Tethered Tungsten-Alkylidenes for the Synthesis of Cyclic Polynorbornene via Ring Expansion Metathesis: Unprecedented Stereoselectivity and Trapping of Key Catalytic Intermediates

Abstract

This report describes an approach for preparing tethered tungsten-imido alkylidene complexes featuring a tetra-anionic pincer ligand. Treating the tungsten alkylidyne [^tBuOCO]W≡C^tBu(THF)₂ (1) with isocyanates (RNCO; R = ^tBu, Cy, and Ph) leads to cycloaddition occurring exclusively at the C═N bond to generate the tethered tungsten-imido alkylidenes (6-NR). Unanticipated intermediates reveal themselves, including the discovery of [(O₂C^tBuC═)W(η²-(N,C)-RNCO)(THF)] (11-R) and an unprecedented decarbonylation product [(^tBuOCO)W(≡NR)(^tBuCCO)] (14-R), on the pathway to the formation of 6-NR. Complex 11-R is kinetically stable for sterically bulky isocyanate R = ^tBu (11-^tBu) and is isolated and characterized by single-crystal X-ray diffraction. Finally, adding to the short list of catalysts capable of ring expansion metathesis polymerization (REMP), complexes 6-NR and 11-^tBu are active for the stereoselective synthesis of cyclic polynorbornene.