Application of Linear Free Energy Relationships (LFER) to pKaH+ of Benzimidazolium Cations: Chemical Education Perspective

Application of Linear Free Energy Relationships (LFER) to pKaH+ data in water at 25o C of deprotonation of protonated fused ring systems like benzimidazolium cations is carried out in the present work. With a good comparison of the sites of substituents with reference to a functional group in benzene ring and the imidazolium ring, an excellent Hammett correlation is observed for the deprotonation of (pKaH+) of protonated fused ring systems like benzimidazolium cations. For the three substituents OH, MeO and Me at position 4 in the benzimidazole satisfy the correlation with I values. A positive Hammet  values of 1.93 indicates that electron withdrawing substituents facilitate the deprotonation. Under the same conditions a Taft * value of 1.11 is obtained for the deprotonation of 2-substituted-benzimidazolium cations. The available pKaH+ data in 5% aq. ethanol at 30o C of 2-methyl benzimidazolium cations and 2-(hydroxyethyl) benzimidazolium cations also followed Hammett correlation. The lower Hammett  value of 0.89 for 2-(hydroxyethyl) benzimidazolium cation series than that of 1.78 of 2-methyl benzimidazolium cation series is explained in terms of strong intramolecular hydrogen bonding in 2-(hydroxyethyl) benzimidazolium cation which resists the easy deprotonation. Deprotonation of 1-substituted-benzimidazolium cations did not follow Hammett relation.

Peculiarity of Symmetric Ring Systems with Double Y-Junctions and the magnetic effects

We discuss quantum dynamics in the ring systems with double Y-junctions in which two arms have same length. The node of a Y-junction can be parametrized by U(3). Considering mathematically permitted junction conditions seriously, we formulate such systems by scattering matrices. We show that the symmetric ring systems, which consist of two nodes with the same parameters under the reflection symmetry, have remarkable aspects that there exist localized states inevitably, and resonant perfect transmission occurs when the wavenumber of an incoming wave coincides with that of the localized states, for any parameters of the nodes except for the extremal cases in which the absolute values of components of scattering matrices take 1. We also investigate the magnetic disturbance to the symmetric ring systems.

Synthesis and characterization of some new heterocyclic compounds from α, β- unsaturated carbonyl system

An efficient and facile one pot synthesis of 3,4-dihydropyrimidinones (Belinelli compound) from furfural, acetylacetone and urea under solvent free conditions was performed, resulted in promising yield. These compounds reacted with benzaldehyde and furfural to give the corresponding chalcones. Chalcones are used to synthesize several derivatives like pyrazolines isoxazoles and pyrimidines having different heterocyclic ring systems. Keywords: Belinelli compounds; Chalcone; Pyrazolines isoxazoles; Pyrimidines

Synthesis of a Hexameric Magnesium 4-pyridyl Complex with Cyclohexane-like Ring Structure via Reductive C-N Activation

The reaction of [{(Arnacnac)Mg}2] (Arnacnac = HC{MeC(NAr)}2, Ar = 2,6-diisopropylphenyl, Dip, or 2,6-diethylphenyl, Dep) with 4-dimethylaminopyridine (DMAP) at elevated temperatures afforded the hexameric magnesium 4-pyridyl complex [{(Arnacnac)Mg(4-C5H4N)}6] via reductive cleavage of the DMAP C-N bond. The title compound contains a large s-block organometallic cyclohexane-like ring structure comprising tetrahedral (Arnacnac)Mg nodes and linked by linear 4-pyridyl bridging ligands, and the structure is compared with other ring systems. [(Dipnacnac)Mg(DMAP)(NMe2)] was structurally characterised as a by-product.

