Copper-Mediated Selective Hydroxylation of a Non-activated C−H Bond in Steroids: A DFT Study of Schönecker's Reaction

Expanding the Clip-and-Cleave Concept: Approaching Enantioselective C-H Hydroxylations by Copper Imine Complexes Using O2 and H2O2 as Oxidants

Copper-mediated aromatic and aliphatic C-H hydroxylations using benign oxidants (O2 and H2O2) have been studied intensively in recent years to meet the growing demand for efficient and green C-H functionalizations. Herein, we report an enantioselective variant of the so-called clip-and-cleave concept for intramolecular ligand hydroxylations by the application of chiral diamines as directing groups. We tested the hydroxylation of cyclohexanone and 1-acetyladamantane under different oxidative conditions (CuI/O2; CuI/H2O2; CuII/H2O2) in various solvents. As an outstanding example, we obtained (R)-1-acetyl-2-adamantol with a yield of 37% and >99:1 enantiomeric excess from hydroxylation in acetone using CuI and O2. Low-temperature stopped-flow UV-vis measurements in combination with density functional theory (DFT) computations revealed that the hydroxylation proceeds via a bis(μ-oxido) dicopper intermediate. The reaction product represents a rare example of an enantiopure 1,2-difunctionalized adamantane derivative, which paves the way for potential pharmacological studies. Furthermore, we found that 1-acetyladamantane can be hydroxylated in a one-pot reaction under air with isolated yields up to 36% and enantiomeric ratios of 96:4 using CuII/H2O2 in MeOH.

Modelling on a Biomimetic [Cu-O-Cu]2+-mediated Methane-to-Methanol Conversion Unveils the Site for Methane Activation

The Cu-O-Cu core exhibits methane-to-methanol conversion, mirroring the reactivity of the copper-containing enzyme pMMO. Herein, we computationally examined the reactivity of a biomimetic Cu-O-Cu core towards methane-to-methanol conversion. The oxygen atom of the Cu-O-Cu core abstracts hydrogen present in the C-H bond of methane. The spin density at the bridging oxygen helps to abstract hydrogen from the C-H bond. We modulated the spin density of the bridging oxygen by substituting only a single copper atom of the Cu-O-Cu core by metals (M) such as Fe, Co, and Ag. These substitutions result in bimetallic [Cu-O-M]2+ models. We observed that the energy barriers for the C-H activation step and the subsequent rebound step vary with the metal M. [Cu-O-Ag]2+ exhibits the highest reactivity for M2M conversion, while [Cu-O-Fe]2+ displays the lowest reactivity. To understand the different reactivity of these models towards M2M conversion, we employed distortion-interaction analysis, orbital analysis, spin density analysis, and quantum theory of atoms in molecules analysis. Orbital analysis reveals that all four adducts follow a hydrogen atom transfer mechanism for C-H activation. Further, spin density analysis reveals that a higher spin density on the bridging oxygen leads to a lower C-H activation barrier.

