Designing ligands to achieve robust atom-transfer oxidation catalysts

318 Abstracts F31 DESIGNING LIGANDS TRANSFER OXIDATION T. J. C o l l i n s , a M . J. B a r t o s , a S. Kauffmann, a E. M i i n c k , a C. E. U f f e l m a n a and L. J a m e s W r i g h t a TO ACHIEVE ROBUST ATOMCATALYSTS. W . G o r d o n - W y l i e , a B. G. F o x , a K. E. F. R i c k a r d , b S. T . W e i n t r a u b , c E. S. a Departments of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15206 and b The University of Auckland, Private Bag 92019, Auckland, New Zealand, c Department of Pathology, University of Texas at San Antonio, San Antonio, TX 78284. Several decades of investigation into the mechanisms of oxidations catalyzed by iron enzymes has created a knowledge base that challenges inorganic chemists to develop Fe-oxo complexes that are both stable and able to oxidize C-H bonds. One approach arises from the development of organic ligands that are oxidation resistant.[1] The rationale for the presented study begins with the observation that Mn m complexes of strongly-donating, oxidationresistant ligands react with alkyl-hydroperoxides to produce stable MnV(O) complexes, e.g. 1.[2] Therefore, in examining the Fe chemistry of the tetraamide macrocycles[3, 4] we were led to ask if stable Fe(O) complexes could be produced in a similar manner. Yellow-orange, five-coordinate, S = 3/2, Felh starting complexes with the same macrocycle, e.g., 2, react in acetonitrile with t-butylhydroperoxide (TBHP) to give a deep blue species. This species has been shown to be the S = 1 FelV-cyano complex, 3: 13C-labeling studies demonstrate that the cyano ligand is obtained from the nitrile solvent. These findings led us to conduct a general study of nitrile oxidations by the 3/TBHP system. The study has been focused on attaining quantitative mass balance for the fate of the Fe complex, the TBHP, and the solvent/substrate: nitrile solvents that possess a - C - H bonds (D°298 = 86.5-93 kcal.mol q) are substrates. I o.jlOlv,' \ N o°"\ o.._l,l,, C' o The structures and spectroscopy of the oxidized Fe complexes encountered will be presented in the context of a description of the complicated reactivity among 2, the TBHP oxidant, the tetraamido-N-ligand, the nitrile solvent-substrates, and the byproducts and intermediate products of file reaction system. Analysis of the C-H bond oxidation chemistry will include discussions of the relative roles of free radical autoxidation and metal-catalyzed oxidation, the characterization of sensitive peroxide products, ligand degradation and ligand design. The presentation will conclude with a discussion of the suicide inhibition encountered and of the implications of the work for ligand refinement aimed at producing nonbiological, metalloredox-active catalysts for C - H bond oxidations. A serious and possibly general consequence of the abandonment of the protein for biomimetic oxidations that involve intermediate carbon radicals will be noted. 1. T. J. Collins, Acc. Chem. Res., 27, 279 (1994) 2. T. J. Collins, R. D. Powell, C. Slebodnick and E. S. Uffelman, J. Am. Chem. Soc., 112, 899 (1990) 3. K. L. Kostka, B. G. Fox, M. P. Hendrich, T. J. Collins, C. E. F. Rickard, L. J. Wright and E. Mtinck, J. Am. Chem. Soc., 115, 6746 (1993) 4. T. J. Collins, B. G. Fox, Z. G. Hu, K. L. Kostka, E. M~nck, C. E. F. Rickard and L. J. Wright, J. Am. Chem. Soc., 114, 8724 (1992)