Novel Ruthenium(II) N‐Heterocyclic Carbene Complexes as Catalyst Precursors for the Ring‐Opening Metathesis Polymerization (ROMP) of Enantiomerically Pure Monomers: X‐ray Structures, Reactivity, and Quantum Chemical Considerations

Abstract

Four chiral, enantiomerically pure monomers, exo,exo‐N,N‐(norborn‐5‐ene‐2,3‐dicarbimido)‐L‐valine ethyl ester (exo‐1),endo,endo‐N,N‐(norborn‐5‐ene‐2,3‐dicarbimido)‐L‐valineethyl ester (endo‐1), exo,exo‐N,N‐(norborn‐5‐ene‐2,3‐dicarbimido)‐L‐valine‐tert‐butylamide (exo‐2), and endo,endo‐N,N‐(norborn‐5‐ene‐2,3‐dicarbimido)‐L‐valine‐tert‐butylamide (endo‐2), were subjected to ring‐opening metathesis polymerization (ROMP) with Ru(CF3CO2)2(IMesH2)(p‐cymene) (3), Ru(CF3CO2)2(IMes)(p‐cymene) (4), RuCl2(IMes)(p‐cymene) (5), Ru(PCy3)(CF3CO2)2(p‐cymene) (6), Ru(CF3CO2)2(p‐cymene)·CF3COOAgPCy3 (6a), Ru(CF3CO2)2(PPh3)(p‐cymene) (7), Ru(CF3CO2)2(IMes)(PhNC)3 (8), and Ru(CF3CO2)2(IMesH2)(PhNC)3 (9) (IMes = 1,3‐dimesitylimidazol‐2‐ylidene, IMesH2 = 1,4‐dimesityl‐4,5‐diyhdroimidazolin‐2‐ylidene, PCy3 = tricyclohexylphosphane). X‐ray structures of precatalysts 3 and 6–9 are presented. Compounds 3 and 4 displayed significant ROMP activity, allowing for the controlled, yet nonliving synthesis of the corresponding polymers with polydispersity indices (PDIs) in the range of 1.17–2.14. In all cases the exo isomers of compounds 1 and 2 were polymerized by preference. While poly(endo‐1) was formed in an all‐trans form, poly(exo‐1) and poly(exo‐2) were produced in their cis/trans forms with a cis content of around 40 %. Calculations carried out at the B3LYP/LACVP* level suggest two possible mechanisms for the increased reactivity of the 2,3‐R2‐exo,exo isomers of norborn‐5‐ene‐2,3‐dicarbimido derivatives resulting in the formation of the ROMP‐active RuIV alkylidene. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Cationic versus Neutral RuIIN‐Heterocyclic Carbene Complexes as Latent Precatalysts for the UV‐Induced Ring‐Opening Metathesis Polymerization

“…The 1 H and 13 C NMR and mass spectroscopic data of compounds 1-3 reveal the presence of one N-heterocyclic carbene (NHC) ligand, four tBuCN ligands, and two inequivalent anionic ligands (CF 3 SO 3 À and Cl À , respectively), suggesting cationic Ru II complexes for 1, 2, and 3. This is also supported by the 19 F NMR data for compounds 1 and 2, which show two inequivalent trifluorosulfonates (d = À78.9 and À79.0 ppm for 1; d = À78.8, À79.0 ppm for 2). Compound 4 a is not selectively formed but exists also in form of a neutral isomer, 4 b, which has three tBuCN ligands.…”

“…However, the NMR spectroscopic data on the polymers prepared by 1-4 clearly show ROMP-derived structures (see the Supporting Information). This and theoretical investigations [19] strongly suggest the formation of Ru-based Grubbs-type initiators. The ciscontent of the double bonds in poly(6)-poly(9) prepared by the action of the UV-activated precatalysts 1 and 2 varied to a significant extent with the different precatalysts used.…”

“…Further photolysis of these species in the presence of monomer then finally leads to the release of a second tBuCN ligand, coordination of the monomer and, most probably thorough a 1,2-sigmatropic H shift, [19] to the active Ru À alkylidene complex (Scheme 2).…”

Latent olefin metathesis catalysts

“…17 Exo-norbornene derivatives were polymerized faster than their endo-congeners, but the non-quantitative nature of the initiation of the ruthenium precatalysts yielded ''non-living'', though controlled polymerizations. Replacement of the chlorine ligands by trifluoro acetate ligands, as well as the incorporation of N-heterocyclic carbene ligands, furthermore proved to be of capital importance for the straightforward in situ formation of the active catalyst.…”

Cationic RuII Complexes with N‐Heterocyclic Carbene Ligands for UV‐Induced Ring‐Opening Metathesis Polymerization

“…We commenced our investigations with [Ru(IMesH 2 )-(CF 3 CO 2 )(tBuCN) 4 [31,32] (L = IMes or IMesH 2 , 1,3-dimesityl-4,5-diyhdroimidazolin-2-ylidene) by reaction with excess tBuCN. Both compounds can be handled in air.…”