Discrete Bridging and Terminal Copper Carbenes in Copper-Catalyzed Cyclopropanation

The Cu(I) β-diketiminate [Me2NN]Cu(η2-ethylene) (2) catalyzes the cyclopropanation of styrene
with N2CPh2 to give 1,1,2-triphenylcyclopropane in 67% yield. Addition of N2CPh2 to 2 equiv of 2 allows for
the isolation of the dicopper carbene {[Me2NN]Cu}2(μ-CPh2) (3) in which the diphenylcarbene moiety is
symmetrically bound between two [Me2NN]Cu fragments (Cu−C = 1.922(4) and 1.930(4) Å) with a
Cu−Cu separation of 2.4635(7) Å. In toluene-d8 solution, 3 reversibly dissociates a [Me2NN]Cu fragment
to give [Me2NN]Cu(toluene) and the terminal carbene [Me2NN]CuCPh2. Dicopper carbene 3 reacts with
3 equiv of styrene to give 1,1,2-triphenylcyclopropane and 2 equiv of [Me2NN]Cu(η2-styrene) within minutes.
DFT studies with simplified ligands indicate a stronger Cu−C π-back-bonding interaction from two Cu(I)
centers to the carbene acceptor orbital in a dicopper carbene than that present in a monocopper carbene.
Nonetheless, the terminal carbene [Me3NN]CuCPh2 (8) that possesses a p-methyl group on each
β-diketiminato N-aryl ring may be isolated and exhibits a shortened Cu−C distance of 1.834(3) Å. The
stoichiometric cyclopropanation of styrene by 8 in 1,4-dioxane is first-order in both copper carbene 8 and
styrene with activation parameters ΔH⧧ = 10.4(3) kcal/mol and ΔS⧧ = −32.3(9) cal/mol·K. In 1,4-dioxane,
8 decomposes to Ph2CCPh2 via first-order kinetics with activation parameters ΔH⧧ = 21(1) kcal/mol and
ΔS⧧ = −8(3) cal/mol·K. Arene solutions of thermally sensitive terminal carbene 8 decompose to [Me3NN]Cu(arene), which reacts with 8 still present in solution to give the more thermally stable {[Me3NN]Cu}2(μ-CPh2).