Professor and George A. and Judith A. Olah Nobel Laureate Chair in Hydrocarbon Chemistry
Organic and Polymer Chemistry
Ph.D., 1978, University of Southern California
M.S., 1974, Indian Institute of Technology, Madras, India
B.S., 1972, Bangalore University, India
Office: LHI 203
Phone: (213) 740-5984
Email: gprakash@usc.edu
Research Focus
Synthetic Organic, Mechanistic, Hydrocarbon and Energy Related Chemistry
Professor Prakash and his group’s research interests cover a wide range of subjects in the area of selective fluorinations, oxidations, energetic materials, reductions, stereoselective reactions, electrochemical synthesis, hydrocarbon activation and isomerization, anthropogenic CO2 based fuels and feed-stocks, direct oxidation fuel cells, lithium ion battery electrolytes, iron batteries, flow batteries, electrochemistry, polymer chemistry, superacid catalyzed reactions, stable carbocation chemistry, application of ab initio and DFT theory and NMR chemical shift calculations. Our group also utilizes extensively all modern spectroscopic and analytical tools in organic structure characterization as well as in mechanistic studies. Synthetic Organic Chemistry Our efforts in this area mainly emphasize the development of new reactions and reagents, which greatly benefit practicing synthetic organic chemists. Although our goal is not target molecule synthesis oriented, development of single step selective and stereoselective transformations is of immense value in general organic synthesis. We have developed many fluorination protocols based on pyridinium polyhydrogen fluorides (ionic liquids) as room temperature nucleophilic fluorinating agents. Many of the methods replace the use of highly toxic HF and elemental fluorine. Selective trifluoromethylations, difluoromethylations and difluoromethylenations have been achieved using the trifluoromethyltrimethylsilane (the Ruppert-Prakash Reagent). Related fluoroalkylations using sulfur-based reagents have shown great promise. Such fluorinated methods have gained wide use in the area of small molecule based drug discovery. A number of silicon reagents such as trialkylsilanes, trimethylsilyl nitrile and azide have been developed as useful synthons. A new ionic hydrogenation method using trialkylsilanes as reducing agent has resulted in general-purpose ether and sulfide synthesis method. We have adopted the method for the preparation of polymers and crown ethers. Increasingly, superacids serve as excellent high acidity medium for electrophilic reactions (so called superelectrophilic activation). Using CF3SO3H or BF3-H2O as a high acidity and non-oxidizing medium, a host of new reactions for deactivated aromatics such as iodination, acylation, nitration etc., are being developed. Use of solid strong acid catalysts such as Nafion-HR (an ionomeric perfluoroalkane type sulfonic acid) are also being investigated. New precursors for carbene based photoaffinity probes were also developed. New polymer bead chemistry at nanoscale (nanochemistry) has been ongoing. The nanometer scale polymer spheres with pendant surface functionalities serve as hosts for metal nanoparticles, which can be used as high surface area catalysts.
Mechanistic Studies
These studies involve generation of reactive electrophilic intermediates such as carbocations, carbodications, halonium ions, diazonium ions, oxonium ions, acylium ions, thioacylium ions, nitrenium ions, silicenium ions and selenonium ions in low nucleophilicity highly acidic solvent systems and their characterization using low temperature broad-band nuclear magnetic resonance spectroscopy (1H, 2H, 13C, 19F, 17O, 29Si, 77Se, 15N, 35Cl, etc.,). Other techniques such as infrared and X-ray photoelectron spectroscopy are also employed to characterize their structures. Using the above methods, a wide variety of trivalent (classical) and bridged (non-classical) carbocations have been characterized along with some new aromatic cationic systems. Several empirical correlations to relate positive charge density and chemical shifts were developed. Other reactive intermediates studied include carbanions and oxonium ylides. In conjunction with these studies a wide array of two-dimensional NMR techniques and special pulse sequences are routinely employed. Solid-state 13C NMR spectra of carbocation salts are also routinely obtained using cross polarization magic angle spinning techniques (CPMAS). In this area ab initio and DFT calculations are routinely employed to delineate structure and energetics of complex carbocation structures. The minimized structures are also used in NMR chemical shift calculations.
Hydrocarbon and Energy Related Chemistry
Utilization of saturated hydrocarbons including methane as raw materials to synthesize value added compounds. Use of hydrocarbons and oxygenated hydrocarbons (particularly methanol) as fuels in direct oxidation fuel cells. Hydrocarbon isomerization, functionalization (nitration, carbonylation, hydroxylation, sulfuration, etc.,) and synthesis of polycyclic cage hydrocarbons. Polymerization of ethylene and α-olefins to polyolefins by aluminum subhalide chemistry. Polymerization of unusual monomers. Proton conducting polymer electrolytes for fuel cells and lithium ion batteries. Superacid facilitated upgrading of fossil fuels (such as coal, methane, etc.) to industrially useful feedstocks. Electrochemical reduction of anthropogenic carbon dioxide to methanol and related derivatives. Development of CO2 capture and carbon neutral energy cycles through the Methanol EconomyTM concept. Development of rechargeable aqueous batteries based on iron- nickel and organic redox flow materials.