Intermolecular Addition Research Articles

Iron-Catalyzed Radical Intermolecular Addition of Unbiased Alkenes to Aldehydes.

The intermolecular reductive radical coupling of aldehydes with nonactivated alkenes, employing metal hydride atom transfer (MHAT) catalysis with a combination of FeII and FeIII salts, is described. This constitutes the first use of aldehydes as viable acceptor groups in MHAT reactions. The insights gained in this study led to the reexamination of the previously reported intramolecular version of the reaction, and the addition of FeII salts allowed the development of a more efficient second-generation approach.

Electrochemical Synthesis of Quinazolinones by the Metal-Free and Acceptor-Free Dehydrogenation of 2-Aminobenzamides

An efficient approach has been developed for the construction of quinazolin-4(3H)-ones by the selective anodic dehydrogenative oxidation/cyclization of benzylic chlorides and 2-aminobenzamides. The method features acceptor-free and metal-free dehydrogenation of amines to imines; a subsequent intermolecular addition provides the products in moderate to good yields.

Visible-Light-Mediated Photoredox Reactions in the Total Synthesis of Natural Products

In the last two decades, the field of photoredox catalysis (PRC) has grown impressively with reports of new synthetic methodologies and more efficient versions of known free-radical reactions. The impressive success of visible-light-mediated photoredox catalysis is, in great part, due to its low environmental impact, mild reaction conditions, clean reactions, and inexpensive methodologies. These features have allowed photoredox catalysis to emerge as a powerful tool in the synthesis of natural products; much excellent work was reported between 2011 and 2015. Since 2016, a number of more efficient and impressive total syntheses of natural products featuring photoredox catalysis have been reported. In this review, we summarize the recent synthetic applications of photoredox catalysis in the total synthesis of natural products between 2016 and 2020.1 Introduction2 Intermolecular Additions from Functionalized Substrates2.1 Intermolecular Additions from Alkyl Halides2.2 Intermolecular Additions from Alcohols and Carboxylic Acids3 Cyclizations from Functionalized Substrates3.1 Cyclizations of Carbon-Centered Radicals3.2 Cyclizations of Nitrogen-Centered Radicals4 Intramolecular Cyclization from Non-functionalized N–H Bonds4.1 Type I Radical Cascade4.2 Type II Radical Cascade4.3 Type III Radical Cascade5 Functionalization of Imines and Enamines6 Cycloadditions7 Miscellaneous7.1 Dehalogenation and Reductive Decarboxylation7.2 Thiyl Radical Promoted Cascade8 Conclusions and Perspectives

Selective Synthesis of 2-Indolyl-3-oxoindolines or 2-(2-Aminophenyl)quinolines through Cu(II)- or Bi(III)-Catalyzed Tunable Dimerizations of 2-Alkynylanilines.

A novel and selective synthesis of 2-indolyl-3-oxoindolines or 2-(2-aminophenyl)quinolines through tunable dimerizations of 2-alkynylanilines is presented. Mechanistically, the formation of 2-indolyl-3-oxoindolines involves a Cu(OAc)2/O2-promoted intramolecular cyclization of 2-alkynylaniline to give the required indole and 3H-indol-3-one intermediates followed by the indolylation of 3H-indol-3-one. On the other hand, the formation of 2-(2-aminophenyl)quinolines is believed to go through a Bi(OTf)3/MesCO2H-catalyzed intermolecular N-nucleophilic addition between two 2-alkynylaniline molecules to give an enamine intermediate followed by its intramolecular C-nucleophilic addition/annulation. Notable features of these new methods include easily obtainable substrates, economical catalysts and oxidant, controllable selectivity, and high versatility toward diverse products.

A complementary approach to conjugated N-acyliminium formation through photoredox-catalyzed intermolecular radical addition to allenamides and allencarbamates.

An intermolecular radical addition, using photoredox catalysis, to allenamides and allencarbamates is reported. This transformation synthesizes N-acyl-N’-aryl-N,N’-allylaminals, and proceeds by a conjugated N-acyliminium intermediate that previously has principally been generated by electrophilic activation methods. The radical adds to the central carbon of the allene giving a conjugated N-acyliminium that undergoes nucleophilic addition by arylamines and alcohols.

