論文

Photoinduced acceptorless dehydrogenative cross-coupling between the benzylic C(sp3)-H bonds of N-benzylanilines and the allylic C(sp3)-H bonds of alkenes was developed through the combined use of an Ir-based photosensitizer, silyl thiol, and 1,2,3-triazolium amidate as catalysts. This protocol exploits the fast and reversible hydrogen atom transfer between α-aminoalkyl radicals and the silyl thiol to facilitate the cross-coupling, directly routing complex aniline derivatives from readily available, less functionalized starting materials under mild conditions.

Cations and anions are fundamental species whose interactions have been thoroughly studied and widely applied in organic synthesis, particularly to control the selectivity in chemical transformations. Strategies exploiting ion pairing have proven to be applicable to a broad range of reactions that proceed via anionic intermediates and cationic species. Considering the general versatility of ion pairs in constructing well-organized transition structures, we utilized these powerful attractive interactions to construct catalyst structures. This account summarizes the design and applications of ion-paired chiral ligands and chiral 1,2,3-triazolium salts.

A catalytic 1,1-dicarbofunctionalization of electron-deficient olefins was effected on the basis of the three-component coupling reactions involving olefins bearing vicinal electron-withdrawing groups, potassium cyanide, and an alkyl halide, which afforded geminally cyanoalkylated products in high yields via conjugate cyanation, 1,2-proton transfer, and enolate alkylation. The use of suitable chiral phase-transfer catalysts enabled asymmetric induction in this transformation.

Acceptorless dehydrogenative cross-coupling between two carbon–hydrogen (C–H) bonds has garnered increasing attention in synthetic chemistry. Cross-coupling of this type does not require the use of functionalized reactive reagents such as organic halides and organometallics. In addition, acceptorless dehydrogenation can proceed without external oxidants, releasing molecular hydrogen as the sole by-product. These features enable target molecules to be accessed with fewer synthetic steps and reduced waste. For the development of dehydrogenative cross-coupling reactions, viable methodologies for the cleavage of C–H bonds are crucial. Hydrogen-atom transfer (HAT) catalysis has significant potential because a diverse range of hydrocarbons can be subjected to HAT. This Perspective describes the use of photoinduced HAT catalysis in acceptorless dehydrogenative reactions, with the aim of providing an opportunity to consider the future prospects of this methodology. [Figure not available: see fulltext.]

The stereoselective cyanoalkylation of electron-deficient olefins with potassium cyanide and alkyl halides was developed based on the utilization of modular chiral 1,2,3-triazolium salts featuring a hydrogen bond-donor ability as catalysts. The reaction involving multiple carbon-carbon bond formations proceeds via the enantioselective conjugate addition of a cyanide ion and the consecutive catalyst-controlled diastereoselective alkylation of intermediary chiral triazolium enolates. Control experiments revealed that the use of a properly tuned chiral triazolium ion as a catalyst and the presence of the cyano functionality in the intermediary enolate are of crucial importance for achieving high levels of acyclic absolute and relative stereocontrol.

A catalytic asymmetric Strecker reaction of ketoimines with potassium cyanide (KCN) is developed. Control experiments reveal the involvement of a retro-Strecker reaction, which inevitably decreases the product yield and enantioselectivity. The use of a protic additive in combination with a chiral 1,2,3-triazolium ion as a catalyst allows for the highly efficient and enantioselective cyanide addition to various ketoimines.