Highly Efficient Nickel-Catalyzed 2-Methyl-3-butenenitrile Isomerization: Applications and Mechanistic Studies Employing the TTP Ligand Family

• Published in: Organometallics Vol. 30; no. 10; pp. 2790 - 2809
• Main Authors: Tauchert, Michael E, Warth, Daniel C. M, Braun, Sebastian M, Gruber, Irene, Ziesak, Alexandra, Rominger, Frank, Hofmann, Peter
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 23.05.2011; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Inorganic & Nuclear; Chemistry, Organic; Physical Sciences; Science & Technology; AUXILIARY BASIS-SETS; DIPHOSPHINE LIGANDS; ATOMS LI; APPROXIMATION; CN BOND-CLEAVAGE; HYDROCYANATION; COMPLEXES; REGIOSELECTIVITY; GAUSSIAN-BASIS SETS; HYDROFORMYLATION
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om200164f

A series of sterically and electronically fine-tuned, chelating diphosphine ligands were synthesized. The ligands are analogues of Triptyphos (TTP, 1), all based upon a variably 9,10-two-carbon-bridged 9,10-dihydroanthracene scaffold. These new TTP-type ligands were employed in the Ni(0)-catalyzed isomerization of 2-methyl-3-butenenitrile (2M3BN), one of the key steps of industrial adiponitrile production by the DuPont process. The reaction showed a surprising preference for ligands bearing electron-donating substituents, such as methoxy or methyl groups, in the phenyl para position of the Ni-ligating PPh2 units. Octyltriptyphos (3) afforded the highest 2M3BN-isomerization turnover rate yet reported. A series of deuterium-labeling experiments was performed to investigate the possibility of an isomerization mechanism consisting of a cascade of de- and rehydrocyanation steps, which could be excluded. Using the ethano-bridged ligand 4, complex 16a (4-κP:κP′)Ni(η3-C4H7)CN (supposedly an intermediate of the 2M3BN-isomerization reaction) was isolated, and its solid-state structure was determined by X-ray diffraction analysis. The complete catalytic cycle of 2M3BN isomerization with ligand 4, as suggested by the available experimental evidence, was modeled using DFT methods.