Palladium-catalyzed cycloreductions of haloene-ynes in the presence of triethylsilane

Haloene-ynes 1 with a palladium catalyst would form the alkenylpalladium intermediates via intramolecular Heck reactions, which were reductively cleaved with triethylsilane or formate ligand to give the cycloreduction products 3 in good to excellent yields.     

A new route to 2-C- and 4-C-branched sugars by palladium-indium bromide-mediated carbonyl allylation

Palladium-catalysed carbonyl allylations can be effectively applied to the regio- and diastereoselective synthesis of 2-C- and 4-C-branched sugars from allylic esters or carbonates via the formation of pi-allylpalladium(II) intermediates and their reductive transmetalation with indium(I) bromide.     

Crystalline Organomercuric Acetates Via Organoboranes

Organomercury derivatives have found widespread use as intermediates in organic syntheses.^1-4 The organomercuric halides have been used most extensively due to the ease with which they are prepared, isolated, and manipulated. ^5-8 The organomercuric acetates would be expected to be more reactive than the corresponding halide derivatives⁹ but they have not been utilized extensively because, with the exception of the aryl derivatives, they are not readily available. Arylmercuric acetates can be prepared via electrophilic aromatic substitution reactions^1O,l1 but the preparation of vinyl and alkylmercuric acetates is not as straightforward. The few known vinylmercuric acetates were prepared via transmetallation reactions¹² or destructive reactions of divinylmercury reagents.⁹ Except in rare instances, ^l3,14 the alkylmercuric acetates have not been isolated but have served as intermediates in the synthesis of alkylmercuric halides.⁷     

Palladium-catalyzed coupling of ammonia and lithium amide with aryl halides

A mild, palladium-catalyzed coupling of aryl halides with ammonia or lithium amide to form primary arylamines as the major product is described. These reactions occurred with excellent selectivity for formation of the primary arylamine over formation of the diarylamine (9.5:1 to over 50:1 ratios of arylamine to diarylamine). In addition, the first organopalladium complex with a terminal -NH2 ligand has been isolated. This complex reductively eliminates to form arylamines.     

Facile reductive coupling of benzylic halides with ferrous oxalate dihydrate

Facile reductive coupling of benzylic halides is reported with ferrous oxalate dihydrate in DMF or HMPA under nitrogen atmosphere at 155-160 degrees C. The coupling is proposed to proceed by two successive oxidative additions of benzylic halides to ferrous oxalate to give an intermediate organoiron complex which undergoes concerted dimerization to give the corresponding reductively coupled dimers in high yields.     

Ruthenium‐catalyzed formal alkyl group transfer: Synthesis of quinolines from nitroarenes and alkylammonium halides

Nitroarenes are reductively cyclized with an array of tetraalkylammonium halides and trialkylarnmonium chlorides in the presence of a catalytic amount of a ruthenium catalyst along with tin(II) chloride dihydrate at 180° to afford the corresponding quinolines in moderate to good yields. The addition of tin(II) chloride dihydrate is necessary for the effective formation of quinolines and toluene is the solvent of choice. A reaction pathway involving initial reduction of nitroarenes to anilines and conversion of alkylammonium halides to alkylamines, alkyl group transfer from alkylamines to anilines to form an imine, dimerization of imine, and heteroannulation is proposed for this catalytic process.     

Mechanism of carbon-halogen bond reductive cleavage in activated alkyl halide initiators relevant to living radical polymerization: theoretical and experimental study

The mechanism of reductive cleavage of model alkyl halides (methyl 2-bromoisobutyrate, methyl 2-bromopropionate, and 1-bromo-1-chloroethane), used as initiators in living radical polymerization (LRP), has been investigated in acetonitrile using both experimental and computational methods. Both theoretical and experimental investigations have revealed that dissociative electron transfer to these alkyl halides proceeds exclusively via a concerted rather than stepwise manner. The reductive cleavage of all three alkyl halides requires a substantial activation barrier stemming mainly from the breaking C-X bond. The activation step during single electron transfer LRP (SET-LRP) was originally proposed to proceed via formation and decomposition of RX(?-) through an outer sphere electron transfer (OSET) process (Guliashvili, T.; Percec, V. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1607). These radical anion intermediates were proposed to decompose via heterolytic rather than homolytic C-X bond dissociation. Here it is presented that injection of one electron into RX produces only a weakly associated charge-induced donor-acceptor type radical anion complex without any significant covalent σ type bond character between carbon-centered radical and associated anion leaving group. Therefore, neither homolytic nor heterolytic bond dissociation applies to the reductive cleavage of C-X in these alkyl halides inasmuch as a true radical anion does not form in the process. In addition, the whole mechanism of SET-LRP has to be revisited since it is based on presumed OSET involving intermediate RX(?-), which is shown here to be nonexistent.     

