Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Direct Silyl−Heck Reaction of Chlorosilanes Kazuhiro Matsumoto,‡ Jiadi Huang,‡ Yuki Naganawa, Haiqing Guo, Teruo Beppu, Kazuhiko Sato, Shigeru Shimada,* and Yumiko Nakajima* Interdisciplinary Research Center for Catalytic Chemistry (IRC3), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan S Supporting Information *
ABSTRACT: A nickel complex/Lewis acid combination effectively catalyzed the direct silyl−Heck reaction of chlorosilanes, which are key raw materials in the organosilicon industry, to give synthetically important alkenylsilane products. Trichlorosilanes, dichlorosilanes, and monochlorosilanes underwent the silyl−Heck reaction to afford the corresponding alkenylsilanes in high yields. In the reactions of dichlorosilanes, a single substitution occurred to give monoalkenylsilanes in a highly selective manner.
C
Scheme 1. Silyl−Heck Reactions
hlorosilanes are a key raw material in the organosilicon industry. The first step for the industrial production of organosilicon compounds is the Müller−Rochow “Direct Process,” in which silicon metal reacts with RCl, such as chloromethane and chlorobenzene, to produce chloromethylsilanes or chlorophenylsilanes (e.g., R2SiCl2, RSiCl3, R3SiCl, HRSiCl2).1 Therefore, a wide variety of chlorosilanes are commercially available at much lower cost, compared with other halosilanes and silyl triflates. Thus, the development of direct transformations of chlorosilanes, especially the incorporation of organic groups into the silicon center, has attracted broad attention. Alkenylsilanes are recognized as useful substrates for several organic transformations such as Hiyama cross-coupling2 and Tamao−Fleming oxidation.3 Such compounds are usually prepared by conventional nucleophilic substitution of halosilanes with alkenylmagnesium or alkenyllithium reagents. However, the low functional group tolerance of the method limits the availability of functionalized alkenylsilanes. Alkyne hydrosilylation is also utilized for alkenylsilane synthesis, and regioselective and stereoselective reactions have been developed.4 Considering the wide availability and low cost of alkenes, as well as chlorosilanes, a preferable route to alkenylsilanes would be the silyl−Heck reaction of chlorosilanes (Scheme 1). Pioneering work by Tanaka and co-workers allowed the palladium-catalyzed reaction of iodotrimethylsilane with styrenes to give alkenylsilanes.5 Although the original report was only moderately yielding, Watson and co-workers recently rediscovered the reaction and a rational design of palladium and nickel catalysts led to a significant improvement of the productivity. Under the improved silyl−Heck conditions, iodosilanes and silyl triflates can be employed as silyl sources. Chlorosilanes are also usable only in the presence of lithium iodide, which undergoes halogen exchange in situ with chlorosilanes to form the corresponding iodosilanes.6−8 Nonetheless, to the best of our knowledge, there is no precedent for the direct silyl−Heck reaction of chlorosilanes.9 © XXXX American Chemical Society
One of the reasons for the lack is the high bond dissociation energy (BDE) of Si−Cl bonds (Me3Si−Cl: 113 kcal/mol), which is much higher than the BDE of Si−I bonds (Me3Si−I: 77 kcal/mol) and the BDE of the carbon counterpart, C−Cl bonds (Me3C−Cl: 80 kcal/mol).10 Although the oxidative addition of a Si−Cl bond to a metal seems to be quite difficult because of the extremely high BDE,11 we hypothesized that nucleophilic metals have a great chance to undergo oxidative addition via a SN2 pathway,12,13 because Si−Cl bonds are polarized to a large extent (Pauling electronegativity: Si 1.90, Cl 3.16)14 and nucleophilic substitution of chlorosilanes is the common protocol in organosilicon synthesis. In this communication, we present a direct silyl−Heck reaction of chlorosilanes with styrenes by using a combination of a nickel catalyst and a Lewis acid. We first chose electron-rich Ni(cod)2/2PCy3 as a catalyst and examined the reaction of styrene 1a with Me2SiCl2 2a, the Received: March 15, 2018
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DOI: 10.1021/acs.orglett.8b00847 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters silicon center of which is more electrophilic than that of monochlorosilanes (Table 1).13c−e Me2SiCl2 is the most
Scheme 2. Substrate Scope in the Silyl−Heck Reaction of Dichlorosilanes
Table 1. Optimization of Reaction Conditions
entry
x (mol %)
Lewis acid
y (mol %)
yielda (%)
1 2 3 4 5 6 7 8 9b
10 10 10 10 10 10 10 1 1
none Zn(OTf)2 ZnCl2 AlCl3 Me2AlCl Me3Al Me3Al Me3Al Me3Al
20 20 20 20 20 50 50 50
4 19 5 17 32 58 88 91 90
a
Determined by 1H NMR analysis (600 MHz) in the presence of mesitylene as internal standard. bNiCl2(PCy3)2 instead of Ni(cod)2/ 2PCy3 was used.
