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Yttrium-Catalyzed Regioselective #-C–H Silylation of Methyl Sulfides with Hydrosilanes Yong Luo, Huai-Long Teng, Can Xue, Masayoshi Nishiura, and Zhaomin Hou ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02405 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018
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ACS Catalysis
Yttrium-Catalyzed Regioselective α-C–H Silylation of Methyl Sulfides with Hydrosilanes Yong Luo,† Huai-Long Teng,‡ Can Xue,‡ Masayoshi Nishiura,†,‡ and Zhaomin Hou*,†,‡ †
Organometallic Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Advanced Catalysis Research Group, RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako, Saitama 3510198, Japan ‡
ABSTRACT: The catalytic C−H silylation of sulfides with hydrosilanes is of much interest and importance, but has remained unexplored to date. We report here the C−H silylation of methyl sulfides with hydrosilanes by an yttrium metallocene catalyst. The reaction took place regioselectively at the α-methyl C−H bond, affording the corresponding silylated sulfide derivatives that are difficult to make by other means. KEYWORDS: silylation, silylated sulfides, yttrium complex, sp3 C–H activation, thiasilolanes
Organosilanes play an important role in various fields, such as molecular and materials synthesis, pharmaceuticals, and organic electronics and photonics.1 There has been persistently impetus for the development of efficient and selective methods for the synthesis of various types of organosilane compounds.2 Among possible approaches, the C−H silylation with hydrosilanes through release of H2 is the most straightforward and atom-efficient route. Despite extensive studies and recent advances in this area, the catalytic C−H silylation at a carbon atom directly bonded to a heteroatom has met with only limited success to date (see Scheme 1).3,4 Suginome and coworkers reported the C−H silylation of a methyl group bonded to a boron atom in the presence of a Ru catalyst with the aid of 2(1H-pyrazol-3-yl)aniline as a directing group (Scheme 1a).3a Sato and coworkers achieved the methyl C−H silylation of N,N-dimethyl pyridines by using Rh or Ir catalysts, in which the pyridine moiety served as a directing group (Scheme 1a).3b Very recently, Huang and coworkers realized the intramolecular C−H silylation of heteroatom-bonded methyl groups in a series of ortho-disubstituted benzene derivatives such as 2silylated anisoles and N,N-dimethylanilines by using Ru catalysts without the need for an extra directing group (Scheme 1b).4 Although sulfides are often seen in many natural products, bioactive molecules, and functional materials,5 the catalytic C−H silylation of an organosulfide with a hydrosilane in either an intermolecular or intramolecular fashion has not been reported previously. This is probably because a sulfide unit often acts as a poison to late transition metal catalysts6 and can also easily undergo C−S bond cleavage in the presence of a transition metal complex.7 We report here the regioselective α-C−H silylation of a wide range of methyl sulfides with hydrosilanes by an yttrium metallocene catalyst (Scheme 1c). This protocol represents the first example of sulfide C−H silylation with hydrosilanes and offers an efficient and reliable route for the synthesis of various silylated sulfides.
Scheme 1. Catalytic Silylation of a C(sp3)–H Bond Adjacent to a Heteroatom At first, we examined the silylation of pentyl methyl sulfide (1a) with methylphenylsilane (2a) by using the half-sandwich scandium dibenzyl complex Sc-1 (Chart 1) as a catalyst (5 mol %) at 70 °C in toluene, because Sc-1 was recently found to be a unique catalyst for the C−H alkylation of methyl sulfides with alkenes under the similar conditions.8 However, no silylation product was observed (Table 1, entry 1). This is probably because the reaction of Sc-1 with the hydrosilane 2a may form a polynuclear scandium polyhydride complex, which could be inert toward the C−H activation of the sulfide 1a.9 We then checked the constrained geometry complex (CGC) scandium complex Sc-2, which was previously found
Chart 1. Rare-earth Complexes with Different Ligands
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Table 1. Catalyst Screening and Optimization of Pentyl Methyl Sulfide Silylation with Methylphenylsilanea
Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14c 15d
[Cat.] Sc-1 Sc-2 Sc-3 Y-1 Y-2 Y-3 Y-4 Y-5 Y-5 Y-5 Y-5 Y-5 Y-5 Y-5 Y-5
T 70 70 70 70 70 70 70 70 90 110 90 90 90 90 90
Solvent toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene tolueneb octaneb – – –
Yield n.r n.r. n.r. 56% n.r. < 5% < 5% 62% 65% 65% 71% 71% 81% 45% 64%
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matic primary silane PhSiH3 yielded a mixture of mono- and disilylated products 3d and 3d’, probably because the methylene group between the sulfur and silicon atoms in the monosilylation product 3d is more reactive the methyl group in 1a due to the influence of the electron-withdrawing PhSiH2 unit.14 In contrast, the aliphatic primary silane n-C6H13SiH3 afforded a mixture of the mono-thiomethyl silane compound 3e and the bis-thiomethyl silane 3e’’, probably because the thiomethyl silane compound 3e is more reactive than n-C6H13SiH3. A tertiary silane such as dimethylphenylsilane was inactive under the standard reaction conditions probably because of steric hindrance. Table 2. Silylation of Pentyl Methyl Sulfide with Various Hydrosilanes by Y-5a
a
Reaction condition: 1a (0.6 mmol), 2a (0.2 mmol), [Cat.] (5 mol %), toluene (1 mL), 24 h, isolated yield, unless otherwise noted. b Solvent (0.5 mL). c1a (0.2 mmol). d1a (0.2 mmol), 2a (0.6 mmol).
