Liang Deng's group at the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, recently discovered that a dinuclear cobalt carbonyl complex [(Xantphos)Co2(CO)6] coordinated by the bidentate phosphine ligand Xantphos can effectively catalyze the selective anti-Markovnikov hydrosilylation reaction of terminal alkynes with tertiary silanes. In collaboration with Hui Chen's group at the Institute of Chemistry, Chinese Academy of Sciences, they conducted experimental and theoretical exploration of the reaction mechanism and found that the (Xantphos)(CO)Co site in the dinuclear cobalt catalyst can be viewed as a ligand, cooperating with the Co(CO)n site to achieve the activation transformation of the covalent bond in the catalytic reaction.
Background:
Alkenyl silicon compounds have a wide range of uses in organic synthesis, organometallic chemistry, and materials chemistry. Transition metal-catalyzed hydrosilylation of alkynes with silanes is a simple and effective method for the synthesis of alkenyl silicon compounds. Tertiary silanes are inexpensive and readily available compared to primary and secondary silanes, but they have large steric hindrance and relatively low reactivity. Currently, the 3d metal-catalyzed hydrosilylation of tertiary silanes with alkynes has problems such as difficult control of chemical, regio- and stereoselectivity, and a narrow range of substrate applicability. The development of efficient 3d metal catalysts to achieve highly selective anti-Markovnikov hydrosilylation of tertiary silanes with alkynes is an unresolved challenge in the field of high-yield metal catalysis.
Highlights of this article:
This work achieved a highly selective anti-Markovnikov hydrosilylation reaction of terminal alkynes with tertiary silanes catalyzed by cobalt. The introduction of a bidentate phosphine ligand into the Co2(CO)8 system can effectively improve the β/α selectivity. The structure of the bidentate phosphine ligand has an important influence on the reaction activity and selectivity. When the bidentate phosphine ligand is Xantphos, excellent β/α selectivity can be achieved. [(Xantphos)Co2(CO)6] synthesized by the reaction of Co2(CO)8 with Xantphos showed similar activity and selectivity to the in situ generated catalyst in the catalytic reaction. The substrate universality investigation using [(Xantphos)Co2(CO)6] as the catalyst showed that a series of primary, secondary, and tertiary alkyl-substituted terminal alkynes can undergo anti-Markovnikov hydrosilylation with triethylsilane to form linear alkenyl silicon compounds. The catalytic system has good compatibility with functional groups such as halogens, ethers, and hydroxyls. This catalytic reaction produces anti-Markovnikov hydrosilation products in high yield and selectivity for various aryl alkynes containing electron-withdrawing or electron-donating substituents at the para-, meta-, and ortho-positions. Other tertiary silanes, including HSiBu n 3 , HSiMe2Ph, HSiPh3 , HSi(OSiMe3)2Me, and HSi(OEt)3, also react with phenylacetylene and 1-octyne under this catalytic system to produce the target products. However, selectivity decreases with decreasing silane substituent number.
Reaction mechanism studies have shown that the catalyst [(Xantphos)Co2(CO)6] ( 1 ) reacts equivalently with CyCCH and HSiEt3 to generate a dinuclear cobalt alkyne complex [(Xantphos)(CO)Co(μ-η2:η2-HCCCy)Co(CO)3] ( 2 ) and a mononuclear cobalt hydrogen complex [(Xantphos)Co(CO)2(H)] ( 3 ). The dinuclear cobalt alkyne complex 2 can further react with HSiEt3 to generate an anti-Markovnikov hydrosilation product and a mononuclear zero-valent cobalt complex [(Xantphos)Co(CO)2] ( 4 ), and exhibits better catalytic activity. The mononuclear cobalt hydrogen complex 3 cannot catalyze the reaction. At the same time, combined with a series of deuteration experiments, it is shown that the dinuclear cobalt alkyne species is an active intermediate in the catalytic cycle.
