Recent Advances in Radical-Involved Alkynylation of Unactivated C(sp3)–H Bonds by Hydrogen Atom Abstraction

Remote radical alkynylation of unactivated C(sp3)-H bonds of ethynesulfonamides

We report an efficient protocol for the construction of δ-alkynyl amides via 1,5-hydrogen atom transfer and alkynyl group transfer of ethynesulfonamides. The readily installed ethynesulfonamides serve as both an amidyl radical precursor and an alkyne source. This reaction features excellent site selectivity for tertiary, secondary, primary, and benzylic C(sp3)-H bonds.

Reactions of Tertiary Aliphatic Cations with Silylated Alkynes: Substitution, Cyclization and Unexpected C-H Activation Products

Capozzi's groundbreaking work in 1982 introduced a fascinating reaction involving highly reactive tertiary aliphatic cations and silylated alkynes. This reaction provided an innovative solution to the challenge of coupling a fully substituted tertiary aliphatic fragment with an alkyne moiety. Building upon Capozzi's pioneering efforts, we started an extensive exploration of reaction conditions to expand the initial scope of this reaction. Through meticulous control of the reaction parameters, we uncovered conditions capable of accommodating various functional groups, thereby enhancing the reaction's applicability. Intriguingly, our study revealed remarkably high diastereoselectivities for substrates with substitution in the α-position. Additionally, we made an unexpected discovery: an intriguing C-H activation of a cyclooctane ring furnishing a cyclooctane-fused cyclobutene. These findings not only extend the utility of Capozzi's original concept but also underscore the potential of highly reactive cations in modern organic C-H activation reactions.

Photoinduced copper-catalyzed asymmetric cyanoalkylalkynylation of alkenes, terminal alkynes, and oximes

The asymmetric dicarbofunctionalization of alkenes via a radical relay process can provide routes to diverse hydrocarbon derivatives. Three-component carboalkynylation, limited to particular alkyl halides and using readily available cycloketone oxime esters as redox-active precursors, is restricted by the available pool of suitable chiral ligands for broadening the redox potential window of copper complexes and simultaneously creating the enantiocontrol environment. Herein, we report a new hybrid tridentate ligand bearing a guanidine-amide-pyridine unit for photoinduced copper-catalyzed cyanoalkylalkynylation of alkenes. Leveraging the copper catalyst's redox capability is achieved via merging the electron-rich ligand with a readily organized configuration and enhanced absorption in the visible light range, which also facilitates the enantioselectivity. The generality of the catalyst system is exemplified by the efficacy across a number of alkenes, terminal alkynes and cycloketone oxime esters, working smoothly to give alkyne-bearing nitriles with good yields and excellent enantioselectivity. A mechanistic study reveals that the chiral copper catalyst meets the requirements of possessing sufficient reduction ability, good light absorption properties, and appropriate steric hindrance.

CoH-catalyzed asymmetric remote hydroalkylation of heterocyclic alkenes: a rapid approach to chiral five-membered S- and O-heterocycles

Saturated heterocycles, which incorporate S and O heteroatoms, serve as fundamental frameworks in a diverse array of natural products, bioactive compounds, and pharmaceuticals. Herein, we describe a unique cobalt-catalyzed approach integrated with a desymmetrization strategy, facilitating precise and enantioselective remote hydroalkylation of unactivated heterocyclic alkenes. This method delivers hydroalkylation products with high yields and excellent stereoselectivity, representing good efficiency in constructing alkyl chiral centers at remote C3-positions within five-membered S/O-heterocycles. Notably, the broad scope and good functional group tolerance of this asymmetric C(sp3)-C(sp3) coupling enhance its applicability.

Dealkenylative Alkynylation Using Catalytic FeII and Vitamin C

In this paper, we report the synthesis of alkyl-tethered alkynes through ozone-mediated and FeII-catalyzed dealkenylative alkynylation of unactivated alkenes in the presence of alkynyl sulfones. This one-pot reaction, which employs a combination of a catalytic FeII salt and l-ascorbic acid, proceeds under mild conditions with good efficiency, high stereoselectivity, and broad functional group compatibility. In contrast to our previous FeII-mediated reductive fragmentation of α-methoxyhydroperoxides, the FeII-catalyzed process was devised through a thorough kinetic analysis of the multiple competing radical (redox) pathways. We highlight the potential of this dealkenylative alkynylation through multiple post-synthetic transformations and late-stage diversifications of complex molecules, including natural products and pharmaceuticals.

