Theoretical studies of borazynes and azaborines

Four isomers of didehydroborazine, B3N3H4, borazyne, and three isomers of azaborine, C4H4BN, are studied by DFT, CCSD, and CCSD(T) computational methods. The singlets of 1,2-borazyne (I) and 1,4-borazyne (IV) have angles about the boron of ca. 150 degrees . In 1,2-azaborine (V), the angles are ca. 140 degrees , while the N angles are ca. 112 degrees except in IV (127 degrees ) and 1,4- azaborine (VII, 120 degrees ). These geometries are almost reversed in the triplets. The 1,3-borazyne (III) shows more bicyclic character than the corresponding m-benzyne, with an N-N distance of 1.7 A. In all cases I was found to be lower in energy than the 2,4-borazyne (II), III, and IV. The order of stability of the azaborines is V > VII > 1,3- azaborine (VI). The nucleus-independent chemical shift (NICS) indicates that all isomers of borazyne are less aromatic than benzene and all isomers of azaborine are about as aromatic as benzene. Time-dependent DFT (TD-DFT) is used to study the excited states of the singlets. The significant structural differences between the singlet and triplet states suggest long phosphoresence lifetimes.     

A One-Pot synthesis of 1,2-Dihydro-1-arylnaphtho[1,2-e][1,3]oxazine-3-one Derivatives catalyzed by Perchloric Acid Supported on Silica (HClO4/SiO2) in the absence of solvent

1,2-Dihydro-1-arylnaphtho[1,2-e][1,3]oxazine-3-one derivatives were synthesized in high yields using a facile and one-pot condensation of 2-naphthol, aromatic aldehydes and urea catalyzed by perchloric acid supported on silica under thermal solvent-free conditions.     

A novel dilithiation approach to 3,4-dihydro-2H-1,3-benzothiazines, 3,4-Dihydro-2H-1,3-benzoxazines,and 2,3,4,5-tetrahydro-1,3-benzothiazepines

3,4-Dihydro-2H-1,3-benzothiazines 4, 3,4-dihydro-2H-1,3-benzoxazines 9, and 2,3,4,5-tetrahydro-1,3-benzothiazepines 6 were synthesized by directed ortho-lithiation of thiophenols and phenols and by side-chain lithiation of substituted thiophenols, respectively, in one-pot by reacting with N,N-bis[(benzotriazol-1-yl)methyl]amines 3 as 1,3-biselectrophile synthons.     

Nucleophilic aromatic substitution reactions of 1,2-dihydro-1,2-azaborine

Could go either way: The addition of nucleophiles to the parent 1,2-dihydro-1,2-azaborine and subsequent quenching with an electrophile generates novel substituted 1,2-azaborine derivatives. Mechanistic studies are consistent with two distinct nucleophilic aromatic substitution pathways depending on the nature of the nucleophile.     

Recent advances in azaborine chemistry

The chemistry of organoboron compounds has been primarily dominated by their use as powerful reagents in synthetic organic chemistry. Recently, the incorporation of boron as part of a functional target structure has emerged as a useful way to generate diversity in organic compounds. A commonly applied strategy is the replacement of a CC unit with its isoelectronic BN unit. In particular, the BN/CC isosterism of the ubiquitous arene motif has undergone a renaissance in the past decade. The parent molecule of the 1,2-dihydro-1,2-azaborine family has now been isolated. New mono- and polycyclic B,N heterocycles have been synthesized for potential use in biomedical and materials science applications. This review is a tribute to Dewar's first synthesis of a monocyclic 1,2-dihydro-1,2-azaborine 50 years ago and discusses recent advances in the synthesis and characterization of heterocycles that contain carbon, boron, and nitrogen.     

Nouvelle voie d'acces aux dihydro-3,4-2H-benzoxazines-1,3

3,4-Dihydro-2H-1,3-benzoxazines were prepared from primary amines and dihalocompounds by a method which can lead to compounds dissymetrically substituted in positions 2 and 4.     

Concise synthesis and anti-HIV activity of pyrimido[1,2-c][1,3]benzothiazin-6-imines and related tricyclic heterocycles

3,4-Dihydro-2H,6H-pyrimido[1,2-c][1,3]benzothiazin-6-imine (PD 404182) is a virucidal heterocyclic compound active against various viruses, including HCV, HIV, and simian immunodeficiency virus. Using facile synthetic approaches that we developed for the synthesis of pyrimido[1,2-c][1,3]benzothiazin-6-imines and related tricyclic derivatives, the parallel structural optimizations of the central 1,3-thiazin-2-imine core, the benzene part, and the cyclic amidine part of PD 404182 were investigated. Replacement of the 6-6-6 pyrimido[1,2-c][1,3]benzothiazin-6-imine framework with 5-6-6 or 6-6-5 derivatives led to a significant loss of anti-HIV activity, and introduction of a hydrophobic group at the 9- or 10-positions improved the potency. In addition, we demonstrated that the PD 404182 derivative exerts anti-HIV effects at an early stage of viral infection.     

