Oxidative Aromatization of 4,7-Dihydro-6-nitroazolo[1,5-a]pyrimidines: Synthetic Possibilities and Limitations, Mechanism of Destruction, and the Theoretical and Experimental Substantiation

The reaction tolerance of the multicomponent process between 3-aminoazoles, 1-morpholino-2-nitroalkenes, and aldehydes was studied. The main patterns of this reaction have been established. Conditions for the oxidation of 4,7-dihydro-6-nitroazolo[1,5-a]pyrimidines were selected. Previous claims that the 4,7-dihydro-6-nitroazolo[1,5-a]pyrimidines could not be aromatised have now been refuted. Compounds with an electron-donor substituent at position seven undergo decomposition during oxidation. The phenomenon was explained based on experimental data, electro-chemical experiment, and quantum-chemical calculation. The mechanism of oxidative degradation has been proposed.

N/N Bridge Type and Substituent Effects on Chemical and Crystallographic Properties of Schiff-Base (Salen/Salphen) Niii Complexes

In total, 13 ligands R-salen (N,N’-bis(5-R-salicylidene)ethylenediamine (where R = MeO, Me, OH, H, Cl, Br, NO2) and R-salphen (N,N’-bis(5-R-salicylidene)-1,2-phenylenediamine (where R = MeO, Me, OH, H, Cl, Br) and their 13 nickel complexes NiRsalen and NiRsalphen were synthesized and characterized using IR (infrared) spectroscopy, mass spectrometry, elemental analysis, magnetic susceptibility, NMR (nuclear magnetic resonance), UV-vis (ultraviolet-visible) spectroscopy, cyclic voltammetry, and X-ray crystal diffraction. Previous studies have shown that all complexes have presented a square planar geometry in a solid state and as a solution (DMSO). In electrochemical studies, it was observed that in N/N aliphatic bridge complexes, the NiII underwent two redox reactions, which were quasi-reversible process, and the half-wave potential followed a trend depending on the ligand substituent in the 5,5’-R position. The electron-donor substituent—as -OH, and -CH3 decreased the E1/2 potential—favored the reductor ability of nickel. The crystals of the complexes NiMesalen, NiMeOsalen, NiMeOsalphen, and Nisalphen were obtained. It was shown that the crystal packaging corresponded to monoclinic systems in the first three cases, as well as the triclinic for Nisalphen. The Hirshfeld surface analysis showed that the packaging was favored by H∙∙∙H and C∙∙∙H/H∙∙∙C interactions, and C-H∙∙∙O hydrogen bridges when the substituent was -MeO and π-stacking was added to an aromatic bridge. Replacing the N/N bridge with an aromatic ring decreased distortion in square-planar geometry where the angles O-Ni-N formed a perfect square-planar.

Gold-Catalyzed Partial Hydrogenation of Activated Alkynes Mediated by Triphenylphosphine

Gold(I) can exhibit a cooperative effect with triphenylphosphine, accelerating the triphenylphosphine-mediated partial hydrogenation of activated alkynes. In this protocol, 3-arylpropiolates are selectively reduced to the Z-isomer when the aryl ring bears an electron-donor substituent, whereas 3-arylpropynones are reduced to the E-isomers.

Recent Advances in the Chemistry of Doubly Activated Cyclopropanes: Synthesis and Reactivity

Diactivated cyclopropanes containing two geminal electron withdrawing groups, commonly called as ‘Doubly Activated Cyclopropanes’ are useful synthons for the synthesis of many interesting natural products and functionalized molecules. These geminal electron withdrawing groups (EWG’s) facilitate the regioselective ring opening of cyclopropanes by polarizing the C-C bond adjacent to it. This polarization also allows them to undergo 1,3 dipolar cycloaddition reactions when substituted with a suitable electron donor substituent at the adjacent carbon (donor-acceptor cyclopropanes) in the presence of suitable dipolarophiles. In this review, we discuss the recent advances in the chemistry of doubly activated cyclopropanes: their synthesis, reactions and applications in total synthesis.

Impact of the donor substituent on the optoelectrochemical properties of 6H-indolo[2,3-b]quinoxaline amine derivatives

An opto-electrochemical study of D–A based indolo-quinoxaline amine derivatives was performed by varying the strength of the amine substituent.

The alkyl group is a -I + R substituent

Electronic substituent effects are usually classified as inductive (through σ-bonds) and resonance effects (via π-bonds). The alkyl group has been usually regarded as aσ -electron donor substituent (+I effect, according to the Ingold’s classification). However, a σ-withdrawing, π-donor effect (-I + R pattern) allows explaining the actual electron-withdrawing behavior of alkyl groups when bound to sp³ carbon atoms as well as their well-known electron-releasing properties when attached to sp² or sp atoms. Alkyl substitution effects on several molecular properties (dipole moments, NMR, IR, and UV spectra, reactivity in gas phase and solution) are discussed.

An NBO-TS predictive approach for torquoselectivity of ring opening in 3-substituted cyclobutenes

<p>The torquoselectivity, the inward or outward ring opening of 3-substituted cyclobutenes, is conventionally guided by the donor and/or acceptor ability of the substituent (S). It is typically predicted by estimating the respective ring opening transition state (TS) barriers. While there is no known dissent in regard to the outward rotation of electron-rich substituents from the approaches of TS calculations, the inward rotation was predicted for some electron-accepting substituents and outward for others. To address this divergence in predicting the torquoselectivity, we have used reliable orbital descriptors through natural bond orbital theoretical calculations and demonstrated that (a) interactions <i>n</i><i>_S</i>→s*_C3C4 for a lone pair containing substituent, s_S→s*_C3C4 for a s-donor substituent, s_C3C4→p*_S for a resonance-accepting substituent and s_C3C4→s*_S for a s-acceptor substituent constitute the true electronic controls of torquoselectivity, and (b) reversibility of the ring opening event is an additional important contributor to the observed product distribution.</p>

An NBO-TS predictive approach for torquoselectivity of ring opening in 3-substituted cyclobutenes

<p>The torquoselectivity, the inward or outward ring opening of 3-substituted cyclobutenes, is conventionally guided by the donor and/or acceptor ability of the substituent (S). It is typically predicted by estimating the respective ring opening transition state (TS) barriers. While there is no known dissent in regard to the outward rotation of electron-rich substituents from the approaches of TS calculations, the inward rotation was predicted for some electron-accepting substituents and outward for others. To address this divergence in predicting the torquoselectivity, we have used reliable orbital descriptors through natural bond orbital theoretical calculations and demonstrated that (a) interactions <i>n</i><i>_S</i>→s*_C3C4 for a lone pair containing substituent, s_S→s*_C3C4 for a s-donor substituent, s_C3C4→p*_S for a resonance-accepting substituent and s_C3C4→s*_S for a s-acceptor substituent constitute the true electronic controls of torquoselectivity, and (b) reversibility of the ring opening event is an additional important contributor to the observed product distribution.</p>