Remote functionalization reactions in steroids: discovery and application

Research published between 2001 and 2022 on the functionalization of remote positions of steroids, as well as the use of this technique in the generation of biologically active compounds has been reviewed. In the first section of the analysis established and novel methods for activation of sites deemed to be remote were reported. A series of manganese- (mainly), rhodium-, ruthenium- and osmium-centered porphyrins as catalysts in the presence of PIDA as oxidant have effected hydroxylation at C-1, -5, -6, -7, -11, -14, -15, -16, -17, -20, -24 and -25. Dioxiranes have been utilized in inserting hydroxyl groups at the 5, 12, 14, 15, 16, 17, 20, 24 and 25 positions (tertiary centers for the most part). Alcohols at C-12 and -16 were oxidized further to ketones. The Schönecker oxidation, discovered and developed during the period, has revolutionized the selective functionalization at C-12 of steroids possessing a 17-keto group. In the presence of iron-centered PDP- and MCP-based catalysts, hydrogen peroxide and acetic acid, substrates tended to be hydroxylated at C-6 and -12, with further oxidation to ketones often accompanying this reaction. The hypohalite reaction, utilizing the more modern Suarez conditions (irradiation in the presence of iodine and PIDA), was reported to facilitate the insertion of a hydroxyl moiety five atoms away from an existing alcohol oxygen. Steroidal-3β-diazoacetates tend to decompose on heating with di-rhodium-centered catalysts while activating carbons four or five atoms away. Chromium- and iron-based acetates were observed to functionalize C-5 and -25. Other reactions involving ring cleavage and halogenation, ketone irradiation and α-hydroxylation of ethers were also covered. The syntheses of compounds with marked biological activity from readily available steroids is described in the second section of the study. Cyclopamine, cephalostatin-1, ritterazine B and three polyhydroxypregnanaes (pergularin, utendin and tomentogenin) were generated in sequences in which a key step required hydroxylation at C-12 using the Schönecker reaction. A crucial stage in the preparation of cortistatin A, the saundersioside core, eurysterol A, 5,6-dihydroglaucogenin C, as well as clinostatins A and B involved the functionalization of C-18 or -19 utilizing hypohalite chemistry. The synthetic route to xestobergsterol A, pavonin-4-aglycone and ouagabagenin included a transformation where ketone irradiation played a part in either producing a Δ14 or a C-19 activated steroid. The radical relay reaction, where a 17α-chloro-steroid was formed, was central in the generation of pythocholic acid. The lead tetraacetate reaction was pivotal in the functionalization of C-19 during the synthesis of cyclocitrinol.

Orbital Analysis Captures the Existence of a Mixed-Valent CuIII -O-CuII Active-Site and its Role in Water-Assisted Aliphatic Hydroxylation

The Cu-O-Cu core has been proposed as a potential site for methane oxidation in particulate methane monooxygenase. In this work, we used density functional theory (DFT) to design a mixed-valent CuIII -O-CuII species from an experimentally known peroxo-dicopper complex supported by N-donor ligands containing phenolic groups. We found that the transfer of two-protons and two-electrons from phenolic groups to peroxo-dicopper core takes place, which results to the formation of a bis-μ-hydroxo-dicopper core. The bis-μ-hydroxo-dicopper core converts to a mixed-valent CuIII -O-CuII core with the removal of a water molecule. The orbital and spin density analyses unravel the mixed-valent nature of CuIII -O-CuII . We further investigated the reactivity of this mixed-valent core for aliphatic C-H hydroxylation. Our study unveiled that mixed-valent CuIII -O-CuII core follows a hydrogen atom transfer mechanism for C-H activation. An in-situ generated water molecule plays an important role in C-H hydroxylation by acting as a proton transfer bridge between carbon and oxygen. Furthermore, to assess the relevance of a mixed-valent CuIII -O-CuII core, we investigated aliphatic C-H activation by a symmetrical CuII -O-CuII core. DFT results show that the mixed-valent CuIII -O-CuII core is more reactive toward the C-H bond than the symmetrical CuII -O-CuII core.

Benchmarking Density Functional Theory Methods for Metalloenzyme Reactions: The Introduction of the MME55 Set

We present a new benchmark set of metalloenzyme model reaction energies and barrier heights that we call MME55. The set contains 10 different enzymes, representing eight transition metals, both open and closed shell systems, and system sizes of up to 116 atoms. We use four DLPNO-CCSD(T)-based approaches to calculate reference values against which we then benchmark the performance of a range of density functional approximations with and without dispersion corrections. Dispersion corrections improve the results across the board, and triple-ζ basis sets provide the best balance of efficiency and accuracy. Jacob's ladder is reproduced for the whole set based on averaged mean absolute (percent) deviations, with the double hybrids SOS0-PBE0-2-D3(BJ) and revDOD-PBEP86-D4 standing out as the most accurate methods for the MME55 set. The range-separated hybrids ωB97M-V and ωB97X-V also perform well here and can be recommended as a reliable compromise between accuracy and efficiency; they have already been shown to be robust across many other types of chemical problems, as well. Despite the popularity of B3LYP in computational enzymology, it is not a strong performer on our benchmark set, and we discourage its use for enzyme energetics.

