Design and Synthesis of a Ferrocene-Based Diol Library and Application in the Hetero-Diels-Alder Reaction

Diol scaffolds have been utilized as highly effective catalysts and ligands in a wide range of catalytic asymmetric transformations. For scaffolds to be successful as broadly used motifs, they should be prepared cheaply through facile routes and be easily handled. Herein, the synthesis of a family of planar chiral diols based on a modular and robust three-step route is described, which yields catalytically active diols in >99 % de and >99 % ee, with up to seven different chiral elements. These diols have been characterized by X-ray crystallographic analysis, which provides clear evidence for the likely transition state when applied in the asymmetric hetero-Diels-Alder reaction. Without altering the stereochemistry of the catalyst backbone, it is possible to access both enantiomers of the product by varying the substitution of the catalyst at the α-position.

Cp*Rh(III)-Catalyzed C-H Arylation of Ferrocenethionamides with Aryl Boronic Acids for the Synthesis of Aryl-Ferrocenes

We developed a Cp*Rh(III)-catalyzed C-H arylation of ferrocenethionamides with arylboronic acids for the synthesis of aryl-ferrocenes under mild and base-free conditions, using Ag₂ CO₃ as oxidant. The reaction results in high yields and excellent regioselectivity accommodating a broad scope of substrate range and functional group compatibility, and provides an alternative protocol for the generation of highly functionalized aryl-ferrocene compounds.

Pd(II)-Catalyzed Azine-Assisted Enantioselective Oxidative C-H/C-H Cross-Coupling of Ferrocenes with Various Heteroarenes

A palladium(II)-catalyzed enantioselective oxidative cross-coupling of ferrocenes with heteroarenes is described. Mono-N-protected amino acids can be used as sources of chirality. With azine as an efficient directing group, various substituted planar chiral ferrocenes were obtained via a dual C-H bond activation pathway in medium yields (up to 72%) with good enantioselectivity (up to 89.4:10.6 er) under mild conditions.

Diastereoselective Double C-H Functionalization of Chiral Ferrocenes with Heteroaromatics

Diastereoselective double C-H heteroarylation of chiral ferrocenes provides valuable compounds with multiple functionalities using mild reaction conditions and simple reagents. Pd-Complexes with chiral mono-protected amino acids afforded corresponding heteroarylated ferrocenyl amines in good yields and high diastereomeric purities. In this way, a variety of indole, thiophene, pyrrole, or furan substituents were introduced to the ferrocene moiety. Furthermore, a range of relevant functional groups, for example ketone, ester, chloro, nitro, or silyl, are tolerated by this method. An alternative combination of amino acid and ferrocenyl amine configurations was leveraged to provide the complementary diastereomeric products. The products of C-H heteroarylation can be transformed into corresponding phosphines. Absolute configurations of CH-activation products were confirmed by the combination of X-ray crystallographic analysis and CD spectroscopy. ¹⁹ F NMR kinetic study and DFT calculations provided insights into the reaction mechanism and reasons governing stereoinduction.

Accessing C2-Functionalized 1,3-(Benz)azoles through Transition Metal-Catalyzed C-H Activation

The skeletal presence of 1,3-azoles in a variety of bioactive natural products, pharmacophores, and organic materials demands the derivatization of such heteroarenes regioselectively. Plenty of cross-coupling as well as cyclocondensation reactions have been performed to build up these skeletons but remained commercially unrealizable. A couple of severe drawbacks are faced by these traditional protocols that require a more straightforward strategy to obviate them. Transition metal-catalyzed C-H functionalization has emerged as a superior alternative in that context. 1,3-Azoles and their benzo counterparts have been extensively functionalized exploiting both noble and earth-abundant transition metals. Lately, C-2 functionalization have gained much traction due to the ease of attaining high regioselectivity and installation of synthetically manipulative functionalities. This critical review presents a bird's eye view of all major C-2 functionalization of (benz)azoles catalyzed by a diverse set of metals performed over the past 15 years.

Enantioselective Pallada-Electrocatalyzed C-H Activation by Transient Directing Groups: Expedient Access to Helicenes

Asymmetric pallada-electrocatalyzed C-H olefinations were achieved through the synergistic cooperation with transient directing groups. The electrochemical, atroposelective C-H activations were realized with high position-, diastereo-, and enantio-control under mild reaction conditions to obtain highly enantiomerically-enriched biaryls and fluorinated N-C axially chiral scaffolds. Our strategy provided expedient access to, among others, novel chiral BINOLs, dicarboxylic acids and helicenes of value to asymmetric catalysis. Mechanistic studies by experiments and computation provided key insights into the catalyst's mode of action.

Thioketone-directed rhodium(I) catalyzed enantioselective C-H bond arylation of ferrocenes

Planar chiral ferrocenes have received great attention in both academia and industry. Although remarkable progresses have been made over the past decade, the development of efficient and straightforward methods for the synthesis of enantiopure  planar chiral  ferrocenes remains highly challenging. Herein, we report a rhodium(I)/phosphonite catalyzed thioketone-directed enantioselective C-H bond arylation of ferrocenes. Readily available aryl iodides are used as the coupling partners in this transformation, leading to a series of planar chiral ferrocenes in good yields and excellent enantioselectivities (up to 86% yield, 99% ee). Of particular note, heteroaryl coupled ferrocenes, which are difficult to access with previous approaches, can be obtained in satisfactory results.