Calibration of a fast benzylic radical clock reaction

Modular synthesis of α-branched secondary alkylamines via visible-light-mediated carbonyl alkylative amination

The development of methods for the assembly of secondary α-alkyl amines remains a central challenge to chemical synthesis because of their critical importance in modulating the physical properties of biologically active molecules. Despite decades of intensive research, chemists still rely on selective N-alkylation and carbonyl reductive amination to make most amine products. Here we report the further evolution of a carbonyl alkylative amination process that, for the first time, brings together primary amines, aldehydes and alkyl iodides in a visible-light-mediated multicomponent coupling reaction for the synthesis of a wide range of α-branched secondary alkylamines. In addition to exploring the tolerance and limitations in each reaction component, we also report preliminary applications to the telescoped synthesis of α-branched N-heterocycles and an N-alkylation protocol that is selective for primary over cyclic secondary amines. Our data support a mechanism involving addition of an alkyl radical to an uncharged alkyl imine which, to the best of our knowledge, has not previously been described. We believe that this method will enable practitioners of synthetic chemistry in academic and industrial settings to approach the synthesis of these important molecules in a manner that is streamlined compared to established approaches.

Mechanistic studies on single-electron transfer in frustrated Lewis pairs and its application to main-group chemistry

Advances in the field of frustrated Lewis pair (FLP) chemistry have led to the discovery of radical pairs, obtained by a single-electron transfer (SET) from the Lewis base to the Lewis acid. Radical pairs are intriguing for their potential to enable cooperative activation of challenging substrates (e.g., CH4, N2) in a homolytic fashion, as well as the exploration of novel radical reactions. In this review, we will cover the two known mechanisms of SET in FLPs-thermal and photoinduced-along with methods (i.e., CV, DFT, UV-vis) to predict the mechanism and to characterise the involved electron donors and acceptors. Furthermore, the available techniques (i.e., EPR, UV-vis, transient absorption spectroscopy) for studying the corresponding radical pairs will be discussed. Initially, two model systems (PMes3/CPh3+ and PMes3/B(C6F5)3) will be reviewed to highlight the difference between a thermal and a photoinduced SET mechanism. Additionally, three cases are analysed to provide further tools and insights into characterizing electron donors and acceptors, and the associated radical pairs. Firstly, a thermal SET process between LiHMDS and [TEMPO][BF4] is discussed. Next, the influence of Lewis acid complexation on the electron acceptor will be highlighted to facilitate a SET between (pBrPh)3N and TCNQ. Finally, an analysis of sulfonium salts as electron acceptors will demonstrate how to manage systems with rapidly decomposing radical species. This framework equips the reader with an expanded array of tools for both predicting and characterizing SET events within FLP chemistry, thereby enabling its extension and application to the broader domain of main-group (photo)redox chemistry.

Acid Promoted Tetrafunctionalization of Terminal Alkynes: Geminal Diazidation and Dibromination

The synthesis of complex alkanes by the tetrafunctionalization of alkynes is limited and challenging. Herein, an unprecedented efficient geminal diazidation and dibromination of terminal alkynes is developed, which provides novel access to structurally diverse organic azides. The approach has exclusive chemo- and regioselectivity and features mild reaction conditions, good tolerance of various functional groups, and more crucially, no metal involved in the reaction, thereby benefiting the late-stage decoration of medicinal molecules. A mechanistic study showed that the current geminal diazidation and dibromination proceeds via a radical pathway.

Competition between C-C and C-H Bond Fluorination: A Continuum of Electron Transfer and Hydrogen Atom Transfer Mechanisms

In 2015, we reported a photochemical method for directed C-C bond cleavage/radical fluorination of relatively unstrained cyclic acetals using Selectfluor and catalytic 9-fluorenone. Herein, we provide a detailed mechanistic study of this reaction, during which it was discovered that the key electron transfer step proceeds through substrate oxidation from a Selectfluor-derived N-centered radical intermediate (rather than through initially suspected photoinduced electron transfer). This finding led to proof of concept for two new methodologies, demonstrating that unstrained C-C bond fluorination can also be achieved under chemical and electrochemical conditions. Moreover, as C-C and C-H bond fluorination reactions are both theoretically possible on 2-aryl-cycloalkanone acetals and would involve the same reactive intermediate, we studied the competition between single-electron transfer (SET) and apparent hydrogen-atom transfer (HAT) pathways in acetal fluorination reactions using density functional theory. Finally, these analyses were applied more broadly to other classes of C-H and C-C bond fluorination reactions developed over the past decade, addressing the feasibility of SET processes masquerading as HAT in C-H fluorination literature.

