Synthesis of monoprotected 1,4-diketones by photoinduced alkylation of enones with 2-substituted-1,3-dioxolanes

One-pot synthesis of short-chain cyclic acetals via tandem hydroformylation–acetalization under biphasic conditions

A novel method of producing short-chain acetals via tandem hydroformylation–acetalization under biphasic conditions is developed.

Exploiting single-electron transfer in Lewis pairs for catalytic bond-forming reactions

A single-electron transfer (SET) between tris(pentafluorophenyl)borane (B(C₆F₅)₃) and N,N-dialkylanilines is reported, which is operative via the formation of an electron donor–acceptor (EDA) complex involving π-orbital interactions as a key intermediate under dark conditions or visible-light irradiation depending on the structure of the aniline derivatives. This inherent SET in the Lewis pairs initiates the generation of the corresponding α-aminoalkyl radicals and their additions to electron-deficient olefins, revealing the ability of B(C₆F₅)₃ to act as an effective one-electron redox catalyst.

Radical–ion pair generation from common Lewis pairs and its application to catalytic carbon–carbon bond formation.

Dual Benzophenone/Copper-Photocatalyzed Giese-Type Alkylation of C(sp3 )-H Bonds

Photocatalyzed Giese-type alkylations of C(sp³ )-H bonds are very attractive reactions in the context of atom-economy in C-C bond formation. The main limitation of such reactions is that when using highly polymerizable olefin acceptors, such as unsubstituted acrylates, acrylonitrile, or methyl vinyl ketone, radical polymerization often becomes the dominant or exclusive reaction pathway. Herein, we report that the polymerization of such olefins is strongly limited or suppressed when combining the photocatalytic activity of benzophenone (BP) with a catalytic amount of Cu(OAc)₂ . Under mild and operationally simple conditions, the Giese adducts resulting from the C(sp³ )-H functionalization of amines, alcohols, ethers, and cycloalkanes could be synthesized. Preliminary mechanistic studies have revealed that the reaction does not proceed through a radical chain, but through a dual BP/Cu photocatalytic process, in which both Cu^II and low-valent Cu^I/0 species, generated in situ by reduction by the BP ketyl radical, may react with α-keto or α-cyano intermediate radicals, thus preventing polymerization.

Generation of Dithianyl and Dioxolanyl Radicals Using Photoredox Catalysis: Application in the Total Synthesis of the Danshenspiroketallactones via Radical Relay Chemistry

Visible-light-induced generation of dithianyl and dioxolanyl radicals via selective hydrogen atom transfer (HAT) has been achieved. This radical relay tactic enables remote C(sp³)-H functionalization to permit rapid access to polyol and spiroketal segments, and in turn has been exploited as a key synthetic construct in the total synthesis of the danshenspiroketallactones. The conformational stability of the danshenspiroketallactones has also been defined via experiments and DFT calculations.

Novel mono- and bimetallic organotin(iv) compounds as potential linkers for coordination polymers

Organotin(iv) compounds might be used as linkers for building coordination polymers as suggested by the isolation of [{2-(OCH)C₆H₄}Me₂SnO(O)CC₅H₄N-4]ZnTPP.

Site-Selective Cu-Catalyzed Alkylation of α-Amino Acids and Peptides toward the Assembly of Quaternary Centers

The Cu^I -catalyzed selective α-alkylation of α-amino acid and peptide derivatives with 2-alkyl-1,3-dioxolanes is reported. This oxidative coupling is distinguished by its site-specificity, high diastereoselectivity, and chirality preservation and exhibits absolute chemoselectivity for N-aryl glycine motifs over other amino acid units. Collectively, the method allows for the assembly of challenging quaternary centers, as well as compounds derived from natural products of high structural complexity, which may provide ample opportunities for late-stage functionalization of peptides.

Formal Giese addition of C(sp3)-H nucleophiles enabled by visible light mediated Ni catalysis of triplet enone diradicals

An unprecedented utilization of triplet excited enones in Ni-catalysis enabled a formal Giese addition of C(sp3)-H nucleophiles. This mechanism-based approach has greatly widened the reaction scope, allowing the synthesis of previously inaccessible structures. In this process, the enone diradical acted as two distinct reaction centers, participating in both metalation and hydrogen atom transfer, ultimately furnishing a range of formal Giese addition products in a highly general context. This reaction provides complementary access to traditional 1,4-addition reactions of enones, with a future perspective to develop triplet diradical-based transition metal catalysis.

The past, present, and future of the Yang reaction

Photoexcited ketones have diradical characteristics and are functionally similar to high-spin metal-oxo species that are frequently used to catalyze C-H oxidation. First discovered by Yang in 1958, photoexcited ketones can abstract a hydrogen atom from hydrocarbons inter- or intramolecularly. Coupling with atom-transfer, group-transfer, or radical addition, the Yang reaction can be used to achieve various types of C-H functionalization. We provide in this article an overview of triplet ketone-mediated or catalyzed C-H functionalization reactions.

