Enhanced reactivity in dioxirane C-H oxidations via strain release: a computational and experimental study

Periselectivity and ambimodal transition states in cycloadditions of tetrachloro-o-benzoquinone with 6,6-dimethylfulvene

The reaction mechanism of cycloadditions of tetrachloro-o-benzoquinone with 6,6-dimethylfulvene were systematically investigated with density functional theory calculations. It was found that conditional primary interactions stabilize the ambimodal transition states in the endo pathways. Ambimodal transition states lead to [6 + 4]/[4 + 2] adducts or [4 + 2]/[2 + 4] adducts, which interconvert through 3,3-sigmatropic shift reactions. The substituent effects on periselectivity were also investigated.

Deoxygenation of Oxiranes by λ3 σ3 -Phosphorus Reagents: A Computational, Mechanistic, and Stereochemical Study

The deoxygenation of parent and substituted oxiranes by λ3 σ3 -phosphorus reagents has been explored in detail, therefore unveiling mechanistic aspects as well as regio- and stereochemical consequences. Attack to a ring C atom is almost always preferred over one-step deoxygenation by direct P-to-O attack. In most cases a carbene transfer occurs as first step, leading to a phosphorane and a carbonyl unit that thereafter react in the usual Wittig fashion via the corresponding λ5 σ5 -1,2-oxaphosphetane intermediate. Betaines rarely constitute true minima after the first C-attack to oxiranes, at least in the gas-phase. Use of the heavier derivatives AsMe3 and SbMe3 as oxirane deoxygenating reagents was also mechanistically studied. The thermodynamic tendency of λ3 σ3 -phosphorus reagents to act as oxygen (O-attack) or carbene acceptors (C-attack) was theoretically studied by means of the thermodynamic oxygen-transfer potential (TOP) and the newly defined thermodynamic carbene-transfer potential (TCP) parameters, that were explored in a wider context together with many other acceptor centres.

Radical and Cationic Pathways in C(sp3)-H Bond Oxygenation by Dioxiranes of Bicyclic and Spirocyclic Hydrocarbons Bearing Cyclopropane Moieties

A product and DFT computational study on the reactions of 3-ethyl-3-(trifluoromethyl)dioxirane (ETFDO) with bicyclic and spirocyclic hydrocarbons bearing cyclopropyl groups was carried out. With bicyclo[n.1.0]alkanes (n = 3-6), diastereoselective formation of the alcohol product derived from C2-H bond hydroxylation was observed, accompanied by smaller amounts of products derived from oxygenation at other sites. With 1-methylbicyclo[4.1.0]heptane, rearranged products were also observed in addition to the unrearranged products deriving from oxygenation at the most activated C2-H and C5-H bonds. With spiro[2.5]octane and 6-tert-butylspiro[2.5]octane, reaction with ETFDO occurred predominantly or exclusively at the axial C4-H to give unrearranged oxygenation products, accompanied by smaller amounts of rearranged bicyclo[4.2.0]octan-1-ols. The good to outstanding site-selectivities and diastereoselectivities are paralleled by the calculated activation free energies for the corresponding reaction pathways. Computations show that the σ* orbitals of the bicyclo[n.1.0]alkane cis or trans C2-H bonds and spiro[2.5]octanes axial C4-H bond hyperconjugatively interact with the Walsh orbitals of the cyclopropane ring, activating these bonds toward HAT to ETFDO. The detection of rearranged oxygenation products in the oxidation of 1-methylbicyclo[4.1.0]heptane, spiro[2.5]octane, and 6-tert-butylspiro[2.5]octane provides unambiguous evidence for the involvement of cationic intermediates in these reactions, representing the first examples on the operation of ET pathways in dioxirane-mediated C(sp3)-H bond oxygenations. Computations support these findings, showing that formation of cationic intermediates is associated with specific stabilizing hyperconjugative interactions between the incipient carbon radical and the cyclopropane C-C bonding orbitals that trigger ET to the incipient dioxirane derived 1,1,1-trifluoro-2-hydroxy-2-butoxyl radical.

Strategic application of C-H oxidation in natural product total synthesis

The oxidation of unactivated C-H bonds has emerged as an effective tactic in natural product synthesis and has altered how chemists approach the synthesis of complex molecules. The use of C-H oxidation methods has simplified the process of synthesis planning by expanding the choice of starting materials, limiting functional group interconversion and protecting group manipulations, and enabling late-stage diversification. In this Review, we propose classifications for C-H oxidations on the basis of their strategic purpose: type 1, which installs functionality that is used to establish the carbon skeleton of the target; type 2, which is used to construct a heterocyclic ring; and type 3, which installs peripheral functional groups. The reactions are further divided based on whether they are directed or undirected. For each classification, examples from recent literature are analysed. Finally, we provide two case studies of syntheses from our laboratory that were streamlined by the judicious use of C-H oxidation reactions.

Role of Steric Effects on Rates of Hydrogen Atom Transfer Reactions

The influence of steric effects on the rates of hydrogen atom transfer (HAT) reactions between oxyradicals and alkanes is explored computationally. Quantum chemical density functional theory computations of transition states show that activation barriers and reaction enthalpies are both influenced by bulky substituents on the radical but very little by substituents on the alkane. The activation barriers remain roughly correlated with reaction enthalpies via the Evans-Polanyi relationship even when steric repulsion effects become important, although dispersion effects sometimes stabilize transition states. By making comparisons to previously developed Evans-Polanyi and modified Roberts-Steel relationships, we find that HAT reactions between bulky molecules remain well-described by these relationships.

