Ligand-Accelerated, Copper-Catalyzed Aerobic Oxidative Dehydrogenation of Primary Amines to Nitriles

A highly efficient copper-catalyzed oxidative dehydrogenation of primary amines to access nitriles has been developed. We found that DMAP was an efficient ligand for copper-catalyzed oxidation and molecular oxygen was a green oxidizing agent. This reaction exhibited excellent functional group compatibility and a broad substrate scope. Various benzylic, allylic, and aliphatic amines were selectively and effectively oxidized to the corresponding nitriles in high yields (up to 100%). The ligand DMAP not only accelerated the reaction rate but also enhanced the stability of the catalyst. The practicality of the reaction was illustrated on a gram scale, even at a lower catalyst loading and/or under bubble air conditions.

Convergent Total Synthesis of Ixabepilone and Its Analogues Enabled by Highly Efficient Asymmetric Hydrogenations

The convergent total synthesis of ixabepilone and its analogues in a 13-step longest linear sequence is reported. The crucial chiral centers at challenging C3-O, C8-C and C15-N positions on the scaffold of the ixabepilone were installed via highly efficient asymmetric hydrogenations (up to 95 % yield and up to 99 % e.e.) and all three fragments could be prepared on gram scale. The key aldol reaction bridges the aldehyde and keto fragments with high yield and exquisite diastereo control (8 : 1 d.r.). Finally, a novel RCM reaction conformationally controlled by a bulky silyl group was reported, which allows the facile synthesis of ixabepilone and its analogues.

One-Pot, Telescoped Aryl Nitrile Synthesis from Benzylic Silyl Ethers

A one-pot, telescoped transformation of silyl ethers into cyanides that proceeds via silyl-ether oxidation mediated by nitroxyl-radical catalyst 1 and [bis(trifluoroacetoxy)iodo]benzene followed by an imine formation-oxidation sequence using iodine and aqueous ammonia is reported. This transformation is effective for the site-selective transformation of benzylic and allylic silyl ethers in the presence of other silyl ethers. Using an O-protected oxime and a catalytic amount of triflic acid instead of iodine/aqueous ammonia is also effective for cyanation.

Nickel-catalysed regio- and stereoselective hydrocyanation of alkynoates and its mechanistic insights proposed by DFT calculations

We have developed a nickel-catalysed regio- and stereoselective hydrocyanation of alkynoates that gives syn-β-cyanoalkenes. DFT calculations suggest that a favored transition state promotes Cα-H bond formation for determining regio- and stereoselectivity of the products.

Cobalt-Catalyzed Acceptorless Dehydrogenation of Primary Amines to Nitriles

The direct double dehydrogenation from primary amines to nitriles without an oxidant or hydrogen acceptor is both intriguing and challenging. In this paper, we describe a non-noble metal catalyst capable of realizing such a transformation with high efficiency. A cobalt-centered N,N-bidentate complex was designed and employed as a metal-ligand cooperative dehydrogenation catalyst. Detailed kinetic studies, control experiments, and DFT calculations revealed the crucial hydride transfer, proton transfer, and hydrogen evolution processes. Finally, a tandem outer-sphere/inner-sphere mechanism was proposed for the dehydrogenation of amines to nitriles through an imine intermediate.

Triflylpyridinium Enables Rapid and Scalable Controlled Reduction of Carboxylic Acids to Aldehydes using Pinacolborane

Building up new and efficient methods for the controlled conversion of carboxylic acids to aldehydes is important. Herein, we report a rapid, modular and scalable method for the conversion of carboxylic acids to aldehydes using pinacolborane at ambient temperature, in which a triflylpyridinium reagent is used. The conversion of carboxylic acid to intermediate acylpyridinium by triflylpyridinium is new. A binary pyridine-coordinated boronium complex is generated after reduction. The unprecedented reduction of the acylpyridinium by HBpin opens up a practically direct synthesis of aldehydes from carboxylic acids. Theoretical studies indicate that the reduction of acylpyridinium requires a lower activation free energy than that of the product aldehyde. The synthetic advantage of this protocol is further highlighted by the scalable synthesis of aldehyde via continuous flow process. Configuration retention for chiral acids are presented in those syntheses.

Recent advances of aminoazanium salts as amination reagents in organic synthesis

This review summarizes the utilization of aminoazaniums as amination reagents in organic synthesis, including the amination of aldehydes, boronic esters, olefins, etc.