Nitrene transfer from a sterically confined copper nitrenoid dipyrrin complex

Despite the myriad Cu-catalyzed nitrene transfer methodologies to form new C-N bonds (e.g., amination, aziridination), the critical reaction intermediates have largely eluded direct characterization due to their inherent reactivity. Herein, we report the synthesis of dipyrrin-supported Cu nitrenoid adducts, investigate their spectroscopic features, and probe their nitrene transfer chemistry through detailed mechanistic analyses. Treatment of the dipyrrin CuI complexes with substituted organoazides affords terminally ligated organoazide adducts with minimal activation of the azide unit as evidenced by vibrational spectroscopy and single crystal X-ray diffraction. The Cu nitrenoid, with an electronic structure most consistent with a triplet nitrene adduct of CuI, is accessed following geometric rearrangement of the azide adduct from κ1-N terminal ligation to κ1-N internal ligation with subsequent expulsion of N2. For perfluorinated arylazides, stoichiometric and catalytic C-H amination and aziridination was observed. Mechanistic analysis employing substrate competition reveals an enthalpically-controlled, electrophilic nitrene transfer for primary and secondary C-H bonds. Kinetic analyses for catalytic amination using tetrahydrofuran as a model substrate reveal pseudo-first order kinetics under relevant amination conditions with a first-order dependence on both Cu and organoazide. Activation parameters determined from Eyring analysis (ΔH‡ = 9.2(2) kcal mol-1, ΔS‡ = -42(2) cal mol-1 K-1, ΔG‡298K = 21.7(2) kcal mol-1) and parallel kinetic isotope effect measurements (1.10(2)) are consistent with rate-limiting Cu nitrenoid formation, followed by a proposed stepwise hydrogen-atom abstraction and rapid radical recombination to furnish the resulting C-N bond. The proposed mechanism and experimental analysis are further corroborated by density functional theory calculations. Multiconfigurational calculations provide insight into the electronic structure of the catalytically relevant Cu nitrene intermediates. The findings presented herein will assist in the development of future methodology for Cu-mediated C-N bond forming catalysis.

A Quadruple Catalysis Enabling Intermolecular Branch-Selective Hydroacylation of Styrenes

A quadruple N-heterocyclic carbene/cobalt/photoredox/Brønsted base catalysis to realize branch-selective hydroacylation of styrenes with aromatic and aliphatic aldehydes is demonstrated. This protocol allows access to branched ketones from readily available materials in an atom-economical manner. The quadruple catalysis can transfer a formyl hydrogen of aldehydes as a hydrogen radical equivalent onto the terminal carbon of an alkene by controlled electron and proton transfers.

Light-driven reduction of aromatic olefins in aqueous media catalysed by aminopyridine cobalt complexes

A catalytic system based on earth-abundant elements that efficiently hydrogenates aryl olefins using visible light as the driving-force and H₂O as the sole hydrogen atom source is reported. The catalytic system involves a robust and well-defined aminopyridine cobalt complex and a heteroleptic Cu photoredox catalyst. The system shows the reduction of styrene in aqueous media with a remarkable selectivity (>20 000) versus water reduction (WR). Reactivity and mechanistic studies support the formation of a [Co–H] intermediate, which reacts with the olefin via a hydrogen atom transfer (HAT). Synthetically useful deuterium-labelled compounds can be straightforwardly obtained by replacing H₂O with D₂O. Moreover, the dual photocatalytic system and the photocatalytic conditions can be rationally designed to tune the selectivity for aryl olefin vs. aryl ketone reduction; not only by changing the structural and electronic properties of the cobalt catalysts, but also by modifying the reduction properties of the photoredox catalyst.

A dual catalytic system based on earth-abundant elements reduces aryl olefins to alkanes in aqueous media under visible light. Mechanistic studies allow for rational tunning of the system for the selective reduction of aryl olefins vs ketones and vice versa.

