1,720,984 research outputs found
Gas-phase acylation reactions. Acetylation of phenols by radiolytic acetylium (CH3CO+) ions
No full textThe gas-phase acetylation of representative phenols by CH3CO+ ions from the γ-radiolysis of CH3F/CO mixts. has been investigated in the pressure range from 380 to 760 Torr, and in the presence of a gaseous base (NH3). At the higher system pressures, the competition between the n-type and the π-type nucleophilic centers of the ambident substrates is kinetically controlled and biased in favor of O-acetylation, as demonstrated by the predominant (from ∼80 to ∼97%) formation of aryl acetates with respect to the isomeric hydroxyacetophenones. The isomeric compn. of the ring-acetylated products reflects the electrophilic character and the remarkable positional selectivity of the CH3CO+ ion, which yields predominantly o- and p-hydroxyacetophenones, in proportions depending on the total pressure and on the presence of NH3. In fact, O- as well as o-acetylated substrates, are more stable than the other isomeric ionic intermediates, as indicated by their increased yields at the lowest total pressure used (380 Torr), under conditions of increased thermodn. control of products. The results of competition expts., carried out in the presence of mesitylene as the ref. substrate, allow detn. of a gas-phase acetylation reactivity scale from p-xylene to 2,6-xylenol, this scale is characterized by values ranging over 3 orders of magnitude. Comparison of the gas-phase acetylation of CH3CO+ ions with related reactions occurring in soln. reveals no basic mechanistic differences, the gas-phase reactivity and selectivity of free acetylium ions displaying significant analogies with those of acetylating reactants generated in low-polarity media by the action of poorly co-ordinating Friedel-Crafts catalysts on suitable precursors
Gas-phase acylation reactions. Substrate and positional selectivity of free acetylium ions toward methylbenzenes
No full textFree Ac+ ions, obtained in the dild. gaseous state from the γ-radiolysis of MeF-CO mixts., reacted with methylbenzenes at 380-760 Torr in the presence of a gaseous base (NH3). The gaseous cation is unreactive toward C6H6 and PhMe but acetylates the xylenes and other selected polymethylbenzenes. The relative rates of acetylation were detd. in competition expts., using mesitylene as the ref. substrate. The mechanism of acetylation and subsequent isomerization is discussed, and the substrate and positional selectivity of the free Ac+ ion are evaluated, together with its intrinsic steric requirements. Comparison of the gas-phase results with those of related condensed-phase reactions involving Ac+ salts as 1 of the reactive species reveals no basic mechanistic differences. Some obsd. reactivity and selectivity discrepancies, in particular those concerning acetylation of PhMe, o- and m-xylene, and hemimellitene are outlined, and their possible causes considered
PREDOMINANT O-ALKYLATION IN THE ATTACK OF FREE BENZYL CATION ON 2,6-DIMETHYLANISOLE
No full textLabeled benzyl cation from the decay of side-chain multitritiated toluene was allowed to react with 2,6-dimethylanisole in n-hexane, MeNO2, and Et2O at room temp. The nuclear-decay formed (nucleogenic) benzyl cation displayed high selectivity toward the n-type center of the nucleophiles present, leading to predominant formation of benzyl 2,6-dimethylphenyl ether in n-hexane and MeNO2, via the O-benzylated oxonium intermediate. In n-hexane, the oxonium intermediate underwent partial isomerization, yielding the corresponding ring-substituted products. A mechanism involving a tight sandwich π complex is proposed to account for the solvent effect on product distribution. The mechanistic features of the benzylation process, deduced from the decay expts., are discussed and compared with those of related gas-phase and conventional Friedel-Crafts data
Gas Phase Acid-Induced Nucleophilic Displacement Reactions. Structural Analysis of Intermediate Ions Formed by the Decomposition of beta-Arylethyl Onium Ions
No full textCompetitive Concerted and Stepwise Addition of Free Arylium Ions To Propane In the Gas-phase
