α-Phenylthioaldehydes for the effective generation of acyl azolium and azolium enolate intermediates.

α-Phenylthioaldehydes are readily prepared using a simple multi-step procedure and herein are introduced as a new precursor for the NHC-catalysed generation of acyl azolium and azolium enolate intermediates that are of widespread synthetic interest and utility. Treatment of α-phenylthioaldehydes with an NHC precatalyst and base produces an efficient redox rearrangement via a Breslow intermediate, elimination of thiophenolate, and subsequent rebound addition to the generated acyl azolium to give the corresponding thiol ester. In the presence of an external alcohol, competition between redox rearrangement and redox esterification can be controlled through judicious choice of the N-aryl substituent within the NHC precatalyst and the base used in the reaction. With NEt3 as base, NHCs bearing electron-withdrawing (N-C6F5 or N-C6H2Cl3) substituents favour redox rearrangement, while triazolium precatalysts with electron-rich N-aryl substituents (N-Ph, N-Mes) result in preferential redox esterification. Using DBU, redox esterification is preferred due to transesterification of the initially formed thiol ester product. Additionally, α-phenylthioaldehyde-derived azolium enolates have been used in enantioselective formal [4 + 2]-cycloaddition reactions to access dihydropyridinone heterocycles with high enantioselectivity (up to >95 : 5 dr, 98 : 2 er).

Role of the Meso Substituent in Defining the Reduction of Uranyl Dipyrrin Complexes.

The uranyl complex UVIO2Cl(LMes) of the redox-active, acyclic dipyrrin-diimine anion LMes- [HLMes = 1,9-di-tert-butyl-imine-5-(mesityl)dipyrrin] is reported, and its redox property is explored and compared with that of the previously reported UVIO2Cl(LF) [HLF = 1,9-di-tert-butyl-imine-5-(pentafluorophenyl)dipyrrin] to understand the influence of the meso substituent. Cyclic voltammetry, electron paramagnetic resonance spectroscopy, and density functional theory studies show that the alteration from an electron-withdrawing meso substituent to an electron-donating meso substituent on the dipyrrin ligand significantly modifies the stability of the products formed after reduction. For UVIO2Cl(LMes), the formation of a diamond-shaped, oxo-bridged uranyl(V) dimer, [UVO2(LMes)]2 is seen, whereas in contrast, for UVIO2Cl(LF), only ligand reduction occurs. Computational modeling of these reactions shows that while ligand reduction followed by chloride dissociation occurs in both cases, ligand-to-metal electron transfer is favorable for UVIO2Cl(LMes) only, which subsequently facilitates uranyl(V) dimerization.

Exploring the Redox Properties of Bench-Stable Uranyl(VI) Diamido-Dipyrrin Complexes.

The uranyl complexes UO2(OAc)(L) and UO2Cl(L) of the redox-active, acyclic diamido-dipyrrin anion L- are reported and their redox properties explored. Because of the inert nature of the complexes toward hydrolysis and oxidation, synthesis of both the ligands and complexes was conducted under ambient conditions. Voltammetric, electron paramagnetic resonance spectroscopy, and density functional theory studies show that one-electron chemical reduction by the reagent CoCp2 leads to the formation of a dipyrrin radical for both complexes [Cp2Co][UO2(OAc)(L•)] and [Cp2Co][UO2Cl(L•)].

Synthesis and complexes of a constrained-cavity Schiff-base dipyrrin macrocycle.

A new constrained-cavity [1 + 1] Schiff-base dipyrrin macrocycle comprising an N4 donor-pocket has been synthesised by spontaneous oxidation and in situ crystallisation. Access to Fe(ii) and Zn(ii) complexes is achieved by salt elimination reactions of the lithium salt. All compounds have been characterised by NMR and UV-vis spectroscopy, X-ray crystallography, and DFT analysis.