Electron-rich Aryl Ring Research Articles

Enantioselective Synthesis of α-Aryl Ketones by a Cobalt-Catalyzed Semipinacol Rearrangement.

A highly enantioselective cobalt-catalyzed semipinacol rearrangement of symmetric α,α-diarylallylic alcohols is disclosed. A chiral cobalt-salen catalyst generates a highly electrophilic carbocation surrogate following hydrogen atom transfer and radical-polar crossover steps. This methodology provides access to enantioenriched α-aryl ketones through invertive displacement of a cobalt(IV) complex during 1,2-aryl migration. A combination of readily available reagents, silane and N-fluoropyridinium oxidant, are used to confer this type of reactivity. An exploration into the effect of aryl substitution revealed the reaction tolerates para- and meta-halogenated, mildly electron-rich and electron-poor aromatic rings with excellent enantioselectivities and yields. The yield of the rearrangement diminished with highly electron-rich aryl rings whereas very electron-deficient and ortho-substituted arenes led to poor enantiocontrol. A Hammett analysis demonstrated the migratory preference for electron-rich aromatic rings, which is consistent with the intermediacy of a phenonium cation.

Electrochemistry and Organic Synthesis: New Adventures with Olefin Coupling Reactions and Electrode Surfaces

Electrochemistry has long offered opportunities to explore new modes of reactivity, new approaches for introducing selectivity into synthetic reactions, and new strategies for the synthesis of everything from complex molecules to complex, addressable molecular surfaces. In the talk to be given, two aspects of our efforts along these lines will be detailed. In the first, we will take a look at recent efforts to use anodically-generated reactive intermediates to convert readily available small molecules into functionalized synthetic building blocks and natural product derivatives. Key to these efforts are oxidation reactions that generate reactive radical cation and radical intermediates. In one application, newly available chiral lactols synthesized using methodology developed by the Lin (Cornell) and Miller (Yale) groups are being transformed into C-glycoside and aza-C-glycoside derivatives. The reactions take advantage of oxidation reactions of either enol ether or vinylsulfide groups. The cyclization reactions and the subsequent deprotection strategy needed for converting the cyclization products into a useful synthetic building block will be highlighted. In a second application of an anodic cyclization, the role of thioamides in reactions targeting the chrysosoporazine family of natural products will be discussed. The reactions seek to provide an efficient method for converting lignin derived building blocks into molecules that reduce the efflux of anticancer drugs from cells. Initial cyclization attempts utilizing amide based N-radicals and anodic olefin coupling reactions were not successful. Those reactions consistently led to over-oxidation of the products that contain a highly electron-rich aryl ring. The use of thio-amide derived radicals provides a method to circumvent this problem.In the second topic to be discussed, the question of how one might begin to induce asymmetry into such electrochemical cyclizations will be highlighted. The chemistry focuses on efforts to influence the course of both electrochemical oxidation and reduction reaction and non-redox based transition metal catalyzed reactions with the surface of an electrode.

Quantum Chemical Insight into 1,2-Shift Rearrangement in Bromination of Allylaryls.

In this work, we have provided mechanistic insight into the addition of bromine to an allylic double bond of allylaryl derivatives using experimental and DFT-based electronic structure methods. The experimental yields indicate the influence of the functional group on the aryl ring on the ratio of 1,2-dibromo and 1,3-dibromo adducts formed in the reaction. The optimized geometry and the electron density maps of the allylaryls and their cationic intermediates from DFT simulations revealed that electron-rich aryl rings promoted formation of cationic spiro[2.5] intermediate II, whereas electron-poor aryl rings resulted in formation of bromonium intermediate I. It was observed that electron-rich allylaryls promoted the 1,2-shift of the aryl ring that resulted in bond formation between the carbon atom (C1) on the aryl ring and the central carbon atom (C3) in the allylic double bond and formed spiro[2.5] intermediate II, a trend which was confirmed by harmonic oscillator model of aromaticity index. Also, Wiberg bond order analysis is in good agreement with the experimental work. Thermochemical analysis indicates that smaller C1···C3 distance resulted in favorable values for the difference in free energy change (ΔΔG). The favorable ΔΔG values are a result of higher electron density on the aryl ring, making it more nucleophilic toward C3 carbon and promoting 1,2-shift that led to formation of the spiro[2.5] intermediate. Thus, the underlying mechanism indicates that the electron-rich allylaryls promote the formation of 1,3-dibromo compounds through formation and stabilization of the spiro[2.5] intermediate II.

