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(μ-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(μ-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(μ-O)­(CHR)­(L1)]2. This mixture decomposed over a period of 1–2 h, was much slower to initiate polymerization than (R,S)-[W­(μ-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.