New In situ Generated Ruthenium Catalysts Bearing N-Heterocyclic Carbene Ligands for the Ring-Opening Metathesis Polymerization of Cyclooctene

Abstract

New 1,3‐diarylimidazol(in)ium chlorides bearing phenyl, 1‐naphthyl, 4‐biphenyl, 2‐tolyl, 2,6‐dimethylphenyl, and 3,5‐dimethylphenyl substituents were synthesized. They were combined with [RuCl2(p‐cymene)]2 and potassium tert‐butoxide or sodium hydride to generate the corresponding ruthenium‐N‐heterocyclic carbene complexes in situ. Catalyst precursors derived from imidazol(in)ium salts bearing the 2,4,6‐trimethylphenyl (mesityl) and the 2,6‐diisopropylphenyl groups were also prepared. The catalytic activity of all these species in the photoinduced ring‐opening metathesis polymerization of cyclooctene was investigated. The C4‐C5 double bond in the imidazole ring of the N‐heterocyclic carbene ligands was not crucial to achieve high catalytic efficiencies. The presence or the absence of alkyl groups on the ortho positions of the phenyl rings had a more pronounced influence. Blocking all the ortho positions was a requisite for obtaining efficient catalysts. Failure to do so probably results in the ortho‐metallation of the carbene ligand, thereby altering the coordination sphere of the ruthenium active centers.

Synthesis and structural characterization of Palladium(II) complex bearing N-Heterocyclic Carbene

“…The performance of homogeneous catalysts is highly dependent on the substituent on the N-atoms. The presence and position of methyl groups of the phenyl rings had a more pronounced influence [2].…”

“…The volatiles were removed in vacuo, and the residue was washed with Et 2 O. The crude product was recrystalized from CH 2 …”

“…Particular attention has been devoted to imidazol(in)-2-ylidene derivatives because they can provide extra stability to the metal centers [2]. Among other applications, these complexes have been used as catalyst in reactions such as furanization, cyclopropanation [3], olefin metathesis [4], and hydrogenation [5].…”

Olefin metathesis meets rubber chemistry and technology

“…Higher activity was obtained by Calderon et al using WCl 6 /(C 2 H 5 )AlCl 2 /ethanol as catalyst [30]. However, many other strong catalyst systems were implemented in the last decades for the synthesis of polyoctenamer [17,21,[31][32][33][34][35]. The cis/trans ratio as well as the formation of cyclic polymers strongly depends on the initiator system; polymers with high cis content exhibit a lower melting temperature as well as a lower crystallinity [36].…”

Ruthenium–arene catalysts bearing N-heterocyclic carbene ligands for olefin cyclopropanation and metathesis

“…metathesis polymerization (ROMP) of cyclic olefins when activated by visible light [17]. A 1:2:4 molar association of the [RuCl 2 (p-cymene)] 2 dimer (1), an imidazol(in)ium salt, and a base also provided efficient catalytic systems for various types of metathesis reactions [18,19]. Such a combination required only stable and commercially available reagents to generate active species in situ, thereby affording very simple and straightforward experimental procedures [20].…”