One electron oxidation of the tetragonal Cu(II) complex [Bu4N][LCuOH] at −80 °C generated the reactive intermediate LCuOH, which was shown to be a Cu(III) complex on the basis of spectroscopy and theory (L = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide). The complex LCuOH reacts with dihydroanthracene to yield anthracene and the Cu(II) complex LCu(OH2). Kinetic studies showed that the reaction occurs via H-atom abstraction via a second-order rate law at high rates (cf. k = 1.1(1) M−1s−1 at −80 °C, ΔH‡ = 5.4(2) kcal mol−1, ΔS‡ = −30(2) eu) and with very large kinetic isotope effects (cf. kH/kD = 44 at −70 °C). The findings suggest that a Cu(III)-OH moiety is a viable reactant in oxidation catalysis.

Insight into copper-oxygen species proposed as intermediates in oxidation catalysis is provided by the identification of a Cu(II)-superoxide complex supported by a sterically hindered, pyridinedicarboxamide ligand. A tetragonal, end-on superoxide structure is proposed based on DFT calculations and UV-vis, NMR, EPR, and resonance Raman spectroscopy. The complex yields a trans-1,2-peroxodicopper(II) species upon reaction with [(tmpa)Cu(CH 3 CN)]OTf, and, unlike other known Cu(II)-superoxide complexes, acts as a base rather than an electrophilic (Hatom abstracting) reagent in reactions with phenols.An important first step in copper-promoted aerobic oxidations in biology1 and catalysis2 is the formation of a 1:1 Cu/O 2 adduct, in which the O 2 molecule is activated for subsequent reactions, either with substrate or to form different copper-oxygen species. In one approach aimed at understanding such adducts, synthetic 1:1 Cu/O 2 complexes have been targeted for detailed structural, spectroscopic, and reactivity studies.3 To date, three types have been identified: (a) end-on, triplet Cu(II)-superoxos supported by tetradentate tripodal4 or, in one case, tridentate5 N-donor ligands, (b) side-on, singlet Cu(II)-superoxos supported by facially coordinating tris(pyrazolyl)hydroborates,6 and (c) side-on, singlet Cu(III)-peroxos supported by strongly electron-donating, bidentate β-diketiminates or anilido-imines.3 , 7 A key finding from reactivity studies of type (a) compounds is that they are electrophilic, with the ability to perform biologically relevant H-atom abstractions from phenols and weak C-H bonds; 4b , 5 investigations of the reactivity of type (b) compounds have not been reported, and type (c) compounds are relatively unreactive with external organic substrates. Herein we report that in seeking to expand the repertoire of available 1:1 Cu/O 2 structures for comparative evaluations, we have discovered an end-on Cu(II)-superoxide complex that displays unique characteristics, including a tetragonal geometry and non-electrophilic reactivity.Inspired by a recent report,8 we prepared complex 1 (Scheme 1) by treating N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide9 with NaOMe followed by CuCl 2 in the presence of CH 3 CN.10 In methanol or THF solution, 1 is green, whereas it is red-brown in the presence of CH 3 CN or pyridine. Consistent with this solvatochromism (Figure S2), attempts to obtain crystals of 1 suitable for X-ray crystallographic analysis were complicated by apparent CH 3 CN lability. The addition of 4-tBu-pyridine, however, yielded X-ray quality dark red crystals of 2 (Scheme 1). The complex is square planar with a geometry similar to other known 2,6-pyridinedicarboxamide Cu(II) complexes.11 wtolman@umn.edu. Supporting Information Available: Experimental procedures, spectra, computational details (PDF), and CIF. This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public AccessAuthor Manuscript J Am Chem Soc. Author manuscript; available in PMC 2011 Nov...

Using the Q2MM method, new molecular mechanics parameters were developed to perform initial screening of a chiral library to focus the experimental screening for the rhodium catalyzed hydrogenation of enamides. Computational predictions agree very well with experimental data.

The rhodium catalyzed asymmetric hydrogenation of enamides to generate amino acid products and derivatives is a widely used method to generate unnatural amino acids. The choice of a chiral ligand is of utmost importance in this reaction and is often based on high throughput screening or simply trial and error. A virtual screening method can greatly increase the speed of the ligand screening process by calculating expected enantiomeric excesses from relative energies of diastereomeric transition states. Utilizing the Q2MM method, new molecular mechanics parameters are derived to model the hydride transfer transition state in the reaction. The new parameters were based off of structures calculated at the B3LYP/LACVP** level of theory and added to the MM3* force field. The new parameters were validated against a test set of experimental data utilizing a wide range of bis-phosphine ligands. The computational model agreed with experimental data well overall, with an unsigned mean error of 0.6 kcal/mol against a set of 18 data points from experiment. The major errors in the computational model were due either to large energetic errors at high e.e., still resulting in qualitative agreement, or cases where large steric interactions prevent the reaction from proceeding as expected.