Construction of 3-Oxazolin-5-one via Visible Light-Induced Nondecarboxylative Coupling and Sequential Reactions

3-Oxazolin-5-ones are precursors of nitrile ylides and represent valuable 1,3-dipoles for constructing cyclic imines or pyrrole compounds. Harnessing the power of photocatalysis, we accomplished a straightforward synthesis of 3-oxazolin-5-ones from redox-active esters and secondary nitro compounds. Visible light-induced nondecarboxylative coupling of a redox-active ester, nitro aldol condensation, and subsequent visible light-induced N-oxide deoxygenation were accomplished within 2 h. The reaction mechanism was supported by experimental data and DFT calculations.

Catalysts’ evolution in the asymmetric conjugate addition of nitroalkanes to electron-poor alkenes

This review provides a journey of the catalyst usage for the enantioselective conjugate addition of nitroalkanes to electron-poor olefins from the early attempts to the latest achievements. Selected applications are also reported.

Benzoylguanidines as Anion-Responsive Systems

A series of benzoylguanidinium salts was prepared and the changes in UV/Vis spectra, triggered by the presence of anions, were investigated. All compounds undergo deprotonation with basic anions like dihydrogenphosphate and acetate in acetonitrile. The most pronounced spectral changes were obtained by deprotonation of N¹ -benzoyl-N³ -(p-nitrophenyl) guanidinium chloride which shows the naked-eye visible color change from colorless to yellow. Measured pK_a (BH⁺ ) in acetonitrile ranges from 12-16, which is comparable to the pyridinium cations. The proton transfer equilibria were also tested in acetonitrile/water mixture where all but the most acidic derivatives showed pK_a (BH⁺ ) of 4-6 units which corresponds to apparent association constants of 10⁴ -10⁶  dm³  mol^-1 . UV/Vis spectra of neutral and protonated forms were modelled by the TD-DFT approach using CAM-B3LYP and PBE0 functionals and compared to CC2 results. In the case of CAM-B3LYP, a parameter ω, defining amount of long-range exchange correction, was varied to achieve the best agreement with the experimental spectra. The optimized ω parameters are 0.10 a₀ ^-1 for neutral benzoylguanidines and 0.20 a₀ ^-1 for neutral nitrobenzoyl and protonated systems. The larger ω parameter in the latter is ascribed to more pronounced charge transfer character of the HOMO-LUMO transition - the one responsible for the lowest energy absorption band.

Chiral bifunctional guanidine-catalyzed enantioselective Aza-Henry reaction of isatin-derived ketimines

An efficient asymmetric aza-Henry reaction of isatin-derived ketimines has been achieved by using a chiral guanidine-amide organocatalyst. A series of 3-substituted 3-amino-2-oxindoles was obtained with excellent results (up to 99% yield, 94% ee). Other functionalized derivatives were also conveniently transformed. This metal-free system was convenient, practical, and insensitive to air and moisture. On the basis of the crystal structure of the catalyst and NMR spectra analysis, a bifunctional catalytic model was suggested to explain the origin of the asymmetric process.

Chiral guanidine catalyzed enantioselective reactions

Chiral guanidine catalysts share common characteristics such as high pK(a) values and dual hydrogen-bonding modes of activation, and high catalytic activities and enantioselectivities can often be achieved. The utilization of guanidines as catalysts has been growing at a steady pace. In the past few years, it has attracted tremendous attention through several landmark achievements. This article highlights the development of chiral guanidine catalysis in asymmetric synthesis.

(Thio)urea organocatalysis--what can be learnt from anion recognition?

The present critical review outlines the close relationship and mutual interplay between molecular recognition, active site considerations in enzyme catalysis involving anions, and organocatalysis utilizing explicit hydrogen bonding. These interconnections are generally not made although, as we demonstrate, they are quite apparent as exemplified with pertinent examples in the field of (thio)urea organocatalysis. Indeed, the concepts of anion binding or binding with negatively (partially) charged heteroatoms is key for designing new organocatalytic transformations. Utilizing anions through recognition with hydrogen-bonding organocatalysts is still in its infancy but bears great potential. In turn, the discovery and mechanistic elucidation of such reactions is likely to improve the understanding of enzyme active sites (108 references).

