Catalytic Diastereo- and Enantioselective Fluoroamination of Alkenes

Ritter-enabled catalytic asymmetric chloroamidation of olefins

Intermolecular asymmetric haloamination reactions are challenging due to the inherently high halenium affinity (HalA) of the nitrogen atom, which often leads to N-halogenated products as a kinetic trap. To circumvent this issue, acetonitrile, possessing a low HalA, was used as the nucleophile in the catalytic asymmetric Ritter-type chloroamidation of allyl-amides. This method is compatible with Z and E alkenes with both alkyl and aromatic substitution. Mild acidic workup reveals the 1,2-chloroamide products with enantiomeric excess greater than 95% for many examples. We also report the successful use of the sulfonamide chlorenium reagent dichloramine-T in this chlorenium-initiated catalytic asymmetric Ritter-type reaction. Facile modifications lead to chiral imidazoline, guanidine, and orthogonally protected 1,2,3 chiral tri-amines.

Intermolecular haloamination reactions are challenging due to the high halenium affinity of the nitrogen atom. This is circumvented by using acetonitrile as an attenuated nucleophile, resulting in an enantioselective halo-Ritter reaction.

Enantioselective Construction of Tertiary Fluoride Stereocenters by Organocatalytic Fluorocyclization

1,1-Disubstituted styrenes with internal oxygen and nitrogen nucleophiles undergo oxidative fluorocyclization reactions with in situ generated chiral iodine(III)-catalysts. The resulting fluorinated tetrahydrofurans and pyrrolidines contain a tertiary carbon–fluorine stereocenter. Application of a new 1-naphthyllactic acid-based iodine(III)-catalyst allows the control of tertiary carbon–fluorine stereocenters with up to 96% ee. Density functional theory calculations are performed to investigate the details of the mechanism and the factors governing the stereoselectivity of the reaction.

I(III)-Catalyzed Oxidative Cyclization-Migration Tandem Reactions of Unactivated Anilines

An I(III)-catalyzed oxidative cyclization-migration tandem reaction using Selectfluor as the oxidant was developed that converts unactivated anilines into 3H-indoles is reported herein. The reaction requires as little as 1 mol % of the iodocatalyst and is mild, tolerating pyridine and thiophene functional groups, and the dependence of the diastereoselectivity of the process on the identity of the iodoarene or iodoalkane precatalyst suggests that the catalyst is present for the stereochemical determining C-N bond forming step.

Recent Application of Chiral Aryliodine Based on the 2-Iodo­resorcinol Core in Asymmetric Catalysis

Chiral iodoarenes have been steadily increasing in importance in recent years, especially in enantioselective synthesis and catalysis. Since the development of the concept of chiral iodine(I/III) catalysis, the use of various chiral aryliodine frameworks has been explored in this area. This short review gives an overview of the use of chiral hypervalent iodine(I/III) reagents based on the 2-iodoresorcinol core with two attached two lactic side chains bearing ester or amide groups for the catalytic enantioselective dearomatization of phenol compounds, asymmetric oxidation of alkenes, and enantioselective α-functionalization of carbonyl compounds highlighting the excellent reactivities in terms of yield and enantioselectivity.

1 Introduction

2 Enantioselective Dearomatization of Phenol Derivatives

3 Asymmetric Oxidation of Alkenes

4 Enantioselective α-Functionalization of Carbonyl Compounds

5 Conclusion and Outlook

Enantioselective Aryl-Iodide-Catalyzed Wagner-Meerwein Rearrangements

We report a strategy for effecting catalytic, enantioselective carbocationic rearrangements through the intermediacy of alkyl iodanes as stereodefined carbocation equivalents. Asymmetric Wagner-Meerwein rearrangements of β-substituted styrenes are catalyzed by the C₂-symmetric aryl iodide 1 to provide access to enantioenriched 1,3-difluorinated molecules possessing interesting and well-defined conformational properties. Hammett and kinetic isotope effect studies, in combination with computational investigations, reveal that two different mechanisms are operative in these rearrangement reactions, with the pathway depending on the identity of the migrating group. In reactions involving alkyl-group migration, intermolecular fluoride attack is product- and enantio-determining. In contrast, reactions in which aryl rearrangement occurs proceed through an enantiodetermining intramolecular 1,2-migration prior to fluorination. The fact that both pathways are promoted by the same chiral aryl iodide catalyst with high enantioselectivity provides a compelling illustration of generality across reaction mechanisms in asymmetric catalysis.

