**4. Conclusions**

Since 2000, the time the first seminal publication by List, Lerner, and Barbas III on the intermolecular asymmetric aldol reaction catalyzed by proline appeared, a countless number of papers focused on enamine organocatalysis with the aim to solve a few critical issues inherent in the use of proline. Summarizing, high catalyst loading, long reaction times, solvent limitations owing to proline solubility, variable stereocontrol mainly dependent on the donor-acceptor aldol partners, di fficult and/or expensive product isolation, and catalyst recovery characterize the proline-catalysed aldol protocol. On the other hand, advantages have been previously underlined such as low cost, no toxicity, no need for anhydrous solvents or controlled atmosphere, and process practicality.

Over these two decades, the greatest e fforts have been dedicated to the design and synthesis of new catalysts, mostly sharing with proline the chiral pyrrolidine sca ffold. These derivatives allow to enlarge the platform of solvent candidates, up to enabling the possibility of catalyst recycling. Reaction kinetics improve with shorter reaction times and lower catalyst loadings. If these improvements are beyond doubt, the costs coupled to their preparation are clearly a limiting factor. On the other hand, it is known that a number of common solvents have been questioned in recent years as their hazardous properties have come to light, for example, the environmental, safety, and health issues associated to the use of DCM, toluene, DMSO, and others.

The work presented here shows that very good results can be simply achieved using methanol/water mixtures as reactionmedium. When only water is used, these reactions take place in a typical heterogeneous conditions (emulsions), where the interphase water has as many as about a quarter of the O–H bonds not being involved in hydrogen bonding. According to Jung and Marcus [140], the interactions of these unbound hydroxyl groups with organic reactants and, more importantly, with the transition states, lower the activation energies, enabling rate and yield enhancements. Faster reactions occur in pure methanol because of the homogeneous conditions, which allow all the amount of proline used to participate to catalysis, but this superior reactivity is characterized by a lower stereocontrol. Recent papers evidenced, by DFT calculations, the positive effects of co-additives such as water or methanol in stabilizing the transition states of the aldol reaction, with methanol displaying the larger effects [24,141–143]. These protic additives could directly participate in the reaction mechanism, acting as an active proton transfer relay between the proline carboxylic acid group and the incoming aldehyde. The amount and the nature of the protic additive could significantly change the reactivity and stereoselectivity of this transformation, as transition states with one, two, and even three molecules of the additive have been located and described.

If both methanol and water as pure solvents give largely unsatisfactory results that discouraged further investigations, we demonstrated that methanol/water mixtures provide the high reaction rates (good yields in short reaction times) typical of methanol and the high stereocontrol typical of water. The efficient, simple, and cost-effective reaction protocol proposed, easily scaled up here up to the 100 mmol scale, as well as the safe handling of the methanol/water mixture, positively impact the overall efficiency and sustainability of this proline-catalysed aldol protocol. However, we have to observe that, also following this procedure, the recurring dependence of relative reaction rates and stereochemical outcome on the nature of the donor-acceptor pair has not been overcome. Thus, cyclohexanone is the best donor in terms of reactivity and stereocontrol, while cyclopentanone works faster, but with a much lower stereocontrol. Electron-rich aromatic aldehydes are the slowest reaction acceptors, requiring long reaction times, while electron-poor aldehydes are the best. Nevertheless, given that the usual relative behavior of ketones and aldehydes is confirmed, the aldol protocol in methanol/water can be considered a useful contribution, enabling the achievement of performance never obtained before (also for less reactive compounds), employing the smallest and cheapest organocatalytic species, proline.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/10/6/649/s1, Table S1: Enantioselectivity variation as a function of reaction time; Figure S1: 1H-NMR spectrum of commercial proline; Figure S2: 1H-NMR spectrum of recovered proline employing work-up method C (Table 8); Figure S3: 1H-NMR spectrum of recovered proline employing work-up method D (Table 8), CSP-HPLC separation conditions and chromatograms of aldols **3** (racemic and enantio-enriched), full characterization of *anti* aldol product **3ad** (CSP-HPLC chromatogram of enantio-enriched product, 1H-NMR spectrum, 13C-NMR spectrum, HPLC-MS chromatograms, ESI-MS spectrum).

**Author Contributions:** Conceptualization, M.L., A.Q., and C.T.; methodology, M.L., A.Q., and C.T.; validation, A.M. and A.Q.; investigation, M.G.E., A.T., and A.Q.; resources, M.L., A.Q., and C.T.; data curation, M.L., A.Q., and C.T.; writing—original draft preparation, A.Q.; writing—review and editing, A.M., M.L., A.Q., and C.T.; visualization, A.M. and A.Q.; supervision, C.T.; project administration, M.L. and A.Q.; funding acquisition, M.L., A.Q., and C.T.. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research and the APC were funded by Università di Bologna (RFO) and MIUR (Rome).

**Acknowledgments:** We thank R. Miani for the execution of some experiments.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
