Green Regio‐ and Enantioselective Aminolysis Catalyzed by Graphite and Graphene Oxide under Solvent‐Free Conditions

Abstract

The ring-opening reactions of epoxides with amines were efficiently and regioselectively catalyzed by high-surface-area graphite and graphene oxide under metal-free and solventfree conditions. For epoxides withouta ryl groups, catalytic activity waso bserved only for graphene oxide,a nd hence, the activity must have been due to its acidic groups. For styrene oxide, instead,g raphite and graphene oxide exhibited rather similar catalytic activities, and hence, the activity was mainly due to activation of the electrophilic epoxide by p-stacking interactions with the graphitic p system.T he described aminolysis procedure is green and cheap because the catalyst can be recovered and recycled without loss of efficiency.M oreover, these heterogeneous catalysts exert high stereoselective control in the presence of nonracemic epoxidesa nd provide chiral b-aminoa lcohols with enantiomeric excess values up to 99 %.Epoxider ing-opening reactions are useful methods to provide multifunctionalc ompounds ready to be used as versatile intermediates in total synthesis or as precursors of relevant molecules. As three-membered heterocyclic rings,e poxides are more reactive than ethers owing to ring strain anda re susceptible to attack by ar ange of nucleophiles, including nitrogen (e.g.,a mmonia,a mines, and azides), oxygen (e.g.,w ater,a lcohols, phenols, and acids),a nd sulfur (e.g.,t hiols) containing compounds, and this leads to bifunctional molecules of great industrial value.In particular, epoxy ring-opening reactions with amines are well documented, and with the choice of specific physical parameters (e.g.,h eating, [1,2] microwave, [3][4][5] and ultrasound [5] ), the use of polar reaction media (e.g.,i onic liquids, [6] fluoro alcohols, [7] or water under differentp Hc onditions [8,9] ), or the use of catalysts or activators as homogeneousc atalysts (e.g., Brønsteda cids and bases [6,[9][10][11][12][13][14] ands everalm etal salts and/or complexes [15][16][17][18][19][20][21][22][23][24][25][26][27][28] ), good results have been achieved, although most suffer from poor regioselectivity,h igh temperature, and/ or stoichiometrica mountso ft he catalysta nd the use of excess amounts of reagents.The use of heterogeneous catalyst as solids (e.g.,silica gel, [29] nanosilica, [30] functionalized mesoporouss ilica, [31][32][33] alumina and/or modifieda lumina, [34][35][36][37] nanoaluminos ilicates, [38][39][40][41][42] montmorillonite-K10 clay, [43] sulfated zirconia, [44,45] annotitanium dioxides, [46] heteropolya cids, [47] polyoxometalatei norganic metal oxygen clusters, [48] Amberlyst-15, [49] nanocrystalline zircosilicate, [50] and iron oxides [51] )h as tried to meet the need for more sustainable protocols that assure good regioselective control,o ften lost to competitive polymerization/isomerization of the epoxides.However,ametal-free, highly regioselective procedure is still missing. One of the emerging promises in sustainable chemistry is carbon-based materials, whicha re already used in some important synthetic reactions ...

Graphene‐Based Carbocatalysts for Thermoset Polymers and for Diastereoselective and Enantioselective Organic Synthesis

“…For most of the considered reactions, catalytic behavior is observed for GO but not for HSAG. Notably, HSAG shows catalytic activity comparable to that of GO for a few reactions …”

“…As also discussed in a previous section, the ring‐opening reactions of epoxides with amines are catalyzed only by GO (and not by HSAG) for epoxides that do not contain aryl groups. In these cases, the catalytic activity is possibly due to the acidic groups of GO . However, both GO and HSAG are efficient and regioselective catalysts for epoxides containing an aryl group, such as styrene oxide (Scheme ).…”

“…This kind of heterogeneous catalysis, which is effective for both GO and HSAG and is, hence, based on π‐stacking interactions with the graphitic π system, also shows high stereoselective control for nonracemic epoxides. For instance, chiral β‐amino alcohols with enantiomeric excess values up to 99 % are obtained in both GO‐ and HSAG‐catalyzed reactions …”

“…The fourth section describes reactions that are catalyzed not only by graphene oxide (being acidic and rich of functional groups) but also by HSAG. The observed behavior is rationalized by the predominant catalytic role of π stacking of the aromatic substrates with the graphitic planes . This feature is also determinant for unexpected diastereoselective and enantioselective catalytic behaviors.…”

“…In particular,i ti ss hown that the main drawbacks of carbocatalysts, that is, high catalyst loading (in weight amountsc lose to those of the reactants) and generally poor selectivity,c an often be removed by operating under solventfree conditions. [23][24][25][26][27][28][29] The third section is devoted to the dual role of graphene-based materials,t hat is, as ac atalysta nd as a filler,f or thermoset polymers. [29][30][31][32][33][34][35] The fourth sectiond escribes reactions that are catalyzed not only by graphene oxide (being acidic and rich of functional groups) but also by HSAG.The observed behavior is rationalized by the predominant catalytic role of p stacking of the aromatic substrates with the graphitic planes.…”