Asymmetric Dearomatizing Spirolactonization of Naphthols Catalyzed by Spirobiindane-Based Chiral Hypervalent Iodine Species

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.

Silver-Catalyzed, Chemo- and Enantioselective Intramolecular Dearomatization of Indoles to Access Sterically Congested Azaspiro Frameworks

An asymmetric dearomatization of indoles bearing α-diazoacetamide functionalities was developed for synthesizing high-value spiro scaffolds. A silver phosphate chemoselectively catalyzed the sterically challenging dearomatization, whereas more typically used metal catalysts for carbene transfer reactions, such as a rhodium complex, were not effective and instead resulted in a Büchner ring expansion or cyclopropanation. Mechanistic studies indicated that the spirocyclization occurred through a silver-assisted asynchronous concerted process and not via a silver-carbene intermediate. Analyses based on natural bond orbital population and a distortion/interaction model indicated that the degree of C-Ag mutual interaction is crucial for achieving a high level of enantiocontrol. In addition, an oxidative disconnection of a C(sp³)-C(sp²) bond in the product provided unconventional access to the corresponding chiral spirooxindole.

Hypervalent Iodine(III)-Catalysed Enantioselective α-Acetoxylation of Ketones

An enantioselective catalytic synthesis of α-acetoxylated ketones through I(I)/I(III) catalysis using a resorcinol/lactamide-based chiral iodoarene is reported. Catalyst turnover by in situ generation of the active iodine(III) derivative is achieved by oxidation with mCPBA in the presence of acetic acid. The prior transformation of ketones to easily accessible acetyl enol ethers is beneficial and yields up to 97 % with enantioselectivities up to 88 % ee are obtained using only low catalyst loadings of only 5 mol % under mild reaction conditions.

Synthesis of Polycyclic Cyclohexadienone through Alkoxy-Oxylactonization and Dearomatization of 3'-Hydroxy-[1,1'-biphenyl]-2-carboxylic Acids Promoted by Hypervalent Iodine

Alkox-oxylactonization and dearomatization of 3'-hydroxy-[1,1'-biphenyl]-2-carboxylic acid simultaneously promoted by hypervalent iodine have been developed using stoichiometric PhI(OAc)₂ or a catalytic amount of chiral aryl-λ³-iodane generated in situ. This reaction provides a concise method to synthesize diverse polycyclic cyclohexadienones as potential inhibitors of DNA polymerase under mild reaction conditions.

A Triazole-Substituted Aryl Iodide with Omnipotent Reactivity in Enantioselective Oxidations

A widely applicable triazole‐substituted chiral aryl iodide is described as catalyst for enantioselective oxidation reactions. The introduction of a substituent in ortho‐position to the iodide is key for its high reactivity and selectivity. Besides a robust and modular synthesis, the main advantage of this catalyst is the excellent performance in a plethora of mechanistically diverse enantioselective transformations, such as spirocyclizations, phenol dearomatizations, α‐oxygenations, and oxidative rearrangements. DFT‐calculations of in situ generated [hydroxy(tosyloxy)iodo]arene isomers give an initial rational for the observed reactivity.

Catalytic asymmetric multiple dearomatizations of phenols enabled by a cascade 1,8-addition and Diels-Alder reaction

A direct catalytic asymmetric multiple dearomatization reaction of phenols was disclosed, which provides expedient access to a series of architecturally complex polycyclic compounds bearing four stereogenic centers in high enantiopurity. The key to achieve such a transformation is the combination of a dearomative 1,8-addition of β-naphthols to para-quinone methides generated in situ from propargylic alcohols and a subsequent intramolecular dearomative Diels–Alder reaction. Noteworthily, this protocol enrichs not only the diversity of dearomatized products but also the toolbox of dearomatization strategies.

The first chiral phosphoric acid catalyzed asymmetric multiple dearomatizations of phenols for the synthesis of bridged polycyclic compounds are reported.

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.

Dearomatization of Electron-Deficient Phenols to ortho-Quinones: Bidentate Nitrogen-Ligated Iodine(V) Reagents

Despite their broad utility, the synthesis of ortho-quinones remains a significant challenge, in particular, access to electron-deficient derivatives remains an unsolved problem. Reported here is the first general method for the synthesis of electron-deficient ortho-quinones by direct oxidation of phenols. The reaction is enabled by a novel bidentate nitrogen-ligated iodine(V) reagent, a previously unexplored class of compounds which we have termed Bi(N)-HVIs. The reaction is extremely general and proceeds with excellent regioselectivity for the ortho over para isomer. Functionalization of the ortho-quinone products was examined, resulting in a facile one-pot synthesis of catechols, as well as the incorporation of a variety of heteroatom nucleophiles. This method represents the first synthetic application of Bi(N)-HVIs and demonstrates their potential as a platform for the further development of highly reactive, but also highly tunable, I(V) reagents.

