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

This report describes an approach for preparing tethered tungsten-imido alkylidene complexes featuring a tetraanionic pincer ligand. Treating the tungsten alkylidyne [(BuOCO)-Bu-t]-W (CBu)-Bu-t(THF)(2) (1) with isocyanates (RNCO; R =Bu- (t), 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 [((O2CBuC)-Bu-t=W(eta(2)-(N,C)-RNCO)(THF)] (11-R) and an unprecedented decarbonylation product [((BuOCO)-Bu-t)W( NR)((BuCCO)-Bu-t)] (14-R), on the pathway to the formation of 6-NR. Complex 11-R is kinetically stable for sterically bulky isocyanate R = Bu-t (11-Bu-t) 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-Bu-t are active for the stereoselective synthesis of cyclic polynorbornene.