Abstract
This mini-review presents a general overview of the progress achieved during the last decade on the amalgamation of CuAAC processes (copper-catalyzed azide-alkyne cycloaddition) with the employment of sustainable solvents as reaction media. In most of the presented examples, the use of water, glycerol (Gly), or deep eutectic solvents (DESs) as non-conventional reaction media allowed not only to recycle the catalytic system (thus reducing the amount of the copper catalyst needed per mole of substrate), but also to achieve higher conversions and selectivities when compared with the reaction promoted in hazardous and volatile organic solvents (VOCs). Moreover, the use of the aforementioned green solvents also permits the improvement of the overall sustainability of the Cu-catalyzed 1,3-dipolar cycloaddition process, thus fulfilling several important principles of green chemistry.
Highlights
The employment of copper catalysts, as opposed to the use of stoichiometric amounts of other typical promoters, has permitted the discovery of a plethora of new synthetic protocols within the toolbox of organic chemists [1]
This is due to the following properties: (i) it is an earth-crust abundant first row transition metal, and cheaper than other second or third row precious metals; (ii) it presents a wide variety of oxidations states (Cu(0), Cu(I), Cu(II) or even Cu(III)) with different coordination environments; (iii) it accommodates both hard or soft donor ligands in its coordination sphere, and forms σ- or/and π-interactions with unsaturated organic substrates; (iv) it is an important metal in Biology [5], being the less toxic first row metal; and (v) it is active under both homogenous or heterogeneous [6] conditions
The first work in this field was reported back in 2006 by Sreedhar and co-workers who describe an efficient and reliable CuI-catalyzed three-component reaction that employs terminal alkynes, NaN3 and Baylis-Hillman acetates as starting materials for the high-yielding synthesis of 1,4-disubtitutes triazoles in polyethylene glycol (PEG) at room temperature [36]
Summary
The employment of copper catalysts, as opposed to the use of stoichiometric amounts of other typical promoters (like Brønsted acids, bases, radicals, etc.), has permitted the discovery of a plethora of new synthetic protocols within the toolbox of organic chemists [1]. The first work in this field was reported back in 2006 by Sreedhar and co-workers who describe an efficient and reliable CuI-catalyzed three-component reaction that employs terminal alkynes, NaN3 and Baylis-Hillman acetates as starting materials for the high-yielding synthesis of 1,4-disubtitutes triazoles in PEG at room temperature (see Scheme 4) [36] At this point it is important to note that better yields of the desired triazoles were obtained when hazardous and volatile organic solvents (like THF, CH3 CN or DMSO) were replaced by PEG. Cu-Catalyzed 1,3-Dipolar Cycloaddition of Azides and Alkynes (CuAAC) in Deep Eutectic
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