Late-stage Functionalization for Improving Drug-like Molecular Properties

The development of late-stage functionalization (LSF) methodologies, particularly C-H functionalization, has revolutionized the field of organic synthesis. Over the past decade, medicinal chemists have begun to implement LSF strategies into their drug discovery programs, allowing for the drug discovery process to become more efficient. Most reported applications of late-stage C-H functionalization of drugs and drug-like molecules have been to rapidly diversify screening libraries to explore structure-activity relationships. However, there has been a growing trend toward the use of LSF methodologies as an efficient tool for improving drug-like molecular properties of promising drug candidates. In this review, we have comprehensively reviewed recent progress in this emerging area. Particular emphasis is placed on case studies where multiple LSF techniques were implemented to generate a library of novel analogues with improved drug-like properties. We have critically analyzed the current scope of LSF strategies to improve drug-like properties and commented on how we believe LSF can transform drug discovery in the future. Overall, we aim to provide a comprehensive survey of LSF techniques as tools for efficiently improving drug-like molecular properties, anticipating its continued uptake in drug discovery programs.

Fluoro, Trifluoromethyl, and Trifluoroacetyl Substituent Effects on Cycloaddition Reactivities: Computations and Analysis

The importance of fluoro and trifluoromethyl substituents in drug effectiveness prompted the computational exploration of fluorine-containing substituents in valuable synthetic cycloadditions. Diels-Alder or 1,3-dipolar cycloaddition reactions of typical reactants, cyclopentadiene, N-phenyldiazoacetamide, tetrazine, and N-phenylsydnone involving fluorine-containing substituents (F, CF₃, and COCF₃) were studied with M06-2X density functional theory. Inductive and conjugative effects influence normal and inverse electron-demand reactions differently. These results provide a guide to the design and use of cycloadditions for the introduction of fluoro and trifluoromethyl substituents in synthetic processes.

Rapid and Accurate Estimation of Activation Free Energy in Hydrogen Atom Transfer-Based C-H Activation Reactions: From Empirical Model to Artificial Neural Networks

A well-performing machine learning (ML) model is obtained by using proper descriptors and artificial neural network (ANN) algorithms, which can quickly and accurately predict activation free energy in hydrogen atom transfer (HAT)-based sp³ C-H activation. Density functional theory calculations (UωB97X-D) are used to establish the reaction system data sets of methoxyl (CH₃O·), trifluoroethoxyl (CF₃CH₂O·), tert-butoxyl (tBuO·), and cumyloxyl (CumO·) radicals. The simplified Roberts' equation proposed in our recent study works here [R ² = 0.84, mean absolute error (MAE) = 0.85 kcal/mol]. Its performance is comparable with univariate Mulliken-type electronegativity (χ) with the ANN model. The ANN model with bond dissociation free energy, χ, α-unsaturation, and Nolan buried volume (%V _buried) successively improves R ² and MAE to 0.93 and 0.54 kcal/mol, respectively. It reproduces the test sets of trichloroethoxyl (CCl₃CH₂O·) with R ² = 0.87 and MAE = 0.89 kcal/mol and accurately predicts the relative experimental barrier of the HAT reactions with CumO· and the site selectivity of CH₃O·.

Strategy Evolution in a Skeletal Remodeling and C-H Functionalization-Based Synthesis of the Longiborneol Sesquiterpenoids

Detailed herein are our synthesis studies of longiborneol and related natural products. Our overarching goals of utilizing a "camphor first" strategy enabled by skeletal remodeling of carvone, and late-stage diversification using C-H functionalizations, led to divergent syntheses of the target natural products. Our initial approach proposed a lithiate addition to unite two fragments followed by a Conia-ene or Pd-mediated cycloalkylation reaction sequence to install the seven-membered ring emblematic of the longibornane core. This approach was unsuccessful and evolved into a revised plan that employed a Wittig coupling and a radical cyclization to establish the core. A reductive radical cyclization, which was explored first, led to a synthesis of copaborneol, a structural isomer of longiborneol. Alternatively, a metal-hydride hydrogen atom transfer-initiated cyclization was effective for a synthesis of longiborneol. Late-stage C-H functionalization of the longibornane core led to a number of hydroxylated longiborneol congeners. The need for significant optimization of the strategies that were employed as well as the methods for C-H functionalization to implement these strategies highlights the ongoing challenges in applying these powerful reactions. Nevertheless, the reported approach enables functionalization of every natural product-relevant C-H bond in the longibornane skeleton.

