Electrophilic Process Research Articles

Vinylidene rearrangements of internal borylalkynes via 1,2-boryl migration.

Vinylidene rearrangement of alkynes is a well-established and powerful method for alkyne transformations, while use of borylalkynes has remained largely unexplored. This paper describes vinylidene rearrangements of internal borylalkynes using a cationic ruthenium complex. This rearrangement is applicable to alkynes with both tri-(B(pin), B(dan)) and tetracoordinate (B(mida)) boryl groups, and the reaction rate is dramatically affected by the Lewis acidity of the boryl group. Mechanistic study revealed that the rearrangement proceeds via 1,2-boryl migration regardless of the coordination number of the boron center. The migration mode was elucidated by theoretical calculations to indicate that the migration of the tricoordinate boryl groups is an electrophilic process in contrast to the previous vinylidene rearrangements of internal alkynes with two carbon substituents.

Copper-catalyzed ortho-Monofluorination of Aniline Derivatives with Selectfluor Directed by Picolinic Acid Amides

A novel and facile method for selective ortho-C―H monofluorination of aniline derivatives was achieved using Selectfluor/Cu(OAc)2/HOAc system. The reaction proceeded via electrophilic process with diverse substrates in acceptable moderate to high yields. The position selectivity was controlled by bidentate coordination of picolinic acid amides with copper.

Mechanism of Mechanochemical C-H Bond Activation in an Azobenzene Substrate by PdII Catalysts.

Mechanism of C-H bond activation by various PdII catalysts under milling conditions has been studied by in situ Raman spectroscopy. Common PdII precursors, that is PdCl2 , [Pd(OAc)2 ]3 , PdCl2 (MeCN)2 and [Pd(MeCN)4 ][BF4 ]2 , have been employed for the activation of one or two C-H bonds in an unsymmetrical azobenzene substrate. The C-H activation was achieved by all used PdII precursors and their reactivity increases in the order [Pd(OAc)2 ]3 <PdCl2 (MeCN)2 <PdCl2 <[Pd(MeCN)4 ][BF4 ]2 . In situ Raman monitoring in combination with stepwise ex situ NMR, IR and PXRD experiments has provided direct probing of the reaction mechanism and kinetics, and revealed how liquids of different acid-base properties and proticity as well as selected solids used as additives modify precursors or intermediates and their reactivity. Reaction intermediates that were isolated and structurally characterized agree with the observed species during reaction. In situ Raman spectroscopy has also enabled the derivation of reaction profiles suggesting an electrophilic process which proceeds via a coordination complex (adduct) undergoing deprotonation by a bound or an external base depending on the used PdII precursor. Slow step of the first palladation for two chloride precursors and [Pd(MeCN)4 ][BF4 ]2 is the C-H bond cleavage whereas palladation using [Pd(OAc)2 ]3 depends primarily on breaking of its trimeric structure by the azobenzene substrate and/or liquid additives.

The reaction of dimethyldioxirane with 1,3-cyclohexadiene and 1,3-cyclooctadiene: monoepoxidation kinetics and computational modeling

Abstract The reaction of dimethyldioxirane (1) with excess 1,3-cyclohexadiene (2a) and 1,3-cyclooctadiene (2b) in dried acetone yielded the corresponding monoepoxides in excellent yield. Second-order rate constants for monoepoxidation were determined using UV methodology. The k2 value for the monoepoxidation of 1,3-cyclohexadiene was found to be 1.14±0.06 m -1 s-1, whereas that for the monoepoxidation of 1,3-cycloctadiene was 0.31±0.03 m -1 s-1. Basic density functional calculations at the B3LYP/6-31G level were employed to model the monoepoxidations. As expected, the calculations were consistent with a concerted, electrophilic process with a spiro-transition state. As found for the epoxidation of simple alkenes, the calculated transition-state geometry showed a slight asynchronous tilt of the dioxirane plane relative to that of the remaining alkene portion of the diene and a slight tilt back from the face of the diene. Relative reactivities (relative k2 values) were determined using the difference in the calculated electronic activation energies and were consistent with the experimental relative k2 values without the need to correct for the medium (solvent). Reactivity differences for epoxidation can be quickly predicted by this approach as long as there are reasonable structural similarities between the substrates.

