Halogen ‘dance’: a way to extend the boundaries of arene deprotolithiation

Unsymmetrization of 1,8-Dibromonaphthalenes by Acid-Induced Halogen Dance Reaction

A simple and powerful tool for preparing uncommon bromoarenes via the unsymmetrization of a naphthalene ring was developed. The steric repulsion between the peri-bromo groups of 1,8-dibromonaphthalene distorts the naphthalene ring, allowing for nonelectronical activation. Ring distortion facilitates the 1,2-rearrangement of the bromo group, affording 1,7-dibromonaphthalene upon treatment with trifluoromethanesulfonic acid (halogen dance reaction). For 1,4,5,8-tetrabromonaphthalene, stepwise 1,2-rearrangements proceeded successively to furnish 1,3,5,7-tetrabromonaphthalene. Density functional theory calculations suggest that this reaction is initiated by ipso-protonation, with a subsequent 1,2-rearrangement occurring via the bromonium transition state. Utilizing 1,7-dibromonaphthalene, which is characterized by two C-Br bonds arranged at a 60° angle, a unique metal-organic framework comprising a 52-membered ring network was synthesized.

Ultrafast Halogen Dance Reactions of Bromoarenes Enabled by Catalytic Potassium Hexamethyldisilazide

Lochmann-Schlosser base, a stoichiometric combination of nBuLi and KOtBu, is commonly used as a superbase for deprotonating a wide range of organic compounds. In the present study, we report that catalytic potassium hexamethyldisilazide (KHMDS) exhibits higher catalytic activity than KOtBu for successive bromine-metal exchanges. Accordingly, 1-10 mol% of KHMDS dramatically enhances halogen dance reactions to introduce various electrophiles to bromopyridine, bromoimidazole, bromothiophene, bromofuran, and bromobenzene derivatives with the bromo group translocated from the original position. A dual catalytic cycle is proposed to explain the ultrafast bromine transfer.

Formal Halogen Transfer of Bromoarenes via Stepwise Reactions

A two-step halogen transfer of bromoarenes is reported. Mono-, di-, and tribromoaryllithiums generated through deprotonative lithiation were converted into organozinc species by in situ zincation, which were then subjected to bromination to provide the corresponding di-, tri-, and tetrabromoarenes, respectively, in 41-95% yields. Regioselective bromine-magnesium exchange with ethylmagnesium chloride followed by electrophilic trapping afforded benzene, pyridine, quinoline, pyrimidine, and thiazole derivatives with the bromo group translocated from the original position in 28-86% yields.

Deprotonative Generation and Trapping of Haloaryllithium in a Batch Reactor

A method for the regioselective functionalization of haloarenes through deprotonative lithiation is disclosed. The generated haloaryllithiums were trapped in a batch reactor with a zinc chloride diamine complex to provide organozinc species without aryne formation, which reacted with electrophiles to afford the corresponding products in 38-98% yields. This method was applied to the five-step total synthesis of carbazomycin A on a gram scale in 33% overall yield.

From ferrocene to decasubstituted enantiopure ferrocene-1,1'-disulfoxide derivatives

The functionalization of (R,R)-S,S'-di-tert-butylferrocene-1,1'-disulfoxide by deprotolithiation-electrophilic trapping sequences was studied towards polysubstituted, enantiopure derivatives for which the properties were determined. While the 2,2'-disubstituted ferrocene derivatives were obtained as expected, subsequent functionalization of the 2,2'-di(phenylthio) and 2,2'-bis(trimethylsilyl) derivatives occurred primarily at the 4- or 4,4'-positions. This unusual regioselectivity was discussed in detail in light of pK_a values and structural data. The less sterically hindered 2,2'-difluorinated derivative yielded the expected 1,1',2,2',3,3'-hexasubstituted ferrocenes by the deprotometallation-trapping sequence. Further functionalization proved possible, leading to early examples of 1,1',2,2',3,3',4,4'-octa, nona and even decasubstituted ferrocenes. Some of the newly prepared ferrocene-1,1'-disulfoxides were tested as ligands for enantioselective catalysis and their electrochemical properties were investigated.