Ring Strain Energies of Three-Membered Homoatomic Inorganic Rings El3 and Diheterotetreliranes El2Tt (Tt = C, Si, Ge): Accurate versus Additive Approaches

Accurate ring strain energies (RSEs) for three-membered symmetric inorganic rings El₃ and organic dihetero-monocycles El₂C and their silicon El₂Si and germanium El₂Ge analogues have been computed for group 14–16 “El” heteroatoms using appropriate homodesmotic reactions and calculated at the DLPNO-CCSD-(T)/def2-TZVPP//B3LYP-D4/def2-TZVP(ecp) level. Rings containing triels and Sn/Pb heteroatoms are studied as exceptions to the RSE calculation as they either do not constitute genuine rings or cannot use the general homodesmotic reaction scheme due to uncompensated interactions. Some remarkable concepts already related to the RSE such as aromaticity or strain relaxation by increasing the s-character in the lone pair (LP) of the group 15–16 elements are analyzed extensively. An appealing alternative procedure for the rapid estimation of RSEs using additive rules, based on contributions of ring atoms or endocyclic bonds, is disclosed.

Accurate ring strain energies (RSEs) are calculated for saturated three-membered homoatomic inorganic rings El₃ and diheterotetreliranes El₂Tt (Tt = C, Si, Ge). Aromaticity, lone-pair-induced strain relaxation for the group 15−16 elements, and bond stiffness are extensively discussed as main factors affecting the RSE. An attractive alternative procedure for fast estimation of the RSE using additive rules is proposed.

Electronic control over site-selectivity in hydrogen atom transfer (HAT) based C(sp3)-H functionalization promoted by electrophilic reagents

The direct functionalization of C(sp³)-H bonds represents one of the most investigated approaches to develop new synthetic methodology. Among the available strategies for intermolecular C-H bond functionalization, increasing attention has been devoted to hydrogen atom transfer (HAT) based procedures promoted by radical or radical-like reagents, that offer the opportunity to introduce a large variety of atoms and groups in place of hydrogen under mild conditions. Because of the large number of aliphatic C-H bonds displayed by organic molecules, in these processes control over site-selectivity represents a crucial issue, and the associated factors have been discussed. In this review article, attention will be devoted to the role of electronic effects on C(sp³)-H bond functionalization site-selectivity. Through an analysis of the recent literature, a detailed description of the HAT reagents employed in these processes, the associated mechanistic features and the selectivity patterns observed in the functionalization of substrates of increasing structural complexity will be provided.

Total Synthesis and Late-Stage C-H Oxidations of ent-Trachylobane Natural Products

Herein, we present our studies to construct seven ent-trachylobane diterpenoids by employing a bioinspired two-phase synthetic strategy. The first phase provided enantioselective and scalable access to five ent-trachylobanes, of which methyl ent-trachyloban-19-oate was produced on a 300 mg scale. During the second phase, chemical C-H oxidation methods were employed to enable selective conversion to two naturally occurring higher functionalized ent-trachylobanes. The formation of regioisomeric analogs, which are currently inaccessible via enzymatic methods, reveals the potential as well as limitations of established chemical C-H oxidation protocols for complex molecule synthesis.

Total Synthesis and Late-Stage C-H Oxidations of ent-Trachylobane Natural Products

Herein, we present our studies to construct seven ent-trachylobane diterpenoids by employing a bioinspired two-phase synthetic strategy. The first phase provided enantioselective and scalable access to five ent-trachylobanes, of which methyl ent-trachyloban-19-oate was produced on a 300 mg scale. During the second phase, chemical C-H oxidation methods were employed to enable selective conversion to two naturally occurring higher functionalized ent-trachylobanes. The formation of regioisomeric analogs, which are currently inaccessible via enzymatic methods, reveals the potential as well as limitations of established chemical C-H oxidation protocols for complex molecule synthesis.

Dioxiranes: A Half-Century Journey

Dioxiranes are multi-tasking reagents inheriting mild and selective oxygen transfer attributes. These oxidants are accessed from the reaction of ketones with an oxidant and are employed stoichiometrically or catalytically (in...

Substituted 9-Anthraldehydes from Dibenzocycloheptanol Epoxides via Acid-Catalyzed Epoxide Opening/Semipinacol Rearrangement

Starting from benzaldehyde derivatives, the corresponding dibenzocycloheptenol could be prepared in five steps. Under both substrate (secondary vs tertiary alcohol and the substituents on the aromatic ring(s)) and condition control, the subsequent epoxidation and acid-catalyzed epoxide opening/semipinacol rearrangement/aromatization afforded the corresponding 9-anthraldehydes in good yields, up to 88% over two steps. The presence of the electron-withdrawing group(s) on the aromatic ring(s) suppressed the rate of the epoxidation while the subsequent semipinacol rearrangement step required heating; the presence of the electron-donating group(s), on the other hand, frequently led to the decomposition during the epoxidation. From the mechanistic studies, the semipinacol rearrangement of the epoxide could precede the ionization at the bisbenzylic position, yielding the aldehyde intermediate. The ensuing dehydrative aromatization led to the formation of 9-anthraldehyde. Conversely, nucleophilic addition to the aldehyde and dehydrative aromatization with concomitant loss of formic acid led to anthracene.

