In the investigation of efficient chemical transformations, the carbon-carbon bond-forming reactions play an outstanding role. In this context, organocatalytic processes have achieved considerable attention. 1 Beside their facile reaction course, selectivity, and environmental friendliness, new synthetic strategies are made possible. Particularly, the inversion of the classical reactivity (Umpolung) opens up new synthetic pathways. 2 In nature, the coenzyme thiamine (vitamin B1), a natural thiazolium salt, utilizes a catalytic variant of this concept in biochemical processes as nucleophilic acylations. 3 The catalytically active species is a nucleophilic carbene. 4

Organocatalysis by N-Heterocyclic Carbenes

Asymmetric enamine catalysis.

Small-molecule H-bond donors in asymmetric catalysis.

4.2.8. Reductive Coupling 3109 5. Reaction of Aromatic Compounds 3110 5.1. Electrophilic Substitutions 3110 5.2. Radical Substitution 3111 5.3. Oxidative Coupling 3111 5.4. Photochemical Reactions 3111 6. Reaction of Carbonyl Compounds 3111 6.1. Nucleophilic Additions 3111 6.1.1. Allylation 3111 6.1.2. Propargylation 3120 6.1.3. Benzylation 3121 6.1.4. Arylation/Vinylation 3121 6.1.5. Alkynylation 3121 6.1.6. Alkylation 3121 6.1.7. Reformatsky-Type Reaction 3122 6.1.8. Direct Aldol Reaction 3122 6.1.9. Mukaiyama Aldol Reaction 3124 6.1.10. Hydrogen Cyanide Addition 3125 6.2. Pinacol Coupling 3126 6.3. Wittig Reactions 3126 7. Reaction of R,â-Unsaturated Carbonyl Compounds 3127

Organic Reactions in Aqueous Media with a Focus on Carbon−Carbon Bond Formations: A Decade Update

Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo- and transition-metal catalysis. The most challenging transformation remains the formation of the parent C-F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal-fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost-efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.

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Introduction of Fluorine and Fluorine-Containing Functional Groups

Acyclic azomethine imines would be useful prochiral electrophiles in catalytic asymmetric reactions, but their generation often requires harsh conditions. Here, they are generated under mild conditions in the presence of an axially chiral dicarboxylic acid, with the chiral counterion (the conjugate base) then directing an enantioselective reaction with a diazoacetate nucleophile.

Generation and exploitation of acyclic azomethine imines in chiral Brønsted acid catalysis

Despite the remarkable advances in catalytic asymmetric aziridinations over the past decades, establishing a general procedure for the stereoselective synthesis of trisubstituted aziridines has remained an elusive goal. Chiral N-triflyl phosphoramide-catalyzed reactions of N-α-diazoacyl oxazolidinones and N-Boc imines were developed as a solution to this unmet challenge.

Chiral Brønsted acid-catalyzed asymmetric trisubstituted aziridine synthesis using α-diazoacyl oxazolidinones.

The formation of carbon–halogen bonds in an enantioselective manner is an important reaction because it leads to optically active halogen compounds, which are useful intermediates for further elaboration to other valuable compounds. Within the past few years various enantioselective α-halogenations of carbonyl compounds by asymmetric organocatalysis have been reported. Most importantly, these recent developments have greatly enhanced the synthetic utility of α-halogenations and opened up a promising new frontier in organic synthesis.

Keiji Maruoka

Papers

Generation and exploitation of acyclic azomethine imines in chiral Brønsted acid catalysis

Chiral Brønsted acid-catalyzed asymmetric trisubstituted aziridine synthesis using α-diazoacyl oxazolidinones.

Organocatalyzed direct asymmetric α-halogenation of carbonyl compounds

Virtually Complete Blocking of .alpha.,.beta.-Unsaturated Aldehyde Carbonyls by Complexation with Aluminum Tris(2,6-diphenylphenoxide)

Molecular design of a chiral lewis acid for the asymmetric claisen rearrangement