• Ashoka G Samuelson

      Articles written in Journal of Chemical Sciences

    • Role of orbital symmetry in transition metal promoted ring opening reactions of methylenecyclopropanes and cyclobutenes

      Ashoka G Samuelson

      More Details Abstract Fulltext PDF

      Transition metals catalyse a variety of organic reactions, of which the ring opening of strained ring organic molecules generated a lot of interest. Theoreticians predicted a metal orbital catalysed pathway, which involved concerted bond breaking and bond forming. On the other hand experimentalists were able to show that the reaction was not proceeding through a concerted pathway by intercepting the intermediates involved. There remained, however, two ring systems methylenecyclopropanes and cyclobutenes—whose reactions with metal complexes seemed to be of a concerted nature. An analysis of the reactions of different metal complexes with these ring systems and the theoretical predictions provide a rationale for understanding these reactions.

    • Substituent effects on the insertion of heterocumulenes into copper (I) aryloxides

      Christina Wycliff Santosh P Abraham Ashoka G Samuelson

      More Details Fulltext PDF
    • Structure and reactivity of copper(I) oligomers

      Santosh P Abraham Ashoka G Samuelson M Nethaji

      More Details Abstract Fulltext PDF

      The reactivity of oligomeric copper(I) complexes towards test electrophiles and hydrogen have been studied. The complexes exhibit similar reactivity patterns in most cases. Photochemical irradiation of the tetramer4, led to the isolation of the ligand8, 2, 6-dimethylphenyl-N-phenylthionocarbamate, present in the tetramer. This compound crystallized in the monoclinic system in the P21/n space group witha = 11.992(5) Å, b = 9.886(3) Å,c = 12.783(6) Å, β = 115.91(4)°,V= 1360.37(4) Å{st3}, z = 4. The comparison of the structure of the ligand with the tetramer shows the differences in the structure brought about by coordination. A comparison of the structures of the tetramer4 and the hexamer 5 brings out the differences in the nature of bridging sulphide which considerably alters the properties of the complexes. Differences in the reactivity can be understood in terms of the type of the sulphide bridge present in the complexes. Similarities in the reactivity trends are the result of the dissociation of the oligomers in solution.1HNMR studies reveal that the structures of these complexes in the solution state are different from the solid state and account for the similarities in the reactivity patterns of these complexes.

    • Synthesis, structure and reactivity of a water-soluble copper(I) complex

      D Saravana Bharathi Ashoka G Samuelson N K Lokanath M A Shridhar Sashidara Prasad

      More Details Abstract Fulltext PDF
    • Synthesis, spectroscopic characterization and electronic structure of some new Cu(I) carbene complexes

      Chinnappan Sivasankar Christina Baskaran Ashoka G Samuelson

      More Details Abstract Fulltext PDF

      Reaction of oligomeric Cu(I) complexes [Cu(Μ-S-C(=NR)(O-Ar-CH3)]n with Lewis acids gave Cu(I) carbene complexes, which were characterized by1H and13C NMR spectroscopy. Cu(I) carbene complexes could be directly generated from RNCS, Cu(I)-OAr and Lewis acids; this method can be used to prepare Cu(I) carbene complexes with different substitutents on the carbene carbon. The complexes were unreactive towards olefins and do not undergo cyclopropanation. Electronic structure calculations (DFT) show that the charge on the carbene carbon plays an important role in controlling the reactivity of the carbene complex.

    • On the key role of water in the allylic activation catalysed by Pd (II)bisphosphinite complexes

      Rakesh Kumar Sharma Ashoka G Samuelson

      More Details Abstract Fulltext PDF

      Palladium and platinum complexes of bisphosphinites and bisphosphines derived from mandelic acid have been prepared and characterized. Their ability to catalyze allylation of imines with allyltributylstannane has been studied. Bisphophinite complexes of Pd (II) are shown to be ideal and they work best in the presence of one equivalent of water. The near neutral conditions employed make the catalysts suitable for a wide variety of substrates.