Deciphering Reactivity and Selectivity Patterns in Aliphatic C-H Bond Oxygenation of Cyclopentane and Cyclohexane Derivatives

A kinetic, product, and computational study on the reactions of the cumyloxyl radical with monosubstituted cyclopentanes and cyclohexanes has been carried out. HAT rates, site-selectivities for C-H bond oxidation, and DFT computations provide quantitative information and theoretical models to explain the observed patterns. Cyclopentanes functionalize predominantly at C-1, and tertiary C-H bond activation barriers decrease on going from methyl- and tert-butylcyclopentane to phenylcyclopentane, in line with the computed C-H BDEs. With cyclohexanes, the relative importance of HAT from C-1 decreases on going from methyl- and phenylcyclohexane to ethyl-, isopropyl-, and tert-butylcyclohexane. Deactivation is also observed at C-2 with site-selectivity that progressively shifts to C-3 and C-4 with increasing substituent steric bulk. The site-selectivities observed in the corresponding oxidations promoted by ethyl(trifluoromethyl)dioxirane support this mechanistic picture. Comparison of these results with those obtained previously for C-H bond azidation and functionalizations promoted by the PINO radical of phenyl and tert-butylcyclohexane, together with new calculations, provides a mechanistic framework for understanding C-H bond functionalization of cycloalkanes. The nature of the HAT reagent, C-H bond strengths, and torsional effects are important determinants of site-selectivity, with the latter effects that play a major role in the reactions of oxygen-centered HAT reagents with monosubstituted cyclohexanes.

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

N-Ammonium Ylide Mediators for Electrochemical C-H Oxidation

The site-specific oxidation of strong C(sp³)-H bonds is of uncontested utility in organic synthesis. From simplifying access to metabolites and late-stage diversification of lead compounds to truncating retrosynthetic plans, there is a growing need for new reagents and methods for achieving such a transformation in both academic and industrial circles. One main drawback of current chemical reagents is the lack of diversity with regard to structure and reactivity that prevents a combinatorial approach for rapid screening to be employed. In that regard, directed evolution still holds the greatest promise for achieving complex C-H oxidations in a variety of complex settings. Herein we present a rationally designed platform that provides a step toward this challenge using N-ammonium ylides as electrochemically driven oxidants for site-specific, chemoselective C(sp³)-H oxidation. By taking a first-principles approach guided by computation, these new mediators were identified and rapidly expanded into a library using ubiquitous building blocks and trivial synthesis techniques. The ylide-based approach to C-H oxidation exhibits tunable selectivity that is often exclusive to this class of oxidants and can be applied to real-world problems in the agricultural and pharmaceutical sectors.

Late-Stage Diversification of Natural Products

Late-stage diversification of natural products is an efficient way to generate natural product derivatives for drug discovery and chemical biology. Benefiting from the development of site-selective synthetic methodologies, late-stage diversification of natural products has achieved notable success. This outlook will outline selected examples of novel methodologies for site-selective transformations of reactive functional groups and inert C-H bonds that enable late-stage diversification of complex natural products. Accordingly, late-stage diversification provides an opportunity to rapidly access various derivatives for modifying lead compounds, identifying cellular targets, probing protein-protein interactions, and elucidating natural product biosynthetic relationships.

Molecule-Induced Radical Formation (MIRF) Reactions-A Reappraisal

Radical chain reactions are commonly initiated through the thermal or photochemical activation of purpose-built initiators, through photochemical activation of substrates, or through well-designed redox processes. Where radicals come from in the absence of these initiation strategies is much less obvious and are often assumed to derive from unknown impurities. In this situation, molecule-induced radical formation (MIRF) reactions should be considered as well-defined alternative initiation modes. In the most general definition of MIRF reactions, two closed-shell molecules react to give a radical pair or biradical. The exact nature of this transformation depends on the σ- or π-bonds involved in the MIRF process, and this Minireview specifically focuses on reactions that transform two σ-bonds into two radicals and a closed-shell product molecule.

Computational Exploration of Chiral Iron Porphyrin-Catalyzed Asymmetric Hydroxylation of Ethylbenzene Where Stereoselectivity Arises from π-π Stacking Interaction

The mechanism and origins of stereoselectivity of chiral iron porphyrin-catalyzed asymmetric hydroxylation of ethylbenzene were explored with density functional theory. The hydrogen atom abstraction is the rate- and stereoselectivity-determining step. In good agreement with experimental results, the formation of the (R)-1-phenylethanol product is found to be the most favorable pathway. The transition state of hydrogen atom abstraction which leads to the (S)-1-phenylethanol product is unfavorable by 1.7 kcal/mol compared to the corresponding transition state which leads to the (R)-1-phenylethanol product. Enantioselectivity arises from an attractive π-π stacking interaction between the phenyl group of ethylbenzene substrate and the naphthyl group of the porphyrin ligand.

Continued Progress towards Efficient Functionalization of Natural and Non-natural Targets under Mild Conditions: Oxygenation by C-H Bond Activation with Dioxirane

The successful isolation and characterization of a dioxirane species in 1988 opened up one of the most attractive methods for the efficient oxidation of simple and/or structurally complex molecules. Dioxirane today rank among the most powerful tools in organic chemistry, with numerous applications in commercially important processes. They were quickly recognized as efficient oxygen transfer agents, especially for epoxidations and for a wide range of O-insertion reactions into C-H bonds. Dioxirane possess catalytic activity and appear as highly (chemo-, regio-, and stereo-) selective oxidants, despite their reactivity under mild and strictly neutral conditions being controlled by a combination of steric and electronic factors. In this review, we discuss some of the most recent and significant developments in the selective homogeneous and heterogeneous oxyfunctionalization of non-activated C-H bonds in hydrocarbons of natural and non-natural targets by using isolated dioxirane or, more generally, by using the ketones (i.e., the dioxirane precursors) as organocatalysts.

