Metal and halogen dependence of the rate effect in hydroamination/cyclization of unactivated aminoalkenes: Synthesis, characterization, and catalytic rates of CCC-NHC hafnium and zirconium pincer complexes

Wesley D. Clark
Joon Cho
Henry U. Valle
T. Keith Hollis
Edward J. Valente, University of Portland

Journal of Organometallic Chemistry, 2014, Volume 751, 534-540.

© 2014 Elsevier Publishing

Linked version is the final published version.

Abstract

1,3-Bis(3′-butylimidazol-1′-yl)benzene dibromide (2b) or 1,3-bis(3′-butylimidazol-1′-yl)benzene dichloride (2c) was reacted with a stoichiometric amount of Zr(NMe2)4 or Hf(NMe2)4 yielding four new early transition metal CCC-N-heterocylic carbene (CCC-NHC) pincer complexes. Two of the CCC-NHC pincer complexes were synthesized via a new methodology, which allowed for a room temperature reaction, shorter reaction times, and slightly less Zr(NMe2)4 or Hf(NMe2)4. The molecular structure of 2-(1,3-bis-3′-butylimidazol-1′-yl-2′-ylidene)phenylene)(dimethylamido)(dibromo)zirconium(IV) (3b) was determined by X-ray crystallography. The complexes were evaluated for hydroamination/cyclization of unactivated aminoalkenes yielding pyrrolidines or piperidines. The hydroamination/cyclization rates were dependent upon the halogen or the metal in the complex. The complexes with iodide ligands gave the fastest hydroamination/cyclization rates (I > Br > Cl). The Zr complexes provided faster hydroamination/cyclization rates than the Hf analogs. Use of CCC-NHC ZrI3 complex 5 enhanced the rate of reaction.