N-Heterocyclic Carbene-Catalyzed 1,4-Alkylacylation of 1,3-Enynes

The radical relay coupling reaction recently emerged as a powerful synthetic strategy for producing tetrasubstituted allenes. However, bond-forming processes involving the allenyl radical intermediate are mostly limited to those promoted by transition metals. In this report, we describe that a ketyl radical generated from single-electron oxidation of the Breslow intermediate is an excellent coupling partner of allenyl radicals. An organocatalytic 1,4-alkylacylation of 1,3-enynes occurred smoothly in the presence of an aldehyde, a radical precursor, and an N-heterocyclic carbene catalyst. This transformation showed remarkable tolerance to both aromatic and aliphatic aldehydes, enyne substitution, and diversified radical precursors.

Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins

Many photoactive metal complexes can act as electron donors or acceptors upon photoexcitation, but hydrogen atom transfer (HAT) reactivity is rare. We discovered that a typical representative of a widely used class of iridium hydride complexes acts as an H-atom donor to unactivated olefins upon irradiation at 470 nm in the presence of tertiary alkyl amines as sacrificial electron and proton sources. The catalytic hydrogenation of simple olefins served as a test ground to establish this new photo-reactivity of iridium hydrides. Substrates that are very difficult to activate by photoinduced electron transfer were readily hydrogenated, and structure-reactivity relationships established with 12 different olefins are in line with typical HAT reactivity, reflecting the relative stabilities of radical intermediates formed by HAT. Radical clock, H/D isotope labeling, and transient absorption experiments provide further mechanistic insight and corroborate the interpretation of the overall reactivity in terms of photo-triggered hydrogen atom transfer (photo-HAT). The catalytically active species is identified as an Ir(ii) hydride with an IrII-H bond dissociation free energy around 44 kcal mol-1, which is formed after reductive 3MLCT excited-state quenching of the corresponding Ir(iii) hydride, i.e. the actual HAT step occurs on the ground-state potential energy surface. The photo-HAT reactivity presented here represents a conceptually novel approach to photocatalysis with metal complexes, which is fundamentally different from the many prior studies relying on photoinduced electron transfer.

Synthesis of DPPP- and DPPPEN-Type Bidentate Ligands by Ring-Opening Diphosphination of Methylene- and Vinylcyclopropanes under Visible-Light-Promoted Photoredox Catalysis

A ring-opening diphosphination of methylene- and vinylcyclopropanes with tetraaryldiphosphines (Ar₂P-PAr₂) has been developed to afford the corresponding 1,3-diphenylphosphinopropane- and 1,3-diphenylphosphinopentane-type bidentate ligands, respectively. The reaction proceeds under bromine cation-initiated, visible-light-promoted photoredox catalysis at ambient temperature. Owing to the ready availability of functionalized diphosphines, the electronically diverse MeO- and CF₃-substituted bidentate ligands are also easily prepared.

Pd/light-induced alkyl–alkenyl coupling reaction between unactivated alkyl iodides and alkenylboronic acids

Alkyl–alkenyl coupling reaction between unactivated alkyl iodides and 2-arylalkenylboronic acids utilizing a Pd/light combined system was studied.

Oxidative radical ring-opening/cyclization of cyclopropane derivatives

The ring-opening/cyclization of cyclopropane derivatives has drawn great attention in the past several decades. In this review, recent efforts in the development of oxidative radical ring-opening/cyclization of cyclopropane derivatives, including methylenecyclopropanes, cyclopropyl olefins and cyclopropanols, are described. We hope this review will be of sufficient interest for the scientific community to further advance the application of oxidative radical strategies in the ring-opening/cyclization of cyclopropane derivatives.

Ru-Catalyzed Completely Deoxygenative Coupling of 2-Arylethanols through Base-Induced Net Decarbonylation

Substituted arylethanols can be coupled by using a readily available Ru catalyst in a fully deoxygenative manner to produce hydrocarbon chains in one step. Control experiments indicate that the first deoxygenation occurs through an aldol condensation, whereas the second occurs through a base-induced net decarbonylation. This double deoxygenation enables further development in the use of alcohols as versatile and green alkylating reagents, as well as in other fields, such as deoxygenation and upgrading of overfunctionalized biomass to produce hydrocarbons.

