Catalytic Aziridination of Styrene with Copper Complexes of Substituted 3,7‐Diazabicyclo[3.3.1]nonanones

The copper( II ) complexes of five bispidine‐type ligands {3,7‐diazabicyclo[3.3.1]nonanone; three tetradentate ligands with 2‐pyridyl (L 1 ), 6‐methyl‐2‐pyridyl (L 2 ) or 2‐imidazolyl‐3‐methyl (L 3 ) substituents in 2,4‐positions; two pentadentate derivatives of L 1 with an additional 2‐methylpyridine substituent at N3 (L 4 ) or N7 (L 5 )} have, with one co‐ligand (Cl − ), a ligand‐enforced square pyramidal (L 1,2,3 ) or octahedral (L 4,5 ) geometry. The main structural properties of three of the five [Cu(L)(Cl)] + complexes (L 1,2,3 ) are very similar, with Cu−N3 < Cu−N7 and Cu−Cl ≈ 2.25 Å ( trans to N3); with L 2 Cu−N3 ≈ Cu−N7 and Cu−Cl = 2.22 Å ( trans to N7); with L 5 Cu−N3 < Cu−N7 and Cu−Cl = 2.72 Å ( trans to N7). These structural patterns lead to considerable differences in ligand field and electrochemical properties (range of E ° of approx. 500mV), and the reactivities of the copper( II ) complexes as aziridination catalysts (styrene, PhINTs, CH 3 CN) are strikingly different. While the complex with L 2 is very efficient, the activities of those with L 1 and L 3 are reduced to approx. 50% and 30%, respectively, and those with L 4 and L 5 are inactive. The fact that the maximum TON (maximum turnover number) of Cu II L 2 (19) is much smaller than the maximum TON of Cu I L 2 (47) suggests that in the active form the catalysts are in the Cu I oxidation state, and that the differences in reduction potentials are of major importance for catalysis. The result that CuL 4,5 have no activity in the Cu II state and only a small activity in the reduced form indicates that, apart from the reduction potentials, steric effects might also be of importance. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)