Metallated dihydropyridinates: prospects in hydride transfer and (electro)catalysis

Hydride transfer (HT) is a fundamental step in a wide range of reaction pathways, including those mediated by dihydropyridinates (DHP-s). Coordination of ions directly to the pyridine ring or functional groups stemming therefrom, provides a powerful approach for influencing the electronic structure and in turn HT chemistry. Much of the work in this area is inspired by the chemistry of bioinorganic systems including NADH. Coordination of metal ions to pyridines lowers the electron density in the pyridine ring and lowers the reduction potential: lower-energy reactions and enhanced selectivity are two outcomes from these modifications. Herein, we discuss approaches for the preparation of DHP-metal complexes and selected examples of their reactivity. We suggest further areas in which these metallated DHP-s could be developed and applied in synthesis and catalysis.

Dearomative triple elementalization of quinolines driven by visible light

Organoboron and organosilicon compounds are used not only as synthetic building blocks but also as functional materials and pharmaceuticals, and compounds with multiple boryl and silyl groups are beginning to be used for these purposes. Especially in drug discovery, methodology providing easy stereoselective access to aliphatic nitrogen heterocycles bearing multiple boryl or silyl groups from readily available aromatic nitrogen heterocycles would be attractive. However, such transformations remain challenging, and available reactions have been mostly limited to dearomative hydroboration or hydrosilylation reactions. Here, we report the dearomative triple elementalization (carbo-sila-boration) of quinolines via the addition of organolithium followed by photo-boosted silaboration, affording the desired products with complete chemo-, regio-, and stereoselectivity. The reaction proceeds via the formation of silyl radicals instead of silyl anions. We also present preliminary studies to illustrate the potential of silaboration products as synthetic platforms.

Heterobimetallic Coinage Metal-Ruthenium Complexes Supported by Anionic N-Heterocyclic Carbenes

The lithium complexes [(WCA‐NHC)Li(toluene)] of anionic N‐heterocyclic carbenes with a weakly coordinating borate moiety (WCA‐NHC, WCA=B(C₆F₅)₃, NHC=IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were used for the preparation of silver(I) or copper(I) WCA‐NHC complexes. While the reactions in THF with AgCl or CuCl afforded anionic mono‐ and dicarbene complexes with solvated lithium counterions [Li(THF)_n]⁺ (n=3, 4), the reactions in toluene proceeded with elimination of LiCl and formation of the neutral phosphine and arene complexes [(WCA‐NHC)M(PPh₃)] and [(WCA‐NHC)M(η²‐toluene)] (M=Ag, Cu). The latter were used for the preparation of chlorido‐ and iodido‐bridged heterobimetallic Ag/Ru and Cu/Ru complexes [(WCA‐NHC)M(μ‐X)₂Ru(PPh₃)(η⁶‐p‐cymene)] (M=Ag, Cu, X=Cl; M=Ag, X=I). Surprisingly, these complexes resisted the elimination of CuCl, AgCl, or AgI, precluding WCA‐NHC transmetalation.

C-N and C-H Activation of an N-Heterocyclic Carbene by Magnesium(II) Hydride and Magnesium(I) Complexes

Reactions of the hindered N-heterocyclic carbene, :C{(MesNCH)₂} (IMes; Mes = mesityl), with a series of β-diketiminatomagnesium(II) hydride and dimagnesium(I) complexes were carried out at 80 °C. The reactions involving the magnesium hydrides, [{(^ArNacnac)Mg(μ-H)}₂] [^ArNacnac = [(ArNCMe)₂CH]^-, where Ar = 2,6-diethylphenyl (Dep) or 2,6-diisopropylphenyl (Dip)], proceeded via activation of an exocyclic C-N bond of IMes, giving magnesium imidazolyl compounds [(^ArNacnac)Mg(μ-H)(μ-Imid)Mg(^ArNacnac)] (Imid = [NC₂H₂N(Mes)C]^-) and mesitylene. A low-yield IMes methyl C-H activation product, [(^DepNacnac)Mg(IMes^-H)], was also obtained, via H₂ elimination, from the reaction between IMes and [{(^DepNacnac)Mg(μ-H)}₂]. Reactions between IMes and dimagnesium(I) compounds [{(^ArNacnac)Mg}₂] [Ar = 2,6-dimethylphenyl (Xyl) or Mes] afforded isostructural C-H activation products [(^ArNacnac)Mg(IMes^-H)] but in higher yields. Density functional theory calculations suggest that the reactions do not progress via stable adduct complex intermediates, which are sterically inaccessible.

