Base-mediated 1,3-dipolar cycloaddition of pyridinium bromides with bromoallyl sulfones: a facile access to indolizine scaffolds

An expedient and transition-metal-free synthetic strategy has been developed for the construction of substituted indolizines from a unique combination of pyridinium salts and 2-bromoallyl sulfones. This approach does not compromise with the diverse substitutions on both the pyridinium salts and 2-bromoallyl sulfones. Wide substrate scope, operational simplicity, milder reaction conditions and good to moderate yields are the merits associated with the current approach. Moreover, this method provides two products which are amenable for the generation of a library of key indolizine building blocks.

Thermal decomposition of syn- and anti-dihydropyrenes; functional group-dependent decomposition pathway

Syn and anti dihydropyrene (DHP) are excellent thermochromes, and therefore extensively studied for their thermochromic and photochromic properties, respectively. However, they suffer from thermal decomposition due to thermal instability. In this study, we thoroughly investigated pathways for the thermal decomposition of anti- and syn- dihydropyrenes through computational methods. The decomposition pathways include sigmatropic shift and hemolytic and heterolytic (cationic and anionic) cleavages. The decomposition pathway is influenced not only by the dihydropyrene (syn- or anti-) but also by the functional groups present. For anti-dihydropyrenes, sigmatropic shift is the most plausible pathways for CN and CHO internal groups. The cascade of sigmatropic shifts is followed by elimination to deliver substituted pyrenes. For CH₃- and H- dihydropyrenes, hemolytic cleavage of the internal groups is the most plausible pathway for decomposition to pyrenes. The pathway is changed to heterolytic cleavage when the internal groups on the dihydropyrenes are Cl^-, Br^-, and SMe^-. Comparison of the activation barriers for syn (30.18 kcal mol^-1) and anti (32.10 kcal mol^-1) dimethyldihydropyrenes for radical pathway reveal that decomposition of syn- DHP is more facile over anti-, which is consistent with the experimental observation. The decomposition pathway for syn-dihydropyrene is also hemolytic in cleavage when the internal groups are methyl and hydrogen. Syn-dihydropyrenes (symmetrical or unsymmetrical) bearing CN group do not follow sigmatropic shift, quite contrary to the anti-dihydropyrene. The lack of tendency of the syn-dihydropyrene for sigmatropic shift is rationalized on the planarity of the scaffold. The results of the theoretical study are consistent with the experimental observations. The results here help in understanding the behavior of substituents on the dihydropyrene scaffold, which will be useful in designing new molecules with improved thermal stabilities. Graphical abstract Functional group dependent decomposition pathways of dihydropyrenes.

Acridinedione as selective flouride ion chemosensor: a detailed spectroscopic and quantum mechanical investigation

The use of small molecules as chemosensors for ion detection is rapidly gaining popularity by virtue of the advantages it offers over traditional ion sensing methods. Herein we have synthesized a series of acridine(1,8)diones (7a-7l) and explored them for their potential to act as chemosensors for the detection of various anions such as fluoride (F-), acetate (OAc-), bromide (Br-), iodide (I-), bisulfate (HSO4 -), chlorate (ClO3 -), perchlorate (ClO4 -), cyanide (CN-), and thiocyanate (SCN-). Acridinediones were found to be highly selective chemosensors for fluoride ions only. To investigate in detail the mechanism of selective fluoride ion sensing, detailed spectroscopic studies were carried out using UV-visible, fluorescence and 1H NMR spectroscopy. Fluoride mediated (NH) proton abstraction of acridinedione was found to be responsible for the observed selective fluoride ion sensing. Quantum mechanical computational studies, using time dependent density functional theory (TDDFT) were also carried out, whereupon comparison of acridinedione interaction with fluoride and acetate ions explained the acridinedione selectivity for the detection of fluoride anions. Our results provide ample evidence and rationale for further modulation and exploration of acridinediones as non-invasive chemosensors for fluoride ion detection in a variety of sample types.

