Decennary Update on Oxidative-Rearrangement Involving 1,2-Aryl C-C Migration Around Alkenes: Synthetic and Mechanistic Insights

In recent years, numerous methodologies on oxidative rearrangements of alkenes have been investigated, that produce multipurpose synthons and heterocyclic scaffolds of potential applications. The present review focused on recently established methodologies for oxidative transformation via 1,2-aryl migration in alkenes (2013-2023). Special emphasis has been placed on mechanistic pathways to understand the reactivity pattern of different substrates, challenges to enhance selectivity, the key role of different reagents, and effect of different substituents, and how they affect the rearrangement process. Moreover, synthetic limitations and future direction also have been discussed. We believe, this review offers new synthetic and mechanistic insight to develop elegant precursors and approaches to explore the utilization of alkene-based compounds for natural product synthesis and functional materials.

Three-Component Direct Phosphorylation of Aldehydes and Alkylation of Ketones: Synthesis of γ-Ketophosphine Oxides under Acidic Conditions

An effective and economical acid-promoted three-component reaction for the construction of C-P and C-C bonds for the synthesis of γ-ketophosphine oxides with water as the only byproduct was developed. Detailed mechanistic experiments confirmed that the reaction proceeds by phospha-aldol elimination, in which a benzylic carbocation is generated from the phosphorylation of aldehydes, which then reacts with ketone enolates under acidic conditions.

A Domino Radical Amidation/Semipinacol Approach to All-Carbon Quaternary Centers Bearing an Aminomethyl Group

A photoredox-mediated radical amidation ring-expansion sequence that enables the generation of all-carbon quaternary centers bearing a protected aminomethyl substituent is described. The methodology can be applied to both styrene and unactivated alkene substrates generating structurally diverse sp3 -rich amine derivatives in a concise manner.

Nickel-Catalyzed Radical Ring-Opening Phosphorylation of Cycloalkyl Hydroperoxides Leading to Distal Acylphosphine Oxides

A facile and efficient nickel-catalyzed C-C bond cleavage/phosphorylation of various cycloalkyl hydroperoxides was developed. This radical ring-opening strategy provided practical access to structurally diverse distal ketophosphine oxides in one pot through concurrent C═O/C-P bond formation with high atom economy under mild room temperature and base-free conditions.

Visible light organic photoredox catalytic cascade reactions

Over the past years, impressive progress has been made in the development of organic photoredox catalytic cascade reactions without the participation of expensive and toxic transition metals under visible light irradiation. These transformations highly depend on the in situ generation of various radical species in the photoredox catalytic cycles. Numerous chemically and biomedically valuable building blocks have been synthesized through this efficient and sustainable protocol. In this review, we highlight the recent progress in this blooming area by presenting a series of new catalytic cascade reactions mediated by organic photoredox catalysts and describe their mechanisms and applications which have appeared in the recent literature.

Dibromo-BODIPY as an Organic Photocatalyst for Radical-Ionic Sequences

A new dibrominated 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) is reported as a new metal-free photocatalyst. This BODIPY showed similar optoelectronic, electrochemical, and performance properties to those of Ru(bpy)₃Cl₂, one of the most common photocatalysts in a known radical-ionic transformation, such as the formation of 1,4-dicarbonyl compounds. Moreover, additional sequences in which the generated oxonium ion is trapped by an internal nucleophile were developed using this BODIPY photocatalyst. These new sequences allowed the straightforward preparation of γ-alkoxylactones, monoprotected 1,4-ketoaldehydes, and dihydrofurans. This new catalyst, the methodology, and the forged functional groups could be important tools in organic synthesis.

Radical-mediated rearrangements: past, present, and future

Rearrangement reactions, one of the most significant transformations in organic chemistry, play an irreplaceable role in improving synthetic efficiency and molecular complexity. Concomitant cleavage and reconstruction of chemical bonds can display the great artistry and the glamour of synthetic chemistry. Over the past century, ionic rearrangement reactions, in particular those involving cationic pathways, have represented most of the research. Alongside the renaissance of radical chemistry, radical-mediated rearrangements have recently seen a rapid increase of attention from the chemical community. Many new radical rearrangements that extensively reveal the migratory behaviour of functional groups have been unveiled in the last decade. This Review provides a comprehensive perspective on the area from the past to present achievements, and brings up the prospects that may inspire colleagues to develop more useful synthetic tools based on radical rearrangements.

Recent development and applications of semipinacol rearrangement reactions

As has been well-recognized, semipinacol rearrangement functions as an exceptionally useful methodology in the synthesis of β-functionalized ketones, creation of quaternary carbon centers, and construction of challenging carbocycles. Due to their versatile utilities in organic synthesis, development of novel rearrangement reactions has been a vibrant topic that continues to shape the research field. Recent breakthroughs in novel electrophiles, tandem processes, and enantioselective catalytic transformations further enrich the toolbox of this chemistry and spur the strategic applications of this methodology in natural product synthesis. These achievements will be discussed in this minireview.

