Rhodium Catalysts with a Chiral Cyclopentadienyl Ligand Derived from Natural R‐Myrtenal

Enantiospecific Synthesis of Planar Chiral Rhodium and Iridium Cyclopentadienyl Complexes: Enabling Streamlined and Computer-Guided Access to Highly Selective Catalysts for Asymmetric C-H Functionalizations

Chiral cyclopentadienyl (CpX) metal complexes are frequently used in asymmetric catalysis by virtue of their high reactivity and selectivity. Planar-chiral-only rhodium and iridium cyclopentadienyl complexes are particularly promising due to unrestricted chemical space for CpX ligand design while retaining structural simplicity. However, they are currently still niche because of a lack of efficient synthetic strategies that avoid lengthy chiral auxiliary routes or chiral preparatory HPLC resolution of the complexes. To streamline access to such planar-chiral-only CpX-metal complexes, we designed a straightforward, highly enantiospecific, point-to-planar chirality transfer complexation via facially selective concerted-metalation-deprotonation between metal-carboxylate precursor [M(olefin)2OAc]2 and a chiral cyclopentadiene. This entirely avoids the typical stereoablative complexation of an achiral cyclopentadienyl anion that detrimentally yields a racemate. Exploiting the described enantiospecific complexation protocol and a simple divergent synthetic route to suitable chiral cyclopentadienes, we generated a structurally diverse library of new planar chiral Cp-Rh(I), Cp-Ir(I), Cp-Rh(III), and Cp-Ir(III) complexes. Moreover, the enantiospecific complexation step can be concatenated with a preceding Au-catalyzed cyclization in an efficient one-pot process that likely involves an elaborate point-to-axial-to-point-to-planar chirality transfer. Guided by computational selectivity predictions, the structure of a CpX-Rh complex in our library was tuned to optimize reactivity and selectivity in the asymmetric C-H functionalization of a benzamide with various challenging alkenes. With an optimized CpX-Rh complex in hand, we showcased its excellent catalytic performance and high selectivity for refractory alkene substrates that reacted in poor selectivity with previous CpX-Rh catalysts.

[2.2]Benzoindenophane-Based Chiral Indenyl Ligands: Design, Synthesis, and Applications in Asymmetric C-H Activation

Development of chiral indenyl ligands for asymmetric C-H activation is a longstanding challenge, and extremely few successes have been achieved. In this paper, we describe a class of readily accessible, facilely tunable and user-friendly chiral indenyl ligands featuring a [2.2]benzoindenophane skeleton via a divergent synthesis strategy. The corresponding chiral indenyl rhodium catalysts were successfully applied in the asymmetric C-H activation reaction of O-Boc hydroxybenzamide with alkenes to give various chiral dihydroisoquinolone products (up to 97 % yield, up to 98 % ee). Moreover, the asymmetric C-H activation reaction of carboxylic acids with alkynes was also successfully accomplished, providing a range of axially chiral isocoumarins (up to 99 % yield, up to 94 % ee). Notably, this represents the first example of enantioselective transition metal catalyzed C(sp2)-H activation/oxidative coupling of benzoic acids with internal alkynes to construct isocoumarins. Given many attractive features of this class of indenyl ligands, such as convenient synthesis, high tunability and exclusive face-selectivity of coordination, its applications in more catalytic asymmetric C-H activation and in other asymmetric catalysis are foreseen.

Planar Chiral Rhodium Complex Based on the Tetrahydrofluorenyl Core for Enantioselective Catalysis

A simple four-step route to a chiral tetrahydrofluorenyl rhodium catalyst from naturally occurring (-)-α-pinene was developed. Our approach does not use multistep and time-consuming procedures such as chiral HPLC or diastereomeric resolution. The key to success lies in the face-selective coordination of rhodium to the sterically hindered tetrahydrofluorenyl ligand, giving only one diastereomeric complex. This catalyst proved to be highly efficient for asymmetric C-H annulation of aryl hydroxamates with alkenes (yield up to 95%, 91% ee) at low loading (up to 0.4 mol % based on Rh).

Synthesis, stereochemical assignment, and enantioselective catalytic activity of late transition metal planar chiral complexes

Planar chirality is an important form of molecular chirality that can be utilized to induce enantioselectivity when incorporated into transition metal catalysts. However, due to synthetic constraints, the use of late transition metal planar chiral complexes to conduct enantioselective transformations has been limited. Additionally, the published methods surrounding the stereochemical assignment of planar chiral compounds are sometimes conflicting, making proper assignment difficult. This review aims to provide clarity on the methods available to assign planar chirality and provide an overview on the synthesis and use of late transition metal planar chiral complexes as enantioselective catalysts.

