Scalable On-Demand Production of Purified Diazomethane Suitable for Sensitive Catalytic Reactions

Redox Approaches to Carbene Generation in Catalytic Cyclopropanation Reactions

Transition metal-catalyzed carbene transfer reactions have a century-old history in organic chemistry and are a primary method for the synthesis of cyclopropanes. Much of the work in this field has focused on the use of diazo compounds and related precursors, which can transfer a carbene fragment to a catalyst with concomitant loss of a stable byproduct. Despite the utility of this approach, there are persistent limitations in the scope of viable carbenes, most notably those lacking stabilizing substituents. By coupling carbene transfer chemistry with two-electron redox cycles, it is possible to expand the available starting materials that can be used as carbene precursors. In this Minireview, we discuss emerging catalytic reductive cyclopropanation reactions using either gem-dihaloalkanes or carbonyl compounds. This strategy is inspired by classic stoichiometric transformations, such as the Simmons-Smith cyclopropanation and the Clemmensen reduction, but instead entails the formation of a catalytically generated transition metal carbene or carbenoid. We also present recent efforts to generate carbenes directly from methylene (CR2H2) groups via a formal 1,1-dehydrogenation. These reactions are currently restricted to substrates containing electron-withdrawing substituents, which serve to facilitate deprotonation and subsequent oxidation of the anion.

In-situ Kinetic Studies of Rh(II)-Catalyzed C-H Functionalization to Achieve High Catalyst Turnover Numbers

Detailed kinetic studies on the functionalization of unactivated hydrocarbon sp3 C-H bonds by dirhodium-catalyzed reaction of aryldiazoacetates revealed that the C-H functionalization step is rate-determining. The efficiency of this step was increased by using the hydrocarbon as solvent and using donor/acceptor carbenes with an electron-withdrawing substituent on the aryl donor group. The optimum catalyst for these reactions is the tetraphenylphthalimido derivative Rh2(R-TPPTTL)4 and a further beneficial refinement was obtained by using N,N'-dicyclohexylcarbodiimide as an additive. Under the optimum conditions with a catalyst loading of 0.001 mol %, effective enantioselective C-H functionalization (66-97% yield, 83-97% ee) was achieved of cycloalkanes with a range of aryldiazoacetates as long as the aryldiazoacetate was not to sterically demanding. The reaction with cyclohexane using a catalyst loading of 0.0005 mol % could be recharged twice with additional aryldiazoacetate, resulting in an overall dirhodium catalyst turnover number of 580,000.

Copper(II) Acetate-Induced Oxidation of Hydrazones to Diazo Compounds under Flow Conditions Followed by Dirhodium-Catalyzed Enantioselective Cyclopropanation Reactions

A tandem system comprising in-line diazo compound synthesis and downstream consumption in a rhodium-catalyzed cyclopropanation reaction has been developed. Passing hydrazone through a silica column absorbed with Cu(OAc)₂-H₂O/N,N-dimethylaminopyridine oxidized the hydrazone to generate an aryldiazoacetate in flow. The crude aryldiazoacetate elutes from this column directly into a downstream cyclopropanation reaction, catalyzed by the chiral dirhodium tetracarboxylates, Rh₂(R-p-Ph-TPCP)₄ and Rh₂(R-PTAD)₄. This convenient flow to batch method was applied to the synthesis of a range of 1,2-diarylcyclopropane-1-carboxylates in high yields and with high levels of enantioselectivity.