Difluorination of Functionalized Aromatic Olefins Using Hypervalent Iodine/HF Reagents

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.

Catalytic Ring Expanding Difluorination: An Enantioselective Platform to Access β,β-Difluorinated Carbocycles

Cyclic β,β-difluoro-carbonyl compounds have a venerable history as drug discovery leads, but limitations in the synthesis arsenal continue to impede chemical space exploration. This challenge is particularly acute in the arena of fluorinated medium rings where installing the difluoromethylene unit subtly alters the ring conformation by expanding the internal angle (∠C-CF2-C>∠C-CH2-C): this provides a handle to modulate physicochemistry (e.g. pKa). To reconcile this disparity, a highly modular ring expansion has been devised that leverages simple α,β-unsaturated esters and amides, and processes them to one-carbon homologated rings with concomitant geminal difluorination (6 to 10 membered rings, up to 95 % yield). This process is a rare example of the formal difluorination of an internal alkene and is enabled by sequential I(III)-enabled O-activation. Validation of enantioselective catalysis in the generation of unprecedented medium ring scaffolds is reported (up to 93 : 7 e.r.) together with X-ray structural analyses and product derivatization.

Molecular Iodine as a Catalyst for Alkene Difluorination

The difluorination reaction of alkenes catalyzed by molecular iodine was revealed for the first time. This difluorination reaction affords a simple and practical experimental method and can be applied to many aliphatic and aromatic alkenes bearing synthetically useful functional groups, such as ester, amide, hydroxy, and aryl groups. Preliminary mechanistic studies of this alkene difluorination suggest the existence of two catalytic cycles: the IF-driven cycle and the catalytic cycle by the IF adduct.

Iodobenzene-Catalyzed Synthesis of Fully Functionalized NH-Pyrazoles and Isoxazoles from α,β-Unsaturated Hydrazones and Oximes via 1,2-Aryl Shift

An iodine(III)-catalyzed general method for the synthesis of fully functionalized NH-pyrazoles and isoxazoles from α,β-unsaturated hydrazones and oximes, respectively, via cyclization/1,2-aryl shift/aromatization/detosylation, has been developed. The reaction progresses through an anti-Baldwin 5-endo-trig cyclization. It gives direct access to an advanced intermediate for the preparation of valdecoxib and parecoxib, drugs used for COX-inhibition. In addition, a method for N-alkynylation of pyrazoles has also been developed in the presence of TIPS-EBX.

Regioselective, catalytic 1,1-difluorination of enynes

Fluorinated small molecules are prevalent across the functional small-molecule spectrum, but the scarcity of naturally occurring sources creates an opportunity for creative endeavour in developing routes to access these important materials. Iodine(I)/iodine(III) catalysis has proven to be particularly well-suited to this task, enabling abundant alkene substrates to be readily intercepted by in situ-generated λ3-iodanes and processed to high-value (di)fluorinated products. These organocatalysis paradigms often emulate metal-based processes by engaging the π bond and, in the case of styrenes, facilitating fluorinative phenonium-ion rearrangements to generate difluoromethylene units. Here we demonstrate that enynes are competent proxies for styrenes, thereby mitigating the recurrent need for aryl substituents, and enabling highly versatile homopropargylic difluorides to be generated in an operationally simple manner. The scope of the method is disclosed, together with application in target synthesis (>30 examples, up to >90% yield).

Catalytic, Regioselective 1,4-Fluorodifunctionalization of Dienes

A catalysis-based regioselective 1,4-fluorofunctionalization of trifluoromethyl substituted 1,3-dienes has been developed to access compact, highly functionalized products. The process allows E,Z-mixed dienes to be processed to a single E-alkene isomer, and leverages an inexpensive and operationally convenient I(I)/I(III) catalysis platform. The first example of catalytic 1,4-difluorination is disclosed and subsequently evolved to enable 1,4-hetero-difunctionalization, which allows δ-fluoro-alcohol and amine derivatives to be forged in a single operation. The protocol is compatible with a variety of nucleophiles including fluoride, nitriles, carboxylic acids, alcohols and even water thereby allowing highly functionalized products, with a stereocenter bearing both C(sp³ )-F and C(sp³ )-CF₃ groups, to be generated rapidly. Scalability (up to 3 mmol), and facile post-reaction modifications are demonstrated to underscore the utility of the method in expanding organofluorine chemical space.

