|Ph.D Student||Cohen Irit|
|Subject||Photo-Assisted Amidation Using Charge-Transfer Complexes:|
Direct Photo Amidation of Carboxylic Acids
|Department||Department of Chemistry||Supervisors||Professor Yoav Eichen|
|Professor Alex.m Szpilman|
|Full Thesis text - in Hebrew|
We report the direct amide formation from carboxylic acids and amines assisted by visible light. The process is based on a weakly absorptive complex of triethylamine and tetrachloromethane (TEA•••CCl4). This process is advantageous, as it requires no expensive and toxic transition metals. Any visible light source, such as the mercury arc lamp, blue LED and, crucially, the sun, can be used to enable this process. Moreover, the process can also be scaled up to gram scale.
The addition of triethylamine to a solution containing an excess of tetrachloromethane in dichloromethane results in the formation of a new light absorbing species which is likely to be the 1:1 TEA•••CCl4 complex. Other tri-alkyl amines, such as tributylamine, were also shown to form similar charge-transfer complexes with tetrachloromethane, exhibiting comparable absorption characteristics. In contrast, mono-alkyl amines and di-alkyl amines, such as butylamine and dibutylamine, form much weaker complexes with tetrachloromethane, which exhibit absorption at shorter wavelengths.
The UV-VIS absorption spectra of the charge-transfer complexes show strong absorption at UV wavelengths. However, they also absorb significantly at near- visible wavelengths above 350 nm. The absorption of trimethylamine and tetrachloromethane at these wavelengths, however, is found to be insignificant.
Based on these observations, we contemplate the synthetic application of this complex in photoredox reactions assisted by solar light while removing the UV radiation by using the filtering properties of the borosilicate glass walls of glass vessels. When using a mercury lamp system, we use a filter with cut-off at 400 nm for removing the UV part of the spectrum.
The irradiation of the charge-transfer complex using visible light led to the formation of iminium ion. The carboxylate attack the iminium ion to generate intermediate (similar to hemi-aminal ester). In itermolecular mechanism, the electrophilic carbonyl function is attacked by di-ethyl amine to give intermediate which in turn generates the products diethyl amide, aldehyde and ethylamine. Alternative mechanism is intramolecular, in which the electrophilic carbonyl function is attacked by the amine aminal moiety leading to intermediate, which in turn generates the products ethyl amide and aldehyde.
Ideally, the reaction should be performed under an inert atmosphere, with an excess of amine and potassium carbonate acting mostly as a dispersant and reflector of the incident light, rather than as a base. Therefore, it greatly enhances the distance traveled by the light through the reaction mixture. The reaction can be scaled up to 3 and 10 grams scale, clearly indicating the benefit of using a light harvesting complex with low absorptivity. Whereas secondary and tertiary amines are only formed charge-transfer complexes that absorb in the visible region, preliminary tests demonstrated that a primary amine can form amides in the reaction by using 1,4-diazabicyclo[2.2.2]octane (DABCO) as a sacrificial amine. This also enables us to reduce the amount of nucleophilic amine to 1.5 equivalent.