Highly regio- and diastereoselective halohydroxylation of olefins: a facile synthesis of vicinal halohydrins

Dichlorination of olefins with trichloroisocyanuric acid (TCCA) and tetrabutylammonium chloride (TBACl)

Herein, a new metal-free, molecular chlorine-free, environmentally friendly, atom-economical, short time, inexpensive and simple operation method with mild reaction conditions for chlorination of alkenes, cyclic alkenes, α,β-unsaturated carbonyl compounds, heteroaromatics, and natural products was reported with up to 96% yields using trichloroisocyanuric acid (TCCA) as the electrophilic chlorine source and TBACl as the nucleophilic chlorine source. It was demonstrated with bicyclic alkene benzonorbornadiene that regioselective chlorobromination and dibromination reactions can be carried out through TCCA/TBABr redox reactions, where TCCA acts as an oxidant in the presence of TBABr. The structures of the redox products were confirmed as a result of control experiments conducted with the newly presented DBI/TBACl and DBI/TBABr halogenation pairs.

Additive-free oxychlorination of unsaturated C-C bonds with tert-butyl hypochlorite and water

Herein we report an additive-free protocol for the facile synthesis of α,α-dichloroketones and α-chlorohydrins from various aryl terminal, diaryl internal, and aliphatic terminal alkynes and alkenes, respectively. The commercially available tert-butyl hypochlorite (tBuOCl) was employed as a suitable chlorinating reagent, being accompanied by the less harmful tBuOH as the by-product. In addition, the oxygen atoms in the products came from water rather than molecular oxygen, based on the 18O-labelling experiments. Meanwhile, the diastereoselectivity of the Z- and the corresponding E-alkenes has been compared and rationalized. Using a group of control experiments, the possible mechanisms have been proposed as the initial electrophilic chlorination of unsaturated C-C bonds in a Markovnikov-addition manner in general followed by a nucleophilic addition with water. This work simplified the oxychlorination method with a mild chlorine source and a green oxygen source under ambient conditions.

Alkene Thianthrenation Unlocks Diverse Cation Synthons: Recent Progress and New Opportunities

Oxidative alkene functionalization reactions are a fundamental class of complexity-building organic transformations. However, the majority of established approaches rely on electrophilic reagents that limit the diversity of groups that can be installed. Recent advances have established a new approach that instead relies on the transformation of alkenes into thianthrene-derived cationic electrophiles. These linchpin intermediates can be generated selectively and undergo a diverse array of mechanistically distinct reactions with abundant nucleophiles. Taken together, this unlocks a suite of net oxidative alkene transformations that have been elusive using conventional strategies. This Minireview describes these advances and is organized around the three distinct synthons formally accessible from alkenes via thianthrenation: 1) alkenyl cations; 2) vicinal dications; 3) allyl cations. Throughout the Minireview, we illustrate how thianthrenium salts address key limitations endemic to classic alkene-derived electrophiles and highlight the mechanistic origins of these distinctions wherever possible.

Regiospecific Alkene Aminofunctionalization via an Electrogenerated Dielectrophile

Modular strategies to rapidly increase molecular complexity have proven immensely synthetically valuable. In principle, transformation of an alkene into a dielectrophile presents an opportunity to deliver two unique nucleophiles across an alkene. Unfortunately, the selectivity profiles of known dielectrophiles have largely precluded this deceptively simple synthetic approach. Herein, we demonstrate that dicationic adducts generated through electrolysis of alkenes and thianthrene possess a unique selectivity profile relative to more conventional dielectrophiles. Specifically, these species undergo a single and perfectly regioselective substitution reaction with phthalimide salts. This observation unlocks an appealing new platform for aminofunctionalization reactions. As an illustrative example, we implement this new reactivity paradigm to address a longstanding synthetic challenge: alkene diamination with two distinct nitrogen nucleophiles. Studies into the mechanism of this process reveal a key alkenyl thianthrenium salt intermediate that controls the exquisite regioselectivity of the process and highlight the importance of proton sources in controlling the reactivity of alkenyl sulfonium salt electrophiles.

