1,2,3‐Diazaborinine: A BN Analogue of Pyridine Obtained by Ring Expansion of a Borole with an Organic Azide

A new pathway for the ring expansion reaction of antiaromatic boroles with organic azides is reported. While the reaction usually leads to 1,2‐azaborinines, it was diverted to the formation of a 1,2,3‐diazaborinine by changing the electronic characteristics of the reagents. The isolable azo‐azaborinine intermediate initially formed from the reaction of 1‐(2,3,4,5‐tetraphenylborolyl)ferrocene with 4‐azido‐N,N‐dimethylaniline gradually decomposed to a 1,2,3‐diazaborinine and benzonitrile. Both the spectroscopic properties and the reactivity of the heteroaromatic compound show analogies to pyridine, to which it is isoelectronic. Density functional theory (DFT) calculations provided insight into the mechanism of this unusual transformation.     

The Dewar Isomer of 1,2‐Dihydro‐1,2‐azaborinines: Isolation,Fragmentation, and Energy Storage

The photochemistry of 1,2‐dihydro‐1,2‐azaborinine derivatives was studied under matrix isolation conditions and in solution. Photoisomerization occurs exclusively to the Dewar valence isomers upon irradiation with UV light (>280 nm) with high quantum yield (46 %). Further photolysis with UV light (254 nm) results in the formation of cyclobutadiene and an iminoborane derivative. The thermal electrocyclic ring‐opening reaction of the Dewar valence isomer back to the 1,2‐dihydro‐1‐tert‐butyldimethylsilyl‐2‐mesityl‐1,2‐azaborinine has an activation barrier of (27.0±1.2) kcal mol⁻¹. In the presence of the Wilkinson catalyst, the ring opening occurs rapidly and exothermically (ΔH=(−48±1) kcal mol⁻¹) at room temperature.     

The Dewar Isomer of 1,2‐Dihydro‐1,2‐azaborinines: Isolation,Fragmentation, and Energy Storage

The photochemistry of 1,2‐dihydro‐1,2‐azaborinine derivatives was studied under matrix isolation conditions and in solution. Photoisomerization occurs exclusively to the Dewar valence isomers upon irradiation with UV light (>280 nm) with high quantum yield (46 %). Further photolysis with UV light (254 nm) results in the formation of cyclobutadiene and an iminoborane derivative. The thermal electrocyclic ring‐opening reaction of the Dewar valence isomer back to the 1,2‐dihydro‐1‐tert‐butyldimethylsilyl‐2‐mesityl‐1,2‐azaborinine has an activation barrier of (27.0±1.2) kcal mol^?1. In the presence of the Wilkinson catalyst, the ring opening occurs rapidly and exothermically (ΔH=(?48±1) kcal mol^?1) at room temperature.     

Solution Phase Reactivity of Dibenzo[c,e][1,2]azaborinine: Activation and Insertion into Si-E Single Bonds (E=H,OSi(CH3)3, F,Cl) by a BN-Aryne

The boron-nitrogen analogue of ortho-benzyne, 1,2-azaborinine, is a reactive intermediate that features a formal boron-nitrogen triple bond. We here show by combining experimental and computational techniques that the Lewis acidity of the boron center of dibenzo[c,e][1,2]azaborinine allows interaction with the silicon containing single bonds Si−E through the silicon bonding partner E (E=F, Cl, OR, H). The binding to boron activates the Si−E bonds for subsequent insertion reaction. This shows that the BN-aryne is a ferocious species that even can activate and insert into the very strong Si−F bond.     

Photoionization and Pyrolysis of a 1,4‐Azaborinine: Retro‐Hydroboration in the Cation and Identification of Novel Organoboron Ring Systems

The photoionization and dissociative photoionization of 1,4‐di‐tert‐butyl‐1,4‐azaborinine by means of synchrotron radiation and threshold photoelectron photoion coincidence spectroscopy is reported. The ionization energy of the compound was determined to be 7.89 eV. Several low‐lying electronically excited states in the cation were identified. The various pathways for dissociative photoionization were modeled by statistical theory, and appearance energies AE_0K were obtained. The loss of isobutene in a retro‐hydroboration reaction is the dominant pathway, which proceeds with a reverse barrier. Pyrolysis of the parent compound in a chemical reactor leads to the generation of several yet unobserved boron compounds. The ionization energies of the C₄H₆BN isomers 1,2‐ and 1,4‐dihydro‐1,4‐azaborinine and the C₃H₆BN isomer 1,2‐dihydro‐1,3‐azaborole were determined from threshold photoelectron spectra.     

