An Inexpensive and Recyclable Silver‐Foil Catalyst for the Cyclopropanation of Alkenes with Diazoacetates under Mechanochemical Conditions

Abstract

The diastereoselective cyclopropanation of various alkenes with diazoacetate derivatives can be achieved under mechanochemical conditions using metallic silver foil and a stainless-steel vial and ball system. This solvent-free method displays analogous reactivity and selectivity to solution-phase reactions without the need for slow diazoacetate addition or an inert atmosphere. The heterogeneous silver-foil catalyst system is easily recyclable without any appreciable loss of activity or selectivity being observed. The cyclopropanation products were obtained with excellent diastereoselectivities (up to 98:2 d.r.) and in high yields (up to 96 %).

Two Amphoteric Silver Carbene Clusters

“…[7a, 10} Thefact that silver catalysts allow otherwise poorly reactive olefins to be cyclopropanated is also strong but again only indirect evidence for the participation of silver carbene intermediates. [12,13] Similar arguments pertain to silver-catalyzed Doyle-Kirmse reactions [14] and ring expansions of cyclopropyldiazomethane derivatives. [15] Even attempted asymmetric silver catalysis was not fully conclusive:T he very modest enantioselectivities and/or low yields achieved in intramolecular CÀHi nsertion as well as aziridination reactions upon silver-catalyzed diazo decomposition have indeed been rationalized by assuming that the chosen chiral silver complexes merely act as Lewis acid catalysts rather than as sources of true silver carbenes.…”

“…[1][2][3] Actually,i tc annot be firmly excluded that nascent silver carbenes decompose instantly, especially when carrying electron-withdrawing carbonyl substituents;i nd oing so,t he carbene ligand might be released into solution, where the actual rearrangement could take place in the unbound state. [12,13] Similar arguments pertain to silver-catalyzed Doyle-Kirmse reactions [14] and ring expansions of cyclopropyldiazomethane derivatives. [7a, 10} Thefact that silver catalysts allow otherwise poorly reactive olefins to be cyclopropanated is also strong but again only indirect evidence for the participation of silver carbene intermediates.…”

Mechanochemistry for Synthesis

“…[61] Mechanochemical alkene cyclopropanation with diazoacetates was readily performed using as ac atalyst silver foil that can easily be removed and reused (Figure 4c). [70] Conventional catalysts such as palladium on carbon can also be used, for example,for ball milling catalytic transfer hydrogenation. [71] Importantly,this approach can also enable new reaction selectivity,a si llustrated by nickelcatalyzed acetylene oligomerization which in solution pro-vides benzene derivatives,b ut upon milling with nickel pebbles [9] gave cyclooctatetraenes (Figure 4d [66] c) recyclability of silver foil catalyst in mechanochemical cyclopropanation; [70] d) cyclotetramerization of alkynes [9] using e) nickel pebbles.…”

Ball size or ball mass – what matters in organic mechanochemical synthesis?

“…While this effect is not expected to be universal, it has been documented in a number of organic and organometallic reactions. [27][28][29][30][31][32][33] To exclude this effect, we have therefore also prepared "composite" milling balls, whose outer layers (i.e. those in contact with the sample) are composed of the same material (nylon and polyurethane), with the internal layer composed of varying quantities of lead.…”