Zwitterion-Catalyzed Intermolecular Bromoesterifications

Bromophosphatation as a Mode of Chiral Phosphoric Acid Catalyst Deactivation as Elucidated by Kinetic Profiling

A BINOL-derived chiral phosphoric acid (R)-1 was shown by kinetic profiling to be deactivated during the catalytic bromoesterification of cyclohexene. The products of the deactivation were identified as diastereoisomeric phosphates (R,1R,2R)-3a and (R,1S,2S)-3b and are formed via an alkene bromophosphatation process where the phosphate of 1 behaves as a competitive nucleophile, as confirmed by authentic preparations of 3a and 3b from a stoichiometric bromophosphatation reaction. HPLC separation of the diastereoisomers gave pure 3a whose absolute and relative configurations were proven by single-crystal X-ray diffraction. The 31P{1H} NMR spectrum of phosphate 3a displayed four resonances despite 3a having just one phosphorus atom, and combined VT-NMR and DFT analysis revealed this to be a consequence of rotational isomerism about the 9-phenanthrene (Ar) bearing C3,3'-Ar bonds. Moreover, kinetic studies using variable time normalization analysis (VTNA) of the catalytic cyclohexene bromoesterification showed first order kinetics in all reactants. The amount of phosphates 3a and 3b formed under catalytic bromoesterification conditions was quantified, enabling tracking of the temporal catalyst 1 concentration and hence elucidation of first order kinetics in catalyst 1. A catalytic cycle consistent with these observations is proposed.

Halogen and Chalcogen Activation by Nucleophilic Catalysis

The high utility of halogenated organic compounds has prompted the development of numerous transformations that install the carbon-halogen motif. Halogen functionalities, deemed as "functional and functionalizable" molecules due to their capacity to modulate diverse internal properties, constitute a pivotal strategy in drug discovery and development. Traditional routes to these building blocks have commonly involved multiple steps, harsh reaction conditions, and the use of stoichiometric and/or toxic reagents. With the emergence of solid halogen carriers such as N-halosuccinimides, and halohydantoins as popular sources of halonium ions, the past decade has witnessed enormous growth in the development of new catalytic strategies for halofunctionalization. This review aims to provide a nuanced perspective on nucleophilic activators and their roles in halogen activation. It will highlight critical discoveries in effecting racemic and asymmetric variants of these reactions, driven by the development of new catalysts, activation modes, and improved understanding of chemical reactivity and reaction kinetics.

Trialkylphosphonium Oxoborate as C(sp3 )-H Oxyanion Hole Catalyst for Diels-Alder Reaction

We have developed a catalytic protocol for Diels-Alder reaction using trialkylphosphonium oxoborates as oxyanion hole catalysts. The reaction can be operated under ambient conditions. Dienes could easily polymerize under acidic condition. Nonetheless, these acid-sensitive substrates are compatible with the catalytic protocol and the reaction scope covers a wide range of substrates.

Trialkylphosphonium oxoborates as C(sp3)-H oxyanion holes and their application in catalytic chemoselective acetalization

The use of trialkylphosphonium oxoborates (TOB) as catalysts is reported. The site-isolated borate counter anion in a TOB catalyst increases the availability of C(sp3)-H to interact with electron donor substrates. The catalytic protocol is applicable to a wide range of substrates in the acetalization reaction and provides excellent chemoselectivity in the acetalization over thioacetalization in the presence of alcohols and thiols, which is otherwise hard to achieve using typical acid catalysts. Experimental and computational studies revealed that the TOB catalysts have multiple preorganized C(sp3)-Hs that serve as a mimic of oxyanion holes, which can stabilize the oxyanion intermediates via multiple C(sp3)-H non-classical hydrogen bond interactions.

Applications of the DIB-BBr3 Protocol in Bromination Reactions

Non-symmetrical bromoiodanes are useful for bromination reactions, and some protocols were found to be suitable for specific substrates. Herein, we report the use of a DIB/BBr₃ protocol for various bromination reactions, including electrophilic bromination of arenes, carbonyl C-H monobromination, bromolactonization, bromocarbocyclization, intermolecular bromoetherification of olefin, and light-triggered C(sp³)-H bromination.

Access to Bromo-γ-butenolides via Zwitterion-Catalyzed Rearrangement of Cyclopropene Carboxylic Acids

γ-Butenolides are useful structural motifs in many pharmaceutically relevant compounds. In particular, halogenated γ-butenolides are attractive building blocks because the halogen handles can readily be manipulated to give various functional molecules. In this study, a catalytic synthesis of halogenated γ-butenolides from cyclopropene carboxylic acids was developed using zwitterionic catalysts and N-haloamides as the halogen sources. The catalytic protocol could also be applied to the synthesis of halogenated pyrrolones by using cyclopropene amides as the starting materials.

Solvent and catalyst-free bromofunctionalization of olefins using a mechanochemical approach

Bromofunctionalizations of olefins are an important class of chemical transformations. N-Bromoimide reagents are commonly used in these reactions but catalysts and chlorinated solvents are often employed to achieve a reasonable reaction rate. In this report, we present a solvent and catalyst-free bromofunctionalization of olefins using mechanical force.

Efficient bromofunctionalization of olefins including bromolactonization, bromocycloetherification, and intermolecular bromoesterification were achieved under solvent and catalyst-free conditions using a mechanochemical approach.

Nitroxyl Catalysts for Six-Membered Ring Bromolactonization and Intermolecular Bromoesterification of Alkenes with Carboxylic Acids

We developed a nitroxyl-catalyzed bromoesterification of alkenes with bromo reagents, which includes a six-membered ring bromolactonization of alkenyl carboxylic acids catalyzed by AZADO as the nitroxyl radical catalyst, and an intermolecular bromoesterification of alkenes with carboxylic acids using NMO as the N-oxide catalyst. We also accomplished a remote diastereoselective bromohydroxylation via an AZADO-catalyzed six-membered ring bromolactonization and a subsequent ring cleavage reaction with alkylamines to furnish ε-bromo-δ-hydroxy amides with high diastereoselectivity.

Zwitterion-Catalyzed Isomerization of Maleic to Fumaric Acid Diesters

Fumaric acid diesters are important building blocks for organic synthesis. A class of zwitterionic organocatalysts based on an amide anion/iminium cation charge pair were found to be effective in catalyzing the isomerization of maleic acid diesters to give fumaric acid diesters. Comparison of the performance of different zwitterionic organocatalysts toward the reaction revealed that nonclassical hydrogen bonding was involved in the stabilization of the Michael adduct intermediate.