Hui Chen's team at the Institute of Chemistry, Chinese Academy of Sciences, conducted theoretical calculations to investigate the possible reaction process. These calculations further confirmed the dinuclear cobalt-mediated catalytic cycle. The dinuclear cobalt acetylene silane species (Xantphos)(CO)3Co2(μ-HCCPh)(HSiEt3) (B) undergoes a singlet-to-triplet transition via intramolecular hydrogen atom transfer to form the dinuclear cobalt vinylsilyl intermediate (Xantphos)(CO)3Co2(μ-HCC(H)Ph)(SiEt 3) (C), which is the regioselective and rate-determining step. The dinuclear cobalt vinylsilyl intermediate C forms a C-Si bond through a reductive elimination process to form the dinuclear cobalt olefinic silicon intermediate (Xantphos)(CO)3Co2(μ-H(SiEt3)CC(H)Ph) (D), and the olefinic silicon is removed to form the coordinatively unsaturated dinuclear cobalt carbonyl intermediate (Xantphos)Co2(CO)3 (E). The reaction occurs on the triplet energy surface. The dinuclear cobalt intermediate (Xantphos)Co2(μ-HCCPh)(CO)3 (A) and the cobalt metal B have a long distance and similar atomic spin density, exhibiting the electronic structure characteristics of zero-valent cobalt. The dinuclear cobalt intermediates C, D, and E exhibit high-spin electronic structure characteristics, with the two unpaired electrons mainly distributed on the cobalt metal center coordinated to Xantphos, showing the electronic structure characteristics of high-spin monovalent cobalt. The other cobalt center in dinuclear cobalt intermediate C exhibits low-spin monovalent cobalt, while the other cobalt centers in dinuclear cobalt intermediates D and E exhibit electronic structure characteristics of negative monovalent cobalt. Electronic structure analysis of the dinuclear cobalt intermediates indicates that oxidative addition of the H-Si bond occurs at both cobalt centers, transforming [Co(0)•••Co(0)] 0 to [Co(I)•••Co(I)] 2+, while electron transfer during Si-C bond formation occurs at the Co(CO)2 site ([Co(I)••••••••Co(I)] 2+ to [Co(I)•••Co(-I)]0). The high anti-Markovnikov selectivity for alkynes and tertiary silanes is attributed to a unique cooperative mechanism between the dinuclear cobalts: the (Xantphos)(CO)Co site can be viewed as a ligand, cooperating with the Co(CO)n site to achieve Si-H bond activation and C-Si bond formation in the catalytic reaction.
Summary and Outlook:
In summary, the research groups of Liang Deng and Hui Chen reported a highly selective anti-Markovnikov hydrosilylation of terminal alkynes with tertiary silanes catalyzed by dinuclear cobalt. This catalytic system boasts mild reaction conditions, a broad substrate range, and good functional group tolerance, enabling the efficient synthesis of alkenyl silicon compounds. Mechanistic studies indicate that the dinuclear cobalt alkyne species is an active intermediate in the catalytic cycle. Theoretical calculations indicate that Si-H bond activation, C-H bond formation, and C-Si bond formation occur at one of the cobalt-centered Co(CO)2 sites, while the other cobalt-centered (Xantphos)(CO)Co site acts as a non-redox metal ligand to assist the selective activation and conversion of the cobalt-centered Co(CO)2 site toward tertiary silanes. The excellent selectivity is attributed to the unique synergistic mechanism between the dinuclear metals. The realization of the anti-Markovnikov addition reaction catalyzed by this dinuclear cobalt complex demonstrates the broad prospects of redox-active metal ligands in the design of 3D metal catalysts.
This research result was published as a Research Article in CCS Chemistry . Researchers Liang Deng and Hui Chen from the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, are the corresponding authors of the paper, and Associate Researcher Dongyang Wang from the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, is the first author.
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About the journal: CCS Chemistry is the Chinese Chemical Society's flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem .
About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman's Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/ .
