Browsing by Author "Jeena, Vineet."

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The application of semiconductors as oxidants in synthetic organic chemistry.
(2012) Jeena, Vineet.; Robinson, Ross Stuart.
Oxidation of alcohols is a vital transformation in synthetic organic chemistry as evidenced by their numerous applications in natural product synthesis. However, the traditional oxidants employ hazardous, toxic and malodorous reagents. A welcome addition to the field of alcohol oxidation is the emergence of tandem coupling reactions in which the oxidized alcohol is immediately trapped by an appropriate nucleophile. In previous research within the group, the synthesis of quinoxalines using a photocatalyzed tandem coupling approach was demonstrated. However, an extension of this research to other tandem coupling reactions was unsuccessful due to the high redox potential of the active oxidizing species. Thus, the immediate goal of this project was to develop a photocatalyzed oxidative system that was selective and high yielding. The assembling of this photooxidative system began with the choice of TiO2 and ZnO as the photocatalysts, Alizarin Red S as the dye, silver (I) ions as the electron acceptor and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) as the active oxidizing species. A test reaction was conducted using the dye sensitized TiO2/silver/TEMPOand dye sensitized ZnO/silver/TEMPO systems. While the dye sensitized ZnO system afforded a good yield the dye sensitized TiO2 system produced only a trace amount of product. Thus, a full study was conducted on a range of alcohols using the developed dye sensitized ZnO system. In addition, the oxidation of alcohols using the dye sensitized ZnO system could also be scaled-up with notable success. The developed system was applied to a one-pot tandem Wittig reaction which unfortunately was unsuccessful. The dye sensitized ZnO/silver/TEMPO system was however successful when applied to a pseudo- tandem Wittig reaction. Subsequently, an electron paramagnetic resonance (EPR) study was conducted using the dye sensitized TiO2/silver/TEMPO and the dye sensitized ZnO/silver/TEMPO system. The results of the EPR study supported the proposed mechanism for the dye sensitized ZnO system and revealed a ‘break’ in the photooxidative chain for the dye sensitized TiO2 system which accounted for the low yields obtained. As part of our interest in developing new photocatalyzed oxidative systems, attention was directed towards the application of impure diamondoid powder as a potential tandem coupling reagent. Using the diamondoid powder, the synthesis of highly conjugated quinoxalines was effected in moderate to excellent yields.
The direct oxidation of Cѕр³–h benzyl aryl ketones to 1,2-diaryl diketones as a key step for the multi-component synthesis of 2,4,5-trisubstituted-1h-imidazoles.
(2019) Jayram, Janeeka.; Jeena, Vineet.
Abstract available in PDF.
A green, solvent-free one-pot synthesis of disubstituted quinolines via A3-coupling using 1 mol% FeCl3.
(2016) Naidoo, Shivani.; Jeena, Vineet.
Quinoline derivatives are important compounds due to their broad range of applications in medicinal, synthetic and industrial chemistry. However, current synthetic techniques such as the classical Skraup, Döebner-Miller, Friedländer and Pfitzinger reactions have numerous drawbacks such as the use of expensive reagents/catalysts, strong acidic conditions, long reaction times, high temperatures and the use of environmentally unfriendly solvents. As a result, the development of new synthetic routes to access quinoline derivatives is vital. One such approach is the coupling between an aldehyde, amine and alkyne, known as A3-coupling, which results in the formation of propargylamine, which has been well documented in the literature. The synthesized propargylamines have multiple reaction sites and in the presence of a Lewis acid catalyst results in the formation of quinoline derivatives.
One-pot, multicomponent oxidative synthesis of 2,4,5-trisubstituted imidazoles from internal alkenes using an I2/DMSO system.
(2023) Majola, Nonhlelo.; Jeena, Vineet.
Imidazoles are vital heterocyclic compounds usually incorporated in natural products such as biotin, vitamin B12, histamine, and histidine. 2,4,5-trisubstituted imidazoles, in particular, possess versatile biological and pharmaceutical activities such as antidiabetic, antimalarial, and analgesic properties. A traditional procedure for the synthesis of these elegant compounds involves the cyclocondensation reaction between a 1,2-diketone, an aldehyde, and ammonia in the presence of an acid or metal catalyst. However, this methodology suffers from various shortcomings such as the use of acid or metal catalysts, tedious work-up procedures, use of toxic reagents, and substrate scope limitations. Hence, the development of new methods to synthesize 2,4,5-trisubstituted imidazoles is of vital importance. This study describes the preparation of 2,4,5-trisubstituted imidazoles from alkenes using an environmentally benign iodine/DMSO system. This novel methodology was applied to a broad substrate scope such as substituted benzaldehydes, heterocyclic aldehydes, bulkier aldehydes, and substituted stilbenes, and afforded the target compounds in moderate to high yields under mild reaction conditions. Preliminary mechanistic studies revealed that 1,2-diketone is a key intermediate and that the mechanism is not radical-mediated. It also revealed that the oxygen source is DMSO and that the coupling step is catalyzed by iodine coordination and hydrogen bonding from the solvent. Based on the results obtained from the preliminary mechanistic investigations, a reasonable mechanism is proposed.
Photocatalyzed tandem oxidation reactions and their application in the synthesis of quinoxalines.
(2009.) Jeena, Vineet.; Robinson, Ross Stuart.
No abstract available.
Synthesis of quinoline derivatives by a Doebner-von Miller reaction using a Ag(I)-exchanged Montmorillonite K10 catalyst.
Jayram, Janeeka.; Robinson, Ross Stuart.; Jeena, Vineet.
Quinolines play an important role in organic chemistry, as exemplified by their extensive application as biologically and pharmacologically active compounds. However, current methods available to access quinoline compounds employ harsh reaction conditions and expensive starting materials with varying product yields. The Döebner-von Miller reaction is a valuable asset in the synthesis of various natural and biologically active quinoline derivatives; however, there are various challenges associated with this methodology such as harsh reaction conditions, hazardous reagents (10 M hydrochloric acid), tedious isolation procedures, side products and low yields. Consequently, the need to develop a simple and environmentally friendly route to synthesize quinoline derivatives via the Döebner-von Miller reaction is essential. Silver(I)-exchanged Montmorillonite K10 was evaluated as a potential solid acid catalyst towards the synthesis of quinoline derivatives via the Döebner-von Miller reaction. Using this approach, the Döebner-von Miller reaction was evaluated under various reaction conditions with solvent-free, conventional heating conditions affording the best results. Using the optimized reaction conditions, a series of substituted quinoline derivatives were synthesized in moderate to excellent yields (42-89%) in 3 hours. The scope of our methodology towards both aromatic and aliphatic α, β-unsaturated aldehydes was also evaluated and the system was found to be equally efficient on both the substrates mentioned above. A recycle and reuse study was conducted in order to gain an accurate assessment of the activity of our catalyst and it was shown that it can be utilized several times without any appreciable loss in activity, thus making this procedure more environmentally benign. A literature comparison study was conducted and the yields obtained using the silver(I)-exchanged Montmorillnoite K10 approach were found to be superior to most previously reported approaches via the Döebner-von Miller reaction. In addition, silver(I)-exchanged Montmorillonite K10, was analyzed by Scanning Electron Microscopy (SEM) and Energy Dispersion X-Ray (EDX) analysis. The results of this study indicated a uniform distribution of silver(I) ions on the surface of Montmorillonite K10 with a total silver(I) content of 3.67 weight %.