Superacid-Catalyzed Formylation of Aromatics with Carbon Monoxide

Article abstract of DOI:10.1021/jo00209a025

Superacid-catalyzed formylation of aromatics, including benzene, xylenes, mesitylene, and ethylbenzene, with carbon monoxide was investigated.Under comparable reaction conditions, the yield of aromatic aldehydes increases with increasing acidity of the catalyst systems.With CF3SO3H + HF + BF3 and CF3SO3H + SbF5 high yields of aldehydes were obtained under mild conditions even at atmospheric pressure and at 0 deg C.With CF3SO3H + TaF5 and CF3SO3H comparable conversions were only obtained under CO pressure and by using excess acid.In formylation of toluene, in situ further reaction of protonated tolualdehyde with excess toluene gave isomeric ditolylmethanes via the intermediate formation of ditolylmethyl alcohol, together with traces of tritolylmethane.The ditolylmethyl cation formed via ionization of the alcohol reacts with CO to form the corresponding acyl ion, giving subsequently aldehyde or upon aqueous workup some ditolylacetic acid.The amount of these byproducts was 18-20percent for toluene and 3-5percent for xylenes, and none was observed in reaction with mesitylene.Competing acid-catalyzed disproportionation of starting alkylbenzenes led to somewhat more complex reaction mixtures in the case of xylenes and ethylbenzenes.Whereas high positional selectivity was obtained in all systems studied (91-94percent para isomer), the substrate selectivity (k_T/k_B) varied.With CF3SO3H + HF + BF3 and CF3SO3H + SbF5 it was found lower than in conventional Gatterman-Koch reaction with AlCl3 + HCl or AlCl3 + Cu2Cl2 + HCl and was comparable to that observed in the reaction of HCOF + BF3.Control experiments showed that no decarbonylation of mesitaldehyde and p-tolualdehyde occurs under the reaction conditions.To establish the nature of the electrophilic formylating agent, protolysis of 13C-labeled O-protonated formic acid, H-13C(OH)2⁺, was studied in CF3SO3H + SbF5 + SO2ClF solution in an attempt to observe the elusive formyl cation at low temperature (-80 deg C) by 13C NMR spectroscopy, but the formyl cation was not observed.Only exchanging 13CO could be detected, suggesting that in the superacid media a rapidly equilibrating protosolvated ion is involved.