Gas-phase methylation of phenol and anisole by CH3XCH3 +(X = F, Cl, or Br) ions

Abstract

Dimethylhalogenonium ions CH,XCH3+ (X = F, CI, or Br), obtained in the dilute gas state from the y-radiolysis of the corresponding methyl halide CH,X, were allowed to react with representative ambident substrates, such as phenol and anisole, either neat or in competition with benzene, in the pressure range 100-760 Torr, and in the presence of a gaseous base (NH,). The competition between the nand x-type nucleophilic centres of phenol is kinetically biased in favour of O-methylation, leading to the predominant (CH3FCH3+ 6 : 1 ; CH3CICH3+ 5 : 1 ; CH,BrCH,+ 4 : 1) formation of anisole with respect to cresols under conditions, i.e. high CH,X pressure and in the presence of NH,, which favour kinetic control of the products. For ring methylation, both the selected substrates undergo preferential substitution at the ortho-and para-positions. In general, ortho-attack is favoured at low pressure and in the absence of NH,. The results of competition experiments show that phenol (S) reacts faster than benzene (B) with gaseous CH3XCH3+, the substrate selectivity of the electrophile increasing in the order CH3FCH,+ (k,/k, 2.0) < CH3CICH3+ (k,/k, 13.8) < CH,BrCH,+ (ky/kB 40.9). The same conclusion cannot be reached for anisole because of the occurrence of extensive O-methylation with formation of dimethylphenyloxonium ion, which turns out to be very stable under the conditions used. The results are consistent with a model involving kinetically predominant attack on the oxygen atom of the ambident substrate, which, in the case of CH,FCH,+ ion, can be, preceded by an extensive electrostatic interaction with formation of a ' chelated ' complex. The same interaction helps also explaining the relatively high reactivity of the ring position ortho to the n-donor substituent, observed at low pressure. The substrate and positional selectivity of the CH,XCH,+ ions toward ambident compounds is discussed and compared with related gas-phase results and with conventional Friedel-Crafts methylation data.C n.d. = not detectable (GM < 1 x

Regio- and Stereochemistry of Gas-Phase Acid-Induced Nucleophilic Substitutions on Chiral Allylic Alcohols1

“…As pointed out in the preceding paper,1b ionization of the bulk CH 3 Cl gas leads to the formation of (CH 3 ) 2 Cl + and CH 2 Cl + as the final ionic species completely unreactive toward the CH 3 Cl molecules. − Thermal (CH 3 ) 2 Cl + (Δ H f ° = 184 kcal mol -1 ) ,, behaves as a pure Lewis acid with a distinct affinity for all the n-type nucleophiles present in the gaseous mixture. − The allylic substrate (either M S or E S ) is the n-type nucleophile deliberately added to the gaseous systems, which, however, also contain H 2 O as an ubiquitous impurity either initially introduced in the mixture together with its bulk components or formed from its radiolysis. The average stationary concentration of H 2 O in the radiolytic systems is estimated to be approximately twice as large as that of the allylic substrate. 1a,b Thus, taking into account the relatively high diffusion rate of H 2 O in the gaseous medium, a significant fraction of the radiolytic (CH 3 ) 2 Cl + ions, instead of attacking the allylic alcohol yielding the corresponding O-methylated derivative (either M S Me + or E S Me + ), is trapped by H 2 O, giving rise to CH 3 OH 2 + .…”

“…[44][45][46][47][48][49][50] Thermal (CH 3 ) 2 Cl + (∆H f °) 184 kcal mol -1 ) 43,51,52 behaves as a pure Lewis acid with a distinct affinity for all the n-type nucleophiles present in the gaseous mixture. [53][54][55][56][57][58] The allylic substrate (either M S or E S ) is the n-type nucleophile deliberately added to the gaseous systems, which, however, also contain H 2 O as an ubiquitous impurity either initially introduced in the mixture together with its bulk components or formed from its radiolysis. The average stationary concentration of H 2 O in the radiolytic systems is estimated to be approximately twice as large as that of the allylic substrate.…”

Intracomplex CH3OH Walking around Optically Active 1-Methyl-3-ethylallyl Cations. A Gas-Phase Kinetic Study

“…Nature of the Ionic Reagents. Ionization of gaseous CH 3 Cl is known to generate both (CH 3 ) 2 Cl + and CH 2 Cl + ions as the final ionic species completely unreactive toward their neutral precursor. − Thermal (CH 3 ) 2 Cl + (Δ H f ° = 184 kcal mol -1 ) , behaves as a pure Lewis acid with a distinct affinity for n-type nucleophiles. − The allylic substrate 1S or 2R (0.07−0.10 mol %) and the base B = (CH 3 O) 3 PO (0.11−0.12 mol %) are n-type nucleophiles deliberately added to the gaseous systems, which however contain also H 2 O as an ubiquitous impurity either initially introduced in the mixture together with its bulk components or formed from its radiolysis. As pointed out in the previous paper, the average stationary concentration of H 2 O in the radiolytic systems is estimated to approach the combined concentration of the added n-type nucleophiles.…”

Gas-Phase Aromatic Amination by Protonated Phenylazide. A Mass Spectrometric and Radiolytic Study

“…Owing to the lack of suitable charged reactants, gas-phase aromatic amination is conspicuously missing among the reactions investigated, which include alkylation by carbenium ions [2][3][4] and by dialkylhalonium ions [5,6], nitration by protonated alkyl nitrates [7,8], silylation by trimethylsilyl ions [9,10], etc.…”