- Experimental Determination of Internal Energy Barriers in the Gas-Phase Aromatic Alkylation by Dimethylchloronium Ions
-
The temperature and pressure dependence of the substrate selectivity of the alkylation of mesitylene (M) and p-xylene (X) by radiolytically formed CH3ClCH3+ ions have been investigated in CH3Cl gas at pressure between 50 and 760 Torr in the range 40-140 deg C.The Arrhenius plot of the empirical kM/kX ratio measured at 760 Torr is linear over the entire temperature range investigated, and its slope corresponds to a difference of 2.2 +/- 0.2 kcal mol-1 between the activation energy for the CH3ClCH3+ methylation of the p-xylene and mesitylene.A pressure-dependence study of the same competition reactions carried out at 100 deg C points to 300 Torr as the pressure limit, below which the correspondence between the phenomenological Arrhenius-plot slope and the actual activation-barrier difference is not any longer warranted.This conclusion is further corroborated by a comparison of the present results with those derived for the same reactions from reactant ion monitoring (RIM) "high-pressure" mass spectrometry at 0.5-1.2 Torr.The large discrepancy observed is interpreted as evidence that above 300 Torr the activation mechanism of the CH3ClCH3+ methylation of arenes, a typical ion-molecule process, is essentially thermal and that, below this limit, coexistence of both thermal and electrostatic activation mechanisms as well as incomplete equilibration of the internal energy of the reactants make Arrhenius plots hardly a measure of the activation barriers involved in the gas-phase aromatic alkylations.
- Speranza, Maurizio,Laguzzi, Guiseppe
-
-
Read Online
- Coupling Photocatalysis and Substitution Chemistry to Expand and Normalize Redox-Active Halides
-
Photocatalysis can generate radicals in a controlled fashion and has become an important synthetic strategy. However, limitations due to the reducibility of alkyl halides prevent their broader implementation. Herein we explore the use of nucleophiles that can substitute the halide and serve as an electron capture motif that normalize the variable redox potentials across substrates. When used with photocatalysis, bench-stable, commercially available collidinium salts prove to be excellent radical precursors with a broad scope.
- Rathnayake, Manjula D.,Weaver, Jimmie D.
-
p. 2036 - 2041
(2021/04/05)
-
- The Selective Conversion of Methyl and Ethyl Acetate to High Content Alkyl Aromatic Hydrocarbons over H-ZSM5
-
Abstract: This research is devoted to a catalytic process using the H-ZSM5 catalyst for the conversion of methyl and ethyl acetate to hydrocarbon aromatics. These reactions are carried out in a fixed bed reactor under atmospheric pressure at 370°C. The distribution of products was measured by GC-Mass spectrometer. The variation of weight hourly space velocity (WHSV) on the conversion of these esters to aromatic hydrocarbons showed a significant effect on carbon distribution. The deactivation catalyst by time was monitored using product selectivity and conversion. The production of alkyl and poly alkyl aromatic compounds was formed under controlled conditions. The advantages of these methods are the formation of a higher concentration of octane number booster poly alkyl aromatic compounds (mono-aromatics) from esters as starting materials. Moreover, the catalyst lifetime on stream was investigated and exhibited longer catalyst lifetime for ethyl acetate conversion than methyl acetate.
- Dabbagh, H. A.,Hassanpour, J.
-
p. 1284 - 1290
(2020/12/02)
-
- Decyanation method of nitrile organic compound
-
The invention provides a decyanation method of a nitrile organic compound. The nitrile organic compound shown in a general formula (1), a sodium reagent, crown ether and a proton donor are subjected to decyanation reaction in an organic solvent I to generate an organic compound shown in a general formula (2). According to the method, a Na/15-crown-5/H2O system is adopted, so that nitrile organic matters can be converted into a decyanation product, and the generation of amine byproducts is inhibited. The new method does not need to use liquid ammonia as a solvent, and is safer and more convenient to operate. The required sodium dispersoid is low in price; and the 15-crown-5 can be recycled and repeatedly used. The method has the advantages of good chemical selectivity, wide substrate application range, good functional group compatibility and the like.
- -
-
Paragraph 0051-0054
(2020/02/10)
-
- Reductive Cleavage of Unactivated Carbon-Cyano Bonds under Ammonia-Free Birch Conditions
-
A general protocol for the reductive cleavage of unactivated carbon-cyano bonds in aliphatic nitriles has been achieved under single-electron-transfer conditions using Na/15-crown-5/H2O. Electron is supplied by the electride derived from bench-stable sodium dispersions and recoverable 15-crown-5. H2O provides the proton source and suppresses the reduction of aromatic moieties. Compared with the Na/NH3 electride system generated under traditional Birch conditions, this ammonia-free electride system is more practical and features better reactivity and chemoselectivity for the decyanations of a broad range of aliphatic nitriles.
- An, Jie,Ding, Yuxuan,Luo, Shihui,Ma, Lifu
-
-
- Selective Hydrogenations and Dechlorinations in Water Mediated by Anionic Surfactant-Stabilized Pd Nanoparticles
-
We report a facile, inexpensive, and green method for the preparation of Pd nanoparticles in aqueous medium stabilized by anionic sulfonated surfactants sodium 1-dodecanesulfonate 1a, sodium dodecylbenzenesulfonate 1b, dioctyl sulfosuccinate sodium salt 1c, and poly(ethylene glycol) 4-nonylphenyl-3-sulfopropyl ether potassium salt 1d simply obtained by stirring aqueous solutions of Pd(OAc)2 with the commercial anionic surfactants further treated under hydrogen atmosphere for variable amounts of time. The aqueous Pd nanoparticle solutions were tested in the selective hydrogenation reactions of aryl-alcohols, -aldehydes, and -ketones, leading to complete conversion to the deoxygenated products even in the absence of strong Br?nsted acids in the reduction of aromatic aldehydes and ketones, in the controlled semihydrogenation of alkynes leading to alkenes, and in the efficient hydrodechlorination of aromatic substrates. In all cases, the micellar media were crucial for stabilizing the metal nanoparticles, dissolving substrates, steering product selectivity, and enabling recycling. What is interesting is also that a benchmark catalyst like Pd/C can often be surpassed in activity and/or selectivity in the reactions tested by simply switching to the appropriate commercially available surfactant, thereby providing an easy to use, flexible, and practical catalytic system capable of efficiently addressing a variety of synthetically significant hydrogenation reactions.