One-pot two step synthesis of unsymmetrically substituted indenes from 3,4-diarylbutadiene sulfones

A new one-pot two step synthesis of unsymmetrically substituted indenes from available 3,4-diarylbutadiene sulfones involves SO2 thermal extrusion followed by acid- catalyzed cyclization of the diene formed, the cyclization proceeding selectively at the more electron-rich aryl rings. The procedure is efficient for substrates bearing donor, acceptor, as well as bulky substituents.

Visible-light photoredox-catalyzed C\u2013O bond cleavage of diaryl ethers by acridinium photocatalysts at room temperature

Cleavage of C–O bonds in lignin can afford the renewable aryl sources for fine chemicals. However, the high bond energies of these C–O bonds, especially the 4-O-5-type diaryl ether C–O bonds (~314 kJ/mol) make the cleavage very challenging. Here, we report visible-light photoredox-catalyzed C–O bond cleavage of diaryl ethers by an acidolysis with an aryl carboxylic acid and a following one-pot hydrolysis. Two molecules of phenols are obtained from one molecule of diaryl ether at room temperature. The aryl carboxylic acid used for the acidolysis can be recovered. The key to success of the acidolysis is merging visible-light photoredox catalysis using an acridinium photocatalyst and Lewis acid catalysis using Cu(TMHD)2. Preliminary mechanistic studies indicate that the catalytic cycle occurs via a rare selective electrophilic attack of the generated aryl carboxylic radical on the electron-rich aryl ring of the diphenyl ether. This transformation is applied to a gram-scale reaction and the model of 4-O-5 lignin linkages.

Chemoselective Radiosyntheses of Electron-Rich [18F]Fluoroarenes from Aryl(2,4,6-trimethoxyphenyl)iodonium Tosylates.

Hypervalent diaryliodonium salts have been used to produce various [18F]fluoroarenes. The iodonium salt approach as a labeling precursor has been established to equally afford complex 18F-fluorinated molecules. Because of the inherent two aryl ring system connected to a central iodine atom, safeguarding the chemoselectivity during radiofluorination using diaryliodonium salts is important. Herein, we introduce a superior chemoselective radiosynthesis of [18F]fluoroarenes using an aryl(2,4,6-trimethoxyphenyl)iodonium tosylate as a precursor for 18F-incorporation, even on electron-rich aryl rings.

Regioselectivity of the ortho- and para-semidine, and diphenyline rearrangements

The regioselectivity of the o-semidine, p-semidine, and diphenyline rearrangements of unsymmetrical N,N′-diarylhydrazines was studied experimentally. The results indicate that their electron-rich nitrogen atom is first protonated and then the electron-poor non-protonated nitrogen atom undergoes an N[1,3]-sigmatropic shift to the ortho-position of the electron-rich aryl rings, generating key intermediates. The intermediates can undergo (1) a direct proton transfer to give o-semidines, (2) a second N[1,3]-shift of the electron-poor nitrogen atom and then proton transfer to furnish p-semidines, and (3) a [3,3]-sigmatropic shift and subsequent proton transfer to yield diphenylines. It is the first N[1,3]-sigmatropic shift step that plays an important role in controlling the regioselectivity in the three rearrangements, further determining the structures of o-semidines, p-semidines, and diphenylines. The current results provide new insights into the o/p-semidine and diphenyline rearrangements and useful information for controlling and predicting the structures of the rearrangement products.

