Cation−π Interactions in the Gas Phase Methylation of α,ω-Diphenylalkanes

Abstract

The methylation of α,ω-diphenylalkanes (C6H5(CH2) n C6H5, n = 1−6) has been performed in the gas phase using Me2Cl+ ions as alkylating species and toluene as reference substrate. Both in radiolytic experiments at atmospheric pressure and in FT-ICR measurements at 10-8 Torr, the selected diphenylalkanes reacted faster than toluene, the highest reactivity displayed by 1,3-diphenylpropane. The kinetic pattern of the reaction, conforming to the established scheme of an electrophilic alkylation reaction, is consistent with a rate-determining formation of the σ-complex intermediate, at variance with the tert-butylation of the same series of compounds by Me3C+ ions, occurring at the collisional encounter rate. The kinetic features are explained by a marked effect due to the presence of the second aryl ring, providing additional stabilization of both the ion−neutral collision complex and the σ complex with respect to toluene. Both factors contribute to the δE a of ca. 8 kcal mol-1 for the competition of 1,3-diphenylpropane and toluene found in the temperature dependence study of the Me2Cl+ reaction.

Complexes between Lithium Cation and Diphenylalkanes in the Gas Phase: The Pincer Effect

“…Thus, the presence of the second ring was found to affect the competition between proton and hydride transfer within the [tC 4 H 9 + ···a,w-diphenylalkanes] complexes [25][26][27] and the regiospecificity of the latter process, [26,28] in addition to the kinetics of gas-phase alkylation of a,w-diphenylalkanes under radiolytic conditions ("spectator ring effect"). [29,30] The simultaneous coordination of Fe + ions to the two aromatic rings of homologues a,w-diphenylalkanes has also been invoked to explain the highly regioselective intramolecular dihydrogen abstraction, [31] and a similar coordination of Cr + ions to a,w-diphenylalkanes has been deduced from the reactivity of the bare metal ions with these bidentate ligands. [32] Results and Discussion Experimental lithium cation basicities (LCB values): The experimental determination of the LCB values for Ph-CH(Me)-Ph and Ph-(CH 2 ) n -Ph (n = 2, 3, 7) was based on the nine reliable equilibrium measurements for which the resulting DLCB values and inferred LCB values are reported in Table 1.…”

Cation−π Interactions in Protonated Phenylalkylamines

“…The higher GB of 1,3-diphenylpropane with respect to both its next lower and higher homologue has been ascribed to the possibility, with respect to its nearest homologues, of attaining a parallel arrangement of the neutral and the protonated rings that is allowed by the folding of the (CH 2 ) 3 chain in an all staggered conformation at the C–C bonds . Not only thermodynamic data but also a wealth of kinetic evidence points to the role played by a phenyl group attached to an alkylated arenium ion by a polymethylene chain in approaching, solvating, and reacting with the charged σ complex and also stabilizing the transition states leading to its formation. − Metal ion complexes formed in a gas-phase reaction with α,ω-diphenylalkanes attain a sandwich-type structure by simultaneous coordination of the metal cation with the two aryl rings. The key parameter controlling the occurrence and extent of coordination to both rings is once again the length of the polymethylene chain joining the two rings. − In a similar context, the gas-phase reactivity behavior of α,ω-phenylalkyloxonium ions appears to be largely controlled by the electrostatic stabilization afforded by the approach of the phenyl ring to the oxonium moiety through the folding of the aliphatic chain …”

14 Organic gas-phase ion chemistry

“…Chiavarino et al presented a study of the reaction of a,v-diphenylalkanes (C 6 H 5 (CH 2 ) n C 6 H 5 , n ~1-6) with Me 2 Cl 1 ion. 99 Cation-p interactions were investigated by measuring the kinetics of the methylation reaction, and the authors propose that methylation occurs via formation of a s-complex with MeCl loss.…”