Simple Ruthenium Precatalyst for the Synthesis of Stilbene Derivatives and Ring‐Closing Metathesis in the Presence of Styrene Initiators

Abstract

was found to be an efficient precatalyst, with styrene as an initiating species, for the alkene metathesis of various styrenes into symmetrical and unsymmetrical stilbene derivatives and for the ring-closing metathesis reaction of a sterically hindered olefin leading to a tetrasubstituted cycloolefin.Keywords: alkene metathesis; N-heterocyclic carbenes; ring-closing metathesis; ruthenium; stilbenes; styrenes Stilbene derivatives, in particular hydroxy-or alkoxysubtituted ones are gaining importance due to their biological activities for the prevention of cancer or cardiovascular diseases.[1] They are also an important family of building blocks for the design of conjugated materials with optical properties.[2] Wittig or WittigHorner reactions [3] constitute a priviledged approach for the synthesis of stilbene derivatives but they generate stoichiometric amounts of phosphine oxides hence requiring fastidious purification. Catalytic reactions such as Heck or Suzuki couplings can be used as more efficient ways to synthesize stilbene derivatives.[4] More recently catalytic olefin metathesis [5] appeared as a tool of choice for the efficient and stereoselective synthesis of stilbene derivatives thanks to the emergence of very active catalysts for cross-metathesis.[6] In particular, the ruthenium-based second generation Grubbs catalyst has been shown to efficiently promote the self-and cross-metathesis of styrene derivatives. [7] Other complexes based on molybdenum [8] or osmium [9] have also been described as efficient catalysts for this transformation.We recently reported the intramolecular vinyl C À H bond activation of an N-vinyl-N-heterocyclic carbeneruthenium(II) intermediate leading to a bidentate NHC-alkenyl ruthenium complex (Scheme 1). [10] In an attempt to explore the intermolecular version of this transformation, styrene was reacted with the readily available ruthenium complex[11] [IMes = 1,3bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]. This reaction resulted in the stereoselective formation of E-stilbene in very good yield, thus suggesting a self-metathesis reaction. Here we describe several aspects of RuCl 2 A C H T U N G T R E N N U N G (p-cymene)-A C H T U N G T R E N N U N G (IMes) as catalyst precursor for alkene metathesis reactions. In the absence of an alkene metathesis carbene initiator, complex 1 acts as a pre-catalyst for the metathesis transformation of various styrene derivatives into the corresponding stilbenes including unsymmetrical ones. We show the dual ring-closing metathesis (RCM) vs. cycloisomerisation activity of 1 and that, in the presence of styrene as an activator, complex 1 is effective for the RCM of a sterically hindered diene.

Ruthenium‐Based NHC‐Arene Systems as Ring‐Opening Metathesis Polymerisation Catalysts

“…Whereas the coordination of IMes (1,3-dimesitylimidazol-2-ylidene) to Ru dimer 2 proceeds readily by standard synthetic strategies (complex 4), [38,47] analogous binding of H 2 IMes was found to be more problematic. Regardless of the reaction conditions applied, a mixture of Ru complexes was obtained (Scheme 1).…”

“…1 H and 13 C NMR spectroscopic measurements were performed with a Varian Unity-300 spectrometer. Ru dimer 2, [59] H 2 IMes(H)(CCl 3 ), [60] the bidentate NHC salts [47] and complex 3 [35] were prepared according to literature procedures. (Trimethylsilyl)-diazomethane (TMSD) was purchased from Aldrich as a solution in hexane.…”

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

“…The [RuCl 2 (p-cymene)] 2 dimer (1) was purchased from Strem. The [RuCl 2 (p-cymene)(NHC)] complexes 2-9 were prepared according to literature procedures [17,[30][31][32][33]. All the other chemicals were purchased from Aldrich and used as received.…”

Flash-metathesis for the coupling of sustainable (poly)hydroxyl β-methylstyrenes from essential oils

“…However no improvement was observed (entry 2). As literature precedents demonstrate that using an excess of one of the cross metathesis partners allows for good selectivity, 11,32 we transposed this strategy that had been developed with terminal styrenes under conventional conditions to our solvent-free procedure with β-methylstyrenes. Satisfyingly, using 5 equiv.…”