[(L)Cu(O)] (i.e., cupric-superoxo) complexes, as the first and/or key reactive intermediates in (bio)chemical Cu-oxidative processes, including in the monooxygenases PHM and DβM, have been systematically stabilized by intramolecular hydrogen bonding within a TMPA ligand-based framework. Also, gradual strengthening of ligand-derived H-bonding dramatically enhances the [(L)Cu(O)] reactivity toward hydrogen-atom abstraction (HAA) of phenolic O-H bonds. Spectroscopic properties of the superoxo complexes and their azido analogues, [(L)Cu(N)], also systematically change as a function of ligand H-bonding capability.

The central role of cupric superoxide intermediates proposed in hormone and neurotransmitter biosynthesis by noncoupled binuclear copper monooxygenases like dopamine-β-monooxygenase has drawn significant attention to the unusual methionine ligation of the Cu M ("Cu B ") active site characteristic of this class of enzymes. The copper−sulfur interaction has proven critical for turnover, raising still-unresolved questions concerning Nature's selection of an oxidizable Met residue to facilitate C−H oxygenation. We describe herein a model for Cu M , [( TMG N 3 S)Cu I ] + ([1] + ), and its O 2 -bound analog [The latter is the first reported cupric superoxide with an experimentally proven Cu−S bond which also possesses demonstrated hydrogen atom abstraction (HAA) reactivity. Introduction of O 2 to a precooled solution of the cuprous precursor [1]B(C 6 F 5 ) 4 (−135 °C, 2-methyltetrahydrofuran (2-MeTHF)) reversibly forms [1•O 2 ]B(C 6 F 5 ) 4 (UV/vis spectroscopy: λ max 442, 642, 742 nm). Resonance Raman studies (413 nm) using 16 O 2 [ 18 O 2 ] corroborated the identity of [1•O 2 ] + by revealing Cu−O (446 [425] cm −1 ) and O−O (1105 [1042] cm −1 ) stretches, and extended X-ray absorption fine structure (EXAFS) spectroscopy showed a Cu−S interatomic distance of 2.55 Å. HAA reactivity between [1•O 2 ] + and TEMPO−H proceeds rapidly (1.28 × 10 −1 M −1 s −1 , −135 °C, 2-MeTHF) with a primary kinetic isotope effect of k H /k D = 5.4. Comparisons of the O 2 -binding behavior and redox activity of [1] + vs [2] + , the latter a close analog of [1] + but with all N atom ligation (i.e., N 3 S vs N 4 ), are presented.

The dioxygen reactivity of as eries of TMPA-based copper(I) complexes (TMPA = tris(2-pyridylmethyl)amine), with and without secondary-coordination-sphere hydrogenbonding moieties,w as studied at À135 8 8Ci n2 -methyltetrahydrofuran (MeTHF). Kinetic stabilization of the H-bonded [( ðX 1 ÞðX 2 Þ TMPA)Cu II (O 2 C À )] + cupric superoxide species was achieved, and they were characterized by resonance Raman (rR) spectroscopy. The structures and physical properties of [( ðX 1 ÞðX 2 Þ TMPA)Cu II (N 3 À )] + azido analogues were compared, and the O 2 C À reactivity of ligand-Cu I complexes when an Hbonding moiety is replaced by amethyl group was contrasted. Ad rastic enhancement in the reactivity of the cupric superoxide towards phenolic substrates as well as oxidation of substrates possessing moderate C À Hb ond-dissociation energies is observed, correlating with the number and strength of the H-bonding groups.Oxidation and oxygenation reactions are vital for biological and synthetic processes. [1][2][3][4][5][6][7][8][9] In biology,s ome copper-containing metalloenzymes are capable of performing these reactions. [2,10] Forexample,galactose oxidase (GO) is responsible for the oxidation of primary alcohols to aldehydes (Figure 1A); monooxygenases such as peptidylglycine a-hydroxylating monooxygenase (PHM), dopamine b-monooxygenase (DbM), and lytic polysaccharide monooxygenases (LMPOs) catalytically hydroxylate organic substrates containing strong (85-100 kcal mol À1 )C ÀHb ond dissociation energies (BDEs) using dioxygen ( Figure 1B). [2,[10][11][12] These reactions are important for biosynthesis of human prohormones and neurotransmitters in the former,a nd breaking down polysaccharides in the latter. Acupric superoxide species (that is,Cu II À O 2 C À ,formed from the reaction of copper(I) and dioxygen) is postulated to be involved in the catalytic cycles of each of these enzymes. [11] In GO,t his complex abstracts ah ydrogen atom from anearby Tyrresidue,thereby forming the catalytically active intermediate responsible for substrate oxidation.While the exact nature of the reactive intermediate in LPMOs is still widely debated, [12][13][14] ac onsensus in the literature for PHM and DbMi sthat acupric superoxide is responsible for the initial hydrogen-atom transfer (HAT) from ac arbon substrate. [11] Thus,t he study of synthetic cupric superoxide model complexes to further understand their structures, physical-spectroscopic properties,a nd correlated reactivity toward the hydroxylation of substrates containing strong CÀ Hb onds is of considerable interest in catalysis.Ty pically,t hese primary copper-dioxygen superoxide model species [11,15,16] are difficult to study due to their tendency to form secondary Cu 2 -O 2 adducts (that is, m-1,2peroxo-dicopper(II), side-on peroxodicopper(II), or bis-moxodicopper(III) complexes) in solution. Researchers have been able to prevent the formation of 2:1C u:O 2 adducts through ligand design;the addition of as econdary coordination sphere of sterically bulky groups [17][18][19][20][21...

The present article describes novel oxidative protocols for direct esterification of alcohols. The protocols involve successful demonstrations of both "cross" and "self" esterification of a wide variety of alcohols. The cross-esterification proceeds under a simple transition-metal-free condition, containing catalytic amounts of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy)/TBAB (tetra-n-butylammonium bromide) in combination with oxone (potassium peroxo monosulfate) as the oxidant, whereas the self-esterification is achieved through simple induction of Fe(OAc)2 /dipic (dipic=2,6-pyridinedicarboxylic acid) as the active catalyst under an identical oxidizing environment.

A cuprous chelate bearing a secondary sphere hydrogen bonding functionality, [(PV-tmpa)­CuI]+, transforms •NO(g) to N2O(g) in high-yields in methanol. Ligand derived proton transfer facilitates N–O bond cleavage of a putative hyponitrite intermediate releasing N2O(g), underscoring the crucial balance between H-bonding capabilities and acidities in (bio)­chemical •NO(g) coupling systems.