- La Sorella, Giorgio,Sperni, Laura,Canton, Patrizia,Coletti, Lisa,Fabris, Fabrizio,Strukul, Giorgio,Scarso, Alessandro
-
supporting information
p. 7438 - 7446
(2018/05/29)
-
- Trimethylphosphate as a Methylating Agent for Cross Coupling: A Slow-Release Mechanism for the Methylation of Arylboronic Esters
-
A methyl group on an arene, despite its small size, can have a profound influence on biologically active molecules. Typical methods to form a methylarene involve strong nucleophiles or strong and often toxic electrophiles. We report a strategy for a new, highly efficient, copper and iodide co-catalyzed methylation of aryl- and heteroarylboronic esters with the mild, nontoxic reagent trimethylphosphate, which has not been used previously in coupling reactions. We show that it reacts in all cases tested in yields that are higher than those of analogous copper-catalyzed reactions of MeOTs or MeI. The combination of C-H borylation and this methylation with trimethylphosphate provides a new approach to the functionalization of inert C-H bonds and is illustrated by late-stage methylation of four medicinally active compounds. In addition, reaction on a 200 mmol scale demonstrates reliability of this method. Mechanistic studies show that the reaction occurs by a slow release of methyl iodide by reaction of PO(OMe)3 with iodide catalyst, rather than the typical direct oxidative addition to a metal center. The low concentration of the reactive electrophile enables selective reaction with an arylcopper intermediate, rather than nucleophilic groups on the arylboronate, and binding of tert-butoxide to the boronate inhibits reaction of the electrophile with the tert-butoxide activator to form methyl ether.
- He, Zhi-Tao,Li, Haoquan,Haydl, Alexander M.,Whiteker, Gregory T.,Hartwig, John F.
-
supporting information
p. 17197 - 17202
(2018/12/14)
-
- Reaction routes in catalytic reforming of poly(3-hydroxybutyrate) into renewable hydrocarbon oil
-
Poly(3-hydroxybutyrate) or PHB is an energy storage material of microbial organisms and can be reformed into hydrocarbon oils rich with aromatic compounds. This work investigated the main reaction routes from PHB to the key intermediates and final hydrocarbons. The main sequential reactions under catalysis of phosphoric acid at moderate temperatures (200-230 °C) consist of: (1) decomposition of PHB into crotonic acid, a major monomeric intermediate, (2) deoxygenation of crotonic acid, and (3) combination of the deoxygenated molecules. The oxygen in PHB is removed as CO2 and H2O in stage (2), involving decarboxylation and ketonization of crotonic acid. The main aromatic compounds are formed in stage (3) from propylene and 2,3-dimethyl-2-cyclopenten-1-one as two key intermediates, the former from decarboxylation and the latter from ketonization of crotonic acid. The reaction routes reveal that the formation of aromatics is affected to a great extent by the concentrations of phosphoric acid and water in the reaction, which can be used to control the composition of hydrocarbon oil.
- Kang, Shimin,Yu, Jian
-
p. 30005 - 30013
(2015/05/13)
-
- Arene oxidation with malonoyl peroxides
-
Malonoyl peroxide 7, prepared in a single step from the commercially available diacid, is an effective reagent for the oxidation of aromatics. Reaction of an arene with peroxide 7 at room temperature leads to the corresponding protected phenol which can be unmasked by aminolysis. An ionic mechanism consistent with the experimental findings and supported by isotopic labeling, Hammett analysis, EPR investigations, and reactivity profile studies is proposed.
- Dragan, Andrei,Kubczyk, Tomasz M.,Rowley, Julian H.,Sproules, Stephen,Tomkinson, Nicholas C. O.
-
supporting information
p. 2618 - 2621
(2015/06/16)
-
- Methylbenzene hydrocarbon pool in methanol-to-olefins conversion over zeolite H-ZSM-5
-
The formation and reactivity of a methylbenzenes (MBs) hydrocarbon pool in the induction period of the methanol-to-olefins (MTO) reaction over zeolite H-ZSM-5 was investigated and the mechanistic link of MBs to ethene and propene was revealed. Time evolution analysis of the formed MBs and 12C/13C methanol-switching experiments indicate that in the induction period bulkier compounds such as tetraMB and pentaMB have higher reactivity than their lighter counterparts such as p/m-diMB and triMB. By correlating the distribution of MBs trapped on H-ZSM-5 with ethene and propene, we found that tetraMB and pentaMB favor the formation of propene, while p/m-diMB and triMB mainly contribute to the formation of ethene. On the basis of this relationship, the olefin (ethene and propene) selectivity can be controlled by regulating the distribution of trapped MBs by varying the silicon-to-aluminum ratio of ZSM-5, reaction temperature, and space velocity. The reactivity of MBs and the correlation of MBs with olefins were also verified under steady-state conditions. By observation of key cyclopentenyl and pentamethylbenzenium cation intermediates using in situ solid-state NMR spectroscopy, a paring mechanism was proposed to link MBs with ethene and propene. P/M-diMB and triMB produce ethylcyclopentenyl cations followed by splitting off of ethene, while tetraMB and pentaMB generate propyl-attached intermediates, which eventually produce propene. This work provides new insight into the MBs hydrocarbon pool in MTO chemistry.
