Carbenium Ion Intermediates Research Articles

Aromatization of short chain alkanes on zeolite catalysts

On MFI catalysts, low cost liquefied petroleum gas (LPG ) can be transformed into valuable aromatics (mainly C 6-C 8 benzenics) and into hydrogen. Unfortunately methane and ethane are also produced in significant amounts. A reduction of the production of these unwanted compounds would render the aromatization process economically more attractive. The reaction pathways of propane aromatization were established on H-ZSM-5 pure or loaded with platinum or with gallium. On H-ZSM-5 the first step is the dehydrogenation and the cracking of the reactant through carbonium ion intermediates. The resulting alkenes (propene and ethylene) undergo rapid successive reactions via carbenium ion intermediates: oligomerization, cyclization, hydrogen transfer. The selectivity to aromatics is limited because of the formation of methane by propane cracking and of alkanes by hydrogen transfer. Platinum increases the rate of propane transformation significantly but a higher production of methane and ethane is found on PtH-ZSM-5 catalysts, owing to the hydrogenolysis of alkanes and of alkylaromatics and to the hydrogenation of ethylene on the platinum sites. Gallium improves both the rate and the selectivity of propane aromatization. The aromatization occurs, like on PtH-ZSM-5, through a bifunctional pathway, gallium catalyzing the dehydrogenation of the alkane reactant to alkenes and of naphthenic intermediates to aromatics, and the acid sites catalyzing the oligomerization of light alkenes and the cyclization of C 6-C 8 alkenes. The better selectivity to aromatics is obtained for GaMFI catalysts with gallium species well dispersed within the zeolite and being very active for dehydrogenation (e.g. with gallosilicates or galloaluminosilicates steamed under mild conditions). At high temperature, in presence of hydrogen (hence during aromatization) GaMFI catalysts would undergo various modifications: reduction of gallium species, migration within the zeolite, reaction of these species with the protonic sites. The nature of the gallium active species and their role in the various steps are discussed.

Conversion of light alkanes into aromatic hydrocarbons: VI. Aromatization of C 2-C 4 alkanes on H-ZSM-5 —reaction mechanisms

The conversion of ethane, propane, n-butane, isobutane and of the corresponding alkenes was studied at 530°C on a H-ZSM-5 zeolite (Si/Al=40). Butanes transform about 4 times more rapidly than propane and 100 times more than ethane. From propane and butanes the primary products result from dehydrogenation and cracking, cracking being more rapid than dehydrogenation. Ethane undergoes mainly dehydrogenation and leads slowly to methane. It is suggested than these reactions occur through scission of carbonium ions formed by alkane protonation. The more stable the carbonium ion the higher the reaction rate. The olefinic primary products undergo very rapid reactions through carbenium ion intermediates: oligomerization, cyclization of the oligomers followed by aromatic formation through hydrogen transfer from naphthenes to light alkenes. Whatever the reactant, the limiting step of aromatization is the initial formation of alkenes.

Monomolecular and bimolecular mechanisms of paraffin cracking: n-butane cracking catalyzed by HZSM-5

The cracking of n-butane catalyzed by the zeolite HZSM-5 has been characterized by measurements of the conversion determined with a flow reactor at temperatures of 426–523°C and n-butane partial pressures of 0.01–1.00 atm. The primary products, each formed in a first-order reaction, are H 2 + butenes; methane + propylene; and ethane + ethylene. In the limit approaching zero conversion, each compound in each stated pair was formed at approximately the same rate as the other. Propane and a small amount of isobutane were formed as secondary products in second-order reactions. The results are consistent with the occurrence of two simultaneous mechanisms: (1) a monomolecular mechanism proceeding through the pentacoordinated carbonium ion formed by protonation of the n-butane at the two position and (2) a bimolecular hydride transfer proceeding through carbenium ion intermediates. The former proceeds almost to the exclusion of the latter in the limit approaching zero n-butane conversion. The limiting product distribution characterizes the intrinsic selectivity of the collapse of the carbonium ion; at 496°C, the relative rates of decomposition of the carbonium ion to give H 2 + butenes, methane + propylene, and ethane + ethylene are 30 ± 6, 36 ± 4, and 34 ± 5, respectively, with the corresponding activation energies all being approximately 140 kJ/mol. These results provide the first demonstration of stoichiometric dehydrogenation accompanying paraffin cracking.

