Migration Of Aryl Group Research Articles

Photoredox Catalyzed Conia-Ene-Type Cyclization/Smiles Rearrangement Cascade Reactions to Access Substituted Methylenecarbocycles.

We report a novel visible-light-driven photoredox-catalyzed cascade reaction involving Conia-ene-type cyclization and Smiles rearrangement initiated from alkyne-tethered α-sulfonyl esters. This methodology not only facilitates the rapid synthesis of a broad spectrum of highly substituted methylenecarbocycles but also introduces a new mechanistic pathway with aryl group migration, surpassing the conventional 1,5-hydrogen shift typically observed in Conia-ene reactions.

Site-selective α-C(sp3)-H arylation of dialkylamines via hydrogen atom transfer catalysis-enabled radical aryl migration.

Site-selective C(sp3)-H arylation is an appealing strategy to synthesize complex arene structures but remains a challenge facing synthetic chemists. Here we report the use of photoredox-mediated hydrogen atom transfer (HAT) catalysis to accomplish the site-selective α-C(sp3)-H arylation of dialkylamine-derived ureas through 1,4-radical aryl migration, by which a wide array of benzylamine motifs can be incorporated to the medicinally relevant systems in the late-stage installation steps. In contrast to previous efforts, this C-H arylation protocol exhibits specific site-selectivity, proforming predominantly on sterically more-hindered secondary and tertiary α-amino carbon centers, while the C-H functionalization of sterically less-hindered N-methyl group can be effectively circumvented in most cases. Moreover, a diverse range of multi-substituted piperidine derivatives can be obtained with excellent diastereoselectivity. Mechanistic and computational studies demonstrate that the rate-determining step for methylene C-H arylation is the initial H atom abstraction, whereas the radical ipso cyclization step bears the highest energy barrier for N-methyl functionalization. The relatively lower activation free energies for secondary and tertiary α-amino C-H arylation compared with the functionalization of methylic C-H bond lead to the exceptional site-selectivity.

Photoinduced Group Transposition via Iridium‐Nitrenoid Leading to Amidative Inner‐Sphere Aryl Migration

AbstractWe herein report a fundamental mechanistic investigation into photochemical metal‐nitrenoid generation and inner‐sphere transposition reactivity using organometallic photoprecursors. By designing Cp*Ir(hydroxamate)(Ar) complexes, we induced photo‐initiated ligand activation, allowing us to explore the amidative σ(Ir–aryl) migration reactivity. A combination of experimental mechanistic studies, femtosecond transient absorption spectroscopy, and density functional theory (DFT) calculations revealed that the metal‐to‐ligand charge transfer enables the σ(N−O) cleavage, followed by Ir‐acylnitrenoid generation. The final inner‐sphere σ(Ir–aryl) group migration results in a net amidative group transposition.

Photocatalytic Generation of Trifluoromethyl Nitrene for Alkene Aziridination

AbstractN‐Trifluoromethylated organics may be applied in drug design, agrochemical synthesis, and materials science, among other areas. Yet, despite recent advances in the synthesis of aliphatic, cyclic and heterocyclic N‐trifluoromethyl compounds, no strategy based on trifluoromethyl nitrene has hitherto been explored. Here we describe the formation of triplet trifluoromethyl nitrene from azidotrifluoromethane, a stable and safe‐to‐use precursor, by visible light photocatalysis. The addition of CF3N to alkenes via biradical intermediates afforded previously unknown aziridines substituted with trifluoromethyl group on the nitrogen atom. The obtained aziridines were converted into either N‐trifluoromethylimidazolines, via formal [3+2] cycloaddition with nitriles, mediated by a Lewis acid, or into N‐trifluoromethylaldimines, via ring opening and aryl group migration mediated by a strong Brønsted acid. Our findings open new opportunities for the development of novel classes of N‐CF3 compounds with possible applications in the life sciences.

Photocatalytic Generation of Trifluoromethyl Nitrene for Alkene Aziridination.

