High oxidation state N-heterocyclic carbene molybdenum alkylidene complexes: functional-group tolerant olefin metathesis catalysts

Abstract

Olefin metathesis reactions belong to the most powerful tools for the formation of carbon-carbon double bonds both in organic synthesis and polymer chemistry. Enormous progress has been made both in terms of activity, selectivity and functional-group tolerance of the catalysts. Nonetheless, turn-over numbers (TONs) are still, with very few exceptions, far below 100,000 in most cases below 5,000. The use of modern, molecularly well-defined olefin metathesis catalysts are mostly restricted to high-end pharmaceutical applications. There, both Schrock and Grubbs catalysts find ample use. Therefore, the synthesis of well-defined catalysts which conquered the weakness of the existing systems has received great attention. Chapter 1 The first chapter of this thesis deals with a history of olefin metathesis as well as a short overview on the synthesis and applications of Schrock catalysts. The context also offers a short description about different olefin metathesis reactions and N-heterocyclic carbenes (NHC). Chapter 2 A single-site catalyst, which is highly active both in terms of turn-over numbers (TONs) and turn-over frequency (TOF), which is tolerant toward water, air and functional- groups with high stereo- and regioselectivity remains a challenge in olefin metathesis. In search for olefin active metathesis catalysts based on cheap metals (Mo/W) and fulfill all the above mentioned criteria, the first N-heterocyclic carbene (NHC) complexes of molybdenum imido alkylidene bis(triflate) complexes have been synthesized. Unlike existing bis(triflate) complexes, the novel 5-fold coordinated 16-electron Mo-complexes contain two carbenes, i.e. a Schrock carbene and an NHC. Single crystal X-ray analysis revealed that the above mentioned complexes are distorted square pyramidal with one triflate (OTf) ligand trans to the NHC. In course of a metathesis reaction, this triflate leaves the complex and generates a trigonal bipyramidal cationic 16-electron Mo-NHC complex (19F-NMR studies). The most important observation is this type of catalysts is active in ring-closing metathesis (RCM), cross-metathesis (CM), the cyclopolymerisation of α,ω-diynes and ring-opening metathesis polymerisation (ROMP). Monomers containing functional-groups, e.g., sec-amine, hydroxy, and carboxylic acid moieties, which are not tolerated by the existing variations of Schrock catalysts, can be used. This novel class of catalysts displays substantial activity even at high temperatures (140 °C), e.g., in RCM. Based on the observation that bis(triflate) complexes show a coalescence temperature for the two triflate groups, an activation mechanism based on a Berry-type pseudorotation, i.e. interconversion between trigonal biyramidal (TBP) configurations through a square pyramidal (SP), is proposed. Activation of the catalysts through the release of one triflate in the SP configuration is in full accordance with the observed reactivity of both neutral and cationic Mo-imido alkylidene NHC complexes and with 19F-NMR. Furthermore, reactions of the Mo-NHC bis(triflate) complexes with one equivalent of a fluorinated alkoxide (e.g., -OCH(CF3)2, -OC6F5, -OCCH3(CF3)2) or with AgB(ArF)4 in dichloroethane afforded the corresponding monoalkoxide and the cationic Mo-imido alkylidene NHC complexes. This particular feature is the presence of the NHC ligand, which delocalizes the cationic charge and stabilizes the molybdenum center. The structures of all compounds have been determined by single-crystal X-ray diffraction and their reactivities in various olefin metathesis reactions have been explored. In selected metathesis reactions, TONs up to 545,000 have been reached. Nonetheless, Mo-imido alkylidene NHC complexes with one electron-withdrawing fluorinated alkoxide and the corresponding cationic complexes in which the remaining triflate replaced by AgB(ArF)4 afforded remarkably active and functional-group tolerant metathesis catalysts. Employing different NHCs such as triazole-2-ylidene, benzimidazolylidene and CAAC (Cyclic Alkyl Amino Carbene) provides access to another novel class of Mo-NHC alkylidene complexes. In fact, Mo-imido alkylidene NHC complexes prepared recently display unprecedented functional-group tolerance. Therefore, these catalysts hold great promise in both organic and polymer chemistry. Chapter 3 In this chapter, the first anionic high oxidation state molybdenum (VI) imido bisalkyl alkylidyne complex [(Mo(N-2,6-Me2-C6H3)(CH2CMe3)2(CCMe2Ph)(Mg.Et2O-μ-Cl)2] (29) is reported. It forms via reaction of [Mo(N-2,6-Me2-C6H3)(CH2CMe2Ph)2(O3SCF3)2(DME)], (DME = 1,2-dimethoxyethane) with an excess of neopentylmagnesium chloride. [Mo(N-2,6-(2-iPr)2-C6H3)(CCMe2Ph)(2,5-Me2-pyrrolide)2(1,3-dimesityl-4,5-dihydro-1H-imidazol-3-ium)] (30) is accessible via reaction of Mo(N-2,6-(2-iPr)2-C6H3)(IMesH2)(CHCMe2Ph)(2,6-Me2-pyrrolide)2 with 1,3-dimesitylimidazolidinium chloride in benzene. X-ray studies of both complexes are also presented. While 29 is inactive in alkyne metathesis and in ring-closing metathesis (RCM), it is active in the ring-opening metathesis polymerization (ROMP) of (substituted) norborn-2-ene(s) and in the 1-alkyne polymerization of 2-ethynyl-trimethylsilylbenzene. The ROMP derived polymers display a high cis-content up to ≥ 96%. Both the proposed mesomerism via dπ-pπ interactions through Mo(IV) between a molybdenum amidatoalkylidyne and a molybdenum amidoalkylidenate and a [1,3-H] shift in course of polymerization that accounts for the unique reactivity of 29 are supported by density functional calculations (DFT).