Optimisation and Scope of a Palladium-Catalysed Allylic Alkylation Cascade

<p>The design and development of new chemical reactions is crucial for progress in organic synthesis research. Cascade reactions, involving two or more steps carried out in situ in a single pot, provide a step-efficient and atom-economic route to synthesise polycyclic ring systems. The synthesis of new heterocyclic ring systems provides valuable routes towards complex natural products. Previous work in the Harvey group led to the development of a regioselective palladium-catalysed allylic alkylation (Pd-AA) cascade. This research aims to expand the scope and utility of this existing Pd-AA cascade, by optimising the current reaction conditions and exploring a range of non-symmetric pyran-based bis-electrophiles and nitrogen and sulfur-based β-carbonyl bis-nucleophiles. Isomeric 2,3-unsaturated silyl glycosides based on D-glucose and D-galactose were successfully synthesised. These substrates were assessed as bis-electrophiles in the Pd-AA cascade. The yield of the cascade was successfully optimised with the glucose-derived substrate 4-hydroxy-6-methylpyran-2-one, using Pd₂(dba)₃ and Xantphos, to 87% from the previously reported 77% yield. However, the galactose-derived silyl glycoside formed an undesired pyranone as the major product. Additionally, a series of β-dicarbonyl compounds (4-hydroxy-6-methylpyran-2-one analogues) were assessed as bis-nucleophiles in the Pd-AA cascade, with all of the analogues forming complex mixtures of side products and a fully unsaturated pyranone as the major isolated product.</p>

Optimisation and Scope of a Palladium-Catalysed Allylic Alkylation Cascade

<p>The design and development of new chemical reactions is crucial for progress in organic synthesis research. Cascade reactions, involving two or more steps carried out in situ in a single pot, provide a step-efficient and atom-economic route to synthesise polycyclic ring systems. The synthesis of new heterocyclic ring systems provides valuable routes towards complex natural products. Previous work in the Harvey group led to the development of a regioselective palladium-catalysed allylic alkylation (Pd-AA) cascade. This research aims to expand the scope and utility of this existing Pd-AA cascade, by optimising the current reaction conditions and exploring a range of non-symmetric pyran-based bis-electrophiles and nitrogen and sulfur-based β-carbonyl bis-nucleophiles. Isomeric 2,3-unsaturated silyl glycosides based on D-glucose and D-galactose were successfully synthesised. These substrates were assessed as bis-electrophiles in the Pd-AA cascade. The yield of the cascade was successfully optimised with the glucose-derived substrate 4-hydroxy-6-methylpyran-2-one, using Pd₂(dba)₃ and Xantphos, to 87% from the previously reported 77% yield. However, the galactose-derived silyl glycoside formed an undesired pyranone as the major product. Additionally, a series of β-dicarbonyl compounds (4-hydroxy-6-methylpyran-2-one analogues) were assessed as bis-nucleophiles in the Pd-AA cascade, with all of the analogues forming complex mixtures of side products and a fully unsaturated pyranone as the major isolated product.</p>

Studies of Some Strained-Ring Systems: Tricyclo-Octanes

<p>The present study has been directed towards the synthesis and base promoted rearrangements of some saturated and unsaturated halogenotricyclo-octanes. The preparation of a number of previously unknown 4,4,8- and 4,4,8,8-tetra-halogeno-anti-tricyclo[5,1,0,03,5]- octanes and the corresponding 3,3,8-tri- and 3,3,8,8-tetra-halogeno-anti-tricyclo[5,1,0,02,4] octanes has been accomplished and the stereo-chemistries of their ring systems established. The former tricyclo-octanes are base sensitive and on treatment with potassium t-butoxide give rise to mixtures of (E) - and (Z) -Beta-t-butoxystyrenes. However, the anti-tricyclo[5,1,0,02,4] octanes give no characterisable products on treatment with base. The mechanistic implications of the tricycle-octane - styrene conversion have been studied in some detail and halogenocyclo-octatetraenes and phenylacetylenes have been eliminated as intermediates. The behaviour of several methoxy-substituted 4,4,8,8-tetrahalogeno-anti-tricyclo[5,1,0,03,5] octanes with the same base has been studied, and it has been found that homotropilidenes are formed regiospecifically by 1,4-elimination. These conversions constitute the first known examples of such elimination experienced by ring-fused halogenocyclopropanes. From these studies, evidence is presented which implicates homotropilidenes and bicyclo[4,2,0]octa-2,4-dienes as intermediates in the production of styrenes, and a mechanism for the tricyclo-octane - styrene rearrangement has been proposed. A new and potentially versatile synthesis of 3,3-dihalogeno-anti tricyclo[5,1,0,02,4] oct-5-enes (anti-bishomobenzenes) has been developed. The new route employs readily available starting materials, involves sequential introduction of the cyclopropyl, gem-dihalogenocyclopropyl, and olefinic moieties and proceeds in an overall yield of about 14%. This represents a five-fold increase over the previously reported yields.</p>