Theoretical study of the gas-phase reaction between formyl cation and aromatics

Abstract

Cálculos teóricos da reação do cátion formila (CHO + ) com tolueno, cumeno e p-cresol mostraram que a transferência de próton é termodinamicamente preferida em relação a acilação. No caso do tolueneo, a acilação do anel aromático, para formação do complexo de Wheland, é 11,7 kcal mol -1 mais alta em energia ( H) que a protonação, em nível de cálculo MP4(SDTQ)/ 6-31++G(d,p)//MP2(full)/6-31G(d,p). Esta diferença diminui com a introdução de um grupo hidroxila no anel (p-cresol) ou substituindo o grupo metila por isopropila (cumeno). A protonação do tolueno pelo H 3 + e pelo cátion isoformila é, respectivamente, 88,6 e 84,5 kcal mol -1 mais exotérmica que a acilação em nível MP4(SDTQ)/6-31++G(d,p)//MP2(full)/6-31G(d,p). A afinidade protônica do p-cresol foi calculada como sendo 195,4 kcal mol -1 .Theoretical calculations of the reaction of formyl cation (CHO + ) with toluene, cumene and p-cresol showed that proton transfer is thermodynamically preferred over acylation. For toluene, acylation of the aromatic ring, to form the Wheland complex, is 11.7 kcal mol -1 higher in energy ( H) than protonation, at MP4(SDTQ)/6-31++G(d,p)//MP2(full)/6-31G(d,p) level of theory. This difference reduces upon introduction of a hydroxy group in the ring (p-cresol) or replacing the methyl group by an isopropyl group (cumene). Protonation of toluene by H 3 + and isoformyl cation is 88.6 and 84.5 kcal mol -1 , respectively, more exothermic than acylation, at MP4(SDTQ)/ 6-31++G(d,p)//MP2(full)/6-31G(d,p). The proton affinity of p-cresol was calculated to be 195.4 kcal mol -1 .Keywords: formyl cation, Friedel-Crafts acylation, gas phase ion reaction, proton transfer IntroductionFriedel-Crafts acylation is an important reaction in organic and industrial chemistry.1 Many important chemicals, like fragrances, pharmaceuticals and dyes are industrially produced through acylation steps. In addition, the Gatterman-Koch reaction 2 is a useful procedure to produce benzaldehydes, through the reaction of carbon monoxide and an aromatic compound, catalyzed by a strong Bronsted acid, like HF.Acylation of aromatic compounds has long been studied in the gas phase.3,4 Speranza and Sparapani showed that gas phase acetylation of alkylbenzenes does not follow the same reactivity trend found in solution, 5 suggesting that solvent, counter ion and catalyst play a major role in the reactivity of the CH 3 CO + cation in condensed phase. Strong electrophilic acylium ions, such as CF 3 CO + , are able to acylate deactivated substrates, such as trifluorobenzene, in the gas phase.6 Cerichelli et al. 7 reported that formyl cation behaves as a Bronsted acid in gas phase, exclusively transferring a proton to toluene, rather than producing formylated products. This behavior contrasts with the reactivity of C 2 H 5 + and i-C 3 H 7 + cations in gas phase, both capable of acting as Lewis (alkylation) and Bronsted (protonation) acids toward toluene. In the present study we carried out theoretical calculations on the thermochemistry of acylation (formation of the Whela...

Mechanistic models for the intramolecular hydroxycarbene–formaldehyde conversion and their intermolecular interactions: Theory and chemistry of radicals, mono‐, and dications of hydroxycarbene and related configurations

“…H . In the formyl cation, the theoretical CO and CH distances are 1.154 Å and 1.092 Å, respectively 11. The formyl cation was experimentally obtained with carbon monoxide dissolved in very strong acids as HF‐SbF 5 under pressure 12.…”

Theoretical Studies on Si−C Bond Cleavage in Organosilane Precursors during Polycondensation to Organosilica Hybrids

“…However, calculation of the activation energy based on the transition state structure is generally difficult and time-consuming, even for a small molecular system. Instead, we employ the stability of the σ-complex, which can be defined as the energy difference between the σ-complex and the original substrate, as an index for the occurrence of Si−C bond cleavage, because the reactivity of aromatic molecules for electrophilic substitution has been known to correlate well with the relative stabilities of their σ-complexes. , For example, several studies have revealed that reactivity at the ortho -, meta -, and para -positions sites of monosubstituted benzenes well correlate with the theoretically calculated stabilities of their σ-complexes. – In particular, Szabo et al investigated the nitration of benzene and toluene by ab initio calculations and demonstrated that the higher reactivity of toluene than that of benzene and the ortho/para selectivity could be well explained by the stability of their σ-complexes . They also succeeded in calculations of the transition state structures and activation barriers for σ-complex formation for benzene and toluene with consideration of the desolvation process and showed that the activation barrier order was in good agreement with the stabilities of the σ-complexes.…”

“…13,14 For example, several studies have revealed that reactivity at the ortho-, meta-, and para-positions sites of monosubstituted benzenes well correlate with the theoretically calculated stabilities of their σ-complexes. [15][16][17][18] In particular, Szabo et al investigated the nitration of benzene and toluene by ab initio calculations and demonstrated that the higher reactivity of toluene than that of benzene and the ortho/para selectivity could be well explained by the stability of their σ-complexes. 18 They also succeeded in calculations of the transition state structures and activation barriers for σ-complex formation for benzene and toluene with consideration of the desolvation process 19 and showed that the activation barrier order was in good agreement with the stabilities of the σ-complexes.…”