C(sp3)-H hydroxylation of fluorenes, oxindoles and benzofuranones with a Mg(NO3)2-HP(O)Ph2 oxidation system

A novel oxidation system in which magnesium nitrate [Mg(NO3)2] is used as an oxidant in the presence of diphe-nylphosphine oxide [HP(O)Ph2] permits the C(sp3)-H hydroxylation of fluorenes, oxindoles, and benzofuranones. This method features high efficiency, good functional group tolerance, and operational simplicity. The synthetic utility is highlighted by further transformations to valuable organic materials.

Aliphatic C-H Oxidations for Late-Stage Functionalization

The atomistic change of C( sp3)-H to C( sp3)-O can have a profound impact on the physical and biological properties of small molecules. Traditionally, chemical synthesis has relied on pre-existing functionality to install new functionality, and directed approaches to C-H oxidation are an extension of this logic. The impact of developing undirected C-H oxidation reactions with controlled site-selectivity is that scientists gain the ability to diversify complex structures at sites remote from existing functionality, without having to carry out individual de novo syntheses. This Perspective offers a historical view of why, as recently as 2007, it was thought that the differences between aliphatic C-H bonds of the same bond type (for example, 2° aliphatic) were not large enough to distinguish them preparatively with small-molecule catalysis in the absence of directing groups or molecular recognition elements. We give an account of the discovery of Fe(PDP)-catalyzed non-directed aliphatic C-H hydroxylations and how the electronic, steric, and stereoelectronic rules for predicting site-selectivity that emerged have affected a shift in how the chemical community views the reactivity among these bonds. The discovery that site-selectivity could be altered by tuning the catalyst [i.e., Fe(CF3-PDP)] with no changes to the substrate or reaction now gives scientists the ability to exert control on the site of oxidation on a range of functionally and topologically diverse compounds. Collectively, these findings have made possible the emerging area of late-stage C-H functionalizations for streamlining synthesis and derivatizing complex molecules.

Direct C-C Bond Formation from Alkanes Using Ni-Photoredox Catalysis

A method for direct cross coupling between unactivated C(sp3)-H bonds and chloroformates has been accomplished via nickel and photoredox catalysis. A diverse range of feedstock chemicals, such as (a)cyclic alkanes and toluenes, along with late-stage intermediates, undergo intermolecular C-C bond formation to afford esters under mild conditions using only 3 equiv of the C-H partner. Site selectivity is predictable according to bond strength and polarity trends that are consistent with the intermediacy of a chlorine radical as the hydrogen atom-abstracting species.

The Quest for Selectivity in Hydrogen Atom Transfer Based Aliphatic C-H Bond Oxygenation

Aliphatic C-H bond functionalization is at the frontline of research because it can provide straightforward access to simplified and cost-effective synthetic procedures. A number of these methodologies are based on hydrogen atom transfer (HAT), which, as a consequence of the inert character of C-H bonds, often represents the most challenging step of the overall process. Because the majority of organic molecules contain multiple nonequivalent C-H bonds that display similar chemical properties, differentiating between these bonds with high levels of selectivity represents one of the most challenging issues. Clarification of the factors that govern the relative reactivity of C-H bonds toward HAT reagents is thus of primary importance in order to develop selective functionalization procedures. In this Account we describe, through the combination of kinetic studies employing a genuine HAT reagent such as the cumyloxyl radical, along with oxidations performed with H₂O₂ and iron or manganese catalysts, our contribution toward the development of selective C-H functionalization methodologies. Despite the different nature of these reagents, an oxygen-centered radical and a metal-oxo species, congruent reactivity and selectivity patterns have emerged, providing strong evidence that both reactions proceed via HAT. Consequently, selectivity in this class of metal catalyzed C-H oxidations can be reasonably predicted and synthetically exploited. Amides have been identified as preferential functional groups for governing selectivity on the basis of electronic, steric, and stereoelectronic effects. Torsional effects have proven moreover to be particularly important C-H directing factors in the oxidation of cyclohexane scaffolds where a delicate balance of these effects, in synergistic combination with catalyst design, enables highly chemoselective and enantioselective oxidations. Medium effects have been also shown to govern the relative HAT reactivity of C-H bonds in proximity to polar, hydrogen bond acceptor (HBA) functional groups. By engaging in hydrogen bonding with these groups, fluorinated alcohols strongly deactivate proximal C-H bonds toward HAT-based oxidation. As a result, alcohols, ethers, amines, and amides, which are electron rich and effective proximal C-H activating groups toward HAT reagents in conventional solvents, become oxidatively robust deactivating functionalities that direct C-H oxidation toward remote positions. These deactivating effects enable moreover the accomplishment of product chemoselective methylenic hydroxylations. Overall, clarification of the factors that govern HAT-based reactions has served to provide unique examples of catalytic methodologies for chemoselective and enantioselective oxidation of nonactivated aliphatic C-H bonds of potential utility in organic synthesis.