Bis-N-heterocyclic Carbene Palladium(IV) Tetrachloride Complexes: Synthesis, Reactivity, and Mechanisms of Direct Chlorinations and Oxidations of Organic Substrates

This Article describes the preparation and isolation of novel octahedral CH(2)-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula LPd(IV)Cl(4) [L = (NHC)CH(2)(NHC)] from LPd(II)Cl(2) and Cl(2). In intermolecular, nonchelation-controlled transformations LPd(IV)Cl(4) reacted with alkenes and alkynes to 1,2-dichlorination adducts. Aromatic, benzylic, and aliphatic C-H bonds were converted into C-Cl bonds. Detailed mechanistic investigations in the dichlorinations of alkenes were conducted on the 18VE Pd(IV) complex. Positive solvent effects as well as kinetic measurements probing the impact of cyclohexene and chloride concentrations on the rate of alkene chlorination support a Pd(IV)-Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl(+)-transfer from the pentacoordinated Pd(IV)-intermediate LPd(IV)Cl(3)(+) to olefins. 1-Hexene/3-hexene competition experiments rule out both the formation of π-complexes along the reaction coordinate as well as in situ generated Cl(2) from a reductive elimination process. Instead, a ligand-mediated direct Cl(+)-transfer from LPd(IV)Cl(3)(+) to the π-system is likely to occur. Similarly, C-H bond chlorinations proceed via an electrophilic process with in situ formed LPd(IV)Cl(3)(+). The presence of a large excess of added Cl(-) slows cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both Pd(IV)-Cl ionization and Cl(+)-transfer from LPd(IV)Cl(3)(+). (1)H NMR titrations, T1 relaxation time measurements, binding isotherms, and Job plot analysis point to the formation of a trifurcated Cl(-)···H-C bond in the NHC-ligand periphery as a supramolecular cause for the accelerated chemical events involving the metal center.

Mechanism of the gold-catalyzed cyclopropanation of alkenes with 1,6-enynes

The gold(I)-catalyzed intermolecular cyclopropanation of alkenes with 1,6-enynes is an electrophilic process, which is mechanistically related to the well-known Simmons–Smith reaction proceeding through zinc carbenoids. This cyclopropanation is stereospecific, even in cases where the reaction was found to proceed stepwise.

Probing the Electronic Effect of Carbon Nanotubes in Catalysis: NH3 Synthesis with Ru Nanoparticles

Carbon nanotubes (CNTs) have been shown to modify some properties of nanomaterials and to modify chemical reactions confined inside their channels, which are formed by curved graphene layers. Here we studied ammonia synthesis over Ru as a probe reaction to understand the effect of the electron structure of CNTs on the confined metal particles and their catalytic activity. The catalyst with Ru nanoparticles dispersed almost exclusively on the exterior nanotube surface exhibits a higher activity than the CNT-confined Ru, although both have a similar metal particle size. Characterization with TEM, N(2) physisorption, H(2) chemisorption, temperature-programmed reduction, CO adsorption microcalorimetry, and first-principles calculations suggests that the outside Ru exhibits a higher electron density than the inside Ru. As a result, the dissociative adsorption of N(2), which is an electrophilic process and the rate-determining step of ammonia synthesis, is more facile over the outside Ru than that over the inside one.

Iodination of Organic Compounds with Elemental Iodine in the Presence of Hydrogen Peroxide in Ionic Liquid Media

