Synthesis and Reactivity of Ruthenium Phosphite Indenylidene Complexes

Abstract

The synthesis of the four olefin metathesis precatalysts Caz-1a-d, featuring the NHC ligand N,N'-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (SIMes) and four different phosphites (P((OPr)-Pr-i)(3), P(OPh)(3), P(OEt)(3), and P(OMe)(3)), is reported. The complexes are readily synthesized from commercially available [RuCl2(3-phenylinden-1-ylidene)-(pyridine)(SIMes)] (Ind-III) in yields of up to 88%. These complexes adopt an unusual cis configuration between the phosphite and the NHC ligands. NMR experiments and computational studies confirm that the cis complexes are thermodynamically favored in comparison to their trans counterparts. In addition, the isomerization from trans to cis occurs via a mononuclear and non-dissociative mechanism. Among the four precatalysts, cis-Caz-1a, featuring a P((OPr)-Pr-i)(3) ligand, displays the highest activity in ring-closing metathesis and cross-metathesis transformations. Experiments at low catalyst loadings demonstrated the potential of this catalyst, allowing better conversions than with commonly used commercially available precatalysts

Key processes in ruthenium-catalysed olefin metathesis

“…55 Cazin reported the synthesis of Caz1 via simple ligand displacement (Scheme 6). 56,57 Surprisingly, the product complex featured cis-chloride rather than the expected trans-chloride geometry. Analogous benzylidene complexes exhibit the expected trans-geometry.…”

The Influence of Structure on Reactivity in Alkene Metathesis

“…In 2010, Cazin introduced a new family of ruthenium-indenylidene metathesis pre-catalysts which bore phosphite ligands in place of phosphine ligands; 44 this was followed in 2011 by the disclosure of the corresponding benzylidene variants (from the reaction of G2-derived bis(pyridine) complex G2py) 45 and subsequent full papers in 2012 and 2013 where a range of indenylidene species were reported. 46,47 The synthesis of the indenylidene complexes was accomplished by addition of phosphite to the corresponding pyridine complexes; however, in some cases a mixture of cis-dichloride and transdichloride complexes was obtained (Scheme 7).…”

“…Synthesis of mixed NHC/phosphite indenylidene complexes. 44,46,47 SIMes-bearing complexes are therefore most often isolated as the cis-dichloride isomer, while complexes bearing the bulkier SIPr ligand are typically isolated as the trans-dichloride isomer. The activation parameters for trans-cis isomerization have been determined (∆H ‡ = 22.6 kcal mol -1 ; ∆S ‡ = -4.2 cal K -1 mol -1 );, which the small entropy term suggests that phosphite dissociation is not involved in the trans-cis isomerization; .…”

Cross‐metathesis of fatty acid methyl esters with acrolein: An entry to a variety of bifunctional compounds

“…Therefore, cross‐metathesis of FAMEs with acrolein could in principle lead to fully bio‐sourced compounds. A literature survey showed that acrolein has been rarely used in cross‐metathesis reactions probably because of its lower reactivity as compared to other acrylic derivatives . Herein, we present a thorough study of the cross‐metathesis of several FAMEs with acrolein.…”