Weakened O–H Bonds: Silicon Trick Unlocks Direct Hydrogen Atom Transfer for Chiral Alcohol Synthesis

Alcohols are among the most abundant and versatile feedstocks in organic chemistry, yet their most fundamental bond—the oxygen–hydrogen bond—has long resisted one of the most elegant tools in the radical chemist’s arsenal: direct hydrogen atom transfer, or HAT. The reason is thermodynamic. The O–H bonds of ordinary alcohols carry bond dissociation energies that are simply too high for photocatalysts to cleave under mild, redox-neutral conditions. A research group led by Professor Xiao Shen at Wuhan University has now found a way around this barrier, and the solution lies in an old and beloved concept of physical organic chemistry: the β-silicon effect. Writing in National Science Review, the team reports that a strategically placed silicon atom weakens the O–H bond of α-silyl alcohols by more than eleven kilocalories per mole, opening the door to a photoinduced direct HAT process that feeds directly into an asymmetric nickel-catalyzed cross-coupling.

The significance of the advance becomes clear when one considers how chemists normally activate alcohols for radical chemistry. Classical approaches rely on single-electron transfer (SET) or proton-coupled electron transfer (PCET), both of which depend on the oxidation potential of the substrate or on pre-installed activating groups such as xanthates, oxalates, or N-alkoxyphthalimides. These requirements impose a double penalty: they demand synthetic effort to prepare the activated precursor, and they exclude substrates whose redox potentials fall outside the window accessible to the photocatalyst. A general, redox-neutral method that cleaves the O–H bond directly—without substrate-specific redox tuning or metal pre-activation—has therefore remained an elusive goal in the field.

The Wuhan team’s insight is rooted in computation. Density functional theory calculations revealed that the calculated O–H bond dissociation energy of a representative α-silyl alcohol is only 89.2 kcal/mol, roughly 11.5 kcal/mol lower than that of its non-silylated counterpart. This dramatic bond weakening arises from the β-silicon effect, the well-known ability of a silicon atom positioned beta to a developing radical or cationic center to stabilize that center through hyperconjugative interaction of its σ C–Si bonds with the adjacent orbital. In this case, the effect stabilizes the alkoxy radical formed upon O–H homolysis, rendering direct HAT thermodynamically accessible with a photocatalyst that would be powerless against a normal alcohol.

To translate this computational insight into practice, the researchers selected thioxanthone (TXT) as the HAT photocatalyst. Under mild, redox-neutral conditions, photoexcited thioxanthone abstracts the weakened O–H hydrogen from the α-silyl alcohol, generating an alkoxy radical. What follows is a radical relay of remarkable elegance: the oxygen-centered radical rapidly undergoes a radical Brook rearrangement, a silicon-mediated 1,2-migration that converts the alkoxy radical into a nucleophilic carbon radical. This carbon radical is then intercepted by a nickel catalyst, which performs an enantioconvergent cross-coupling with either acyl chlorides or aryl bromides. The net result is the formation of chiral acyloins and chiral alcohols in high yields with excellent enantioselectivities, all starting from an unactivated O–H bond.

Perhaps the most convincing evidence for the power of the new paradigm comes from a direct experimental comparison with the group’s own earlier work. In a previous study published in Angewandte Chemie, the team had developed SET/PCET conditions for related transformations. Substrates bearing high oxidation potentials—most notably trifluoromethyl-substituted α-silyl alcohols—proved completely inert under those conditions, because the photocatalyst could not oxidize them. Under the new HAT protocol, however, these same stubborn substrates reacted smoothly, delivering the desired product in 84 percent yield. This head-to-head validation demonstrates that the direct HAT pathway operates independently of substrate redox potentials, unlocking a swath of chemical space that electron-transfer-based methods simply cannot reach.

Operational simplicity adds further appeal. The α-silyl alcohol substrates are readily prepared by nucleophilic addition of silyllithium reagents to aldehydes, a straightforward and modular step. After a simple work-up, the crude material can be carried directly into the coupling step without column chromatography, making the entire sequence a one-pot, two-step protocol. Both the acylation with acyl chlorides and the arylation with aryl bromides proceed efficiently under these conditions, furnishing products in good yields and enantioselectivities. The reaction also scales: a 2 millimole-scale run provided product 3a in 72 percent yield with 96 percent enantiomeric excess, with no erosion of stereochemical fidelity as the scale increased.

To showcase the synthetic value of the method in more complex settings, the team subjected the chiral acyloin products to a battery of downstream transformations. The products tolerated selective deoxyfluorination, diastereoselective reduction, Wittig olefination, and vinylation, in each case preserving their stereochemical integrity. Because acyloins are versatile intermediates that sit at the intersection of α-hydroxy ketone chemistry, these derivatizations point toward ready application in pharmaceutical synthesis and natural product construction, where chiral α-hydroxy carbonyl motifs are pervasive.

Mechanistic investigations lend strong support to the proposed direct HAT pathway. A radical clock experiment using a cyclopropane-bearing substrate produced the ring-opened product, a classic signature confirming the generation of an oxygen-centered radical intermediate. Cyclic voltammetry measurements showed that the oxidation potentials of the α-silyl alcohols are significantly higher than the excited-state reduction potential of thioxanthone, effectively ruling out a single-electron-transfer mechanism. Meanwhile, proton NMR titration experiments revealed hydrogen-bonding interactions between thioxanthone and the substrate—interactions that likely pre-organize the complex and facilitate the hydrogen atom transfer event itself.

The computational picture completes the story. Density functional theory calculations confirmed that the O–H bond dissociation energy of the α-silyl alcohol is substantially lower than that of the reduced photocatalyst intermediate INT2, and the computed activation barrier for the HAT step is a remarkably low 4.4 kcal/mol—a barrier easily surmounted at room temperature. On the basis of these experimental and computational data, the authors propose a comprehensive catalytic cycle that integrates the initial HAT event, the radical Brook rearrangement, and the nickel-catalyzed enantioconvergent cross-coupling into a single, redox-neutral sequence.

The work, published in National Science Review with DOI 10.1093/nsr/nwag498, lists Professor Xiao Shen and Dr. Xiaoqian He of Wuhan University as corresponding authors, with Dr. Chenyu Jin and Dr. Yunhong Niu as co-first authors. The research was supported by the National Key Research and Development Program of China and the National Natural Science Foundation of China, with theoretical calculations performed on the supercomputing system at the Supercomputing Center of Wuhan University. For a field that has long treated the alcohol O–H bond as untouchable by direct HAT, the message of this study is transformative: with a strategically placed silicon atom, one of chemistry’s most stubborn bonds becomes a launch pad for asymmetric radical synthesis.

Subject of Research: Direct hydrogen atom transfer activation of α-silyl alcohols via the β-silicon effect for enantioconvergent nickel-catalyzed cross-couplings

Article Title: β-Silicon effect enables direct HAT for enantioconvergent couplings of α-silyl alcohols

Article References: β-Silicon effect enables direct HAT for enantioconvergent couplings of α-silyl alcohols. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: β-silicon effect, hydrogen atom transfer, α-silyl alcohols, photocatalysis, thioxanthone, Brook rearrangement, nickel catalysis, enantioconvergent coupling, radical chemistry, acyloins, bond dissociation energy, redox-neutral synthesis