The gold-catalyzed cyclization of 2-alkynylarylaldehyde cyclic acetals leads to indanone derivatives in good-to-excellent yields. It is possible to use photoimmobilized Ni clusters on a CdS photocatalyst to give imines by the dehydrogenation of amines under visible-light illumination. The process is suitable for primary and secondary amines. An intramolecular asymmetric reductive amination of aminoketones leading to chiral 1,4-diazepanes catalyzed by an imine reductase enzyme has been reported, which proceeds with high enantioselectivity. Aryl acetic acid esters and other active methylene compounds react with nitrosoarenes to give diastereoselective formation of ketonitrones using 2-t-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine catalyst at room temperature. Saturated aliphatic tricycles including an imine function have been cyclized by van Leusen imidazole synthesis using p-toluenesulfonyl-methyl isocyanide (TosMIC). N-(Tosylmethyl) formamide, a decomposition product of TosMIC, was further transformed into N-methyleneformamide under the basic conditions and acts as a catalyst to reduce reaction times and improve yields.
This chapter contains sections titled: Carbanion Structure and Stability Carbanion Reactions Proton-transfer Reactions Miscellaneous Electrophilic Aliphatic Substitution References
Functionalized arenes and heterocycles continue to serve as essential compounds in industry and academic research. Electrophilic aromatic substitution (EAS) remains one of the most important processes for introducing the diverse array of functionality required for new applications, and novel synthetic methods and investigations into the underlying mechanisms of EAS reactions continue to receive much attention. There is a growing use of artificial intelligence and machine learning to assist in identifying likely products of reactions or the best conditions to apply in synthesis. Improved methods for halogenation continue to be an important area of investigation in EAS. Studies on the mechanisms of new bromination methods continue to receive attention. Mechanisms for reactions to introduce sulfur-containing functionality have appeared. Metals are key to many electrophilic substitution processes as catalysts or stoichiometric reagents, and C–H insertion reactions feature prominently.
This Chapter Contains Sections titled: Pericyclic Reactions Reviews Cycloaddition–Cycloreversion Ene Reaction Sigmatropic Shifts Electrocyclic Reactions Molecular Rearrangements Acid Catalysed Base Catalysed Small Ring Rearrangements 1,2-Rearrangement Alkyl Shifts Ring Expansion Ring Opening and Ring Closure Oxidation Reduction Phosphorus Photochemical Reactions Radical and Biradical Thermal Named Reactions Achmatowicz Reaction Balysis–Hillman Beckmann Bergmnan Brook Cloke–Wilson Rearrangement Curtius Dimroth Rearrangement Fischer Indole Friedel–Crafts Heyns Rearrangement Lossen Rearrangement Meyer–Schuster Mislow–Evans Morita–Baylis–Hillman Nazarov Phospho-Fries Ritter Reaction Smiles Stevens Trost–Rychnovsky Reaction Wittig Wolff Rearrangement Metal Catalysed Aluminium Cobalt Copper Gold Iridium Iron Nickel Ruthenium Rhodium Silver Palladium Platinum Vanadium Miscellaneous References This chapter provides a review on the applicability of the frontier orbital theory to transition-metal and lanthanide-mediated intramolecular and metathesis reactions. It reviews of Bronsted acid catalysts for asymmetric induction in pericyclic reactions which has reported promising examples that rely on multiple non-covalent interactions in the transition state. The chapter gives the pericyclic reactions of enediynes, enyne-cumulenes, and yne-diynes. It reports the Claisen rearrangements of 3-oxa-1,5-hexadienes in which one or more carbon atoms were replaced with atoms different than carbon. The regioselectivity of the 2+2-photocycloaddition of 2-pyridone and methylacrylate has been studied using ab initio molecular orbital methods and the results explained from the conformation of parallel-type conical intersections. Computational studies show that the Diels–Alder pathway is favoured in the enzyme environment.
A review of photocatalytic strategies, involving radical intermediates of importance in organic synthesis, has appeared. The fact that photocatalytic cycles are composed of three steps, namely excitation, quenching, and finally restoration, is highlighted. The review has a particular emphasis on the restoration step, exploring how the deactivated form of the catalyst is converted back into the original state, which permits the cycle to start over again. It classifies photocatalytic processes according to what species is responsible for the restoration step, such as the substrate, an intermediate, or a sacrificial agent. The stereoselective bromoboration of acetylene with boron tribromide is investigated. The mechanism was studied by experiments and calculations, and besides the syn-addition mechanism (proceeding via a four-membered transition state) a radical and polar anti-addition mechanism was postulated.