Functionalised N-Heterocyclic Carbene Ligands in Bimetallic Architectures

N-Heterocyclic carbenes (NHCs) have become immensely successful ligands in coordination chemistry and homogeneous catalysis due to their strong terminal σ-donor properties. However, by targeting NHC ligands with additional functionalisation, a new area of NHC coordination chemistry has developed that has enabled NHCs to be used to build up bimetallic and multimetallic architectures. This minireview covers the development of functionalised NHC ligands that incorporate additional donor sites in order to coordinate two or more metal atoms. This can be through the N-atom of the NHC ring, through a donor group attached to the N-atom or the carbon backbone, coordination of the π-bond or an annulated π-donor on the backbone, or through direct metalation of the backbone.

Synthesis and reduction chemistry of mixed-Lewis-base-stabilised chloroborylenes

Mixed-base-stabilised chloroborylenes are easily accessed by twofold reduction of a cyclic (alkyl)(amino)carbene-supported trichloroborane in the presence of a second Lewis base, thus enabling fine-tuning of the electronic properties of the electron-rich borylene centre.

The one-electron reduction of (CAAC^Me)BCl₃ (CAAC^Me = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) yields the dichloroboryl radical [(CAAC^Me)BCl₂]˙. Furthermore, the twofold reduction of (CAAC^Me)BCl₃ in the presence of a range of Lewis bases (L = CAAC^Me, N-heterocyclic carbene, phosphine) yields a series of doubly base-supported (CAAC^Me)LBCl chloroborylenes, all of which were structurally characterised. NMR and UV-vis spectroscopic and electrochemical data for (CAAC^Me)LBCl show that the boron centre becomes more electron-rich and the HOMO–LUMO gap widens as L becomes less π-accepting. A [(CAAC^Me)BCl₂]^– boryl anion coordination polymer was isolated as a potential intermediate in these reductions. In most cases the reduction of the chloroborylenes resulted in the formation of the corresponding hydroborylenes or derivatives thereof, as well as ligand C–H activation products.

Synergic Deprotonation Generates Alkali-Metal Salts of Tethered Fluorenide-NHC Ligands Co-Complexed to Alkali-Metal Amides

Synergic combinations of alkali-metal hydrocarbyl/amide reagents were used to synthesise saturated N-heterocyclic carbene (NHC) ligands tethered to a fluorenide anion through deprotonation of a spirocyclic precursor, whereas conventional bases were not successful. The Li2 derivatives displayed a bridging amide between two Li atoms within the fluorenide-NHC pocket, whereas the Na2 and K2 analogues displayed extended solid-state structures with the fluorenide-NHC ligand chelating one alkali metal centre.

Heterobimetallic aluminate derivatives with bulky phenoxide ligands: a catalyst for selective vinyl polymerization

New aluminates as active catalysts for vinyl polymerization are described, as well as a strategy to crosslinked polymers from GMA in a controlled fashion.

Synthesis of fused B,N-heterocycles by alkyne cleavage, NHC ring-expansion and C-H activation at a diboryne

The addition of alkynes to a saturated N-heterocyclic carbene (NHC)-supported diboryne results in spontaneous cycloaddition, with complete B[triple bond, length as m-dash]B and C[triple bond, length as m-dash]C triple bond cleavage, NHC ring-expansion and activation of a variety of C-H bonds, leading to the formation of complex mixtures of fused B,N-heterocycles.

Aluminates with Fluorinated Schiff Bases: Influence of the Alkali Metal⁻Fluorine Interactions in Structure Stabilization

New heterometallic aluminium-alkali metal compounds have been prepared using Schiff bases with electron withdrawing substituents as ligands. The synthesis of these new species was achieved via the reaction of AlMe₃ with the freshly prepared alkali-metallated ligand. The derivatives formed were characterized by NMR in solution and by single crystal X-ray diffraction in the solid state. Aluminate derivatives with lithium and sodium were prepared and a clear influence of the alkali metal in the final outcome is observed. The presence of a Na···F interaction in the solid state has a stabilization effect and the species [NaAlMe₃L]₂ can de isolated for the first time, which was not possible when using Schiff bases without electron withdrawing substituents as ligands.