Skeletal Ring Contractions via I(I)/I(III) Catalysis: Stereoselective Synthesis of cis-α,α-Difluorocyclopropanes

The clinical success of α,α-difluorocyclopropanes, combined with limitations in the existing synthesis portfolio, inspired the development of an operationally simple, organocatalysis-based strategy to access cis-configured derivatives with high levels of stereoselectivity (up to >20:1 cis:trans). Leveraging an I(I)/I(III)-catalysis platform in the presence of an inexpensive HF source, it has been possible to exploit disubstituted bicyclobutanes (BCBs) as masked cyclobutene equivalents for this purpose. In situ generation of this strained alkene, enabled by Brønsted acid activation, facilitates an unprecedented 4 → 3 fluorinative ring contraction, to furnish cis-α,α-difluorinated cyclopropanes in a highly stereoselective manner (up to 88% yield). Mechanistic studies are disclosed together with conformational analysis (X-ray crystallography and NMR) to validate cis-α,α-difluorocyclopropanes as isosteres of the 1,4-dicarbonyl moiety. Given the importance of this unit in biology and the foundational n_o → π* interactions that manifest themselves in this conformation (e.g., collagen), it is envisaged that the title motif will find application in focused molecular design.

Divergent Synthesis of gem-Difluorinated Oxa-Spirocyclohexadienones by One-Pot Sequential Reactions of p-Hydroxybenzyl Alcohols with Difluoroenoxysilanes

A new efficient formal [2 + 3] cyclization of p-hydroxybenzyl alcohols with difluoroenoxysilanes has been established. This convenient one-pot sequential procedure enables the divergent construction of highly functionalized gem-difluorinated oxa-spirocyclohexadienones under mild conditions. As opposed to the common C1 synthons in previous studies, difluoroenoxysilanes acted as new 3-atom (CCO) synthons for the first time here. The AcOH and H₂O generated in the reaction are critical for the reactions to proceed smoothly.

Expanding organofluorine chemical space: the design of chiral fluorinated isosteres enabled by I(i)/I(iii) catalysis

Short aliphatic groups are prevalent in bioactive small molecules and play an essential role in regulating physicochemistry and molecular recognition phenomena. Delineating their biological origins and significance have resulted in landmark developments in synthetic organic chemistry: Arigoni's venerable synthesis of the chiral methyl group is a personal favourite. Whilst radioisotopes allow the steric footprint of the native group to be preserved, this strategy was never intended for therapeutic chemotype development. In contrast, leveraging H → F bioisosterism provides scope to complement the chiral, radioactive bioisostere portfolio and to reach unexplored areas of chiral chemical space for small molecule drug discovery. Accelerated by advances in I(i)/I(iii) catalysis, the current arsenal of achiral 2D and 3D drug discovery modules is rapidly expanding to include chiral units with unprecedented topologies and van der Waals volumes. This Perspective surveys key developments in the design and synthesis of short multivicinal fluoroalkanes under the auspices of main group catalysis paradigms.

Trifluorinated Tetralins via I(I)/I(III)-Catalysed Ring Expansion: Programming Conformation by [CH2 CH2 ] → [CF2 CHF] Isosterism

Saturated, fluorinated carbocycles are emerging as important modules for contemporary drug discovery. To expand the current portfolio, the synthesis of novel trifluorinated tetralins has been achieved. Fluorinated methyleneindanes serve as convenient precursors and undergo efficient difluorinative ring expansion with in situ generated p‐TolIF₂ (>20 examples, up to >95 %). A range of diverse substituents are tolerated under standard catalysis conditions and this is interrogated by Hammett analysis. X‐ray analysis indicates a preference for the CH−F bond to occupy a pseudo‐axial orientation, consistent with stabilising σ_C−H→σ_C−F* interactions. The replacement of the symmetric [CH₂−CH₂] motif by [CF₂−CHF] removes the conformational degeneracy intrinsic to the parent tetralin scaffold leading to a predictable half‐chair. The conformational behavior of this novel structural balance has been investigated by computational analysis and is consistent with stereoelectronic theory.

Iodine(III)-Mediated Fluorination/Semipinacol Rearrangement Cascade of 2-Alkylidenecyclobutanol Derivatives: Access to β-Monofluorinated Cyclopropanecarbaldehydes

A hypervalent iodine(III)-mediated ring-contractive fluorination reaction of 2-alkylidenecyclobutanol derivatives is presented. The protocol allows the facile synthesis of β-monofluorinated cyclopropanecarbaldehydes via a fluorination/semipinacol rearrangement cascade using nucleophilic Py·HF as the fluorine source. For challenging electron-rich arene substrates, the installation of a protecting group on the free alcohol is pivotal for maintaining the reaction efficiency. The synthetic utility was demonstrated by the scalability of this reaction and further transformations of the products.

Fluorocyclization of Allyl Alcohols and Amines to Access 3-Functionalized Oxetanes and Azetidines

An efficient method to prepare 3-functionalized oxetanes and azetidines has been realized by fluorocyclization of readily available 2-azidoallyl/2-alkoxyallyl alcohols and amines. Notably, this is the first example applying the fluorocyclization strategy to construct four-membered heterocycles. The pendant electron-donating group (-N₃ or -OR) plays a crucial role in polarizing the C═C double bond and facilitating the cyclization process, as verified by DFT and experimental studies.

