Soluble and Supported Molecular CoIII Catalysts for the Regioselective Ring‐Opening of 1, 2‐Epoxyhexane with Methanol. Issue 1 (12th November 2012)

• Record Type: Journal Article
• Authors: Venkatasubbaiah, Krishnan; Feng, Yan; Arrowood, Tina; Nickias, Peter; Jones, Christopher W.
• Is Part Of: ChemCatChem. Volume 5:Issue 1(2013:Jan.)
• Page Start: 201
• Page End: 209
• Publication Date: 2012-11-12
• Subjects: Catalysis -- Periodicals; 541.39505
• DOI: 10.1002/cctc.201200494
• Languages: English
• ISSNs: 1867-3880

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>The regioselective ring‐opening of 1, 2‐epoxyhexane with methanol as a nucleophile is studied using an array of different molecular Co<sup>III</sup> catalysts, specifically <italic>trans</italic>‐Co<sup>III</sup>‐salen‐X (<bold>1</bold>‐X; X=Cl<sup>−</sup>, OTs<sup>−</sup>, BF<sub>4</sub><sup>−</sup>, SbF<sub>6</sub><sup>−</sup>, PF<sub>6</sub><sup>−</sup>), <italic>cis</italic>/<italic>trans</italic>‐Co<sup>III</sup>‐salen‐SbF<sub>6</sub>, Co<sup>III</sup>‐salphen‐SbF<sub>6</sub>, and Co<sup>III</sup>‐porphyrin‐SbF<sub>6</sub>. Catalytic studies show the nature of the ligand and counterion both play a significant role in influencing reaction rates, and to a lesser extent, the regioselectivity of the ring‐opening reaction, with Co<sup>III</sup>‐porphyrin‐SbF<sub>6</sub> as the most active and Co<sup>III</sup>‐salphen‐SbF<sub>6</sub> the least active soluble molecular catalysts. Unlike in the classical epoxide hydrolytic kinetic resolution reaction, non‐coordinating, non‐nucleophilic counterions proved most effective, and <italic>trans</italic>‐Co<sup>III</sup>‐salen‐Cl, which gives very high initial rates in hydrolytic kinetic resolution, shows very low activity in epoxide ring‐opening with methanol. Supported soluble and insoluble unsymmetrical <italic>trans</italic>‐Co<sup>III</sup>‐salen‐X catalysts are, thus, synthesized to evaluate cooperativity and stability of the Co<sup>III</sup>‐salen species towards epoxide ring‐opening with methanol. Soluble supported <italic>trans</italic>‐Co<sup>III</sup>‐salen‐X (X=SbF<sub>6</sub> and OTs) shows better activity and selectivity in the title reaction than monomeric <italic>trans</italic>‐Co<sup>III</sup>‐salen‐SbF<sub>6</sub> catalyst because of the cooperativity introduced through the catalyst design. The stability of insoluble catalysts is evaluated by catalytic recycling experiments. The supported insoluble catalysts successfully are recovered and reused up to 5 times, showing reduced activity but unchanged selectivity after each cycle. Deactivation is attributed to several different causes based on elemental analysis and UV/Vis spectroscopic analysis of the used catalysts, with counterion and cobalt loss playing major roles.</p> </abstract>