Regiocontrolled halogen dance of 2,5-dibromopyrroles using equilibrium between dibromopyrrolyllithiums

A regiocontrolled halogen dance reaction of 2,5-dibromopyrroles is described. An N,N-dimethylsulfamoyl group on the pyrrole nitrogen was especially effective for facilitating interconversion of the resulting 2,3- and 2,4-dibromopyrrolyllithiums, rendering the smooth halogen dance reaction. This method was applicable to the formal synthesis of atorvastatin.

Continuous‐Flow Divergent Lithiation of 2,3‐Dihalopyridines: Deprotolithiation versus Halogen Dance

We describe herein the first halogen dance (HD) in continuous flow on 2-chloro-3-bromopyridine by selectively trapping a (pyridin-4-yl)lithium species that is known to undergo the halogen-dance process. In addition, this lithiated intermediate was trapped at lower temperature before the HD occurs. The HD process was extended to fluoro-iodopyridines by using various electrophiles to afford 28 examples with yields ranging from 42 to 97 % with very short residence times. Finally, scale up of the reaction was demonstrated, affording a promising space-time yield (STY) of 4.2 kg.h^-1 .L^-1 .

Synthesis of Polysubstituted Ferrocenesulfoxides

The purpose of the study is to design synthetic methodologies, especially directed deprotometalation using polar organometallic reagents, to access polysubstituted ferrocenesulfoxides. From enantiopure 2-substituted (SiMe₃, PPh₂) S-tert-butylferrocenesulfoxides, a third substituent was first introduced at the 5 position (SiMe₃, I, D, C(OH)Ph₂, Me, PPh₂, CH₂NMe₂, F) and removal of the trimethylsilyl group then afforded 2-substituted ferrocenesulfoxides unreachable otherwise. Attempts to apply the “halogen dance” reaction to the ferrocenesulfoxide series led to unexpected results although rationalized in light of calculated pK_a values. Further functionalizations were also possible. Thus, new enantiopure, planar chiral di- and trisubstituted ferrocenes have been obtained, in addition to several original 2-substituted, 2,3- and 2,5-disubstituted, 2,3,5-trisubstituted and even 2,3,4,5-tetrasubstituted ferrocenesulfoxides, also enantiopure.

Direct C-H Carboxylation Forming Polyfunctionalized Aromatic Carboxylic Acids by Combined Brønsted Bases

CO₂ fixation into electron-deficient aromatic C-H bonds proceeds with the combined Brønsted bases LiO-t-Bu and LiO-t-Am/CsF/18-crown-6 (t-Am = CEtMe₂) under a CO₂ atmosphere to afford a variety of polyfunctionalized aromatic carboxylic acids.

The chemistry of ferrocenesulfonyl fluoride revealed

The first general route toward polysubstituted ferrocenesulfonyl fluorides is described. Merging deprotometallations, 'halogen dance' reaction, Sonogashira, Suzuki-Miyaura and Negishi cross-couplings with SuFEx chemistry allowed original ferrocenes of an unprecedented diversity to be obtained.

Total Synthesis of Lamellarins G, J, L, and Z Using One-Pot Halogen Dance/Negishi Coupling

A bottom-up synthesis of lamellarins G, J, L, and Z was achieved via one-pot halogen dance/Negishi coupling of a lithiated dibromopyrrole derivative. The easily accessible dibromopyrrole bearing an ester moiety underwent halogen dance smoothly at -78 °C within 10 min. The resultant α-pyrrolyllithium was transmetalated to the corresponding organozinc species, which was then coupled with an aryl iodide in the presence of catalytic palladium to provide the fully substituted pyrrole. Subsequent halogen-lithium exchange was performed to incorporate a boronate group exclusively at the β position proximal to the ester moiety. This synthetic intermediate allowed stepwise diarylation for the total synthesis of lamellarins G, J, L, and Z.

Nucleophilic C-H Etherification of Heteroarenes Enabled by Base-Catalyzed Halogen Transfer

We report a general protocol for the direct C-H etherification of N-heteroarenes. Potassium tert-butoxide catalyzes halogen transfer from 2-halothiophenes to N-heteroarenes to form N-heteroaryl halide intermediates that undergo tandem base-promoted alcohol substitution. Thus, the simple inclusion of inexpensive 2-halothiophenes enables regioselective oxidative coupling of alcohols with 1,3-azoles, pyridines, diazines, and polyazines under basic reaction conditions.

