Abstract
Although highly selective complexes for the cross-metathesis of olefins, particularly oriented toward the productive metathesis of Z-olefins, have been reported in recent years, there is a constant need to design and prepare new and improved catalysts for this challenging reaction. In this work, guided by density functional theory (DFT) calculations, the performance of a Ru-based catalyst chelated to a sulfurated pincer in the olefin metathesis was computationally assessed. The catalyst was designed based on the Hoveyda–Grubbs catalyst (SIMes)Cl2Ru(=CH–o–OiPrC6H4) through the substitution of chlorides with the chelator bis(2-mercaptoimidazolyl)methane. The obtained thermodynamic and kinetic data of the initiation phase through side- and bottom-bound mechanisms suggest that this system is a versatile catalyst for olefin metathesis, as DFT predicts the highest energy barrier of the catalytic cycle of ca. 20 kcal/mol, which is comparable to those corresponding to the Hoveyda–Grubbs-type catalysts. Moreover, in terms of the stereoselectivity evaluated through the propagation phase in the metathesis of propene–propene to 2-butene, our study reveals that the Z isomer can be formed under a kinetic control. We believe that this is an interesting outcome in the context of future exploration of Ru-based catalysts with sulfurated chelates in the search for high stereoselectivity in selected reactions.
Highlights
In the beginning of olefin metathesis, such reactions were performed employing undefined mixtures of molybdenum and tungsten salts adsorbed on alumina under harsh conditions and additives.[1,2] subsequent investigations focused on detailed descriptions of metathesis catalysts to obtain high control over the reaction, which led to the first well-defined Schrock catalysts.[3,4] This discovery encouraged the development of a family of catalysts with early transition metals.[5,6] these species showed some operational issues related to oxophilicity, solvents, as well as limited tolerance to moisture or a number of different functional groups, even though some air-stable and user-friendly complexes were obtained.[7]
As ruthenium and rhenium share some of chemical properties (e.g., a well-defined Re(VII) complex or the oxide Re2O7 have been demonstrated to be active catalysts for olefin metathesis74,75), we hypothesize that HNC S can be relatively strongly chelated to ruthenium
Reactions were monitored by the dihedral angle φ defined in Scheme 3, which accounts for the styrene rotation in the first step of the pathway, and the bond distances dn related to the bond formation and rupture of the reacting olefin carbon atoms, useful in the description of the 2,2-cycloaddition and 2,2-cycloreversion steps
Summary
In the beginning of olefin metathesis, such reactions were performed employing undefined mixtures of molybdenum and tungsten salts adsorbed on alumina under harsh conditions and additives.[1,2] subsequent investigations focused on detailed descriptions of metathesis catalysts to obtain high control over the reaction, which led to the first well-defined Schrock catalysts.[3,4] This discovery encouraged the development of a family of catalysts with early transition metals.[5,6] these species showed some operational issues related to oxophilicity, solvents, as well as limited tolerance to moisture or a number of different functional groups, even though some air-stable and user-friendly complexes were obtained.[7]. This strategy was previously implemented by Hoveyda et al via the substitution of chlorides by a catechothiolate ligand (see Scheme 2c), which resulted in a highly Z-selective catalyst and high yields for ROMP and ROCM.[38] It is observed that this approach focused on stereoselectivity differs from other sulfur chelates previously reported, in which the ether R2O→Ru in HG catalysts is replaced by the catalytic aactthivioiteyt.h39e−r4R4 2ISt →shoRuul,drbeesuclltainrigfieind,ehnohwaenvceerm, tehnattswine aimed at the formation of an active species that resembles the structure depicted in Scheme 2b In this regard, several authors have previously reported predictive catalysis based on DFT calculations verified by experimental evidence.[45−55]
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