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G K Surya Prakash

Publications and source records attributed to G K Surya Prakash.

At least 19 recordsLinked to original sources
PubMed2007

Efficient one-pot synthesis of fluorinated benzimidazolines, benzothiazolines, benzoxazolines, and dihydrobenzoxazinones using gallium(III) triflate as a catalyst.

One-pot synthesis of fluorinated benzimidazolines, benzothiazolines, benzoxazolines, and dihydrobenzoxazinones was easily achieved under mild conditions in high yields and purity through gallium(III) triflate mediated condensation-cyclization. Introduction of fluorine atoms favors the formation of the five-membered heterocycles over seven-membered heterocycles. [reaction: see text].

Journal Article↗
PubMed2006

Comparative study of the hypercoordinate ions C7H9+ and C8H9+ by the ab initio/GIAO-CCSD(T) method.

A comparative study of the hypercoordinate square-pyramidal carbocations C7H9+ and C8H9+ was performed by the ab initio/GIAO-CCSD(T) method. The structures and 13C NMR chemical shifts of the cations were calculated at the GIAO-CCSD(T)/tzp/dz//MP2/cc-pVTZ level. The bishomo square pyramidal structure 1 was calculated for C7H9+ at the MP2/cc-pVTZ level. The calculated 13C NMR chemical shifts of structure 1 agree extremely well with the experimental values. However, unlike for C7H9+ both the bishomo square pyramidal structure 3 and the trishomocyclopropenium type structure 4 were found to be minima on the potential energy surface of C8H9+. They are very close energetically with cation 3, only 0.7 kcal/mol less stable than cation 4 at the MP2/cc-pVTZ//MP2/cc-pVTZ + ZPE level. Neither structure 3 nor 4 yields NMR spectra that agree with experiment. However, a weighted average of the two reproduces the observed NMR spectrum of C8H9+ (at -80 degrees C) quite well.

Journal Article↗
PubMed2006

Facile synthesis of TMS-protected trifluoromethylated alcohols using trifluoromethyltrimethylsilane (TMSCF3) and various nucleophilic catalysts in DMF.

Organofluorine compounds are becoming increasingly important in different fields, such as material science, agro chemistry, and the pharmaceutical industry. Nucleophilic trifluoromethylation is one of the widely used methods to incorporate a trifluoromethyl moiety into organic molecules. We have carried out extensive studies to develop varieties of easily accessible nucleophilic catalysts to promote such reactions. TMS-protected trifluoromethylated alcohols were prepared from both aldehydes and ketones in excellent yields using catalytic amount of amine N-oxide. Carbonate and phosphate salts also showed efficient catalytic activity toward this reaction. These reactions were highly solvent dependent, and DMF was found to be the most suitable one among the various solvents studied. All these reactions proceeded under very mild conditions, giving clean products and avoiding the use of any fluoride initiators or expensive catalysts, and extremely water-free conditions. The mechanism for the reaction is discussed in detail. DFT calculations were performed on the possible reaction intermediates using the Gaussian 03 program at B3LYP/6-311+G* level to support the proposed mechanism.

Alcohols↗
PubMed2006

Chiral alpha-branched benzylic carbocations: diastereoselective intermolecular reactions with arene nucleophiles and NMR spectroscopic studies.

The chiral benzylic alcohols 1-6 were prepared and subjected to S(N)1-type displacement reactions with various arene nucleophiles in acidic medium. Under optimized conditions (HBF(4).OEt(2), CH(2)Cl(2), -78 degrees C --> r.t.) the corresponding 1,1-diarylalkanes 11-18 and 20 were obtained in good chemical yields (48-99%). The facial diastereoselectivity of the reaction is high (d.r. = 91/9-97/3) when the substrate bears a stereogenic carbon center -CHtBuMe in the alpha-position to the electrophilic carbon atom. If the starting material was enantiomerically pure, no significant racemization was observed (94% ee --> 92% ee). The reactions proceed stereoconvergently as demonstrated by the conversion of the separated diastereoisomers syn-1a and anti-1a in separate reactions to the same product syn-11 (d.r. = 97/3). Further evidence for long-lived chiral benzylic carbocations as reaction intermediates was obtained from NMR studies in superacidic medium. The chiral cation 24 was generated in SO(2)ClF as the solvent at -70 degrees C employing SbF(5) as the Lewis acid and characterized by its (1)H and (13)C NMR spectra. NOE measurements suggest a preferred conformation in which the diastereotopic faces of the cation are differentiated by the two carbon substituents R and Me at the stereogenic carbon center in the alpha-position. The hypothesis is further supported by the observation that the diastereoselectivity of the substitution reaction decreases if the bulky tert-butyl (R = tBu) substituent in the substrate 1a is replaced by a smaller ethyl group (2a, R = Et).