Visible-Light-Induced Trifluoromethylation of Isonitrile-Substituted Methylenecyclopropanes: Facile Access to 6-(Trifluoromethyl)-7,8-Dihydrobenzo[k]phenanthridine Derivatives

A new visible-light-induced trifluoromethylation of isonitrile-substituted methylenecyclopropanes is developed. A range of substituted 6-(trifluoromethyl)-7,8-dihydrobenzo[k]phenanthridine derivatives are readily furnished by this newly developed tandem reaction with moderate to good yields. This reaction allows the direct formation of two six-membered rings and three new C-C bonds, including the C-CF3 bond, under visible light irradiation.

Evolution of neutral organic super-electron-donors and their applications

In recent times, metal-free chemistry has received significant attention due to its inherent qualities and its potential savings in the costs of (i) reagents and (ii) environmental treatments of residues. In this context, recently developed neutral organic electron-donors have shown an ability to perform challenging reductions that are traditionally the preserve of reactive metals and metal-based complexes, under mild reaction conditions. Hence, this feature article is aimed at describing the evolution of neutral organic super-electron-donors and their rapidly developing applications in electron-transfer reactions.

Direct, catalytic monofluorination of sp³ C-H bonds: a radical-based mechanism with ionic selectivity

Recently, our group unveiled a system in which an unusual interplay between copper(I) and Selectfluor effects mild, catalytic sp(3) C-H fluorination. Herein, we report a detailed reaction mechanism based on exhaustive EPR, (19)F NMR, UV-vis, electrochemical, kinetic, synthetic, and computational studies that, to our surprise, was revealed to be a radical chain mechanism in which copper acts as an initiator. Furthermore, we offer an explanation for the notable but curious preference for monofluorination by ascribing an ionic character to the transition state.

A bromine-radical mediated three-component reaction comprising allenes, electron-deficient alkenes and allyl bromides: facile synthesis of 2-bromo-1,7-dienes

A bromine-radical mediated three-component coupling reaction was effectively achieved by the use of allenes, electron-deficient alkenes, and allyl bromides and led to the synthesis of 2-bromo-1,7-dienes in good to high yields. This protocol was extended to the three-component process using alkylidenecyclopropane, which gave 2-bromo-1,8-diene along with alkylidenecyclopentane.

Thermal C2−C6Cyclization of Enyne−Allenes. Experimental Evidence for a Stepwise Mechanism and for an Unusual Thermal Silyl Shift

Enyne-allenes 4a-c bearing various cyclopropyl systems as radical clock reporter groups at the allene terminus have been synthesized and subjected to thermal C2-C6 cyclization. The ratio of ene versus formal Diels-Alder products could be rationalized on the basis of steric effects. Only the thermolysis of 4c, equipped with the fast diphenylcyclopropylcarbinyl radical clock, afforded a 1,3-butadienyl benzofulvene clearly formed via cyclopropyl ring opening. This finding provides unambiguous evidence for a stepwise mechanism of the C2-C6 cyclization making it possible to suggest a lifetime for the intermediate diradical of >1x10(-10) s (at 170 degrees C). An interesting corollary was the isolation of an unexpected silyl shift product in the thermolysis of all three enyne-allenes that allows explanation of the loss of the TIPS group in some of the Diels-Alder products. For a full understanding of the mechanism, silyl and hydrogen shift processes were interrogated using DFT.

Cyclopropylcarbinyl → Homoallyl-Type Ring Opening of Ketyl Radical Anions. Structure/Reactivity Relationships and the Contribution of Solvent/Counterion Reorganization to the Intrinsic Barrier

[reaction: see text] Following a protocol developed by Mathivanan, Johnston, and Wayner (J. Phys. Chem. 1995, 99, 8190-8195), the radical anions of several cyclopropyl- and oxiranyl-containing carbonyl compounds were generated in an effort to measure the rate constants for their ring opening (k(o)) by laser flash photolysis. The results of these experiments are compared to those obtained from earlier electrochemical studies, and the combined data set is used to rationalize the kinetics of radical anion ring opening in a general context by using Saveant's theory pertaining to stepwise dissociative electron transfer (Acc. Chem. Res. 1993, 26, 455-461). Compared to cyclopropylcarbinyl --> homoallyl rearrangements of neutral free radicals, at comparable driving force, the radical anion ring openings are slightly slower. The small difference in rate is attributed to the contribution of an additional, approximately 2 kcal/mol, solvent reorganization component for the radical anion rearrangements. The solvent reorganization energy for ring opening of these radical anions is believed to be small because the negative charge does not move appreciably in the progression reactant --> transition state --> product.

