The reaction of silylformamidine with various halo‐ and ethoxycarbonyl substituted thiophenes was studied. It proceeded via C−H bond insertion of the nucleophilic diaminocarbene that exists in an equilibrium with silylformamidine affording aminals. The reaction proceeded more readily at the 2(5)‐position compared to the 3(4) one. The number of halogens at the thiophenes markedly influences the reaction. The more halogens, the higher the reaction rate. Even one electron acceptor group at the thiophene ring is enough to promote the reaction. As a rule, the reaction does not require solvents or catalysts. Hydrolyses of the aminals provides a novel convenient approach to substituted thiophene aldehydes.
A novel two‐step method for formylation of fluoropyridines with silylformamidine 1 under catalyst‐free conditions was developed. A series of possible 18 fluoropyridines featuring one to four fluorine atoms were subjected to the reaction with 1 existing in equilibrium with its carbenic form 1′.12 Fluoropyridines were shown to react via C‐H insertion. The reaction proceeded either at β‐ or γ‐positions affording the corresponding aminals. The more fluorine atoms in pyridines, the easier the reaction proceeded. We also hypothesized that the pyridines in which the fluorine was substituted by other halogens would react in a similar manner. To test the hypothesis, a set of 3,5‐disubstituted pyridines with various combination of halogen atoms was prepared. 3,5‐Difluoropyridine was taken as a compound for comparison. All the pyridines in the series also reacted likewise. In most cases, hydrolysis of the aminals afforded the corresponding aldehydes. As DFT calculations indicate, the reaction mechanism includes deprotonation of pyridine by 1′ as a strong base and the following rearrangement of the formed tight ionic pair to the final product. An alternative reaction pathway involving addition of 1′ to the pyridine carbon with the following hydrogen transfer via a three‐membered transition state structure required much higher activation energy.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.