- Wang, Chao,Xu, Jun,Qi, Guodong,Gong, Yanjun,Wang, Weiyu,Gao, Pan,Wang, Qiang,Feng, Ningdong,Liu, Xiaolong,Deng, Feng
-
p. 127 - 137
(2015/11/09)
-
- Pentafluorophenyl Silver: Structure and Bonding of Arene Solvates
-
The synthesis and full characterization of a series of AgC6F5·arene complexes (arene = RC6H5 with R = Me, Et; R2C6H4, R3C6H3, R4C6H2, and R5C6H with R = Me) are presented. The structure and bonding are discussed on the basis of X-ray and theoretical studies with respect to the substitution pattern at the arene. Additionally, the structure of neat AgC6F5 and a facile way to generate crystals of pure AgC6F5 are reported. (Figure Presented).
- Ibad, Muhammad Farooq,Schulz, Axel,Villinger, Alexander
-
supporting information
p. 3893 - 3901
(2015/08/19)
-
- One-pot production of hydrocarbon oil from poly(3-hydroxybutyrate)
-
Poly(3-hydroxybutyrate) (PHB) is an energy storage material of many microbial species, and has been found to be an effective feedstock for production of renewable hydrocarbon oils. A high oil yield (up to 38.2 wt%) was obtained in a phosphoric acid (H3PO4) solution at mild temperatures (165-240 °C). PHB and crotonic acid (C4H 6O2), a dominant thermal degradation product of PHB, were deoxygenated mainly via decarboxylation, generating similar liquid and gaseous products. Carbon dioxide and propylene were the major products in gas phase with little CO formation. The hydrocarbon oil (C4-C16) is a mixture of alkanes, alkenes, benzenes and naphthalenes. Aromatics (C10-C15) were the major hydrocarbons in a 100 wt% H3PO4 solution, while alkenes and alkanes (C4-C9) were favored in diluted solutions (50 wt% to 85 wt% H 3PO4). The concentration of H3PO4 was a key factor that affected the oil composition and yield. A highly efficient decarboxylation of crotonic acid at 220 °C for 3 hours resulted in 70.8 wt% of oxygen being removed as CO2 and 57.0 wt% of carbon being recovered as hydrocarbon oil. The H3PO4 solution can be repeatedly used for high yield oil production. This work shows that a type of new biological feedstock can be used to produce renewable hydrocarbon oil in an efficient one-pot reaction. This journal is the Partner Organisations 2014.
- Kang, Shimin,Yu, Jian
-
p. 14320 - 14327
(2014/04/17)
-
- Cross-coupling reaction with lithium methyltriolborate
-
We newly developed lithium methyltriolborate as an air-stable white solid that is convenient to handle. The good performance of this triolborate for metal-catalyzed bond-forming reactions was demonstrated in palladium-catalyzed cross-coupling reactions wi
- Yamamoto, Yasunori,Ikizakura, Kazuya,Ito, Hajime,Miyaura, Norio
-
p. 430 - 439
(2013/03/13)
-
- Acid-, water- and high-temperature-stable ruthenium complexes for the total catalytic deoxygenation of glycerol to propane
-
The ruthenium aqua complexes [Ru(H2O)2(bipy) 2](OTf)2, [cisRu(6,6′-Cl2-bipy) 2(OH2O)2)] (OTf)2, [Ru(H 2O)2(phen)2](OTf)2, [Ru(H 2O)3(2,2′:6′,2″-terpy)](OTf) 2 and [Ru(H2O)3(Phterpy)] (OTf)2 (bipy = 2,2′-bipyridine; OTf- = triflate; phen = phenanthroline; terpy = terpyridine; Phterpy = 4′-phenyl-2,2′: 6′,2″-terpyridine) are water- and acid-stable catalysts for the hydrogenation of aldehydes and ketones in sulfolane solution. In the presence of HOS(O)2CF3 (triflic acid) as a dehydration co-catalyst they directly convert 1,2-hexanediol to n-hexanol and hexane. The terpyridine complexes are stable and active as catalysts at temperatures ≥ 250°C and in either aqueous sulfolane solution or pure water convert glycerol into n-propanol and ultimately propane as the final reaction product in up to quantitative yield. For the terpy complexes the active catalyst is postulated to be a carbonyl species [(4′-R-2,2′:6′,2″-terpy)Ru(CO) (H2O)2]-(OTf)2 (R = H, Ph) formed by the decarbonylation of aldehydes (hexanal for 1,2-hexanediol and 3-hydroxypropanal for glycerol) generated in the reaction mixture through acid-catalyzed dehydration. The structure of the dimeric complex [{(4′-phenyl-2,2′: 6′,2″-terpy)Ru(CO)}2(μ-OCH3) 2](OTf)2 has been determined by single crystal X-ray crystallography (Space group P1 (a = 8.2532(17); b = 12.858(3); C = 14.363(3) A; α = 64.38(3); β = 77.26(3); γ = 87.12(3)°, R = 4.36%).
- Taher, Deeb,Thibault, Michelle E.,Di Mondo, Domenico,Jennings, Michael,Schlaf, Marcel
-
supporting information; experimental part
p. 10132 - 10143
(2010/04/03)
-
- Accurate oxidation potentials of benzene and biphenyl derivatives via electron-transfer equilibria and transient kinetics
-
(Graph Presented) Nanosecond transient absorption methods were used to determine accurate oxidation potentials (Eox) in acetonitrile for benzene and a number of its alkyl-substituted derivatives. Eox values were obtained from a combination of equilibrium electron-transfer measurements and electron-transfer kinetics of radical cations produced from pairs of benzene and biphenyl derivatives, with one member of the pair acting as a reference. Using a redox-ladder approach, thermodynamic oxidation potentials were determined for 21 benzene and biphenyl derivatives. Of particular interest, Eox values of 2.48 ± 0.03 and 2.26 ± 0.02 V vs SCE were obtained for benzene and toluene, respectively. Because of a significant increase in solvent stabilization of the radical cations with decreasing alkyl substitution, the difference between ionization and oxidation potentials of benzene is ～0.5 eV larger than that of hexamethylbenzene. Oxidation potentials of the biphenyl derivatives show an excellent correlation with substituent σ+ values, which allows Eox predictions for other biphenyl derivatives. Significant dimer radical cation formation was observed in several cases and equilibrium constants for dimerization were determined. Methodologies are described for determining accurate electrontransfer equilibrium constants even when dimer radical cations are formed. Additional equilibrium measurements in trifluoroacetic acid, methylene chloride, and ethyl acetate demonstrated that solvation differences can substantially alter and even reverse relative Eox values.