Molecular Manipulations of a Utility Nitrogen-Heterocyclic Carbene by Sodium Magnesiate Complexes and Transmetallation Chemistry with Gold Complexes

Expanding the scope and applications of anionic N-heterocyclic carbenes (NHCs), a novel series of magnesium NHC complexes is reported using a mixed sodium-magnesium approach. Sequential reactivity of classical imidazol- 2-ylidene carbene IPr with NaR and MgR₂ (R=CH₂ SiMe₃ ) affords [(THF)₃ Na(μ-IPr^- )MgR₂ (THF)] (2) [IPr^- =:C{[N(2,6-iPr₂ C₆ H₃ )]₂ CHC] containing an anionic NHC ligand, whereas surprisingly sodium magnesiate [NaMgR₃ ] fails to deprotonate IPr affording instead the redistribution coordination adduct [IPr₂ Na₂ MgR₄ ] (1). Compound 2 undergoes selective C2-methylation when treated with MeOTf furnishing novel abnormal NHC complex [{aIPr^Me MgR₂ }₂ ] (3). Dissolving 3 in THF led to the dissociation of this complex into MgR₂ and aIPr^Me with the latter isomerizing to the olefinic NHC IPr=CH₂ . The ability of 2 and 3 to transfer their anionic and abnormal NHC ligands, respectively to Au^I metal fragments has been investigated allowing the isolation and structural characterization of [RAu(μ-IPr^- )MgR(THF)₂ ] (4) and [aIPr^Me AuR] (5) respectively. In both cases transfer of an alkyl R group is observed. However while 3 can also transfer its abnormal NHC ligand to give 5, in 4 the anionic NHC still remains coordinated to Mg via its C4 position, whereas the {AuR} fragment occupies the C2 position previously filled by a donor-solvated {Na(THF)₃ }⁺ cation.

NHC Pd(II) and Ag(I) Complexes: Synthesis, Structure, and Catalytic Activity in Three Types of C-C Coupling Reactions

Four bis-benzimidazolium salts, 1,4-bis[1'-(N-R-benzimidazoliumyl)methyl]-2,3,5,6-tetramethylbenzene 2X- (L 1 H 2 ·(PF 6 ) 2 : R = ethyl, X = PF6; L 2 H 2 ·Br 2 : R = picolyl, X = Br; L 3 H 2 ·Br 2 : R = benzyl, X = Br; and L 4 H 2 ·Br 2 : R = allyl, X = Br), and their three N-heterocyclic carbene (NHC) Pd(II) and Ag(I) complexes, L 1 Pd 2 Cl 4 (1), L 2 Ag 2 Br 2 (2), and L 4 (AgBr) 2 (3), as well as one anionic complex L 3 H 2 ·(Ag 4 Br 8 ) 0.5 (4), have been synthesized and characterized. Complex 1 adopts a funnel-like type of structure, complex 2 adopts a cyclic structure, and complex 3 is an open structure. In the crystal packing of 1-4, one-dimensional polymeric chains and two-dimensional supramolecular layers are formed via intermolecular weak interactions, including hydrogen bonds, π-π interactions, and C-H···π contacts. The catalytic activities of NHC Pd(II) complex 1 in three types of C-C coupling reactions (Suzuki-Miyaura, Heck-Mizoroki, and Sonogashira reactions) were studied. The results show that this catalytic system is efficient for these C-C coupling reactions.

Schiff-base -ate derivatives with main group metals: generation of a tripodal aluminate metalloligand

A new type of -ate derivatives is described, where the alkali metal exerts a strong influence on the final outcome.

NormalandabnormalNHC coordination in cationic hydride iodide complexes of aluminium

Sterically demanding NHC aluminium hydride iodide complexes react with one equivalent of NHC to cationic mixednormal–abnormalNHC Al^IIIcomplexes.