Enantioselective Synthesis of 3-Fluorochromanes via Iodine(I)/Iodine(III) Catalysis

The chromane nucleus is common to a plenum of bioactive small molecules where it is frequently oxidized at position 3. Motivated by the importance of this position in conferring efficacy, and the prominence of bioisosterism in drug discovery, an iodine(I)/iodine(III) catalysis strategy to access enantioenriched 3-fluorochromanes is disclosed (up to 7:93 e.r.). In situ generation of ArIF2 enables the direct fluorocyclization of allyl phenyl ethers to generate novel scaffolds that manifest the stereoelectronic gauche effect. Mechanistic interrogation using deuterated probes confirms a stereospecific process consistent with a type IIinv pathway.

Catalytic Enantioselective Synthesis of Difluorinated Alkyl Bromides

We report an iodoarene-catalyzed enantioselective synthesis of β,β-difluoroalkyl bromide building blocks. The transformation involves an oxidative rearrangement of α-bromostyrenes, utilizing HF-pyridine as the fluoride source and m-CPBA as the stoichiometric oxidant. A catalyst decomposition pathway was identified, which, in tandem with catalyst structure-activity relationship studies, facilitated the development of an improved catalyst providing higher enantioselectivity with lower catalyst loadings. The versatility of the difluoroalkyl bromide products was demonstrated via highly enantiospecific substitution reactions with suitably reactive nucleophiles. The origins of enantioselectivity were investigated using computed interaction energies of simplified catalyst and substrate structures, providing evidence for both CH-π and π-π transition state interactions as critical features.

Fluorohydration of alkynes via I(I)/I(III) catalysis

Substrate specificity is ubiquitous in biological catalysis, but less pervasive in the realm of small-molecule catalysis. Herein, we disclose an intriguing example of substrate specificity that was observed whilst exploring catalysis-based routes to generate α-fluoroketones from terminal and internal alkynes under the auspices of I(I)/I(III) catalysis. Utilising p-TolI as an inexpensive organocatalyst with Selectfluor^® and amine/HF mixtures, the formation of protected α-fluoroketones from simple alkynes was realised. Whilst the transient p-TolIF₂ species generated in situ productively engaged with pentynyl benzoate scaffolds to generate the desired α-fluoroketone motif, augmentation or contraction of the linker suppressed catalysis. The prerequisite for this substructure was established by molecular editing and was complemented with a physical organic investigation of possible determinants.

1,4-Fluoroamination of 1,3-Enynes en Route to Fluorinated Allenes

The regioselective synthesis of fluorinated allenes via a metal-free 1,4-fluoroamination of 1,3-enynes is presented. This method employs commercially available N-fluorobenzenesulfonimide serving as both the nitrogen source and the fluorine source, affording access to various tetrasubstituted allenes in a straightforward and atom-economic pathway. Preliminary mechanistic studies and theoretical studies revealed that this reaction might undergo an intimate ion-pair mechanism.

Fluorofunctionalizations of C-C Multiple Bonds and C-H Bonds

In spite of only a few naturally occurring products having one or more fluorine atoms, organofluorine compounds have been widely utilized in pharmaceutical, agrochemical, and functional material science fields due to the characteristic properties of the fluorine atom. Therefore, the development of new methods for the introduction of fluorine-containing functional groups has been a long-standing research topic. This article discusses our contributions to this area. The first topic is on the trifluoromethylations of C-C multiple bonds using Togni reagent based on our working hypothesis that hypervalent iodine could be activated by coordination of the carbonyl moiety to the Lewis acid catalyst. The second topic relates to asymmetric fluorofunctionalization of alkenes. A newly designed phase-transfer catalyst consisting of a carboxylate anion functioning as a phase-transfer agent and a primary hydroxyl group as a site that captures the anionic substrate was revealed to be an effective catalyst for asymmetric fluorolactonization. Inspired by the mechanistic studies of fluorolactonization, we produced a linked binaphthyl dicarboxylate catalyst, which catalyzes the 6-endo-fluorocyclization and the deprotonative fluorination of allylic amides in a highly enantioselective manner. The third topic is on C-H fluorofunctionalizations using either catalysis or photoactivation. Benzylic trifluoromethylation, which is still a rare reaction, using Togni reagent and aromatic C-H trifluoromethylation using Umemoto reagent under simple photoirradiation conditions were achieved. In addition, the Csp³-H fluorination of alkyl phthalimide derivatives is demonstrated.

