No abstract

A set of nickel(III) peroxo complexes bearing tetraazamacrocyclic ligands, [Ni(III)(TBDAP)(O2)](+) (TBDAP = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane) and [Ni(III)(CHDAP)(O2)](+) (CHDAP = N,N'-dicyclohexyl-2,11-diaza[3.3](2,6)pyridinophane), were prepared by reacting [Ni(II)(TBDAP)(NO3)(H2O)](+) and [Ni(II)(CHDAP)(NO3)](+), respectively, with H2O2 in the presence of triethylamine. The mononuclear nickel(III) peroxo complexes were fully characterized by various physicochemical methods, such as UV-vis, electrospray ionization mass spectrometry, resonance Raman, electron paramagnetic resonance, and X-ray analysis. The spectroscopic and structural characterization clearly shows that the NiO2 cores are almost identical where the peroxo ligand is bound in a side-on fashion. However, the different steric properties of the supporting ligands were confirmed by X-ray crystallography, where the CHDAP ligand gives enough space around the Ni core compared to the TBDAP ligand. The nickel(III) peroxo complexes showed reactivity in the oxidation of aldehydes. In the aldehyde deformylation reaction, the nucleophilic reactivity of the nickel(III) peroxo complexes was highly dependent on the steric properties of the macrocyclic ligands, with a reactivity order of [Ni(III)(TBDAP)(O2)](+) < [Ni(III)(CHDAP)(O2)](+). This result provides fundamental insight into the mechanism of the structure (steric)-reactivity relationship of metal peroxo intermediates.

One of the most important aspects of protein function is the motion that occurs in response to substrate binding. [1] In the dynamics of enzyme catalysis, multiple weak hydrogenbonding interactions [2] in the polypeptide that are controlled by interrelated enthalpy and entropy changes play a significant role in governing the conformational changes that take place. [3] In contrast, the development of asymmetric organocatalysts has rarely focused on hydrogen-bond donors [4][5][6][7][8] that have conformationally flexible scaffolds [9][10][11] as a likely consequence of difficulties in controlling the conformation of acyclic skeletons. [12] However, recently our research group has successfully demonstrated the utility of conformationally flexible guanidine/bisthiourea organocatalysts 1 for organocatalytic carbon-carbon bond-forming reactions. [9] Herein, we describe studies that have led to the development of new acyclic C 3 -linked guanidine/bisthiourea organocatalysts 2. Analysis of these processes shows that the catalytic effect resides in a trade off between enthalpies and entropies of activation and reveals the existence of dramatic concentration effects. This investigation has uncovered a unique catalytic stereodiscrimination process controlled only by differences in the activation entropies.The primary aim of this study was to extend our newly developed organocatalytic system to asymmetric 1,4-additions reactions of nitroolefins. [13] A plausible interaction mode for the catalytic reactions of nitroolefins with nucleophilic anions is shown in Scheme 1. In the reactive complex involving an acyclic guanidine/bisthiourea organocatalyst, the thiourea moiety can interact with the nitro group in the acceptor and ionic interactions with the guanidinium cation can orient a nucleophilic anion. [14] We envisaged that a long chiral spacer between the two centers in the catalyst would be required for the promotion of the 1,4-addition reactions that take advantage of these synergistic proximity effects.In the current study, we initially selected catalytic asymmetric Friedel-Crafts (FC) reactions [15,16] of phenol derivatives. [17][18][19] Although a variety of electron-rich aromatic compounds such as indoles, pyrroles, and furans have been successfully utilized as nucleophiles in 1,4-addition processes, [15,16] asymmetric reactions of phenol derivatives have been rarely studied. The difficulty in employing phenol derivatives in these processes could be a result of two intrinsic factors that are related to the fact that phenoxide anions generated in situ 1) often promote ligand exchange with metal catalysts, [17] and 2) can participate in reactions that take place with low levels of chemo-and regioselectivity. In 2007, Chen and co-workers developed the first 1,4-type of FC reaction of naphthols with nitroolefins that utilize cinchona-based thiourea catalysts. These processes give ortho-selective FC products with 85-95 % ee. [18a] However, the undesired dimeric furans that are formed in these reactions cannot be easily se...