abundant organosilicon source, because it is the major product in the direct process. In the absence of any additional activators, only a trace amount of alkenylsilane 3aa was detected by 1H NMR analysis (Table 1, entry 1). To further activate Me2SiCl2 toward nucleophiles, we screened some Lewis acids and determined that Zn(OTf)2, ZnCl2, and AlCl3 show slight positive effects, giving the desired product 3aa in 5%−19% yields (Table 1, entries 2−4).15 Further screening revealed that alkylaluminum species such as Me2AlCl and Me3Al were more effective, and alkenylsilane 3aa was obtained in higher yields of 32% and 58%, respectively (Table 1, entries 5 and 6).16 A satisfactory yield of 88% was achieved with 50 mol % Me3Al (Table 1, entry 7), and the catalyst loading of Ni(cod)2 could be reduced to 1 mol % without reduction in product yield (Table 1, entry 8). Finally, we found that bench-stable NiCl2(PCy3)2 was also effective for the silyl−Heck reaction with no deterioration of the yield (Table 1, entry 9). Notably, monoalkenylated product 3aa was obtained in a highly selective manner, supporting our working hypothesis that less-electrophilic monochlorosilanes are much less reactive than dichlorosilanes.17 The alkenylchlorosilane product thus obtained is highly useful, because it has both alkenyl and chloro functionalities. The NiCl2(PCy3)2/Me3Al system was successfully applied to the silyl−Heck reaction of various styrene derivatives (Scheme 2). Alkenylchlorosilane products 3 were transformed to the corresponding alkenylisopropoxysilanes 4 for ease of isolation. Styrene derivatives with either electron-donating methyl and tert-butyl groups or an electron-withdrawing fluorine atom at the para-position successfully underwent the reaction to give the corresponding alkenylsilanes 4ba, 4ca, and 4ea in good yields. However, the use of 4-methoxystyrene 1d resulted in a diminished yield of 31%, probably due to coordination of the MeO group to Me3Al. In the reaction of 4-chlorostyrene 1f, C− Cl methylation by nickel-catalyzed cross-coupling with Me3Al was found to occur preferentially over the desired silyl−Heck reaction.18 Thus, the reaction with 100 mol % Me3Al afforded 4-methylstyrylsilane 4ba (54%) instead of 4-chlorostyrylsilane 4fa via cross-coupling and silyl−Heck reaction. The ortho-
a
4-Methylstyrylsilane (4ba) was obtained in the reaction of 4chlorostyrene (1f) with 100 mol % Me3Al.
methyl-substitution is tolerated under the reaction conditions, affording the corresponding alkenylsilane 4ga in high yield. A good yield of 60% was observed in the reaction of 2vinylnaphthalene 1h. However, either aliphatic olefins or disubstituted olefins such as 1-octene and trans-β-methylstyrene did not give the desired alkenylsilanes. Other dichlorosilanes such as Ph2SiCl2 2b and MePhSiCl2 2c also underwent the reaction with styrene to give alkenylsilanes 4ab (92%) and 4ac (66%), respectively. Not only dichlorosilanes, but also trichlorosilanes are suitable for this silyl−Heck reaction (Scheme 3). The reaction of MeSiCl3 5 with 0.9 equiv styrene 1a at lower temperature of 60 °C gave monoalkenylated silane 6 with good selectivity. On the other hand, when 2 equiv of styrene was used in the reaction at 90 °C, dialkenylated silane 7 was obtained as the major product. Although the reaction of Me3SiCl 8a was very sluggish under the standard conditions, re-examination of the conditions considerably improved the reaction of monochlorosilanes (Scheme 4). With 5 mol % NiCl2(PCy3)2 and iPr2NEt instead of Et3N at 120 °C, MePh2SiCl 8b and Me2PhSiCl 8c as well as Me3SiCl underwent the silyl−Heck reaction with styrene 1a to give the corresponding alkenylsilanes 9aa−9ac in 68%−80% yields. Based on the reaction mechanism proposed by Watson, and on our experimental observations, we postulate the following B
DOI: 10.1021/acs.orglett.8b00847 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
crude reaction mixture (see the SI). Given that the produced Me2AlCl can also function as a Lewis acid, alkenylsilane can be produced in sufficient yields, even in the presence of a substoichiometric amount of Me3Al (50 mol %). In conclusion, we have developed the first example of a direct silyl−Heck reaction of chlorosilanes by the combination of nickel catalyst and aluminum Lewis acid. Under the cooperative catalysis, trichlorosilanes, dichlorosilanes, and monochlorosilanes can be transformed to the corresponding alkenylsilanes in high yields with high selectivity. We believe that the cooperative system offers numerous opportunities for direct and catalytic transformations of chlorosilanes, which will open up new access to various organosilicon compounds. Further applications and mechanistic studies are ongoing in our laboratory.
Scheme 3. Silyl−Heck Reaction of Trichlorosilanes
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ASSOCIATED CONTENT
S Supporting Information *
Scheme 4. Silyl−Heck Reaction of Monochlorosilanes
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00847. Experimental procedures and spectral data (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected].
mechanism for the nickel-catalyzed direct silyl−Heck reaction of chlorosilanes with styrenes (Scheme 5). First, precatalyst
ORCID
Yuki Naganawa: 0000-0002-3041-7638 Shigeru Shimada: 0000-0001-7081-6759 Yumiko Nakajima: 0000-0001-6813-8733
Scheme 5. Proposed Catalytic Cycle
Author Contributions ‡
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the “Development of Innovative Catalytic Processes for Organosilicon Functional Materials” project (Project Leader: K. Sato) from the New Energy and Industrial Technology Development Organization (NEDO).
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REFERENCES
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DOI: 10.1021/acs.orglett.8b00847 Org. Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.orglett.8b00847 Org. Lett. XXXX, XXX, XXX−XXX
