Switched Stereocontrol in Grubbs−Hoveyda Complex Catalyzed ROMP Utilizing Proton-Switched NHC Ligands

Abstract

Grubbs-Hoveyda and Grubbs III type complexes with ferrocenyl-or -NEt 2 -substituted NHC ligands were synthesized according to standard procedures. The electron donation of the NHC ligands in the respective ruthenium complexes can be modulated by oxidation of the ferrocenyl moiety or by protonation of the amino group. The neutral and the respective cationic (oxidized or protonated) ruthenium complexes were tested in the ROMP of norbornene. The change in the electron donation of the NHC ligands upon protonation leads to a significant change in the doublebond geometry (from E/Z ratio = 0.78 to E/Z = 1.04) and in the microstructure of the resulting polynorbornene. Consequently, addition of acid and protonation of the living catalyst attached to the polymer chain during the polymerization reaction allows fine-tuning the E/Z ratio of the resulting polynorbornene. † Dedicated to Dietmar Seyferth in recognition of his tremendous contributions to organometallic chemistry and Organometallics.

Cis-Selective Ring-Opening Metathesis Polymerization with Ruthenium Catalysts

“…Many of these monomers have been polymerized with very high cis selectivity and tacticity control using Mo- and W-derived catalysts, but formed predominantly trans polymers when (PCy 3 ) 2 Cl 2 Ru=CHPh was used. 13 Gratifyingly, we found that in almost every case, 2 yielded a polymer with high cis content approaching 90%. In the cases where cis -selectivity with 2 at RT was below that value, conducting ROMP at −20 °C increased %cis by 6–15% (Table 1).…”

“…This is particularly surprising since the closely analogues (PCy 3 ) 2 Cl 2 Ru=CHPh is known to give poly-5 with only 11% cis double bonds. 13 In contrast to poly-5 and poly-9 prepared by Mo-based catalysts, 10 no long-range order was observed using either of the Ru-based initiators. With 2 , the formation of atactic polymers can be explained by fast carbene epimerization relative to the rate of propagation.…”

“…Although % cis varies significantly with catalyst, monomer, solvent, 11 and temperature, 12 Ru-based initiators, such as (PCy 3 ) 2 Cl 2 Ru=CHPh, give almost exclusively trans polymers. 13,14 Indeed, this has been a serious limitation for previous generations of Ru-based metathesis catalysts, as highlighted by Schrock and coworkers. 10c The best literature examples of stereoselective ROMP with Ru catalysts include alternating copolymerization of norbornene with cycloalkenes to give polymers with up to 75% cis double bonds.…”

Synthesis and study of olefin metathesis catalysts supported by redox-switchable diaminocarbene[3]ferrocenophanes

“…The reaction mixture was stirred for 5 min at ambient temperature. (PPh 3 ) 2 Cl 2 Ru(3-phenylindenylid-1-ene) (9) (158 mg, 0.178 mmol) was added and the vial was re-sealed with a Teflon lined cap. The solution was stirred at ambient temperature for 1 h and then concentrated under reduced pressure to afford a brown solid.…”

“…Plenio also reported efforts toward using ligand oxidation as a means to bias the intrinsic E : Z selectivities displayed by a Ru catalyst containing a ligand bearing pendant ferrocenyl substituents. 9 Although a handful of redox-switchable ligands have been studied, 10,11 their utility in controlling catalytic reactions is still rather limited. [5][6][7]12,13 This deficiency may be, at least partially, due to the fact that many of the aforementioned ligands are bi-or multi-dentate, which confines their range of possible geometries and catalytically active transition metal complexes into which they may be incorporated.…”