Counter-Anions Rendered Weak-Interactions Perturb the Stability of Tyrosinase-Mimicked Peroxo-Dicopper(II) Active Site: Unraveling Computational Indicators

It has been observed in literature that the stability of tyrosinase-mimicked μ-η2 :η2 -peroxo-dicopper(II) (P) can be perturbed in presence of counter-anions (CAs) such as PhCO2 - , CF3 SO3 - , TsO- and SbF6 - . In this work, we unravel computational indicators using density functional theory to screen and study the stability of P in experimentally-reported cases. These indicators are Gibbs energies, geometrical parameters such as distances and angles, independent gradient model based on Hirshfeld partition (IGMH) generated data, orbitals' overlap, and distortion-interaction (DI) energies. Our DFT computed Gibbs energies indicate that P is stable in case of PhCO2 - and TsO- . CF3 SO3 - allows P and its isoelectronic species bis-μ-oxo-dicopper (O) to coexist. SbF6 - shows that O is in excess. Our indicators reveal that the stability of P in case of PhCO2 - and TsO- is due to the better placing of P and its CA, thus leading to better interactions and overlap of orbitals. Other indicator displays that the plane of Cu2 O2 core in P is more bend in PhCO2 - and TsO- cases as compared to the plane in the other two cases. In addition, the IGMH-based indicator displays higher values in the case of PhCO2 - and TsO- than the other CAs.

Recent advances in computational studies on Cu-catalyzed aerobic reactions: cooperation of copper catalysts and dioxygen

O2, one of the ideal oxidants, suffers from low solubility, low oxidizability, low selectivity and a triplet ground state when applied in organic synthesis. Biomimetic copper catalysis has been demonstrated to be a powerful method for activating and transforming O2 to conduct aerobic reactions for a long time. On the other hand, the structures of Cu-O2 complexes are complex with diverse downstream reactions, whereas active copper intermediates were rarely identified by experimental methods, making the mechanisms of many Cu-catalyzed aerobic reactions far from clear. In this context, computational studies emerged as an effective alternative to mechanistic studies on Cu-catalyzed aerobic reactions. This review introduces the relevant computational studies since 2012, focusing on showing the cooperation of copper catalysts and O2 in dehydrogenation, oxygenation and coupling reactions.

Synthesis of modified bile acids via palladium-catalyzed C(sp3)-H (hetero)arylation

A Pd(II)-catalyzed strategy for the diastereo- and regioselective (hetero)arylation of unactivated C(sp3)-H bonds in bile acids is accomplished with aryl and heteroaryl iodides under solvent-free conditions using the 8-aminoquinoline auxiliary as a directing group. This methodology demonstrated excellent functional group tolerance with respect to aryl/heteroaryl iodides on O-protected N-(quinolin-8-yl)cholyl/deoxycholyl amides to afford β-C(sp3)-H (hetero)arylated products in good-to-excellent yields. Moreover, the 8-aminoquinoline (AQ) auxiliary can easily be removed to obtain modified bile acids.

Computational Insights of Selective Intramolecular O-atom Transfer Mediated by Bioinspired Copper Complexes

The stereoselective copper-mediated hydroxylation of intramolecular C-H bonds from tridentate ligands is reinvestigated using DFT calculations. The computational study aims at deciphering the mechanism of C-H hydroxylation obtained after reaction of Cu(I) precursors with dioxygen, using ligands bearing either activated (L¹ ) or non-activated (L² ) C-H bonds. Configurational analysis allows rationalization of the experimentally observed regio- and stereoselectivity. The computed mechanism involves the formation of a side-on peroxide species (P) in equilibrium with the key intermediate bis-(μ-oxo) isomer (O) responsible for the C-H activation step. The P/O equilibrium yields the same activation barrier for the two complexes. However, the main difference between the two model complexes is observed during the C-H activation step, where the complex bearing the non-activated C-H bonds yields a higher energy barrier, accounting for the experimental lack of reactivity of this complex under those conditions.

Site-selective C-H hydroxylation of pentacyclic triterpenoids directed by transient chiral pyridine-imino groups

Pentacyclic triterpenoids (PTs) constitute one of the biggest families of natural products, many with higher oxidation state at the D/E rings possess a wide spectrum of biological activties but are poorly accessible. Here we report a site-selective C-H hydroxylation at the D/E rings of PTs paving a way toward these important natural products. We find that Schönecker and Baran's Cu-mediated aerobic oxidation can be applied and become site-selective on PT skeletons, as being effected unexpectedly by the chirality of the transient pyridine-imino directing groups. To prove the applicability, starting from the most abundant triterpenoid feedstock oleanane, three representative saponins bearing hydroxyl groups at C16 or C22 are expeditiously synthesized, and barringtogenol C which bears hydroxyl groups at C16, C21, and C22 is synthesized via a sequential hydroxylation as the key steps.