C-H oxygenation at tertiary carbon centers using iodine oxidant

An oxidation system in which iodic acid (HIO3) is used as an oxidant in the presence of N-hydroxyphthalimide (NHPI) permitted the selective hydroxylation of tertiary C-H bonds and the lactonization of carboxylic acids containing a tertiary carbon center. These reactions are operationally simple and proceed under metal-free conditions using commercially available reagents, thus offering an ideal tool for the efficient oxidation of C-H bonds at tertiary carbon centers.

Aliphatic C-H Bond Oxidation with Hydrogen Peroxide Catalyzed by Manganese Complexes: Directing Selectivity through Torsional Effects

Substituted N-cyclohexyl amides undergo aliphatic C-H bond oxidation with H₂O₂ catalyzed by manganese complexes. The reactions are directed by torsional effects leading to site-selective oxidation of cis-1,4-, trans-1,3-, and cis-1,2-cyclohexanediamides. The corresponding diastereoisomers are unreactive under the same conditions. Competitive oxidation of cis- trans mixtures of 4-substituted N-cyclohexylamides leads to quantitative conversion of the cis-isomers, allowing isolation and successive conversion of the trans-isomers into densely functionalized oxidation products with excellent site selectivity and good enantioselectivity.

Hydrogen Atom Transfer from Alkanols and Alkanediols to the Cumyloxyl Radical: Kinetic Evaluation of the Contribution of α-C-H Activation and β-C-H Deactivation

A kinetic study on the reactions of the cumyloxyl radical (CumO^•) with a series of alkanols and alkanediols has been carried out. Predominant hydrogen atom transfer (HAT) from the α-C-H bonds of these substrates, activated by the presence of the OH group, is observed. The comparable k_H values measured for ethanol and 1-propanol and the increase in k_H measured upon going from 1,2-diols to structurally related 1,3- and 1,4-diols is indicative of β-C-H deactivation toward HAT to the electrophilic CumO^•, determined by the electron-withdrawing character of the OH group. No analogous deactivation is observed for the corresponding diamines, in agreement with the weaker electron-withdrawing character of the NH₂ group. The significantly lower k_H values measured for reaction of CumO^• with densely oxygenated methyl pyranosides as compared to cyclohexanol derivatives highlights the role of β-C-H deactivation. The contribution of torsional effects on reactivity is evidenced by the ∼2-fold increase in k_H observed upon going from the trans isomers of 4- tert-butylcyclohexanol and 1,2- and 1,4-cyclohexanediol to the corresponding cis isomers. These results provide an evaluation of the role of electronic and torsional effects on HAT reactions from alcohols and diols to CumO^•, uncovering moreover β-C-H deactivation as a relevant contributor in defining site selectivity.

Organocatalytic nitrenoid transfer: metal-free selective intermolecular C(sp3)-H amination catalyzed by an iminium salt

This report details the first organocatalytic method for nitrenoid transfer and its application to intermolecular, site-selective C(sp3)-H amination. The method utilizes a trifluoromethyl iminium salt as the catalyst, iminoiodinanes as the nitrogen source, and substrate as the limiting reagent. Activated, benzylic, and aliphatic substrates can all be selectively functionalized in yields up to 87%. A mechanistic proposal for the observed reactivity supported by experimental evidence invokes the intermediacy of a diaziridinium salt or related organic nitrenoid, species that have not been previously explored for the purpose of C-H amination. Finally, examples of late-stage functionalization of complex molecules highlight the selectivity and potential utility of this catalytic method in synthesis.

The mechanism of directed Ni(ii)-catalyzed C-H iodination with molecular iodine

This computational study reveals electrophilic cleavage pathways for substrates with N,N-bidentate directing centers in Ni(ii)-catalyzed C–H iodination with molecular iodine.

The density functional theory method is used to elucidate the elementary steps of Ni(ii)-catalyzed C(sp²)–H iodination with I₂ and substrates bearing N,N′-bidentate directing centers, amide-oxazoline (AO) and 8-aminoquinoline (AQ). The relative stability of the lowest energy high- and low-spin electronic states of the catalyst and intermediates is found to be an important factor for all of the steps in the reaction. As a result, two-state reactivity for these systems is reported, where the reaction is initiated on the triplet surface and generates a high energy singlet nickelacycle. It is shown that the addition of Na₂CO₃ base to the reaction mixture facilitates C–H activation. The presence of I₂ in the reaction provides the much needed driving force for the C–H activation and nickelacycle formation and ultimately reacts to form a new C–I bond through either a redox neutral electrophilic cleavage (EC) pathway or a one-electron reductive cleavage (REC) pathway. The previously proposed Ni(ii)/Ni(iv) and homolytic cleavage pathways are found to be higher in energy. The nature of the substrate is found to have a large impact on the relative stability of the lowest electronic states and on the stability of the nickelacycle resulting from C–H activation.

