Syntheses and Structures of Magnesium and Zinc Boraamidinates: EPR and DFT Investigations of Li, Mg, Zn, B, and In Complexes of the [PhB(NtBu)2]•- Anion Radical

Stereoselective Domino Rearrangement peri-Annulation of Cinchona Alkaloid Derivatives with 8-Bromo-1-naphthyl Grignard

The unexpected domino coupling and rearrangement of the Cinchona alkaloid skeleton has been found to occur in the reaction of 9-chloro-9-deoxy-alkaloids with Grignards from peri-dihalogenonaphthalene. The cyclization and migration of the central quinuclidinylmethyl group (C9) from position C-4′ to position C-3′ of quinoline formed a novel chiral ring system of 5-aza-7H-benzo[no]tetraphene, yielding products of unlike configuration. The proposed reaction pathway involves radical intermediates.

Aluminium alkyl complexes supported by imino-phosphanamide ligand as precursors for catalytic guanylation reactions of carbodiimides

Herein, we report the synthesis, characterisation, and application of three aluminium alkyl complexes, [κ²-{NHI^RP(Ph)NDipp}AlMe₂] (R = Dipp (2a), Mes (2b); tBu (2c), Dipp = 2,6-diisopropylphenyl, Mes = mesityl, and tBu = tert-butyl), supported by unsymmetrical imino-phosphanamide [NHI^RP(Ph)NDipp]⁻ [R = Dipp (1a), Mes (1b), tBu (1c)] ligands as molecular precursors for the catalytic synthesis of guanidines using carbodiimides and primary amines. All the imino-phosphanamide ligands 1a, 1b and 1c were prepared in good yield from the corresponding N-heterocyclic imine (NHI) with phenylchloro-2,6-diisopropylphenylphosphanamine, PhP(Cl)NHDipp. The aluminium alkyl complexes 2a, 2b and 2c were obtained in good yield upon completion of the reaction between trimethyl aluminium and the protic ligands 1a, 1b and 1c in a 1 : 1 molar ratio in toluene via the elimination of methane, respectively. The molecular structures of the protic ligands 1b and 1c and the aluminium complexes 2a, 2b and 2c were established via single-crystal X-ray diffraction analysis. Complexes 2a, 2b and 2c were tested as pre-catalysts for the hydroamination/guanylation reaction of carbodiimides with aryl amines to afford guanidines at ambient temperature. All the aluminium complexes exhibited a high conversion with 1.5 mol% catalyst loading and broad substrate scope with a wide functional group tolerance during the guanylation reaction. We also proposed the most plausible mechanism, involving the formation of catalytically active three-coordinate Al species as the active pre-catalyst.

Three aluminium alkyl complexes, [κ²-{Im^RNP(Ph)NDipp}AlMe₂] (2a–2c), supported by unsymmetrical imino-phosphanamide were synthesised and utilised as competent precatalysts for the hydroamination of carbodiimides under ambient conditions.

An Unsymmetric Imino-Phosphanamidinate Ligand and its Y(III) Complex: Synthesis, Characterization, and Catalytic Hydroboration of Carbonyl Compounds

An imino-phosphanamide ligand, [NHI^iPr2Me2P(Ph)NH-2,6-ⁱPr₂C₆H₃] (LH), containing two different N-substituents was prepared by the direct reaction of the lithium salt of N-heterocyclic imine (NHI) with phenylchloro-2,6-diisopropylphenyl phosphanamine, PhP(Cl)NH-2,6-ⁱPr₂-C₆H₃. Reaction of LH with Y(N(SiMe₃)₂)₃ afforded the heteroleptic complex, [{L}Y(N(SiMe₃)₂)₂] (1), by elimination of HN(SiMe₃)₂. Compound 1 was characterized by multinuclear NMR and X-ray crystallography. In the complex, the Y(III) center was found to be tetracoordinate in a distorted tetrahedral geometry. The ligand, imino-phosphanamidinate, [L]^-, functions in a chelating manner, and its coordination to Y(III) results in a distorted 4-membered YPN₂ ring. As a proof of principle of its activity, 1 was used as a precatalyst for the hydroboration of various aldehydes and ketones using HBpin as the hydrogen source. The hydroboration reaction was rapid and clean even with low catalyst loadings (0.01-0.1 mol %). In addition, a very good functional group tolerance was observed in these reactions.

Bora-amidinate as a cooperative ligand in group 2 metal catalysis

Syntheses and crystal structures of the monomeric bora-amidinate (bam) complexes ^DIPPNBN-Mg·(THF)₃ and ^DIPPNBN-Ca·(THF)₄ are presented; ^DIPPNBN = HB[N(2,6-iPr₂-C₆H₃)]₂. The simplicity of their ¹H NMR spectra in THF-d₈ suggest that their monomeric solid state structures are retained in solution. ^DIPPNBN-Mg·(THF)₃ in C₆D₆, however, is in equilibrium with a dimeric species. Calculations (B3PW91/6-311++G**) reveal a very high localized negative charge (NPA: -1.103) on the N atoms in ^DIPPNBN-Mg. The strongly basic properties of the bam ligand are in agreement with catalytic activity of these complexes in the intramolecular alkene hydroamination. A mechanism is proposed in which the bam ligand is non-innocent and cooperative, playing an active role in substrate deprotonation and product protonation.

