[SiH 2 Ph 3 ] with 1,1′-DPE gave [K(18-crown-6)][SiPh 3 ] and Ph2CHCH3. Therefore, we believe that a metal silyl species is responsible for the anti-Markovnikov regioselectivity. (5) (a) Spielmann, J.; Harder, S. Chem. - Eur. J. 2007, 13, 8928− 8938. (b) Molander, G. A.; Romero, A. C. Chem. Rev. 2002, 102, 2161−2186. (6) (a) Leich, V.; Lamberts, K.; Spaniol, T. P.; Okuda, J. Dalton Trans. 2014, 43, 14315−14321. (b) Leich, V.; Spaniol, T. P.; Maron, L.; Okuda, J. Chem. Commun. 2014, 50, 2311−2314. (7) Wu, T. C.; Wittenberg, D.; Gilman, H. J. Org. Chem. 1960, 25, 596−598. (8) Spielmann, J.; Buch, F.; Harder, S. Angew. Chem., Int. Ed. 2008, 47, 9434−9438. (9) Harder, S. Chem. Rev. 2010, 110, 3852−3876. (10) LaPointe, A. M.; Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1997, 119, 906−917. (11) (a) Clark, D. L.; Gordon, J. C.; Huffman, J. C.; Vincent-Hollis, R. L.; Watkin, J. G.; Zwick, B. D. Inorg. Chem. 1994, 33, 5903−5911. (b) Fukin, G. K.; Lindeman, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2002, 124, 8329−8336. (c) Smith, J. D. Adv. Organomet. Chem. 1999, 43, 267−348. (12) (a) Simpson, C. K.; White, R. E.; Carlson, C. N.; Wrobleski, D. A.; Kuehl, C. J.; Croce, T. A.; Steele, I. M.; Scott, B. L.; Young, V. G.; Hanusa, T. P.; Sattelberger, A. P.; John, K. D. Organometallics 2005, 24, 3685−3691. (b) Quisenberry, K. T.; Gren, C. K.; White, R. E.; Hanusa, T. P.; Brennessel, W. W. Organometallics 2007, 26, 4354− 4356. (c) Torvisco, A.; Decker, K.; Uhlig, F.; Ruhlandt-Senge, K. Inorg. Chem. 2009, 48, 11459−11465. (d) Jochmann, P.; Davin, J. P.; Maslek, S.; Spaniol, T. P.; Sarazin, Y.; Carpentier, J.-F.; Okuda, J. Dalton Trans. 2012, 41, 9176−9181. (e) Lichtenberg, C.; Spaniol, T. P.; Peckermann, I.; Hanusa, T. P.; Okuda, J. J. Am. Chem. Soc. 2013, 135, 811−821. (13) Buncel, E.; Venkatachalam, T. K. J. Organomet. Chem. 2000, 604, 208−210. (14) Bausch, M. J.; Gong, Y. J. Am. Chem. Soc. 1994, 116, 5963− 5964. (15) Miao, Q. J.; Fang, Z.-P.; Cai, G. P. Catal. Commun. 2003, 4, 637−639. (16) Coqueret, X.; Wegner, G. Organometallics 1991, 10, 3139−3145. (17) Caseri, W.; Pregosin, P. S. J. Organomet. Chem. 1988, 356, 259− 269. (18) Fasulo, M. E.; Lipke, M. C.; Tilley, T. D. Chem. Sci. 2013, 4, 3882−3887.
Scheme 4. Modified Silyl Migration Mechanism
In conclusion, the alkali-metal silyl [K(L)SiPh3] (1; L = 18crown-6 ether)6a was shown to catalyze the hydrosilylation of activated CC double bonds. We have provided experimental evidence that a silyl migration mechanism, as first proposed by Harder et al. for s-block metal catalysts,4 is responsible for the anti-Markovnikov regioselectivity. The structure of the isolated addition product [K(L)Ph2CCH2SiPh3] (2) supports this regioselectivity. A value for KIE of kH/kD = 3.1 confirms σbond metathesis (protonolysis) as the rate-determining, reversible step in the hydrosilylation cycle. This model could be useful for the design of other earth-abundant metal catalysts3,4 for selective hydrosilylation of CC double bonds leading to anti-Markovnikov products.
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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/acs.organomet.6b00160. Experimental details, details of the kinetic experiments, NMR spectra for all compounds, and crystallographic data for 2 (CCDC reference number 1454773) (PDF) Crystallographic data for 2 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail for J.O.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Cluster of Excellence “Tailor-Made Fuels from Biomass” for financial support and C. Lhotzky for experimental assistance.
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REFERENCES
(1) (a) Marciniec, B. Coord. Chem. Rev. 2005, 249, 2374−2390. (b) Marciniec, B. Hydrosilylation: A Comprehensive Reviews on Recent Advances; Springer: Berlin, Germany, 2010. (2) (a) Speier, J. L. Adv. Organomet. Chem. 1979, 17, 407−447. (b) Hitchcock, P. B.; Lappert, M. F.; Warhurst, N. J. W. Angew. Chem., Int. Ed. Engl. 1991, 30, 438−440. (c) Lewis, L. N.; Stein, J.; Gao, Y.; Colborn, R. E.; Hutchins, G. Platinum Met. Rev. 1997, 41, 66−75. (3) (a) Nakajima, Y.; Shimada, S. RSC Adv. 2015, 5, 20603−20616. (b) Revunova, K.; Nikonov, G. I. Dalton Trans. 2015, 44, 840−866. (4) Buch, F.; Brettar, J.; Harder, S. Angew. Chem., Int. Ed. 2006, 45, 2741−2745. In this reference, Harder et al. proposed a yet another mechanism to explain the anti-Markovnikov regioselectivity through hypervalent hydridosilicate intermediates. We observed that the reaction of the hypervalent hydridosilicate [K(18-crown-6)] D
DOI: 10.1021/acs.organomet.6b00160 Organometallics XXXX, XXX, XXX−XXX