to be an active catalyst for the C−H silylation of anisoles,10 but did not find a sulfide silylation product under the same conditions (Table 1, entry 2). The scandium metallocene complex bearing two C5Me5-ligands (Sc-3) was previously reported to be active for the silylation of methane,11 but exhibited no activity for the silylation of 1a under the above conditions (Table 1, entry 3). In contrast, the yttrium metallocene analog Y-1 showed a significant catalytic activity for the C−H silylation of 1a under the same conditions, which afforded the αsilylated product 3a in 56% yield (Table 1, entry 4). Encouraged by this result, we investigated several yttrium metallocene complexes bearing different Cp ligands. The yttrium complexes with the less sterically demanding C5H5 (Y-2), C5H4tBu (Y-3), and 1,3-Me2C5H3 (Y-4) ligands were not effective under the same conditions (Table 1, entries 5-7). However, the C5Me4-ligated analog Y-512 gave the desired product 3a in 62% yield (Table 1, entry 8), which is even higher than that with Y-1. Raising the reaction temperature from 70 °C to 90 °C (or 110 °C) slightly increased the yield of 3a (65%) with Y-5 as a catalyst (Table 1, entries 9 and 10). When the reaction was carried out in a more concentrated solution (less solvent), a significantly higher yield of 3a (71%) was obtained (Table 1, entry 11). Finally, a neat condition (no solvent) gave the highest yield (81%) (Table 1, entry 13). Changing the molar ratio of sulfide 1a and silane 2a from 3:1 to 1:1 and 1:3 gave lower yields of 3a (Table 1, entries 14 and 15).13 Next, we investigated the silylation of pentyl methyl sulfide 1a with different hydrosilanes under the optimized reaction conditions (Table 2). Similar to methylphenylsilane, diphenylsilane also worked well with 1a in the presence of Y-5, affording the corresponding α-methyl silylated sulfide product 3b in 83% isolated yield. Dibutylsilane was less active, giving 3c in 30% yield under same conditions. The use of the aro-
a Reaction conditions: 1a (1.2 mmol), 2 (0.4 mmol), Y-5 (5 mol %), 90 °C, 24 h, isolated yield.
