High Oxidation State Tungsten Oxo, Tungsten Thio, and Tungsten Imido Alkylidene NHC Complexes for Olefin Metathesis of Functional Olefins

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Both neutral and cationic high-oxidation state tungsten oxo, tungsten thio and tungsten imido alkylidene N-heterocyclic carbene (NHC) complexes of the general formula W(E)(CHCMe2R)(NHC)X2 and [W(E)(CHCMe2R)(NHC)X+ Y-] (E = O, S, NR1; R1 = tBu, Ad, Ar; R = CH3, Ph) shall be prepared. Besides of the substitution of oxygen by S and NR1, the use of different NHCs with substantially different sigma-donor capability shall be investigated. Variations shall include but not be limited to NHC = imidazol-2-ylidenes, imidazolidin-2-ylidenes, cyclic alkyl aminocarbenes (CAACs), cyclic (amino)(aryl)carbenes (CAArCs), tetrazolin-2-ylidenes, pyrazolin-4-ylidenes, triazol-2-ylidenes and R6-carbodiphosphoranes (R = alkyl, phenyl). In terms of the anionic X-ligand, both weak and stronger sigma-donor ligands such as (fluorinated) alkoxides, thiolates, thiophenolates and iodide shall be used. This will allow identifying the influence of the E-, NHC- and the X-ligand on both the reactivity and functional group tolerance in various olefin metathesis reactions. These will comprise of ring-closing metathesis (RCM), homo-metathesis, cross-metathesis (CM), self-metathesis (SM), ring-opening cross-metathesis (ROCM) and ene-diyne metathesis reactions as well as of ring-opening metathesis polymerization (ROMP). In HM, SM, CM, ene-diyne and ROCM reactions, but also in ROMP, special attention will be devoted to stereoselectivity, an issue that shall be addressed with the aid of novel ansa-type NHCs and NHCs with different sigma-donor capability. In particular, the issue of Z-selectivity shall be addressed and the structure (TB vs. SP) of the tungstacyclobutane shall be elucidated. The work proposed here is not only intended to broaden the understanding about tungsten oxo/thio/imido alkylidene NHC complexes, but also to offer a new class of functional group tolerant, highly active, stereo- and regioselective catalysts for olefin metathesis-based C-C coupling reactions. Last but not least, we will apply the elaborated concept to the synthesis of highly stereoregular polymers via ROMP.