Rendering Schrock‐type Molybdenum Alkylidene Complexes Air Stable: User‐Friendly Precatalysts for Alkene Metathesis

Abstract

Complex 1 and related tetracoordinate molybdenum alkylidenes developed by Schrock and co-workers range amongst the most powerful alkene metathesis catalysts known to date. [1,2] They are distinguished by exceptional reactivity, good functional group tolerance and a remarkable scope, encompassing olefins that are considered difficult substrates on steric and/or electronic grounds. [3,4] The modularity of the established synthesis route allowed the operative motif of 1 to be systematically edited and hence gave rise to several generations of descendant catalysts with tailored properties. Most notable among them are Schrock alkylidenes with chiral alkoxide or diolate ligands and/or alkylidenes that are chiral at metal for all sorts of asymmetric metathesis reactions, [3,5] as well as catalysts that impose high Z-selectivity on certain metathesis reactions. [6] As complex 1 and relatives, however, are very sensitive to air and moisture, their handling is clearly more demanding than that of the competing ruthenium carbenes introduced by Grubbs. [7] To overcome this disadvantage, we now present a simple way of rendering prototype Schrock alkylidenes largely air-stable, which facilitates their handling and may hence encourage more widespread use of these able catalysts in preparative laboratories.Based on recent observations made in the alkyne metathesis arena, [8,9] solutions of 1 in toluene were reacted at room temperature with equimolar amounts of 1,10-phenanthroline or 2,2'-bipyridine. The resulting crystalline adducts 2 and 3 turned out to be surprisingly robust in the solid state as well as in solution. [10] Their integrity can be easily checked by NMR spectroscopy due to the characteristic downfield shift of the alkylidene proton from d H (CD 2 Cl 2 ) = 12.19 ppm in the parent complex 1 to d H (CD 2 Cl 2 ) = 13.88 and 13.74 ppm in 2 and 3, respectively. According to this distinctive signature, a sample of the bipyridine adduct 3 stored at the bench in an open flask was fully intact after eight weeks, whereas the corresponding phenanthroline adduct 2 was found to be somewhat less stable (only ca. 50 % intact after four weeks). Other prototype molybdenum alkylidenes such as the triphenylsilanolate complex 4 [11] and the chiral variant 6 [12] also form adducts that remain intact when stored without any precautions for extended periods (Scheme 1).The structure of the particularly robust complex 3 in the solid state is depicted in Figure 1. The coordination geometry about the Mo VI center can be described as distorted octahedral, with the imido ligand and one of the two inequivalent hexafluoro-tert-butanolates residing trans to each other, even though the corresponding N3-Mo1-O2 angle (167.04(4)8) strongly deviates from linearity. Likewise, the angle between the alkylidene C19 carbon and the trans-positioned N2 atom of the bipyridine is reduced to 167.02(4)8. The distortion of the ligand sphere also manifests in very uneven bond distances between the Mo center and the N atoms of the chelating bipyridine unit. The fact...

Molecular and Silica‐Supported Mo and W d0 Imido‐Methoxybenzylidene Complexes: Structure and Metathesis Activity

“…For instance, 6-coordinated adducts of Mo imidoalkylidenes with bipyridine have been found to be airstable and highly active in metathesis upon activation with ZnCl 2 . [11] While o-alkoxybenzylidene initiating ligands have been widely used for Ru-based catalysts (so-called HoveydaÀ Grubbs catalysts, Figure 1), [4,5] they are much less employed in Schrock-type systems. The first examples of 5-coordinated Mo [12] and W [13,14] complexes bearing the pendant alkoxy group attached to the alkylidene ligand were synthesized in the 1990s, with only few additional reports being published since then, [15 -17] mostly focusing on structure rather than catalytic performance.…”

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

“…With the aim of development of catalysts able for efficient transformations of the olefins under mild conditions a large number of ruthenium and molybdenum complexes were synthesized. [1][2][3] Among systems of ultimate practical importance complex 1, reported by Hoveyda, [4] was demonstrated to combine high stability and activity in metathesis reactions, duplicated further in a palette of related structures bearing chelating benzylidene ligands ( Figure 1). [5] In numerous modifications of the parent structure, general trends associated with electronic [6] and steric [7] activation of the catalysts were based on weakening of the O···Ru interaction that facilitates opening of the chelate ring and entering of the released propagating species into the catalytic cycle.…”

Katalyse und Totalsynthese: eine persönliche Zwischenbilanz

“…[92,93] Im wachsenden Bewusstsein der Bedeutung der Alkenmetathese hat unsere Gruppe auch versucht, durch die Entwicklung neuer und/oder verbesserter Katalysatoren zum Fortschritt auf diesem Gebiet beizutragen. [94][95][96] Ebenso haben wir Vorschläge für eine bessere Handhabung der bekannten Komplexe [97] sowie zur Durchführung der Reaktion in verträglicheren Medien gemacht. [98] Wichtig wurden vor allem die von uns aufgezeigten neuen Wege zu Carbenkomplexen.…”