Enantioselective 1,3-Dipolar Cycloaddition of Cyclic Enones Catalyzed by Multifunctional Primary Amines: Beneficial Effects of Hydrogen Bonding†

Recently, chiral highly acidic Bronsted acids have emerged as powerful catalysts for enantioselective C-C and C-X bond-forming reactions. Their strong acidity renders them valuable tools for the activation of imines, carbonyl compounds, and other weakly basic substrates. As a result, new perspectives are opened and highly stereoselective transformations based on the concept of chiral contact-ion-pair catalysis can be realized. This Minireview gives an overview of the design and application of these new organocatalysts and presents recent results in this rapidly growing field.

Modulating the acidity: highly acidic Brønsted acids in asymmetric catalysis.

A growing world population and increasing energy demands are without a doubt the most important challenges for mankind within this century. Catalysis is a key technology in various fields of chemistry and offers the potential to increase the material output of chemical synthesis without an unreasonable increase in the energy necessary for the production of new materials. Hence, the various disciplines of catalysis are “sustainable” by definition. However, for a wide range of catalytic transformations to take the necessary next step from academic research to industrial application, the catalytic processes do not only have to be sustainable with regard to the substrate-to-product conversions, but they also have to be sustainable with regard to parameters such as solvent, energy source, and the nature of the catalyst. Currently, the vast majority of transition-metal-catalyzed homogenous reactions are based upon late and expensive transition-metal complexes. In the present review we summarize the current state of the art in sustainable metal catalysis, that is, catalysis based upon inexpensive biorelevant metals such as Ca, Mg, V, Mo, Mn, Fe, Co, Ni, Cu, Zn, B, Si, and Se. These metals are part of nature’s catalytic toolbox and have experienced a tremendous comeback in metal catalysis within the past 10 years.

Biorelevant Metals in Sustainable Metal Catalysis—A Survey

Chirale, hoch acide Bronsted-Sauren haben sich als metallfreie Katalysatoren in der organischen Synthese etabliert und konnten mittlerweile in einer Vielzahl enantioselektiver C-C- und C-X-Bindungsknupfungen eingesetzt werden. Die hohe Aciditat dieser neuen Katalysatoren macht sie zu nutzlichen Hilfsmitteln fur die Aktivierung verschiedener Imine, Carbonyle und weiterer schwach basischer Substrate. Durch die Bildung chiraler Kontaktionenpaare ergeben sich vollig neue Aktivierungsmoglichkeiten fur die asymmetrische Katalyse. Dieser Kurzaufsatz gibt einen Uberblick uber das rationale Design dieser neuen Organokatalysatoren und beschreibt verschiedenste, kurzlich erschienene Anwendungen.

Modulation der Acidität – hoch acide Brønsted‐Säuren in der asymmetrischen Katalyse

2.2.2. Hetero [4 + 2] Cycloadditions 1711 2.2.3. Other [4 + 2] Cycloaddition Reactions 1712 3. [n + m] Cycloaddition Reactions 1713 3.1. [3 + 2] Dipolar Cycloadditions 1713 3.1.1. 1,3-Dipolar Cycloaddition of Nitrones 1713 3.1.2. Metal-Catalyzed [3 + 2] Cycloadditions 1714 3.2. [5 + 2] Cycloadditions 1714 3.3. [8 + 2] Cycloadditions 1715 3.4. [4 + 3] Cycloadditions 1716 3.5. [6 + 3] Cycloadditions 1717 3.6. [6 + 4] Cycloadditions 1717 4. Molecular Rearrangements 1717 4.1. Ring Rearrangements 1717 4.2. Schmidt Reactions 1719 4.3. Pinacol Rearrangements 1720 4.4. Claisen/Diels-Alder Cascade Process 1720 4.5. Polonovski Rearrangements 1720 4.6. Prins-Type Reactions 1720 5. Free-Radical Reactions 1722 5.1. Intramolecular Radical Cycloadditions 1722 5.2. Other Radical Reactions 1723 6. Transition-Metal-Catalyzed Cyclization Reactions 1724 6.1. Gold-Catalyzed Reactions 1724 6.2. Cu(II)-Catalyzed Cyclizations 1724 6.3. Rhodium-Catalyzed Reactions 1724 6.3.1. Rh(II)-Catalyzed Reactions 1724 6.3.2. Rh(I)-Catalyzed Reactions 1725 6.4. Palladium-Catalyzed Reactions 1725 7. Other Reactions 1726 7.1. Iodine-Promoted Cyclizations 1726 7.2. Pictet-Spengler Cyclizations 1727 7.3. Nucleophilic Substitutions 1727 7.4. Nucleophilic Additions 1729 7.5. Aldol-Type Condensation Reactions 1730 7.6. Friedel-Crafts Cyclizations 1731 7.7. Ring-Closing Metathesis 1731 8. Important Classes of Compounds 1732 8.1. Reactions in Synthesis of FR66979 and FR900482 1732

Novel syntheses of bridge-containing organic compounds.

allo-Threonine-derived O-acyl-B-phenyl-oxazaborolidinones are demonstrated to be powerful and highly enantioselective Lewis acid catalysts for the Diels-Alder reaction of simple acyclic enone dienophiles, expanding the scope of ketone dienophiles and dienes. With 10 to 20 mol % of catalyst, the Diels-Alder adducts are obtained in 76-98% ee with high endo-selectivity. The catalyst exhibits high activity for the reaction with the less reactive beta-substituted dienophiles and the less reactive 1,3-cycohexadiene and 1,3-butadiene derivatives. The application of the catalysts to the Diels-Alder reaction of furan is also described.

Ram Shanker Singh

Papers

Oxazaborolidinone-Catalyzed Enantioselective Diels-Alder Reaction of Acyclic α,β-Unsaturated Ketones

Enantioselective Diels–Alder Reaction of Acyclic Enones Catalyzed byallo-Threonine-Derived Chiral Oxazaborolidinone

Oxazaborolidinone‐Catalyzed Enantioselective Diels—Alder Reaction of Acyclic α,β‐Unsaturated Ketones.