We then examined the scope of sulfides by using diphenylsilane 2b as a silylation agent. Table 3 summarizes the reaction results of various alkyl methyl sulfides with 2b. No matter whether the sulfide substrates contain branched or cyclic or aryl substituents, the silylation selectively took place at the αmethyl group, affording the desired products (3f−3k) in good yields (73−83%). The bromo substituent (3l) was compatible with the catalyst.15 A phenyl ether unit did not affect the silylation at the methyl sulfide (3m), probably due to the steric hindrance of the phenyl group. In the case of 1,4bis(methylthiomethyl)cyclohexane, which contains two SMe groups, the monosilylation product 3o was selectively obtained in 75% yield. Methyl sulfides having a sterically demanding secondary (3p), cyclic (3q and 3r), or tertiary alkyl group (3s) directly bonded to the sulfur atom were also efficiently silylated at the SMe group. Non-methyl sulfides such as ethyl propyl sulfide and tetrahydrothiophene did not afford the desired silylation products under the standard reaction conditions, probably due to catalyst deactivation through C–S bond cleavage.13,16
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ACS Catalysis
Table 3. Silylation of Various Alkyl Methyl Sulfides with Diphenylsilane by Y-5a
(5 mol %), the heterocyclic [1,3]-thiasilolane product 4a was formed in 81% yield through intramolecular C–H silylation at Table 4. Silylation of Various Aryl Methyl Sulfides with Diphenylsilane by Y-5a
Ph
S
Ph SiH Ph
S
t
Ph
3v, 79%[b]
Ph
Ph SiH Ph
S MeS
Ph SiH
S
S Br
3y, 78% Ph SiH Ph Ph
Me2HSi
S
Me SiH 4
3ab, 83% Ph SiH Ph
S
Ph SiH
S
Ph
3ac, 70%
S
Ph SiH Ph
S
S
SiHMe2
Ph SiH Ph
Ph SiH Ph
N
3af, 85%
3ae, 83%
Ph SiH Ph
3ad, 87%
Ph
i-Pr
Ph SiH Ph
3aa, 45%[b]
3z, 78% S
Ph SiH Ph
3x, 76%[b]
Ph
N
i-PrO
The reactions of aryl methyl sulfides with 2b are summarized in Table 4. The silylation of phenyl methyl sulfide exclusively occurred at the methyl sp3 C–H bond, giving 3t in 82% isolated yield, while a sp2 C–H silylation product was not observed. This is in sharp contrast with the Sc-catalyzed silyation of anisole with hydrosilanes, in which the reaction selectively took place at the ortho sp2 C–H bond.10 Similarly, the reaction of p-tolyl methyl sulfide with 2b afforded the corresponding SMe C–H silylation product 3u in 83% yield. The solid structure of 3u was confirmed by an X-ray diffraction study. In the case of p-tert-butylphenyl methyl sulfide and p-biphenyl methyl sulfide, a small amount of toluene was used as a solvent to dissolve the solid sulfide substrates, which led to efficient formation of the desired silylation products 3v (79%) and 3w (69%), respectively. In the case of p-bis(methylthio)benzene, only the mono-silylation product 3x (76%) was observed. Isopropoxy (3y), pyrrolidinyl (3z), bromo (3aa), and silyl (Me2HSi, 3ab and PhMeHSi, 3ac) substituents at the aromatic ring of the aryl methyl sulfides were all compatible with the catalyst. Substituents at the meta-positions of the phenyl group in the sulfides did not hamper the C–H silylation at the SMe unit (see 3ad-3ah), while in the case of ortho-substituted phenyl methyl sulfides such as 2-methylphenyl methyl sulfide, only a trace amount of the desired silylation product was formed probably due to steric hindrance (3ai). In an attempt to explore the synthetic utility of the silylated sulfide products obtained in this work, we examined the annulation of several aryl methyl sulfide silylation products by using B(C6F5)3 as a catalyst (Table 5).17 When 3ag was heated in chlorobenzene at 120 ºC for 3 h in the presence of B(C6F5)3
3
3w, 69%[c]
Ph SiH Ph
S
Reaction conditions: 1 (1.2 mmol), 2b (0.4 mmol), Y-5 (5 mol %), 90 °C, 24 h, isolated yield. bToluene (0.5 mL) was used to dissolve the solid sulfide substrate.
S
Ar
3u, 83% Ph SiH Ph
S
a
Ph SiH
S
3t, 82%
Bu
neat, 90 C - H2
2b
1
Ph SiH Ph
Y-5
Ar SMe + Ph2SiH2
3ag, 82%
S
Ph SiH Ph
3ai, trace
3ah, 77% a
Reaction conditions: 1 (1.2 mmol), 2b (0.4 mmol), Y-5 (5 mol %), 90 °C, 24 h, isolated yield, unless otherwise noted. bToluene (0.2 mL) was used as a solvent to dissolve the solid sulfide substrate. cToluene (1.0 mL) was used to dissolve the solid sulfide substrate.