- Merkel, Paul B.,Luo, Pu,Dinnocenzo, Joseph P.,Farid, Samir
-
experimental part
p. 5163 - 5173
(2009/12/06)
-
- PROCESS FOR THE PRODUCTION OF A HYDROCARBON
-
A process for the production of a hydrocarbon which comprises contacting, in a reactor, methanol and/or dimethyl ether with a catalyst comprising a metal halide, such as a zinc halide, in which the methanol and/or dimethyl ether is contacted with the catalyst in the presence of at least one phosphorus compound having at least one P-H bond.
- -
-
Page/Page column 19-20; 25-26
(2008/06/13)
-
- Process for the production of mesitylene and durene
-
A process is described for the contemporaneous preparation of mesitylene and durene, characterized in that mesitylene and durene are obtained exclusively starting from pseudo-cumene without the use of any further chemical compound, operating in continuous, at a temperature ranging from 210 to 400° C., at a pressure ranging from 1 to 50 bar, with a weight space velocity ranging from 0.1 to 20 hours?1 and in the presence of a catalyst based on crystalline metal-silicates in acid form. After the recovery of mesitylene and durene from the reaction raw product, some of the remaining components of the raw product itself are recycled and fed to the reactor together with the pseudo-cumene.
- -
-
-
- Process for the production of mesitylene and durene
-
A process is described for the contemporaneous preparation of mesitylene and durene, characterized in that mesitylene and durene are obtained exclusively starting from pseudo-cumene without the use of any further chemical compound, operating in continuous, at a temperature ranging from 210 to 400°C, at a pressure ranging from 1 to 50 bar, with a weight space velocity ranging from 0.1 to 20 hours-1 and in the presence of a catalyst based on crystalline metal-silicates in acid form. After the recovery of mesitylene and durene from the reaction raw product, some of the remaining components of the raw product itself are recycled and fed to the reactor together with the pseudo-cumene.
- -
-
-
- 1,4 Additions with lithium bis(methylenecyclopropyl)cuprate
-
Addition of lithium bis(methylenecyclopropyl) cuprate to α,β-unsaturated ketones provides an efficient route to methylenecyclopropyl ketones which on treatment with TiCl4 give a range of cyclised products.
- Peron, Guillaume,Norton, David,Kitteringham, John,Kilburn, Jeremy D.
-
p. 347 - 349
(2007/10/03)
-
- Flash photolysis study of a Friedel-Crafts alkylation. Reaction of the photogenerated 9-fluorenyl cation with aromatic compounds
-
A combination of flash photolysis and product analysis is employed to investigate the reaction of aromatic compounds (ArH) with the 9-fluorenyl cation (Fl+) photogenerated from 9-fluorenol in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP).The availability of the photochemical route to Fl+ means that the reaction of benzylic-type cation with ArH can be directly followed by flash photolysis.An additional feature with electron-rich ArH is that the cyclohexadienyl cation is observed to grow as Fl+ decays.Thus both cationic intermediates of a Friedel-Crafts alkylation are observed in the same experiment.The formation of the cyclohexadienyl cation is demonstrated to be reversible, or at least quasi-reversible, with the kinetic analysis furnishing absolute rate constants for the formation of this cation as well as for its loss of H+ and Fl+.Values of kH:kD for benzene:benzene and toluene:toluene are ca. 1.5 and demonstrate that Fl+ addition is at least partly reversible with these compounds as well.The Hammett ρ+ value obtained for a series of the less electron-rich ArH is -8, indicative of a transition state with considerable cyclohexadienyl cation character.Anisole shows a negative deviation from from Hammett correlation line, explained by the addition of Fl+ to ArH becoming encounter-controlled.This behaviour is dramatically illustrated in a comparison of data for Fl+ and Br2.For the less electron-rich ArH, rate constants for the two electrophiles are parallel.However, from m-xylene through pentamethylbenzene, the rate with Fl+ is unchanged, while the rate with Br2 increases over 1000-fold.The concept of encounter control with Fl+ is strongly supported by the absolute rate constants, which for the electron-rich ArH are all in the range 1-2 E9 dm3 mol-1 s-1, a magnitude typical of diffusion-controlled reactions.The electron-rich ArH also show no intermolecular selectivity since their reactions are encounter-controlled, but have a high intramolecular selectivity.It is suggested that a factor influencing the latter is the reversibility of formation of the cyclohexadienyl cation from the encounter complex.
- McClelland, Robert A.,Cozens, Frances L.,Li, Jianhui,Steenken, Steen
-
p. 1531 - 1544
(2007/10/03)
-
- The Effect of Spiltover Hydrogen on the Stabilization of Catalytic Activities of Y-Type Zeolite and Pillared Montmorillonite for the Disproportionation of 1,2,4-Trimethylbenzene
-
Disproportionation of 1,2,4-trimethylbenzene was carried out at 200 deg C to investigate the effect of hydrogen spillover on the stabilization of the catalytic activities of ultrastable Y (USY) zeolite and pillared montmorillonite.The catalytic activity of pillared montmorillonite, the acidity of which is mainly due to Lewis acid sites, became stable against deactivation in the presence of spiltover hydrogen.There was a less pronounced effect of spiltover hydrogen on the deactivation of USY catalyst treated at 400 deg C.When the surface acidity of USY was converted from Broensted to Lewis acidity by treatment at temperatures as high as 750 deg C, however, the deactivation was effectively suppressed by spiltover hydrogen.We concluded from these results that the stabilization of activity by spiltover hydrogen is linked to Lewis acidity, irrespective of the type of catalyst.
