Scalable Synthesis of Enantiomerically Pure Bicyclo[2.2.2]octadiene Ligands

Rhodium-Catalyzed Asymmetric Annulation of Unactivated Alkynes with 3-(ortho-Boronated Aryl) Conjugated Enones: Enantioselective Synthesis of 2,3-Disubstituted Indenes

A rhodium-catalyzed tandem arylation/cyclization reaction of 3-(ortho-boronated aryl) conjugated enones with unactivated alkynes is reported. By using a rhodium(I)/chiral-diene complex as the catalyst, the protocol was processed smoothly to provide various 2,3-disubstituted indene compounds in high yields with excellent regioselectivities and enantioselectivities. The approach outlined herein is appealing, as simple diarylalkynes, diakylalkynes, and alkyl(aryl)alkynes are the starting materials.

Chiral Diene Ligands in Asymmetric Catalysis

Asymmetric catalysis has emerged as a general and powerful approach for constructing chiral compounds in an enantioselective manner. Hence, developing novel chiral ligands and catalysts that can effectively induce asymmetry in reactions is crucial in modern chemical synthesis. Among such chiral ligands and catalysts, chiral dienes and their metal complexes have received increased attention, and a great progress has been made over the past two decades. This review provides comprehensive and critical information on the essential aspects of chiral diene ligands and their importance in asymmetric catalysis. The literature covered ranges from August 2003 (when the first effective chiral diene ligand for asymmetric catalysis was reported) to October 2021. This review is divided into two parts. In the first part, the chiral diene ligands are categorized according to their structures, and their preparation methods are summarized. In the second part, their applications in asymmetric transformations are presented according to the reaction types.

Dearomative Synthesis of Chiral Dienes Enables Improved Late-Stage Ligand Diversification

An efficient synthesis of chiral nonracemic diene ligands is facilitated by an enantioselective dearomative intermolecular arene cyclopropanation of anisole. The functionality of the resulting cycloheptatriene engenders distinct chemical environments in a downstream tricyclic bis(enol) triflate that permits selective late-stage functionalization. The synthesis of diverse C₁- and pseudo-C₂-symmetric dienes is therefore viable by iterative palladium-catalyzed cross-coupling reactions. The ligands provide moderate to high selectivities in known Rh(I)-mediated asymmetric transformations.

Synthesis, characterization and computational evaluation of bicyclooctadienes towards molecular solar thermal energy storage

Molecular solar-thermal energy storage (MOST) systems are based on photoswitches that reversibly convert solar energy into chemical energy. In this context, bicyclooctadienes (BODs) undergo a photoinduced transformation to the corresponding higher energy tetracyclooctanes (TCOs), but the photoswitch system has not until now been evaluated for MOST application, due to the short half-life of the TCO form and limited available synthetic methods. The BOD system degrades at higher temperature via a retro-Diels-Alder reaction, which complicates the synthesis of the compounds. We here report a cross-coupling reaction strategy that enables an efficient synthesis of a series of 4 new BOD compounds. We show that the BODs were able to switch to the corresponding tetracyclooctanes (TCOs) in a reversible way and can be cycled 645 times with only 0.01% degradation. Half-lives of the TCOs were measured, and we illustrate how the half-life could be engineered from seconds to minutes by molecular structure design. A density functional theory (DFT) based modelling framework was developed to access absorption spectra, thermal half-lives, and storage energies which were calculated to be 143-153 kJ mol-1 (0.47-0.51 MJ kg-1), up to 76% higher than for the corresponding norbornadiene. The combined computational and experimental findings provide a reliable way of designing future BOD/TCO systems with tailored properties.

Asymmetric Synthesis of Chiral Bicyclo[2.2.1]hepta-2,5-diene Ligands through Rhodium-Catalyzed Asymmetric Arylative Bis-cyclization of a 1,6-Enyne

A series of novel chiral diene ligands (1R,4S)-L₁, which are based on the bicyclo[2.2.1]heptadiene skeleton and are substituted with methyl and an ester group at the bridgehead carbons, were synthesized through rhodium-catalyzed asymmetric arylative bis-cyclization of 1,6-enyne 1 as a key step. The rhodium catalyst with one of the (1R,4S)-L₁ ligands was used for the asymmetric bis-cyclization of 1 giving bicyclic product (1S,4R)-2 of 99% ee, which is a synthetic precursor of (1S,4R)-L₁ ligands.