Catalyst-free regioselective hydroxyfluorination and aminofluorination of α,β-unsaturated ketones

The catalyst-free direct regioselective α-fluoro-β-hydroxylation and α-fluoro-β-amidation of α,β-unsaturated ketones has been developed. Various α,β-unsaturated ketones react with Selectfluor in water and acetonitrile to give α-fluorohydrins and α-fluoroamides respectively with moderate to good yields. The mechanistic studies revealed the possibility of a radical based pathway.

An umpolung strategy to react catalytic enols with nucleophiles

The selective synthesis of α-functionalized ketones with two similar enolizable positions can be accomplished using allylic alcohols and iridium(III) catalysts. A formal 1,3-hydrogen shift on allylic alcohols generates catalytic iridium-enolates in a stereospecific manner, which are able to react with electrophiles to yield α-functionalized ketones as single constitutional isomers. However, the employment of nucleophiles to react with the nucleophilic catalytic enolates in this chemistry is still unknown. Herein, we report an umpolung strategy for the selective synthesis of α-alkoxy carbonyl compounds by the reaction of iridium enolates and alcohols promoted by an iodine(III) reagent. Moreover, the protocol also works in an intramolecular fashion to synthesize 3(2H)-furanones from γ-keto allylic alcohols. Experimental and computational investigations have been carried out, and mechanisms are proposed for both the inter- and intramolecular reactions, explaining the key role of the iodine(III) reagent in this umpolung approach.

Nucleophiles cannot be directly reacted with enolates due to polarity mismatching. Here, the authors developed an umpolung strategy for the selective synthesis of α-alkoxy carbonyl compounds by reaction of iridium enolates with nucleophilic alcohols promoted by an iodine(III) reagent.

Chiral Hypervalent Iodines: Active Players in Asymmetric Synthesis

Asymmetric organocatalytic oxidations have been witnessed to an impressive development in the last years thanks to the establishment of important chiral hypervalent iodines(III/V). Many different approaches involving both stoichiometric and catalytic versions have provided a fundamental advance in this area within asymmetric synthesis. The easily handing, nontoxic, mild, environmentally friendly (green oxidants), and high stability that are features of these reagents have been applied to many reactions and also have allowed exploring further unprecedented enantioselective transformations. The intention of the present review is thus to highlight as a whole the many approaches utilized up to date to prepare chiral iodines(III/V), as well as their reactivity in a comprehensive manner.

Hypervalent iodane mediated reactions of N-acetyl enamines for the synthesis of oxazoles and imidazoles

A hypervalent iodane reagent used for the intramolecular cyclization of N-acetyl enamines and intermolecular cyclocondensation of enamines and nitriles was investigated. The reaction was performed under mild conditions and gave oxazoles and imidazoles, respectively, in moderate to excellent yields. This transformation exhibits good reactivity, selectivity and functional group tolerance. The selectivity of the intra- or intermolecular reaction is dependent on the structure of N-acetyl enamines.

Enantioselective 5-exo-Fluorocyclization of Ene-Oximes

The enantioselective 5-exo-fluorocyclization of ene-oxime compounds was demonstrated under phase-transfer catalysis. Although deprotonative fluorinations competed, the chemical yields and the ee values of the desired isoxazoline products were generally moderate to good. The absolute stereochemistry of the major isomer was determined to be S by comparison with the literature after transformation of the product to the corresponding iodinated isoxazoline.

Alkene Difunctionalization Using Hypervalent Iodine Reagents: Progress and Developments in the Past Ten Years

Hypervalent iodine reagents are of considerable relevance in organic chemistry as they can provide a complementary reaction strategy to the use of traditional transition metal chemistry. Over the past two decades, there have been an increasing number of applications including stoichiometric oxidation and catalytic asymmetric variations. This review outlines the main advances in the past 10 years in regard to alkene heterofunctionalization chemistry using achiral and chiral hypervalent iodine reagents and catalysts.

α-Arylation of Carbonyl Compounds through Oxidative C-C Bond Activation

A synthetically useful approach for the direct α-arylation of carbonyl compounds through a novel oxidative C-C bond activation is reported. This mechanistically unusual process relies on a 1,2-aryl shift and results in all-carbon quaternary centers. The transformation displays broad functional-group tolerance and can in principle also be applied as an asymmetric variant.