Relativistic effects at the Cu2O2 core - a density functional theory study

This work presents for the first time the impact of relativistic effects on the Cu₂O₂ peroxide dicopper(ii)-bis(μ-oxo) dicopper (iii) equilibrium bonding motif in existing tyrosinase model complexes [Cu₂(btmgp)₂(μ-O)₂]²⁺ and [Cu₂{HC(tBuPz)₂Py}₂(O₂)]²⁺ [S. Herres et al., Inorg. Chim. Acta, 2005, 358, 1089; S. Herres-Pawlis et al., Eur. J. Inorg. Chem., 2005, 3815; A. Hoffmann et al., Angew. Chem., Int. Ed., 2013, 52, 5398]. We use density functional theory (TPSSh/cc-pVTZ+D3BJ) with various relativistic approaches (ECPs, DKH, ZORA) to tackle the question of impact and find, as already known for small model complexes in the literature, that the O core is more stabilized with respect to the P core by the relativistic influence [D. G. Liakos and F. Neese, J. Chem. Theory Comput., 2011, 7, 1511]. Whereas DKH and ZORA yield similar results, ECPs underestimate the predicted effect. By calculating the functional of the relativistic overlap change, we can exactly identify the relativistic changes at the molecular orbital level and narrow down the affected MOs. Thus, by performing a detailed bond analysis utilizing the overlap population, we are able to rationalize the effects at the chemical level by finding relativistic bond stabilization in both isomers, which is more dominating in the O core. In conclusion, relativistic effects already play an important role in dicopper cores and should be considered.

Decoding the Mechanism of Intramolecular Cu-Directed Hydroxylation of sp3 C-H Bonds

The use of copper in directed C-H oxidation has been relatively underexplored. In a seminal example, Schönecker showed that copper and O2 promoted the hydroxylation of steroid-containing ligands. Recently, Baran (J. Am. Chem. Soc. 2015, 137, 13776) improved the reaction conditions to oxidize similar substrates with excellent yields. In both reports, the involvement of Cu2O2 intermediates was suggested. In this collaborative article, we studied the hydroxylation mechanism in great detail, resulting in the overhaul of the previously accepted mechanism and the development of improved reaction conditions. Extensive experimental evidence (spectroscopic characterization, kinetic analysis, intermolecular reactivity, and radical trap experiments) is provided to support each of the elementary steps proposed and the hypothesis that a key mononuclear LCuII(OOR) intermediate undergoes homolytic O-O cleavage to generate reactive RO• species, which are responsible for key C-H hydroxylation within the solvent cage. These key findings allowed the oxidation protocol to be reformulated, leading to improvements of the reaction cost, practicability, and isolated yield.

Acid/base triggered interconversion of μ-η2:η2-peroxido and bis(μ-oxido) dicopper intermediates capped by proton-responsive ligands

CuII2(μ-η2:η2-peroxido) and CuIII2(μ-oxido)2 cores represent key intermediates in copper/dioxygen chemistry, and they are mechanistically important for biological hydroxylation and oxidation reactions mediated by dinuclear (type III) copper metalloenzymes. While the exact nature of the active species in different enzymes is still under debate, shifting equilibria between Cu x /O2 species is increasingly recognized as a means of switching between distinct reactivity patterns of these intermediates. Herein we report comprehensive spectroscopic, crystallographic and computational analysis of a family of synthetic CuII2(μ-η2:η2-peroxido) and CuIII2(μ-oxido)2 dicopper complexes with a bis(oxazoline) (BOX) capping ligand. In particular, we demonstrate that a reversible peroxido/bis(μ-oxido) interconversion of the [Cu2O2] core can be triggered by peripheral (de)protonation events on the ligand backbone. As the copper ions in the enzymes are typically supported by histidine imidazoles that offer a backside N atom amenable to potential (de)protonation, it is well conceivable that the shifting of equilibria between the [Cu2O2] species in response to changes in local pH is biologically relevant.