Mechanistic Studies of Hoveyda–Grubbs Metathesis Catalysts Bearing S‐, Br‐, I‐, and N‐coordinating Naphthalene Ligands

Abstract

Derivatives of the Hoveyda-Grubbs complex bearing S-, Br-, I-, and N-coordinating naphthalene ligands were synthesized and characterized with NMR and X-ray studies. Depending on the arrangement of the coordinating sites on the naphthalene core, the isomeric catalysts differ in activity in model metathesis reactions. In particular, complexes with the RuCH bond adjacent to the second aromatic ring of the ligand suffer from difficulties experienced on their preparation and initiation. The behavior most probably derives from steric hindrance around the double bond and repulsive intraligand interactions, which result in abnormal chemical shifts of benzylidene protons observed with (1) H NMR. Furthermore EXSY studies revealed that the halogen-chelated ruthenium complexes display an equilibrium, in which major cis-Cl2 structures are accompanied with small amounts of isomeric forms. In general, contents of the minor forms, measured at 80 °C, correlate with the observed activity trends of the catalysts, although some exceptions complicate the mechanistic picture. We assume that for the family of halogen-chelated metathesis catalysts the initiation mechanism starts with the cis-Cl2 ⇌trans-Cl2 isomerization, although further steps may become rate-limiting for selected systems.

Initiation efficacy of halo-chelated cis-dichloro-configured ruthenium-based second-generation benzylidene complexes in ring-opening metathesis polymerization

“…A similar, but more pronounced deactivating effect was found in the related derivative bearing the nitro group in ortho position to the iodine atom [17]. Further mechanistic studies using NMR spin-exchange spectroscopy at 80°C confirmed the occurrence of the anticipated fast cis-/transdichloro equilibrium and allowed for the determination of the position of the equilibrium under these conditions [18]. A correlation of the share of the trans-dichloro isomers in the equilibrium with the catalytic activity of the complexes led to the formulation of two general statements: (1) bromo-chelating compounds display a higher content of the trans-dichloro isomers going along with a higher catalytic activities (compared to analogous iodo-chelating complexes); and (2) effects of electronic tuning remain partially unclear-e.g., the content of the trans isomer increases for the NO 2 derivatives, while the catalytic activity decreases.…”

“…Based on the experimental determination of the trans isomers content of pre-initiators 1-8 at 80°C in CDCl 3 performed previously [18], data are listed in Table 2, the shares of the trans forms in toluene at 80°C and in dichloromethane at 25°C were estimated. Although these estimations have to be taken with precautions (cf.…”

“…Table 1, entries 2 and 11), M n values decrease in case of the halo-chelated pre-initiators (cf. Table 1, entries [12][13][14][15][16][17][18][19]. Still 1 is the pre-initiator with the highest initiation efficacy but variations of the M n value of different poly9 became smaller than at room temperature as the decrease in M n is bigger in case of using electronically modified initiators 3, 5, 6, and 7 (decrease by the factor of 1.8-2.6) than in case of the unsubstituted congeners 1, 2, 4, and 8 (decrease by the factor of 1.4-1.5).…”

“…Starting materials M2 and M31 were obtained from Umicore. Initiators 1-8 were prepared according to literature [14,[16][17][18]. Benchmark monomer dimethyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate (9) was synthesized via a Diels-Alder reaction according to literature procedure [31].…”

“…Estimation of the content of the trans isomer in different solvents at different temperatures based on experimental findings[18] InitiatorContent of the trans-form/% M n /kg mol -1 Content of the corresponding trans isomer at 80°C in CDCl 3 , data taken from Ref [18]. b No trans isomer detected c Calculated assuming that DG trans-cis in toluene is approx.…”