Table 5. Annulation of Some Aryl Methyl Sulfide Silylation Products by B(C6F5)3a
a
Reaction conditions: 3 (0.1 mmol), B(C6F5)3 (5 mol %), PhCl (0.5 mL), 120 °C, isolated yield.
the para-position of the aminophenyl group. In the case of 3z, a longer reaction time was needed to give the annulation prod-
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uct 4b (70%), in agreement with the previous observation that C–H silylation at the meta position of an aminophenyl ring was less favored than that at the para position.17 The annulation products 4c and 4d, which do not have an amino substituent at the aryl group, were also efficiently formed through the intramolecular aromatic C–H silylation of 3ae and 3u, respectively, although the analogous intermolecular silylation of arenes without an electron-donating amino-substituent was difficult under the similar conditions.17 These results demonstrate that the yttrium-catalyzed α-C−H silylation of aryl methyl sulfides followed by the B(C6F5)3-catalyzed annulation of the resulting silylation products may constitute an atomefficient cascade route for the construction of heterocyclic [1,3]-thiasilolanes.18 Y-5
Ph S
H2
PhSMe
Y Ph2SiH2 C6D6, rt
C6D6, 60 C
H
e SM Ph , rt D6 C6
Y
Y
S Ph B
A
C6D6, 80 C Y PhS Me C6 D 6 , 80 C
S
Ph SiH
S
Ph2SiH2 2b C6D6, 60 C
C6D6, 60 C Ph
3t
no reaction
B
B
methyl silylation product 3t in 30 min with regeneration of A. In contrast, no reaction was observed between C and Ph2SiH2 under the same conditions.20 These results could explain the present yttrium-catalyzed regioselective α-methyl silylation of methyl sulfides with hydrosilanes. In summary, we have demonstrated that an yttrium metallocene complex such as Y-5 can serve as a unique catalyst for the regioselective α-C−H silylation of a wide range of methyl sulfides with hydrosilanes. Some key reaction intermediates have been isolated and structurally characterized, which provided important insights into the mechanistic aspects. This protocol features high atom-efficiency, broad substrate scope, high yields, and simple reaction conditions, offering a useful route for the synthesis of a series of silylated sulfides that are difficult to prepare by other methods. Moreover, the combination of the yttrium-catalyzed intermolecular C−H silylation with the B(C6F5)3-catalyzed intramolecular C−H silylation may constitute a unique cascade for the construction of heterocyclic [1,3]-thiasilolane skeletons from simple aryl methyl sulfides and secondary silanes.
ASSOCIATED CONTENT
Y Me C
Ph2SiH2
Ph
Y
B
H
H Y
S Ph
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C
Scheme 2. Stoichiometric Reactions of Y-5 with Diphenylsilane and Phenyl Methyl Sulfide, Which Demonstrates a Catalytic Cycle for the Sulfide α-Methyl C−H Silylation To gain information on the reaction mechanism of the present catalytic C−H silylation, some stoichiometric reactions were carefully examined (Scheme 2). The reaction of Y-5 with 5 equiv of diphenylsilane (2b) in benzene-d6 at room temperature for 1.5 h led to precipitation of a dimeric yttrium hydride complex A in 30% yield. Addition of 10 equimolar amounts of PhSMe in benzene-d6 resulted in complete disappearance of A in 40 min at 60 °C as monitored by 1H NMR. When the reaction mixture was concentrated under reduced pressure, the mixed phenylthiomethyl/hydride complex B was obtained as white solid, which on recrystallization from hexane at room temperature afforded colorless crystals in 42% yield. An X-ray diffraction analysis revealed that B adopts a binuclear structure, in which the two Y atoms are bridged by one H ligand and one PhSCH2 ligand. The PhSCH2 group in B is bonded to one Y atom with the CH2 unit to form a Y−C covalent bond and to the other Y atom with the S atom through a noncovalent (coordination) bonding interaction. At room temperature, further reaction of B with PhSMe took place to give the bis(phenylthiomethyl) complex B′, which upon heating at 80 °C afforded the ortho-metalation product C (50% NMR yield). Compound C could also be obtained in 70% yield by heating B with 10 equimolar amounts of PhSMe in benzene-d6 at 80 °C for 12 h.19 The reaction of either B or B’ with 5 equiv of Ph2SiH2 in benzene-d6 at 60 °C quantitatively yielded the α-
Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org Experimental details, characterization data, and mechanism investigation X-ray data of Y-5 X-ray data of 3u X-ray data of 4b X-ray data of complex B X-ray data of complex B′ X-ray data of complex C
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected].
ORCID Zhaomin Hou: 0000-0003-2841-5120
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported in part by a Grant-in-Aid for Scientific Research (S) (No. 26220802) and a Grant-in-Aid for Scientific Research on Innovative Areas (17H06451) from JSPS. We thank Mrs. Akiko Karube for micro elemental analysis, and Dr. Takemichi Nakamura for high resolution mass spectrometry (HRMS) analysis.
