A longstanding research goal has been to understand the nature and role of copper–oxygen intermediates within copper-containing enzymes and abiological catalysts. Synthetic chemistry has played a ...pivotal role in highlighting the viability of proposed intermediates and expanding the library of known copper–oxygen cores. In addition to the number of new complexes that have been synthesized since the previous reviews on this topic in this journal (Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev. 2004, 104, 1013–1046 and Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047–1076), the field has seen significant expansion in the (1) range of cores synthesized and characterized, (2) amount of mechanistic work performed, particularly in the area of organic substrate oxidation, and (3) use of computational methods for both the corroboration and prediction of proposed intermediates. The scope of this review has been limited to well-characterized examples of copper–oxygen species but seeks to provide a thorough picture of the spectroscopic characteristics and reactivity trends of the copper–oxygen cores discussed.
The stretching frequency, ν(Cu–O), of the CuOH2+ core in the complexes LCuOH (L = N,N′-bis(2,6-diisopropyl-4-R-phenyl)pyridine-2,6-dicarboxamide, R = H or NO2, or ...N,N′-bis(2,6-diisopropylphenyl)-1-methylpiperidine-2,6-dicarboxamide) was determined to be ∼630 cm–1 by resonance Raman spectroscopy and verified by isotopic labeling. In efforts to use Badger’s rule to estimate the bond distance corresponding to ν(Cu–O), a modified version of the rule was developed through use of stretching frequencies normalized by dividing by the appropriate reduced masses. The modified version was found to yield excellent fits of normalized frequencies to bond distances for >250 data points from theory and experiment for a variety of M–X and X–X bond distances in the range ∼1.1–2.2 Å (root mean squared errors for the predicted bond distances of 0.03 Å). Using the resulting general equation, the Cu–O bond distance was predicted to be ∼1.80 Å for the reactive CuOH2+ core. Limitations of the equation and its use in predictions of distances in a variety of moieties for which structural information is not available were explored.
With the goal of understanding how distal charge influences the properties and hydrogen atom transfer (HAT) reactivity of the CuOH2+ core proposed to be important in oxidation catalysis, the ...complexes M3SO3LCuOH (M = K(18-crown-6)+ or K(crypt-222)+) and NMe3LCuOHX (X = BArF 4 – or ClO4 –) were prepared, in which SO3 – or NMe3 + substituents occupy the para positions of the flanking aryl rings of the supporting bis(carboxamide)pyridine ligands. Structural and spectroscopic characterization showed that the CuOH+ cores in the corresponding complexes were similar, but cyclic voltammetry revealed the E 1/2 value for the CuOH2+/CuOH+ couple to be nearly 0.3 V more oxidizing for the NMe3LCuOH2+ than the SO3LCuOH− species, with the latter influenced by interactions between the distal −SO3 – substituents and K+ or Na+ counterions. Chemical oxidations of the complexes generated the corresponding CuOH2+ species as evinced by UV–vis spectroscopy. The rates of HAT reactions of these species with 9,10-dihydroanthracene to yield the corresponding Cu(OH2)2+ complexes and anthracene were measured, and the thermodynamics of the processes were evaluated via determination of the bond dissociation enthalpies (BDEs) of the product O–H bonds. The HAT rate for SO3LCuOH− was found to be ∼150 times faster than that for NMe3LCuOH2+, despite finding approximately the same BDEs for the product O–H bonds. Rationales for these observations and new insights into the roles of supporting ligand attributes on the properties of the CuOH2+ unit are presented.