Iodo-transformations using the reagent system I2/H2O2 were studied in the water miscible ionic liquid (IL) 1-butyl-3-methyl imidazolium tetrafluoroborate (bmimBF4) and in water immiscible IL, 1-butyl-3-methyl imidazolium hexafluorophosphate (bmimPF6). Two different forms of H2O2 as mediators of iodination were investigated, namely 30% aq. H2O2 and urea-H2O2 (UHP) in solid form. The role of the oxidant during the course of a reaction could be distinguished based on the amount of reagent required for the most efficient transformation. Two types of iodo-functionalizations through an electrophilic process were observed depending on the structure of the substrates. Whereas ring iodination took place in the case of dimethoxy- and trimethoxy-benzenes, with arylalkyl ketones the alkyl group α to the carbonyl was regioselectively iodinated. The results were further evaluated in comparison with iodination using the reagent system I2/H2O2 in water as medium, and under solvent-free reaction conditions, in terms of efficiency, selectivity, mechanism, and the ‘green’ aspects. The reusability/recycling of water immiscible bmimPF6 was investigated for 1,3,5-trimethoxy benzene (1b), which required a 1/0.5/0.6 molar ratio of substrat/I2/oxidant, and for 1,2,3-trimethoxy benzene (1f), which required a 1/1/1 ratio for complete iodine introduction. In addition, the efficiency of iodination was tested by varying the substrates, and employing the recycled hydrophobic IL bmimPF6.

Solvent‐Free Iodination of Organic Molecules Using the I2/Urea—H2O2 Reagent System.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Solvent-free iodination of organic molecules using the I2/urea–H2O2reagent system

Introduction of iodine under solvent-free conditions into several aromatic compounds activated toward electrophilic functionalization was found to proceed efficiently using elemental iodine in the presence of a solid oxidizer, the urea-H(2)O(2) (UHP) adduct. Two types of iodo-functionalization through an electrophilic process were observed: iodination of an aromatic ring, and side-chain iodo-functionalization in the case of arylalkyl ketones. Two reaction routes were established based on the required substrate : iodine : oxidizer ratio for the most efficient iodo-transformation, and the role of UHP was elucidated in each route. The first, requiring a 1 : 0.5 : 0.6 stoichiometric ratio of substrate to iodine to UHP, followed the atom economy concept in regard to iodine and was valid in the case of aniline, 4-t-Bu-phenol, 1,2-dimethoxy benzene, 1,3-dimethoxy benzene, 1,2,3-trimethoxy benzene, 1,2,4-trimethoxy benzene, 1,3,5-trimethoxy benzene, 1-indanone and 1-tetralone. The second reaction route, where a 1 : 1 : 1 stoichiometric ratio of substrate : I(2) : UHP was needed for efficient iodination, was suitable for side-chain iodo-functionalization of acetophenone and methoxy-substituted acetophenones. Moreover, addition of iodine to 1-octene and some phenylacetylenic derivatives was found to proceed efficiently without the presence of any oxidizer and solvent at room temperature.

Stabilization mechanism of antimony mercaptides in poly(vinyl chloride): ab initio theory studies

Ab initio quantum theory has been used to study stabilizing mechanism of antimony mercaptides in poly(vinyl chloride) (PVC). Analysis of composition of the frontier orbitals, dipole moment of the systems, charge on Sb and its bonded atoms in antimony mercaptides, indicates that these compounds could absorb the HCl produced during the decomposition of PVC. The mechanism of stabilization is electrophilic followed by a nucleophilic step. Also they could substitute the Cl in PVC to prevent the production of HCl, thus preventing the decomposition of PVC. This mechanism is nucleophilic followed by an electrophilic process. The results from theoretical calculation match well with the test of the stabilizing effect on a PVC product containing these stabilizers. The structures of some organoantimony compounds are also reported.

NO(2)(+) nitration mechanism of aromatic compounds: electrophilic vs charge-transfer process