N-Trifluoromethylated organics may be applied in drug design, agrochemical synthesis, and materials science, among other areas. Yet, despite recent advances in the synthesis of aliphatic, cyclic and heterocyclic N-trifluoromethyl compounds, no strategy based on trifluoromethyl nitrene has hitherto been explored. Here we describe the formation of triplet trifluoromethyl nitrene from azidotrifluoromethane, a stable and safe-to-use precursor, by visible light photocatalysis. The addition of CF3 N to alkenes via biradical intermediates afforded previously unknown aziridines substituted with trifluoromethyl group on the nitrogen atom. The obtained aziridines were converted into either N-trifluoromethylimidazolines, via formal [3+2] cycloaddition with nitriles, mediated by a Lewis acid, or into N-trifluoromethylaldimines, via ring opening and aryl group migration mediated by a strong Brønsted acid. Our findings open new opportunities for the development of novel classes of N-CF3 compounds with possible applications in the life sciences.

New synthesis of β-aryl-GABA drugs

A new synthetic method for the preparation of β-aryl-GABA drugs is reported. The key step for the construction of α,β-unsaturated-β-aryl-γ-lactam derivatives relies on a novel cascade radical reaction. An N-propargyl aryl amide was rapidly constructed starting from an aryl carboxylic acid and propargyl amine. Protection of the amide N–H with a Boc group, followed by a cascade radical aryl migration reaction, produced the key intermediate, an unsaturated β-aryl-γ-lactam. Further catalytic hydrogenation and deprotection gave rolipram. Meanwhile, coupling of the glycine methyl ester and propargyl bromide, followed by benzoylation gave the radical precursor. The radical reaction resulted in the production of the unsaturated β-aryl-γ-lactam with the glycine moiety. After aminolysis reactions, phenylpiracetam and phenylpiracetam hydrazide were produced, respectively. This synthetic protocol provides a general and simple approach for preparing racemates or chiral forms of β-aryl-GABA drugs and analogues.

Coordination-Induced Migration of Aryl Groups in the Reactions of a Dialkylsilanone with a Series of Triarylboranes.

Although carbonyl compounds are activated by the coordination of a Lewis acid to the carbonyl oxygen atom, a similar activation process of R2 Si=O species remains unclear. We report herein the reactions of a silanone (1, Scheme 1) with a series of triarylboranes to afford the corresponding boroxysilanes. Experimental results and computational studies indicate that the electrophilicity of the unsaturated silicon atom is enhanced by the complexation of 1 with triarylboranes, and the subsequent aryl migration from the boron atom to the electrophilic silicon atom proceeds.

Boron Trifluoride Etherate-Controlled Reactions of Methyl Enol Ethers: Selective Synthesis of Dihydrofuro[3,2-c]chromenone and Furo[3,2-c]chromenone Derivatives

AbstractA facile one-step synthetic approach to dihydrofuro-[3,2-c]chromenones and furo[3,2-c]chromenones by the reaction of methyl enol ethers with 4-hydroxycoumarins under metal-free conditions is presented. Dihydrofuro[3,2-c]chromenones and furo[3,2-c]-chromenones were selectively obtained by controlling the stoichiometry of boron trifluoride diethyl etherate. An unexpected aryl-group migration followed by aromatization of the furan moiety, leading to a variety of furo[3,2-c]chromenone derivatives in good yields, is reported.

Aryl Transfer Strategies Mediated by Photoinduced Electron Transfer.

Radical aryl migrations are powerful techniques to forge new bonds in aromatic compounds. The growing popularity of photoredox catalysis has led to an influx of novel strategies to initiate and control aryl migration starting from widely available radical precursors. This review encapsulates progress in radical aryl migration enabled by photochemical methods─particularly photoredox catalysis─since 2015. Special attention is paid to descriptions of scope, mechanism, and synthetic applications of each method.

Radical 1,4-Aryl Migration Enabled Remote Cross-Electrophile Coupling of α-Amino-β-Bromo Acid Esters with Aryl Bromides.

We report an unprecedented, efficient nickel-catalysed radical relay for the remote cross-electrophile coupling of β-bromo-α-benzylamino acid esters with aryl bromides via 1,4-aryl migration/arylation cascades. β-Bromo-α-benzylamino acid esters are considered as unique molecular scaffolds allowing for aryl migration reactions, which are conceptually novel variants for the radical Truce-Smiles rearrangement. This reaction enables the formation of two new C(sp3 )-C(sp2 ) bonds using a bench-stable Ni/bipyridine/Zn system featuring a broad substrate scope and excellent diastereoselectivity, which provides an effective platform for the remote aryl group migration and arylation of amino acid esters via redox-neutral C(sp3 )-C(sp2 ) bond cleavage. Mechanistically, this cascade reaction is accomplished by combining two powerful catalytic cycles consisting of a cross-electrophile coupling and radical 1,4-aryl migration through the generation of C(sp3 )-centred radical intermediates from the homolysis of C(sp3 )-Br bonds and the switching of the transient alkyl radical into a robust α-aminoalkyl radical.