Two new ligand sets, pipMeLH2 and NO2LH2 (pipMeL = N,N′-bis(2,6-diisopropylphenyl)-1-methylpiperidine-2,6-dicarboxamide, NO2L = N,N′-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide), ...are reported which are designed to perturb the overall electronics of the copper(III)–hydroxide core and the resulting effects on the thermodynamics and kinetics of its hydrogen-atom abstraction (HAT) reactions. Bond dissociation energies (BDEs) for the O–H bonds of the corresponding Cu(II)–OH2 complexes were measured that reveal that changes in the redox potential for the Cu(III)/Cu(II) couple are only partially offset by opposite changes in the pK a, leading to modest differences in BDE among the three compounds. The effects of these changes were further probed by evaluating the rates of HAT by the corresponding Cu(III)–hydroxide complexes from substrates with C–H bonds of variable strength. These studies revealed an overarching linear trend in the relationship between the log k (where k is the second-order rate constant) and the ΔH of reaction. Additional subtleties in measured rates arise, however, that are associated with variations in hydrogen-atom abstraction barrier heights and tunneling efficiencies over the temperature range from −80 to −20 °C, as inferred from measured kinetic isotope effects and corresponding electronic-structure-based transition-state theory calculations.
Kinetic studies of the reactions of two previously characterized copper(iii)-hydroxide complexes (LCuOH and
LCuOH, where L =
,
'-bis(2,6-diisopropylphenyl)-2,6-pyridine-dicarboxamide and
L =
,
...'-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide) with a series of
substituted phenols (
ArOH where X = NMe
, OMe, Me, H, Cl, NO
, or CF
) were performed using low temperature stopped-flow UV-vis spectroscopy. Second-order rate constants (
) were determined from pseudo first-order and stoichiometric experiments, and follow the trends CF
< NO
< Cl < H < Me < OMe < NMe
and LCuOH <
LCuOH. The data support a concerted proton-electron transfer (CPET) mechanism for all but the most acidic phenols (X = NO
and CF
), for which a more complicated mechanism is proposed. For the case of the reactions between
ArOH and LCuOH in particular, competition between a CPET pathway and one involving initial proton transfer followed by electron transfer (PT/ET) is supported by multiwavelength global analysis of the kinetic data, formation of the phenoxide
ArO
as a reaction product, observation of an intermediate LCu(OH
)
species derived from proton transfer from
ArOH to LCuOH, and thermodynamic arguments indicating that initial PT should be competitive with CPET.
The copolymerization of vinyl benzyl alcohol (VBA) and carbon monoxide (CO) to give a new polyester poly(VBA-CO) has been achieved via palladium-catalyzed hydroesterification. Reaction conditions ...involve moderate temperatures, moderate to low CO pressures, and low catalyst loadings to give a low molar mass (M n ∼ 3–4 kg/mol) polymer as a ∼2:1 mixture of linear to branched repeat units. The polymer molar mass increase is consistent with a step-growth polymerization mechanism, and ester yields of >97% are achieved within 24 h. However, increases in M n cease beyond 16 h. Control experiments indicate that the degree of polymerization is limited due to a combination of side reactions such as alcoholic end-group oxidation, hydroxycarbonylation, and alcohol acetylation, which lead to the degradation of monomeric and polymeric end groups. When a less promiscuous substrate is used such as 10-undecenol, higher molar masses (M n ∼ 16 kg/mol) are achieved. This method has the potential to be a mild route to new polyester architectures with appropriate mitigation of side reactions.
The conversion of methyl and ethyl lactate to their corresponding alkyl 2-(propionyloxy)propanoate esters was achieved by Pd catalyzed hydroesterificative coupling with carbon monoxide (CO) and ...ethylene (C2H4) at moderate temperatures and CO/C2H4 pressures. A screening of reaction conditions showed that the reaction could be carried out at low loadings of catalyst, which was generated in situ from inexpensive and commercially available reagents. High conversions and product yields were obtained in a variety of solvents and even under neat conditions. Product analysis identified transesterification to be the primary competing reaction, which could be mitigated by changing solvents, as well as minimizing the amount of excess acid present in solution. Pyrolysis of methyl and ethyl 2-(propionyloxy)propanoate transformed these esters into their respective acrylates suitable for subsequent polymerization, along with propionic acid as a valuable coproduct.
