Acid‐Mediated Formation of Radicals or Baeyer–Villiger Oxidation from Criegee Adducts

Exhaustive Baeyer–Villiger oxidation: a tailor-made post-polymerization modification to access challenging poly(vinyl acetate) copolymers

The discovery of exhaustive (nearly quantitative) post-polymerization modifications (PPM) relies heavily on the efficiency of their corresponding small-molecule protocols. However, the direct translation of existing small-molecule protocols into PPM methods has never been guaranteed due to the intrinsic differences between small-molecule substrates and polymers. Herein, we introduce the direct optimization on polymers (DOP) as a complementary approach to developing exhaustive PPM reactions. As proof of the DOP concept, we present an exhaustive Baeyer–Villiger (BV) post-modification which cannot be accessed by conventional approaches. This user-friendly methodology provides general access to synthetically challenging copolymers of vinyl acetate and more activated monomers (MAMs) including both statistical and narrow-dispersed block copolymers. Furthermore, a scalable one-pot copolymerization/exhaustive BV post-modification procedure was developed to produce such materials showing improved performance over regular PVAc.

Exhaustive Baeyer–Villiger (BV) oxidation, which was developed by a direct optimization on polymers (DOP) approach, provides a general solution for preparing synthetically challenging poly(vinyl acetate) statistical and block copolymers.

Concomitant Functionalization of Two Different Ketones by Merging Brønsted Acid Catalysis and Radical Relay Coupling

In previous literature, incorporation of functional groups at the α-position of unactivated carbonyl compounds was mainly restricted to one kind of corresponding precursors. Herein, we report the concomitant functionalization of...

Interrupted reactions in chemical synthesis

Interrupted reactions reroute established processes to new and often unanticipated end points. Of particular interest are the cases in which a known reactive intermediate takes on a new reaction pathway, either because this pathway is lower in energy or because the conventional pathway is no longer available. Through analysis of documented cases, we aim to dissect the known interrupted reactions and trace their mechanistic origins. As new chemical processes are being discovered at a seemingly ever-increasing pace, it is likely that new interrupted reactions will continue to emerge. Our hope is that the cases considered in this Review will help identify new classes of these fascinating transformations.

Oxidative α-acyloxylation of acetals with cyclic diacyl peroxides

Selective functionalization of the non-activated acetal α-position with formal retaining of the acetal fragment was realized using cyclic diacyl peroxides.

Synthesis of unstrained Criegee intermediates: inverse α-effect and other protective stereoelectronic forces can stop Baeyer-Villiger rearrangement of γ-hydroperoxy-γ-peroxylactones

How far can we push the limits in removing stereoelectronic protection from an unstable intermediate? We address this question by exploring the interplay between the primary and secondary stereoelectronic effects in the Baeyer-Villiger (BV) rearrangement by experimental and computational studies of γ-OR-substituted γ-peroxylactones, the previously elusive non-strained Criegee intermediates (CI). These new cyclic peroxides were synthesized by the peroxidation of γ-ketoesters followed by in situ cyclization using a BF3·Et2O/H2O2 system. Although the primary effect (alignment of the migrating C-Rm bond with the breaking O-O bond) is active in the 6-membered ring, weakening of the secondary effect (donation from the OR lone pair to the breaking C-Rm bond) provides sufficient kinetic stabilization to allow the formation and isolation of stable γ-hydroperoxy-γ-peroxylactones with a methyl-substituent in the C6-position. Furthermore, supplementary protection is also provided by reactant stabilization originating from two new stereoelectronic factors, both identified and quantified for the first time in the present work. First, an unexpected boat preference in the γ-hydroperoxy-γ-peroxylactones weakens the primary stereoelectronic effects and introduces a ∼2 kcal mol-1 Curtin-Hammett penalty for reacquiring the more reactive chair conformation. Second, activation of the secondary stereoelectronic effect in the TS comes with a ∼2-3 kcal mol-1 penalty for giving up the exo-anomeric stabilization in the 6-membered Criegee intermediate. Together, the three new stereoelectronic factors (inverse α-effect, misalignment of reacting bonds in the boat conformation, and the exo-anomeric effect) illustrate the richness of stereoelectronic patterns in peroxide chemistry and provide experimentally significant kinetic stabilization to this new class of bisperoxides. Furthermore, mild reduction of γ-hydroperoxy-γ-peroxylactone with Ph3P produced an isolable γ-hydroxy-γ-peroxylactone, the first example of a structurally unencumbered CI where neither the primary nor the secondary stereoelectronic effect are impeded. Although this compound is relatively unstable, it does not undergo the BV reaction and instead follows a new mode of reactivity for the CI - a ring-opening process.