Ruthenium-catalyzed transformation of aryl and alkenyl triflates to halides

Aryl triflates were transformed to aryl bromides/iodides simply by treating them with LiBr/NaI and [Cp*Ru(MeCN)(3)]OTf. The ruthenium complex also catalyzed the transformation of alkenyl sulfonates and phosphates to alkenyl halides under mild conditions. Aryl and alkenyl triflates undergo oxidative addition to a ruthenium(II) complex to form η(1)-arylruthenium and 1-ruthenacyclopropene intermediates, respectively, which are transformed to the corresponding halides.     

Expedient synthesis of novel β-ketoesters from the Mizoroki–Heck coupling of ethyl 3-ethoxyacrylate with aryl and pyridyl halides

It is well known that β-ketoesters are useful intermediates for the synthesis of a range of heterocyclic templates. While there are many useful synthetic methods available to access these intermediates, there are still opportunities for the discovery of useful methodologies for their construction from novel starting materials. In this regard, we report on the discovery of a facile Pd-catalyzed Mizoroki–Heck coupling of ethyl 3-ethoxyacrylate with aryl and heteroaryl halides to form substituted alkoxyacrylates which can be hydrolyzed to form novel aryl and heteroaryl β-ketoesters.     

Alkali-Metal Aminotroponiminates: Selectivities and Equilibria in Reversible Radical Coupling of Delocalized π-Electron Systems

Aminotroponiminates (ATIs) have recently been shown to belong to the growing class of redox-active ligands. The choice of the metal center allowed to switch between reversible electron transfer (M=Rh) and reductively induced dimerization (M=Na). Here, we investigate if the reductively induced dimerization of ATIs is a more general phenomenon for their alkali-metal complexes. Lithium ATI complexes are shown to undergo reductively induced dimerizations, which are equilibrium reactions and chemically reversible. The choice of the metal center (Li vs. Na), the substitution pattern at the nitrogen atoms of the ATI ligands, and the solvent critically influence the regioselectivity and diastereoselectivity of the radical-dimerization reactions. Potassium ATIs are shown to be susceptible to side reactions, more specifically a reduction accompanied by hydrogen-atom transfer. Products and intermediates of the reductively induced dimerizations were characterized by techniques including NMR and EPR spectroscopy, cyclic voltammetry, DFT calculations, single-crystal X-ray diffraction, and mass spectrometry.     

Palladium-catalyzed heteroannulation of 1,3-dienes to form alpha-alkylidene-gamma-butyrolactones

alpha-Alkylidene-gamma-butyrolactones are readily prepared by the palladium-catalyzed heteroannulation of a variety of 1,3-dienes by alpha-iodo and alpha-bromo acrylic acids. The best results are obtained by employing a catalytic amount of the sterically hindered chelating alkyl phosphine D-t-BPF [(di-tert-butylphosphino)ferrocene]. In most cases, this process is highly regioselective. The reaction is believed to proceed via (1) oxidative addition of the vinylic halide to Pd(0), (2) organopalladium addition to the less hindered end of the 1,3-diene to form a pi-allylpalladium intermediate, and (3) nucleophilic displacement of the palladium by the carboxylate ion.     

Stereoselective Syntheses of (E)-7, (Z)-9 Dodecadien-1-Yl Acetate

Two convenient synthetic methods for (E)-7, (Z)-9 dodecadien-1-yl acetate, via palladium-copper catalysed réactions of acetylenic intermediates with vinylic halides, are presented.     

Pd-catalyzed Negishi coupling of pyrazole triflates with alkyl zinc halides

A mild, functional group tolerant palladium-catalyzed Negishi coupling of pyrazole triflates and nonaflates with alkyl, benzyl, and aryl zinc halides has been developed. It allows quick access to 3-substituted pyrazole analogs at late stage via common synthetic intermediates.     

