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Iron-Catalyzed Dihydrosilylation of Alkynes: Efficient Access to Geminal Bis(silanes) Meng-Yang Hu, Jie Lian, Wei Sun, Tian-Zhang Qiao, and Shou-Fei Zhu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02127 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019
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Journal of the American Chemical Society
Iron-Catalyzed Dihydrosilylation of Alkynes: Efficient Access to Geminal Bis(silanes) Meng-Yang Hu,1 Jie Lian,1 Wei Sun,1 Tian-Zhang Qiao,1 and Shou-Fei Zhu*,1 State Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China 1
Supporting Information Placeholder ABSTRACT: Geminal bis(silanes) are versatile synthetic building blocks owing to their stability and propensity to undergo a variety of transformations. However, the scarcity of catalytic methods for their synthesis limits their structural diversity and thus their utility for further applications. Herein we report a new method for synthesis of geminal bis(silanes) by means of iron-catalyzed dihydrosilylation of alkynes. Iron catalysts were distinctly superior to the other tested catalysts, which clearly demonstrates that novel reactivity can be found by using iron catalysts. This method features 100% atom economy, regiospecificity, mild reaction conditions, and readily available starting materials. Using this method, we prepared a new type of geminal bis(silane) with secondary silane moieties, the Si–H bonds of which can easily undergo various transformations, facilitating the synthetic applications of these compounds. Preliminary mechanistic studies demonstrated that the reaction proceeds via two iron-catalyzed hydrosilylation reactions, the first generating β-(E)vinylsilanes and the second producing geminal bis(silanes).
atom economy and produce only geminal bis(silanes) with quaternary silyl groups, which are difficult to transform further. Hydrosilylation is a promising method for forming C– Si bonds owing to its high efficiency and 100% atom economy,7 and in fact hydrosilylation of quaternary vinylsilanes has been studied by several research groups; however, the regioselectivity of these reactions is generally poor (Scheme 1c).8 Herein we report a new method of synthesizing geminal bis(silanes) by means of iron-catalyzed dihydrosilylation of alkynes. Iron catalysts were distinctly superior to the other types of catalysts that we tested, a result that clearly demonstrates that novel reactivities can be found by using iron catalysts. Our method features 100% atom economy, regiospecificity, mild reaction conditions, and readily available starting materials. More important, this method allows the highly efficient synthesis of previously unreported geminal bis(silanes) with secondary silyl groups, the Si–H bonds of which can undergo various transformations, giving this method great potential synthetic utility. a) Palladium-catalyzed insertion of benzylic carbenes into Si-Si bonds NNHTs
Iron catalysis has attracted considerable attention for two main reasons: (1) iron is abundant, inexpensive, and biocompatible, and thus iron catalysts meet the requirements for green and sustainable chemistry applications, and (2) the unique electronic structures of iron give it the potential to mediate transformations that cannot be achieved with other catalysts. The development of new iron-catalyzed reactions and elucidation of the mechanisms of iron catalysis are among the most important topics in this field.1 Organosilanes are widely used in organic synthesis and materials science.2 In particular, geminal bis(silanes) are versatile synthetic building blocks owing to their stability and propensity to undergo a variety of transformations.3 However, the scarcity of reliable catalytic methods for their preparation has limited their structural diversity and thus the development of new transformations of these compounds. Syntheses based on stoichiometric reactions generally use tBuLi, sBuLi, or other bases, which either show poor selectivity or generate large quantities of waste.4 Geminal bis(silanes) have also been prepared by means of palladium-catalyzed insertion of benzylic carbenes into Si–Si bonds (Scheme 1a)5 and by copper-catalyzed double C(sp3)–Si coupling of geminal dibromides (Scheme 1b),6 but these two methods have poor
R
+
R'
[Pd]
FMe2Si SiMe2F
FMe2Si
SiMe2F R'
R
b) Copper-catalyzed double C(sp3)-Si coupling of geminal dibromides Br R
+ Br
SiMe2Ph
[Cu]
Me2PhSi Bpin
R
SiMe2Ph
c) Transition-metal-catalyzed hydrosilylation of vinylsilanes + R'3SiH
SiR3
SiR'3
[M] Me
+ R'3Si SiR3 poor selectivity
SiR3
d) Iron-catalyzed dihydrosilylation of alkynes (This work)
R1
SiH2R2
[Fe]
R2SiH3
+
R1
(2.2 equiv) Ph Si O O Ph Si Si SiOO C7H15 Ph
SiH2R2
85-95% yield regiospecific
C7H15 Ph
SiX2Ph SiX2Ph C6H13 X = OR3, F
OH Ar C6H13
Ph C6H13
Scheme1. Catalytic Synthesis of Geminal Bis(silanes).
