Molybdenum Alkylidyne Silyloxy N‐Heterocyclic Carbene Complexes – Highly Active Alkyne Metathesis Catalysts that can be Handled in Air

Abstract

A series of molybdenum alkylidyne silyloxy N-heterocyclic carbene (NHC) complexes of the general formula [Mo(�C-(R))(OSiPh 3 ) 3 (NHC)] (R = tBu, 4-methoxyphenyl, 2,4,6-trimethylphenyl; NHC = 1,3-diisopropylimidazol-2-ylidene, 1,3-dicyclohexylimidazol-2-ylidene, 1,3-dicyclohexyl-4,5-dihydroimidazol-2ylidene, 1,3-dimethylimidazol-2-ylidene, 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene) was synthesized. Single crystal X-ray analyses revealed that with increasing steric demand of the alkylidyne group, enhanced air-stability of the complexes in the solid-state is achieved with the most stable complex (R = 2,4,6trimethylphenyl, NHC = 1,3-diisopropylimidazol-2-ylidene) being stable in air for 24 h without showing signs of decomposition in 1 H NMR. In contrast to previously reported air-stable molybdenum-based complexes, the novel catalysts proved to be highly active in alkyne metathesis, allowing for turnover numbers (TONs) of up to 6000 without further activation, and tolerant towards several functional groups such as tosyl, ether, ester, thioether and nitro moieties. Their air stability allows for facile handling of the catalysts in air and even after exposure to ambient atmosphere for one week, the most stable representative still displayed high productivity in alkyne metathesis.

From the Glovebox to the Benchtop: Air-Stable High Performance Molybdenum Alkylidyne Catalysts for Alkyne Metathesis

“…On the other hand, ligation must be reversible, ideally without any extra chemical or physical stimulus to obviate the need for a separate preactivation step. Therefore, phenanthroline or bipyridine were sorted out, ,, and monodentate donors less “sticky” than the NHCs used by the Buchmeiser group were sought . Since (canopy) molybdenum silanolate catalysts tolerate many different Lewis basic groups likely because their binding to the Mo­(+6) center is reversible, we saw a window of opportunity to find the right match.…”

“…The alkyne metathesis reaction itself then needs to be carried out in 1,2-dichloroethane under a nitrogen atmosphere at 80 °C to enforce (partial) decomplexation of the stabilizing NHC ligand (no activity was observed at room temperature). Under these conditions, good turnover numbers were secured at low catalyst loadings, at least for barely functionalized substrates …”

“…The underlying concept was recently extended by the group of Buchmeiser, using N-heterocyclic carbenes (NHCs) instead of phenanthroline to form meta-stable adducts 1 · NHC . , A particular complex of this type (Ar = 2,4,6-trimethylphenyl; NHC = 1,3-diisopropyl­imidazol-2-ylidene) was reported to show a lifetime in the solid state of more than a week on the bench; therefore, it can be weighed and transferred in air. The alkyne metathesis reaction itself then needs to be carried out in 1,2-dichloroethane under a nitrogen atmosphere at 80 °C to enforce (partial) decomplexation of the stabilizing NHC ligand (no activity was observed at room temperature).…”

“…Therefore, phenanthroline or bipyridine were sorted out, 18,19,49 and monodentate donors less "sticky" than the NHCs used by the Buchmeiser group were sought. 21 Since (canopy) molybdenum silanolate catalysts tolerate many different Lewis basic groups likely because their binding to the Mo(+6) center is reversible, 32 we saw a window of opportunity to find the right match. Among the possible candidates, pyridine (or its commercial derivatives) was deemed a good starting point.…”

“…Under these conditions, good turnover numbers were secured at low catalyst loadings, at least for barely functionalized substrates. 21 Other authors pursued different approaches toward userfriendly setups. A notable report by Zhang and co-workers showed that the (presumed) tripodal catalyst 4 generated in situ allowed the subsequent alkyne metathesis reactions to be carried out in air at 70 °C; however, hepatotoxic and potentially carcinogenic CCl 4 is needed as the solvent to ensure optimal results.…”