Chiral recognition in bicyclic guanidines

A theoretical study of chiral recognition in bicyclic guanidines has been carried out by means of B3LYP/6-31+G(d,p) DFT calculations. A series of complexes between protonated 4,8-dimethyl-1,5,7-triazabicyclodecene (DTBD) and 2,5-disubtituted chiral cyclopentanones have been evaluated for chiral recognition, both in the gas phase and in benzene solution as per the polarizable continuum model (PCM) and analyzed by AIM and NBO methodologies. An inversion in the sense of chiral recognition has been observed between gas phase and solvated results for cyclopentanone complexes. Among the different correlations found (i.e. between electron density, hydrogen bond distance, second-order perturbation energy), a linear correlation has been established between the chiral recognition energy and different molecular parameters.

Organocatalytic Asymmetric Nitroaldol Reaction: Cooperative Effects of Guanidine and Thiourea Functional Groups

Catalytic enantio- and diastereoselective nitroaldol reactions were explored by using designed guanidine-thiourea bifunctional organocatalysts under mild and operationally simple biphasic conditions. These catalytic asymmetric reactions have a broad substrate generality with respect to the variety of aldehydes and nitroalkanes. Based on this catalytic nitroaldol process, straightforward syntheses of cytoxazone and 4-epi-cytoxazone were achieved. These catalytic nitroaldol reactions require KI as an additive for highly asymmetric induction; it operates by inhibiting the retro mode of the reaction. On the basis of studies of structure and catalytic-activity relationships, a plausible guanidine-thiourea cooperative mechanism and a transition state of the catalytic reactions are proposed. Drastic substituent effects on the catalytic properties of this catalyst may lead to the development of new chiral surfactants.

Kinetic and equilibrium study of the deprotonation of 4-nitrophenyl[bis(ethylsulphonyl)]methane by organic bases in acetonitrile in the presence of common cation BH+ and tetrabutylammonium perchlorate

The results of kinetic and equilibrium experiments with the set of reaction of proton abstraction from 4-nitrophenyl[bis(ethylsulphonyl)]methane in acetonitrile are reported. Two strong organic bases are used: 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD).

The rates of proton transfer reaction have been measured by T-jump method in the presence of perchlorate of the appropriate base as a common cation BH+ and supporting electrolyte-tetrabutylammonium perchlorate (TBAP) in the temperature range between 20–40°C are: kH=1.32×107−2.00×107 and 2.82×107−4.84×107 dm 3mol−1s−1 for MTBD and TBD respectively. The enthalpies of activation ΔHMTBD≠=13.5 and ΔHTBD≠=18.1 kJmol−1. The entropies of activation are negative: ΔSMTBD≠=−62.3 and ΔSTBD≠=−40.3 Jmol−1K−1.

The change of the absorbance of the anion of 4-nitrophenyl[bis9ethylsulphonyl)]methane at the temperature 25°C in the presence of common cation BH+ gives the equilibrium constants K=705 and 906 M−1 for MTBD and TBD respectively.

Kinetic and equilibrium results are discussed. The possible mechanism of proton transfer reaction between 4-nitrophenyl[bis(ethylsulphonyl)]methane and cyclic organic bases: MTBD and TBD in acetonitrile is proposed.

Strong bicyclic guanidine base-promoted wittig and horner-wadsworth-emmons reactions

A convenient procedure to effect the Wittig and Horner-Wadsworth-Emmons reactions employs guanidine TBD and MTBD as base-promoters; mild reaction conditions, high efficiency, and facile isolation of the final products make the present methodology, at least in some cases, a practical alternative to known procedures.