A Highly Efficient Solvent-Free Asymmetric Direct Aldol Reaction Organocatalyzed by Recoverable (s)-Binam-L-Prolinamides. ESI-MS Evidence of the Enamine-Iminium Formation

Recoverable (Sa)-binam-l-prolinamide in combination with benzoic acid is used as catalysts in the direct aldol reaction between cycloalkyl, alkyl, and α-functionalized ketones and aldehydes under solvent-free reaction conditions. Three different methods are assayed: simple conventional magnetic stirring, magnetic stirring after previous dissolution in THF and evaporation, and ball mill technique. These procedures allow one to reduce not only the amount of required ketone to 2 equiv but also the reaction time to give the aldol products with regio-, diastereo-, and enantioselectivities comparable to those in organic or aqueous solvents. Generally anti-isomers are mainly obtained with enantioselectivities up to 97%. The reaction can be carried out under these conditions also using aldehydes as nucleophiles, yielding after in situ reduction of the aldol products the corresponding chiral 1,3-diols with moderate to high enantioselectivities mainly as anti-isomers. The aldol reaction has been studied by the use of positive ESI-MS technique, providing the evidence of the formation of the corresponding enamine−iminium intermediates.

Spectroelectrochemical Study of the Photoinduced Catalytic Formation of 4,4′-Dimercaptoazobenzene from 4-Aminobenzenethiol Adsorbed on Nanostructured Copper

Surface-enhanced raman scattering (SERS) spectra of self-assembled monolayers of 4-aminobenzenethiol (4-ABT) on copper (Cu) and silver (Ag) surfaces decorated with Cu and Ag nanostructures, respectively, have been obtained with lasers at 532, 632.8, 785, and 1064 nm. Density functional theory (DFT) has been used to obtain calculated vibrational frequencies of the 4-ABT and 4,4′-dimercaptoazobenzene (4,4′-DMAB) molecules adsorbed on model Cu surfaces. The features of the SERS spectra depend on the electrode potential and the type and power density of the laser. SERS spectra showed the formation of the 4,4′-DMAB on the nanostructured Cu surface independently of the laser employed. For the sake of comparison SERS spectra of a self-assembled monolayer of the 4-ABT on Ag surfaces decorated with Ag nanostructures have been also obtained with the same four lasers. When using the 532 and 632.8 nm lasers, the 4,4′-DMAB is formed on Cu surface at electrode potentials as low as −1.0 V (AgCl/Ag) showing a different behavior with respect to Ag (and others metals such as Au and Pt). On the other hand, the surface-enhanced infrared reflection absorption (SEIRA) spectra showed that in the absence of the laser excitation the 4,4′-DMAB is not produced from the adsorbed 4-ABT on nanostructured Cu in the whole range of potentials studied. These results point out the prevalence of the role of electron–hole pairs through surface plasmon activity to explain the obtained SERS spectra.

Synthesis of Chromen[4,3-b]pyrrolidines by Intramolecular 1,3-Dipolar Cycloadditions of Azomethine Ylides: An Experimental and Computational Assessment of the Origin of Stereocontrol

Azomethine ylides, generated from imine-derived O-cinnamyl or O-crotonyl salicylaldeyde and α-amino acids, undergo intramolecular 1,3-dipolar cycloaddition, leading to chromene[4,3-b]pyrrolidines. Two reaction conditions are used: (a) microwave-assisted heating (200 W, 185 °C) of a neat mixture of reagents, and (b) conventional heating (170 °C) in PEG-400 as solvent. In both cases, a mixture of two epimers at the α-position of the nitrogen atom in the pyrrolidine nucleus was formed through the less energetic endo-approach (B/C ring fusion). In many cases, the formation of the stereoisomer bearing a trans-arrangement into the B/C ring fusion was observed in high proportions. Comprehensive computational and kinetic simulation studies are detailed. An analysis of the stability of transient 1,3-dipoles, followed by an assessment of the intramolecular pathways and kinetics are also reported.