Di(imino)aryltin(IV) dichlorides as tectons for heterometallic coordination compounds

Hydrolysis of [2-{(CH2O)2CH}C6H4]2SnCl2 (1) [prepared from 2-[(CH2O)2CH]C6H4MgBr and SnCl4, in 2 : 1 molar ratio] gave [2-(O=CH)C6H4]2SnCl2 (2). Treatment of with the appropriate amine, in the absence of a solvent or catalyst, resulted in the isolation in high yields of [2-(RN[double bond, length as m-dash]CH)C6H4]2SnCl2 [R = 2'-C10H7 (3), 2',4',6'-Me3C6H2 (4), PhCH2 (5), Me2NCH2CH2 (6), 2'-PyCH2 (7)]. The reaction of with [Pd(COD)Cl2] provided the heterometallic species [Cl2Pd{2-(2'-PyCH2N[double bond, length as m-dash]CH)C6H4}2SnCl2] (8). The compounds were characterised by multinuclear NMR spectroscopy in solution, and mass spectrometry and IR spectroscopy in the solid state. The molecular structures of 1, 2, 4-7 and 8·CH3CN were established by single-crystal X-ray diffraction. For all diorganotin(IV) dichlorides intramolecular O→Sn or Nimine→Sn coordination results in hypercoordinated species with a distorted octahedral (C,E)2SnCl2 core (E = O, N). The presence of intramolecular N→Sn interactions in solution, responsible for the restriction of free rotation of mesityl groups around the C-N(=C) single bonds, is suggested by (1)H and (13)C NMR data. In the heterometallic dinuclear complex 8 the octahedral coordination around tin is preserved as in 7 and the nitrogen atoms from the pyridyl groups in the pendant arms are coordinated to palladium, leading to a trans-square planar PdCl2N2 core.

Brønsted acid cocatalysts in photocatalytic radical addition of α-amino C-H bonds across Michael acceptors

In marked contrast to the variety of strategies available for oxidation and nucleophilic functionalization of methylene groups adjacent to amines, relatively few approaches for modification of this position with electrophilic reaction partners have been reported. In the course of an investigation of the reactions of photogenerated α-amino radicals with electrophiles, we made the surprising observation that the efficiency of radical photoredox functionalization of N-aryl tetrahydroisoquinolines is dramatically increased in the presence of a Brønsted acid cocatalyst. Optimized conditions provide high yields and efficient conversion to radical addition products for a range of structurally modified tetrahydroisoquinolines and enones using convenient household light sources and commercially available Ru(bpy)3Cl2 as a photocatalyst. Our investigations into the origins of this unexpected additive effect have demonstrated that the carbon-carbon bond-forming step is accelerated by TFA and is a rare example of Brønsted acid catalysis in radical addition reactions. Moreover, a significant conclusion arising from these studies is the finding that product formation is dominated by radical chain processes and not by photocatalyst turnover. Together, these findings have important implications for the future design and mechanistic evaluation of photocatalytic radical processses.

Sesamin as a co-initiator for unfilled dental restorations

A natural component, sesamin (SA), was used to replace conventional amine as co-initiator for dental composite. A combination of camphorquinone (CQ) and SA was employed to initiate the photopolymerization of 2-2-bis[4-(2-hydroxy-3-methacryloxyprop-1-oxy)phenyl] propane/triethylene glycol dimethacrylate (70/30wt.%). The kinetics was recorded by real-time Fourier transform infrared spectroscopy. The mechanical properties were measured by dynamic mechanical analysis, the cell toxicity was investigated by MTT assay and a mixture of CQ and ethyl 4-N,N-dimethylaminobenzoate (EDMAB) was used as control in the same photocuring condition. The results indicated that the addition of SA as co-initiator greatly improved the rate of polymerization and final double-bond conversion (DC) when compared with the system initiated by CQ alone. Compared with EDMAB, the final DC of the CQ/SA system (71%) was slightly lower than that of CQ/EDMAB (76%); SA resulted in approximately the same storage modulus at around 37 degrees C, but a slightly higher glass transition temperature. SA produced lower yellowing effect and good in vitro biocompatibility. The water sorption and solubility for two mixtures were very close and within the range of the ISO 4049 specification. These results suggest that SA is an effective alternative co-initiator to conventional amine. The natural compound characteristics of SA make it more promising than amine in dental resin formulations.

