Synthesis of Tungsten Oxo Alkylidene Biphenolate Complexes and Ring-Opening Metathesis Polymerization of Norbornenes and Norbornadienes

We have synthesized and characterized tungsten oxo alkylidene biphenolate complexes with the formulas W(O)(CHR)(rac-biphenolate)-(PPhMe2) and (R,S)-[W(mu-O)(CHR)(biphenolate)](2) (R = CMe2Ph; biphenolate = L1 or L2 in the text). They behave as initiators for the stereoselective (cis,isotactic) polymerization of 2,3-dicarbomethoxy-5-norbornadiene and eight enantiomerically pure 5-substituted norbornenes with a cis,isotactic precision of 95-98% in most cases. The active initiators are 14e W(O)(CHR)(biphenolate) complexes, which are formed through either dissociation of PPhMe2 from the phosphine adducts or scission of the heterochiral dimer. Addition of B(C6F5)(3) (one per W) to (R,S)-[W(mu-O)(CHR)(L1)](2) led to formation of what we propose to be monomeric W[OB(C6F5)(3)](CHR)(L1) in equilibrium with B(C6F5)3 and (R,S)-[W(mu-O)(CHR)(L1)](2). This mixture decomposed over a period of 1-2 h, was much slower to initiate polymerization than (R,S)-[W(mu-O)(CHR)(L1)](2), and was much less stereoselective. Polymerization of five of the monomers with the imido alkyidene initiator, W(N-2,6-Me2C6H3)(CHCMe2Ph)(rac-L1), gave virtually identical results compared to the results obtained with oxo complexes.