A Missing Relative: A Hoveyda–Grubbs Metathesis Catalyst Bearing a Peri-Substituted Naphthalene Framework

Abstract

Molecular scaffolds of polycyclic aromatic hydrocarbons can serve as unique tools to control the molecular and electronic structure of coordination compounds. Herein, we report the synthesis and properties of a Hoveyda−Grubbs metathesis catalyst bearing a chelating benzylidene ligand assembled on peri-substituted naphthalene. In contrast to other reported naphthalene-based complexes (Barbasiewicz, M.; Grela, K. et al. Chem. Eur. J. 2008, 14, 9330−9337), it exhibits a very fast initiation behavior, attributed to a distorted molecular structure and reduced π-electron delocalization within the chelate ring.

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

“…To reveal the riddle we focused on structural similarities between the complexes and tricyclic aromatic hydrocarbons (PAHs). [10] In particular, angular structures of complexes 28, 35 and 36 resembled molecule of phenanthrene, which displays interesting 1 H NMR properties concerning protons located in a so-called "bay region". [46] Exceedingly small H···H distances in the area result in deshielding effect (van der Waals shift) of their resonances, which amounts to ca.…”

“…In many issues it resembled its ether-chelated analogue 2 c, with slanting core of the ligand and high chemical shift value of benzylidene proton (18.32 ppm vs. 18.66 ppm for 2 c). [10] However, the catalyst appeared to be inactive in model DEDAM cyclization at 25 8C and 55 8C, and only at 80 8C in [D 8 ]toluene it initiated slowly giving 24 % of conversion of the substrate after 1 h. [30] The activity clearly contrasted with properties of 2 c, which was very fast initiator at RT.…”

“…Complex 2 a, bearing angular tricyclic structure, remained inactive at RT, [9] while peri-substituted naphthalene catalyst 2 c was an excellent initiator, outstanding from 1 and its benzoanalogue 2 b. [10] The unique behavior was attributed to electronic effects operating in the chelate rings, and degree of p-electron delocalization leading some of the them into inactive Fisher-type or aromatic structures. [9] Although, nature [11] and magnitude [12] of the electronic effects was questioned, it remained undoubted that the naphthalene ligands differ in electronic and molecular structures, and both factors actually influence the initiation process.…”

Studies on synthesis of quinonylidene Hoveyda‐type complexes

“…Sterically demanding ligand 4 was constructed using the AlCl 3 ‐catalysed Friedel–Crafts alkylation of 1,4‐dimethoxybenzene with 2,5‐dichloro‐2,5‐dimethylhexane, followed by Vilsmayer formylation. Naphthalene derivatives 6 and 7 were obtained by methylation of isomeric 1,2‐ and 2,3‐dihydroxynaphthalenes, followed by ortho ‐directed metalation and reaction with DMF [15b] . So‐formed aldehydes were subjected to Wittig reagents (Me or Et) under usual conditions…”

“…Following our research programme [5a,b,15] focused on the Hoveyda‐type metathesis catalysts, we considered chelating ligand of the complexes as a potential redox‐active fragment. The benzylidene ring, incorporating two alkoxy or hydroxy substituents, can be recognized as a reduced hydroquinone form, which upon oxidation may transform into an unprecedented quinonylidene ligand (Scheme ) …”