Origin of Differences in Regioselectivity in the (Cyclo-) Polymerization of alpha, omega-Diynes and 1-Alkynes Between Tetra- and Penta-Coordinated Mo/W Imido/Oxo Alkylidene N-Heterocyclic Carbene Complexes and Implications for Ene-yne Metathesis

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The regioselectivity of olefin/alkyne metathesis reactions strongly depends on the structure and geometry of both, the pre-catalyst and the active species. In that regards, the questions, whether metathesis starts from and proceeds via pentacoordinated or tetracoordinated metal alkylidenes is of utmost importance. Since the regioselectivity in cyclopolymerization of diynes with pentacoordinated neutral and cationic metal imido alkylidene N-heterocyclic carbene (NHC) complexes differs substantially from the one of tetracoordinated, cationic ones, it is of high interest to reveal the origin of these differences in regioselectivity. For these purposes, we wish to study in detail the cyclopolymerization of alpha, omega-diynes as well as the polymerization of 1-alkynes using a tailored set of tetra- and pentacoordinated Mo/W imido/oxo alkylidene NHC complexes. Apart from the elucidation of the polymer structure in terms of cis/trans-content and structure of the repeat unit, these studies shall mainly focus on kinetic investigations to clarify whether either an associative or a dissociative mechanism is operative. Temperature-dependent NMR measurements of the rate constants will allow for additionally providing quantitative thermodynamic data on delta-S‡ at the transition state, which will provide unambiguous information about the molecularity of the process and thus about an associative or dissociative mechanism, respectively. Finally, these findings shall be correlated with those on the regio- and chemoselectivity in ene-yne metathesis catalyzed by the same set of (pre-) catalysts, again based on kinetic studies. The comprehensive picture that is to be generated will help for designing future, regioselective olefin and alkyne metathesis catalysts.