Radical 1,4‐Aryl Migration Enabled Remote Cross‐Electrophile Coupling of α‐Amino‐β‐Bromo Acid Esters with Aryl Bromides

AbstractWe report an unprecedented, efficient nickel‐catalysed radical relay for the remote cross‐electrophile coupling of β‐bromo‐α‐benzylamino acid esters with aryl bromides via 1,4‐aryl migration/arylation cascades. β‐Bromo‐α‐benzylamino acid esters are considered as unique molecular scaffolds allowing for aryl migration reactions, which are conceptually novel variants for the radical Truce–Smiles rearrangement. This reaction enables the formation of two new C(sp3)−C(sp2) bonds using a bench‐stable Ni/bipyridine/Zn system featuring a broad substrate scope and excellent diastereoselectivity, which provides an effective platform for the remote aryl group migration and arylation of amino acid esters via redox‐neutral C(sp3)−C(sp2) bond cleavage. Mechanistically, this cascade reaction is accomplished by combining two powerful catalytic cycles consisting of a cross‐electrophile coupling and radical 1,4‐aryl migration through the generation of C(sp3)‐centred radical intermediates from the homolysis of C(sp3)−Br bonds and the switching of the transient alkyl radical into a robust α‐aminoalkyl radical.

Radical Aryl Migration from Boron to Carbon.

Radical aryl migration reactions represent a unique type of organic transformations that involve the intramolecular migration of an aryl group from a carbon or heteroatom to a C- or heteroatom-centered radical through a spirocyclic intermediate. Various elements, including N, O, Si, P, S, Sn, Ge, and Se, have been reported to participate in radical aryl migrations. However, radical aryl migration from a boron center has not been reported to date. In this communication, radical 1,5-aryl migration from boron to carbon in aryl boronate complexes is presented. C-radicals readily generated through radical addition onto alkenyl aryl boronate complexes are shown to engage in 1,5-aryl migration reactions to provide 4-aryl-alkylboronic esters. As boronate complexes can be generated in situ by the reaction of alkenylboronic acid esters with aryl lithium reagents, the aryl moiety is readily varied, providing access to a series of arylated products starting from the same alkenylboronic acid ester via divergent chemistry. Reactions proceed with high diastereoselectivity under mild conditions, and also the analogous 1,4-aryl shifts are feasible. The suggested mechanism is supported by DFT calculations.

Copper-Catalyzed Radical Aryl Migration Approach for the Preparation of Cyanoalkylsulfonylated Oxindoles/Cyanoalkyl Amides.

A copper-catalyzed radical cross-coupling of oxime esters and activated alkenes is accomplished for the synthesis of cyanoalkylsulfonylated oxindoles and cyanoalkyl amides via an aryl migration strategy. Specifically, the subsequent mechanism research indicates that the unique desulfonylation and sulfone addition processes were involved in the transformation. This transformation is identified as having good functional group applicability with two different quaternary stereocenter in a regioselective manner, which is controlled by the substituent group of the nitrogen.

Mobile aryl groups on graphene

Molecular Diffusion Phenyl groups on aromatic rings can migrate to an adjacent carbon atom under acidic conditions through an ipso arenium ion intermediate. He and Swager show that analogous migration of aryl groups on p-doped graphene surfaces can occur if functional groups weaken the bond to graphene. They patterned graphene with a copper mask to create circular islands of phenyl, 4-trifluoromethylphenyl, and 4-methoxyphenyl groups. Upon oxidative etching of the copper, which leads to p-doping of the graphene, Raman imaging showed that the aryl pattern remained for the first two groups. For 4-methoxyphenyl, migration of these groups several micrometers from their original position occurred through a random walk. J. Am. Chem. Soc. 142 , 17876 (2020).