Synthesis of 2H-pyrroles via the 1,3-dipolar cycloaddition reaction of nitrile ylides with acrylamides

3,4-Dihydro-2H-pyrrole derivatives were synthesized by the 1,3-dipolar cycloaddition reaction of nitrile ylides with acrylamides. Acrylamide substitution patterns and benzimidoyl chloride equilibration were investigated.     

Cyclization of fluoroalkyl-containing 1,3-dicarbonyl compounds with ethylenediamine hydroperchlorate to 1,4-diazepines

2,3-Dihydro-1H-1,4-diazepines and 1,2,3,4-tetrahydro-1,4-diazepin-5-ones were obtained by direct condensation of fluorine-containing 1,3-diketones and 1,3-keto ethers, respectively, with ethylenediamine monohydroperchlorate. The structure of the compounds was confirmed by¹H NMR, mass spectrometry and molecular weight determination.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 890–896, April, 1991.     

Radikalische Cyclisierungen von Alkenyl-substituierten 4,5-Dihydro-1,3-thiazol-5-thiolen

Radical Cyclizations of Alkenyl-Substituted 4,5-Dihydro-1,3-thiazole-5-thiols Heating of 5-alkenyl- or 5-alkinyl-4,5-dihydro-1,3-thiazole-5-thiols of type 5 in the presence of a radical initiator gave dithiaspirobicycles in fair-to-excellent yield (Scheme 3). Under analogous conditions, the 4,5-dihydro-4-vinyl-1,3-thiazole-5-thiol 5d underwent a cyclization to give the annellated dithiabicycle 7 (Scheme 4). In this reaction, a minor product 8 was formed by an unknown reaction mechanism. The structure of 8 was established by X-ray crystallography. The starting 1,3-thiazole-5-thiols 5 have been synthesized by carbophilic alkylation of me C?S group of 1,3-thiazole-5(4H)-thiones with Grignard-reagents or alkylcuprates. The thiazolethiones were obtained by the reaction of 3-amino-2H-azirines with thiobenzoic acid followed by sulfurization and cyclization. The 4-benzyl derivative 1b was thermally rearranged via 1,3-benzyl migration to yield the benzyl (1,3-thiazol-5-yl) sulfide 11 (Scheme 5).     

Synthesis and isolation of 5,6-dihydro-4H-1,3-oxazine hydrobromides by autocyclization of N-(3-bromopropyl)amides

5,6-Dihydro-4H-1,3-oxazine hydrobromides have been synthesized by the nucleophilic autocyclo-O-alkylation of N-(3-bromopropyl)amides under neutral conditions in chloroform. It is found that electron-donating amide α-substituents influence the autocyclization efficiency.     

Highly stereoselective addition reactions of metallated trans-1,3-dithiane-1,3-dioxide to aldehydes

The sodium anion of trans-1,3-dithiane-1,3-dioxide reacts with unhindered aromatic and heteroaromatic aldehydes to give adducts with 95:5 to 97:3 diastereoselectivity.     

Palladium-catalyzed cross-coupling of cyclopropylmethyl N-tosylhydrazones with aromatic bromides: an easy access to multisubstituted 1,3-butadienes

Direct synthesis of 1,1-disubstitued 1,3-butadienes has been efficiently realized from the cross-coupling of cyclopropylmethyl N-tosylhydrazones with aromatic bromides by means of PdCl₂(MeCN)₂ as catalyst. 1,1,4-Trisubstitued 1,3-butadiene derivatives were obtained in up to 70% yields through a one-pot procedure catalyzed by Pd(OAc)₂ in the presence of excessive amount of aromatic bromides. The present methodology provides an easy and efficient route to multisubstituted 1,3-butadienes.     

Reactions of cyclic 1,3-dicarbonyl compounds with 1,2(1,4)-dihydro-1-methyl-2(4)-methylene-N-heterocycles. A new access to 6,12-methano-dibenz[d,g]-[1,3]oxazocinones

Summary The enamine-type methylene-N-heterocycles1–5 react with cyclic 2-ethoxymethylene-1,3-dicarbonyl compounds6 to give 2-[2-(hetarylidene)ethylidene]-1,3-dicarbonyl compounds7–14. The result of the reactions between 1,2-dihydro-1-methyl-2-methylene-quinoline (1a) and cyclic 1,3-dicarbonyl compounds depends on the nature of the dihydro intermediatesA/B. Dehydrogenation of keton intermediatesA results in 2-(1,2-dimethyl-4(1H)-quinolylidene)-1,3-dicarbonyl compounds17–21. Enol intermediatesB with 6-membered dicarbonyl ring form 6,12-methano-dibenz-[d,g][1,3]oxazocinones22–25.¹H NMR spectra and X-ray structure analysis prove the structure of23.