Organometallic compounds of Group III. 49. Reductive deoxygenation of ketones and secondary alcohols by organoaluminum Lewis acids

The reductive deoxygenation of ketones and secondary alcohols to the corresponding methylene hydrocarbons has been achieved in good to excellent yield by the combined action of an aluminum hydride source and a strongly Lewis-acidic aluminum reagent. Such reductions were successful with diaryl ketones, alkyl aryl ketones, and dialkyl ketones, as exemplified by the reduction of benzophenone, acetophenone and 5-nonanone, respectively. The corresponding secondary alcohols of these ketones, namely benzhydrol, l-phenyl-l-ethanol, and 5-nonanol, could also be converted into their respective methylene hydrocarbons by Lewis-acidic sources of aluminum hydride. All such reductions of ketones could be conducted in a single reaction flask in a one-, two-, or three-step process. In the one-step process, which is most suitable for diaryl ketones, i-BuAIClz may be employed as both the hydride source and the Lewis acid. For alkyl aryl ketones a two-step process, consisting first of reduction with i-BuzAIH and then treatment with AlBr3 (with or without catalysis by Cp2TiClz), leads to better yields. Finally, for dialkyl ketones a three-step process proved to be preferred, wherein a sequential treatment with i-BuzAIH, AIBr? and then additional i-BuzAIH (with a Ni(aca& catalyst) gives the highest conversion to alkane. If required, residual alkene may be removed by a brief catalytic hydrogenation or treatment with BHgTHF. The ease of deoxygenating the foregoing ketones and secondary alcohols appears to be governed by the ease of forming, and the relative stability of, the corresponding carbenium ion intermediates, namely Ar2HC+ > ArRHC+ > &HC+. The driving force for such deoxygenations by these aluminum reagents undoubtedly is the exothermic formation of the dialuminoxane system, RzA1-O-A1R2.

I.r. study of the interaction of n-butanols with HZSM-5 zeolites

The interaction of 1- and 2-butanol with HZSM-5 zeolites with different Si/Al molar ratios was studied in the temperature range 298–473 K by means of i.r. spectroscopy. The dehydration of the alcohols was accompanied by isomerization of the formed olefins. The ratio between the isomers was estimated by the integral intensity of the i.r. bands. The results were explained on the basis of intermediate carbenium ion formation.

ChemInform Abstract: Active Sites and Intermediates for Isomerization and Cracking of Cyclohexane on HY.

Abstract Initiation of both isomerization and cracking reactions of cyclohexane on HY at 400°C occurs at Bronsted sites. The absence of molecular hydrogen as a product indicates that protonation of the feed molecule occurs, and the resulting carbonium ion can rearrange through a cracking-realkylation process, or undergo ring cleavage leading to acyclic products. No direct evidence was obtained which would indicate the existence of a chain mechanism proceeding through cyclic carbenium ion intermediates. These observations support a previously proposed mechanism for initiation of cracking of linear paraffins at Bronsted sites.

Active sites and intermediates for isomerization and cracking of cyclohexane on HY

Initiation of both isomerization and cracking reactions of cyclohexane on HY at 400°C occurs at Brønsted sites. The absence of molecular hydrogen as a product indicates that protonation of the feed molecule occurs, and the resulting carbonium ion can rearrange through a cracking-realkylation process, or undergo ring cleavage leading to acyclic products. No direct evidence was obtained which would indicate the existence of a chain mechanism proceeding through cyclic carbenium ion intermediates. These observations support a previously proposed mechanism for initiation of cracking of linear paraffins at Brønsted sites.

An MNDO molecular orbital study of the reactions of protonated oxirane derivatives (XCHCH2OH+, X = CN, Cl, CH3, Ph) with simple nucleophiles. Implications for regioselectivity in the reactions of electrophiles with nucleic acid bases

AbstractThe gas‐phase analogs of the classical SN1 and SN2 reactions of nucleophiles with a series of protonated oxiranes bearing the substituents CN, Cl, Me, and Ph were studied using MNDO semiempirical molecular orbital theory. The transition states for nucleophilic attack on the intact rings are calculated to become progressively more “SN1‐like,” as the ability of the substituent group to stabilize the corresponding ring opened carbenium ion increases. At the same time the activation barriers become progressively smaller and their relative values for different nucleophiles approach the order expected on the basis of a purely electrostatic attraction between the reacting moieties. Exactly the opposite trends are calculated for the transition states for nucleophilic attack on the intermediate carbenium ions. As the stabilities of the latter increase the extent of bond formation, and transfer of charge to the incoming nucleophile also increase. Here, the relative barriers for attack by different nucleophiles approach the order expected on the basis of a superposition of both covalent and electrostatic interactions in which the former dominate. These results support a previously suggested rationalization for the enhanced reactivities of certain alkylating agents towards the exocyclic oxygen atoms, rather than the ring nitrogens, of the nucleic acid bases. They also suggest a new explanation for the tendencies of aralkylating electrophiles to modify the exocyclic amino groups of the nucleic acid bases: sites which are unreactive towards simple alkylating agents.