Highly diastereoselective synthesis of enantioenriched anti-α-allyl-β-fluoroamines

A highly diastereoselective synthesis of anti-α-allyl-β-fluoroamines has been developed involving enantioselective α-fluorination of aldehydes followed by a diastereoselective Petasis allyl borono-Mannich reaction. The products are obtained generally in good overall yields for the two steps and with drs of 97 : 3-99 : 1 and ees of 86-92%. Selected products were converted to 3-, 5- and 6-membered ring heterocycles, the latter two types incorporating an exo-cyclic fluorine.

Catalytic, Enantioselective 1,2-Difluorination of Cinnamamides

The enantio- and diastereoselective synthesis of 1,2-difluorides via chiral aryl iodide-catalyzed difluorination of cinnamamides is reported. The method uses HF-pyridine as a fluoride source and mCPBA as a stoichiometric oxidant to turn over catalyst, and affords compounds containing vicinal, fluoride-bearing stereocenters. Selectivity for 1,2-difluorination versus a rearrangement pathway resulting in 1,1-difluorination is enforced through anchimeric assistance from a N- tert-butyl amide substituent.

A cheap metal for a challenging task: nickel-catalyzed highly diastereo- and enantioselective hydrogenation of tetrasubstituted fluorinated enamides

Nickel-catalyzed asymmetric hydrogenation of challenging tetrasubstituted fluorinated enamides has been achieved, affording chiral α-fluoro-β-amino esters in high yields with excellent diastereo- and enantioselectivities (up to 98% yield, >99 : 1 dr, up to >99% ee). Deuterium-labeling experiments and control experiments were conducted to probe the mechanism, and the results indicated that the acidity of the solvent plays a critical role in the control of diastereoselectivity by trapping the adduct of nickel hydride to C[double bond, length as m-dash]C bonds via protonolysis, giving the hydrogenation product with stereospecific syn-selectivity. This protocol provides efficient access to chiral α-fluoro-β-amino esters which have important potential applications in organic synthesis and medicinal chemistry.

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.

Difluorination of Functionalized Aromatic Olefins Using Hypervalent Iodine/HF Reagents

The hypervalent iodine/HF reagent consisting of PhIO and HF·py was found to be effective for fluorination of functionalized aromatic olefins bearing synthetically important carbonyl and hydroxyl groups. Fluorination of 1,3-diphenyl-2-propen-1-one with PhIO/HF·py reagent in CH₂Cl₂ at room temperature gave 3,3-difluoro-1,2-diphenyl-1-propanone in high yield. Other α-aryl-α,β-unsaturated ketones underwent the fluorination to yield aryl 2,2-difluoroethyl ketone derivatives in good to high yields. Catalytic fluorination of α-aryl-α,β-unsaturated ketones using a p-TolI/HF·py/mCPBA reagent system also worked well. Moreover, the fluorination of cinnamyl alcohol derivatives by PhIO/HF·py reagent proceeded smoothly to afford 2-aryl-3,3-difluoro-1-propanols in moderate yields.

Diastereoselective α-Fluorination of N- tert-Butanesulfinyl Imidates

A diastereoselective α-fluorination of N- tert-butanesulfinyl imidates was developed. Deprotonation of N- tert-butanesulfinyl imidates with lithium hexamethyldisilazide generates aza-enolates that can be intercepted, with excellent diastereocontrol, by the inexpensive electrophilic fluorinating agent NFSI. This protocol was applied to the preparation of synthetically useful trans-2-fluoro-cyclohexamine with high enantiomeric purity (99.5% ee).

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.

Enantioselective, Catalytic Vicinal Difluorination of Alkenes

The enantioselective, catalytic vicinal difluorination of alkenes is reported by I^I /I^III catalysis using a novel, C₂ -symmetric resorcinol derivative. Catalyst turnover via in situ generation of an ArI^III F₂ species is enabled by Selectfluor oxidation and addition of an inexpensive HF-amine complex. The HF:amine ratio employed in this process provides a handle for regioselective orthogonality as a function of Brønsted acidity. Selectivity reversal from the 1,1-difluorination pathway (geminal) to the desired 1,2-difluorination (vicinal) is disclosed (>20:1 in both directions). Validation with electron deficient styrenes facilitates generation of chiral bioisosteres of the venerable CF₃ unit that is pervasive in drug discovery (20 examples, up to 94:06 e.r.). An achiral variant of the reaction is also presented using p-TolI (up to >95 % yield).