## 1 Introduction
** Generation of dihydroxyacetone from glycerol by transfer hydrogenation (TH). In order to minimize the decomposition of dihydroxyacetone, [PERSON] and co-workers also explored the ability of the iridium(I) complex **1** (Figure 1), previously described as an efficient catalyst for TH reactions under base-free conditions in isopropanol media, to promote the reduction of acetophenone in glycerol [46]. Unfortunately, complex **1** turned out to be almost inactive (yields \(<8\%\)). In contrast, when benzaldehyde, a more reactive carbonyl compound, was used as the starting material it could be reduced to benzyl alcohol under these base-free conditions, albeit with moderate conversion (up to 46%). Once again, despite the absence of base, the dihydroxyacetone formed still underwent degradation, its final proportion being well below than that of benzyl alcohol (12%-24% _vs._ 18%-46%). The hydrogen transfer from glycerol to benzaldehyde was also explored using a catalytic system composed of the ruthenium(II) dimer [{RuCl(\(\upmu\)-Cl)(\(\eta^{6}\)-\(p\)-cymene)}\(\flat_{2}\)] and KOH (2 mol% of Ru, 12 mol% of base) [47]. Both conventional oil-bath heating (70 \({}^{\circ}\)C) and microwaves (MW) irradiation (domestic oven, 900 W) led to moderate yields in benzyl alcohol (33%-48%) after 24 h, and complete transformation of benzaldehyde could only be achieved by increasing the ruthenium loading to 25 mol% [48]. In contrast, when the same reaction was performed under ultrasounds (US) irradiation, using a combination of KOH and NaOH (1-2 mol%), quantitative formation of benzyl alcohol was achieved at 60 \({}^{\circ}\)C in 3 h with only 1 mol% of [{RuCl(\(\upmu\)-Cl)(\(\eta^{6}\)-\(p\)-cymene)}\(\flat_{2}\)] (Scheme 4) [49]. The authors argued that, under sonochemical conditions, the dispersion of the base in glycerol is optimal and this would explain the shorter reaction time observed. In all these reactions, conversion of glycerol to dihydroxyacetone was proposed, but in no case experimental evidences of their detection and quantification were given. A series of Ir(III) and Ir(I) complexes containing \(N\)-heterocyclic carbene (NHC) ligands **2-5** (Figure 2) were studied by [PERSON] and co-workers as potential catalysts for the transfer hydrogenation of carbonyl compounds in glycerol [50, 51]. In the presence of KOH (1 equivalents), all of them (2.5 mol% of Ir) were active in the reduction of acetophenone and benzaldehyde, leading to the desired alcohols in low to excellent yields after 7 h of heating in an oil-bath at 120 \({}^{\circ}\)C [51]. In general, the  
Figure 1: Structure of the iridium(I) complex **1**. reduction of benzaldehyde proceeded with comparatively higher yields than benzophenone (66%-99% _vs._ 25%-80%), with complexes **3** and **5a** being particularly effective for the reduction of the former, and **3** and **4b** for the latter. Under the same experimental conditions complex **3** was also able to reduce benzophenone in 91% yield, albeit a longer reaction time was in this case required [50]. In the search of activity improvements, the reduction of acetophenone and benzaldehyde by complexes **2-5** was studied using MW and US activation instead of classical oil-bath heating [51]. In particular, the use of ultrasounds had a tremendous impact in shortening the reaction times, and very good results were obtained in the reduction of benzaldehyde even with a lower iridium loading (e.g., 73% yield of benzyl alcohol was achieved after 5 min using 1 mol% of complex **4a**). Again, optimal dispersion of the base in glycerol was evoked to explain the good results obtained under sonochemical conditions. However, we must note that partial decomposition of complexes **2-5** into catalytically inactive Ir(0)-containing nanoparticles was observed using MW and US techniques.