Discrete bridging and terminal copper carbenes in copper-catalyzed cyclopropanation

The Cu(I) beta-diketiminate [Me2NN]Cu(eta(2)-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)(mu-CPh2) (3) in which the diphenylcarbene moiety is symmetrically bound between two [Me2NN]Cu fragments (Cu-C = 1.922(4) and 1.930(4) Angstrom) with a Cu-Cu separation of 2.4635(7) Angstrom. In toluene-d(8) 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(eta(2)-styrene) within minutes. DFT studies with simplified ligands indicate a stronger Cu-C pi-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 beta-diketiminato N-aryl ring may be isolated and exhibits a shortened Cu-C distance of 1.834(3) Angstrom. The stoichiometric cyclopropanation of styrene by 8 in 1,4-dioxane is first-order in both copper carbene 8 and styrene with activation parameters DeltaH(not similar or equal to) = 10.4(3) kcal/mol and DeltaS(not similar or equal to) = -32.3(9) cal/mol-K. In 1,4-dioxane, 8 decomposes to Ph2C=CPh2 via first-order kinetics with activation parameters DeltaH(not similar or equal to) = 21(1) kcal/mol and DeltaS(not similar or equal to) = -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)-(mu-CPh2).