Sunlight Assisted Direct Amide Formation via a Charge-Transfer Complex

Natural photosynthesis carried out by plants is one of the foundations of life on Earth.¹ In recent years, solar energy was successfully harnessed for artificial photosynthesis of man made materials, mostly on research lab scale.² Solar energy is also successfully harnessed for producing heat and electricity.³ All these processes are considered sustainable, ecofriendly, and take advantage of the zero cost energy source, the sun.^3,4

Herein, we report the use of solar light induced activation of weakly absorbing charge-transfer complexes, formed between amines and carbon tetrachloride, for the photo-synthesis of amines (Scheme 1).⁵

Advantages of this process include mild reaction conditions, transition metal free synthesis, and inexpensive commercially available starting materials. The low absorption coefficient of the complex allows deep light penetration and thus easily scale-up to at least grams scale.

Scheme 1: Photo-assisted synthesis of amides from amines and carboxylic acids using R₃N:•••CCl₄ charge transfer complexes.

Mechanistic investigations of the system reveal the formation of an iminium ion through light activation of the charge transfer complex between amine and CCl₄. The iminium ion acts as a combined dehydrating agent and activator of the carboxylic acid. The mechanism is supported by extensive experimental studies, and computations support the proposed key charge-transfer complex. The principles outlined herein are currently finding applications in other settings in our laboratory, the results of which will be reported in due course.

References

1. Robinowitch, E.; Govindjee. Photosynthesis, John Wiley & Sons, Inc., New York, 2014.
2. Marcus et al. Chem. Rev. 2014, 114, 11863.
3. Mekhilef et al. Sust. Energ. Rev. 2011, 15, 1777.
4. Prier et al. Rev. 2013, 113, 5322.
5. Cohen, I. Mishra, A. K.; Parvari, G.; Edrei, R.; Dantus, M.; Eichen, Y.; Szpilman, A. M. Commun. 2017, 53, 10128.