A One-Pot Procedure for The Synthesis of Aromatic Aldehydes in a Heterogeneous Medium

Aromatic aldehydes were obtained from halogenated compounds and DMSO in solvent free medium. The method involves a Kornblum's oxydation of organic halide in mild conditions using microwaves. An inorganic base such as solid potassium bicarbonate is used. The procedure is a smooth alternative to obtain aromatic aldehydes in heterogeneous medium. The important benefits of the method are also the absence of catalysts, the low time, and the good yield of the synthesis.

Kornblum reaction of aromatic aldehydes synthesis becomes possible under microwaves. All reagents were well mixed in a beaker. In the resulting heterogeneous medium during the irradiation period hot spots are created. They are similar to catalytic centers and allow the reaction to proceed rapidly [21].
The irradiation time was 2-4 minutes, then the product was extracted with diethyl ether. Evaporation of the extraction solvent leads to aromatic aldehyde (table 1).
The Kornblum oxidation reaction takes place on the surface of the potassium bicarbonate granules. The resulting transition state of lower energy increases the reaction velocity of arylmethyl bromide oxidation. The reaction mechanism was previously depicted and is shown below [22]: The electron impact mass spectrum of 4-(N,N-dimethylamino) benzaldehyde was studied ( fig. 1) and shows the presence of the molecular ion peak at m/z 149. This peak is assigned to the unfragmented radical cation M .
 and it is also called the parent's peak or the molecular ion [16]. A very low satellite peak (M+1) at m/z 150, near the parent ion peak, is due to the isotopic distribution of the elements in 4-(N,N-dimethylamino) benzaldehyde. The molecular ion peak and the base peak are different. The base beak (tallest peak, I%=100) was identified at m/z 148 and corresponds to (4-(dimethylamino)phenyl)(oxo)methylium cation. Three fragmentation pathways in electron impact mass spectra of 4-(N,N-dimethylamino)benzaldehyde stand out (A, B, C, scheme 1). https://doi.org/10.37358/RC.20.2.7893 In the first fragmentation pathway (A), the molecular radical cation M .

Scheme 1. Fragmentation mechanism of 4-(N,N-dimethylamino) benzaldehyde
In the second fragmentation pathway (B), the molecular ion M .
 . of p-(N,N-dimethylamino) benzaldehyde, by an α cleavage of the covalent bond, releases a dimethylamino radical to form the stable benzoyl cation (m/z 105) according to Stevenson's rule. Furthermore, the latter losses a molecule of carbon monoxide and forms the phenyl cation (m/z 77). This radical cation is a high-energy, unstable specie and releases an acetylene molecule from the Dewar valence isomer structure to give an antiaromatic cyclobutadiene cation (m/z 51). The loss of cyclopropenylidene, or 3-carbon ring carbene, C3H2, leads to the aromatic cyclopropenyl cation (m/z 39).
The third fragmentation pathway (C) leads to the charge-delocalized cation of N-(cyclohepta-2,6-dien-4-yn-1ylidene)-N-methylmethanaminium (m/z 132) by hydroxyl radical cleavage from the molecular ion. Having an electronic structure in which the positive charge can be delocalized to eight atoms, this cation has an important relative abundance in the mass spectrum of the compound. In any case, this cation at m/z 132 (pathway C) is more stable than the cation at m/z 90 (pathway A) as it results from the height of their peaks.

Conclusions
Aromatic aldehydes can be obtained from arylmethyl bromides by an easy protocol. The Kornblum oxidation carried out with DMSO in the presence of potassium bicarbonate in a heterogeneous medium occurs within a few minutes. The removal of the compounds from the reaction medium is not difficult and the yield of the synthesis is high. Last but not least, using microwaves, energy consumption is much lower than conventional approach.