ChemInform Abstract: Bisindolines from the Reaction of 3,5‐Dimethoxyaniline with Vicinal Diones.

Substituted bisindolines were synthesized from the reaction of 3,5-dimethoxyaniline and vicinal diones (2,3-butanedione, benzil, and 4-methylbenzil). The reaction of 3,5-dimethoxyaniline with 2,3-butanedione under polar acidic conditions proceeded with an unanticipated double cyclization reaction forming 2,2′-dimethyl-5,5′,7,7′-tetramethoxybisindole, (+/−)-1. The reaction of 3,5-dimethoxyaniline with benzil under similar conditions did not form a bisindoline, but instead formed cis-1,2-diphenyl-2-[(3,5-dimethoxyphenyl)imino]ethanone, 2. When the solvent, acid, and reaction temperature were changed for the reaction of 3,5-dimethoxyaniline with benzil, 3,3-diphenyl-4,6-dimethoxyindolin-2-one, 3 (an isomer of 2), and 2,2′-diphenyl-5,5′,7,7′-tetramethoxybisindole, (+/−)-4, were isolated as major and minor products, respectively. Similarly, the reaction of 3,5-dimethoxyaniline with 4-methylbenzil produced a 1 : 1 mixture of cis-1-(4-methylphenyl)-2-phenyl-2-[(3,5-dimethoxyphenyl)-imino]ethanone, 6a, and cis-1-(phenyl)-2-(4-methylphenyl)-2-[(3,5-dimethoxyphenyl)imino]ethanone, 6b, under polar acidic conditions and 3-(4-methylphenyl)-3-phenyl-4,6-dimethoxyindolin-2-one, (+/−)-7 and 2-(4-methylphenyl)-2′-phenyl-5,5′,7,7′-tetramethoxybisindole, (+/−)-8, under apolar acidic conditions at elevated temperature. We propose the mechanism of bisindoline and indolinone formation to include a Friedel–Crafts type electrophilic attack on the electron rich aryl ring at the ring closing C–C bond forming steps, and further propose a common carbocationic monocyclized intermediate.

ChemInform Abstract: Selectivity in Garratt—Braverman Cyclization of Aryl‐/Heteroaryl‐Substituted Unsymmetrical Bis‐Propargyl Systems: Formal Synthesis of 7′‐Desmethylkealiiquinone.

Unsymmetrical bis-propargyl ethers and sulfonamides containing various combinations of aryl/heteroaryl substituents at the acetylene termini were synthesized, and their reactivity under basic conditions was studied. Moderate to high (chemo)selectivity was observed, which followed a trend opposite to that reported earlier for the corresponding sufones. The major products obtained in most cases (except with indole) were formed via participation of the heteroaryl ring or the less electron rich aryl ring. The selectivity observed in imidazole-based systems was exploited to complete a formal total synthesis of 7′-desmethylkealiiquinone, an analogue of the marine alkaloid kealiiquinone.

Selectivity in Garratt–Braverman Cyclization of Aryl-/Heteroaryl-Substituted Unsymmetrical Bis-Propargyl Systems: Formal Synthesis of 7′-Desmethylkealiiquinone

Unsymmetrical bis-propargyl ethers and sulfonamides containing various combinations of aryl/heteroaryl substituents at the acetylene termini were synthesized, and their reactivity under basic conditions was studied. Moderate to high (chemo)selectivity was observed, which followed a trend opposite to that reported earlier for the corresponding sufones. The major products obtained in most cases (except with indole) were formed via participation of the heteroaryl ring or the less electron rich aryl ring. The selectivity observed in imidazole-based systems was exploited to complete a formal total synthesis of 7'-desmethylkealiiquinone, an analogue of the marine alkaloid kealiiquinone.

