Complete Field Guide to Asymmetric BINOL-Phosphate Derived Brønsted Acid and Metal Catalysis: History and Classification by Mode of Activation; Brønsted Acidity, Hydrogen Bonding, Ion Pairing, and Metal Phosphates

Iron Catalysis in Organic Synthesis

Isatins As Privileged Molecules in Design and Synthesis of Spiro-Fused Cyclic Frameworks

Flow chemistry involves the use of channels or tubing to conduct a reaction in a continuous stream rather than in a flask Flow equipment provides chemists with unique control over reaction parameters enhancing reactivity or in some cases enabling new reactions This relatively young technology has received a remarkable amount of attention in the past decade with many reports on what can be done in flow Until recently, however, the question, “Should we do this in flow?” has merely been an afterthought This review introduces readers to the basic principles and fundamentals of flow chemistry and critically discusses recent flow chemistry accounts

The Hitchhiker's Guide to Flow Chemistry

The legacy of Gilbert Newton Lewis (1875-1946) pervades the lexicon of chemical bonding and reactivity. The power of his concept of donor-acceptor bonding is evident in the eponymous foundations of electron-pair acceptors (Lewis acids) and donors (Lewis bases). Lewis recognized that acids are not restricted to those substances that contain hydrogen (Bronsted acids), and helped overthrow the "modern cult of the proton". His discovery ushered in the use of Lewis acids as reagents and catalysts for organic reactions. However, in recent years, the recognition that Lewis bases can also serve in this capacity has grown enormously. Most importantly, it has become increasingly apparent that the behavior of Lewis bases as agents for promoting chemical reactions is not merely as an electronic complement of the cognate Lewis acids: in fact Lewis bases are capable of enhancing both the electrophilic and nucleophilic character of molecules to which they are bound. This diversity of behavior leads to a remarkable versatility for the catalysis of reactions by Lewis bases.

Lewis Base Catalysis in Organic Synthesis

A catalytic asymmetric intramolecular homologation of simple ketones with α-diazoesters was firstly accomplished with a chiral N,N′-dioxide–Sc(OTf)3 complex. This method provides an efficient access to chiral cyclic α-aryl/alkyl β-ketoesters containing an all-carbon quaternary stereocenter. Under mild conditions, a variety of aryl- and alkyl-substituted ketone groups reacted with α-diazoester groups smoothly through an intramolecular addition/rearrangement process, producing the β-ketoesters in high yield and enantiomeric excess.

Catalytic Asymmetric Intramolecular Homologation of Ketones with α-Diazoesters: Synthesis of Cyclic α-Aryl/Alkyl β-Ketoesters

Highly enantioselective synthesis of 1,3-bis(hydroxymethyl)-2-oxindoles was established through chiral NdIII-N,N′-dioxide in up to 93% yield and >99% ee under mild conditions. The key features of this approach include using cheap and readily available formalin as a hydroxymethylation C1 unit and unprotected 3-substituted-2-oxindoles as nucleophiles directly. The initial C-aldol product from 3-substituted-2-oxindole was converted to the corresponding 1,3-bis(hydroxymethyl)-2-oxindole derivative immediately under the reaction conditions. Moreover, the catalyst was water-tolerant, and ligand and Nd(OTf)3 could be easily recovered and reused with no loss in catalytic activity and enantioselectivity. The synthetic utility of this methodology was accomplished with the synthesis of key intermediates of physostigmine and coerulescine.

Highly enantioselective synthesis of 1,3-bis(hydroxymethyl)-2-oxindoles from unprotected oxindoles and formalin using a chiral NdIII complex

Although great success has been achieved in asymmetric Claisen rearrangement for the synthesis of chiral γ,δ-unsaturated carbonyl compounds bearing vicinal tertiary-quaternary stereocenters, the development of asymmetric versions for stereodivergent construction of adjacent quaternary-quaternary stereocenters remains a formidable challenge because of the high steric hindrance. Here we report a catalytic enantioselective dearomatization Claisen rearrangement of allyl furyl ethers catalyzed by chiral N,N′-dioxide-NiII complex catalysts. A variety of chiral γ,δ-unsaturated carbonyl compounds bearing vicinal quaternary-quaternary stereocenters were obtained with excellent outcomes under mild conditions. Furthermore, we disclosed that by matching the configuration of the catalysts and the alkene unit of the substrates, four stereoisomers of the products could be prepared in excellent yields and stereoselectivities. Finally, the fascination of this strategy was demonstrated by stereodivergent synthesis of bioactive natural products hyperolactones B, C, and their epimers. A possible catalytic model was proposed to explain the origin of the asymmetric induction. Stereodivergent construction of adjacent quaternary-quaternary stereocenters remains a formidable synthetic challenge. Here, the authors report a nickel-catalyzed enantioselective dearomatization Claisen rearrangement leading to vicinal all-carbon stereocentres and apply it to the stereodivergent synthesis of bioactive hyperolactones.

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Stereodivergent synthesis of vicinal quaternary-quaternary stereocenters and bioactive hyperolactones.

The first enantioselective conjugate addition of silyl ketene imines to in situ generated indol-2-ones was performed in the presence of a chiral N,N′-dioxide/NiII catalyst. This method provides efficient access to chiral β-alkyl nitriles bearing congested vicinal all-carbon quaternary stereocenters in up to 90 % yield with 23:1 d.r. and 98 % ee. The products enable facile transformations to chiral pyrroloindoline frameworks and spirocyclohexane oxindole derivatives. A possible transition state was also proposed to explain the origin of the asymmetric induction.

Nickel-Catalyzed Conjugate Addition of Silyl Ketene Imines to In Situ Generated Indol-2-ones: Highly Enantioselective Construction of Vicinal All-Carbon Quaternary Stereocenters

A simple chiral diamine catalyst (1a) was successfully applied in the asymmetric Michael reaction between cyclic dimedone and α,β-unsaturated ketones. Both acyclic enones with aryl or alkyl β-substituents and cyclic enones were tolerated well in the reaction. The desired adducts were obtained in high yields (up to 98%) with excellent enantioselectivities (up to 97% ee). The additives were found to increase the reactivity dramatically. The biologically active 2,4-disubstituted polyhydroquinoline scaffold was conveniently prepared through an ammoniation from the generated 3,4-dihydropyran product.

Xiaohua Liu

Papers

Catalytic Asymmetric Intramolecular Homologation of Ketones with α-Diazoesters: Synthesis of Cyclic α-Aryl/Alkyl β-Ketoesters

Highly enantioselective synthesis of 1,3-bis(hydroxymethyl)-2-oxindoles from unprotected oxindoles and formalin using a chiral NdIII complex

Stereodivergent synthesis of vicinal quaternary-quaternary stereocenters and bioactive hyperolactones.

Nickel-Catalyzed Conjugate Addition of Silyl Ketene Imines to In Situ Generated Indol-2-ones: Highly Enantioselective Construction of Vicinal All-Carbon Quaternary Stereocenters

Enantioselective Synthesis of 3,4-Dihydropyran Derivatives via Organocatalytic Michael Reaction of α,β-Unsaturated Enones