Ruthenium-Catalyzed Isomerization of ortho-Silylanilines to Their para Isomers

The catalytic ortho to para transposition of a silyl group in aniline derivatives is described. [RuCl₂(p-cymene)]₂/BINAP in conjunction with a Cu(OAc)₂ additive serves as a potent catalytic system. This method is also applicable to the isomerization of 2-silylpyrrole derivatives to the corresponding 3-silyl isomers.

"Snapshot" Trapping of Multiple Transient Azolyllithiums in Batch

Recent developments in flow microreactor technology have allowed the use of transient organolithium compounds that cannot be realized in a batch reactor. However, trapping the transient aryllithiums in a "halogen dance" is still challenging. Herein is reported the trapping of such short-lived azolyllithiums in a batch reactor by developing a finely tuned in situ zincation using zinc halide diamine complexes. The reaction rate is controlled by the appropriate choice of diamine ligand. The reaction is operationally simple and can be performed at 0 °C with high reproducibility on a multigram scale. This method was applicable to a wide range of brominated azoles allowing deprotonative functionalization, which was used for the concise divergent syntheses of both constitutional isomers of biologically active azoles.

Sterically hindered ortho-substituted phosphatriptycenes as configurationally stable P-chirogenic triarylphosphines

ortho-Substituted and unsymmetrical 9-phospha-triptycenes were synthesized via two synthetic approaches involving densely functionalized ortho-halogenated triarylmethane or phosphine precursors. ortho-Substituents imposed a considerable steric shielding due to the tricyclic cage-shaped structure with the aryl rings p-systems orthogonal to the phosphorus electron pair. A series of Au(i) and Rh(i) complexes were analysed in the solid state to determine Tolman electronic parameters, cone angles and buried volumes of these unprecedented functionalized phosphines. Quantum chemical calculations of electronic and steric descriptors revealed that these cage-shaped phosphines are electron-poor and that single methyl substituent is enough to provide the largest effect on steric shielding reported so far in triarylphosphines. An unsymmetrically substituted 9-phosphatriptycene was resolved by chiral HPLC, opening the avenue towards stable P-chirogenic triarylphosphines with unlimited configurational stability for new catalyst development in asymmetric transition-metal catalysis.

Base-catalyzed aryl halide isomerization enables the 4-selective substitution of 3-bromopyridines

The base-catalyzed isomerization of simple aryl halides is presented and utilized to achieve the 4-selective etherification, hydroxylation and amination of 3-bromopyridines. Mechanistic studies support isomerization of 3-bromopyridines to 4-bromopyridines proceeds via pyridyne intermediates and that 4-substitution selectivity is driven by a facile aromatic substitution reaction. Useful features of a tandem aryl halide isomerization/selective interception approach to aromatic functionalization are demonstrated. Example benefits include the use of readily available and stable 3-bromopyridines in place of less available and stable 4-halogenated congeners and the ability to converge mixtures of 3- and 5-bromopyridines to a single 4-substituted product.

Convergent Total Synthesis of Lamellarins and Their Congeners

A convergent total synthesis of lamellarins S and Z is described. The synthesis features a halogen dance of an easily accessible α,β-dibromopyrrole promoted by an ester moiety. The resultant β,β'-dibromopyrrole undergoes a ligand-controlled Suzuki-Miyaura coupling to provide a range of diarylated pyrrole derivatives. The established synthetic method was also applicable to the synthesis of ningalin B and lukianols A and B.

Sulfonyl Group Dance: A Tool for the Synthesis of 6-Azido-2-sulfonylpurine Derivatives

9-Substituted 2-chloro-6-sulfonylpurines provide 6-azido-2-sulfonylpurine derivatives with 61-83% yields when treated with sodium azide. Under optimized reaction conditions, the title compounds are obtained in a one-pot process, which involves a sequential treatment of 2,6-dichloropurines with a selected sodium sulfinate and sodium azide. Such a sulfonyl group dance (functional group swap) results from a cascade of S_NAr reactions, which are facilitated by azidoazomethine-tetrazole (azide-tetrazole) tautomeric equilibrium. The formation of Meisenheimer-type intermediates as tetrazolopurine tautomers was supported by various spectroscopic methods, including ¹⁵N NMR.