Regioselective Ring Opening Reactions of 1-Aminocyclopropenes via Carbenium Ion and Carbene Intermediates

Abstract The reaction of 1-(disubstituted amino)-2,3-diphenylcyclopropenium salt with phenyl- and alkylmagnesium halides afforded, regioselectively, 1-aminocyclopropenes in good yields. The reactions of these 1-aminocyclopropenes were studied under acidic and nonacidic conditions to yield regioselective ring-opening products (C2–C3 and C1–C3 bond fissions of the cyclopropene) associated with the carbenium ions and carbene intermediates, respectively.

ChemInform Abstract: The Role of the 2‐Methyl Substituent in Governing Stereoselective Formation of the E Isomer (II) in the Synthesis of 4‐Hydroxy‐2‐methyltamoxifen (1‐(4‐(2‐(Dimethylamino)ethoxy)phenyl)‐1‐(4‐hydroxy‐2‐methylphenyl)‐2‐phenylbut‐1‐ene).

AbstractAcid treatment of either diastereomer of (I) ((Ia) has already been published together with its X‐ray structure; (Ib)·toluene (1:1) has space group P21/c with Z=4) leads predominantly to the E olefin (II), consistent with a mechanism proceeding by way of a common carbenium ion intermediate.

Kinetics and selectivity of reactions of 1‐alkenes on HY zeolite: The influence of chain length

AbstractCatalytic reactions of 1‐alkenes on HY have been studied at 400°C. Skeletal isomerization to the corresponding rnonornethylalkene is the dominant initial process. Cracking and coke formation are also observed as initial processes. Initial cracking selectivity can be accounted for by monomolecular β‐scission processes involving a carbenium ion intermediate. However, the initial cracking product distribution for l‐hexene must be explained by assuming some contribution from a dimerization‐cracking mechanism.The kinetics of these processes have been fitted by a model previously used to describe the reactions of n‐alkanes. It is found that the rate constants for both skeletal isomerization and cracking increase with chain length of the feed alkene.

Stereochemical effects in the gas-phase pinacol rearrangement. 2. Ring contraction versus methyl migration in cis- and trans-1,2-dimethylcyclohexane-1,2-diol

The gas-phase pinacol rearrangement of cis- and trans-1,2-dimethylcyclohexane-1,2-diols, promoted by D/sub 3//sup +/, CH/sub 5//sup +//C/sub 2/H/sub 5//sup +/ and t-C/sub 4/H/sub 9//sup +/ ions, was studied by mass spectrometric and radiolytic methods in the pressure range 0.5-760 Torr. When product isomerization is inhibited, by using N(CH/sub 3/)/sub 3/ as a trapping reagent at high pressure, mixtures of 2,2-dimethylcyclohexanone and 1-acetyl-1-methylcyclopentane were recovered from the reaction. In methane, the trend of the measured relative rates for ring contraction (k/sub 5/), methyl or hydroxyl group migration (k/sub 6/) versus the rearrangement rate of pincaol itself (k/sub p/), is k/sub 6/(trans) approx. k/sub 5/(trans) greater than or equal to k/sub 5/(cis) > k/sub 6/(cis) greater than or equal to k/sub p/. No evidence for the formation of an intermediate carbenium ion was found. Stereochemical aspects of the mechanism are discussed and compared with solution data.

ChemInform Abstract: Stereoselective Olefin Formation from the Dehydration of 1‐(p‐Alkoxyphenyl)‐1,2‐diphenylbutan‐1‐ols: Application to the Synthesis of Tamoxifen.

AbstractGrignard reaction of the ketones (I) or (IV) affords the butanols (III) or (VI).