Reactivity of bis(organoamino)phosphanes with magnesium(ii) compounds

The reactivity of bis(organoamino)phosphanes with magnesium(ii) reagents is reported. The outcome of the reactions is influenced by a tautomeric H-shift in ligand backbones.

Solid state and solution studies of lithium tris(n-butyl)magnesiates stabilised by Lewis donors

Donor complexes of the synthetically important lithium magnesiate LiMg(ⁿBu)₃ have been prepared and characterised.

Carbene-Stabilized Main Group Radicals and Radical Ions

Shortly after their discovery at the end of the 80s, stable singlet carbenes have been recognized as excellent ligands for transition metal based catalysts, and as organo-catalysts in their own right. At the end of the 2000s, it has been shown that they can coordinate main group elements in their zero oxidation state, and even activate small molecules. This review covers examples in the literature dealing with the most recent application of stable singlet carbenes, namely their use to stabilize radicals and radical ions that are otherwise unstable.

Syntheses, X-ray structures, and redox behaviour of the group 14 bis-boraamidinates M[PhB(μ-N-t-Bu)2]2 (M = Ge, Sn) and Li2M[PhB(μ-N-t-Bu)2]2 (M = Sn, Pb)

The solid-state structures of the complexes M[PhB(μ-N-t-Bu)2]2 (1a, M= Ge; 1b, M = Sn) were determined to be spirocyclic with two orthogonal boraamidinate (bam) ligands N,N′-chelated to the group 14 centre. Oxidation of 1b with SO2Cl2 afforded the thermally unstable, blue radical cation {Sn[PhB(μ-N-t-Bu)2]2}•+, identified by electron paramagnetic resonance (EPR) spectroscopy supported by density functional theory (DFT) calculations, whereas the germanium analogue 1a was inert towards SO2Cl2. The reaction between Li2[PhB(μ-N-t-Bu)2]2 and SnCl2 or PbI2 in 2:1 molar ratio in diethyl ether produced the novel heterotrimetallic complexes Li2Sn[PhB(μ-N-t-Bu)2]2 (2b) and (Et2O·Li)LiPb[PhB(μ-N-t-Bu)2]2 (2c·OEt2), respectively. By contrast, treatment of Li2[PhB(μ-N-t-Bu)2]2 with C4H8O2·GeCl2 yielded the germanium(IV) complex 1a via a redox process. The X-ray structures of 2b and 2c·THF revealed polycyclic arrangements in which one bam ligand is N,N′-chelated to the Sn(II) or Pb(II) atom and one of the Li+ cations, while the second bam ligand exhibits a unique bonding mode, bridging all three metal centres. The fluctional behaviour of 2b was investigated by variable temperature, multinuclear NMR spectroscopy.

New Bonding Modes for Boraamidinate Ligands in Heavy Group 15 Complexes: Fluxional Behavior of the 1:2 Complexes, LiM[PhB(NtBu)2]2 (M = As, Sb, Bi)

The reactions of MCl3 with Li2[PhB(NtBu)2] in 1:1, 1:1.5, and 1:2 molar ratios in diethyl ether produced the monoboraamidinates ClM[PhB(NtBu)2] (1a, M = As; 1b, M = Sb; 1c, M = Bi), the novel 2:3 boraamidinate complexes [PhB(NtBu)2]M-micro-N(tBu)B(Ph)N(tBu)M[PhB(NtBu)2] (2b, M = Sb; 2c, M = Bi), and the bisboraamidinates LiM[PhB(NtBu)2]2 (3a, 3a.OEt2, M = As; 3b, M = Sb; 3c.OEt2, M = Bi), respectively. The 2:3 complexes 2b and 2c were also observed in the reactions carried out in a 1:2 molar ratio at room temperature. All complexes have been characterized by multinuclear NMR spectroscopy (1H, 7Li, 11B, and 13C) and by single-crystal X-ray structural determinations. The molecular units of the mono-boraamidinates 1a-c are isostructural, but their crystal packing is distinct as a result of stronger intermolecular close contacts going from 1a to 1c. In the novel 2:3 bam complexes 2b and 2c, each metal center is N,N'-chelated by a bam ligand and these two [M(bam)]+ units are bridged by the third [bam]2- ligand. The structures of the unsolvated bis-boraaminidate complexes 3a and 3b consist of [Li(bam)]- and [M(bam)]+ monomeric units linked by Li-N and M-N bonds to give a tricyclic structure. Solvation of the Li+ ion by diethyl ether results in a bicyclic structure composed of four-membered BN2As and six-membered BN3AsLi rings in 3a.OEt2. In contrast, the analogous bismuth complex 3c.OEt2 exhibits a tetracyclic structure. Variable-temperature NMR studies reveal that the nature of the fluxional behavior of 3a-c in solution is dependent on the group 15 center.