Fluoroalkylation of Allylic Alcohols with Concomitant (Hetero)aryl Migration: Access to Fluoroalkylated Ketones and Evaluation of Antifungal Action against Magnaporthe grisea

Fluoroalkyl(hetero)arylation of allylic alcohols with perfluoroalkyl iodide or difluoroacetic acid as fluoroalkyl source was developed herein, delivering functionalized ketones containing an α‐quaternary center. Formation of the fluoroalkyl radical was followed by its intermolecular addition to alkenes and the migration of a (hetero)aryl group with the concomitant generation of a ketone group to finish the domino sequence. Mechanistic studies indicated that the aryl migration proceeded through neophyl rearrangement in a radical pathway. Moreover, preliminary antifungal evaluation against Magnaporthe grisea is also described for the first time.

Computationally‐Guided Investigation of Dual Amine/pi Lewis Acid Catalysts for Direct Additions of Aldehydes and Ketones to Unactivated Alkenes and Alkynes

AbstractDual amine/pi Lewis acid catalyst systems have been reported for intramolecular direct additions of aldehydes/ketones to unactivated alkynes and occasionally alkenes, but related intermolecular reactions are rare and not presently of significant synthetic utility, likely due to undesired coordination of enamine intermediates to the metal catalyst. We reasoned that bulky metal ligands and bulky amine catalysts could minimize catalyst poisoning and could facilitate certain examples of direct intermolecular additions of aldehydes/ketones to alkenes/alkynes. Density Functional Theory (DFT) calculations were performed that suggested that pyridine‐2,6‐bis(oxazoline) (PyBOX)‐Pt(II) catalysts for alkene/alkyne activation could be combined with MacMillan's imidazolidinone organocatalyst for aldehyde/ketone activation to facilitate desirable C−C bond formations, and certain reactions were calculated to be more exergonic than catalyst poisoning pathways. Consistent with the calculations, preformed enamines generated from the MacMillan imidazolidinone did not displace ethylene from a biscationic (t‐Bu)PyBOX‐Pt2+ complex. The desired intermolecular C−C bond formation was not observed under several different conditions, but this novel catalytic system facilitated an intramolecular C−C bond formation (Conia‐ene type reaction) with a formyl alkyne substrate.

C-N and C-O Bond Formation in Copper-Catalyzed/Mediated sp3 C-H Activation: Mechanistic Studies from Experimental and Computational Aspects.

Mechanistic studies on Cu-catalyzed/mediated sp3 C-H amidation and acetoxylation are investigated from experimental and computational aspects. The concerted metalation-deprotonation (CMD) mechanism rather than a radical-involved pathway is proved to occur in amidation and acetoxylation reactions, and this is the rare example of the CMD mechanism involved in the more challenging sp3 C-H activations. Theoretical calculations demonstrated that CMD is the rate-determining step either for methylic or benzylic positions in amidation and acetoxylation reactions, and intermolecular nucleophilic addition of acetate anions is more favorable than the ring opening of β-lactams and intramolecular acetoxylation. These mechanistic studies on the divergent and condition-dependent product formation are critical for developing Cu-promoted C-H functionalization via the CMD mechanism.

Convergent Total Synthesis of Hikizimycin Enabled by Intermolecular Radical Addition to Aldehyde.

