Umpolung Asymmetric 1,5-Conjugate Addition via Palladium Hydride Catalysis.

Electronically matched nucleophilic 1,6-conjugate addition has been well studied and widely applied in synthetic areas. In contrast, nucleophilic 1,5-conjugate addition represents an electronically forbidden process and is considered unfeasible. Here, we describe modular protocols for 1,5-conjugate addition reactions via palladium hydride catalysis. Both palladium and synergistic Pd/organocatalyst systems are developed to catalyze 1,5-conjugate reaction, followed by inter- or intramolecular [3+2] cyclization. A migratory 1,5-addition protocol is established to corroborate the feasibility of this umpolung concept. The 1,5-addition products are conveniently transformed into a series of privileged enantioenriched motifs, including polysubstituted tetrahydrofuran, dihydrofuran, cyclopropane, cyclobutane, azetidine, oxetane, thietane, spirocycle and bridged rings. Preliminary mechanistic studies corroborate the involvement of palladium hydride catalysis.

Single-molecule magnet behaviour in a dysprosium-triradical complex.

The first triradical bridged binuclear penta-spin single-molecule magnet, [Dy2(hfac)6(BTR)], based on a nitronyl nitroxide triradical was synthesized and characterized. Theoretical calculations revealed Dy-radical antiferromagnetic coupling and radical-radical ferromagnetic coupling. The SMM behavior is found to originate from the exchange states, rather than the isolated Dy3+ center.

A family of lanthanide compounds with reduced nitronyl nitroxide diradical: syntheses, structures and magnetic properties.

A novel nitronyl nitroxide diradical based on the pyrazine ring, BNITPz (1, BNITPz = 2,2'-(3,6-dimethylpyrazine-2,5-diyl)bis(1-hydroxy-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl-3-oxide)) and six new dinuclear compounds based on the reduced form of 1, namely [Ln(hfac)3]2(BHIMPz)(H2O)2 (Ln = Gd (2Gd), Dy (3Dy), Ho (4Ho), Er (5Er), Tm (6Tm), Yb (7Yb); BHIMPz = 2,2'-(3,6-dimethylpyrazine-2,5-diyl)bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide); hfac = hexafluoroacetylacetonate) were prepared and characterized. Single crystal X-ray crystallographic analyses confirmed the diradical structure of 1, and revealed that complexes 2Gd-7Yb are isostructural and crystallize in the triclinic space group P1[combining macron]. The reduced form BHIMPz was confirmed by its longer N-O bond lengths and the characteristic N-H stretching absorption at 3500-3300 cm-1 of the IR spectroscopy of 2Gd-7Yb. In 2Gd-7Yb, the lanthanide ions are eight-coordinated in a square antiprism environment (D4d). Two symmetric related Ln(iii) ions are bridged by the reduced BHIMPz ligand to form a dinuclear structure. Magnetic measurements revealed the antiferromagnetic exchange interaction between the two radicals of the diradical ligand 1 through the pyrazine ring. For complexes 2Gd to 7Yb with the reduced non-radical ligand, their magnetic properties are consistent with the isolated LnIII centers. Alternating-current (ac) magnetic susceptibility investigations indicate that complexes 3Dy and 7Yb exhibit field-induced frequency dependence, which indicates the presence of slow relaxation of the magnetization. Ab initio calculations were performed on these compounds to understand their magnetic anisotropy and their different magnetic dynamics.

Syntheses, structures and magnetic properties of the lanthanide complexes of the pyrimidyl-substituted nitronyl nitroxide radical.

Four 2p-4f LnIII-radical complexes, [(NIT-2-Pm)Ln(hfac)3]·0.5C7H16 (NIT-2-Pm = 2-pyrimidyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide, hfac = hexafluoroacetylacetonato, Ln = Tb (1), Dy (2), Ho (3) and Er (4)), and four 4f-2p-4f LnIII-radical-LnIII complexes, [(µ-NIT-2-Pm)Ln2(hfac)6(H2O)2]·0.5C7H16 (Ln = Tb (5), Dy (6), Ho (7) and Er (8)) have been synthesized and characterized structurally and magnetically. These compounds can be selectively obtained by controlling the reaction ratio of Ln(hfac)3·2H2O to the radical ligand NIT-2-Pm. The crystal structures show that in the former four complexes 1-4, the NIT-2-Pm radical acts as a terminal bidentate ligand chelating to one LnIII ion, while in 5-8, the NIT-2-Pm acts as a bridging ligand linking two LnIII ions to form a binuclear three-spin system. Magnetic studies revealed that complexes 1-4 and 6 show frequency-dependent ac magnetic susceptibilities, suggesting a possible single-molecule magnet behavior. To the best of our knowledge, complexes 3 and 4 are the first Ho-NIT and Er-NIT compounds showing slow magnetic relaxation. Compounds 5-8 represent a rare family of compounds showing the NIT bridged 4f-2p-4f three-spin motif, while complex 6 is a rare NIT bridged multinuclear lanthanide compound possessing SMM-like behaviour. Ab initio calculations were performed on all these complexes. The fitting of the magnetic susceptibilities of these compounds suggests weak antiferromagnetic coupling between the LnIII and NIT radical in 1-8 and weak ferromagnetic LnIII-LnIII interactions in 5-8.

N-heterocyclic carbene/Lewis acid strategy for the stereoselective synthesis of spirocyclic oxindole-dihydropyranones.

Under the cooperative catalysis of NHC/Lewis acid, the mild, straightforward [4 + 2] annulation of α-bromo-α,ß-unsaturated aldehydes bearing γ-H with isatin derivatives gave spirocyclic oxindole-dihydropyranones stereoselectively. This approach is particularly attractive due to the concise construction, avoidance of external oxidants, and the potential utilization value of final products in molecular biology and pharmacy.

NHC-catalyzed oxidative γ-addition of α,ß-unsaturated aldehydes to isatins: a high-efficiency synthesis of spirocyclic oxindole-dihydropyranones.

This manuscript discloses an efficient construction of the spirocyclic oxindole-dihydropyranone scaffold via the N-heterocyclic carbene (NHC)-catalyzed oxidative γ-functionalization of α,ß-unsaturated aldehydes bearing γ-H with isatin derivatives. The ready availability of the starting materials, easy work-up, mild reaction conditions and the potential utilization value of the products make this strategy attractive.