The nitration of methylnaphthalenes with NO(2)BF(4) and NOBF(4) was examined in order to shed light on the controversial aromatic nitration mechanism, electrophilic vs charge-transfer process. The NO(2)(+) nitration of 1,8-dimethylnaphthalene showed a drastic regioselectivity change depending on the reaction temperature, where ortho-regioselectivity at -78 degrees C and para-regioselectivity at 0 degrees C were considered to reflect the electrophilic and the direct or alternative charge-transfer process, respectively, because the NO(+) nitration through the same reaction intermediates as in the NO(2)(+) nitration via a charge-transfer process resulted in para-regioselectivity regardless of the reaction temperature. The NO(2)(+) nitration of redox potential methylnaphthalenes higher than 1,8-dimethylnaphthalene gave a similar ortho-regioselectivity enhancement to 1,8-dimethylnaphthalene at lower temperature, thus reflecting the electrophilic process. On the other hand, the NO(2)(+) nitration of redox potential methylnaphthalenes lower than 1,8-dimethylnaphthalene showed para-regioselectivity similar to the NO(+) nitration, indicating the direct or alternative charge-transfer process. In the presence of strong acids where the direct charge-transfer process will be suppressed by protonation, the ortho-regioselectivity enhancement was observed in the NO(2)(+) nitration of 1,8-dimethylnaphthalene, suggesting that the direct charge-transfer process could be the main process to show para-regioselectivity. These experimental results imply that the NO(2)(+) nitration proceeds via not only electrophilic but also direct charge-transfer processes, which has been considered to be unlikely because of the high energy demanding process of a bond coordination change between NO(2)(+) and NO(2). Theoretical studies at the MP2/6-31G(d) level predicted ortho- and para-regioselectivity for the NO(2)(+) nitration via electrophilic and charge-transfer processes, respectively, and the preference of the direct charge-transfer process over the alternative one, which support the experimental conclusion

Reaction of [Pd2Cl2(μ-dppm)2] with Arenesulfonyl Azides. Kinetics and Mechanism of Formation of Arenesulfonylimido-Bridged A-Frame Adducts

Substituted arenesulfonyl azides react with [Pd2Cl2(μ-dppm)2] in an entropy-controlled electrophilic process, affording an imido-bridged A-frame adduct via N2 extrusion. The Hammett relationship is valid for the 4-substituents.

Regiospecific Bromination of 3-Methylindoles with NBS and Its Application to the Concise Synthesis of Optically Active Unusual Tryptophans Present in Marine Cyclic Peptides(1).

A regiospecific bromination of substituted 3-methylindoles at either the C(3) alkyl moiety or the C(2) position was achieved via a free radical bromination or electrophilic process, respectively. The regiospecificity of the bromination could be controlled by variation of both the substituent and the N(1) protecting group on the indole ring. In addition, enantiospecific syntheses of 5-methoxytryptophan (20) and 5-hydroxy-6-chlorotryptophan (21c) as well as concise syntheses of optically active 2-bromotryptophan ethyl esters 26a,b or their substituted derivatives in three steps from bifunctional dibromoindoles were achieved via the above regiospecific process.

The hydration of mesitylketene in aqueous solution: detection of acid catalysis for an aromatic ketene

Mesitylketene was generated flash photolytically in aqueous solution by the photo-Wolff reaction of 2,4,6-trimethyldiazoacetophenone and also by rearrangement of mesitylynol obtained through photodecarbonylation of mesitylhydroxycyclopropenone, and rates of hydration of this ketene were measured in dilute perchloric acid, sodium perchlorate, and sodium hydroxide solutions as well as in concentrated sodium perchlorate and perchloric acid solutions. In dilute solution only an uncatalyzed reaction and a sodium-hydroxide-catalyzed process were observed, both of which could be attributed to nucleophilic attack, by water and by hydroxide ion, respectively, at the ketene carbonyl carbon atom. In concentrated sodium perchlorate solutions, a mild decrease in reaction rate with increasing salt concentration was observed, as expected on the basis of decreasing water activity and a consequent slowing of the uncatalyzed reaction. A similar mild decrease was found in perchloric acid solutions up to [Formula: see text] but this then gave way to a rate increase that became dominant above [Formula: see text] This appearance of acid catalysis indicates a change in reaction mechanism from nucleophilic attack of water to an electrophilic process involving rate-determining protonation on the β-carbon atom of the ketene group. Analysis of the acid-catalyzed reaction rate by the Cox–Yates method gives the catalytic coefficient [Formula: see text] This, when compared with [Formula: see text] for ketene itself, shows that the mesityl group retards acid-catalyzed hydration by a factor of 2200, and consequently the acid-catalyzed reaction of this, and other aromatic ketenes as well, becomes apparent only under strongly acidic conditions. Keywords: mesitylketene, ketene hydration, acid catalysis, Cox–Yates excess acidity correlation.