From ferrocene to fluorine-containing penta-substituted derivatives and all points in-between; or, how to increase the available chemical space

In spite of the growing interest in fluorine-containing compounds, and the improvements in materials, optical and biological properties that can arise from substitution of a phenyl ring by ferrocene within a molecular scaffold, synthetic strategies that allow the efficient preparation of fluoroferrocene derivatives are scarce. Following conversion of ferrocene to fluoroferrocene, we have developed routes to fluorine-containing di-, tri-, tetra- and penta-substituted ferrocene derivatives to extend the available chemical space. Our approach is based on the identification of suitable reagents and conditions to achieve fluorine-directed deprotometalation, and exploitation of the halogen 'dance' rearrangement in the ferrocene series.

Synthesis of a dihalogenated pyridinyl silicon rhodamine for mitochondrial imaging by a halogen dance rearrangement

Background: Since their first synthesis, silicon xanthenes and the subsequently developed silicon rhodamines (SiR) gained a lot of attention as attractive fluorescence dyes offering a broad field of application. We aimed for the synthesis of a fluorinable pyridinyl silicon rhodamine for the use in multimodal (PET/OI) medical imaging of mitochondria in cancerous cells.

Results: A dihalogenated fluorinatable pyridinyl rhodamine could be successfully synthesized with the high yield of 85% by application of a halogen dance (HD) rearrangement. The near-infrared dye shows a quantum yield of 0.34, comparable to other organelle targeting SiR derivatives and absorbs at 665 nm (ε_max = 34 000 M⁻¹cm⁻¹) and emits at 681 nm (τ = 1.9 ns). Using colocalization experiments with MitoTracker^® Green FM, we could prove the intrinsic targeting ability to mitochondria in two human cell lines (Pearson coefficient >0.8). The dye is suitable for live cell STED nanoscopy imaging and shows a nontoxic profile which makes it an appropriate candidate for medical imaging.

Conclusions: We present a biocompatible, nontoxic, small molecule near-infrared dye with the option of subsequent radiolabelling and excellent optical properties for medical and bioimaging. As a compound with intrinsic mitochondria targeting ability, the radiolabelled analogue can be applied in multimodal (PET/OI) imaging of mitochondria for diagnostic and therapeutic use in, e.g., cancer patients.

Asymmetric synthesis of hetero-1,2,3,4,5-pentasubstituted ferrocenes

The first synthesis of enantioenriched ferrocenes bearing five different substituents on the same cyclopentadienyl ring is described through an unprecedented asymmetric halogen 'dance' reaction. This work not only extends the chemistry of fluoroferrocene derivatives and constitutes the very first entry into the world of enantioenriched hetero-pentasubstituted ferrocenes, it also lays the foundation for research in other metallocene families.

Regiocontrolled Halogen Dance of Bromothiophenes and Bromofurans

The LDA (lithium diisopropylamide)-promoted regiocontrolled halogen dance of α-bromothiophenes and α-bromofurans is described. Bromothiophenes bearing a diethyl acetal moiety undergo selective deprotonation at the β-position adjacent to the bromo group. In contrast, oxazoline, ester, and amide groups act as directing groups in the initial lithiation step to generate a carbanion at the β-position neighboring the directing group to exclusively give the other regioisomer. These results can be applied to the regiocontrolled halogen dance of bromofuran derivatives.

5-Methyl-2-thienylcalcium iodide

The reduction of 2-bromo- and 3-bromothiophene with calcium powder gives impure thienylcalcium complexes due to interference of various subsequent metalation and calcium-halogen exchange reactions as well as ether degradation. Therefore, calcium-iodine exchange succeeds via the reaction of trimethylsilylmethylcalcium halide with 3-iodothiophene in tetrahydrofuran at -78 °C in the presence of chloro-trimethylsilane, leading to the quantitative formation of 3-(trimethylsilyl)thiophene. The synthesis and isolation of a thienylcalcium halide, a heavy Grignard reagent with a thienyl group, is possible via the reduction of 2-iodo-5-methylthiophene with activated calcium in THF. Substitution of the thf ligands in 5-methyl-2-thienylcalcium iodide ([5-MeThien-2-Ca(thf)4I], 1) by dissolution in tetrahydropyran (THP) leads to the thp complex of 5-methyl-2-thienylcalcium iodide ([5-MeThien-2-Ca(thp)4I], 2). A low field shift of the calcium-bound carbon atom is characteristic of thienylcalcium complexes.