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803. (b) Sadow, A. D.; Tilley, T. D. Synthesis and Characterization of Scandium Silyl Complexes of the Type Cp*2ScSiHRR′. σ-Bond Metathesis Reactions and Catalytic Dehydrogenative Silation of Hydrocarbons. J. Am. Chem. Soc. 2005, 127, 643. (12) Xue, C.; Luo, Y.; Teng, H.; Ma, Y.; Nishiura, M.; Hou, Z. Ortho-Selective C−H Borylation of Aromatic Ethers with Pinacolborane by Organo Rare-Earth Catalysts. ACS Catal. 2018, 8, 5017. (13) Compared to the methyl sulfide 1a, the silylated product 3a could more easily undergo C–S bond cleavage reaction with an yttrium hydride species, thus deactivating the catalyst. The use of an excess of 1a could suppress such catalyst-deactivation process. For examples of the C–S bond cleavage of non-methyl sulfides by rareearth hydrides, see: Deelman, B.-J.; Booij, M.; Meetsma, A.; Teuben, J. H.; Kooijman, H.; Spek, A. L. Activation of Ethers and Sulfides by Organolanthanide Hydrides. Molecular Structures of (Cp*2Y)2(µOCH2CH2O)(THF)2 and (Cp*2Ce)2(µ-O)(THF)2. Organometallics 1995, 14, 2306. (14) The use of an excess of 1a (3 equiv) did not significantly increase the yield of 3d’, probably because of catalyst deactivation through C–S bond cleavage (see also ref. 13). (15) The slightly lower yield of 3l could be due to possible debromination by an yttrium hydride species. See: Qian, C.; Zhu, C.; Zhu, D. Carbon–Halogen Bond Cleavage Reaction Catalyzed by Organoyttrium Hydride (in situ) and Lanthanide Alkoxides. Appl. Organomet. Chem. 1995, 9, 457. (16) Methyl ethers were not suitable for this reaction under the same conditions due to C–O bond cleavage. See also ref. 13. (17) (a) Ma, Y.; Wang, B.; Zhang, L.; Hou, Z. Boron-Catalyzed Aromatic C–H Bond Silylation with Hydrosilanes. J. Am. Chem. Soc. 2016, 138, 3663. (b) Yin, Q.; Klare, H. F. T.; Oestreich, M. FriedelCrafts-Type Intermolecular C−H Silylation of Electron-Rich Arenes Initiated by Base-Metal Salts. Angew. Chem. Int. Ed. 2016, 55, 3204. (18) For stoichiometric approaches to heterocyclic [1,3]-thiasilolanes, see: a) Reich, H. J.; Dykstra, R. R. Structure Effects of Ion Pair Separation: Planar and Pyramidal Sulfur and Silicon Substituted Carbanions. J. Am. Chem. Soc. 1993, 115, 7041. (b) Borisenko, K. B.; Samdal, S.; Suslova, E. N.; Sipachev, V. A.; Shishkov, I. F.; Vilkov, L. V. Molecular Structure and Pseudorotation in 3,3-Dimethyl-3silatetrahydrothiophene from a Joint Gas-Phase Electron Diffraction and Ab Initio Molecular Orbital Study. Acta Chem. Scand. 1998, 52, 975. (c) Durka, K.; Kliś, T.; Serwatowski, J.; Woźniak, K. Influence of the Silyl Group on the Reactivity of Some Ortho-Lithiated Aryl Alkyl Sulfides. Organometallics 2013, 32, 3145. (19) The formation of an ortho-metalated product (C5Me5)2Y(C6H4SMe-o) in the reaction of [(C5Me5)2Y(µ-H)]2 with PhSMe was reported previously. See: Booij, M.; Deelman, B.-J.; Duchateau, R.; Postma, D. S.; Meetsma, A.; Teuben, J. H. CarbonHydrogen Activation of Arenes and Substituted Arenes by the Yttrium Hydride (Cp*2YH)2: Competition Between Cp* Ligand Metalation, Arene Metalation, and H/D Exchange. Molecular Structures of Cp*2Y(μ-H)(μ-η1,η5-CH2C5Me4)YCp* and Cp*2Y(o-C6H4PPh2CH2). Organometallics 1993, 12, 3531. (20) The inertness of C toward Ph2SiH2 is in sharp contrast with the reaction of an analogous anisyl scandium species with a hydrosilane, which rapidly gave the corresponding silylated anisole product (see ref. 10). This demonstrates again the difference in reactivity between a sulfide and an ether.
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