Bimodal Evans-Polanyi Relationships in Dioxirane Oxidations of sp3 C-H: Non-perfect Synchronization in Generation of Delocalized Radical Intermediates

The selectivities in C-H oxidations of a variety of compounds by DMDO have been explored with density functional theory. There is a linear Evans-Polanyi-type correlation for saturated substrates. Activation energies correlate with reaction energies or, equivalently, BDEs (ΔH^‡_sat = 0.91*BDE - 67.8). Unsaturated compounds, such as alkenes, aromatics, and carbonyls, exhibit a different correlation for allylic and benzylic C-H bonds (ΔH^‡_unsat = 0.35*BDE - 13.1). Bernasconi's Principle of Non-Perfect Synchronization (NPS) is found to operate here. The origins of this phenomenon were analyzed by a Distortion/Interaction model. Computations indicate early transition states for H-abstractions from allylic and benzylic C-H bonds, but later transition states for the saturated. The reactivities are mainly modulated by the distortion energy and the degree of dissociation of the C-H bond. While the increase in barrier with higher BDE is not unexpected from the Evans-Polanyi relationship, two separate correlations, one for saturated compounds, and one for unsaturated leading to delocalized radicals, were unexpected.

Navigating the Chiral Pool in the Total Synthesis of Complex Terpene Natural Products

The pool of abundant chiral terpene building blocks (i.e., "chiral pool terpenes") has long served as a starting point for the chemical synthesis of complex natural products, including many terpenes themselves. As inexpensive and versatile starting materials, such compounds continue to influence modern synthetic chemistry. This review highlights 21st century terpene total syntheses which themselves use small, terpene-derived materials as building blocks. An outlook to the future of research in this area is highlighted as well.

Type II Anion Relay Chemistry: Conformational Constraints To Achieve Effective [1,5]-Vinyl Brook Rearrangements

The design, synthesis, and evaluation of bifunctional linchpins, conformationally anchored on six-membered rings to achieve efficient [1,5]-Brook rearrangements involving vinyl silanes have been achieved. The restrained linchpins were subsequently exploited in type II anion relay chemistry (ARC) to permit both alkylations and Pd-mediated cross-coupling reactions (CCR) of sp² stabilized carbanions. DFT calculations were then employed to understand the mechanism and reactivity trends of [1,4]- and [1,5]-vinyl Brook rearrangements to provide insight on the role of the required copper reagent and the substrate geometry.

Highly Enantioselective Oxidation of Nonactivated Aliphatic C-H Bonds with Hydrogen Peroxide Catalyzed by Manganese Complexes

Monosubstituted cycloalkanes undergo regio- and enantioselective aliphatic C-H oxidation with H₂O₂ catalyzed by biologically inspired manganese catalysts. The reaction furnishes the corresponding ketones resulting from oxidation at C3 and C4 methylenic sites (K₃ and K₄, respectively) leading to a chiral desymmetrization that proceeds with remarkable enantioselectivity (64% ee) but modest regioselectivity at C3 (K₃/K₄ ≈ 2) for tert-butylcyclohexane, and with up to 96% ee and exquisite regioselectity toward C3 (up to K₃/K₄ > 99) when N-cyclohexylalkanamides are employed as substrates. Efficient H₂O₂ activation, high yield, and highly enantioselective C-H oxidation rely on the synergistic cooperation of a sterically bulky manganese catalyst and an oxidatively robust alkanoic acid. This represents the first example of nonenzymatic highly enantioselective oxidation of nonactivated methylenic sites. Furthermore, the principles of catalyst design disclosed in this work constitute a unique platform for further development of stereoselective C-H oxidation reactions.

Cu-Catalyzed aromatic C-H imidation with N-fluorobenzenesulfonimide: mechanistic details and predictive models

The LCuBr-catalyzed C-H imidation of arenes by N-fluorobenzenesulfonimide (NFSI), previously reported by us, utilizes an inexpensive catalyst and is applicable to a broad scope of complex arenes. The computational and experimental study reported here shows that the mechanism of the reaction is comprised of two parts: (1) generation of the active dinuclear CuII-CuII catalyst; and (2) the catalytic cycle for the C-H bond imidation of arenes. Computations show that the LCuIBr complex used in experiments is not an active catalyst. Instead, upon reacting with NFSI it converts to an active dinuclear CuII-CuII catalyst that is detected using HRMS techniques. The catalytic cycle starting from the CuII-CuII dinuclear complex proceeds via (a) one-electron oxidation of the active catalyst by NFSI to generate an imidyl radical and dinuclear CuII-CuIII intermediate, (b) turnover-limiting single-electron-transfer (SET1) from the arene to the imidyl radical, (c) fast C-N bond formation with an imidyl anion and an aryl radical cation, (d) reduction of the CuII-CuIII dinuclear intermediate by the aryl radical to regenerate the active catalyst and produce an aryl-cation intermediate, and (e) deprotonation and rearomatization of the arene ring to form the imidated product. The calculated KIE for the turnover-limiting SET1 step reproduces its experimentally observed value. A simple predictive tool was developed and experimentally validated to determine the regiochemical outcome for a given substrate. We demonstrated that the pre-reaction LCuX complexes, where X = Cl, Br and I, show a similar reactivity pattern as these complexes convert to the same catalytically active dinuclear CuII-CuII species.

