Clean and Efficient Benzylic C−H Oxidation in Water Using a Hypervalent Iodine Reagent: Activation of Polymeric Iodosobenzene with KBr in the Presence of Montmorillonite-K10

Organo-photocatalytic C-H bond oxidation: an operationally simple and scalable method to prepare ketones with ambient air

Oxidative C-H functionalization with O₂ is a sustainable strategy to convert feedstock-like chemicals into valuable products. Nevertheless, eco-friendly O₂-utilizing chemical processes, which are scalable yet operationally simple, are challenging to develop. Here, we report our efforts, via organo-photocatalysis, in devising such protocols for catalytic C-H bond oxidation of alcohols and alkylbenzenes to ketones using ambient air as the oxidant. The protocols employed tetrabutylammonium anthraquinone-2-sulfonate as the organic photocatalyst which is readily available from a scalable ion exchange of inexpensive salts and is easy to separate from neutral organic products. Cobalt(ii) acetylacetonate was found to be greatly instrumental to oxidation of alcohols and therefore was included as an additive in evaluating the alcohol scope. The protocols employed a nontoxic solvent, could accommodate a variety of functional groups, and were readily scaled to 500 mmol scale in a simple batch setting using round-bottom flasks and ambient air. A preliminary mechanistic study of C-H bond oxidation of alcohols supported the validity of one possible mechanistic pathway, nested in a more complex network of potential pathways, in which the anthraquinone form - the oxidized form - of the photocatalyst activates alcohols and the anthrahydroquinone form - the relevant reduced form of the photocatalyst - activates O₂. A detailed mechanism, which reflected such a pathway and was consistent with previously accepted mechanisms, was proposed to account for formation of ketones from aerobic C-H bond oxidation of both alcohols and alkylbenzenes.

Oxidative Cleavage of Csp3-Csp2 and Csp3-H Bonds with KOtBu: Highly Robust and Practical Synthesis of Diaryl/(het-Ar) Ketones

Herein, we report an efficient and practical approach for synthesizing diaryl(het) ketones from R-CO-CHR-Ar through a simultaneous oxidative cleavage of C-C and C-H bonds using KO^tBu. This method enables synthesizing a variety of unsymmetrical and symmetrical (hetero)aryl ketones in excellent yields, which are otherwise difficult to make. Besides, we synthesized natural products using this method.

Synthesis and reactivity of azole-based iodazinium salts

A systematic investigation of imidazo- and pyrazoloiodazinium salts is presented. Besides a robust synthetic protocol that allowed us to synthesize these novel cyclic iodonium salts in their mono- and dicationic forms, we gained in-depth structural information through single-crystal analysis and demonstrated the ring opening of the heterocycle-bridged iodonium species. For an exclusive set of dicationic imidazoiodaziniums, we show highly delicate post-oxidation functionalizations retaining the hypervalent iodine center.

Progress in organocatalysis with hypervalent iodine catalysts

Hypervalent iodine compounds as environmentally friendly and relatively inexpensive reagents have properties similar to transition metals. They are employed as alternatives to transition metal catalysts in organic synthesis as mild, nontoxic, selective and recyclable catalytic reagents. Formation of C-N, C-O, C-S, C-F and C-C bonds can be seamlessly accomplished by hypervalent iodine catalysed oxidative functionalisations. The aim of this review is to highlight recent developments in the utilisation of iodine(III) and iodine(V) catalysts in the synthesis of a wide range of organic compounds including chiral catalysts for stereoselective synthesis. Polymer-, magnetic nanoparticle- and metal organic framework-supported hypervalent iodine catalysts are also described.

A Rh(iii)-catalyzed C–H activation/regiospecific annulation cascade of benzoic acids with propargyl acetates to unusual 3-alkylidene-isochromanones

We developed a new approach to synthesize isochromanones with benzoic acids and propargyl acetates, which introducing an unusual exocyclic C–C double bond at the 3-position with high regioselectivity and moderate to excellent yields.

(Diacetoxyiodo)benzene-Mediated C-H Oxidation of Benzylic Acetals

A useful oxidation of C-H bond of benzylic acetals has been achieved. This method avoids the use of stoichiometric metals and is compatible with the presence of both electron-donating and electron-withdrawing substituents on the aromatic ring. Oxidation was carried out by rapid microwave irradiation of benzylic acetals with PhI(OAc)₂ as the oxidant. This led to the oxidation of acetals into 2-acetoxy-1,3-dioxolanes. Furthermore, this transformation protocol encompasses a wide range of valuable conversions of these useful synthons into different carboxylic acid derivatives.