Bisindolines from the reaction of 3,5-dimethoxyaniline with vicinal diones

Substituted bisindolines were synthesized from the reaction of 3,5-dimethoxyaniline and vicinal diones (2,3-butanedione, benzil, and 4-methylbenzil). The reaction of 3,5-dimethoxyaniline with 2,3-butanedione under polar acidic conditions proceeded with an unanticipated double cyclization reaction forming 2,2′-dimethyl-5,5′,7,7′-tetramethoxybisindole, (+/−)-1. The reaction of 3,5-dimethoxyaniline with benzil under similar conditions did not form a bisindoline, but instead formed cis-1,2-diphenyl-2-[(3,5-dimethoxyphenyl)imino]ethanone, 2. When the solvent, acid, and reaction temperature were changed for the reaction of 3,5-dimethoxyaniline with benzil, 3,3-diphenyl-4,6-dimethoxyindolin-2-one, 3 (an isomer of 2), and 2,2′-diphenyl-5,5′,7,7′-tetramethoxybisindole, (+/−)-4, were isolated as major and minor products, respectively. Similarly, the reaction of 3,5-dimethoxyaniline with 4-methylbenzil produced a 1 : 1 mixture of cis-1-(4-methylphenyl)-2-phenyl-2-[(3,5-dimethoxyphenyl)-imino]ethanone, 6a, and cis-1-(phenyl)-2-(4-methylphenyl)-2-[(3,5-dimethoxyphenyl)imino]ethanone, 6b, under polar acidic conditions and 3-(4-methylphenyl)-3-phenyl-4,6-dimethoxyindolin-2-one, (+/−)-7 and 2-(4-methylphenyl)-2′-phenyl-5,5′,7,7′-tetramethoxybisindole, (+/−)-8, under apolar acidic conditions at elevated temperature. We propose the mechanism of bisindoline and indolinone formation to include a Friedel–Crafts type electrophilic attack on the electron rich aryl ring at the ring closing C–C bond forming steps, and further propose a common carbocationic monocyclized intermediate.

Single-step syntheses of no-carrier-added functionalized [18F]fluoroarenes as labeling synthons from diaryliodonium salts.

Radiotracers labelled with short-lived fluorine-18 (t(1/2) = 109.7 min) are keenly sought for biomedical imaging with positron emission tomography (PET). The radiotracers are mostly required at high specific radioactivities, necessitating their radiosyntheses from cyclotron-produced no-carrier-added [(18)F]fluoride ion. PET radiotracers encompass wide structural diversity and molecular weight. Hence, diverse (18)F-labeling methodology is needed to accomplish the required radiosyntheses in a simple and rapid manner. A useful strategy is to introduce nucleophilic [(18)F]fluoride ion first into a labeling synthon that may then be applied to label the target radiotracer. Here, we show that various functionalized [(18)F]fluoroarenes may be rapidly synthesized as labeling synthons through single-step reactions of appropriate diaryliodonium salts with [(18)F]fluoride ion. Decay-corrected radiochemical yields (RCYs) varied with position of functional group, choice of electron-rich aryl ring in the diaryliodonium salt, and choice of anion. Under best conditions, (18)F-labeled fluorobenzaldehydes, fluorobenzyl halides, fluorobenzoic acid esters and fluorophenyl ketones were obtained selectively in 40-73%, 20-55%, 46-89% and 81-98% RCYs, respectively. This versatile straightforward methodology will enhance the scope for producing structurally complex, yet useful, PET radiotracers.

Cu(II)-Promoted Transformations of α-Thienylcarbinols into Spirothienooxindoles: Regioselective Halogenation of Dienyl Sulfethers Containing Electron-Rich Aryl Rings

Under the promotion of Cu(II) salts, the α-thienylcarbinols with an N-phenyl carbonyl group at the other α-position are converted into three different ranges of spirothienooxindoles involving dearomatizing Friedel-Crafts reaction. In addition, the unprecedented regioselective CuX(2)-mediated C-H functionalization/halogenation of dienyl sulfether containing electron-rich aryl rings is presented.