The effect of 19-electron formation constants on the electrochemistry and electron transfer induced substitution reactions of cyclopentadienylmetal halide complexes

The reduction of cyclopentadienylmetal halide complexes is generally considered to involve addition of an electron to an orbital that is antibonding with respect to the metal-halide bond. Subsequent metal-halide bond cleavage yields the halide and an organometallic radical. At inert electrodes, this radical is reduced further to an 18-electron anion. This series of reactions constitutes a prototypical ECE mechanism. Chemical reduction can be used to divert the radical into other pathways such as electron transfer chain catalyzed substitution. Attempts to initiate such reductively induced substitution reactions of CpFe(CO)₂I and Cp′Mo(CO)₃I give very different results, suggesting that these very similar complexes are reduced via substantially different mechanisms. Very likely, the molybdenum complex reacts via a DISP mechanism instead of ECE. The difference in electrochemical reduction mechanism as well as the different reactivity toward reductively induced substitution are explained in terms of a difference in the formation constants of 19-electron intermediates.     

Nickel-catalyzed arylation of acrolein diethyl acetal: a substitute to the 1,4-addition of arylhalides to acrolein

[reaction: see text] In the presence of catalytic amount of NiBr(2) as catalyst precursor, organic halides are reductively coupled at 70 degrees C with acrolein diethyl acetal to give (Z)- and (E)-enolethers by allylic deplacement of an alkoxy group. Subsequent hydrolysis affords beta-arylated aldehydes.     

Alkylation of magnesium sulfinates: a direct transformation of functionalized aromatic/heteroaromatic halides into sulfones

[reaction: see text] Sulfinate alkylation is one of the conventional methods for sulfone synthesis. The alkylation of magnesium sulfinates, which are easily accessible via reactions of organomagnesium intermediates with sulfur dioxide, provides a convenient route for sulfone preparation. In this communication, we report a preliminary study of the alkylation of arylmagnesium sulfinates. An application of this reaction to directly transform functionalized aromatic/heteroaromatic halides into sulfones is also described.     

Palladium-catalyzed tandem cyclization/Suzuki coupling of 1,6-enynes

[reaction: see text] A Pd(0)-catalyzed 1,6-enyne cyclization-arylation cascade reaction was effected via pi-allylpalladium intermediate formation and subsequent Suzuki coupling to give cyclic products with stereodefined exocyclic double bonds.     

A Photochemical Organocatalytic Strategy for the α-Alkylation of Ketones by using Radicals

Reported herein is a visible-light-mediated radical approach to the α-alkylation of ketones. This method exploits the ability of a nucleophilic organocatalyst to generate radicals upon S_N2-based activation of alkyl halides and blue light irradiation. The resulting open-shell intermediates are then intercepted by weakly nucleophilic silyl enol ethers, which would be unable to directly attack the alkyl halides through a traditional two-electron path. The mild reaction conditions allowed functionalization of the α position of ketones with functional groups that are not compatible with classical anionic strategies. In addition, the redox-neutral nature of this process makes it compatible with a cinchona-based primary amine catalyst, which was used to develop a rare example of enantioselective organocatalytic radical α-alkylation of ketones.     

Palladium-catalyzed cascade cyclization-coupling reactions of 2-bromo-1,6-enynes with organoboronic acids

2-Bromo-1,6-enynes 5 with a palladium catalyst would form the alkenylpalladium intermediates via an intramolecular Heck reaction, which were cross-coupled with various organoboronic acids 8 to give the cyclization-coupling products 6 in synthetically valuable yields.     

Regioselective Synthesis of Polycyclic and Heptagon-embedded Aromatic Compounds through a Versatile π-Extension of Aryl Halides

A versatile π-extension reaction was developed based on the three-component cross-coupling of aryl halides, 2-haloarylcarboxylic acids, and norbornadiene. The transformation is driven by the direction and subsequent decarboxylation of the carboxyl group, while norbornadiene serves as an ortho-C−H activator and ethylene synthon via a retro-Diels–Alder reaction. Comprehensive DFT calculations were performed to account for the catalytic intermediates.