Superacid and H-Zeolite Mediated Reactions of Benzaldehyde with Aromatic Compounds and Cyclohexane. The Role of Mono- and Dicationic Intermediates

Abstract

Reactions of benzaldehyde (1) and related compounds-3-pyridinecarboxaldehyde (9) and acetophenone with benzene, chlorobenzene, o-dichlorobenzene and cyclohexane in the presence of superacids or H-form zeolite, HUSY were (re)investigated. In accordance with previous data, 1 condenses with benzene in triflic acid (CF 3 SO 3 H) as well as on HUSY to give triphenylmethane. At elevated temperature 1 also reacts with chlorobenzene but is inert towards o-dichlorobenzene. While 1 does not react with cyclohexane in CF 3 SO 3 H)SbF 5 , 9 undergoes readily selective ionic hydrogenation to give 3-pyridilcarbinol (14). In the presence of excess of AlCl 3 , however, as well as on HUSY, 1 reacts with cyclohexane to produce toluene. The mechanistic aspects of these reactions are discussed. Opposed to earlier, repeatedly cited suggestions that the reactivity of 1 involves, as in case of 9, superelectrophilic diprotonated species, we show that a monocationic intermediate is the best rationale for these results.

Triflic‐Acid‐Mediated Polycondensation of Carbonyl Compounds with Aromatic Hydrocarbons – A Theoretical Study

“…However, according to the report, [11] the reaction rate increases with the acidity of the media. As we mentioned above, the explanations of this fact based on diprotonation of benzaldehyde [13] or protonation of the s complex 7a is inconsistent with the calculation results.…”

“…[12] Evidently, diprotonation of carbonyl compounds having electron-withdrawing substituents is even more difficult. In a recent paper [13] an alternative explanation of enhanced reactivity of benzaldehyde with benzene in superacid media was suggested where diprotonated benzaldehyde was no longer considered as the reactive species. However, neither experimental nor theoretical evidence was given to favor this mechanism.…”

“…Once the s complex 7a is formed, there is a possibility of proton abstraction by triflate anion to produce carbinol 8a. Another reaction path proposed by the authors [13] to explain the dependence of the reaction rate on the acidity of the medium involves the protonation of 7a at oxygen to give a dication followed by the proton abstraction from sp 3 carbon to produce monoprotonated carbinol 9a. The calculations show that the protonation of 7a is unlikely for two reasons; firstly, this is an endergonic reaction (DG ¼ 8.4 kcal Á mol…”

“…[7] In fact, acetophenone does not react at all with benzene and toluene in 100 molar excess of TFSA even at 130 8C. [13] Figure 8 and 9 show the Gibbs reaction energy profiles for the reactions of 1b and 1c with biphenyl. 1b is a much stronger base than 1c due to the electron-donating effect of CH 3 in 1b.…”

Utilization of the hydroxyalkylation reaction to prepare bis(benzocrown ethers)

“…As such, functionalized benzocrown ethers may be readily prepared using Friedel-Crafts type chemistry. 3 We and others have recently demonstrated that the hydroxylalkylation condensation is an outstanding synthetic method leading triarylmethanes, 4 functionalized heterocycles, 5 and condensation polymers. 6 The hydroxyalkylation reaction typically involves the condensation of an aldehyde or ketone with activated aromatic compounds, such as aryl ethers or phenols.…”