@article { ISI:000284515800005, title = {Combined experimental and density functional theory study on the Pd-mediated cycloisomerization of o-alkynylnitrobenzenes - synthesis of isatogens and their evaluation as modulators of ROS-mediated cell death}, journal = {European Journal of Organic Chemistry}, number = {31}, year = {2010}, month = {NOV}, pages = {5955-5966}, publisher = {WILEY-V C H VERLAG GMBH}, address = {PO BOX 10 11 61, D-69451 WEINHEIM, GERMANY}, abstract = {

Highly selective cycloisomerization of o-alkynylnitrobenzenes, leading to isatogens, has been achieved by employment of a Pd-II complex. This reaction is very general and functional-group-tolerant. The possible mechanism of this reaction was investigated with the help of DFT calculations. Three possible pathways - namely, the addition of the nitro group either in (i) 5-exo-dig or (ii) 6-endo-dig mode and (iii) halopalladation - and subsequent intramolecular events have been considered and studied in detail. These investigations revealed that pathway (i) is the favored route to isatogen formation. A preliminary screening of the available isatogens reveals the 2-alkylisatogens to be novel ROS scavengers capable of inhibiting cellular necroptosis.

}, keywords = {Anthranil, cyclization, Density functional calculations, isatogens, Nitro-alkyne cycloisomerization, Palladium}, issn = {1434-193X}, doi = {10.1002/ejoc.201000769}, author = {Ramana, Chepuri V. and Patel, Pitambar and Vanka, Kumar and Miao, Benchun and Degterev, Alexei} } @article { ISI:000343819200016, title = {Nature of the active site in ziegler-natta olefin polymerization catalysis systems - a computational investigation}, journal = {European Journal of Inorganic Chemistry}, number = {29}, year = {2014}, month = {OCT}, pages = {5063-5076}, publisher = {WILEY-V C H VERLAG GMBH}, address = {BOSCHSTRASSE 12, D-69469 WEINHEIM, GERMANY}, abstract = {

Pull quantum chemical calculations with density functional theory (DET) show that a principal role of donors in Ziegler-Nana (ZN) oh-din polymerization catalysts is to coordinate to the metal center at the active sites on the MgCl2 surface. Thereby, the behavior of the catalyst is modulated to favor insertion over termination and, thus, polymerization occurs. This is shown to be true for a range of different donors. The calculations indicate that active sites that feature anionic chloride ligands at the titanium center (the conventional model for the active site) would lead to lower-molecular-weight riolymers. If an -OC2H5 group were present instead of a chloride ligand, the active site would much more effectively produce long chain polymers. Therefore, the current work provides important new insights into the nature of the ZN polymerization process.

}, keywords = {Alkene polymerization, Density functional calculations, heterogeneous catalysis, Insertion, Polymers}, issn = {1434-1948}, doi = {10.1002/ejic.201402180}, author = {Kumawat, Jugal and Gupta, Virendra Kumar and Vanka, Kumar} } @article { ISI:000379531300011, title = {Effect of donors on the activation mechanism in ziegler-natta catalysis: a computational study}, journal = {Chemcatchem}, volume = {8}, number = {10}, year = {2016}, month = {MAY}, pages = {1809-1818}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {

Full quantum chemical calculations, using density functional theory (DFT), have been conducted to explain the effect of donors on the {\textquoteleft}{\textquoteleft}activation mechanism{\textquoteright}{\textquoteright} in the Ziegler-Natta (Z-N) catalyst system. In the activation mechanism, the inactive (TiCl4)-Cl-IV catalyst converts into the active (TiCl2Et)-Cl-III catalyst with the help of the AlEt3 present in the system. The donors that have been considered in this study are: ethyl benzoate (eb), two representative diether cases, a phthalate donor, and a silyl ester donor. The results indicate that eb and the diether donor cases donor have a negative effect on the barriers for the activation mechanism. However, the eb donor can be displaced from the MgCl2 surface by AlEt3, which matches experimental observations. For the phthalate, silyl ester and TiCl3-OC4H8Cl cases, the results indicate that a significant induction period would be present in Z-N systems employing such donors or having such a catalytic center, before catalysis could commence.

}, keywords = {Alkylation, aluminum, Density functional calculations, surface chemistry, titanium}, issn = {1867-3880}, doi = {10.1002/cctc.201600281}, author = {Kumawat, Jugal and Gupta, Virendra Kumar and Vanka, Kumar} } @article {45734, title = {Electronic control on linear versus branched alkylation of 2-/3-aroylbenzofurans with acrylates: combined DFT and synthetic studies}, journal = {Chemistry-A European Journal}, volume = {23}, year = {2017}, month = {JUN }, pages = {7570-7581}, type = {Article}, abstract = {

Investigations on the factors that govern unusual branched alkylation of 2-aroylbenzofurans with acrylates by Ru-catalyzed carbonyl-directed C-H activation has been carried out by calculating the kinetics associated with the two key steps-the coordination of the acrylate with the intermediate ruthenacycle and the subsequent migratory insertion reaction-studied with the help of DFT calculations. Eight possible orientations for each mode of alkylation have been considered for the calculations. From these calculations, it has been understood that there is a synergistic operation of the steric and electronic effects favoring the branched alkylation. Further DFT investigations on the alkylation of the isomeric 3-aroylbenzofurans indicated a preference for the linear alkylation and this has been verified experimentally. Overall, the observed/calculated complementary selectivity in the alkylation of 2-/3-aroylbenzofurans with acrylates reveals that the substrate-dependent charge distribution of the Ru-C bond in the intermediate ruthenacycle is an important determining factor and thus the current work opens up a new domain of substrate design for controlling regioselectivity.