A Class of Readily Tunable Planar-Chiral Cyclopentadienyl Rhodium(III) Catalysts for Asymmetric C-H Activation

Chiral half-sandwich cyclopentadienyl rhodium(III) (CpRh^III ) complexes are powerful catalysts for promoting asymmetric C-H activation reactions. Their preparation normally involved linking or embedding the Cp motif to or into a certain chiral backbone to forge the so-called chiral Cp ligand. However, preparation of a planar-chiral CpRh^III catalyst bearing a non-chiral Cp ligand remains a formidable challenge and is rarely reported. We describe herein an unusual class of planar-chiral rhodium catalysts bearing non-chiral Cp ligands. Different from existing ones, this catalyst is readily tunable. Ten planar-chiral only CpRh^III catalysts were prepared with ease, and successfully used in two enantioselective C-H activation reactions. Given its convenient synthesis and high structural tunability, these catalysts are expected to find more utilities in asymmetric C-H activation.

Formation of a Naphthalene Framework by Rhodium(III)-Catalyzed Double C–H Functionalization of Arenes with Alkynes: Impact of a Supporting Ligand and an Acid Additive

An efficient protocol has been developed for the synthesis of larger condensed arenes from aromatic hydrocarbons and internal alkynes. This protocol uses readily available [CpRhI2]n as a catalyst and Cu(OAc)2 as an oxidant and proceeds smoothly through undirected double C–H activation. The addition of trifluoroacetic acid has a crucial positive impact on the reaction selectivity and the yields of the target products. In contrast to the previously reported catalytic systems, the new conditions allow the use of both dialkyl- and diarylacetylenes with the same high efficiency.

Teaching cyclopentadienyl how to leave: a case study of [CpIr(COD)Br]+ complex

This work was motivated by a recent report that describes the substitution of the cyclopentadienyl ring in [CpIr(COD)Br]+ with P(OMe)3 in mild conditions. We have shown that the first step...

Easy Access to Versatile Catalytic Systems for C-H Activation and Reductive Amination Based on Tetrahydrofluorenyl Rhodium(III) Complexes

On the basis of the 1,2,3,4-tetrahydrofluorenyl ligand, a simple approach was developed to new effective rhodium catalysts for the construction of C-C and C-N bonds. The halide compounds [(η⁵ -tetrahydrofluorenyl)RhX₂ ]₂ (2 a: X=Br; 2 b: X=I) were synthesized by treatment of the bis(ethylene) derivative (η⁵ -tetrahydrofluorenyl)Rh(C₂ H₄ )₂ (1 a) with halogens. An analogous reaction of the cyclooctadiene complex (η⁵ -tetrahydrofluorenyl)Rh(cod) (1 b) with I₂ is complicated by the side formation of [(cod)RhI]₂ . The reaction of 2 b with 2,2'-bipyridyl leads to cation [(η⁵ -tetrahydrofluorenyl)Rh(2,2'-bipyridyl)I]⁺ (3). The halide abstraction from 2 a,b with thallium or silver salts allowed us to prepare sandwich compounds with incoming cyclopentadienyl, dicarbollide and mesityleneligands [(η⁵ -tetrahydrofluorenyl)RhCp]⁺ (4), (η⁵ -tetrahydrofluorenyl)Rh(η-7,8-C₂ B₉ H₁₁ ) (5), and [(η⁵ -tetrahydrofluorenyl)Rh(η-mesitylene)]²⁺ (6). The structures of 1 b, 2 b ⋅ 2I₂ , 3PF₆ , 4TlI₄ , 5, and [(cod)RhI]₂ were determined by X-ray diffraction. Compounds 2 a,b efficiently catalyze the oxidative coupling of benzoic acids with alkynes to selectively give isocoumarins or naphthalenes, depending on the reaction temperature. Moreover, they showed moderate catalytic activity in other annulations of alkynes with aromatic compounds (such as benzamide, acetanilide, etc.) which proceed through CH activation. Compound 2 b also effectively catalyzes the reductive amination of aldehydes and ketones in the presence of carbon monoxide and water via water-gas shift reaction, giving amines in high yields (67-99 %).

A novel AIE fluorescent probe based on myrtenal for Cu2+ detection in a near-perfect aqueous medium and bioimaging in vegetables and zebrafish

An AIE-active fluorescent probe MHTS with good sensitivity and selectivity for the detection of Cu2+ was synthesized from myrtenal.

Cp*Ir(iii)/chiral carboxylic acid-catalyzed enantioselective C–H alkylation of ferrocene carboxamides with diazomalonates

An achiral Cp*Ir(iii)/chiral carboxylic acid-catalysed enantioselective C–H alkylation of ferrocene carboxamides with diazomalonates was achieved, providing planar chiral alkylated ferrocenes in up to 94 : 6 er.