Synthesis of Rhodium Complexes with Chiral Diene Ligands via Diastereoselective Coordination and Their Application in the Asymmetric Insertion of Diazo Compounds into E-H Bonds

A new method for the synthesis of chiral diene rhodium catalysts is introduced. The readily available racemic tetrafluorobenzobarrelene complexes [(R₂ -TFB)RhCl]₂ were separated into two enantiomers via selective coordination of one of them with the auxiliary S-salicyl-oxazoline ligand. One of the resulting chiral complexes with an exceptionally bulky diene ligand [(R,R-iPr₂ -TFB)RhCl]₂ was an efficient catalyst for the asymmetric insertion of diazoesters into B-H and Si-H bonds giving the functionalized organoboranes and silanes with high yields (79-97 %) and enantiomeric purity (87-98 % ee). The stereoselectivity of separation via auxiliary ligand and that of the catalytic reaction was predicted by DFT calculations.

Synthesis of Functionalized Bicyclo[2.2.2]octan-2-ones Skeleton via Tandem Process of Amino Acid Involved Formal [4 + 2] and Decarboxylative-Mannich Sequence

An efficient approach to a functionalized bicyclo[2.2.2]octan-2-one scaffold has been developed through a one-pot cascade process including amino acid involved successive Michael addition and decarboxylative-Mannich sequence. Starting from α,β-unsaturated ketones and amino acids, a series of desired products 7a-7m and 8a-8o were obtained with moderate yields. In addition, the tandem process was reasonably explained by the results of DFT calculations.

Asymmetric Total Syntheses of Di- and Sesquiterpenoids by Catalytic C-C Activation of Cyclopentanones

To show the synthetic utility of the catalytic C-C activation of less strained substrates, described here are the collective and concise syntheses of the natural products (-)-microthecaline A, (-)-leubehanol, (+)-pseudopteroxazole, (+)-seco-pseudopteroxazole, pseudopterosin A-F and G-J aglycones, and (+)-heritonin. The key step in these syntheses involve a Rh-catalyzed C-C/C-H activation cascade of 3-arylcyclopentanones, which provides a rapid and enantioselective route to access the polysubstituted tetrahydronaphthalene cores presented in these natural products. Other important features include 1) the direct C-H amination of the tetralone substrate in the synthesis of (-)-microthecaline A, 2) the use of phosphoric acid to enhance efficiency and regioselectivity for problematic cyclopentanone substrates in the C-C activation reactions, and 3) the direct conversion of serrulatane into amphilectane diterpenes by an allylic cyclodehydrogenation coupling.

Enantioselective Conjugate Addition of Aryl Halides and Triflates to Electron-Deficient Olefins via Nickel- and Rhodium-Catalyzed Sequential Relay Reactions

Asymmetric conjugate addition of aryl halides or aryl triflates to electron-deficient olefins was realized by sequential Miyaura borylation and Hayashi-Miyaura conjugate addition in one pot. A nickel-catalyzed borylation of aryl halides or triflates and a rhodium-chiral diene complex catalyzed enantioselective conjugate addition was executed as a pair of relay reactions as a more efficient and greener protocol.

Enantioselective Formal [4+1] Cycloaddition of Diazoarylacetates and the Danishefsky's Diene: Stereoselective Synthesis of (-)-1,13-Herbertenediol

Rodium chiral diene complex-catalyzed enantioselective cycloaddition of aryl α-diazoarylacetates and electron-enriched Danishefsky-type dienes afforded highly functionalized and optically enriched cyclopentenones in excellent yields (up to 97% yield) and with good to excellent enantioselectivities (60-92% ee). (-)-1,13-Herbertenediol was successfully synthesized in an overall 25% yield employing the optically enriched cyclopentenone with an all-carbon quaternary center as the key intermediate.

Access to Chiral HWE Reagents by Rhodium-Catalyzed Asymmetric Arylation of γ,δ-Unsaturated β-Ketophosphonates

Asymmetric arylation of γ,δ-unsaturated β-ketophosphonates with arylboronic acids is reported. By using the ( R)-diene* ligated rhodium(I) chloride complex as a catalyst under none basic conditions, the corresponding β-ketophosphonates bearing a δ-chiral center were obtained in high yields (up to 99%) with good to excellent enantioselectivities (up to >99% ee). The enantioenriched products can be readily converted to diverse chiral β'-aryl enones by the Horner-Wadsworth-Emmons reaction.

