• Nem Singh

      Articles written in Journal of Chemical Sciences

    • Identification and characterization of intermediates in the formation of the cyclobutadiene linked dimeric cobalt sandwich compound [($\eta^5$-RCp)Co($\eta^4$-C4Ph3)]2 [R = H, CH3C(O)O]

      Nem Singh Anil J Elias

      More Details Abstract Fulltext PDF

      The synthesis of novel cyclobutadiene linked dimers of [$\eta^5$-RCp]Co($\eta^4$-C4Ph4) was reported recently from a three-component reaction of 1,4-diphenylbutadiyne, diphenylacetylene and [$\eta^5$-RCp]Co(PPh3)2 [R = H or MeOC(O)]. The chiral bis oxazoline derivative of this dimer was found to form cobalt oxazolinyl palladacycles which were excellent chiral catalysts for the Overman-Claisen rearrangement of trichloroacetimidates, similar to the well-known COP catalysts. In this paper, we report reactions carried out for identifying the intermediates in the formation of the dimeric sandwich compound {[$\eta^5$-MeOC(O)Cp]Co($\eta^4$-C4Ph3)}2 (1). On varying the stoichiometric ratio of the reactants of the three component reaction, two new cobalt sandwich compounds [$\eta^5$-MeOC(O)Cp]Co[$\eta^4$-C4Ph3(C≡CPh)] (2) and [$\eta^5$-MeOC(O)Cp]Co[$\eta^4$-C4-Ph2(C≡CPh)2] (3) were found to form as viscous oily liquids. The identity of 2 was ascertained by oxidizing its alkyne moiety to yield crystalline [$\eta^5$-MeOC(O)Cp]Co{$\eta^4$-C4Ph3[C(O)-C(O)Ph]} (4) which was structurally characterized. Further, the role of phenylethynyl derived monomeric sandwich compound 2 as a precursor to the dimeric compound 1 was established by reacting 2 with diphenylacetylene and CpCo(COD) which yielded the first example of an unsymmetrical cyclobutadiene linked dimeric cobalt sandwich compound {$\eta^5$-[MeOC(O)Cp]Co($\eta^4$-C4Ph3)}[($\eta^5$-Cp)Co($\eta^4$-C4Ph3)] (5) which was also structurally characterized.

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