Chloroacetate Promotes and Participates in the Oxidative Annulation of Pyridines/Isoquinoline by Using Oxygen as the Oxidant

The aerobic oxidative annulation of chalcones, pyridines/isoquinoline and ethyl chloroacetate to indolizines was achieved by cascade reaction. Various functional groups on chalcones were tolerated. And different pyridines derivatives could also be suitable substrates. Ethyl chloroacetate is an essential component in participating of the oxidative annulation process. Overall, this protocol is very practical and efficient by using molecular oxygen as oxidant with high selectivity for the annulation product.

Catalytic σ-activation of carbon–carbon triple bonds: reactions of propargylic alcohols and alkynes

A variety of organic transformations have been developed based on conceptually novel catalytic σ-activation of CC bonds of alkynes.

Gas hydrates model for the mechanistic investigation of the Wittig reaction “on water”

Water in action! A gas hydrate model consisting of 20 water molecules nicely illustrates acceleration of cis-Wittig reaction over trans-Wittig reaction "on water". "Bucky" water is a perfect model for describing chemical reactions "on water".

Dyotropic rearrangement of bridgehead substituents in closed dithienylethenes; conjugated verses non-conjugated analogues

Type I dyotropic rearrangement reactions of halogen and methyl substituents at the bridgehead position of diarylethenes and dihydroarylethenes have been studied through density functional theory at B3LYP/6-31+G(d) level. The calculations have been performed to explore the dyotropic rearrangement as a possible factor for the elusive nature of halogenated dithienylethenes (closed). The dyotropic rearrangement process in closed dithienylethenes is then compared with the dihydro analogues. Moreover, the effect of hetero atom and conjugation is also explored through quantum mechanical calculations.

An I2-catalyzed oxidative cyclization for the synthesis of indolizines from aromatic/aliphatic olefins and α-picoline derivatives

A novel I₂-catalyzed intermolecular oxidative tandem cyclization reaction of aromatic/aliphatic olefins and α-picoline derivatives has been achieved for the synthesis of indolizines under metal-free conditions.

Mechanistic insight of TiCl4catalyzed formal [3 + 3] cyclization of 1,3-bis(silyl enol ethers) with 1,3-dielectrophiles

The mechanism of TiCl₄mediated formal [3 + 3] cyclization of 1,3-bis(silyl enol ethers) with 1,3-dielectrophiles is studied at the B3LYP level of density functional theory (DFT) to rationalize the experimental regioselectivity.

Aromaticity of azines through dyotropic double hydrogen transfer reaction

Density functional theory calculations have been performed at B3LYP/6-31+G (d) level to quantify the aromaticities of mono- to triazines through dyotropic double hydrogen transfer (DDHT) reaction. The reaction was chosen such that the azines are products of double hydrogen dyotropic rearrangement, and activation barriers and energies of the reactions were functions of the aromaticities of azines. Small activation barriers and high energies of reactions were characteristic of the reactions delivering highly aromatic azines. Synchronicity, reaction energies and energies of activation have been analyzed, and the aromaticity values obtained thereof were compared with the aromaticity values from other geometric and magnetic criteria. Energies of activation were found superior to the energies of reaction for the determination of the aromaticities. Aromaticities of most of the azines were comparable to the aromaticity of benzene. Activation barriers and reaction energies for the dyotropic reactions delivering contiguous or polynitrogeneous azines had thermodynamic contributions arising from the contiguous nature of azines, in addition to the aromaticity related thermodynamic contributions. Moreover, the aromaticity values of azines are also affected by the fusion of azine to the reaction center. When corrected for these factors, the aromaticities of azines using energies of activation for DDHT correlated nicely with the aromaticities of azines reported in the literature through NICS(0)πzz and some other energetic methods.

Silver-catalyzed cyclization of 2-pyridyl alkynyl carbinols with isocyanides: divergent synthesis of indolizines and pyrroles

Divergent syntheses of indolizines and 2,4-disubstituted pyrroles by the silver-catalyzed cyclization of 2-pyridyl alkynyl carbinols with isocyanides are reported.

Palladium-catalyzed carbonylative cyclization/arylation cascade for 2-aroylindolizine synthesis

An efficient synthesis of densely substituted 2-aroylindolizines via the palladium-catalyzed carbonylative cyclization/arylation is reported. This transformation proceeds via the 5-endo-dig cyclization of 2-propargylpyridine triggered by an aroyl Pd complex. It produced diversely substituted 2-aroylindolizines in good to excellent yields.