Copper-Catalyzed Intermolecular Enantioselective Radical Oxidative C(sp3 )-H/C(sp)-H Cross-Coupling with Rationally Designed Oxazoline-Derived N,N,P(O)-Ligands

The intermolecular asymmetric radical oxidative C(sp³ )-C(sp) cross-coupling of C(sp³ )-H bonds with readily available terminal alkynes is a promising method to forge chiral C(sp³ )-C(sp) bonds because of the high atom and step economy, but remains underexplored. Here, we report a copper-catalyzed asymmetric C(sp³ )-C(sp) cross-coupling of (hetero)benzylic and (cyclic)allylic C-H bonds with terminal alkynes that occurs with high to excellent enantioselectivity. Critical to the success is the rational design of chiral oxazoline-derived N,N,P(O)-ligands that not only tolerate the strong oxidative conditions which are requisite for intermolecular hydrogen atom abstraction (HAA) processes but also induce the challenging enantiocontrol. Direct access to a range of synthetically useful chiral benzylic alkynes and 1,4-enynes, high site-selectivity among similar C(sp³ )-H bonds, and facile synthesis of enantioenriched medicinally relevant compounds make this approach very attractive.

Hydroalkylation of Unactivated Olefins via Visible-Light-Driven Dual Hydrogen Atom Transfer Catalysis

Radical hydroalkylation of olefins enabled by hydrogen atom transfer (HAT) catalysis represents a straightforward means to access C(sp³)-rich molecules from abundant feedstock chemicals without the need for prefunctionalization. While Giese-type hydroalkylation of activated olefins initiated by HAT of hydridic carbon-hydrogen bonds is well-precedented, hydroalkylation of unactivated olefins in a similar fashion remains elusive, primarily owing to a lack of general methods to overcome the inherent polarity-mismatch in this scenario. Here, we report the use of visible-light-driven dual HAT catalysis to achieve this goal, where catalytic amounts of an amine-borane and an in situ generated thiol were utilized as the hydrogen atom abstractor and donor, respectively. The reaction is completely atom-economical and exhibits a broad scope. Experimental and computational studies support the proposed mechanism and suggest that hydrogen-bonding between the amine-borane and substrates is beneficial to improving the reaction efficiency.

Cluster Preface: Radicals – by Young Chinese Organic Chemists

(left) received his B.S. degree from Xiamen University in 2003 under the supervision of Prof. Pei-Qiang Huang, and his Ph.D. degree from the Shanghai Institute of Organic Chemistry in 2008 under the supervision of Prof. Guo-Qiang Lin. After postdoctoral research at ­Gakushuin University, Japan with Prof. Takahiko Akiyama, he moved to the University of Texas Southwestern Medical Center, working as a postdoctoral fellow with Prof. John R. Falck and Prof. Chuo Chen. He was appointed as a professor at Soochow University, China in December 2013. He is currently the Head of the Organic Chemistry Department at Soochow University. His current research interests include radical-mediated transformations, in particular radical ­rearrangements, and their applications in the construction of natural products and biologically active compounds. Xin-Yuan Liu (right) obtained his B.S. degree from Anhui Normal University (AHNU) in 2001. He continued his research studies at both the Shanghai Institute of Organic Chemistry (SIOC), CAS and AHNU under the joint supervision of Prof. Dr. Shizheng Zhu and Prof. Dr. Shaowu Wang, obtaining his master’s degree in 2004. After a one-year stint in Prof. Gang Zhao’s laboratory at SIOC, he joined Prof. Dr. Chi-Ming Che’s group at The University of Hong Kong (HKU) and earned his Ph.D. degree in 2010. He subsequently undertook postdoctoral studies in Prof. Che’s group at HKU and in Prof. Carlos F. Barbas III’s group at The Scripps Research Institute. At the end of 2012, he began his independent academic career at the Southern University of Science and Technology (SUSTech) and was promoted to a tenured Full Professor of SUSTech in 2018. His research interests are directed towards the design of novel chiral anionic ligands to solve radical-involved asymmetric reactions.