- Matsuda, Takeshi,Seki, Hidetoshi,Kikuchi, Eiichi
-
-
- One-Electron Oxidation of Alkylbenzenes in Acetonitrile by Photochemically Produced NO3.: Evidence for an Inner-Sphere Mechanism
-
The reaction between NO3. and polyalkylbenzenes was studied using 308-nm laser flash photolysis of cerium(IV) ammonium nitrate in the presence of the alkylbenzenes in acetonitrile solution.For all benzenes, with the exception of monoalkylbenzenes and o- and m-xylene, the reaction with NO3. was found to yield the corresponding radical cations and to proceed in an apparently straightforward bimolecular manner.For monoalkylbenzenes and o- and m-xylene, radicals were seen which are derived from the parents by formal loss of H. from the side chain of the aromatic.This reaction proceeds via a complex between the aromatic and NO3. with the decomposition of the complex being rate determining at higher concentrations of aromatic (rate constants for decomposition between 6 * 105 and 4 * 107 s-1).In the complex, electron transfer from the aromatic to NO3. is suggested to be concerted with deprotonation of the incipient radical cation.Formation of a complex between NO3. and aromatics is likely even in those cases where radical cations are observed, with the assumption that in these cases the complex decomposition rate is greater than 6 * 107 s-1.
- Giacco, Tiziana Del,Baciocchi, Enrico,Steenken, Steen
-
p. 5451 - 5456
(2007/10/02)
-
- Efficiencies of photoinduced electron-transfer reactions: Role of the Marcus inverted region in return electron transfer within geminate radical-ion pairs
-
In photoinduced electron-transfer processes the primary step is conversion of the electronic energy of an excited state into chemical energy retained in the form of a redox (geminate radical-ion) pair (A + D →hν A?-/D?+). In polar solvents, separation of the geminate pair occurs with formation of free radical ions in solution. The quantum yields of product formation, from reactions of either the free ions, or of the geminate pair, are often low, however, due to the return electron transfer reaction (A?-/D?+ → A + D), an energy-wasting step that competes with the useful reactions of the ion pair. The present study was undertaken to investigate the parameters controlling the rates of these return electron transfer reactions. Quantum yields of free radical ion formation were measured for ion pairs formed upon electron-transfer quenching of the first excited singlet states of cyanoanthracenes by simple aromatic hydrocarbon donors in aceonitrile at room temperature. The free-ion yields are determined by the competition between the rates of separation and return electron transfer. By assuming a constant rate of separation, the rates of the return electron transfer process are obtained. These highly exothermic return electron transfer reactions (-ΔG-et = 2-3 eV) were found to be strongly dependent on the reaction exothermicity. The electron-transfer rates showed a marked decrease (ca. 2 orders of magnitude in this ΔG-et range) with increasing exothermicity. This effect represents a clear example of the Marcus "inverted region". Semiquantum mechanical electron-transfer theories were used to analyze the data quantitatively. The electron-transfer rates were found also to depend upon the degree of charge delocalization within the ions of the pair, which is attributed to variations in the solvent reorganization energy and electronic coupling matrix element. Accordingly, mostly on the basis of redox potentials, one can vary the quantum yield of free-ion formation from a few percent to values approaching unity. Use of a strong donor with a strong acceptor to induce reactions based on electron transfer is likely to be inefficient because of the fast return electron transfer in the resulting low-energy ion pair. A system with the smallest possible driving force for the initial charge-separation reaction results in a high-energy, and therefore long-lived ion pair, which allows the desired processes to occur more efficiently. The use of an indirect path based on secondary electron transfer, a concept called "cosensitization", results in efficient radical-ion formation even when the direct path results in a very low quantum yield.
- Gould, Ian R.,Ege, Deniz,Moser, Jacques E.,Farid, Samir
-
p. 4290 - 4301
(2007/10/02)
-
- THE USE OF 1,1,2,2,3,3,4,4-OCTAPHENYLTETRASILANE IN THE IONIC HYDROGENOLYSIS OF ALKYL HALIDES
-
1,1,2,2,3,3,4,4-Octaphenyltetrasilane is a hydride ion donor in the ionic hydrogenolysis of alkyl halides which are capable of forming relatively stable carbocations.The reaction takes place at room temperature in the presence of catalytic amounts of aluminum chloride and leads to the formation of the corresponding hydrocarbon with a high yield.In the light of the properties of octaphenyltetrasilane a method was developed for hydrogenolysis in a heterogenous medium in a column filled with a mixture of octaphenyltetrasilane and aluminum chloride.The spent silane is regenerated by reduction with lithium aluminum hydride and is used repeatedly
- Bolestova, G. I.,Shevchenko, N. V.,Parnes, Z. N.,Vol'pin, M. E.
-
p. 2224 - 2227
(2007/10/02)
-
- The Triflic Acid-Catalysed Deacylation and Decarboxylation of Polymethylbenzenecarbonyl Derivatives under Mild Conditions
-
Sterically hindered acylarenes are deacylated to arenes in good yields on heating in boiling 1,2-dichloroethane containing a catalytic amount of triflic acid and water.Hindered arenecarboxylic acids undergo decarboxylation under the same conditions to give arenes in high yields.