ACS Paragon Plus Environment
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internal standard. c Isolated yield of 6a is 90%. solvent. e Toluene as solvent.
Figure 1. Catalysts used in this study. Ar
Ar
N Fe N
Cl
N
Cl
N
Fe
Cl
Ar N
Cl
N
Fe
Cl
Me
Cl
Me
Ar N Fe
Cl Cl
N Ar
Ar Ar C3 C1a Ar = 2,4,6-(Me)3C6H2 C4 C2 C1b Ar = 2,4,6-(Et)3C6H2 (Ar = 2,4,6-Et3C6H2) (Ar = 2,6-iPr C H ) (Ar = 2,6-iPr C H ) 2 6 3 2 6 3 i C1c Ar = 2,4,6-( Pr)3C6H2 Me
Ph
Ph N
Fe
N
Cl
N Ph
Ph
Fe
Cl
Ar C6 i (Ar = 2,6- Pr2C6H3)
O Si Me Me
2
Ph P
Fe Cl
Fe P
Cl
N
Me
Pt
Ph
PiPr2 N
Cl
N
C5
Me Me Si O Me Si Me
Ar N
Cl
Ph
PPh2 OMe PdCl2
Cl Ph
Ph3P
Ni
Ph3P
C10
Cl Cl
C12
C11
C9
Cl
C8
C7
H2PtCl6.6H2O
(Karstedt's catalyst)
Table 1. Iron-Catalyzed Dihydrosilylation of Pent-4-yn1-ylbenzene with PhSiH3: Optimization of Reaction Conditions. Ph
Ph
3
1a
+
PhSiH3 2a (2.2 equiv)
solvent, 30 oC, 6 h
1 2c 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19d 20e
catalyst C1a C1b C1c C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C1b C1b C1b C1b C1b C1b
3
5a
SiH2Ph Ph
SiH2Ph
3
reductant EtMgBr EtMgBr EtMgBr EtMgBr EtMgBr EtMgBr EtMgBr EtMgBr EtMgBr EtMgBr none none none none MeMgCl NaBHEt3 LiAlH4 LiCH2TMS EtMgBr EtMgBr
Ph
d
Benzene as
We started by carrying out hydrosilylation reactions between pent-4-yn-1-ylbenzene (1a) and PhSiH3 in THF (Table 1). First, we evaluated iron catalysts with various ligands (Figure 1, C1– C8). EtMgBr was used to reduce the Fe(II) complexes to the active low-valent iron species.9 These reactions can generate many possible products: monohydrosilylated products 3a, 4a, and 5a (depending on the regioselectivity and stereoselectivity), desired dihydrodsilylation product 6a, and hydrogenation product 7a. Controlling the selectivity is the key to making this reaction useful. Although most of the tested iron catalysts showed poor activity or selectivity, catalyst C1b, which has a 2,9-diaryl-1,10-phenanthroline ligand,10 gave desired geminal bis(silane) 6a in 93% yield by NMR and 90% isolated yield (Table 1, entries 1–10). It is worth mentioning that catalysts based on other metals, which have been widely used in hydrosilylation,7 gave only complex mixtures containing none of the desired product (entries 11–14). Replacing the iron in C1b with other metals afforded complexes that were inactive for the hydrosilylation reaction (see Table SI for details). In addition to EtMgBr, reductants such as MeMgCl, NaBHEt3, LiAlH4, and LiCH2TMS also promoted the hydrosilylation but gave only moderate to poor selectivity for geminal bis(silanes) (Table 1, entries 15–18). Hydrosilylation promoted by C1b in the presence of EtMgBr could also be performed in benzene or toluene with satisfactory yields (entries 19 and 20).
SiH2Ph
3
SiH2Ph Ph 4a
3a
catalyst (5 mol %) reductant (12 mol %)
6a
entrya
Ph
SiH2Ph
3
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Scheme 2. Iron-Catalyzed Dihydrosilylation of Terminal Alkynes: Substrate Scopea C1b (5 mol %) EtMgBr (12 mol %)
SiH2Ph
3
7a
conv. (%)b
3a/4a/5a/6a/7ab
100 100 100