Heterolytic (2 e) vs Homolytic (1 e) Oxidation Reactivity: N-H versus C-H Switch in the Oxidation of Lactams by Dioxirans

Dioxiranes are powerful oxidants that can act via two different mechanisms: 1) homolytic (H abstraction and oxygen rebound) and 2) heterolytic (electrophilic oxidation). So far, it has been reported that the nature of the substrate dictates the reaction mode independently from the dioxirane employed. Herein, we report an unprecedented case in which the nature of the dioxirane rules the oxidation chemoselectivity. In particular, a switch from C-H to N-H oxidation is observed in the oxidation of lactams moving from dimethyl dioxirane (DDO) to methyl(trifluoromethyl)dioxirane (TFDO). A physical organic chemistry study, which combines the oxidation with two other dioxiranes methyl(fluoromethyl)dioxirane, MFDO, and methyl(difluoromethyl)dioxirane, DFDO, with computational studies, points to a diverse ability of the dioxiranes to either stabilize the homo or the heterolytic pathway.

Fine Control over Site and Substrate Selectivity in Hydrogen Atom Transfer-Based Functionalization of Aliphatic C-H Bonds

The selective functionalization of unactivated aliphatic C-H bonds over intrinsically more reactive ones represents an ongoing challenge of synthetic chemistry. Here we show that in hydrogen atom transfer (HAT) from the aliphatic C-H bonds of alkane, ether, alcohol, amide, and amine substrates to the cumyloxyl radical (CumO^•) fine control over site and substrate selectivity is achieved by means of acid-base interactions. Protonation of the amines and metal ion binding to amines and amides strongly deactivates the C-H bonds of these substrates toward HAT to CumO^•, providing a powerful method for selective functionalization of unactivated or intrinsically less reactive C-H bonds. With 5-amino-1-pentanol, site-selectivity has been drastically changed through protonation of the strongly activating NH₂ group, with HAT that shifts to the C-H bonds that are adjacent to the OH group. In the intermolecular selectivity studies, trifluoroacetic acid, Mg(ClO₄)₂, and LiClO₄ have been employed in a orthogonal fashion for selective functionalization of alkane, ether, alcohol, and amide (or amine) substrates in the presence of an amine (or amide) one. Ca(ClO₄)₂, that promotes deactivation of amines and amides by Ca²⁺ binding, offers, moreover, the opportunity to selectively functionalize the C-H bonds of alkane, ether, and alcohol substrates in the presence of both amines and amides.

Cooperative Reactivity in an Extended-Viologen-Based Cyclophane

A tetracationic pyridinium-based cyclophane with a box-like geometry, incorporating two juxtaposed alkyne functions, has been synthesized. The triple bonds are reactive through cycloadditions toward dienes and azides, promoted by the electron-withdrawing nature of the pyridinium rings, as well as by the strain inherent in the cyclophane. The cycloadditions proceeded in high yields, with the cyclophane reacting faster than its acyclic analogue. While the cyclophane contains two reactive triple bonds, there is no evidence for a stable monofunctional intermediate-only starting material and the difunctional product have been detected by (1)H NMR spectroscopy. Molecular modeling of the energy landscape reveals a lower barrier for the kinetically favored second cycloaddition compared with the first one. This situation results in tandem cascading reactions within rigid cyclophanes, where reactions at a first triple bond induce increased reactivity at a distal second alkyne.

Nineteen-step total synthesis of (+)-phorbol

Phorbol, the flagship member of the tigliane diterpene family, has been known for over 80 years and has attracted attention from many chemists and biologists owing to its intriguing chemical structure and the medicinal potential of phorbol esters. Access to useful quantities of phorbol and related analogues has relied on isolation from natural sources and semisynthesis. Despite efforts spanning 40 years, chemical synthesis has been unable to compete with these strategies, owing to its complexity and unusual placement of oxygen atoms. Purely synthetic enantiopure phorbol has remained elusive, and biological synthesis has not led to even the simplest members of this terpene family. Recently, the chemical syntheses of eudesmanes, germacrenes, taxanes and ingenanes have all benefited from a strategy inspired by the logic of two-phase terpene biosynthesis in which powerful C-C bond constructions and C-H bond oxidations go hand in hand. Here we implement a two-phase terpene synthesis strategy to achieve enantiospecific total synthesis of (+)-phorbol in only 19 steps from the abundant monoterpene (+)-3-carene. The purpose of this synthesis route is not to displace isolation or semisynthesis as a means of generating the natural product per se, but rather to enable access to analogues containing unique placements of oxygen atoms that are otherwise inaccessible.

Diels-Alder Reactivities of Benzene, Pyridine, and Di-, Tri-, and Tetrazines: The Roles of Geometrical Distortions and Orbital Interactions

The cycloadditions of benzene and ten different azabenzenes (pyridine, three diazines, three triazines, and three tetrazines) with the ethylene dienophile have been explored with density functional theory (M06-2X) and analyzed with the distortion/interaction model. Activation barriers correlate closely with both distortion energies and interaction energies over an activation energy range of 45 kcal/mol. The replacement of CH with N increases Diels-Alder reactivity due not only to the more favorable orbital interaction, but also to a decrease in distortion energy. The rates of reactions are greatly influenced by the nature of the bonds being formed: two C-C bonds > one C-C bond, and one C-N bond > two C-N bonds. The activation energy of Diels-Alder reactions correlates very well with reaction energies and with the NICS(0) values of the aromatic dienes. The distortion energy of the Diels-Alder reaction transition states mostly arises from the diene out-of-plane distortion energy.