Direct Synthetic Route to Functionalized 1,2‐Azaborinines

A new catalytic synthetic route to functionalized 1,2‐azaborinines has been developed by the [2+2]/[2+4] cycloaddition reactions of di‐tert‐butyliminoboranes and alkynes in presence of a rhodium catalyst. The first examples of ferrocene‐functionalized azaborinines have been synthesized using this strategy. Moreover, the regioselectivity of this reaction can be controlled by the formation of an intermediate rhodium 1,2‐azaborete complex, which results in the isolation of the first azaborinine boronic ester. The isolation of an NH‐containing BN isostere by elimination of isobutene from an N(tBu) group under thermolytic conditions has also been achieved. Theoretical studies give further insight into the formation of 1,2‐azaborinines and the elimination of isobutene from the N(tBu) group.     

Thermal and Photolytic Transformation of NHC–B,N‐Heterocycles: Controlled Generation of Blue Fluorescent 1,3‐Azaborinine Derivatives and 1H‐Imidazo[1,2‐a]indoles by External Stimuli

NHC–B,N‐heterocyclic compounds have been found to act as convenient precursors for obtaining either 1,3‐azaborinine or 1H‐imidazo[1,2‐a]indole derivatives, which are two different and rare classes of compounds. The formation of these two classes of compounds from the NHC–B,N‐heterocycles is highly selective depending on the external stimuli employed, and the resulting products have been studied for their interesting chemical and photophysical properties. The mechanism and possible reaction pathways of the unusual transformation are established by computational studies.     

Regioselective Catalytic and Stepwise Routes to Bulky,Functional‐Group‐Appended,and Luminescent 1,2‐Azaborinines

The regioselective syntheses of 1,2‐azaborinines is achieved using an unsymmetrical iminoborane through both catalytic and stepwise modular routes. The 1,2‐azaborinine ring can be selectively functionalized in the 4‐ and/or 6‐position through control of the stepwise reaction sequence, allowing access to vinyl‐functionalized and redox‐active, luminescent, donor‐functionalized 1,2‐azaborinines. The electrochemistry and photochemistry of a tetraarylamine‐substituted 1,2‐azaborinine are studied. Cyclic voltammetry of this compound, relative to a non‐B,N‐substituted reference molecule, showed an additional oxidation wave assigned to the oxidation of the azaborinine ring, while emission spectroscopy indicated that the azaborinine was significantly more fluorescent than the reference.     

Synthesis of fused pyrimidines with N‐, S‐heterocyclic moieties by double‐annelation reaction

A variety of tri‐ and tetracyclic hetero systems were obtained by reaction of heteroaromatic ortho‐aminoesters or ortho‐aminonitriles with iminothioether, yielding double‐annelation of a thiazolo[3,2‐a]‐pyrimidine, pyrimido[2,l‐b]thiazine, imidazo[1,2‐a]pyrimidine, and pyrimido[1,2‐a]pyrimidine moieties in a one‐pot process.     

Solvent- and temperature-dependent functionalisation of enantioenriched aziridines

