In recent years, the growing environmental awareness has prompted an
increasing demand for eco-friendlier technologies, acting consequently as a main driving
force for exploring greener methodologies. One of the most fundamental approaches in
organic synthesis is the activation of the carbonyl group. This step is essential for the
performance of a plethora of organic reactions. Herein, we provide a brief overview of
molecular halogens and N-halamines in the context of their application as acid- and
metal-free catalysts. Susceptibility of the carbonyl group to the nucleophilic attack allows
the construction of numerous organic compounds. The existence of oxygen lone pairs
puts the carbonyl moiety into the context of a Lewis base, prone to activation in the
presence of a Lewis acid, which has been observed and extensively investigated over the
last decade. The noncovalent interactions – halogen bonds – provided by halogen atoms
in haloorganic compounds may be assumed responsible for their catalytic activity.
Molecular halogens and N-halamines act as convenient, easily-handled, low-priced
catalysts/mediators and, more importantly, the ones that can presumably act as Lewis
acids. These characteristics prompted this class of compounds in the research focus
aiming at substantial advances in organic synthesis. Finally, an optimal reaction method
(where the carbonyl moiety activation represents an essential step) should meet the
following criteria: i) an easily-manipulable, low-cost, non-metal, water- and air-tolerant
catalyst, ii) mild and solvent-free reaction conditions, iii) no need for simultaneous water
removal, and, iv) stoichiometric amounts of activators or large excesses of reagents.
Keywords: Carbonyl group, Catalysis, Green chemistry, Halogen bond,
Molecular halogens, N-halamines.
