Iridium-Catalyzed Regioselective Hydroalkynylation of Internal Alkenes Directed by an Oxime

Theoretical Study on the Mechanism of Ru(II)-Catalyzed Intermolecular [3 + 2] Annulation between o-Toluic Acid and 3,5-Bis(trifluoromethyl)benzaldehyde: Octahedral vs Trigonal Bipyramidal

Density functional theory was utilized to investigate the mechanism of Ru(II)-catalyzed aromatic C-H activation and addition of aromatic aldehydes. The proposed catalytic cycle consists of C-H bond activation, aldehyde carbonyl insertion for C-C coupling, lactonization for the formation of the final product, product separation, and catalyst recovery. Our calculations suggest that Ru(OAc)2(PCy3) (referred to as CAT) is the most favorable active catalyst, facilitating the C-H bond activation to form a five-membered ring cycloruthenium intermediate (INT2). Subsequently, the aromatic aldehyde reactant 2a enters the Ru coordination sphere, accelerating the C-C coupling and lactonization for the formation of the final product. The involvement of acetate assists in the final product separation, while INT1 re-enters the Ru coordination sphere to initiate a new catalytic cycle. Utilizing the energetic span model, the apparent activation free energy barrier was computed to be 34.3 kcal mol-1 at 443 K. Furthermore, exploration of the reaction mechanism in the absence of phosphine ligands identified Ru(OAc)2(p-cymene) as the most favorable active catalyst. The derived apparent activation free energy barrier offers a comprehensive explanation for the experimentally observed yields. Additionally, we have examined the disparities between the octahedral and trigonal bipyramidal structures of the catalysts concerning their effects on the reaction mechanisms and apparent activation free energy barriers.

Chiral bisphosphine Ph-BPE ligand: a rising star in asymmetric synthesis

Chiral 1,2-bis(2,5-diphenylphospholano)ethane (Ph-BPE) is a class of optimal organic bisphosphine ligands with C2-symmetry. Ph-BPE with its excellent catalytic performance in asymmetric synthesis has attracted much attention of chemists with increasing popularity and is growing into one of the most commonly used organophosphorus ligands, especially in asymmetric catalysis. Over two hundred examples have been reported since 2012. This review presents how Ph-BPE is utilized in asymmetric synthesis and how powerful it is as a chiral ligand or even a catalyst in a wide range of reactions including applications in the total synthesis of bioactive molecules.

Coordination Assistance: A Powerful Strategy for Metal-Catalyzed Regio- and Enantioselective Hydroalkynylation of Internal Alkenes

ConspectusAlkenes are versatile compounds that are readily available on a large scale from industry or through organic synthesis. The widespread occurrence of alkenes provides the continuous impetus for the development of catalytic asymmetric alkene hydrofunctionalizations, which enables expeditious construction of complex chiral molecules from readily available starting materials. Catalytic asymmetric hydrofunctionalization of internal alkenes presents a notable challenge, due to their low reactivity, many potential side reactions, and the simultaneous control of the regio-, diastereo-, and enantioselectivities.Dehydroamino acids and enamides are among the first substrates that provide notable enantioselectivities in catalytic asymmetric hydrogenation. The crucial importance of an amide coordinating group is established by a series of classical mechanistic studies. This initial success greatly stimulated further development for catalytic hydrogenation and hydrofunctionalization. Building on these pioneering works in asymmetric hydrogenation as well as related hydrofunctionalizations, we have adopted coordination assistance as a powerful tool to address the challenges associated with the asymmetric hydrofunctionalization of internal alkenes. Using a functional group on the alkene substrate as a native coordinating group, a two-point binding mode of the substrate to the metal center effectively enhances the reactivity and facilitates the control of regio-, diastereo- and enantioselectivities. Through this strategy, we have developed a number of alkene hydrofunctionalization methods with excellent regio-, diastereo-, and enantiocontrols.In this Account, we summarize the recent advance in our lab using coordination assistance as a key element to achieve regio- and enantioselective hydroalkynylation of internal alkenes. First, we describe our early work aimed at controlling the regio- and enantioselectivity of hydroalkynylation using disubstituted enamide as the substrate. Both α- and β-alkynylation were achieved by channeling the reaction pathway into a Chalk-Harrod or modified Chalk-Harrod mechanism. Next, we discuss the further development of catalysts to achieve regiodivergent and enantioselective hydroalkynylation of trisubstituted enamide to access vicinal stereocenters and quaternary carbon stereocenters. We also discuss the hydroalkynylation of α,β-unsaturated amides to achieve unconventional site-selectivity through a combination of alkene isomerization and regioselective hydroalkynylation. This provides the basis for the construction of a remote quaternary carbon stereocenter through catalytic hydroalkynylation of trisubstituted β,γ-unsaturated amides. We further show that this controlling principle is applicable to terminal alkene with a coordinating group as well. A ligand-controlled mechanism shift is discussed for the enantioselective alkynylation at the terminal and internal position of 1,1,-disubstituted alkenes. Finally, we briefly mention the application of coordination assistance to other hydrofunctionalizations such as hydroboration and hydrosilylation, where previously inaccessible reactivity and selectivity were achieved. Collectively, these catalytic methods demonstrate the power of coordination assistance for enantioselective hydrofunctionalizations. We anticipate that this strategy will create a platform to enable diverse enantioselective alkene transformations.