Dimethylzinc-initiated radical reactions

Developments in modern organic synthesis owe much to the field of radical chemistry. Mild reaction conditions, high selectivity, good functional group tolerance and high product yield are features that have made reactions involving radical species indispensable tools for synthetic chemists. In part, the discovery of new radical initiators has led to the efficiency that now characterizes most radical reactions. This Account describes our investigations of radical reactions initiated by dimethylzinc. In 2001, we unexpectedly observed this reaction while investigating the amidophosphane-copper-catalyzed asymmetric addition of dimethylzinc to N-sulfonyl imines with tetrahydrofuran (THF) as reaction solvent. However, instead of adding the desired methyl group to the N-sulfonyl imine, we produced the THF adduct in excellent yield. This result laid the foundation for our discovery of novel modes of reactivity. Further investigations of the unexpected addition reaction revealed that a trace amount of air was needed for reaction progress, indicating that radical intermediates were involved. Indeed, controlled injection of air into the reaction flask by a syringe pump through a sodium hydroxide tube afforded the products in good to excellent yield. In addition, the reaction proved to be chemoselective for a C=N bond over a C=O bond, as well as for 1,4-addition over 1,2-addition. We developed asymmetric variants of the radical addition reaction of ethers to imines using chiral N-sulfinyl imines to produce the adducts in reasonably high stereoselectivity (up to 11:1). A 93:7 diastereomeric ratio of the adduct was obtained when bis(8-phenylmenthyl) benzylidenemalonate was used in the radical addition of ethers to C=C bonds. Interestingly, in the presence of dimethylzinc and air, arylamines, alkoxyamines, and dialkylhydrazines react with THF to give amino alcohols, oximes, and hydrazones, respectively, in moderate to high yields. We performed a tin-free intermolecular addition of functionalized primary alkyl groups, generated from their corresponding iodides, to N-sulfonyl imines using dimethylzinc, air, boron trifluoride diethyl etherate, and a catalytic amount of copper(II) triflate. Direct C-H bond cleavage from cycloalkanes was also feasible in the presence of dimethylzinc, air, and boron trifluoride diethyl etherate to give the corresponding cycloalkyl radicals, which were suitable nucleophiles for N-sulfonyl imines. In all of the above reactions, dimethylzinc was a superior radical initiator than other conventional initiators such as dibenzoyl peroxide, diethylzinc, and triethylborane. We hope the coming decades will witness the report of other novel radical initiators that would complement the reactivity modes of existing ones.

A natural component as coinitiator for unfilled dental resin composites

A natural component, 1,3-benzodioxole (BDO), was used for the purpose of replacing the conventional amine for dental composite. Camphorquinone (CQ)/BDO was used to initiate the photopolymerization of urethane dimethacrylate (UDMA)/triethylene glycol dimethacrylate (TEGDMA) (70/30 wt %). The kinetics was recorded by real-time Fourier transformation infrared spectroscopy (FTIR). The mechanical properties were measured by dynamic mechanical analyzer (DMA), and CQ/ethyl 4-N,N-dimethylaminobenzoate (EDMAB) mixture was used as control in the same photocuring condition. The results indicated that, the addition of BDO as coinitiator greatly improved the rate of polymerization and final double bond conversion (DC), when compared with the system initiated by CQ alone. BDO and EDMAB were found to reach almost the same final DC (75%), though the kinetics of two systems was different. Comparing with EDMAB, BDO brought approximately the same glass transition temperature (Tg), but slightly higher storage modulus around 37 degrees C. The water sorption and solubility for two mixtures were almost the same and within the range of the ISO 4049's standards. These results suggested that BDO was an effective alternative to conventional amine for coinitiator. And the human diet characteristics of BDO made it more promising than amine in the dental resin formulations.

Photoinduced-electron-transfer chemistry: from studies on PET processes to applications in natural product synthesis

The application of photoinduced electron transfer (PET) for the construction of heterocyclic ring systems is an appealing route in synthetic organic photochemistry. Electronically excited carbonyl chromophors in ketones, aldehydes, amides, or imides are strong electron acceptors that oxidize alkenes, amines, thioethers, or carboxylates. In subsequent steps, the radical anions formed thereof either are operating as secondary electron donors and initiate a photon-driven chain reaction or combine with electrophilic species and form products. These reactions are applied in the synthesis of heterocyclic compounds. The basic structures of these target molecules are bicyclic tertiary amines from the pyrrolizidine, benzopyrrolizidine, and indolizidine families, cyclic oligopeptides, macrocyclic ring systems, and many more.

The photomediated reaction of alkynes with cycloalkanes

In the presence of a photomediator such as benzophenone, alkynes with electron-withdrawing groups react with cycloalkanes to give vinyl cycloalkanes. The reaction involves the regiospecific addition of a photochemically generated cycloalkyl radical to the beta-carbon of the alkyne. The stereochemical outcome of the reaction depends on the nature of the photomediator and alkyne used.

The rapid photomediated alkenylation of 2-alkyl-1,3-dioxolanes with alkynes: a stereoelectronically assisted chain reaction

The photomediated reaction of alkynes such as DMAD with 1,3-dioxolanes leads to remarkably rapid alkenylation; evidence is presented that these reactions occur via a chain mechanism.

Radical addition of ethers to imines initiated by dimethylzinc

Ethers undergo addition to imines in the presence of dimethylzinc and air through a radical process. [reaction: see text]