Copper(I) catalysis of olefin photoreactions. 14. A copper(I)-catalyzed photobicyclization route to exo-1,2-polymethylene- and 7-hydroxynorbornanes. Nonclassical 2-bicyclo[3.2.0]heptyl and 7-norbornyl carbenium ion intermediates

Copper(I) catalysis of olefin photoreactions. 14. A copper(I)-catalyzed photobicyclization route to exo-1,2-polymethylene- and 7-hydroxynorbornanes. Nonclassical 2-bicyclo[3.2.0]heptyl and 7-norbornyl carbenium ion intermediates

The influence of oxonium ion and carbenium ion stabilities on the Alcohol/H-ZSM-5 interaction

We have examined the adsorption of methanol, ethanol, 1-propanol, 1-butanol, 2-propanol, and 2-methyl-2-propanol on H-ZSM-5 using temperature-programmed desorption (TPD) and thermogravimetrie analysis (TGA). For each of the alcohols except 1-butanol, there exists a clearly defined desorption state corresponding to one alcohol molecule per Al atom in the zeolite. For 2-methyl-2-propanol and 2-propanol, this molecule at the Al site completely reacts to olefin products and water; however, reaction occurs at 295 K for 2-methyl-2-propanol, while 2-propanol does not react until 360 K. Most of the methanol, ethanol, and 1-propanol adsorbed at the Al sites at this coverage desorbs unreacted. A substantial amount of the 1-butanol adsorbed near the Al sites reacts, but differs from the other alcohols in that it reacts through a dibutyl ether intermediate. These results can be understood in terms of the stabilities of the carbenium ion intermediates which can be formed during reaction. A potential energy scheme, based on gas-phase proton affinity, is developed. This scheme provides a formalism for explaining relative reactivities of alcohols on H-ZSM-5 in TPD.

Chemistry of di- and tetrahydropyrans. Communication 5. Ritter reactions in the pyran series

1. Reaction of 4-methylenetetra- and 4-methyl-5,6-dihydropyrans with aceto-, benzo-and acrylonitriles under the conditions of the Ritter reaction leads to the formation of 4-acylamino-4-methyltetrahydropyran in 45–65% yield. 2. Since protonation of isomeric mixtures of substituted 5,6-, 3,6-dihydro-, and 4-methylenetetrahydropyrans generates the same intermediate carbenium ions, reaction of either of these materials with nitriles in conc. H2SO4 leads to the same amides in the heterocyclic series.

The influence of intermediate carbenium ion stabilization on the mechanism of the acid-catalysed hydrolysis of α-acetoxystyrenes

The hydrolysis of α-acetoxystyrenes 1a-f in 37% sulfuric acid and 1e at different acid strengths to the corresponding acetophenone 2a-f exhibits pseudo-first-order kinetics. Apparent rate constants ( k h) have been measured spectroscopically at 23 °C by following the change in absorbance with time at a wavelength of 280 nm. Good linearity has been found for the log ( k h) versus log [H +] plot, but not for the plot against H 0. A linear relationship also exists between log ( k h) and the electron density of the terminal vinylic carbon atom. The mechanistic implications of the data are discussed.

Mechanism of the interaction of the superacid HGeCl3 with olefins

1. The addition of HGeCl3 to simple olefins leads to the formation of Markownikoff and anti-Markownikoff adducts, which is explained by competition of ionic and free-radical mechanisms of the reaction. In this case the proportion of ionic addition of HGeCl3 increases in the presence of substituents in the olefin that effectively stabilize the intermediate carbenium ion. 2. Free-radical interaction of HGeCl3 with isobutylene was confirmed by the isolation of a telomer, the structure of which is also simultaneously evidence of the participation of dichlorogermylene in the reaction. 3. Anti-Markownikoff addition of HGeCl3 to 1-methylcyclohexene is stereospecific anti-addition, leading to a product with a diequatorial arrangement of the inserted radicals.

Induction period of the conversion of methanol to hydrocarbons on a nickel loaded faujasite: Evidence for a suppression of the autocatalytic step

The conversion of methanol to hydrocarbons on nickel loaded faujasites can be retarded for many hours time-on-stream. The induction period is interpreted to be due to a suppression of the autocatalytic step of the reaction. An autocatalytic mechanism is proposed based on the β-scission of branched carbenium ion intermediates.

Discrete introduction of the electrophile and nucleophile in Ad E -reactions of dicobalt hexacarbonyl complexes of conjugated enynes.

The dicobalt hexacarbonyl complex of the enyne isopropenylacetylene ( 1 ) is effective in permitting stepwise Ad E reactions to the double bond of 1 via stabilization of the intermediate carbenium ion.