Enzyme-Inspired Lysine-Modified Carbon Quantum Dots Performing Carbonylation Using Urea and a Cascade Reaction for Synthesizing 2‑Benzoxazolinone

Catalysts
as the dynamo of chemical reactions along with solvents
play paramount roles in organic transformations in long-lasting modes.
Thus, developing effective and biobased catalysts in nontoxic solvents
is highly in demand. In this report, carbon quantum dots (CQDs) functionalized
with l-lysine (Lys-CQDs) were generated from entirely nature-derived
materials; they were demonstrated to be a promising catalyst for C–N
bond formation in choline chloride urea (ChCl/U), a natural deep eutectic
solvent (NADES). Among a number of synthesized CQDs, Lys-CQD turned
out to be a powerful catalyst in the model reaction with aniline to
afford phenyl urea. This type of transformation is important because
aniline as a nucleophile has low activity, and urea is a very weak
electrophile but an abundant natural source of the carbonyl moiety
at the same time. The optimized reaction was performed under a highly
desirable condition without using tedious and toxic workup processes
at a low temperature (37 °C for aliphatic amines and 60 °C
for aniline derivatives), as well as by embracing the broad scope
of products in good to high yields even with weak nucleophiles such
as aniline. A proposed acid-activated mechanism was suggested for
the model reaction that was further confirmed by detecting ammonia
as the leaving group. To show further multifunctionality of the catalyst,
a cascade catalysis approach was developed for synthesizing 2-benzoxazolinone,
which was furnished in a two-step transformation, starting from 2-aminophenol.
Using X-ray crystallography, the structure of the final product in
the cascade reaction was also determined. The catalyst was characterized
using various analytical techniques including SEM, TEM, AFM, XRD,
IR spectroscopy, UV–vis spectroscopy, DLS, and fluorescence
spectroscopy. Measuring the acid/base sites by back titration, the
catalyst was shown to be highly functionalized by the lysine functional
group. The size of the catalyst was determined to be in the range
of 1–8 nm, having a well-dispersed surface. In all, Lys-modified
CQD, as a metal-free catalyst, was synthesized, characterized, and
optimized for carbonylation, as well as a cascade reaction, under
mild conditions. The whole process including catalyst synthesis and
organic transformations is economically competitive and fulfills all
requirements toward viability.