}, keywords = {acrylates, Alkylation, aroylbenzofuran, C-H activation, Density functional calculations}, issn = {0947-6539}, doi = {10.1002/chem.201700643}, author = {Srinivas, Kolluru and Dangat, Yuvraj and Kommagalla, Yadagiri and Vanka, Kumar and Ramana, Chepuri V.} } @article {44204, title = {Beyond hydrofunctionalisation: a well-defined calcium compound catalysed mild and efficient carbonyl cyanosilylation}, journal = {Chemistry-A European Journal}, volume = {24}, year = {2018}, month = {JAN}, pages = {1269-1273}, type = {Article}, abstract = {

Organocalcium compounds have been reported as efficient catalysts for various transformations, for cases in which one of the substrates contained an E-H (E=B, N, Si, P) bond. Here, we look at the possibility of employing an organocalcium compound for a transformation in which none of the precursors has a polar E-H bond. This study demonstrates the utilization of a well-defined amidinatocalcium iodide, [PhC(NiPr)(2)CaI] (1) for cyanosilylation of a variety of aldehydes and ketones with Me3SiCN under ambient conditions without the need of any co-catalyst. The reaction mechanism involves a weak adduct formation between 1 and Me3SiCN leading to the activation of the Si-C bond, which subsequently undergoes sigma-bond metathesis with a C=O moiety. Such a mechanistic pathway is unprecedented in alkaline earth metal chemistry. Experimental and computational studies support the mechanism.

}, keywords = {Ca catalysis, carbonyls, cyanosilylation, Density functional calculations, transition-metal free}, issn = {0947-6539}, doi = {10.1002/chem.201705795}, author = {Yadav, Sandeep and Dixit, Ruchi and Vanka, Kumar and Sen, Sakya S.} } @article {47608, title = {Computational insights into the role of external and local electric fields in macrocyclic chemical and biological systems}, journal = {ChemPhysChem}, volume = {22}, year = {2021}, month = {DEC}, pages = {2484-2492}, type = {Article}, abstract = {The investigation of the role of the electric field in systems of widespread interest employing computational techniques is an emerging area of research. The outcome of applying an oriented external electric field (OEEF) on the geometric and electronic properties of the chemically unique pi-conjugated cyclic carbon ring compounds has been explored with density functional theory (DFT). Distinct changes in the structural and electronic features of such ring compounds are observed upon the application of OEEFs. Importantly, the calculations indicate that a mixed aliphatic-aromatic conjugated ring converts from a singlet to a triplet after the application of an OEEF, suggesting potential applications in optoelectronics for such molecules, without the need for photochemically induced change in the spin state. Furthermore, the influence of built-in local electric fields (LEFs) present in naturally occurring macrocyclic systems such as valinomycin has also been explored. Static and ab initio molecular dynamics (AIMD) calculations indicate that LEFs are the primary driving factor in determining the energetically favoured position of counter anions such as chloride (Cl-) in the potassium (K+) and sodium (Na+) coordinated valinomycin macrocycle structures: they exist inside the cage in the case of K+ sequestration by valinomycin and outside for Na+. This divergence has been proposed to be the determining factor for the selectivity of the valinomycin macrocycle for binding a K+ cation over Na+.}, keywords = {counter anions, Density functional calculations, electric field, ion selectivity, macrocyclic systems}, issn = {1439-4235}, doi = {10.1002/cphc.202100581}, author = {Mukherjee, Anagh and Ghule, Siddharth and Vanka, Kumar} } @article {48714, title = {"Weak" C-H{\textbullet}{\textbullet}{\textbullet}S interaction drives enantioselectivity in cinchona alkaloid complex catalyzed thiocyanation}, journal = {Chemistry-An Asian Journal}, volume = {18}, year = {2023}, month = {JUL}, pages = {e202300321}, type = {Article}, abstract = {

The great success of asymmetric organocatalysis has made it one of the most important advancements made in chemistry in the past two decades. A significant achievement in this context is the asymmetric organocatalysis of the thiocyanation reaction. In the current study, computational studies with density functional theory have been done in order to understand an interesting experimental finding: the reversal of enantioselectivity from R to S when the electrophile is changed from beta-keto ester to oxindole for the thiocyanation reaction with the cinchona alkaloid complex catalyst. The calculations reveal an unusual fact - the principal reason for the reversal is the presence of the C-H center dot center dot center dot S noncovalent interaction, which is present only in the major transition states in each of the two nucleophile cases. Only recently has it been realized that the supposedly weak C-H center dot center dot center dot S noncovalent interaction has the properties of a hydrogen bond, and the fact that this interaction is the cause of enantioselectivity has relevance, because of the large number of asymmetric transformations involving the sulphur heteroatom.

}, keywords = {Density functional calculations, Enantioselectivity, noncovalent interactions, organocatalysis, reaction mechanisms}, issn = {1861-4728}, doi = {10.1002/asia.202300321}, author = {Banerjee, Subhrashis and Vanka, Kumar} }