Difluorination of α-(bromomethyl)styrenes via I(I)/I(III) catalysis: facile access to electrophilic linchpins for drug discovery

Simple α-(bromomethyl)styrenes can be processed to a variety of 1,1-difluorinated electrophilic building blocks via I(I)/I(III) catalysis. This inexpensive main group catalysis strategy employs p-TolI as an effective organocatalyst when combined with Selectfluor® and simple amine·HF complexes. Modulating Brønsted acidity enables simultaneous geminal and vicinal difluorination to occur, thereby providing a platform to generate multiply fluorinated scaffolds for further downstream derivatization. The method facilitates access to a tetrafluorinated API candidate for the treatment of amyotrophic lateral sclerosis. Preliminary validation of an enantioselective process is disclosed to access α-phenyl-β-difluoro-γ-bromo/chloro esters.

Modern approaches towards the synthesis of geminal difluoroalkyl groups

This review will cover the importance of and most recent approaches toward geminal difluoroalkyl groups. Transition metal-mediated, photochemical, organocatalytic, and other methods as well as their mechanistic implications will be discussed, with special emphasis on applications to biologically-relevant compounds.

Alkene Vicinal Difluorination: From Fluorine Gas to More Favoured Conditions

Vicinal difluorinated alkanes are entities relevant to medicinal chemistry that are accessed through the difluorination of alkenes. This reaction has advanced from the use of highly reactive and unsafe reagents, which provide poor functional-group tolerance and selectivity, to the use of safer and more selective reagents that facilitate access to a broader scope of substrates. In this review article, we describe the details of these developments.

1 Introduction

2 Strategy 1: Ambiphilic Fluorine Sources

3 Strategy 2: Oxidant and Fluoride

4 Conclusions and Outlook

Kitamura Electrophilic Fluorination Using HF as a Source of Fluorine

This review article focused on the innovative procedure for electrophilic fluorination using HF and in situ generation of the required electrophilic species derived from hypervalent iodine compounds. The areas of synthetic application of this approach include fluorination of 1,3-dicarbonyl compounds, aryl-alkyl ketones, styrene derivatives, α,β-unsaturated ketones and alcohols, homoallyl amine and homoallyl alcohol derivatives, 3-butenoic acids and alkynes.

A DFT study on the mechanism and origins of the ligand-controlled regioselectivity of a palladium-catalyzed dearomatic reaction of 1-(chloromethyl)naphthalene with phenylacetonitrile

DFT calculations demonstrated that the bulky ligand ^tBuPPh₂ generates para-substituted products, while the small ligand Me₂PPh provides ortho-substituted products.

Difluorinative ring expansions of benzo-fused carbocycles and heterocycles are achieved withp-(difluoroiodo)toluene

Fluorination of exocyclic alkenes and allenes withp-TolIF₂gives ring-expanded β,β-difluoridesviaa 1,2-phenyl shift.

Catalytic Geminal Difluorination of Styrenes for the Construction of Fluorine-rich Bioisosteres

A geminal difluorination of alkenes based on I(I)/I(III) catalysis is disclosed, which is compatible with a range of electronically and substitutionally diverse styrenes (27 examples, up to 89% yield). Employing inexpensive p-TolI as the organocatalyst, turnover is enabled by Selectfluor-mediated oxidation to generate the ArIF₂ species in situ. Extension to include α-substituted styrenes bearing fluorine-containing groups is disclosed and provides an expansive platform for the generation of fluorine-rich architectures.

Mechanism and Origins of Chemo- and Stereoselectivities of Aryl Iodide-Catalyzed Asymmetric Difluorinations of β-Substituted Styrenes

The mechanism of the aryl iodide-catalyzed asymmetric migratory geminal difluorination of β-substituted styrenes ( Banik et al. Science 2016, 353, 51 ) has been explored with density functional theory computations. The computed mechanism consists of (a) activation of iodoarene difluoride (ArIF2), (b) enantiodetermining 1,2-fluoroiodination, (c) bridging phenonium ion formation via SN2 reductive displacement, and (d) regioselective fluoride addition. According to the computational model, the ArIF2 intermediate is stabilized through halogen-π interactions between the electron-deficient iodine(III) center and the benzylic substituents at the catalyst stereogenic centers. Interactions with the catalyst ester carbonyl groups (I(III)+···O) are not observed in the unactivated complex, but do occur upon activation of ArIF2 through hydrogen-bonding interactions with external Brønsted acid (HF). The 1,2-fluoroiodination occurs via alkene complexation to the electrophilic, cationic I(III) center followed by C-F bond formation anti to the forming C-I bond. The bound olefin and the C-I bond of catalyst adopt a spiro arrangement in the favored transition structures but a nearly periplanar arrangement in the disfavored transition structures. Multiple attractive non-covalent interactions, including slipped π···π stacking, C-H···O, and C-H···π interactions, are found to underlie the high asymmetric induction. The chemoselectivity for 1,1-difluorination versus 1,2-difluorination is controlled mainly by (1) the steric effect of the substituent on the olefinic double bond and (2) the nucleophilicity of the carbonyl oxygen of substrate.