Selective benzylic C-H monooxygenation mediated by iodine oxides

A method for the selective monooxdiation of secondary benzylic C–H bonds is described using an N-oxyl catalyst and a hypervalent iodine species as a terminal oxidant. Combinations of ammonium iodate and catalytic N-hydroxyphthalimide (NHPI) were shown to be effective in the selective oxidation of n-butylbenzene directly to 1-phenylbutyl acetate in high yield (86%). This method shows moderate substrate tolerance in the oxygenation of substrates containing secondary benzylic C–H bonds, yielding the corresponding benzylic acetates in good to moderate yield. Tertiary benzylic C–H bonds were shown to be unreactive under similar conditions, despite the weaker C–H bond. A preliminary mechanistic analysis suggests that this NHPI-iodate system is functioning by a radical-based mechanism where iodine generated in situ captures formed benzylic radicals. The benzylic iodide intermediate then solvolyzes to yield the product ester.

Aerobic oxidation of fluorene to fluorenone over Co–Cu bimetal oxides

Aerobic oxidation of fluorene to fluorenone was achieved over Co–Cu bimetal oxides using O₂ as an oxidant in the absence of a radical initiator. Co–Cu bimetal oxides showed better catalytic performance than CuO and Co₃O₄.

Pd(TFA)2-catalyzed direct arylation of quinoxalinones with arenes

Pd(TFA)₂-catalyzed cross-dehydrogenative coupling reaction for the direct C-3 arylation of quinoxalin-2-ones with arenes is described.

Dehydrogenative Transformation of Alcoholic Substrates in Aqueous Media Catalyzed by an Iridium Complex Having a Functional Ligand with α-Hydroxypyridine and 4,5-Dihydro-1H-imidazol-2-yl Moieties

A new catalytic system that employs water as an environmentally friendly solvent for the dehydrogenative oxidation of alcohols and lactonization of diols has been developed. In this catalytic system, a water-soluble dicationic iridium complex having a functional ligand that comprises α-hydroxypyridine and 4,5-dihydro-1H-imidazol-2-yl moieties exhibits high catalytic performance. For example, the catalytic dehydrogenative oxidation of 1-phenylethanol in the presence of 0.25 mol % of the iridium catalyst and base under reflux in water proceeded to give acetophenone in 92% yield. Additionally, under similar reaction conditions, the iridium-catalyzed dehydrogenative lactonization of 1,2-benzenedimethanol gave phthalide in 98% yield.

Selective carboxylation of reactive benzylic C-H bonds by a hypervalent iodine(III)/inorganic bromide oxidation system

An oxidation system comprising phenyliodine(III) diacetate (PIDA) and iodosobenzene with inorganic bromide, i.e., sodium bromide, in an organic solvent led to the direct introduction of carboxylic acids into benzylic C–H bonds under mild conditions. The unique radical species, generated by the homolytic cleavage of the labile I(III)–Br bond of the in situ-formed bromo-λ³-iodane, initiated benzylic carboxylation with a high degree of selectivity for the secondary benzylic position.

Cu(I) Coordination Polymers as the Green Heterogeneous Catalysts for Direct C-H Bonds Activation of Arylalkanes to Ketones in Water with Spatial Confinement Effect

To develop coordination polymers (CPs) as catalysts to selectively catalyze the reaction of C-H bond activation of arylalkanes to their homologous ketones, three new Cu(I)-based coordination polymers (Cu^I-CPs) [CuI(aas-TPB)]_n (1), [CuBr(ass-TPB)CH₃CN]_n (2), and {[Cu(ass-TPB)]Cl}_n (3) (TPB = N,N,N-tris(3-pyridinyl)-1,3,5-benzenetricarboxamide) were synthesized. Structural variations from a herringbone fashion one-dimensional framework of 1 to a two-dimensional framework of 2 containing a 48-membered macrocycle and a cationic three-dimensional framework of 3 filled with Cl^- anions were observed arising from the different halogen ions (I^-, Br^-, and Cl^-). 1-3 were used as the green heterogeneous catalysts to catalyze direct C-H bond activation reactions of arylalkanes to ketones under mild reaction conditions with water as solvent. Handy product separation, convenient reaction procedures, and recyclability of these catalysts make the catalytic system fascinating. Moreover, the Cu^I-CPs performed the reaction with high regioselectivity due to the unique spatial confinement effect of CPs.

A unified photoredox-catalysis strategy for C(sp3)-H hydroxylation and amidation using hypervalent iodine

We report a unified photoredox-catalysis strategy for both hydroxylation and amidation of tertiary and benzylic C-H bonds. Use of hydroxyl perfluorobenziodoxole (PFBl-OH) oxidant is critical for efficient tertiary C-H functionalization, likely due to the enhanced electrophilicity of the benziodoxole radical. Benzylic methylene C-H bonds can be hydroxylated or amidated using unmodified hydroxyl benziodoxole oxidant Bl-OH under similar conditions. An ionic mechanism involving nucleophilic trapping of a carbocation intermediate by H2O or CH3CN cosolvent is presented.

