Neutral eosin Y photocatalysis: Flow chemistry creates exclusive silanes

As an important organic molecule, silane is widely used in various fields of life and technology, from coatings, adhesives, gels, rubber and other materials to the research and development of pesticides and drugs. Recently, Wu Jie’s group at the National University of Singapore developed a new method for selective stepwise polyhydrosilane functionalization using direct hydrogen atom transfer based on light-neutral eosin Y, and on March 9, 2023, the results were published as “Stepwise on-demand functionalization of multihydrosilanes enabled by a hydrogen-atom-transfer.” photocatalyst based on eosin Y”, published in the journal Nature Chemistry, the first authors of the paper are Fan Xuanzi and Zhang Muliang.

Organosilicon compounds can typically be synthesized from various silicon electrophiles such as silyl chloride compounds, or nucleophiles such as silicon boron reagents. However, there are usually many problems with the controlled functionalization of polychlorosilanes, such as the reaction often does not stop at monosubstitution, and the raw material is very sensitive to water and difficult to preserve; Silicon-boron reagents are often only able to prepare a single functionalized product. Stable and easy-to-preserve hydrosilanes are an ideal class of silican-containing synthetic building blocks for the direct and efficient introduction of various silane compounds. In the past few decades, the research on Si-H bond functionalization catalyzed by transition metals has become increasingly mature, and the more efficient catalysts are copper, iridium, rhodium, etc., and the functional grouping of silanes can be realized with the use of ligands. However, the synthesis of low-cost, non-heavy metal residues, low waste, easy to scale, and green chemistry for silane compounds (e.g., arylsilicon, silyl chloride, alcohol, silyl ether, silicodeuterium compounds) has rarely been reported. At present, the limited silane conversion strategy is still the biggest bottleneck restricting the application of silane, and it is one of the research hotspots in synthetic chemistry.

Hydrosilanes are generally reactive, highly reactive and may trigger a series of side reactions during activation, so the difficult problem in the synthesis of highly functional organosilicon compounds is how to activate hydrosilanes controllably and selectively (Figure 1), especially heteroatom-substituted silica reagent synthesis. At present, there are few types of reactions developed based on the radical strategy Si-H activation, which require additional oxidants and have limited synthetic applications. Silane activation based on single-electron transfer (SET) is limited by the redox potential of the photocatalyst, thereby limiting the substrate range. One of the most feasible ways to activate hydrosilanes is to extract hydrogen atoms from hydrosilanes to generate silyl radicals by extracting hydrogen atoms from hydrosilanes under photocatalytic conditions by hydrogen atom transfer (HAT). This approach has obvious advantages in terms of atomic economy and simplified synthesis steps; However, it turns out that achieving selective activation of Si-H bonds and functionalizing the diversity of hydrosilanes in the presence of active C-H bonds remains a formidable challenge.

Figure 1: Stepwise functionalization of polyhydrosilanes as needed

Based on the experience of C-H bond activation in direct hydrogen atom transfer catalyzed in neutral eosin Y, Wu Jie’s group tried to realize the functional grouping of hydrosilanes. Si-H activation independent of redox potential is achieved by generating silicon-based radicals through a direct HAT process. After neutral eosin Y absorbs a blue light sub (~63 kcal/mol), it can grab the bond energy C-H bond containing about 90 kcal/mol to produce the corresponding free radical. And neutral eosin Y is a widely available cheap organic small molecule, absorbing visible light, so it can be used as an ideal Si-H bond-activated HAT photocatalyst, hydrogen atom transfer photocatalysis can directly extract hydrogen atoms from hydrosilane to generate silicon-based radicals, omitting the redox step and broadening the range of applicable substrates.

In the article, the authors report the diversity functionalization of neutral eosin Y activated Si-H bonds, demonstrating a single selectivity between Si-H activation and C-H activation, and a gradual selectivity for monofunctionalization of polyhydrosilanes. They first demonstrated eight different types of new transformations (Figure 2), such as hydroalkylation of various alkenes and alkynes, cross-dehydrogenation coupling (CDC) with styrene and (hetero)aromatics, allylation, oxidation, deuterated and tandem bromination substitution reactions, etc. Eoson Y catalysts play multiple roles in these transformations, including the formation of silicon-based radical intermediates and the promotion of reverse HAT or photoredox-mediated dehydrogenation processes. In addition, the authors also studied in detail the possible reaction mechanisms involved in the catalyst cycle, and designed a series of mechanism experiments to verify, including free radical capture experiments, cyclic voltammetry measurements, determination of hydrogen atom sources by gas chromatography mass spectrometry (GC-MS) analysis, kinetic isotope effect (KIE) studies and transient absorption spectroscopy measurements.

Figure 2: Diversity functionalization of Si-H bonds

The authors then designed a simple microfluidic reactor that utilizes its excellent mixing efficiency and precise residence time control to achieve selective single- or dual-function of dihydrosilanes and trihydrosilanes. The reactor can directly achieve production on a scale of 10 g. It is worth noting that this reaction does not achieve high selectivity in conventional flask reactors. The authors further developed stepwise modifications of polyhydrosilanes to precisely control the stepwise localization of different functional groups, allowing for a series of previously difficult to prepare organosilicon compounds with four different substituents in a controlled and on-demand manner (Figure 3). Finally, the authors demonstrate a range of conversion applications for the prepared silicon products, such as cross-coupling, closed-loop metathesis, cyclization reactions, and various types of polymerization based on silicon-containing units.

Figure 3: Synthesis of silicone reagents by stepwise modification of trihydrosilanes


In this study, the authors propose a new method for selective and controlled stepwise functionalization of polyhydrosilanes using a direct hydrogen atom transfer strategy based on neutral eosin Y, which has achieved a significant breakthrough in the field of silicon chemistry. Eosin Y has proven to be highly suitable for Si-H activation and is capable of efficiently and selectively activating Si-H bonds in a wide range of compounds where active C-H bonds are present. Activated silicon radicals react with a range of bulk chemical feedstocks to produce functionalized, high value-added silane compounds. The introduction of flow chemistry ensures the progressive activation of polyhydrosilanes with high selectivity. This scheme provides a simple and universal and green new way for the selective introduction of various functional groups to synthesize a variety of silane products, which is expected to be widely used in scientific research and industry. (Source: Science Network)

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