Hikizimycin (1), which exhibits powerful anthelmintic activity, has the most densely functionalized structure among nucleoside antibiotics. A central 4-amino-4-deoxyundecose of 1 possesses 10 contiguous stereocenters on a C1-C11 linear chain and is decorated with a cytosine base at C1 and a 3-amino-3-deoxyglucose at C6-OH. These distinctive structural features of 1 make it an extremely challenging target for de novo construction. Herein, we report a convergent total synthesis of 1 from four known components: 3-azide-3-deoxyglucose derivative 4, bis-TMS-cytosine 5, d-mannose 9, and d-galactose derivative 10. We first designed and devised a novel radical coupling reaction between multiply hydroxylated aldehydes and α-alkoxyacyl tellurides. The generality and efficiency of this process was demonstrated by the coupling of 7c and 8, which were readily accessible from two hexoses, 9 and 10, respectively. Et3B and O2 rapidly induced decarbonylative radical formation from α-alkoxyacyl telluride 8, and intermolecular addition of the generated α-alkoxy radical to aldehyde 7c yielded 4-amino-4-deoxyundecose 6-α with installation of the desired C5,6-stereocenters. Subsequent attachments of the cytosine with 5 and of the 3-azide-3-deoxyglucose with 4 were realized through selective activation of the C1-acetal and selective deprotection of the C6-hydroxy group. Finally, the 3 amino and 10 hydroxy groups were liberated in a single step to deliver the target 1. Thus, the combination of the newly developed radical-coupling and protective-group strategies minimized the functional group manipulations and thereby enabled the synthesis of 1 from 10 in only 17 steps. The present total synthesis demonstrates the versatility of intermolecular radical addition to aldehyde for the first time and offers a new strategic design for multistep target-oriented syntheses of various nucleoside antibiotics and other bioactive natural products.

Visible-Light-Mediated Intermolecular Radical Conjugate Addition for the Construction of Vicinal Quaternary Carbon Centers.

The visible light-driven organophotoredox catalysis is reported for the construction of vicinal quaternary carbon centers. Intermolecular conjugate addition of alkyl radicals, derived from 2,2-disubstituted dihydroquinazolinones, to Michael acceptors under blue light irradiation and rhodamine B catalysis allows the facile assembly of diverse, vicinal secondary/quaternary, tertiary/quaternary, and quaternary/quaternary carbon centers at room temperature. Our method provides a synthetically versatile protocol since both 2,2-disubstituted dihydroquinazolinones and Michael acceptors can be conveniently prepared from readily available ketones.

Photocatalytic Synthesis of Polycyclic Indolones.

In this work, a photocatalytic strategy for a rapid and modular access to polycyclic indolones starting from readily available indoles is reported. This strategy relies on the use of redox‐active esters in combination with an iridium‐based photocatalyst under visible‐light irradiation. The generation of alkyl radicals through decarboxylative single electron reductions enables intramolecular homolytic aromatic substitutions with a pending indole moiety to afford pyrrolo‐ and pyridoindolone derivatives under mild conditions. Furthermore, it was demonstrated that these radicals could also be engaged into cascades consisting of an intermolecular Giese‐type addition followed by an intramolecular homolytic aromatic substitution to rapidly assemble valuable azepinoindolones.

Tertiary Amine Synthesis by Radical Carbonyl Alkylative Amination

Highly efficient access to structurally diverse branched tertiary amines is a long-standing challenge in synthetic chemistry. In a recent paper published in Nature, Gaunt and co-workers disclose a practical strategy for the construction of synthetically intractable tertiary amines through visible-light-mediated radical carbonyl alkylative amination of aldehydes, amines, and alkyl halides.

Radical addition-transfer of xanthates to 3,3,3-trifluoro-1-propene. A versatile and inexpensive access to trifluoromethyl substituted building blocks

The controlled intermolecular addition of a series of xanthates to 3,3,3-trifluoro-1-propene represents a convenient, versatile, and inexpensive route to highly functional organofluorine derivatives. The initial radical adducts can be further transformed into original trifluoromethyl substituted building blocks and relevant chemical intermediates.

First Zinc Bromide Promoted Annulative Domino Reactions between Enamines and Cyclic Morita–Baylis–Hillman Alcohols: Synthesis of N,O-Ketals

A new efficient ZnBr2-mediated annulative domino reaction between enamines and cyclic Morita–Baylis–Hillman (MBH) alcohols is disclosed. The process involves a tandem sequence (intermolecular conjugate addition of enamines to MBH alcohols and intramolecular nucleophilic addition of the hydroxyl moiety to the transiently generated iminium ion), affording the corresponding N,O-ketals diastereoselectively in good yields.

C-H Sulfonylation via 1,3-Rearrangement of Sulfonyl Group in N-Protected 3-Bis-sulfonimidoindole Derivatives Using Fluorine Reagent.