Self-Standing Photo-Crosslinked Hydrogel Construct: in vitro Microphysiological Vascular Model

Modeling of the human vascular microphysiological system (MPS) has gained attention due to precise prediction of drug response and toxicity during drug screening process. Developing a physiologically equivalent vascular MPS still remains complex as it demands the recapitulation of dynamic structural and biological microenvironment similar to native vasculature. Hence, an ideal MPS would involve developing perfusable 3D in vitro models with multilayered human vascular cells encapsulated in a matrix to regulate the vascular tone resembling the native. Several attempts to model such anatomically accurate physiological and pathological blood vessels often fail to harmonize the essential vascular microenvironment. For instance, conventional microfluidic-based approaches employed for vascular MPS, though offering creation of perfusable channel, do not replicate the vascular hierarchical cellular arrangement due to planar geometry and confluent monolayered cell seeding. Also, recent advances with 3D biofabrication strategies are still limited by fabrication of small-diameter constructs and scalability besides post-processing techniques that indirectly distort the structural integrity of the hydrogel tubular constructs. These existing limitations toward fabricating a relevant vascular MPS demand a facile and mechanically stable construct. Hence, the present study is aimed toward developing a stable viable self-standing perfusable hydrogel construct by a rapid and scalable strategy toward vascular MPS application. The fabricated tubular constructs were found to be structurally stable with end-to-end perfusability exhibiting their potential as self-standing perfusable structures. Also, the construct exhibited nonhemolytic behavior with perfusion of red blood cells inside the luminal channel. The present study evidences creation of a dual-crosslinked stable, viable self-standing hydrogel construct with multilayered homogenous distribution of viable smooth muscle cells throughout the construct, thereby demonstrating its applicability as a promising 3D in vitro vascular microphysiological system.

Alkene homologation via visible light promoted hydrophosphination using triphenylphosphonium triflate

A hydrophosphination reaction of alkenes with triphenylphosphonium triflate under photocatalytic conditions is described. The reaction is promoted by naphthalene-fused N-acylbenzimidazole and is believed to proceed through intermediate formation of a phosphinyl radical cation. The resulting phosphonium salts are directly involved in the Wittig reaction leading to homologated alkenes.

Shining Light on the Light-Bearing Element: A Brief Review of Photomediated C–H Phosphorylation Reactions

Organophosphorus compounds have numerous useful applications, from versatile ligands and nucleophiles in the case of trivalent organophosphorus species to therapeutics, agrochemicals and material additives for pentavalent species. Although phosphorus chemistry is a fairly mature field, the construction of C–P(V) bonds relies heavily on either prefunctionalized substrates such as alkyl or aryl halides, or requires previously oxidized bonds such as C=N or C=O, leading to potential sustainability issues when looking at the overall synthetic route. In light of the recent advances in photochemistry, using photons as a reagent can provide better alternatives for phosphorylations by unlocking radical mechanisms and providing interesting redox pathways. This review will showcase the different photomediated phosphorylation procedures available for converting C–H bonds into C–P(V) bonds.

1 Introduction

1.1 Organophosphorus Compounds

1.2 Phosphorylation: Construction of C–P(V) Bonds

1.3 Photochemistry as an Alternative to Classical Phosphorylations

2 Ionic Mechanisms Involving Nucleophilic Additions

3 Mechanisms Involving Radical Intermediates

3.1 Mechanisms Involving Reactive Carbon Radicals

3.2 Mechanisms Involving Phosphorus Radicals

3.2.1 Photoredox: Direct Creation of Phosphorus Radicals

3.2.2 Photoredox: Indirect Creation of Phosphorus Radicals

3.2.3 Dual Catalysis

3.3 Photolytic Cleavage

4 Conclusion and Outlook

TfOH-Catalyzed Phosphinylation of 2,3-Allenols into γ-Ketophosphine Oxides

The first facile and efficient acid-catalyzed direct coupling of a wide range of unprotected 2,3-allenols with arylphosphine oxides was developed, offering a general, one-step approach for the synthesis of structurally diverse γ-ketophosphine oxides accompanied by concurrent C-P/C═O bond formation with remarkable functional group tolerance and complete atom-economy under metal- and additive-free conditions. Mechanistic studies showed that this transformation involved a rearrangement and a phospha-Michael reaction.

Visible-light-driven metal-free aerobic synthesis of highly diastereoselective phosphinoylpyrroloindoles

A tandem C–P and C–C bond formation promoted by visible light at room temperature in a highly diastereoselective manner is described.

Defluorinative C(sp3 )-P Bond Construction for the Synthesis of Phosphorylation gem-Difluoroalkenes under Catalyst- and Oxidant-Free Conditions

Defluorinative C(sp³ )-P bond formation of α-trifluoromethyl alkenes with phosphine oxides or phosphonates have been achieved under catalyst- and oxidant-free conditions, giving phosphorylation gem-difluoroalkenes as products. α-Trifluoromethyl alkenes bearing various of aryl substituents such as halogen, cyano, ester and heterocyclic groups are available in this transformation. The results of control experiments demonstrated that the mechanism of dehydrogenative/defluorinative cross-coupling reactions was not a radical route, but might be an S_N 2' process involving phosphine oxide anion.

Visible light-mediated CP bond formation reactions

Organophosphorus compounds have attracted continuous attention in materials science, agrochemical and pharmaceutical fields due to their unique bioactivities. Thus, the development of novel and robust manners for the construction new CP bond has therefore gained great interests in synthetic organic chemistry. Because of their intrinsic sustainability and green chemistry character, visible light-induced photoredox catalysis has been widely applied in the construction of new chemical bonds, including the formation of CP bond. In this review, we summarized recent achievements in CP bond formation reactions initiated by visible light-induced photoredox catalysis, which mainly focusing on the discussion of reaction design and the mechanism.

Visible-light-promoted oxidation/condensation of benzyl alcohols with dialkylacetamides to cinnamides

An oxidative cross-coupling reaction of benzyl alcohols with dialkylacetamides was developed to construct cinnamides under visible-light-enabled photocatalytic conditions.

A copper-catalyzed radical coupling/fragmentation reaction: efficient access to β-oxophosphine oxides

The development of a novel copper-catalyzed three-component radical coupling/fragmentation cascade reaction to generate diverse β-oxophosphine oxides is reported.