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Issue 17, 2005
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Substituent effects and the mechanism of alkene metathesis catalyzed by ruthenium dichloride catalysts

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Abstract

Density functional theory calculations are reported concerning the dissociative mechanism for alkene metathesis by ruthenium dichloride catalysts, including both bisphosphine and diaminocarbene/phosphine complexes. The calculations use a hierarchy of models, ranging from [(L)(PH3)Ru(Cl)2(CH2)] (L = PH3 or diaminocarbene) through the larger [(L)(PMe3)Ru(Cl)2(CHPh)] to the “real” [(L)(PCy3)Ru(Cl)2(CHPh)]. Calculations show that the rate-limiting step for metathesis is either ring closing from an alkene complex to form a ruthena-cyclobutane, or ring-opening of the latter intermediate to form an isomeric alkene complex. The higher efficiency of the diaminocarbene based catalysts is due to the stabilization of the formal +IV oxidation state of the ruthenium centre in the metallacycle. This effect is partly masked in the smaller model systems due to a previously unnoticed stereoelectronic effect. The calculations do not reproduce the experimental observation whereby the initiation step, phosphine dissociation, is more energetically demanding and hence slower for the diaminocarbene-containing catalyst system than for the bisphosphine. Further calculations on the corresponding bond energies using a variety of DFT and hybrid DFT/molecular mechanics methods all find instead a larger phosphine dissociation energy for the bisphosphine catalyst. This reversed order of binding energies would in fact be the one expected based on the stronger trans influence of the diaminocarbene ligand. The discrepancy with experiment is small and could have a number of causes which are discussed here.

Graphical abstract: Substituent effects and the mechanism of alkene metathesis catalyzed by ruthenium dichloride catalysts

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Publication details

The article was received on 16 May 2005, accepted on 27 Jun 2005 and first published on 20 Jul 2005


Article type: Paper
DOI: 10.1039/B506929G
Citation: Dalton Trans., 2005,0, 2849-2858
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    Substituent effects and the mechanism of alkene metathesis catalyzed by ruthenium dichloride catalysts

    A. C. Tsipis, A. G. Orpen and J. N. Harvey, Dalton Trans., 2005, 0, 2849
    DOI: 10.1039/B506929G

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