FeCl3 catalyzed intermolecular reaction between enol ethers and anilines: Access to 2,3-substituted indoles through aryl group migration

An intermolecular FeCl3 catalyzed reaction between anilines and enol ethers is described. A variety of enol ethers and aromatic amines undergo a CC bond formation followed by cyclization via CN bond formation to afford the 2,3-disubstituted indoles, involving an unexpected aryl group migration. In this methodology, anilines act as bis-nucleophiles, wherein the initial attack occurs at the α-position of enol ether from the ortho position of aniline followed by the subsequent reaction of the amine moiety of aniline at the β-position, leading to the indole framework. This method is simple, obviates the use of expensive/hazardous transition-metal catalysts, and offers a wide range of substrate scope.

Copper-Catalyzed Oxidative Fragmentation of Alkynes with NFSI Provides Aryl Ketones.

A copper-catalyzed oxidative cleavage reaction of alkynes using NFSI and TBHP was described. Various terminal and internal alkyne substrates were employed to render quick access to aryl ketone products in moderate to good yields. NFSI not only functioned as N-centered radical precursors but also engaged in the aryl group migration. Mechanistic studies also suggested the important role of water in the title reactions.

Fluoroalkylation of Allylic Alcohols with Concomitant (Hetero)aryl Migration: Access to Fluoroalkylated Ketones and Evaluation of Antifungal Action against Magnaporthe grisea

Fluoroalkyl(hetero)arylation of allylic alcohols with perfluoroalkyl iodide or difluoroacetic acid as fluoroalkyl source was developed herein, delivering functionalized ketones containing an α‐quaternary center. Formation of the fluoroalkyl radical was followed by its intermolecular addition to alkenes and the migration of a (hetero)aryl group with the concomitant generation of a ketone group to finish the domino sequence. Mechanistic studies indicated that the aryl migration proceeded through neophyl rearrangement in a radical pathway. Moreover, preliminary antifungal evaluation against Magnaporthe grisea is also described for the first time.

Acid‐catalyzed decomposition and stability of diazofuranones: Experimental and mechanistic study

AbstractReactions of substituted 4‐diazotetrahydrofurane‐3‐ones with various acids (pKa < 3) irrespective of acid strength give rise to elimination of nitrogen with subsequent 1,2‐nucleophilic migration of aryl (alkyl) groups from C‐5 to C‐4 position of heterocyclic core with quantitative formation of regioisomeric 3(2Н)furanones (yields up to 99%). Regioselectivity of this acid‐catalyzed process increases with the decreasing of solvent polarity and strongly depends on the nature of substituents in the aryl groups. The suggested mechanism of this reaction via intermediate RN2+ cation was confirmed by density functional theory (DFT) calculations at the PBE0/6‐31+G(d) functional due to good agreement between calculated and experimental data on the acid stability of diazoketones in different solvents.

Theoretical Study of Substituent Effect in Aryl Group Migration in (para-C6H4X)Mn(CO)5 Complexes

In this study, we report substituent effect on aryl group migration in (para-C6H4X)Mn(CO)5 complexes using mpw1pw91 quantum chemical calculations. These calculations reveal good linear relationships between barrier energy (ΔE), activation energy (ΔH‡), activation free energy (ΔG‡) values and rate constants with Hammett constants of X-substituents. The occupancy values of Mn–COcis and Mn–C(O)-(para-C6H4X) bonds in reactant, transition state and product were calculated by Natural bond orbital (NBO) method.

Enantioselective 1,2-Anionotropic Rearrangement of Acylsilane through a Bisguanidinium Silicate Ion Pair.

Highly enantioselective bisguanidinium-catalyzed tandem rearrangements of acylsilanes are reported. The acylsilanes were activated via an addition of fluoride on the silicon to form a penta-coordinate anionic silicate intermediate. The silicate then underwent alkyl or aryl group migration from the silicon atom to the neighboring carbonyl carbon atom (1,2-anionotropic rearrangement), followed by [1,2]-Brook rearrangement to provide the secondary alcohols in high yields with excellent enantioselectivities (up to 95% ee). The isolation of an α-silylcarbinol intermediate as well as DFT calculations revealed that the 1,2-anionotropic rearrangement occurred via a bisguanidinium silicate ion pair, which is the stereodetermining step. The chiral center formed is then retained without inversion through the subsequent [1,2]-Brook rearrangement. Crotyl acylsilanes were smoothly transformed into homoallylic linear crotyl alcohols with retention of E/Z geometry, and no branched alcohols were detected. This clearly suggested that the 1,2-anionotropic rearrangement occurred through a three-membered instead of a five-membered transition state.