Chiral Olefins as Steering Ligands: Syntheses of C1-Symmetric Dibenzo[a,e]cyclooctenes (Rdbcot)

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.

Asymmetric Synthesis of Chiral Aza-macrodiolides via Iridium-Catalyzed Cascade Allylation/Macrolactonization

Iridium-catalyzed cascade allylation/macrolactonization between vinylethylene carbonate (VEC) and isatoic anhydride derivatives was successfully developed, readily generating a wide range of C₂-symmetric chiral macrodiolides bearing 14-membered rings in moderate to good yields with excellent diastereoselectivities and enantioselectivities (generally 99% ee). Control experiments revealed that racemic VEC as the precursor of electrophilic iridium-π-allyl species underwent kinetic resolution process. This expedient protocol features easily available substrates, excellent stereoselective control, and high step economy.

Heteroleptic Rare-Earth Tris(metallocenes) Containing a Dibenzocyclooctatetraene Dianion

Isolable heteroleptic tris(metallocenes) containing five-membered and larger rings remain extremely scarce. The utilization of tripositive rare-earth-metal ions with ionic radii >1 Å allowed access to unprecedented and sterically congested dibenzocyclooctatetraenyl (dbCOT) metallocenes, [K(crypt-222)][Cp^tet₂RE(η²-dbCOT)] (RE = Y (1), Dy (2); Cp^tet = tetramethylcyclopentadienyl), through a salt metathesis reaction involving Cp^tet₂RE(BPh₄) and the potassium salt of the dbCOT dianion. The solid-state structures were investigated by single-crystal X-ray diffraction, magnetometry, and IR spectroscopy and provided evidence for the first crystallographically characterized (dbCOT)^2- anion in a complex containing d- or f-block metals. Remarkably, the (Cp^tet)^- ligands force a distortion from planarity within the (dbCOT)^2- moiety, engendering a rare η²-bonding motif, as opposed to the classical η⁸ conformation observed in complexes bearing a (COT)^2- ion. The η² coordination mode was proven crystallographically between 100 and 298 K and computationally (DFT and NBO). Furthermore, nucleus independent chemical shift (NICS) calculations uncovered significant ring current within the dbCOT ligand. The solution-state properties of 1 and 2 were analyzed via cyclic voltammetry, NMR, and UV-vis spectroscopy. Cyclic voltammograms of 1 and 2 exhibit a quasi-reversible feature indicating the accessibility of complexes with dbCOT in two oxidation states (dbCOT^2-/3-•). Importantly, the dysprosium congener, 2, is a zero-field single-molecule magnet (SMM).

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.

Palladium(ii) catalysed cascade strategy for the synthesis of dibenzo[5,6:7,8]cycloocta[1,2-b]indol-10-ols/-10(15H)-ones: easy access to 1,3,5,7-cyclooctatetraenes (COTs)

An atom-economic Pd(ii)-catalysed cascade cyclisation of 2-(biphenylethynyl)anilines tethered to an aldehyde or cyano group leads to the formation of dibenzo[5,6:7,8]cycloocta[1,2-b]indol-10-ols 6 or dibenzo[5,6:7,8]cycloocta[1,2-b]indol-10(15H)-ones 8 with high yields (up to 95%). The reaction proceeds via amino-palladation of the alkyne followed by nucleophilic addition onto the aldehyde/cyano group. Treatment of 6 with p-TsOH·H₂O smoothly provided cyclooctatetraene (COT) derivatives 7.

Enantioselective Iron-Catalyzed Cross-[4+4]-Cycloaddition of 1,3-Dienes Provides Chiral Cyclooctadienes

Chiral cyclooctadienes are a frequently occurring scaffold in natural products and specialty chemicals, and are used as ligands in asymmetric catalysis. Accessing substituted cyclooctadienes in an efficient asymmetric fashion has been notoriously challenging. We report an iron-catalyzed enantioselective cross-[4+4]-cycloaddition of 1,3-dienes to form substituted cyclooctadienes under very mild conditions. A highly tailored chiral α-diimine iron complex is key for the success of the transformation providing a balanced performance between reactivity, excellent cross-selectivity and very high enantioselectivity. Steric maps of the complexes help accounting for the observed selectivity. The developed method allows rapid and atom-economic access to novel differently functionalized cyclooctadienes in very high yields and enantioselectivities.

Chiral Discrimination in Rhodium(I) Catalysis by 2,5-Disubstituted 1,3a,4,6a-Tetrahydropenatalene Ligands-More Than Just a Twist of the Olefins?

Chiral dienes are useful ligands in a number of asymmetric transition-metal-catalyzed reactions. Here, we evaluate the efficiency of 2,5-disubstituted 1,3a,4,6a-tetrahydropentalenes as ligands to rhodium(I). 2,5-Dibenzyl and diphenyl tetrahydropentalenes were synthesized in two steps and resolved, either chromatographically, or through fractional crystallization of diastereomeric rhodium(I) salts. When evaluated in a 1,4-arylation reaction, the 2,5-dibenzyl ligand gave up to 99% ee. The use of a well-defined rhodium complex as catalyst, Cs₂CO₃ as the base, and toluene/water as solvent was found to have a pronounced beneficial effect on the selectivity of the reaction. The homologous 2,5-diphenyl ligand on the other hand proved to be highly prone to racemization/loss of chirality during catalysis. Control experiments reveal that this rearrangement proceeds via a rhodium-mediated 1,3-hydride shift. Implications for ligand design and catalysis are discussed.

Computational modelling of the enantioselectivity in the asymmetric 1,4-addition of phenylboronic acid to a bulky, doubly pro-chiral maleimide catalyzed by a Rh/chiral diene complex

Computational chemistry is a powerful tool for understanding chemical reactions used for the synthesis of chiral compounds.

Computational modelling of the enantioselectivity in the asymmetric 1,4-addition reaction catalyzed by a Rh complex of a S-chiral disulfoxide

Computational chemistry is a powerful tool for understanding chiral catalysis and can also aid future catalyst design.

Rhodium dinaphthocyclooctatetraene complexes: synthesis, characterization and catalytic activity in [5+2] cycloadditions

Rh COT in the act: a Ni(0)-catalyzed [2+2+2+2] cycloaddition provides a high-yielding, scalable synthesis of the ligand dinaphtho[a,e]cyclooctatetraene (dnCOT). dnCOT complexation with Rh(I) gives [Rh(dnCOT)(MeCN)(2)]SbF(6), an excellent catalyst for [5+2] cycloadditions of vinylcyclopropanes and π-systems with impressive functional group compatibility.

Alternatives to organoboron reagents in rhodium-catalyzed conjugate additions

The rhodium-catalyzed conjugate addition of organoboron reagents to alkene acceptors has emerged as an important methodology in organic synthesis. The process results in the formation of new carbon-carbon bonds, and in principle, can create new stereocenters at both the alpha and beta carbon atoms of the acceptor. Other organometallics are known to participate in the key transmetalation to rhodium and are becoming more established in stereoselective synthesis (notably arylzinc reagents). This Focus Review will describe recent developments in the use of alternative organometallic donors to organoboron reagents in rhodium-catalyzed conjugate addition reactions and their growing applications in synthesis.