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Application of Photoactive Diruthenium Catalyst for New Organic Reactions: Syntheses of Imines, Isoquinolines, and Vinylstannanes

Application of Photoactive Diruthenium Catalyst for New Organic Reactions: Syntheses of Imines, Isoquinolines, and Vinylstannanes
Gupta, Sreya
Date Issued
Light had the potential to serve as an inexpensive, abundant, renewable, and nonpolluting reagent for chemical transformations. Thus developing new methodologies by employing metal-catalyst activated by visible light is the pivotal interest in the field of organic synthesis. In this thesis, we investigated important chemical transformation catalyzed by carbonyl bridged diruthenium complexes which is activated by visible light source. In chapter 1, Ru-catalyzed synthesis of N-unsubstituted imines from easily available alkyle azides under the photolytic condition has described. N-unsubstituted imines were generated by removal of N2 gas from azide precursors and subsequent migration of hydrogen atom under the house hold fluorescent light. This novel protocol has shown broad synthetic scope to construct various N-unprotected imines including unprecedented enolizable substrates. Mild reaction conditions used in this protocol even allowed obtaining fundamental physical parameters of N-unprotected imines, such as, existence of dynamic geometric isomers in the solution phase and dissociation energy of imino N-H bond. This novel synthetic route was successfully applied in one-pot imine generation/allylation sequence by using allylboron reagents
directly N-unprotected allylamines were formed without any undesirable side reactions. The above one-pot sequence has shown greater efficiency over prevailing methodologies in terms of enantio- and diastereoselectivity. Furthermore, this protocol also effective to synthesize N-unsubstituted imidates from various azidoethers. This is the first report of imidate synthesis in neutral reaction conditions which allowed constructing imidates having acid-labile functionality. Chapter 2 describes the Ru/Rh-tandem catalysis to synthesize isoquinoline from arylazides and internal alkynes by direct C-H bond activation. N-unsubstituted arylimines generated from the corresponding azides by the ruthenium catalysis were used as a directing group for rhodium-catalyst to activate the C-H bond at ortho-position of aromatic ring. Subsequent addition of various internal alkynes produces N-annulated isoquinolines. The essential merits of this cascade protocol are broad scope of applicable benzylic azides, including those having carbonyl and chiral functionalities. Additionally, this protocol is completely atom-economical as N2-gas is the only the byproduct. Furthermore, N-unsubstituted imidate synthesized by Ru-catalyst II from the corresponding azidoethers, also turned out to be the successful entity for direct synthesis of 1-alkoxyisoquinolines. The mechanism of the above transformation was established through the isolation and characterization (X-ray crystallography structure) of the important unknown intermediates. Previously we reported the hydrosilylation of aldehydes1 by using ruthenium catalyst under fluorescent light condition. We expected this catalyst can activate other metal-hydride species too. In the chapter 3, ruthenium-catalyzed addition of tin hydride to various internal alkynes (hydrostannation) under fluorescent light was described. The reaction was initiated by the formation of ruthenium hydride and tin radical species under the illumination of light. In the termination step, tin-radical and the hydrogen from ruthenium hydride species were added to the alkynes in anti-stereoselective way. In general, control the regio- and setreoselectivity of hydrostannation reaction is the formidable challenge. However, in above ruthenium photocatalyzed system, (Z)-β-selective vinylstannanes were obtained selectively for various internal alkynes and shown better activity over well-known radical initiators such as AIBN (azobisisobutyronitril) or BEt3 (triethylborane). To obtain insight into the hydrostannation mechanism, we conducted various experiments. The outcome of various control experiments strongly suggested the insolvent of radical mechanism and the formation of ruthenium-hydride (VI) as a key intermediate species under the fluorescent light. Additionally, the recyclability of the ruthenium catalyst added extra advantage of this novel methodology. 1 Do, Y.
Han, J.
Rhee, Y. H.
Park. J. Adv. Synth. Catal. 2011, 353, 3363.
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