Chiral phosphoric acid catalyzed aminative dearomatization of α-naphthols/Michael addition sequence

Asymmetric dearomatization reactions have recently emerged as a powerful tool for the rapid build-up of the molecular complexity. Chiral three-dimensional polycyclic molecules bearing contiguous stereogenic centers can be synthesized from readily available planar aromatic feedstocks. Here we report that an intermolecular asymmetric dearomatization reaction of α-naphthols bearing a tethered nucleophile at the C4 position of the naphthol ring is achieved by a chiral phosphoric acid. The reaction proceeds via a highly chemo- and regioselective aminative dearomatization/Michael addition sequence, affording a wide array of functionalized cyclic ketones in good yields (up to 93%) with excellent enantioselectivity (up to >99% ee). The catalyst loading can be reduced to 0.1 mol%. Preliminary mechanistic investigations identify that the enantioselectivity is established in the dearomatization step, while the Michael addition is the rate-limiting step. A working model accounting for the origin of the stereochemistry is proposed based on DFT calculations.

On the origins of stereoselectivity in the aminocatalytic remote alkylation of 5-alkylfurfurals

In the manuscript, computational studies on the remote alkylation of 5-alkylfurfurals proceeding via formation of the corresponding trienamine intermediate are presented. By the means of density functional theory (DFT) calculations and the symmetry-adapted perturbation theory (SAPT) method, interesting insights into the mechanism of the reaction have been provided explaining the influence and contribution of different molecular interactions on the observed reactivity as well as on the enantio- and diastereoselectivity of the process. The studies have been extended to the thiophene analogue of the starting furfural derivative and the results obtained verified experimentally.

Rhodium(III)-Catalyzed [4+2] Annulation via C-H Activation: Synthesis of Multi-Substituted Naphthalenone Sulfoxonium Ylides

A convenient Rh(III)-catalyzed C-H activation and cascade [4+2] annulation for the synthesis of naphthalenone sulfoxonium ylides has been developed. This method features perfect regioselectivity, mild and redox-neutral reaction conditions, and broad substrate tolerance with good to excellent yields. Preliminary mechanistic experiments were conducted and a plausible reaction mechanism was proposed. The new type naphthalenone sulfoxonium ylides could be further transformed into multi-substituted naphthols, which demonstrates the practical utility of this methodology.

DFT mechanistic investigation into phenol dearomatization mediated by an iodine(iii) reagent

Density functional theory (DFT) was utilized to investigate the mechanistic aspects of the oxidative dearomatization of phenols mediated by an iodine(iii) reagent. In this article, we will show that the conventional mechanism in which an iodine(iii) phenolate is proposed as the key intermediate is not operative, and the process is promoted if the phenolate ligand is dearomatized on the iodine(iii) center. The dearomatized phenolate is calculated to be a more potent reductant than phenolate itself. In such a case, the reaction is capable of proceeding via two competitive mechanisms (dissociative and associative). Consistent with the experimental findings, we found that while the less polar solvents considerably disfavor the dissociative mechanism, they have an insignificant effect on the associative one. The energetic order of these two mechanisms is calculated to be influenced by the nature of the counter anion coordinated to the iodine(iii) center.

Synthesis, characterisation, and reactivity of novel pseudocyclic hypervalent iodine reagents with heteroaryl carbonyl substituents

New heteroatom-containing pseudocyclic iodanes are being introduces as alternatives to the Koser reagent.

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.

Chiral Cyclohexyl-Fused Spirobiindanes: Practical Synthesis, Ligand Development, and Asymmetric Catalysis

1,1'-Spirobiindane has been one type of privileged skeleton for chiral ligand design, and 1,1'-spirobiindane-based chiral ligands have demonstrated outstanding performance in various asymmetric catalysis. However, the access to enantiopure spirobiindane is quite tedious, which obstructs its practical application. In the present article, a facile enantioselective synthesis of cyclohexyl-fused chiral spirobiindanes has been accomplished, in high yields and excellent stereoselectivities (up to >99% ee), via a sequence of Ir-catalyzed asymmetric hydrogenation of α,α'-bis(arylidene)ketones and TiCl₄ promoted asymmetric spiroannulation of the hydrogenated chiral ketones. The protocol can be performed in one pot and is readily scalable, and has been utilized in a 25 g scale asymmetric synthesis of cyclohexyl-fused spirobiindanediol (1 S,2 S,2' S)-5, in >99% ee and 67% overall yield for four steps without chromatographic purification. Facile derivations of (1 S,2 S,2' S)-5 provided straightforward access to chiral monodentate phosphoramidites 6a-c and a tridentate phosphorus-amidopyridine 11, which were evaluated as chiral ligands in several benchmark enantioselective reactions (hydrogenation, hydroacylation, and [2 + 2] reaction) catalyzed by transition metal (Rh, Au, or Ir). Preliminary results from comparative studies showcased the excellent catalytic performances of these ligands, with a competency essentially equal to the corresponding well-established privileged ligands bearing a regular spirobiindane backbone. X-ray crystallography revealed a close resemblance between the structures of the precatalysts 20 and 21 and their analogues, which ultimately help to rationalize the almost identical stereochemical outcomes of reactions catalyzed by metal complexes of spirobiindane-derived ligands with or without a fused cyclohexyl ring on the backbone. This work is expected to stimulate further applications of this type of readily accessible skeletons in development of chiral ligands and functional molecules.