The Synthetic Approaches to 1,2-Chlorohydrins

This short review highlights the hitherto realised synthetic approaches towards organic 1,2-chlorohydrins by functionalisation of alkenes (i.e., 1,2-chlorohydroxylation), which is the most prominent access route to this class of compounds. Also, some other synthetic approaches involving the reduction of α-chloroketones, the epoxide opening ring by chloride anions and the utilisation of Grignard reagents for the synthesis of these compounds and chlorination of allylic alcohols are highlighted. Finally, enzymatic reactions for the formation of chlorohydrins are briefly summarised followed by a short view on natural products containing this moiety.

1 Introduction

2 Applications for the Synthesis of 1,2-Chlorohydrins

2.1 Chlorohydroxylation of Alkenes

2.2 Reduction of Chloroketones

2.3 Metalorganic Reagents

2.4 Epoxide Ring Opening

2.5 Chlorination of Allylic Alcohols

2.6 Biochemical Methods

2.7 Selected Applications in Natural Product Total Synthesis

3 Conclusion

Amphiphilic Indoles as Efficient Phase-Transfer Catalysts for Bromination in Water

Brominated compounds are important, but they are usually prepared in organic solvents. Here, efficient amphiphilic indole-based phase-transfer organocatalysts were developed for environmentally benign bromination reactions in water. As test reactions, hydroxybromination of olefins and aromatic bromination could be conducted in a greener and more sustainable manner compared with methods using organic solvents, producing the corresponding bromides in good yields. Some pure products could be obtained without column chromatography.

Route exploration and synthesis of the reported pyridone-based PDI inhibitor STK076545

The enzyme protein disulfide isomerase (PDI) is essential for the correct folding of proteins and the activation of certain cell surface receptors, and is a promising target for the treatment of cancer and thrombotic conditions. A previous high-throughput screen identified the commercial compound STK076545 as a promising PDI inhibitor. To confirm its activity and support further biological studies, a resynthesis was pursued of the reported β-keto-amide with an N-alkylated pyridone at the α-position. Numerous conventional approaches were complicated by undesired fragmentations or rearrangements. However, a successful 5-step synthetic route was achieved using an aldol reaction with an α-pyridone allyl ester as a key step. An X-ray crystal structure of the final compound confirmed that the reported structure of STK076545 was achieved, however its lack of PDI activity and inconsistent spectral data suggest that the commercial structure was misassigned.

Mechanistic Insights into the Origin of Stereoselectivity in an Asymmetric Chlorolactonization Catalyzed by (DHQD)2PHAL

Electrophilic halofunctionalization reactions have undergone a resurgence sparked by recent discoveries in the field of catalytic asymmetric halocyclizations. To build mechanistic understanding of these asymmetric transformations, a toolbox of analytical methods has been deployed, addressing the roles of catalyst, electrophile (halenium donor), and nucleophile in determining rates and stereopreferences. The test reaction, (DHQD)₂PHAL-catalyzed chlorocyclization of 4-arylpent-4-enoic acid with 1,3-dichloro-5,5-dimethylhydantoin (DCDMH), is revealed to be first order in catalyst and chlorenium ion donor and zero order in alkenoic acid substrate under synthetically relevant conditions. The simplest interpretation is that rapid substrate-catalyst binding precedes rate-limiting chlorenium attack, controlling the face selectivity of both chlorine attack and lactone closure. ROESY and DFT studies, aided by crystal structures of carboxylic acids bound by the catalyst, point to a plausible resting state of the catalyst-substrate complex predisposed for asymmetric chlorolactonization. As revealed by our earlier labeling studies, these findings suggest modes of binding in the (DHQD)₂PHAL chiral pocket that explain the system's remarkable control over rate- and enantioselection-determining events. Though a comprehensive modeling analysis is beyond the scope of the present work, quantum chemical analysis of the fragments' interactions and candidate reaction paths point to a one-step concerted process, with the nucleophile playing a critical role in activating the olefin for concomitant electrophilic attack.