    • Metathesis of carbon dioxide and phenyl isocyanate catalysed by group(IV) metal alkoxides: An experimental and computational study

      Akshai Kumar Ashoka G Samuelson

      More Details Abstract Fulltext PDF

      The insertion reactions of zirconium(IV) 𝑛-butoxide and titanium(IV) 𝑛-butoxide with a heterocumulene like carbodiimide, carbon dioxide or phenyl isocyanate are compared. Both give an intermediate which carries out metathesis at elevated temperatures by inserting a second heterocumulene in a head-to-head fashion. The intermediate metallacycle extrudes a new heterocumulene, different from the two that have inserted leading to metathesis. As the reaction is reversible, catalytic metathesis is feasible. In stoichiometric reactions heterocumulene insertion, metathesis and metathesis cum insertion products are observed. However, catalytic amounts of the metal alkoxide primarily led to metathesis products. It is shown that zirconium alkoxides promote catalytic metathesis (isocyanates, carbon dioxide) more efficiently than the corresponding titanium alkoxide. The difference in the metathetic activity of these alkoxides has been explained by a computational study using model complexes Ti(OMe)4 (1bTi) and Zr(OMe)4 (1bZr). The computation was carried out at the B3LYP/LANL2DZ level of theory.

    • Computational tools for mechanistic discrimination in the reductive and metathesis coupling reactions mediated by titanium(IV) isopropoxide

      Akshai Kumar Ashoka G Samuelson

      More Details Abstract Fulltext PDF

      A theoretical study has been carried out at the B3LYP/LANL2DZ level to compare the reactivity of phenyl isocyanate and phenyl isothiocyanate towards titanium(IV) alkoxides. Isocyanates are shown to favour both mono insertion and double insertion reactions. Double insertion in a head-to-tail fashion is shown to be more exothermic than double insertion in a head-to-head fashion. The head-to-head double insertion leads to the metathesis product, a carbodiimide, after the extrusion of carbon dioxide. In the case of phenyl isothiocyanate, calculations favour the formation of only mono insertion products. Formation of a double insertion product is highly unfavourable. Further, these studies indicate that the reverse reaction involving the metathesis of N,N'-diphenyl carbodiimide with carbon dioxide is likely to proceed more efficiently than the metathesis reaction with carbon disulphide. This is in excellent agreement with experimental results as metathesis with carbon disulphide fails to occur. In a second study, multilayer MM/QM calculations are carried out on intermediates generated from reduction of titanium(IV) alkoxides to investigate the effect of alkoxy bridging on the reactivity of multinuclear Ti species. Bimolecular coupling of imines initiated by Ti(III) species leads to a mixture of diastereomers and not diastereoselective coupling of the imine. However if the reaction is carried out by a trimeric biradical species, diastereoselective coupling of the imine is predicted. The presence of alkoxy bridges greatly favours the formation of the d,l (±) isomer, whereas the intermediate without alkoxy bridges favours the more stable meso isomer. As a bridged trimeric species, stabilized by bridging alkoxy groups, correctly explains the diastereoselective reaction, it is the most likely intermediate in the reaction.

    • Synthesis and unexpected reactivity of [Ru(𝜂6 -cymene)Cl2 (PPh2 Cl)], leading to [Ru(𝜂6 -cymene)Cl2 (PPh2H)] and [Ru(𝜂6-cymene)Cl2 (PPh2OH)] complexes

      Arun Kumar Pandiakumar Ashoka G Samuelson

      More Details Abstract Fulltext PDF

      The reaction of [Ru(𝜂6-cymene)Cl2]2 and PPh2Cl in the ratio 1:2 gives a stable [Ru(𝜂6-cymene) Cl2(PPh2Cl)] complex. Attempts to make the cationic [Ru(𝜂6-cymene)Cl(PPh2Cl)2]Cl with excess PPh2Cl and higher temperatures led to adventitious hydrolysis and formation of [Ru(𝜂6-cymene)Cl2 (PPh2OH)]. Attempts to make a phosphinite complex by reacting [Ru(𝜂6-cymene)Cl2]2 with PPh2Cl in the presence of an alcohol results in the reduction of PPh2Cl to give [Ru(𝜂6-cymene)Cl2(PPh2H)] and the expected phosphinite. The yield of the hydride complex is highest when the alcohol is 1-phenyl-ethane-1,2-diol. All three half-sandwich complexes are characterized by X-ray crystallography. Surprisingly, the conversion of chlorodiphenylphosphine to diphenylphosphine is mediated by 1-phenyl-ethane-1,2-diol even in the absence of the ruthenium half-sandwich precursor.

  • Journal of Chemical Sciences | News

© 2017-2019 Indian Academy of Sciences, Bengaluru.