Versatile Donor‐Acceptor Cyclopropenes in Metal Carbene Transformations

Abstract

Select transition metal compounds catalyze metal vinylcarbene formation from cyclopropenes, and their documented reactions include both intermolecular and intramolecular C−H insertion and cyclopropanation, as well as [3+3]‐cycloaddition. Although known to undergo carbene‐like transformations for decades, the uses of cyclopropenes as reactive alternatives to diazo compounds under mild conditions has been limited. However, recently developed donor‐acceptor cyclopropenes that are conveniently accessed from enoldiazoacetates and enoldiazoacetamides are effective metallo‐vinylcarbene precursors. They provide entry to highly stereoselective metal carbene transformations under reaction conditions that are milder than those required for dinitrogen extrusion from diazo compounds.

Six‐Membered Cyclic Nitroso Acetals: Synthesis and Studies of the Nitrogen Inversion Process of N‐Silyloxy‐3,6‐dihydro‐2H‐1,2‐oxazines

“…Metal enol carbenoids 5 are known to exist in equilibrium with the corresponding cyclopropenes 6 . [11d], Therefore, competitive [3+2]‐cycloaddition between nitronate 1a and cyclopropene 6 leading to cyclopropane‐annulated isoxazolidine 7 might be expected. However, the obtained NMR data is inconsistent with the structure of 7 and evidences the occurrence of rearrangement and the formation of aziridine 4 .…”

“…This increases the rate of the 5 → 6 transformation, thus also decreasing the concentration of 5 . [11d] Nitronates 1 should also be considered as a Lewis base because of the N ‐oxide moiety. Thus, they are also able to deactivate Rh species.…”

“…The effect should be more significant in the case of Lewis‐acidic Rh catalysts and was observed for rhodium trifluoroacetate (Entry 16). [11d] In accordance with the equilibrium between 5 and 6 is the formation of adduct 3aa in the Rh‐catalyzed reaction between nitronate 1 and preformed cyclopropene 6 [Scheme , Equation (2)].…”

“…Recently, we reported the first examples of a formal [3+3]‐cycloaddition of nitronates with donor–acceptor cyclopropanes . Herein, we report the [3+3]‐cycloaddition of silyl nitronates 1 with rhodium enol carbenoid species, generated in situ from the corresponding enol diazoacetates 2 .…”

Cycloaddition reactions of enoldiazo compounds

“…Donor–acceptor cyclopropenes, 186 generated in situ from enoldiazo compounds, are not only direct participants in CPEC reactions 81,132,169–177 but also important intermediates (in equilibrium with the corresponding metallo-enolcarbenes) in MECC reactions (detected during 1 H NMR monitoring). 102,122,132,152,167,168 Furthermore, the cyclopropenes, preformed from enoldiazo compounds under catalytic or thermal conditions, have proven to be effective metallo-enolcarbene precursors in several MECC reactions.…”