Main Group Metal-Mediated Transformations of Imines

Main-group-metal-mediated transformations of imines have earned a valued place in the synthetic chemist's toolbox. Their versatility allows the simple preparation of various nitrogen containing compounds. This review will outline the early discoveries including metallation, addition/cyclisation and metathesis pathways, followed by the modern-day use of imines in synthetic methodology. Recent advances in imine C-F activation protocols are discussed, alongside revisiting "classic" imine reactivity from a sustainable perspective. Developments in catalytic methods for hydroelementation of imines have been reviewed, highlighting the importance of s-block metals in the catalytic arena. Whilst stoichiometric transformations in alternative reaction media such as deep eutectic solvents or water have been summarised. The incorporation of imines into flow chemistry has received recent attention and is summarised within.

Lithium-Bromide Exchange versus Nucleophilic Addition of Schiff's base: Unprecedented Tandem Cyclisation Pathways

By exploring lithium-bromide exchange reactivity of aromatic Schiff's bases with tert-butyllithium (tBuLi), we have revealed unprecedented competitive intermolecular and intramolecular cascade annulation pathways, leading to valuable compounds, such as iso-indolinones and N-substituted anthracene derivatives. A series of reaction parameters were probed, including solvent, stoichiometry, sterics and organolithium reagent choice, in order to understand the influences that limit such ring-closing pathways. With two viable reactivity options for the organolithium on the imine; namely, nucleophilic addition or lithium-bromide exchange, a surprising competitive nature was observed, where nucleophilic addition dominated, even under cryogenic conditions. Considering the most commonly used solvents for lithium-bromide exchange, tetrahydrofuran (THF) and diethyl ether (Et₂ O), contrasting reactivity outcomes were revealed with nucleophilic addition promoted in THF, while Et₂ O yielded almost double the conversion of cyclic products than in THF.

Sodium Diisopropylamide: Aggregation, Solvation, and Stability

The solution structures, stabilities, physical properties, and reactivities of sodium diisopropylamide (NaDA) in a variety of coordinating solvents are described. NaDA is stable for months as a solid or as a 1.0 M solution in N,N-dimethylethylamine (DMEA) at -20 °C. A combination of NMR spectroscopic and computational studies show that NaDA is a disolvated symmetric dimer in DMEA, N,N-dimethyl-n-butylamine, and N-methylpyrrolidine. Tetrahydrofuran (THF) readily displaces DMEA, affording a tetrasolvated cyclic dimer at all THF concentrations. Dimethoxyethane (DME) and N,N,N',N'-tetramethylethylenediamine quantitatively displace DMEA, affording doubly chelated symmetric dimers. The trifunctional ligands N,N,N',N″,N″-pentamethyldiethylenetriamine and diglyme bind the dimer as bidentate rather than tridentate ligands. Relative rates of solvent decompositions are reported, and rate studies for the decomposition of THF and DME are consistent with monomer-based mechanisms.

(μ-1,2-dimethoxyethane-κ2O:O')bis[(1,2-dimethoxyethane-κ2O,O')tris(1,1,1,5,5,5-hexafluoro-4-oxopent-2-en-2-olato-κ2O,O')cerium(III)]

A previous analysis [Fatila et al. (2012). Dalton Trans. 41, 1352-1362] of the title complex, [Ce(2)(C(5)HF(6)O(2))(6)(C(4)H(10)O(2))(3)], had identified it as Ce(hfac)(3)(dme)(1.5) according to the (1)H NMR integration [hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate (1,1,1,5,5,5-hexafluoro-4-oxopent-2-en-2-olate) and dme = 1,2-dimethoxyethane]; however, it was not possible to determine the coordination environment unambiguously. The structural data presented here reveal that the complex is a binuclear species located on a crystallographic inversion center. Each Ce(III) ion is coordinated to three hfac ligands, one bidentate dme ligand and one monodentate (bridging) dme ligand, thus giving a coordination number of nine (CN = 9) to each Ce(III) ion. The atoms of the bridging dme ligand are unequally disordered over two sets of sites. In addition, in two of the -CF(3) groups, the F atoms are rotationally disordered over two sets of sites. This is the first crystal structure of a binuclear lanthanide β-diketonate with a bridging dme ligand.