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

Abstract

We have synthesized and characterized tungsten oxo alkylidene biphenolate complexes with the formulas W(O)(CHR)(rac-biphenolate)-(PPhMe 2 ) and (R,S)-[W(μ-O)(CHR)(biphenolate)] 2 (R = CMe 2 Ph; 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 PPhMe 2 from the phosphine adducts or scission of the heterochiral dimer. Addition of B(C 6 F 5 ) 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(C 6 F 5 ) 3 ](CHR)(L1) in equilibrium with B(C 6 F 5 ) 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-Me 2 C 6 H 3 )(CHCMe 2 Ph)(rac-L1), gave virtually identical results compared to the results obtained with oxo complexes.

Effect of Lewis Acids on the Catalyst Activity for Alkene Metathesis, Z-/E- Selectivity and Stability of Tungsten Oxo Alkylidenes

“…In the subsequent years, the Schrock group reported other tungsten oxo alkylidenes and the role of ancillary ligands on the catalytic activity and product selectivity were deeply discussed (i.e. 15 and 16) [36][37][38][39][40][41][42]. Moreover, Buchmeiser and co-workers synthetized a series of very active cationic tungsten oxo alkoxy alkylidene bearing an N-heterocyclic carbene (NHC) as fourth ligand (17)(18) [43][44][45].…”

Oxo 2-Adamantylidene Complexes of Mo(VI) and W(VI)

“…Preliminary studies of the polymerization of DCMNBD by 4 W (2% in CDCl 3 ) showed that polymerization is complete in 10 min to give >99% cis , syndiotactic -poly­(DCMNBD), a result that is characteristic of certain other imido MAP initiators and analogous to the selective polymerization of DCMNBD by ( R , S )-[W­(μ-O)­(CHCMe 2 Ph)­(biphenolate)] 2 to afford the cis isotactic polymer as a consequence of dissociation of the dimer into monomers . DCMNBD is also immediately polymerized by 2 W , but the resulting polymer is approximately a 1:1 mixture of cis and trans structures, consistent with facile cleavage of the dimer into 14-electron monomers, but those are not cis-selective initiators, as expected.…”

“…The adamantylidene ligand is slightly disordered in the structural study, but the WC bond length (1.896(2) Å) could be determined accurately. The structure of 2 W is similar to that of ( R , S )-[W­(μ-O)­(CHCMe 2 Ph)­(biphenolate)] 2 , which initiates polymerization of 5,6-dicarbomethoxynorbornadiene (DCMNBD) to form the cis isotactic polymer selectively through facile dissociation of the dimer to a small degree into 14-electron monomeric units …”

“…The structure of 2 W is similar to that of (R,S)-[W(μ-O)(CHCMe 2 Ph)(biphenolate)] 2 , which initiates polymerization of 5,6-dicarbomethoxynorbornadiene (DCMNBD) to form the cis isotactic polymer selectively through facile dissociation of the dimer to a small degree into 14-electron monomeric units. 9 Bispyrrolide or bis-2,5-dimethylpyrrolide complexes have been invaluable for preparing and evaluating catalysts in situ and/or isolating a variety of derivatives formed through protonation of one or both pyrrolide ligands. 1c The reaction between 2 W and 2 equiv of LiMe 2 Pyr (Me 2 Pyr = 2,5dimethylpyrrolide) in benzene at 80 °C proceeds smoothly to give a deep-red solution containing W(O)(Adene)-(Me 2 Pyr) 2 (3 W ) and a precipitate of LiOR F9 .…”

Ring Opening Metathesis Polymerization of a New Monomer Derived from a Nitroso Diels–Alder Reaction

“…The few reports that do exist are almost exclusively focused on aza Diels-Alder reactions. [40][41][42][43][44][45][46] However, the utilization of nitroso Diels-Alder adducts as monomers for ROMP could allow for the preparation of some interesting, polar functional group-containing "polyolefins," materials of high interest to the polymer science community (Figure 1d). [47] This is especially interesting as it could possibly lead to polymers containing primary amines, materials that are challenging to prepare with ROMP.…”