Kinetics and Thermodynamics of Reactions Involving Criegee Intermediates: An Assessment of Density Functional Theory and Ab Initio Methods Through Comparison with CCSDT(Q)/CBS Data

Reactions involving Criegee intermediates (CIs, R₁ R₂ COO) are important in atmospheric ozonolysis models. In recent years, density functional theory (DFT) and CCSD(T)-based ab initio methods are increasingly being used for modeling reaction profiles involving CIs. We obtain highly accurate CCSDT(Q)/CBS reaction energies and barrier heights for ring-closing reactions involving atmospherically important CIs (R₁ /R₂ = H, Me, OH, OMe, F, CN, cyclopropene, ethylene, acetaldehyde, and acrolein). We use this benchmark data to evaluate the performance of DFT, double-hybrid DFT (DHDFT), and ab initio methods for the kinetics and thermodynamics of these reactions. We find that reaction energies are more challenging for approximate theoretical procedures than barrier heights. Overall, taking both reaction energies and barrier heights into account, only one of the 58 considered DFT methods (the meta-GGA MN12-L) attains near chemical accuracy, with root-mean-square deviations (RMSDs) of 3.5 (barrier heights) and 4.7 (reaction energies) kJ mol^-1 . Therefore, MN12-L is recommended for investigations where CCSD(T)-based methods are not computationally feasible. For reaction barrier heights performance does not strictly follow Jacob's Ladder, for example, DHDFT methods do not perform better than conventional DFT methods. Of the ab initio methods, the cost-effective CCSD(T)/CBS(MP2) approach gives the best performance for both reaction energies and barrier heights, with RMSDs of 1.7 and 1.4 kJ mol^-1 , respectively. All the considered Gaussian-n methods show good performance with RMSDs below the threshold of chemical accuracy for both reaction energies and barrier heights, where G4(MP2) shows the best overall performance with RMSDs of 2.9 and 1.5 kJ mol^-1 , respectively. © 2019 Wiley Periodicals, Inc.

Acid promoted radical-chain difunctionalization of styrenes with stabilized radicals and (N,O)-nucleophiles

A radical chain difunctionalization of styrenes does not require a catalyst, but activation of benzoyl peroxide by a strong acid.

Facile access to evodiakine enabled by aerobic copper-catalyzed oxidative rearrangement

Oxidation as a fundamentally important method for the synthesis of complex structures is difficult to achieve in a selective manner. Evodiakine, a complex natural product possessing an unprecedented ring system (6/5/5/7/6), has a high oxidation state without a practical solution. Herein, we report the first synthesis of evodiakine via aerobic copper-catalyzed late-stage functionalization of evodiamine.

Radical addition of ketones and cyanide to olefinsviaacid catalyzed formation of intermediate alkenyl peroxides

γ-Cyanoketones are formed by double radical addition from olefins, ketones and sulfonyl cyanidesviareactive alkenyl peroxide intermediates.

Flexibility and lability of a phenyl ligand in hetero-organometallic 3d metal-Sn(iv) compounds and their catalytic activity in Baeyer-Villiger oxidation of cyclohexanone