C-H Activation to Make Highly Functionalized Anthracenes

The authors present the tetraarylation of anthraquinone using ruthenium-catalyzed C-H bond activation. The products of the arylation can then be converted into acenes using known methods, arriving at the tetra- or even hexa-substituted anthracene derivatives. The scope of the arylation is remarkable, with electron-poor and electron-rich aryl rings being viable substrates. The proposed mechanism of the arylation is shown above and consists of oxidative cleavage of the aryl C-H bond by ruthenium(0) (A), followed by hydrometalation of pinacolone (B). Transmetal-ation (C) with the arylboronic ester followed by reductive elimination (D) provides the arylation product and regenerates the active ruthenium catalyst.

Pd-Catalyzed Cyclization to Form Benzo-pyranones from Enediynes

This paper describes the synthesis of substituted dibenzo[b,d]pyran-6-ones by a Pd-catalyzed, one-step, tandem cyclization reaction from enediynes. For the 14 cases studied, yields vary from poor to modest; furthermore, interesting chlorinated and minor byproducts are formed in a few cases. A mechanism involving the formation of a vinyl palladium intermediate which undergoes 6-endo cyclization to yield the product is proposed. A 5-exo cyclization to the observed chlorinated product is observed when the terminal alkene substituent is a sterically hindered alkyl group or an electron-rich aryl ring. The starting methyl 2-[6-substituted-3(Z)-hexen-1,5-diynyl]benzo­ates were prepared in moderate to excellent yield by the well-established Sonogashira methodology.

π‐Face Donor Properties of N‐Heterocyclic Carbenes in Grubbs II Complexes

The electron-donating properties of eighteen N-heterocyclic carbenes (N,N'-bis(2,6-dimethylphenyl)imidazol)-2-ylidene and the respective dihydro ligands) with 4,4'-R substituted aryl rings (4,4'-R = NEt2, OMe, Me, H, SMe, F, Cl, Br, I) in the respective Grubbs II complexes were studied using electrochemical techniques. The nature of the 4-R substituent has a strong influence on the RuII/III redox potentials ranging between DeltaE1/2= +0.196 and +0.532 V. Three unsymmetrical Grubbs II complexes with 4-R not equal to 4-R' were also synthesized. Dynamic NMR spectroscopy revealed the restricted rotation around the (NHC)C-Ru bond (DeltaG = 89 kJ mol(-1) at 333 K) resulting in two atropisomers, respectively, with an isomer ratio close to unity. Each of the isomers, that is the two orientations of the 4-R/4-R' substituted mesityl ring with respect to the R=CHPh unit, gives rise to different redox potentials (4-R = NEt2, 4-R' = Br: DeltaE1/2= +0.232 and +0.451 V). In the oxidized Grubbs II complex (4-R = NEt2, H) and in the cathodic isomer the electron rich aryl ring is located above the Ru=CHPh unit. This orientational effect provides clear evidence for strong pi-pi through-space interactions in the RuIII complexes, assuming that the alternative through-bond transfer of electron density is equally efficient in both isomers.

Intramolecular anodic olefin coupling reactions: Initial studies concerning the use of electron-rich aryl rings

Intramolecular anodic olefin coupling reactions involving electron-rich aryl rings were examined and shown to afford fused bicyclic products. When alkoxysubstituted phenyl rings were used, the reactions benefited from the use of either controlled potential electrolysis conditions or a vinyl sulfide initiating group. Coupling reactions involving heteroatomic aryl rings also led to good yields of cyclized product.

ChemInform Abstract: A New Synthesis of 3‐Amino‐1‐aryl‐2‐pyrazolin‐5‐ones.

AbstractAus β‐Nitropropionsäureethylester (I) entstehen in Gegenwart von Alkoholat mit Diazoniumsalzen (II) die Hydrazone (III), die nach katalytischer Hydrierung zu (IV) zu den Pyrazolinonen (V) cyclokondensieren.