Recent advances in homogeneous enantioselective Diels–Alder reactions catalyzed by chiral transition-metal complexes

Exo-Selective and Enantioselective Photoenolization/Diels-Alder Reaction

An unusual exo-selective photoenolization/Diels-Alder reaction of electron-rich 2-methylbenzaldehydes and dienophiles containing a benzoyl group at its α position was reported herein. The chiral TADDOL-type ligand plays a key role in this process: (1) accelerating the reaction; (2) controlling the enantioselectivity; and (3) improving and tuning the diastereoselectivity of the reaction.

Chiral disulfonimides: a versatile template for asymmetric catalysis

Since the emergence of pseudo-C2-symmetric chiral phosphoric acids (CPA), much work has been done to utilize these systems in stereoselective, organocatalytic processes. Despite the success in this field, reasonably basic substrates such as imines are often required to achieve appreciable activation. In order to access a wider variety of potential reaction partners, many related organocatalysts with enhanced Brønsted acidity have since been developed. Chiral disulfonimides (DSIs) have materialized as one such powerful class of organocatalysts and have been shown to expand the list of potential substrates to include aldehydes and ketones via Brønsted, Lewis, or bifunctional acid activation. This versatility renders DSIs amenable to an impressive scope of reaction types, typically with remarkable stereoselectivity induced by asymmetric counteranion-directed catalysis (ACDC). This review serves to provide a complete analysis of the successful applications, mechanistic insights, and unmet challenges exhibited to date in DSI-catalyzed and -assisted processes.

Rh-Catalyzed Cycloisomerization of 1,7-Ene-Dienes to Synthesize trans-Divinylpiperidines: A Formal Intramolecular Addition Reaction of Allylic C-H Bond into Dienes

An originally designed Rh-catalyzed [4 + 2] cycloaddition reaction of nitrogen-tethered 1,7-ene-dienes turned out to be a cycloisomerization reaction, which involves allylic C-H activation/alkene insertion into Rh-H bond/reductive elimination processes. Deuterium labeling experiments gave support to the proposed mechanism. This unexpected cycloisomerization reaction provides an efficient way to synthesize trans-divinylpiperidines from easily accessed linear 1,7-ene-dienes.

Asymmetric Catalysis Using Chiral Salen-Metal Complexes: Recent Advances

Chiral salen-metal complexes are among the most versatile asymmetric catalysts and have found utility in fields ranging from materials chemistry to organic synthesis. These complexes are capable of inducing chirality in products formed from a wide variety of chemical processes, often with close to perfect stereoinduction. Salen ligands are tunable for steric as well as electronic properties, and their ability to coordinate a large number of metals gives the derived chiral salen-metal complex very broad utility in asymmetric catalysis. This review primarily summarizes developments in chiral salen-metal catalysis over the last two decades with particular emphasis on those applications of importance in asymmetric synthesis.

Ruthenium-catalyzed Bis-Homo-Diels-Alder reaction: searching for commercially available catalysts and expanding the scope of reaction

Commercially available ruthenium catalyst, Cp*RuCl(COD), was found to be active in catalyzing Bis-Homo-Diels-Alder [2+2+2] cycloaddition reactions between 1,5-cyclooctadiene and various alkynes giving moderate to good yields (35%–92%). The presence of electron donating groups, especially hydroxyl groups, greatly enhanced the reactivity of the alkyne moiety in the cycloaddition. The reaction was also found to be successful even in the presence of bulky substituents on the alkynes.

Organocatalytic oxa-Diels–Alder reaction of α,β-unsaturated ketones under non-classical conditions

Non-classical activation techniques improve the organocatalysed oxa-Diels–Alder reaction of aldehydes with enones.

Asymmetric 1,3-dipolar cycloaddition reactions between enals and nitrones catalysed by half-sandwich rhodium or iridium diphosphane complexes

The aqua complexes [(η⁵-C₅Me₅)M(PP*)(H₂O)][SbF₆]₂(M = Rh, Ir; PP* = chiral diphosphane) catalyse the asymmetric 1,3-dipolar cycloaddition of nitrones to α,β-unsaturated aldehydes. Enantioselectivity up to 99% ee was achieved.

Chiral-at-metal complexes and their catalytic applications in organic synthesis

This tutorial review provides an introduction to the synthesis and characterization of chiral-at-metal complexes and their catalytic application in organic transformations. The synthetic access to these architectures either via chiral resolution or by employment of chiral ligands is described, characterization techniques for the complexes are referenced and the application of the R/S nomenclature is explained. Racemization and epimerization processes are often observed for the title compounds; the article gives mechanistic insights to these processes and describes how to recognize and document them. Finally, key catalytic applications in organic synthesis are presented and how the molecular architectures of the chiral-at-metal complexes lead to stereodifferentiation and, thus to enantiomeric excesses in the products.

Organocatalyzed highly atom economic one pot synthesis of tetrahydropyridines as antimalarials

A highly atom economic one pot synthesis of tetrahydropyridines was achieved by L-proline/TFA catalysed multicomponent reaction of beta-keto-esters, aromatic aldehydes and anilines. The synthesized compounds were screened against Plasmodium falciparum in vitro and one of them showed antimalarial activity with MIC as low as 0.09 microg/mL.

Chiral pyridylamino-ruthenium(II) complexes: synthesis, structure and catalytic properties in Diels-Alder reactions

Half-sandwich complexes [(eta(6)-arene)RuCl(pyam)][SbF(6)] (pyam = L(n) = N-(2-pyridylmethyl)-(R)-1-phenylethylamine (L(1)), N-(2-pyridylmethyl)-(R)-1-naphthylethylamine (L(2)), N-(2-quinolylmethyl)-(R)-1-naphthylethylamine (L(3)), N-(2-pyridylmethyl)-(R)-1-cyclohexylethylamine (L(4)), N-(2-pyridylmethyl)-(1R,2S,4R)-1-bornylamine (L(5))) have been synthetised and characterised. Treatment of these compounds with AgSbF(6) generates dicationic complexes [(eta(6)-arene)Ru(pyam)(H(2)O)](2+) which act as enantioselective catalysts for the Diels-Alder reactions of methacrolein and cyclopentadiene. The catalytic reactions occur quickly at room temperature with good exo : endo selectivity (from 84 : 16 to 98 : 2) and moderate enantioselectivity (up to 74% ee). The molecular structures of the chloride complexes (R(Ru),S(N),R(C))-[(eta(6)-p-MeC(6)H(4)iPr)RuClL(1)][SbF(6)], (R(Ru),S(N),S(C2))-[(eta(6)-p-MeC(6)H(4)iPr)RuClL(5)][SbF(6)], and that of the aqua complex (R(Ru),S(N),S(C2))-[(eta(6)-p-MeC(6)H(4)iPr)RuL(5)(H(2)O)][SbF(6)](2), were determined by X-ray diffractometric methods. The distinctive variations observed in the molecular structures of these complexes only concern the puckering parameters of the metallacycle and the relative disposition of substituents within this ring. A clear trend to localise the most steric demanding substituents at equatorial positions is evident from the structural study.