No full textLabeled tolylium ions from the decay of ring-multitritiated toluene have been allowed to react with propane in the gas phase, at pressures ranging from 20 to 744 Torr, yielding isomeric propyltoluenes as the major addn. products. The relative compn. of the n- vs. iso-propyltoluenes (σp:σs 3.2-4.7) is found to depend appreciably upon the total pressure of the system and the presence of added bases (NH3 or CH3OH). Pressure and base effects on the isomeric distribution of both n- and iso-propyltoluenes are also investigated. The results are consistent with a reaction pattern involving preliminary formation of an electrostatic adduct, wherein fast hydride-ion transfer from a secondary C-H bond of C3H8 to the arylium ion takes place. The same mechanism does not seem operative when the first interaction occurs between the tolylium ions and a primary C-H bond of the substrate. The behavior of isomeric tolylium ions towards propane is discussed and compared with related studies involving unsubstituted phenylium ions. A mechanistic model is proposed for both reactions, which accounts for the apparent discrepancy between the indiscriminate affinity of arylium ions for any kind of substrate and their abnormally high site selectivities
Gas-phase heteroaromatic substitution. 2. Electrophilic methylation of pyrrole and N-methylpyrrole by MeXMe+ (X = fluoro, chloro) ions
No full textThe gas-phase methylation of pyrrole (I) and N-methylpyrrole (II) by Me2X+ (X = F, Cl) from the γ radiolysis of MeX is examd. at 50 to 760 torr, in the presence of a thermal radical scavenger (O2) and variable concns. of an added base (NMe3). Both the reactivity of I or II relative to C6H6, used as the ref. substrate in competition expts., and the isomeric distribution of their methylated derivs. depend markedly on the total pressure of the system and the concn. of NMe3. The apparent kpyrrole/kC6H6 ratios increase from 0.2 (I)-0.3 (II), in neat MeF at 50 torr, to over 0.4 (I)-1.0 (II), at 760 torr contg. 10 torr of NMe3. Concurrently, the isomeric distribution of the methylated products changes from β:α:N = 80%:13%:7% (from I) and β:α = 65%:35% (from II) to β:α:N = 50%:15%:35% and β:α = 70%:30%. This is consistent with a methylation mechanism involving kinetically predominant Me2F+ attack on the β-C atoms of the pyrrole rings, and subsequent isomerization of the resulting excited intermediates to the thermodynamically most stable 3-methylpyrrole protonated on the 2-position. The substrate and positional selectivity of the gas-phase methylation and the mechanism of the intramol. isomerization are related to theor. predictions on heteroarom. reactivity and are compared with the available data of related methylation reactions, occurring both in the gaseous and condensed phase
GAS-PHASE HETEROAROMATIC SUBSTITUTION .7. METHYLATION OF 5-MEMBERED HETEROAROMATIC RINGS BY FREE METHYL CATIONS
No full textThe results of a study of the reactions of a powerful alkylating electrophile, the CT3+ ion from the β-decay of CT4, with gaseous pyrrole, N-methylpyrrole, furan, and thiophene are reported. In all the systems investigated, two major categories of labeled products are recovered: the methylated and the tritiated substrates. The nature and the yields of the methylated products, as well as their isomeric compn., suggest that the gas-phase alkylation reaction of simple heteroaroms. by the CT3+ cation is regulated by factors related to electron mixing between the HOMO of the substrate and the LUMO of the electrophile in the transition state, a behavior that ranks CT3+ as a soft Lewis acid. For harder alkylating species, such as CH3FCH3+, the orienting properties of the heteroarom. substrates are mainly detd. by electrostatic interactions established within the encounter pair. Furan and thiophene give, although for different reasons, similar product distributions with both hard and soft alkylating cations
Decay synthesis of carrier-free 3-methylfuran-t and 2-methylfuran-t
No full textCarrier-free 2- and 3-(methyl-t3)furan and furan-t were prepd. by nuclear decay of CT4 in the presence of gaseous furan and 3 mol % O at 100° and 150 tor for 1 mo
Gas Phase Acid-Induced Nucleophilic Displacement Reactions. Structural and Stereochemical Evidence for the Existence and Relative Stability of Alkylenebenzenium Ions in the Gas Phase
No full textReactions of phenylium ions with gaseous hydrocarbons. 1. Methane, ethane, and propane
No full textThe reaction of p-TC6H4+ (I), generated from the nuclear β-decay of p-T2C6H4, with CH4, C2H6, or C3H8 (at 10-100 torr partial pressures) gives the corresponding TC6H4R (R = Me, Et, Pr, Me2CH). The reactions of I show that it is very reactive towards pure σ-type substrates and that these reactions are C-H bond regioselective. The stability of TC6H5R+, intermediates in the reactions of I, and their fragmentation- and isomerization-mechanism are discussed. These studies are compared to those from ion-cyclotron mass spectral studies at 10-5 torr. The effect of NH3 or MeOH on the reactions of I with alkanes is also discussed
- …