The reactivity of mononuclear metal-hydroperoxo adducts has fascinated researchers in many areas due to their diverse biological and catalytic processes. In this study, a mononuclear cobalt(III)-peroxo complex bearing a tetradentate macrocyclic ligand, [CoIII(Me3-TPADP)(O2)]+ (Me3-TPADP = 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane), was prepared by reacting [CoII(Me3-TPADP)(CH3CN)2]2+ with H2O2 in the presence of triethylamine. Upon protonation, the cobalt(III)-peroxo intermediate was converted into a cobalt(III)-hydroperoxo complex, [CoIII(Me3-TPADP)(O2H)(CH3CN)]2+. The mononuclear cobalt(III)-peroxo and -hydroperoxo intermediates were characterized by a variety of physicochemical methods. Results of electrospray ionization mass spectrometry clearly show the transformation of the intermediates: the peak at m/z 339.2 assignable to the cobalt(III)-peroxo species disappears with concomitant growth of the peak at m/z 190.7 corresponding to the cobalt(III)-hydroperoxo complex (with bound CH3CN). Isotope labeling experiments further support the existence of the cobalt(III)-peroxo and -hydroperoxo complexes. In particular, the O-O bond stretching frequency of the cobalt(III)-hydroperoxo complex was determined to be 851 cm−1 for 16O2H samples (803 cm−1 for 18O2H samples) and its Co-O vibrational energy was observed at 571 cm−1 for 16O2H samples (551 cm−1 for 18O2H samples; 568 cm−1 for 16O22H samples) by resonance Raman spectroscopy. Reactivity studies performed with the cobalt(III)-peroxo and -hydroperoxo complexes in organic functionalizations reveal that the latter is capable of conducting oxygen atom transfer with an electrophilic character, whereas the former exhibits no oxygen atom transfer reactivity under the same reaction conditions. Alternatively, the cobalt(III)-hydroperoxo complex does not perform hydrogen atom transfer reactions, while analogous low-spin Fe(III)-hydroperoxo complexes are capable of this reactivity. Density function theory calculations indicate that this lack of reactivity is due to the high free energy cost of O-O bond homolysis that would be required to produce the hypothetical Co(IV)-oxo product.

One of the most important aspects of protein function is the motion that occurs in response to substrate binding. [1] In the dynamics of enzyme catalysis, multiple weak hydrogenbonding interactions [2] in the polypeptide that are controlled by interrelated enthalpy and entropy changes play a significant role in governing the conformational changes that take place. [3] In contrast, the development of asymmetric organocatalysts has rarely focused on hydrogen-bond donors [4][5][6][7][8] that have conformationally flexible scaffolds [9][10][11] as a likely consequence of difficulties in controlling the conformation of acyclic skeletons.[12] However, recently our research group has successfully demonstrated the utility of conformationally flexible guanidine/bisthiourea organocatalysts 1 for organocatalytic carbon-carbon bond-forming reactions.[9] Herein, we describe studies that have led to the development of new acyclic C 3 -linked guanidine/bisthiourea organocatalysts 2. Analysis of these processes shows that the catalytic effect resides in a trade off between enthalpies and entropies of activation and reveals the existence of dramatic concentration effects. This investigation has uncovered a unique catalytic stereodiscrimination process controlled only by differences in the activation entropies.The primary aim of this study was to extend our newly developed organocatalytic system to asymmetric 1,4-additions reactions of nitroolefins.[13] A plausible interaction mode for the catalytic reactions of nitroolefins with nucleophilic anions is shown in Scheme 1. In the reactive complex involving an acyclic guanidine/bisthiourea organocatalyst, the thiourea moiety can interact with the nitro group in the acceptor and ionic interactions with the guanidinium cation can orient a nucleophilic anion. [14] We envisaged that a long chiral spacer between the two centers in the catalyst would be required for the promotion of the 1,4-addition reactions that take advantage of these synergistic proximity effects.In the current study, we initially selected catalytic asymmetric Friedel-Crafts (FC) reactions [15,16] of phenol derivatives. [17][18][19] Although a variety of electron-rich aromatic compounds such as indoles, pyrroles, and furans have been successfully utilized as nucleophiles in 1,4-addition processes, [15,16] asymmetric reactions of phenol derivatives have been rarely studied. The difficulty in employing phenol derivatives in these processes could be a result of two intrinsic factors that are related to the fact that phenoxide anions generated in situ 1) often promote ligand exchange with metal catalysts, [17] and 2) can participate in reactions that take place with low levels of chemo-and regioselectivity. In 2007, Chen and co-workers developed the first 1,4-type of FC reaction of naphthols with nitroolefins that utilize cinchona-based thiourea catalysts. These processes give ortho-selective FC products with 85-95 % ee.[18a] However, the undesired dimeric furans that are formed in these reactions cannot be easily separat...