A Comparison of the Stability and Reactivity of Diamido- and Diaminocarbene Copper Alkoxide and Hydride Complexes

The mononuclear N‐heterocyclic carbene (NHC) copper alkoxide complexes [(6‐NHC)CuOtBu] (6‐NHC=6‐MesDAC (1), 6‐Mes (2)) have been prepared by addition of the free carbenes to the tetrameric tert‐butoxide precursor [Cu(OtBu)]4, or by protonolysis of [(6‐NHC)CuMes] (6‐NHC=6‐MesDAC (3), 6‐Mes (4)) with tBuOH. In contrast to the relatively stable diaminocarbene complex 2, the diamidocarbene derivative 1 proved susceptible to both thermal and hydrolytic ring‐opening reactions, the latter affording [(6‐MesDAC)Cu(OC(O)CMe2C(O)N(H)Mes)(CNMes)] (6). The intermediacy of [(6‐MesDAC)Cu(OH)] in this reaction was supported by the generation of Cu2O as an additional product. Attempts to generate an isolable copper hydride complex of the type [(6‐MesDAC)CuH] by reaction of 1 with Et3SiH resulted instead in migratory insertion to generate [(6‐MesDAC‐H)Cu(P(p‐tolyl)3)] (9) upon trapping by P(p‐tolyl)3. Migratory insertion was also observed during attempts to prepare [(6‐Mes)CuH], with [(6‐Mes‐H)Cu(6‐Mes)] (10) isolated, following a reaction that was significantly slower than in the 6‐MesDAC case. The longer lifetime of [(6‐Mes)CuH] allowed it to be trapped stoichiometrically by alkyne, and also employed in the catalytic semi‐reduction of alkynes and hydrosilylation of ketones