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 tetra-anionic pincer ligand. Treating
the tungsten alkylidyne [tBuOCO]­WCtBu­(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 [(O2CtBuC)­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.