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Reaktionen cyclischer 1,3-Dicarbonylverbindungen mit 1,2(1,4)-Dihydro-1-methyl-2(4)-methylen-N-heterocyclen. Ein neuer Zugang zu 6,12-Methano- dibenz[d,g][1,3]oxazocinonen

Zusammenfassung Aufgrund ihres Enamincharakters reagieren die Methylen-N-heterocyclen1–5 mit cyclischen 2-Ethoxymethylen-1,3-dicarbonylverbindungen6 zu den 2-[2-(Hetaryliden)ethyliden]-1,3-dicarbonylverbindungen7–14. Das Ergebnis der Reaktionen zwischen 1,2-Dihydro-1-methyl-2-methylen-chinolin (1a) und cyclischen 1,3-Dicarbonylverbindungen hängt von der Natur der zwischenzeitlich entstehenden DihydroverbindungenA/B ab. Die Intermediat-KetoneA gehen durch Dehydrierung während der Reaktion in die 2-(1,2-Dimethyl-4(1H)chinolyliden)-1,3-dicarbonylverbindungen17–21 über. Die Intermediat-EnoleB mit sechsgliedrigem Dicarbonylring bilden in intramolekularer Reaktion die 6,12-Methano-dibenz[d,g][1,3]oxazocinone22–25, deren Struktur am Beispiel der Verbindung23 durch¹H-NMR sowie durch Röntgenkristallstrukturanalyse bewiesen wird.

     

Electrophilic aromatic substitution reactions of 1,2-dihydro-1,2-azaborines

The aromatic boron-nitrogen heterocycle 1,2-dihydro-1,2-azaborine undergoes classical electrophilic substitution. These reactions allow easy functionalization to provide a variety of 3- and 5-substituted derivatives. [reaction: see text].     

Synthesis of the novel thieno[4,3,2-ef][1,4]benzoxazepine ring system: 4,5-Dihydro-3-(4-pyridinyl)thieno[4,3,2-ef][1,4]benzox-azepine maleate

4,5-Dihydro-3-(4-pyridinyl)-thieno[4,3,2-ef][1,4]benzoxazepine maleate 2 has been synthesized from 3-amino-4-fluorobenzo[fc]thiophene by employing an intramolecular nucleophilic aromatic fluoride displacement. In the presence of strong base and heat, 2 rearranges to form the isomeric hemiaminal, 3,4-dihydro-4-methyl-3-(4-pyridinyl)thieno[4,3,2-ef][1,3]benzoxazine 10 . A proposed mechanism for this rearrangement is discussed.     

Numerical Investigation on 1,3-Butadiene/Propyne Co-pyrolysis and Insight into Synergistic Effect on Aromatic Hydrocarbon Formation

A numerical investigation on the co-pyrolysis of 1,3-butadiene and propyne is performed to explore the synergistic effect between fuel components on aromatic hydrocarbon formation.A detailed kinetic model of 1,3-butadiene/propyne co-pyrolysis with the sub-mechanism of aromatic hydrocarbon formation is developed and validated on previous 1,3-butadiene and propyne pyrolysis experiments.The model is able to reproduce both the single component pyrolysis and the co-pyrolysis experiments,as well as the synergistic effect between 1,3-butadiene and propyne on the formation of a series of aromatic hydrocarbons.Based on the rate of production and sensitivity analyses,key reaction pathways in the fuel decomposition and aromatic hydrocarbon formation processes are revealed and insight into the synergistic effect on aromatic hydrocarbon formation is also achieved.The synergistic effect results from the interaction between 1,3-butadiene and propyne.The easily happened chain initiation in the 1,3-butadiene decomposition provides an abundant radical pool for propyne to undergo the H-atom abstraction and produce propargyl radical which plays key roles in the formation of aromatic hydrocarbons.Besides,the 1,3-butadiene/propyne co-pyrolysis includes high concentration levels of C3 and C4 precursors simultaneously,which stimulates the formation of key aromatic hydrocarbons such as toluene and naphthalene.     

Biphilic properties of substituted triphenylphosphoranylideneacetonitriles bearing 4,5-Di(arylsulfonyl)-1,3-thiazol-2-yl fragments at the ylide center

Phosphonium ylides stabilized by nitrile and 4,5-di(arylsulfonyl)-1,3-thiazol-2-yl fragments show biphilic reactivity. They react with aromatic aldehydes upon heating revealing a noticeable activity of ylide center and are condensed with typical nucleophiles at C⁴ center of the thiazole ring with the elimination of arylsulfonyl group under mild conditions. New 1,3-thiazole derivatives were obtained whose structures were proved by chemical, spectal and X-ray structural investigations.     

Direct oxidative conversion of alkyl halides into nitriles with molecular iodine and 1,3-diiodo-5,5-dimethylhydantoin in aq ammonia

Various benzylic halides were smoothly and directly converted into the corresponding aromatic nitriles in high yields using molecular iodine and 1,3-diiodo-5,5-dimethylhydantoin, respectively, in aq ammonia. Similarly, primary alkyl halides were also converted into corresponding nitriles in moderate to good yields using molecular iodine and 1,3-diiodo-5,5-dimethylhydantoin in aq ammonia, although a long reaction time was required. The present reaction is a new method for the preparation of aromatic nitriles from benzylic halides and a new method for the conversion of alkyl halides into corresponding nitriles with retention of the number of carbon atoms.