741291-43-0Relevant articles and documents
Double geminal C-H activation and reversible α-elimination in 2-aminopyridine iridium(III) complexes: The role of hydrides and solvent in flattening the free energy surface
Clot, Eric,Chen, Junyi,Lee, Dong-Heon,Sung, So Young,Appelhans, Leah N.,Faller, Jack W.,Crabtree, Robert H.,Eisenstein, Odile
, p. 8795 - 8804 (2007/10/03)
[H2Ir(OCMe2)2L2]BF4 (1) (L = PPh3), a preferred catalyst for tritiation of pharmaceuticals, reacts with model substrate 2-(dimethylamino)pyridine (py-NMe2; py = 2-pyridyl) to give chelate carbene [H 2Ir(py-N(Me)CH=)L2]BF4 (2a) via cyclometalation, H2 loss, and reversible α-elimination. Agostic intermediate [H2Ir(py-N(Me)CH2-H)L2]BF 4 (4a), seen by NMR, is predicted (DFT(B3PW91) computations) to give C-H oxidative addition to form the alkyl intermediate [(H)(η2- H2)Ir(py-N(Me)CH2-)L2]BF4. Loss of H2 leads to the fully characterized alkyl [HIr(OCMe 2)(py-N(Me)CH2-)L2]BF4 (3a Me2CO), which loses acetone to give alkylidene hydride 2a by rapid reversible α-elimination. 2a rapidly reacts with excess H2 in d6-acetone to generate [H2Ir(OC(CD3) 2)2L2]BF4 (1-d12), 3a(CD3)2CO, and py-NMe2 in a 1:1:1 ratio, showing reversibility and accounting for the selective isotope exchange catalyzed by 1. Reaction of 1 with py-N(CH2)4 gives the fully characterized carbene 2c. A cis-L2 carbene intermediate, cis-2c, observed by NMR, reacts with CO via retro α-elimination to give the alkyl 3cCO, while the trans isomer, 2c, does not react; retro α-elimination thus requires the Ir-H bond to be orthogonal to the carbene plane. Consistent with experiment, computational studies show a particularly flat PE surface with activation of the agostic C-H bond giving a less stable H2 complex, then formation of a kinetic carbene complex with cis-L, only seen experimentally for py-N(CH2)4. Hydrides at key positions, together with gain or loss of solvent and H2, flatten the PE (AG) surfaces to allow fast catalysis.
