Stabilization of Molybdenum Metathesis Catalysts: The Fürstner Approach

Utility of Mo and W based metathesis catalysts is well documented, both the activity and selectivity have been demonstrated many times in the literature and also in our blog. On the other hand, the air/moisture sensitivity of the Mo and W-catalysts are widely debated and there are several approaches to overcome this hurdle.

One option is the physical encapsulation of Schrock alkylidenes, which we developed at XiMo, to formulate air stable, easy-to-handle XiMoPac pellets. You can learn more here.

Another option is the chemical stabilization method, developed by Prof. Alois Fürstner (Max Planck Institute) earlier. [1] This method stabilized several catalysts including the widely used bisalkoxy catalyst X002  by complexation with bidentate N-heterocycles, such as 1,10-phenanthroline or 2,2’-bipyridine. The resulting adduct (X002-Phen or X002-BiPy) is bench stable, and easy to prepare: one just need to mix equimolar amounts of adduct and N-heterocycles in toluene at room temperature.

These stabilized complexes are lacking the metathesis catalytic activity even at higher temperatures.  Luckily, the bidentate N-ligands can be readily removed. Upon treatment of X002-Phen (and related complexes) with one equivalent of anhydrous ZnCl2 in toluene at ≤ 100 °C, the parent Schrock alkylidene X002 complex is released.  (Scheme 1.)

Scheme 1.

The catalytic activity was fully recovered even in the presence of the precipitated bipyridine·ZnCl2 or phenanthroline·ZnCl2 by-products. The value of this in situ activation process is demonstrated by the formation of various sterically hindered and tetrasubstituted products; by the cyclization of electronically disfavored alkenes (vinylsilane, enol ether, enoate), by the easy conversion of an azide-containing substrate as well as a desymmetrization reaction and kinetic resolution, where the outcome was comparable with the reactivity of the parent Schrock alkylidene complexes (such as X002). (Scheme 2.)

Scheme 2.

The parent active complex has already been marketed by XiMo in bulk (X002) and in paraffin formulated pellets (XiMoPac-Mo007).

References:

[1] Heppekausen, J.; Fürstner, A. Angewandte Chemie, 2011, 123, 34, 7975-7978.

09. January, 2018

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