Halohydroxylation of alkenyl MIDA boronates: switchable stereoselectivity induced by B(MIDA) substituent

A switchable stereoselectivity in the halohydroxylation of alkenyl MIDA boronates was found. C–B(MIDA) bond hyperconjugation plays the key role.

Electrophilic chloro(ω-alkoxy)lation of alkenes employing 1-chloro-1,2-benziodoxol-3-one: facile synthesis of β-chloroethers

A four-component reaction for electrophilic chloro(ω-alkoxy)lation of alkenes has been described. The stable chloro-iodine(iii) reagent and SOCl2 were used as electrophilic and nucleophilic chlorine sources, respectively. This approach provides a straightforward way to synthesize various useful β-chloro ω-chloroalkyl ethers from a wide range of alkenes, including electron-deficient, aromatic and unactivated alkenes. The synthetic applications of this approach were also explored in some useful transformations.

Halofunctionalization of alkenes by vanadium chloroperoxidase from Curvularia inaequalis

The vanadium-dependent chloroperoxidase from Curvularia inaequalis is a stable and efficient biocatalyst for the hydroxyhalogenation of a broad range of alkenes into halohydrins. Up to 1 200 000 TON with 69 s^-1 TOF were observed for the biocatalyst. A bienzymatic cascade to yield epoxides as reaction products is presented.

Efficient conversion of alkenes to chlorohydrins by a Ru-based artificial enzyme

Activation of a ruthenium complex by its insertion into a protein scaffold leads to an efficient non natural transformation of alkenes into α-hydroxy-β-chloro chlorohydrins.

Immobilization of Amano lipase from Pseudomonas fluorescens on silk fibroin spheres: an alternative protocol for the enantioselective synthesis of halohydrins

Amano lipase from Pseudomonas fluorescens immobilized on silk fibroin spheres and used in the enzymatic kinetic resolution of halohydrins, to obtain optically active epoxides, important precursors in the synthesis of antifungal azoles.

Use of Bromine and Bromo-Organic Compounds in Organic Synthesis

Bromination is one of the most important transformations in organic synthesis and can be carried out using bromine and many other bromo compounds. Use of molecular bromine in organic synthesis is well-known. However, due to the hazardous nature of bromine, enormous growth has been witnessed in the past several decades for the development of solid bromine carriers. This review outlines the use of bromine and different bromo-organic compounds in organic synthesis. The applications of bromine, a total of 107 bromo-organic compounds, 11 other brominating agents, and a few natural bromine sources were incorporated. The scope of these reagents for various organic transformations such as bromination, cohalogenation, oxidation, cyclization, ring-opening reactions, substitution, rearrangement, hydrolysis, catalysis, etc. has been described briefly to highlight important aspects of the bromo-organic compounds in organic synthesis.

Difunctionalization of Alkenes Using 1-Chloro-1,2-benziodoxol-3-(1H)-one

Difunctionalization of alkenes with 1-chloro-1,2-benziodoxol-3-(1H)-one (1) was investigated. Various additional nucleophiles were tested, and oxychlorination, dichlorination, azidochlorination, chlorothiocyanation, and iodoesterfication were demonstrated. The oxychlorination product was obtained efficiently when the reaction was operated in water. Dichlorination occurred in the presence of a Lewis basic promoter, such as 4-phenylpyridine N-oxide, as an additive. The reaction with in situ-generated azido anion afforded azidochlorinated compounds with a chlorine atom at the terminal position, while the reaction with trimethylsilyl isothiocyanate produced chlorothiocyanation adducts with a chlorine atom at the benzylic position. On the other hand, when 1 was treated with tetra-n-butylammonium iodide prior to the addition of alkenes, only iodoesterification occurred selectively. These mild reactions enable convenient site-selective difunctionalizations of substrates having two alkene moieties. NMR experiments suggested that the electrophilic reactive species in each reaction varied depending on the nature of the added nucleophile.

The vicinal functionalization of olefins: a facile route to the direct synthesis of β-chlorohydrins and β-chloroethers

A general and green approach for the highly regio- and stereoselective synthesis of α-hydroxy-β-chloro and α-methoxy-β-chloro derivatives from olefins using inexpensive and industrially acceptable reagents is reported.