Activation/Driving Force Relationships for Cyclopropylcarbinyl → Homoallyl-Type Rearrangements of Radical Anions

By using direct and indirect electrochemical methods, rate constants (ko) for cyclopropane ring opening of radical anions derived from the one-electron reduction of trans-1-benzoyl-2-phenylcyclopropane, trans-1-benzoyl-2-vinylcyclopropane, 2-methylenecyclopropyl phenyl ketone, spiro[anthracene-9,1'-cyclopropan-10-one], 3-cyclopropylcyclohex-2-en-1-one, and 3-(1-methylcyclopropyl)cyclohex-2-en-1-one were determined. Qualitatively, rate constants for ring opening of these (and other cyclopropyl- and cyclobutyl-containing radical anions) can be rationalized on the basis of the thermodynamic stability of the radical anion, the ability of substituents on the cyclopropyl group to stabilize the radical portion of the distonic radical anion, and the stability of the enolate portion of the distonic radical anion. On the basis of this notion, a thermochemical cycle for estimating deltaG(o) for ring opening was presented. For simple cyclopropyl-containing ketyl anions, a reasonable correlation between log(ko) and deltaG(o) was found, and stepwise dissociative electron transfer theory was applied to rationalize the results. Activation energies calculated with density functional theory (UB3LYP/6-31+G*) correlate reasonably well with measured log(ko). The derived log(ko) and deltaG(o) and log(ko) vs E(a) plots provide the basis for a "calibration curve" to predict rate constants for ring opening of radical anions derived from carbonyl compounds, in general.

Thermolysis of 1-(1-aryl-1-bromomethyl)cyclopropyl bromides: a reinvestigation

Two compounds, the (Z)- and (E)-isomers of 2,4-dibromo-1-p-tolyl-1-butene 2a and 3a, respectively, were isolated in 65% total yield when 1-(1-bromo-1-p-tolylmethyl)cyclopropyl bromide (1a) was heated at 150 degrees C for 1 h. 1,1-Dibromo-2-p-tolylcyclobutane (4a), previously reported to be the only product in this reaction, was not detected. The phenyl analogue of 1a reacted similarly and gave the (Z)- and (E)-isomers of 2,4-dibromo-1-phenyl-1-butene 2b and 3b, respectively, in 60% yield. A rationale for the reaction is presented.

Cyclopropylcarbinyl-type ring openings. Reconciling the chemistry of neutral radicals and radical anions

Cyclopropylcarbinyl --> homoallyl and related rearrangements of radical ions (a) are frequently used as mechanistic "probes" to detect the occurrence of single electron transfer in chemical and biochemical processes, (b) provide the basis for mechanism-based drug design, and (c) are important tools in organic synthesis. Unfortunately, these rearrangements are poorly understood, especially with respect to the effect of substrate structure on reactivity. Frequently, researchers assume that the same factors which govern the reactivity of neutral free radicals also pertain to radical ions. The results reported herein demonstrate that in some cases structure-reactivity trends in radical ion rearrangements are very different from neutral radicals. For radical ions, delocalizations of both charge and spin are important factors governing their reactivity.

Chemoselective C-H oxidation of alcohols to carbonyl compounds with iodosobenzene catalyzed by (salen)chromium complex

Primary and secondary alcohols with benzylically and allylically activated C-H bonds are chemoselectively oxidized to the corresponding carbonyl compounds by the (salen)Cr(III) complex I as the catalyst and iodosobenzene as the oxygen source; the oxidizing species is the Cr(V) oxo complex. Allylic alcohols with fully substituted double bonds give appreciable amounts of epoxides besides the C-H oxidation products enones, while saturated alcohols are less readily oxidized.