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 astainless-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 %).The transition-metal-catalyzed decomposition of diazoacetates has been used in various synthetic strategies to afford natural products and other biologically active compounds, many requiring high enantioselectivity.[1] Thec atalytic intermolecular [2+ +1] cyclopropanation of alkenes in the presence of diazoacetates is awell-studied transformation that is often used as ab aseline reaction for testing the activity and selectivity of newly developed catalysts.[2] Whereas several transition-metal salts have been shown to be catalytically active for the cyclopropanation of alkenes in the presence of carbene precursors,dirhodium(II) salts are widely recognized as the benchmark catalysts for these transformations as they enable the cyclopropanation of aw ide range of substrates with high chemo-, regio-, and stereoselectivity.[3] At the same time,asignificant financial cost is incurred by these preciousmetal catalysts.T oa lleviate some of the economic concerns associated with homogeneous catalysts,s everal reports have sought to immobilize them on as olid support to render the catalysts recoverable and recyclable for future use.[4] Still, multistep ligand syntheses and the cost of the solid support place an additional burden on yielding ah igher catalytic activity (e.g.,improved turnover numbers and frequencies) to recover the initial cost of investment. Furthermore,o ver the catalyst lifetime,r hodium metal is leached from the solid support into the product mixture,a nd the support must be continually charged with additional catalyst, which constitutes an additional expense to the operation costs.[5] Alternatively,r eplacing precious-metal catalysts with more earthabundant materials would have many wide-ranging benefits for developing sustainable processes.T owards this goal, Pericàs and co-workers have developed ap olystyrene-supported tris(triazolyl) methyl copper(I) complex for carbene transfer reactions of ethyl diazoacetate to aw ide range of organic compounds both in batch and in continuous-flow setups.[6] More recently,J ones,D avies,a nd co-workers also developed af low reactor system for highly enantioselective CÀHi nsertion reactions with immobilized chiral dirhodium salts. [7] Tr ansition-metal-catalyzed reactions under mechanochemical conditions have been shown to have many advantages over traditional solution-based methods.[8] Furthermore, many research groups...