Journal Article↗
PubMed2006

Ab Initio/GIAO-CCSD(T) study of bicyclic and related strained olefins. Structures and 13C NMR chemical shifts.

Bicyclic and related strained olefins were studied by the ab initio/GIAO-CCSD(T) method. Structures and (13)C NMR chemical shifts of the olefins were calculated using ab initio/GIAO-CCSD(T) method. The delta(13)C of the olefinic carbons of the yet unknown bicyclo[1.1.0]but-1,3-ene 1 and bicyclo[2.1.0]pent-1(4)-ene 2 were computed to be 69.4 and 212.4, respectively, at the GIAO-CCSD(T)/qzp/tzp//MP2/cc-pVTZ level. The delta(13)C of the olefinic carbons of the intriguing (larger and also yet unknown) tricyclo[3.3.1.0(3,7)]non-3(7)-ene 6 and cubene 7 were calculated to be 172.5 and 187.4, respectively, at the GIAO-CCSD(T)/tzp/dz//MP2/cc-pVTZ level. In a related study, the relative energies of the various conformers of ethylene were computed and were found to correlate extremely well with the (13)C NMR chemical shifts, reflecting the linear dependence of the (13)C NMR chemical shifts on the internal strain of the molecules.

Journal Article↗
PubMed2006

Electrophilic intermediates and their reactions in superacids.

Acid-catalyzed reactions have played a major role in hydrocarbon chemistry involving electron-deficient intermediates such as carbocations, carbodications, onium ions, etc. The pioneering discovery of the use of superacids by George A. Olah, in the early 1960s, to characterize such intermediates under so-called long-lived stable ion conditions led to the understanding of their structures and reactivity patterns much more clearly. Continuing studies in this area in the past 30 years have resulted in a paradigm shift in comprehending the stability and reactivity of electrophilic intermediates in superacid media.

Acids↗
PubMed2006

BF3.2CF3CH2OH (BF3.2TFE), an efficient superacidic catalyst for some organic synthetic transformations.

BF3.2CF3CH2OH complex was found to be a very effective superacidic catalyst comparable in acid strength to at least that of 100% anhydrous sulfuric acid for various acid-catalyzed organic transformations such as isomerizations, rearrangements, ionic hydrogenation of various ketones, and aromatics with triethylsilane and nitration of aromatics with metal nitrate. Studies of the pivalaldehyde-methyl isopropyl ketone rearrangement and the benzopinacol to phenanthrene transformation suggest that the complex has an acidity comparable to that of 100% anhydrous sulfuric acid. The structure and properties of the 1:2 boron trifluoride-trifluoroethanol complex have been further studied using NMR (1H, 13C, 19F, 11B) and DFT calculations at the B3LYP/6-311++G//B3LYP/6-31G level.

Journal Article↗
PubMed2006

Theoretical study of AlH(2+)n (n=1-7) dications.

The structures and stability of AlH(2+)n (n = 1-7 )dications were calculated at the ab initio MP2/aug-cc-pVTZ level of theory. The dications AlH(2+) 1 and AIH(2+2) 2 were characterized to be unstable thermodynamically. However, these and the stable dications, AlH(2+)n (n = 3-7) 3-7 have considerable kinetic barriers for deprotonation. Each of the structures 3-7 contains one or more two-electron three-center (2e-3c) bonds. Aluminum atoms of these dications carry most of the positive charges, as indicated by NBO charge calculations.

Aluminum Compounds↗
PubMed2006

Ab initio/GIAO-CCSD(T) study of propenoyl (H2C=CH-CO+) and isopentenoyl ((CH3)2C=CH-CO+) cations and their superelectrophilic protonated dications.