A highly stereo‐ and regioselective functionalisation of chiral non‐racemic aziridines is reported. By starting from a parent enantioenriched aziridine and finely tuning the reaction conditions, it is possible to address the regio‐ and stereoselectivity of the lithiation/electrophile trapping sequence, thereby allowing the preparation of highly enantioenriched functionalised aziridines. From chiral N‐alkyl trans‐2,3‐diphenylaziridines (S,S)‐ 1 a , b , two differently configured chiral aziridinyllithiums could be generated (trans‐ 1 a , b‐Li in toluene and cis‐ 1 a , b‐Li in THF), thus disclosing a solvent‐dependent reactivity that is useful for the synthesis of chiral tri‐substituted aziridines with different stereochemistry. In contrast, chiral aziridine (S,S)‐ 1 c showed a temperature‐dependent reactivity to give chiral ortho‐lithiated aziridine 1 c‐ ortho ‐Li at ?78 °C and α‐lithiated aziridine 1 c‐α‐Li at 0 °C. Both lithiated intermediates react with electrophiles to give enantioenriched ortho‐ and α‐functionalised aziridines. The reaction of all the lithiated aziridines with carbonyl compounds furnished useful chiral hydroxyalkylated derivatives, the stereochemistry of which was ascertained by X‐ray and NMR spectroscopic analysis. The usefulness of chiral non‐racemic functionalised aziridines has been demonstrated by reductive ring‐opening reactions furnishing chiral amines that bear quaternary stereogenic centres and chiral 1,2‐, 1,3‐ and 1,5‐aminoalcohols. It is remarkable that the solvent‐dependent reactivity observed with (S,S)‐ 1 a , b permits the preparation of both the enantiomers of amines ( 11 and ent‐ 11 ) and 1,2‐aminoalcohols ( 13 and ent‐ 13 ) starting from the same parent aziridine. Interestingly, for the first time, a configurationally stable chiral α‐lithiated aziridine ( 1 c‐α‐Li ) has been generated at 0 °C. In addition, ortho‐hydroxyalkylated aziridines have been easily converted into chiral aminoalkyl phthalans, which are useful building blocks in medicinal chemistry.     

Triflic Acid Mediated Cascade Cyclization of Aryldiynes for the Synthesis of Indeno[1,2‐c]chromenes: Application to Dye‐Sensitized Solar Cells

A new triflic acid (TfOH)‐mediated cascade cyclization of ortho‐anisole‐substituted aryldiynes is described for the construction of indeno[1,2‐c]chromenes. The cascade cyclization proceeds through an unusual TfOH‐induced alkyne‐alkyne cyclization followed by nucleophilic attack of the methoxy group on the benzylidene cation, which is completely different to the cyclization of ortho‐aniline‐ or ortho‐thioanisole‐substituted aryldiynes. A new class of organic dyes with the indeno[1,2‐c]chromene framework as both donor and π‐linker were synthesized. These compounds exhibit high photovoltaic performances in dye‐ sensitized solar cells (DSCs).     

Ruthenium‐Catalyzed Oxidant‐Free Allylation of Aromatic Ketoximes with Allylic Acetates at Room Temperature

Substituted aromatic ketoximes reacted efficiently with allylic acetates in the presence of {[RuCl₂(p‐cymene)]₂} and AgSbF₆ in 1,2‐dichloroethane at ambient temperature, providing ortho‐allyl aromatic ketoximes in a highly regioselective manner without an oxidant. In the reaction, the acetate group of allyl acetate acts as a base to activate the C?H bond of aromatics. Later, ortho‐allyl aromatic ketoximes were converted into ortho‐allyl aromatic ketones in the presence of HCl.     

Kinetic Analysis of a Two‐Phase EC Mechanism with Concomitant Lateral Processes

Electrochemical transfer of azo dye fast red TR across the water/1,2 – dichloroethane (DCE) interface followed by the coupling reaction with 1‐naphthylamine in the organic phase is studied. Pseudo first order rate constants of this reaction were obtained by fitting the experimental I_p?/I_p+ ratio under different experimental conditions with the theoretical values reported by Nicholson and Shain. The occurrence of lateral processes is demonstrated (partition of the azo dye non assisted by potential and electrochemical transfer of the protons generated in the coupling reaction), which constrains the accurate determination of the kinetic parameters.     

ortho‐Benzoxylation of N‐Alkyl Benzamides with Aromatic Acids Catalyzed by Ruthenium(II) Complex

A highly regioselective ortho‐benzoxylation of N‐alkyl benzamides with aromatic acids in the presence of [{RuCl₂(p‐cymene)}₂], AgSbF₆, and (NH₄)₂S₂O₈ in 1,2‐dichloroethane at 100 °C for 24 h affording ortho‐benzoxylated N‐alkyl benzamides by C?H bond activation is described. Further, Ru‐catalyzed alkenylation is done at the ortho C?H bond of benzoxylated N‐alkyl benzamides with alkenes in water solvent. Subsequently, the benzoxyl moiety of N‐alkyl benzamides was converted into a hydroxyl group in the presence of base or acid. A possible reaction mechanism was proposed to account for the present coupling reaction.     