A C-H sulfonylation of N-protected 3-bis-sulfonimidoindole derivatives via a 1,3-rearrangement of a sulfonyl group on a bis-sulfonimide moiety using tetra-n-butylammonium fluoride (TBAF) was developed to provide 2-sulfonyl-3-sulfonamidoindole derivatives in high yields as a novel Csp2-H functionalization of heterocycles. The rearrangement of the sulfonyl group proceeded through an intermolecular addition of the desorbed sulfinyl ion from the bis-sulfonimide moiety in the substrate.

Synthesis of Eight‐Membered Nitrogen Heterocycles via a Heterogeneous PtI2‐Catalyzed Cascade Cycloaddition Reaction of δ‐Aminoalkynes with Electron‐Deficient Alkynes

AbstractA novel heterogeneous PtI2‐catalyzed cascade reaction of δ‐aminoalkynes was developed for the synthesis of various eight‐membered nitrogen heterocycles in excellent yields. The reaction proceeds via a hydration of δ‐aminoalkynes and subsequent intramolecular cyclization and intermolecular addition as well as ring‐expansion cascade reaction with another electron‐deficient alkynes. This method has the advantages of simple operation and mild reaction conditions. And the simple PtI2 with no any supports could be readily recycled through simple centrifugation without substantial loss of activity in 1,2‐dichloroethane (DCE). The recyclability of PtI2 may be ascribed to its insolubility in DCE. The reaction constitutes a formal [6+2]‐cycloaddition.magnified image

Oxidative Addition of Alkenyl and Alkynyl Iodides to a AuI Complex

AbstractThe first isolated examples of intermolecular oxidative addition of alkenyl and alkynyl iodides to AuI are reported. Using a 5,5′‐difluoro‐2,2′‐bipyridyl ligated complex, oxidative addition of geometrically defined alkenyl iodides occurs readily, reversibly and stereospecifically to give alkenyl‐AuIII complexes. Conversely, reversible alkynyl iodide oxidative addition generates bimetallic complexes containing both AuIII and AuI centers. Stoichiometric studies show that both new initiation modes can form the basis for the development of C−C bond forming cross‐couplings.

Oxidative Addition of Alkenyl and Alkynyl Iodides to a AuI Complex.

The first isolated examples of intermolecular oxidative addition of alkenyl and alkynyl iodides to AuI are reported. Using a 5,5'-difluoro-2,2'-bipyridyl ligated complex, oxidative addition of geometrically defined alkenyl iodides occurs readily, reversibly and stereospecifically to give alkenyl-AuIII complexes. Conversely, reversible alkynyl iodide oxidative addition generates bimetallic complexes containing both AuIII and AuI centers. Stoichiometric studies show that both new initiation modes can form the basis for the development of C-C bond forming cross-couplings.

Intermolecular Addition of Carbon-Centered Radicals to Ynamides. A Regio- and Stereoselective Route to Persubstituted α-Iodo-enamides.

Rather surprisingly, C-C bond formation through "intermolecular" radical addition to internal ynamides has never been reported. Actually, ynamides are excellent acceptors for "electrophilic" carbon-centered radicals. These processes enable the introduction of functionalized alkyl chains at Cβ, groups that have not yet been introduced via the addition of organometallics. Radical carboiodination affords persubstituted α-iodo-enamides in moderate to high yield. The addition is totally stereoselective. Theoretical support to the mechanism and the scope and limitation of the reaction are discussed.

Enantioselective Construction of Quaternary Stereogenic Centers by the Addition of an Acyl Anion Equivalent to 1,3-Dienes.

We report the enantioselective formation of quaternary stereogenic centers by the intermolecular addition of malononitrile, an acyl anion equivalent, and related pronucleophiles to several 1,3-disubstituted acyclic 1,3-dienes in the presence of a Pd-PHOX catalyst. Products are obtained in up to 88% yield and 99:1 er and in most cases are formed as a single regioisomer. The products' malononitrile unit undergoes oxidative functionalization to afford β,γ-unsaturated carbonyls bearing internal olefins and α-quaternary stereogenic centers.