Catalyst design based on agostic interactions: synthesis, characterization, and catalytic activity of bis(pyrazolyl)borate copper complexes

Agostic interactions are often used to activate inert C-H bonds, and thus facilitate new reactions. We report the first example of designed catalysts based on the agostic interaction. Novel copper(I) complexes [BBN(pz(x))(2)]Cu(PPh3)(n) (BBN = 9-borabicyclo[3.3.1]nonane; pz(x) = 3-substituted pyrazole; x = H, n = 2; x = Me, n = 1) and {[BBN(pz(iPr))(2)]Cu}(2) have been synthesized and characterized. Single crystal studies of the three compounds show weak intramolecular C-H center dot center dot center dot Cu interactions which can be assigned as agostic or anagostic interactions. Catalytic studies of these complexes toward carbenoid insertion into N-H bonds indicate these weak interactions act as a switch which will be turned on if interacting with the substrate and off if eliminating the product and regenerating the weak interaction. The process of the switch turning on or off, which is related to the catalytic effect, is found to be influenced by both steric effects and the solvent: a less sterically hindered catalyst in non-coordinating benzene results in high yield, while a more sterically hindered catalyst in coordinating THF results in relatively low yield.