On the Origin of E-Selectivity in the Ring-Opening Metathesis Polymerization with Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes

Abstract

The understanding and control of stereoselectivity is a central aspect in ring-opening metathesis polymerization (ROMP). Herein, we report detailed quantum chemical studies on the reaction mechanism of E -selective ROMP of norborn-2-ene (NBE) with Mo( N -2,6-Me 2 -C 6 H 3 )(CHCMe 3 )(IMes)(OTf) 2 ( 1 , IMes = 1,3-dimesitylimidazol-2-ylidene) as a first step to stereoselective polymerization. Four different reaction pathways based on an ene syn or ene anti approach of NBE to either the syn - or anti -isomer of the neutral precatalyst have been studied. In contrast to the recently established associative mechanism with a terminal alkene, where a neutral olefin adduct is formed, NBE reacts directly with the catalyst via [2 + 2] cycloaddition to form molybdacyclobutane with a reaction barrier about 30 kJ mol –1 lower in free energy than via the formation of a catalyst–monomer adduct. However, the direct cycloaddition of NBE was only found for one out of four stereoisomers. Our findings strongly suggest that this stereoselective approach is responsible for E -selectivity and point toward a substrate-specific reaction mechanism in olefin metathesis with neutral Mo imido alkylidene N -heterocyclic carbene bistriflate complexes.

Olefin Metathesis and Stereoselective Ring-Opening Metathesis Polymerization with Neutral and Cationic Molybdenum(VI) Imido and Tungsten(VI) Oxo Alkylidene Complexes Containing N-Chelating N-Heterocyclic Carbenes

“…ROMP allows for determining the true E - or Z -selectivity of a catalyst, since particularly with norbornenes and norbornadienes after monomer insertion no further E/Z -isomerization can take place. ,, Four different enantiopure monomers were tested; their structures are shown in Figure . Both W2 and Mo4 were used; the results obtained are listed in Table .…”

“…The area of stereoselective ring-opening metathesis polymerization − (ROMP) by well-defined group 5, 6 and group 8 metal alkylidenes has blossomed into a mature field over the last two decades, given the large number of publications and that all possible stereoregular structures can now be accessed, though with different ease and generality. In this regard, neutral and cationic molybdenum imido, tungsten imido, and tungsten oxo alkylidene N-heterocyclic carbene (NHC) complexes have evolved as highly active and stereoselective catalysts. ,,,− Recently, we reported on molybdenum imido alkylidene complexes containing N-, O-, and C-chelating NHCs. , In view of their unique reactivity but also stability, we extended our research to cationic molybdenum and tungsten­(VI) imido and oxo alkylidene complexes containing N-chelating NHCs. Notably, to date, no neutral or cationic tungsten­(VI) oxo alkylidene complexes containing N-chelating N-heterocyclic carbene have been reported.…”

PyConSolv: A Python Package for Conformer Generation of (Metal-Containing) Systems in Explicit Solvent

“…73,74 It was shown during the investigation of the reaction mechanism, that conformer generation is crucial for explaining the reactivity. 16 For the Mo-based catalyst, the structures generated with P y C o n S o l v u s e d B P 8 6 / d e f 2 -S V P / D 3 B J + C P C M -(CH 2 Cl 2 ) 31,69,70,72,81 for the force-field parametrization. The trajectory was aligned on the molybdenum, the carbon of the aryl group, and the NHC carbon bound to the molybdenum.…”

“…Molybdenum imido alkylidene N-heterocyclic carbene (NHC) catalysts represent one avenue of performing olefin metathesis. , It was shown during the investigation of the reaction mechanism, that conformer generation is crucial for explaining the reactivity …”

“…While the importance of conformers is well-known and a key feature in drug design pipelines, , it has gathered some attention in the field of homogeneous catalysis as early as the 1990s. − However, conformer searches are by no means widespread in standard mechanistic studies today. A systematic exploration of the conformational space, by generating rotamers, can be employed for simple systems, with few degrees of freedom, but quickly becomes unfeasible for larger molecules.…”