Enantioselective Reactions Catalyzed by N‐Heterocyclic Carbenes
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Abstract Enantioselective organocatalysis by using N‐heterocyclic carbenes (NHCs) has emerged as a powerful strategy for the construction of complex chiral molecules. Recently, remarkable advances have been made in enantioselective NHC‐catalyzed reactions that involve diverse activation intermediates, such as Breslow intermediates, homoenolate intermediates, α,β‐unsaturated acylazoliums, azolium enolates, and azolium dienolates, which are generated by the nucleophilic addition of NHCs to aldehydes. Furthermore, NHC‐catalyzed reactions that proceed through non‐covalent bonding interactions have also been developed.Keywords:
Nucleophilic Addition
A range of carbon-donor nucleophiles add to the arene ring in (arene)Mn(CO)2L+ cations to give neutral cyclohexadienyl complexes that liberate monofunctionalized arenes upon oxidative removal of the metal. Treatment of the cyclohexadienyl complexes with the nitrosonium salt NOPF6 affords cationic metal nitrosyl complexes that are attacked by a second nucleophile to give cis- and trans-difunctionalized 1,3-cyclohexadienes. When the metal center is chiral, this procedure provides a route to enantiomerically pure cyclohexadienes. 1. Introduction 2. Mechanism of Nucleophilic Addition 3. Single Nucleophilic Addition 4. Double Nucleophilic Addition 5. Conclusions
Cationic polymerization
Nucleophilic Addition
Oxidative addition
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We report an experimental and computational study of 3-silylarynes. The addition of nucleophiles yield ortho-substituted products as a result of aryne distortion, but meta-substituted products form predominately when the nucleophile is large. Computations correctly predict the preferred site of attack observed in both nucleophilic addition and cycloaddition experiments. Nucleophilic additions to 3-tert-butylbenzyne, which is not significantly distorted, give meta-substituted products.
Aryne
Nucleophilic Addition
Distortion (music)
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The synthesis of the first four-membered N-heterocyclic carbene is described. Depending on the substituents on the nitrogen atoms, it is possible to characterize at room temperature the carbene dimer or the free carbene. Crystallographic analyses are provided for these carbene species.
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Abstract Azolium cations are widely employed in organocatalysis to catalyse highly valuable synthetic processes in the presence of a base. These reactions are called “N‐heterocyclic carbene catalysis”, based on the assumption that they are initiated by the formation of a free carbene through deprotonation, which can then react with the substrates and thereby affect their reactivity to obtain the desired products. However, we herein provide evidence that an electrophilic aromatic substitution mechanism is energetically more favourable, in which the azolium cation reacts directly with the substrate, avoiding the formation of the free carbene in solution.
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This chapter contains sections titled: Introduction Addition of O-Nucleophiles Addition of Water: Synthesis of Aldehydes from Terminal Alkynes Addition of Alcohols Addition of Carboxylic Acids Addition of Carbamates Addition of Carbonates Addition of N-Nucleophiles Addition of Hydrazines Hydroamination Addition of P-Nucleophiles: Hydrophosphination Hydrosilylation Addition of CH Bond to Alkynes Conclusions
Hydroamination
Hydrosilylation
Nucleophilic Addition
Addition reaction
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Nucleophilic Addition
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The chemical reactivity of the simple condensates 1 and 2, derived from 3-formylchromone, was studied towards some nucleophilic reactions.Reactions of compounds 1 and 2 with some nucleophilic reagents mainly proceed via nucleophilic addition at the exocyclic vinyl bond followed by either elimination or cyclization during the course of reactions.A variety of products were obtained depending on the substrate and the nucleophile used.O O CN CO 2 Et 1 O O CN CN 2 Figure 1
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Carbenoid
Dig
Nucleophilic Addition
Tandem
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Singlet carbenes are not always isolable and often even elude direct detection. When they escape observation, their formation can sometimes be evidenced by in situ trapping experiments. However, is carbene-like reactivity genuine evidence of carbene formation? Herein, using the first example of a spectroscopically characterized cyclic (amino)(aryl)carbene (CAArC), we cast doubt on the most common carbene trapping reactions as sufficient proof of carbene formation.
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