- Keumi, Takashi,Morita, Toshio,Ozawa, Yoshihiro,Kitajima, Hidehiko
-
p. 599 - 601
(2007/10/02)
-
- Twelve-electron organochromium species: Synthesis and characterization of high-spin square-planar chromium(II) alkyls and aryls
-
Alkylation of the chromium phosphine complex [CrCl2(dippe)]2 (dippe = 1,2-bis(diisopropyl-phosphino)ethane) with Grignard or dialkylmagnesium reagents gives the unusual 12-electron chromium(II) alkyls cis-CrR2(dippe) (R = CH2CMe3, CH2SiMe3, C6H2Me3). The unidentate phosphine complex trans-Cr(C6H2Me3)2(PMe 3)2 may be prepared similarly by treatment of CrCl2(THF) with dimesitylmagnesium in the presence of PMe3. All of these four-coordinate alkyls possess magnetic moments characteristic of high-spin species, μ = 4.9 μB, and exhibit broadened 1H NMR resonances consistent with the paramagnetism. X-ray crystal structures of Cr(CH2CMe3)2(dippe) and Cr(CH2SiMe3)2(dippe) reveal tetrahedrally distorted square-planar structures, where the dihedral angle between the C-Cr-C and P-Cr-P planes is larger in the neopentyl complex, 28.4° vs 15.8°, due to steric effects. For similar reasons, the Cr-C and Cr-P distances in the neopentyl compound of 2.149 (8) and 2.556 (2) A? are slightly longer than those of 2.128 (4) and 2.517 (1) A? in the (trimethylsilyl)methyl derivative, and the C-Cr-C angle is also larger in the neopentyl complex at 101.9 (3)° vs 95.7 (2)°. In contrast to these results, the mesityl complex Cr(C6H2Me3)2(PMe3) 2 is essentially flat: the ipso carbon atoms and the phosphorus atoms are +0.9° and -0.9° out of the mean CrC2P2 plane, respectively. The Cr-C and Cr-P distances in this molecule are 2.130 (6) and 2.462 (2) A?, while all the C-Cr-P angles fall between 89.3 (2)° and 90.8 (2)°. Crystal data: for Cr(CH2CMe3)2(dippe), space group C2/c, a = 15.867 (5) A?, b = 10.401 (4) A?, c = 18.154 (6) A?, β = 106.67 (3)°, V = 2870 (2) A?3, Z = 4, RF = 5.9%, RwF = 7.7% on 126 variables and 1205 data; for Cr(CH2SiMe3)2(dippe), space group C2/c, a = 15.666 (4) A?, b = 10.983 (3) A?, c = 18.099 (6) A?, β = 102.43 (2)°, V = 3041 (1) A?3, Z = 4, RF = 4.1% RwF = 4.6% on 128 variables and 1472 data; for Cr(C6H2Me3)2(PMe3) 2, space group P21/n, a = 9.710 (2) A?, b = 14.121 (3) A?, c = 19.446 (3) A?, β = 98.36 (1)°, V = 2638.0 (9) A?3, Z = 4, RF = 5.1%, RwF = 6.1% on 260 variables and 2199 data. The chromium alkyls are able to polymerize ethylene slowly at 25°C and 10 atm.
- Hermes, Ann R.,Morris, Robert J.,Girolami, Gregory S.
-
p. 2372 - 2379
(2008/10/08)
-
- Substituent and Ring Size Dependence of the 4J(Me-C-C-H) Coupling Constant
-
The magnitude of the 4J proton-proton coupling constant across the fragment CH3-C-C-H (a probe of the bond order between the central sp2-sp2 hybridized carbon atoms) has been found to be essentially independent of substitution on a toluene fragment and the size of the ring containing the ortho-allylic fragment.The coupling constant is sensitive to direct substitution on the ortho-allilyc fragment, especially where the substituent is placed α to methyl group.KEY WORDS - 4J proton-proton coupling constant ortho-benzylic coupling constant ring size dependence substituent dependence
- Collins, M. J.,Hatton, P. M.,Sternbell, S.,Tansey, C. W.
-
p. 824 - 828
(2007/10/02)
-
- Trifluoroacetic Acid-catalysed Transacylation of Arenes by Acylpentamethylbenzene
-
Facile transacylation between acylpentamethylbenzene and activated arenes such as anisole was found to occur in boiling trifluoroacetic acid (TFA).The mechanism for the transacylation between acetylpentamethylbenzene (AcPMB) and anisole with TFA was elucidated by means of product isolation and kinetics.The reaction proceeds via reversible protodeacetylation of AcPMB involving an ipso-protonated intermediate B to give pentamethylbenzene and acetic trifluoroacetic anhydride followed by irreversible acetylation of anisole by the mixed anhydride.The mechanism resulting in an ipso-protonated intermediate B was deduced from the reaction of acetylmesitylene with TFA as well as from the rate of deacetylation of 3,5-dideuterioacetylmesitylene with TFA.The formation of such an intermediate was also independently confirmed by 13C n.m.r. spectroscopic syudies on AcPMB in superacid solutions under stable conditions.
- Keumi, Takashi,Morita, Toshio,Shimada, Takanobu,Teshima, Naomi,Kitajima, Hidehiko,Prakash, G. K. Surya
-
p. 847 - 852
(2007/10/02)
-
- Gas-Phase Cationic Methylation of Biphenyl and Methylbiphenyls. A Mass Spectrometric and Radiolytic Study
-
Alkylation of biphenyl and methylbiphenyls by Me2F+ and Me2Cl+ ions has been studied in the gas phase by a combination of radiolytic and mass spectrometric techniques, including chemical ionization and collisionally induced dissociation spectrometry.The results, in particular, the disproportionately high extent of ortho substitution, conform to a mechanistic model envisaging, in addition to direct substitution, the preliminary formation of an electrostatically bound adduct between the symmetric, bidentate electrophile and both rings of the substrate.Formation of the adduct increases the local concentration of the electrophile, hence the rate of alkylation at the ortho positions of biphenyl.