Iodine(III) Reagents in Radical Chemistry

The chemistry of hypervalent iodine(III) compounds has gained great interest over the past 30 years. Hypervalent iodine(III) compounds show valuable ionic reactivity due to their high electrophilicity but also express radical reactivity as single electron oxidants for carbon and heteroatom radical generation. Looking at ionic chemistry, these iodine(III) reagents can act as electrophiles to efficiently construct C–CF₃, X–CF₃ (X = heteroatom), C–R_f (R_f = perfluoroalkyl), X–R_f, C–N₃, C–CN, S–CN, and C–X bonds. In some cases, a Lewis or a Bronsted acid is necessary to increase their electrophilicity. In these transformations, the iodine(III) compounds react as formal “CF₃⁺”, “R_f⁺”, “N₃⁺”, “Ar⁺”, “CN⁺”, and “X⁺” equivalents. On the other hand, one electron reduction of the I(III) reagents opens the door to the radical world, which is the topic of this Account that focuses on radical reactivity of hypervalent iodine(III) compounds such as the Togni reagent, Zhdankin reagent, diaryliodonium salts, aryliodonium ylides, aryl(cyano)iodonium triflates, and aryl(perfluoroalkyl)iodonium triflates. Radical generation starting with I(III) reagents can also occur via thermal or light mediated homolysis of the weak hypervalent bond in such reagents. This reactivity can be used for alkane C–H functionalization. We will address important pioneering work in the area but will mainly focus on studies that have been conducted by our group over the last 5 years.

We entered the field by investigating transition metal free single electron reduction of Togni type reagents using the readily available sodium 2,2,6,6-tetramethylpiperidine-1-oxyl salt (TEMPONa) as an organic one electron reductant for clean generation of the trifluoromethyl radical and perfluoroalkyl radicals. That valuable approach was later successfully also applied to the generation of azidyl and aryl radicals starting with the corresponding benziodoxole (Zhdankin reagent) and iodonium salts. In the presence of alkenes as radical acceptors, vicinal trifluoromethyl-, azido-, and arylaminoxylation products result via a sequence comprising radical addition to the alkene and subsequent TEMPO trapping. Electron-rich arenes also react with I(III) reagents via single electron transfer (SET) to give arene radical cations, which can then engage in arylation reactions. We also recognized that the isonitrile functionality in aryl isonitriles is a highly efficient perfluoroalkyl radical acceptor, and reaction of R_f-benziodoxoles (Togni type reagents) in the presence of a radical initiator provides various perfluoroalkylated N-heterocycles (indoles, phenanthridines, quinolines, etc.). We further found that aryliodonium ylides, previously used as carbene precursors in metal-mediated cyclopropanation reactions, react via SET reduction with TEMPONa to the corresponding aryl radicals. As a drawback of all these transformations, we realized that only one ligand of the iodine(III) reagent gets transferred to the substrate. To further increase atom-economy of such conversions, we identified cyano or perfluoroalkyl iodonium triflate salts as valuable reagents for stereoselective vicinal alkyne difunctionalization, where two ligands from the I(III) reagent are sequentially transferred to an alkyne acceptor.

Finally, we will discuss alkynyl-benziodoxoles as radical acceptors for alkynylation reactions. Similar reactivity was found for the Zhdankin reagent that has been successfully applied to azidation of C-radicals, and also cyanation is possible with a cyano I(III) reagent. To summarize, this Account focuses on the design, development, mechanistic understanding, and synthetic application of hypervalent iodine(III) reagents in radical chemistry.

Visible light mediated efficient oxidative benzylic sp(3) C-H to ketone derivatives obtained under mild conditions using O2

A photooxygenation of benzylic sp(3) C-H reaction has been demonstrated using O2 mediated by visible light. This protocol provides a simple and mild route to obtain ketones from benzylic sp(3) C-H bonds. Various benzylic sp(3) C-H bonds can be transformed into the desired ketone derivatives in moderate to good yields. The (18)O2 labelling experiments demonstrated that the oxygen introduced into ketone originated from dioxygen. A plausible mechanism has been proposed accordingly.

Advances in Synthetic Applications of Hypervalent Iodine Compounds

The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C-C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of using hypervalent iodine compounds as an environmentally sustainable alternative to heavy metals.

Two catalytic systems of l-proline/Cu(ii) catalyzed allylic oxidation of olefins with tert-butyl hydroperoxide

Allylic oxidation of olefins by the two catalytic system of l-proline/Cu(ii).