Structures of superelectrophilic protonated propenoyl (H2C=CH-COH2+) and isopentenoyl ((CH3)2C=CH-COH2+) dications and their parent cations were calculated using ab initio methods at the MP2/6-311+G and MP2/cc-pVTZ levels. Energies were calculated using Gaussian-2 (G2) theory. The alpha-carbon (Calpha) protonated 3 and 7 were found to be the global minima for protonated propenoyl and isopentenoyl dications, respectively. 13C NMR chemical shifts of the cations were also calculated using the GIAO-CCSD(T), GIAO-MP2 and GIAO-SCF methods. 13C NMR chemical shifts of the related tert-butyl cation ((CH3)3C+) and protonated tert-butyl dication ((CH3)2CCH4(2+)) were also computed at the same level to compare and explore the effect of the additional charge in dications.

Journal Article↗
PubMed2005

Oxygenation of alkane C-H bonds with methyl(trifluoromethyl)dioxirane: effect of the substituents and the solvent on the reaction rate.

[Chemical reaction: See text] The mechanism of the oxygenation of alkane C-H bonds with methyl(trifluoromethyl)dioxirane (1a) is studied through the effect of the substituent and solvent on the rate of oxygenation of 2-substituted adamantanes (2). The results suggest a remarkable electron deficiency at the reacting carbon atom in the transition state leading to the regular oxygenation products. The linearity of the Hammett plot reveals that the reaction mechanism does not change within a range of 0.15-0.67 units of sigma(I). A change in the solvent does not affect the distribution of the products, indicating a through-bond transmission of the substituent effect as the origin of the deactivation of the substrate.

Journal Article↗
PubMed2005

Ionic liquid and solid HF equivalent amine-poly(hydrogen fluoride) complexes effecting efficient environmentally friendly isobutane-isobutylene alkylation.

Isoparaffin-olefin alkylation was investigated using liquid as well as solid onium poly(hydrogen fluoride) catalysts. These new immobilized anhydrous HF catalysts contain varied amines and nitrogen-containing polymers as complexing agents. The liquid poly(hydrogen fluoride) complexes of amines are typical ionic liquids, which are convenient media and serve as HF equivalent catalysts with decreased volatility for isoparaffin-olefin alkylation. Polymeric solid amine:poly(hydrogen fluoride) complexes are excellent solid HF equivalents for similar alkylation acid catalysis. Isobutane-isobutylene or 2-butene alkylation gave excellent yields of high octane alkylates (up to RON = 94). Apart from their excellent catalytic performance, the new catalyst systems significantly reduce environmental hazards due to the low volatility of complexed HF. They represent a new, "green" class of catalyst systems for alkylation reactions, maintaining activity of HF while minimizing its environmental hazards.

Journal Article↗
PubMed2005

The question of C- vs. O-silylation of ketenes: electrophilic triethylsilylation of diphenylketene.

Electrophilic triethylsilylation of diphenylketene leads to exclusive C-silylation giving the diphenyl(triethylsilyl)acetyl cation in the solution phase even though density functional theory calculations at the B3LYP/6-311+G* level indicate that the O-silylation of diphenylketene is preferred over C-silylation by 5.4 kcal/mol in the gas phase. On the other hand, in the case of the parent ketene, similar density functional theory calculations show that C-silylation is preferred over O-silylation by 8.2 kcal/mol.

Journal Article↗
PubMed2005

Comparative ab initio study of the structures and stabilities of the ethane dication C2H6(2+) and its silicon analogues Si2H6(2+) and CSiH6(2+).

Ab initio MP2/6-311G and QCISD(T)/6-311G levels as well as Gaussian-2 theory were used to perform a comparative study of the structures and stabilities of the ethane dication C(2)H(6)(2+) and its silicon analogues Si(2)H(6)(2+) and CSiH(6)(2+). Similar to previous HF/6-31G results, our present calculations also indicate that the two-electron three-center (2e-3c) bonded carbonium-carbenium structure 1 is more stable than the doubly hydrogen bridged diborane-type structure 2 by about 12 kcal/mol. For the silicon analogue Si(2)H(6)(2+) the calculations, however, indicate that the 2e-3c bonded siliconium-silicenium structure 8 is about 9 kcal/mol less stable than doubly hydrogen bridged structure 9. Similar results were also computed for carbon-silicon mixed CSiH(6)(2+) dication structures. These studies are in agreement with the more electropositive character of silicon compared to carbon. Possible dissociation paths of the minimum structures were also calculated.

Journal Article↗