- Cacace, Fulvio,Petris, Giulia de,Fornarini, Simonetta,Giacomello, Pierluigi
-
p. 7495 - 7501
(2007/10/02)
-
- "IPSO" AROMATIC ALKYLATION IN THE GAS PHASE. INTERMEDIACY AND STRUCTURE GASEOUS HEPTAALKYLBENZENIUM IONS
-
The structure of heptaalkylbenzenium ions has been investigated in the gas phase with complementary techniques, i. e., chemical ionization mass spectrometry, alkylation of hexaalkylbenzenes by radiolytically generated cations, and collisianally induced dissociation mass spectrometry.The results allow us to assign the ?-complex ( Wheland intermedisate ) structure to the free, gaseous heptaalkylbenzenium ions, in agreement with the established structure of their salts in solution and in the solid state.The heptaalkylbenzenium ions represent the charged intermediates of the gas-phase "ipso"-substitution reactions, whose occurence has been demonstrated and whose mechanism is briefly discussed.
- Attina, Marina,Cacace, Fulvio,Petris, Giulia de,Fornarini, Simonetta,Giacomello, Pierluigi
-
p. 2297 - 2302
(2007/10/02)
-
- Selective and Mild Deacylation of Hindered Acylarenes with Aqueous Trifluoroacetic Acid
-
Sterically hindered acylarenes are deacylated to arenes in quantitative yields on heating in boiling 85percent trifluoroacetic acid.Hindered arenecarboxylic acids undergo decarboxylation under the same conditions to give arenes in high yields.
- Keumi, Takashi,Morita, Toshio,Inui, Yoko,Teshima, Naomi,Kitajima, Hidehiko
-
p. 979 - 980
(2007/10/02)
-
- Rates and Mechanism of Proton Transfer from Transient Carbon Acids. The Acidites of Methylbenzene Cations
-
The fast rates of proton transfer from various methylbenzene cation radicals to a series of substitued pyridine bases are successfully measured in acetonitrile solutions.The technique utilizes the production of the cation radical as a transient intermediate during the electron-transfer oxidation of the methylbenzene with an iron(III) oxidant.Complete analysis of the complex kinetics affords reliable values of the deprotonation rate constants k2 which span a range from 3 x 102 to more than 2 x 107 M-1 s-1.The relative acidities of the cation radicals of hexamethylbenzene, pentamethylbenzene, durene, and prehnitene can be obtained from the Broensted correlation of the deprotonation rate constants with the pyridine base strengths and the standard oxidation potentials of the methylarenes.An estimate of the acidity constant for the hexamethylbenzene cation radical is based on several empirical extrapolations to that of the toluene cation radical previously evaluated by Nicholas and Arnold on thermochemical grounds.The kinetic acidities of the various methylarene cation radicals are also examined in the context of the Marcus equation, as applied to proton transfer.The mechanism of proton transfer from these labile carbon acids is discussed with regard to the electronic effects relevant to the methylarene oxidation potential and the pyridine base strength, the kinetic isotope effects with deuterated methyl groups, the salt effects in acetonitrile, and the steric effects of ortho substituents on pyridine.
- Schlesener, C. J.,Amatore, C.,Kochi, J. K.
-
p. 7472 - 7482
(2007/10/02)
-
- The gas phase reactivity of chloronium ions by high pressure mass spectroscopy
-
The reactions of CH3ClCH3+ (I) and CH3ClCH2Cl+ (II) with a range of bases have been studied in a high pressure ion source.Reactant ion monitoring has been used to obtain the relative reactivities.Transfer of CH3+ from I and CH2Cl+ from II are the only reactions observed with N- and O-containing bases.Following addition of CH2Cl+ from II, the lower alkyl aromatics (ArH) yield either ArHCH2Cl+ or the benzyl ions ArCH2+ and ArCHCl+ while CH3+ transfer leads only to ArHCH3+.The reactivities of the alkyl aromatics increase with increasing exothermicity (benzene --> mesitylene) and there is an increasingly negative temperature coefficient of reactivity in the same order.
- Stone, John A.,Splinter, William M.,Splinter, Dena E.
-
p. 1373 - 1379
(2007/10/02)
-
- A MILD REDUCTION OF BENZYL ALCOHOLS WITH DIPHOSPHORUS TETRAIODIDE
-
On treatment with diphosphorus tetraiodide in boiling benzene, benzyl alcohols are smoothly reduced to parent hydrocarbons in good to moderate yields.
- Suzuki, Hitomi,Tani, Hiroyuki,Kubota, Hirohisa,Sato, Naofumi,Tsuji, Junko,Osuka, Atsuhiro
-
p. 247 - 248
(2007/10/02)
-
- Mise au point d'une reaction simple de caracterisation du reseau poreux des zeolithes de taille de pore intermediaire
-
Over three protonic zeolithes : HY, HZSM-5 and H mordenite, the initial selectivities of the transformations of xylenes and 1,2,4-trimethylbenzene have been compared.The ratio of disproportionation to isomerization rates which is close to 1 on Y zeolite, is 3 to 20 times lower on mordenite and equal to zero on ZSM-5 zeolite.The ratio of the amount of hydrocarbon transformed into coke to the amount isomerized is 2 times greater for Y zeolite than for mordenite and 30 to 90 times greater than for ZSM-5 zeolite.The distribution of the primary products of isomerization depends very much on the zeolite : the smaller the pore size, the more favoured will be the smallest isomer.Thus by m-xylene isomerization over Y zeolite, ortho and para-xylenes are obtained in equal quantities, whereas over mordenite and above all over ZSM-5 zeolite, the smallest para isomer is definitely favoured.The distribution of the disproportionation primary products is also quite different over mordenite and over Y zeolite.For example, over Y zeolite, the 1,3,5- and 1,2,4-trimethylbenzenes are formed in their equilibrium mixture, whereas over mordenite, the 1,3,5-trimethylbenzene is practically not formed.These observations can be explained by the three types of shape selectivity defined by Csicsery (Zeolite chemistry and catalysis, Ed.J.A.Rabo, ACS monograph, 171, Washington, 1976, p. 680) : selectivity due to i) the difficulty of access of the reactant to active sites; ii) the difficulty of product desorption; iii) the difficulty of formation of bulky intermediates.The reactions of these aromatic hydrocarbons are therefore of great interest for characterizing the pore structure of intermediate pore size zeolites such as mordenite and ZSM-5 zeolite.The m-xylene transformations are shown to be model reactions the best adapted for this characterization.The selectivity of m-xylene isomerization is related to the pore size : the smaller the pores, the more favoured p-xylene will be.The distribution of the disproportionation products (trimethylbenzenes) as well as the ratio of the rate of disproportionation (a bimolecular reaction) to the rate of isomerization (an intramolecular reaction) are directly related to the maximum space available for the formation of intermediates or of transition states, and are therefore directly related to the cavity size or to the size of the channel intersections.This model reaction which is easy to use leads to more detail information than the constraint index (as defined by Mobil investigators) which is related only to the pore size.