The single compartmental Schiff base N,N'-ethylenebis(salicylaldimine) (H₂L) and [SnPh₂Cl₂] were utilized to synthesize heterobimetallic 3d metal-Sn complexes, the Co^IIISn^IV compound [{SnPhCl₂}(1κO²N²,2κO²-μ-L)(μ-OMe){CoPh}] (1), the Ni^IISn^IV compound [{SnPh₂Cl₂}(1κO²N²,2κO²-μ-L)Ni] (2) and the Cu^IISn^IV compound [{SnPh₂Cl₂}(1κO²N²,2κO²-μ-L)Cu] (3). Attempting to prepare the ethoxido bridged compound analogous to 1 (in ethanol) gives the phenylcobalt(iii) complex [Co(κO²N²)Ph(H₂O)] (1A). Single crystal X-ray structure analyses reveal that 1 is derived from an intermetallic (Sn to Co) phenyl shift and that 1A is a transmetallated product; in compounds 2 and 3, the phenyl groups remain coordinated to Sn^IV but one of the π rings interacts with the 3d-metal. Thus, while systems 1 and 1A show the lability of the phenyl ligand, 2 and 3 reveal its flexible nature. Theoretical DFT calculations demonstrate that the conceivable Ph group shift occurs in the oxidized Co^III intermediate [{Sn^IVPh₂Cl₂}(κO₂N₂-μ-L){Co^III(MeO)}] (5) rather than in the corresponding Co^II species [{Sn^IVPh₂Cl₂}(κO₂N₂-μ-L){Co^II(MeOH)}] (4). Their catalytic studies in the Baeyer-Villiger oxidation of cyclohexanone into ε-caprolactone with two different oxidants reveal that the sacrificial aldehyde method (with dioxygen/benzaldehyde) is better than that with aqueous H₂O₂ (30%). The effects of various reaction parameters such as solvent, catalyst amount, temperature, time and heating method were studied allowing the achievement of yields up to 83% with 89% selectivity.

ATRA-like alkylation–peroxidation of alkenes with trichloromethyl derivatives by the combination of tBuOOH and NEt3

This protocol provides a unique and innovative approach to the construction of α-tert-butylperoxy-β-dichloromethyl alkanes, employing CHCl₃ as an alkylating reagent to provide a –CHCl₂ group, and TBHP as an oxidant to provide an α-^tBuOO group.

Alkylation–peroxidation of α-carbonyl imines or ketones catalyzed by a copper salt via radical-mediated Csp3–H functionalization

The catalytic alkylation–peroxidation of α-carbonyl imines or ketones was enabled by a simple copper salt via radical-mediated C_sp3–H functionalization.

Difunctionalization of alkenes with iodine and tert-butyl hydroperoxide (TBHP) at room temperature for the synthesis of 1-(tert-butylperoxy)-2-iodoethanes

We developed a direct vicinal difunctionalization of alkenes with iodine and TBHP at room temperature. This iodination and peroxidation in a one-pot synthesis produces 1-(tert-butylperoxy)-2-iodoethanes, which are inaccessible through conventional synthetic methods. This method generates multiple radical intermediates in situ and has excellent regioselectivity, a broad substrate scope and mild conditions. The iodine and peroxide groups of 1-(tert-butylperoxy)-2-iodoethanes have several potential applications and allow further chemical modifications, enabling the preparation of synthetically valuable molecules.

Oxidative Rearrangement Coupling Reaction for the Functionalization of Tetrahydro-β-carbolines with Aromatic Amines

The observation of an unexpected oxidative rearrangement coupling reaction led to the development of a novel method for the efficient functionalization of tetrahydro-β-carbolines (THβCs). The treatment of THβCs with photogenerated singlet oxygen (¹ O₂ ) afforded unstable dioxetanes, which underwent further transformation to form new bonds in the presence of trifluoroacetic acid. This operationally simple protocol exhibits broad functional-group tolerance and is suitable for the late-stage functionalization of complex druglike molecules.

Cobalt salt-catalyzed carbocyclization reactions of α-bromo-N-phenylacetamide derivatives

A Co(ii)-catalyzed carbocyclization reaction for the synthesis of 4-substituted quinolin-2-(1H)-ones is described.

Acid-Catalyzed Activation of Peroxyketals: Tunable Radical Initiation at Ambient Temperature and Below

This study details how peroxyketals, commercially available thermal initiators, and structurally related peroxides are activated in the presence of an acid catalyst to generate radicals at room temperature and below. This simple combination of two substrates was shown to efficiently initiate a variety of radical processes. This phenomenon is rationalized by the acid-catalyzed in situ formation of highly unstable alkenyl peroxides which readily decompose into initiating radical species.

Synthesis of 1,4-Diketones from β-Oxo Esters and Enol Acetates by ­Cerium-Catalyzed Oxidative Umpo­lung Reaction

Cyclic β‐oxo esters are converted with enol acetates in a cerium‐catalyzed, oxidative Umpolung reaction to furnish 1,4‐diketones with up to 95 % yield. Atmospheric oxygen is the oxidant in this process, which can be regarded as ideal from economic and ecological points of view. Further advantages of this new C–C coupling reaction are its operational simplicity and the application of nontoxic and inexpensive CeCl₃·7H₂O as precatalyst.