Rhodium-Catalyzed Enantioposition-Selective Hydroarylation of Divinylphosphine Oxides with Aryl Boroxines

The rhodium-catalyzed hydroarylation of divinylphosphine oxides (RP(O)(CH=CH₂ )₂ ) with aryl boroxines ((ArBO)₃ ) gives the corresponding monoarylation products (RP(O)(CH=CHAr)CH₂ CH₃ ) in high yields. One of the two vinyl groups in the phosphine oxide undergoes oxidative arylation while the other one is reduced to an ethyl moiety. These reactions proceed with high selectivity in terms of the enantiotopic vinyl groups in the presence of (R)-DTBM-segphos/Rh to give the P-stereogenic monoarylation products with high enantioselectivity.

Iron-Catalyzed C-O Bond Activation: Opportunity for Sustainable Catalysis

Oxygen-based electrophiles have emerged as some of the most valuable cross-coupling partners in organic synthesis due to several major strategic and environmental benefits, such as abundance and potential to avoid toxic halide waste. In this context, iron-catalyzed C-O activation/cross-coupling holds particular promise to achieve sustainable catalytic protocols due to its natural abundance, inherent low toxicity, and excellent economic and ecological profile. Recently, tremendous progress has been achieved in the development of new methods for functional-group-tolerant iron-catalyzed cross-coupling reactions by selective C-O cleavage. These methods establish highly attractive alternatives to traditional cross-coupling reactions by using halides as electrophilic partners. In particular, new easily accessible oxygen-based electrophiles have emerged as substrates in iron-catalyzed cross-coupling reactions, which significantly broaden the scope of this catalysis platform. New mechanistic manifolds involving iron catalysis have been established; thus opening up vistas for the development of a wide range of unprecedented reactions. The synthetic potential of this sustainable mode of reactivity has been highlighted by the development of new strategies in the construction of complex motifs, including in target synthesis. The most recent advances in sustainable iron-catalyzed cross-coupling of C-O-based electrophiles are reviewed, with a focus on both mechanistic aspects and synthetic utility. It should be noted that this catalytic manifold provides access to motifs that are often not easily available by other methods, such as the assembly of stereodefined dienes or C(sp² )-C(sp³ ) cross-couplings, thus emphasizing the synthetic importance of this mode of reactivity.

Iron-Catalyzed C-C Cross-Couplings Using Organometallics

Over the last decades, iron-catalyzed cross-couplings have emerged as an important tool for the formation of C-C bonds. A wide variety of alkenyl, aryl, and alkyl (pseudo)halides have been coupled to organometallic reagents, the most currently used being Grignard reagents. Particular attention has been devoted to the development of iron catalysts for the functionalization of alkyl halides that are generally challenging substrates in classical cross-couplings. The high functional group tolerance of iron-catalyzed cross-couplings has encouraged organic chemists to use them in the synthesis of bioactive compounds. Even if some points remain obscure, numerous studies have been carried out to investigate the mechanism of iron-catalyzed cross-coupling and several hypotheses have been proposed.

Rhodium(I)-catalyzed asymmetric carbene insertion into B-H bonds: highly enantioselective access to functionalized organoboranes

A unique rhodium(I)-catalyzed asymmetric B-H insertion of α-diazo carbonyl compounds with easily available amine-borane adducts was achieved using a newly developed C1-symmetric chiral diene as ligand. This first Rh(I)-carbene-directed B-H insertion example represents an attractive and promising approach for synthesis of highly enantioenriched organoboron compounds, allowing for the efficient construction of α-boryl esters and ketones with excellent enantioselectivities (up to 99% ee) under exceptionally mild conditions.

Lipase catalyzed desymmetrization of roof shape cis-11,12-bis(hydroxymethyl)-9,10-dihydro-9,10-ethanoanthracene

Biocatalyzed desymmetrization of roof shape meso cis-11,12-bis(hydroxymethyl)-9,10-dihydro-9,10-ethanoanthracene has been achieved.

A chiral bicyclo[2.2.2]octa-2,5-diene ligand substituted with the ferrocenyl group and its use for rhodium-catalyzed asymmetric 1,4-addition reactions

New chiral diene ligand, Fc,Ph-bod, showed higher enantioselectivity than Ph-bod in the rhodium-catalyzed asymmetric 1,4-addition of aryl- and alkenylboronic acids.