Synthesis,Enantiomeric Conformations,and Stereodynamics of Aromatic ortho‐Substituted Disulfones

Aromatic ortho‐disulfone derivatives are readily accessible from diiodide precursors by Cu^I‐mediated reaction with sodium sulfinate salts (DMF, 110°). The sulfonyl substituents adopt in solution and in the solid state two enantiomeric conformations (λ and δ) as evidenced by ³¹P‐ and ¹H‐NMR data of the chiral D₃‐symmetric tris{4,5‐bis[(4‐methylphenyl)sulfonyl]benzene‐1,2‐diolato(2?)‐κO,κO′}phosphate(v) anion ( 3a ) and 1,2‐bis(camphor‐10‐sulfonyl)‐4,5‐dimethoxybenzene ((=1,2‐bis{{[(1S,4R)‐7,7‐dimethyl‐2‐oxobicyclo[2.2.1]hept‐1‐yl]methyl}sulfonyl}‐4,5‐dimethoxybenzene; 6c ). X‐Ray structure analysis of 1,2‐dimethoxy‐4,5‐bis(methylsulfonyl)benzene ( 6a ) and 1,2‐dimethoxy‐4,5‐bis(4‐methylphenyl)sulfonyl]benzene ( 6b ) confirmed in the solid state the preferred chiral orientation of the sulfonyl groups. Dynamic conformational isomerism was detected for 6c in its ¹H‐NMR in the temperature range of 110°, the corresponding free energy being 19.8 kcal?mol^?1.     

A Synthesis Detour to Planar‐Diastereoisomeric Ferrocene Derivatives around an Unexpected Rearrangement of ortho‐Lithiated Kagan's Template [S(S)]‐(p‐Tolylsulfinyl)ferrocene

Usually, ortho lithiation of Kagan's template 1 and quenching with electrophiles leads highly diastereoselectively to planar‐chiral 1,2‐disubstituted ferrocenes. Surprisingly, lithiation of 1 with lithium diisopropylamide (LDA) followed by addition of paraformaldehyde afforded regioisomer (+)‐{[S(S)]‐[4‐(2‐hydroxyethyl)phenyl]sulfinyl}ferrocene ( 2 ), which was converted to (+)‐{[S(S)]‐{4‐{2‐[(methylsulfonyl)oxy]ethyl}phenyl}sulfinyl}ferrocene ( 3 ) (Scheme 1). The desired diastereoisomer (l)‐1‐(hydroxymethyl)‐2‐(p‐tolylsulfinyl)ferrocene ( 5 ) in turn could also be obtained by ortho lithiation of 1 with LDA but by quenching with DMF to yield aldehyde 4 first, which then was reduced with NaBH₄ to 5 . Finally, target compound (l)‐1‐[(dimethylamino)methyl]‐2‐(p‐tolylsulfinyl)ferrocene ( 6 ) was obtained by substitution of the OH group of 5 under mild conditions or directly by ortho lithiation of 1 with lithio‐2,4,6‐triisopropylbenzene (=2,4,6‐triisopropylphenyl)lithium; LTP) followed by quenching with N,N‐dimethylmethyleneiminium chloride. At low temperatures, reaction of 1 with LDA leads, via the preferred diastereoisomeric transition state ‘exo’‐ 7 and under extrusion of a (diisopropylamine)lithium complex of type 8 , in a highly selective manner, to diastereoisomeric ortho‐lithiated chelate (l)‐ 9 (Scheme 2). The reaction of 1 to 2 is explained by a rearrangement of (l)‐ 9 to {[S(S)] [4‐(lithiomethyl)phenyl]sulfinyl}ferrocene 10 , which is acid‐catalyzed by coordinated diisopropylamine in complexes of type 8 . This rearrangement is not observed if LTP is used as base or, in case LDA is applied, if the electrophile is sufficiently reactive at low temperatures.     