Chemically amplified resists based on polymer side‐chain rearrangement or electrophilic crosslinking

AbstractA variety of resist systems have been studied that owe their sensitivity to the occurrence of catalyzed electrophilic rearrangements or substitution processes. In most designs, polyfunctional or polymer‐bound benzylic ethers, esters, or alcohols are used in the key step of the reaction, which is initiated by protonation of the latent electrophilic moiety, resulting in formation of a carbocationic center. Thermal activation of the electrophilic process leads to the formation of new C–C bonds with liberation of a proton; therefore the overall process is catalytic and amounts to chemical amplification. A variety of polymers and active small molecules that provide access to both negative and positive tone images have been tested, and sensitivities as high as 0.1 mJ/cm2 have been measured. In addition, several image reversal schemes involving either selective silylation of non‐exposed areas or the in situ photogeneration of base have been tested and are presented.

Enantioselective electrophilic bond construction to the α-carbon of α-aminoacids

In this report, we describe three possibilities for aminoacid synthesis using an enantioselective electrophilic process. Thus, enantioselective carboxylation, alkylation and protonation of Schiff bases yield optically active aminoacids with e.e. up to 76%.

Oxidative and electrophilic pathways in the reactions of pentaaquo(organo)chromium(2+) ions with nitrous acid

The reactions of HONO with (H/sub 2/O)/sub 5/CrR/sup 2 +/ complexes occur with a 1:1 stoichiometry at rates ranging from 0.12 (R = CH/sub 2/Br) to 1010 M/sup -2/ s/sup -1/ (R = rho-CH/sub 2/C/sub 6/H/sub 4/CH/sub 3/). The chromium product is Cr(H/sub 2/O)/sub 6//sup 3 +/ for R = alkyl, haloalkyl, and aralkyl but the kinetically stable nitrosyl complex (H/sub 2/O)/sub 5/CrNO/sup 2 +/ for R = ..cap alpha..-hydroxy- and ..cap alpha..-alkoxyalkyl. All of the reactions are believed to proceed by a bimolecular reaction between the nitrosyl cation NO/sup +/ and CrR/sup 2 +/. In the case of the aralkyl complexes, the data are consistent with an electrophilic process; correlations with data in the literature support the assignment of an S/sub E/2 mechanisms. The other reactions are best interpreted in terms of electron transfer to form CrR/sup 3 +/. The fate of the latter depends on the nature of the group R. Intramolecular electron transfer for R = CH/sub 2/OH, CH/sub 2/OCH/sub 3/, and CH(C-H/sub 3/)OC/sub 2/H/sub 5/ results in Cr/sup 2 +/, which is scavenged by NO to give CrNO/sup 2 +/. The rate constant for the reaction between Cr/sup 2 +/ and NO has been independently evaluatedmore » as 2.2 x 10/sup 6/ M/sup -1/ s/sup -1/ by competition methods. All the other CrR/sup 3 +/ complexes decompose by homolysis. The free radical so formed reacts with NO to yield RNO. 57 references, 7 figures, 3 tables.« less

Bromination of octmethylcyclopentene - the irregular reactivity of a sterically hindered cycloalkene

The title compound 7 reacts with bromine under mild conditions (CCI., 20°C) to give the substitution products 9 and/or 10 via an electrophilic process. The tetrabromo compound 11 is formed from 7 and bromine in refluxing CCI, via a radical mechanism.

ChemInform Abstract: Formation of Thioamide Derivatives from Reactions of Isothiocyanates with Oxazol‐2‐amines.

AbstractDie Reaktion der Oxazole (III) (Darstellung siehe Formelschema) mit den Isothiocyanaten (V) führt nicht nur zu den erwarteten Thioharnstoff‐Derivaten (VI); in Abhängigkeit von den jeweiligen Substituenten werden auch größere Mengen der Thioamide (VII) erhalten [mit einem Überschuß (V) entstehen die disubstituierten Produkte (VIII) ‐ die analoge Verbindung wird auch bei äquimolarer Umsetzung zu 26% neben (VIc) und (VIIc) isoliert].