In order to address how diverse metalloprotein active sites, in particular those containing iron and copper, guide O₂binding and activation processes to perform diverse functions, studies of ...synthetic models of the active sites have been performed. These studies have led to deep, fundamental chemical insights into how O₂coordinates to mono- and multinuclear Fe and Cu centers and is reduced to superoxo, peroxo, hydroperoxo, and, after O-O bond scission, oxo species relevant to proposed intermediates in catalysis. Recent advances in understanding the various factors that influence the course of O₂activation by Fe and Cu complexes are surveyed, with an emphasis on evaluating the structure, bonding, and reactivity of intermediates involved. The discussion is guided by an overarching mechanistic paradigm, with differences in detail due to the involvement of disparate metal ions, nuclearities, geometries, and supporting ligands providing a rich tapestry of reaction pathways by which O₂is activated at Fe and Cu sites.
The control of radical reactions to afford selective carbon–carbon bond formation is a significant synthetic challenge with applications ranging from small‐molecule activation to natural product ...synthesis. Oxidation of (IP–)2Al(CH3) (1, IP = iminopyridine) with TrBPh4 (Tr = trityl) afforded the C–C coupled product (IP)(Tr‐IP)Al(CH3)BPh4 (2) in which the trityl radical and the IP– radical have undergone C–C bond formation. In contrast, oxidation of 1 with TrBArF24 {BArF24 = tetrakis(3,5‐trifluoromethyl)phenylborate} or TrB(C6F5)4 affords cationic (IP)(IP–)Al(CH3)BArF24 (3a) or (IP)(IP–)Al(CH3)B(C6F5)4 (3b), respectively . The different reaction outcomes provided by the different counteranions of Tr+ imply that a difference in stability of the products or of the intermediate mixed‐valent (IP)(IP‐)Al(CH3)+ state exists. We speculate that the most likely factor is the difference in solubility afforded by the different anions of the products that are formed. We also show that the formation of stable, cationic biradical complexes is possible and that these complexes do not undergo C–C radical coupling at the IP ligand. Cationic (IP–)2Al(OEt2)BArF24 (4) was obtained by protonolysis of 1 with H(OEt2)2BArF24, and two‐electron oxidation of (IP2–)2Al– (5) afforded (IP–)2Al(thf)BArF24 (6).
Oxidation of (IP–)2Al(CH3) (IP = iminopyridine) with TrBPh4 (Tr = trityl) affords the C–C‐coupled kinetic product (IP)(Tr‐IP)Al(CH3)BPh4 in which the trityl radical and the IP– radical have undergone C–C bond formation. Oxidation of (IP–)2Al(CH3) with TrBArF24 affords the thermodynamic product (IP)(IP–)Al(CH3)BarF24 {BArF24 = tetrakis(3,5‐trifluoromethyl)phenylborate}.
Kinetic studies of the reactions of two previously characterized copper(
iii
)-hydroxide complexes (LCuOH and
NO
2
LCuOH, where L =
N
,
N
′-bis(2,6-diisopropylphenyl)-2,6-pyridine-dicarboxamide and
...NO
2
L =
N
,
N
′-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide) with a series of
para
substituted phenols (
X
ArOH where X = NMe
2
, OMe, Me, H, Cl, NO
2
, or CF
3
) were performed using low temperature stopped-flow UV-vis spectroscopy. Second-order rate constants (
k
) were determined from pseudo first-order and stoichiometric experiments, and follow the trends CF
3
< NO
2
< Cl < H < Me < OMe < NMe
2
and LCuOH <
NO
2
LCuOH. The data support a concerted proton-electron transfer (CPET) mechanism for all but the most acidic phenols (X = NO
2
and CF
3
), for which a more complicated mechanism is proposed. For the case of the reactions between
NO
2
ArOH and LCuOH in particular, competition between a CPET pathway and one involving initial proton transfer followed by electron transfer (PT/ET) is supported by multiwavelength global analysis of the kinetic data, formation of the phenoxide
NO
2
ArO
−
as a reaction product, observation of an intermediate LCu(OH
2
)
+
species derived from proton transfer from
NO
2
ArOH to LCuOH, and thermodynamic arguments indicating that initial PT should be competitive with CPET.
Kinetic studies of the reactions of two previously characterized copper(
III
)-hydroxide complexes with a series of
para
substituted phenols (
X
ArOH where X = NMe
2
, OMe, Me, H, Cl, NO
2
, or CF
3
) indicate competition between concerted CPET and sequential PT/ET pathways.
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