- Gnep, Ngi Suor,Tejada, Jorge,Guisnet, Michel
-
-
- COPPER(I) IODIDE ASSISTED REACTION OF NONACTIVATED IODOARENES WITH SODIUM TRIFLUOROACETATE IN HEXAMETHYLPHOSPHORIC TRIAMIDE. NUCLEAR TRIFLUOROMETHYLATION AND DIARYL ETHER FORMATION
-
Polymethyliodobenzenes react with sodium trifluoroacetate, in the presence of copper(I) iodide, in hexamethylphosphoric triamide at 150-180 deg C, giving a mixture of the corresponding polymethylbenzotrifluoride, parent hydrocarbon, and bis(polymethylphenyl) ether.
- Suzuki, Hitomi,Yoshida, Yoshiki,Osuka, Atsuhiro
-
p. 135 - 136
(2007/10/02)
-
- MERCURY IN ORGANIC CHEMISTRY. 20. ALKYLATION OF ORGANOMERCURIALS VIA ORGANOCOPPER REAGENTS
-
Aryl-, alkenyl-, and alkylmercurials readily cross-couple with primary and secondary alkyl- and alkenylcuprate reagents to provide the first truly general method for the alkylation of a wide variety of organomercurials.
- Larock, R. C.,Leach, D. R.
-
p. 3435 - 3438
(2007/10/02)
-
- Acetyl Exchange between Acetyl Chloride and Sterically Hindered Aryl Ketones under Friedel-Crafts Condition
-
The kinetics of acetyl exchange between acetylmesitylene, or acetyldurene, and (14)C-labelled acetyl chloride have been measured in nitromethane solution in the presence of aluminium chloride.Mechanistic studies using acetylmesitylene as substrate show conclusively that acetyl exchange proceeds, not by acylation-deacylation or deacylation-acylation, but via a synchronous reaction involving an ipso-complex.Theoretical calculations (MNDO) indicate that of three possible synchronous pathways, two are energitically feasible.
- Andreou, Andreas D.,Bulbulian, Roger V.,Gore, Peter H.,Morris, Donald F. C.,Short, Eric L.
-
p. 830 - 837
(2007/10/02)
-
- Electrophilic Aromatic Substitution. Part 26. The Effect of the Cyclopropyl Substituent in Aromatic Detritiation
-
The effects of the ortho-, and para-cyclopropyl and para-1-methylcyclopropyl substituents in acid-catalysed hydrogen exchange have been determined through detritiation of a range of disubstituted aromatics in various acetic acid-trifluoroacetic acid media at 70 deg C.The p-cyclopropyl substituent gives f 13 900 and ?+ -0.473, the latter being in excellent agreement with values obtained in other reactions.The o-cyclopropyl substituent is much less activating, f 1 630, and this confirms that the very high ortho : para ratios obtained in nitration (especially in acetic anhydride) are anomalous, as they are for certain other substituents.The value for log f0/log fp for cyclopropylbenzene in hydrogen exchange (0.77) is significantly lower than for other alkyl- and cycloalkyl-benzenes (each of which gives a value of -0.875 +/- 0.01) implying that the bisected conformation of the o-cyclopropyl group is not quite achieved in the (normally) unhindered exchange reaction.The p-1-methylcyclopropyl substituent is more activating than the p-cyclopropyl substituent (fp39 200,?+ -0.525) just as p-t-butyl is more activating than p-isopropyl in trifluoroacetic acid media, and may be attributed to the same cause viz. the greater inductive and hyperconjugative effect of the methylated species.In these poorly solvating media, the greater importance of C-C hyperconjugation over C-H hyperconjugation is not masked by steric hidrance to solvation.All the cyclopropyl substituents are slightly more activating (relative to methyl) in media containing less trifluoroacetic acid, suggesting that the former are more polarisable.However, since this behaviour parallels, though to a much lesser extent, that of the methoxy-substituent (which is strongly hydrogen-bonded in trifluoroacetic acid), we cannot exclude the possibility that cyclopropyl is similarly, but slightly, hydrogen-bonded.Rate factors were therefore determined in media in which hydrogen bonding is insignificant even for methoxy.The factors were therefore determined in media in which hydrogen bonding is insignificant even for methoxy.The rate versus acidity profile for exchange in a wide range of acetic acid-trifluoroacetic acid media was been determined and shows that at 70 deg C, exchange in acetic acid is 1E8.2 times slower than in trifluoroacetic acid.Between acetic acid and trifluoromethanesulphonic acid, a reactivity of 1E20 is therefore encompassed.
- Fischer, Peter,Taylor, Roger
-
p. 781 - 785
(2007/10/02)
-