Mannich Reaction as a New Route for the Synthesis of Tetrahydro Pyrimido Benzimidazolyl Coumarins

Condensation of coumarin‐4‐acetic acids ( 1 ) with ortho‐phenylenediamine ( 2 ) in anhydrous phosphoric acid afforded 4‐((1H‐benzo[d]imidazol‐2‐yl)methyl)‐2H‐chromen‐2‐ones ( 3 ). Attempted Mannich reaction of 3 with formalin and primary amines resulted in 4‐(2‐phenyl‐1,2,3,4‐tetrahydrobenzo[4,5]imidazo[1,2‐c]pyrimidin‐4‐yl)‐2H‐chromen‐2‐ones ( 6 ). The structures of synthesized compounds were elucidated by analyses including 2D HETCOR and DEPT experiments. Synthesized compounds have been subjected for anti‐inflammatory activity. Compound 6j exhibited promising anti‐inflammatory activity.     

ortho‐Oxygenative 1,2‐Difunctionalization of Diarylalkynes under Merged Gold/Organophotoredox Relay Catalysis

Reported herein is an ortho‐oxygenative 1,2‐difunctionalization of diarylalkynes under merged gold/organophotoredox catalysis to access highly functionalized 2‐(2‐hydroxyaryl)‐2‐alkoxy‐1‐arylethan‐1‐ones. Detailed mechanistic studies suggested a relay process, initiating with gold‐catalyzed hydroalkoxylation of alkynes, to generate enol‐ether followed by a key formal [4+2]‐cycloaddition reaction. The successful application of the present methodology was also shown for the synthesis of benzofurans.     

Reactions of Peroxynitrite with Phenolic and Carbonyl Compounds: Flavonoids are not Scavengers of Peroxynitrite

Peroxynitrite (ONOO⁻, oxoperoxonitrate(1−)), an isomer of nitrate that oxidizes and nitrates biomolecules, is likely to be formed in vivo from the reaction of superoxide with nitrogen monoxide. To determine whether flavonoids scavenge peroxynitrite as postulated in the literature, we studied the reactions of peroxynitrite with phenol, hydroquinone, catechol, and the flavonoid monoHER. These reactions are first‐order with respect to peroxynitrous acid and zero‐order with respect to the organic compounds, and proceed as fast as the isomerization of peroxynitrous acid to nitrate. In vivo, a large fraction of all peroxynitrite is likely to react with carbon dioxide to form an unstable adduct, the 1‐carboxylato‐2‐nitrosodioxidane anion (ONOOCO). The presence of phenolic compounds did not influence the rate of disappearance of this adduct, which was ca. 4×10² s⁻¹. On the basis of these kinetic studies, it can be concluded that flavonoids are not scavengers of peroxynitrite. The products from the reaction of peroxynitrite with hydroquinone (benzene‐1,4‐diol) and catechol (benzene‐1,2‐diol) are para‐benzoquinone and ortho‐benzoquinone, respectively; no nitrated products were found. In a subsequent reaction, ortho‐quinone reacted with nitrite, a common contaminant of peroxynitrite preparations to form 1,2‐dihydroxy‐4‐nitrobenzene. We also investigated whether carbonyl compounds could redirect the reactivity of peroxynitrite toward nitration, as carbon dioxide does. The reaction with acetone is first‐order with respect to peroxynitrite and first‐order with respect to the carbonyl compound. The rate constant is 1.8 M ⁻¹s⁻¹ at neutral pH and 20°; peroxynitrite does not react with the carbonyl compounds dimethyl acetamide, L ‐alanylalanine, or methyl acetate. It is not likely that the carbonyl compounds or the mono‐, di‐, or polyphenolic compounds can scavenge peroxynitrite in vivo.     

The preparation of benzoisothiazolo[1,2‐b][1,2]isoquinolin‐11‐one‐1,1‐dioxides from dilithiated ortho‐toluic acids and lithiated methyl 2‐(aminosulfonyl)benzoate

Select dilithiated ortho‐toluic acids were prepared in excess lithium diisopropylamide and condensed with methyl 2‐(aminosulfonyl)benzoate followed by a twofold cyclization of intermediates to afford benzoisothiazolo[1,2‐b][1,2]isoquinolin‐11‐one‐1,1‐dioxides, a new fused‐ring heterocyclic system.