Alkali and alkaline-earth metal ketyl complexes: isolation, structural diversity, and hydrogenation/protonation reactions

Reduction of Ketones and Coupling of Ketyls by a Zn-Zn-Bonded Compound

The α-diimine-ligated Zn-Zn-bonded compound [K(THF)2]2[LZn-ZnL] (1, L = [(2,6-iPr2C6H3)NC(Me)]22-) displays diverse reactivities toward a variety of ketones. In the reaction of 1 with benzophenone or 4,4'-di-tert-butylbenzophenone, a multielectron transfer process was observed to give bimetallic (Zn/K) complexes with both ketyl radical fragments and C-C coupled pinacolate moieties (products 2 and 3). In contrast, treating 1 with 9-fluorenone only afforded pinacolate complex 5. Moreover, the reactions of 1 with N- or O-heterocycle-functionalized ketones, i.e., di(2-pyridyl)ketone, 2,2-pyrrolidinone, 9-xanthenone, or 10-methyl-9(10H)-acridone, were also carried out. Besides different transformations of the ketone moiety, the heteroatoms (nitrogen or oxygen) are also involved in coordination with zinc or potassium ions, yielding discrete aggregates or polymeric structures of products 6-9.

Pinacol Cross-Coupling Promoted by an Aluminyl Anion

A simple sequential addition protocol for the reductive coupling of ketones and aldehydes by a potassium aluminyl grants access to unsymmetrical pinacolate derivatives. Isolation of an aluminium ketyl complex presents evidence for the accessibility of radical species. Product release from the aluminium centre was achieved using an iodosilane, forming the disilylated 1,2-diol and a neutral aluminium iodide, thereby demonstrating the steps required to generate a closed synthetic cycle for pinacol (cross) coupling at an aluminyl anion.

A 2,2'-bipyridyl calcium complex: synthesis, structure and reactivity studies

A 2,2'-bipyridyl calcium complex based on a tridentate ligand [CH₃C(N-2,6-ⁱPr₂C₆H₃)CHC(CH₃)NCH₂CH₂N(CH₃)₂]Ca(bipy)(THF) (1) was prepared by the reduction of {[CH₃C(N-2,6-ⁱPr₂C₆H₃)CHC(CH₃)NCH₂CH₂N(CH₃)₂]CaI(THF)}₂ with potassium graphite in the presence of 2,2'-bipyridine (bipy). Complex 1 is a good Ca(I)synthon, as shown by its reactivity with I₂, PhCH₂SSCH₂Ph, PhCH₂SeSeCH₂Ph and 9-fluorenone, yielding the calcium iodide complex [CH₃C(N-2,6-ⁱPr₂C₆H₃)CHC(CH₃)NCH₂CH₂N(CH₃)₂]CaI(bipy) (2), calcium thiolate [CH₃C(N-2,6-ⁱPr₂C₆H₃)CHC(CH₃)NCH₂CH₂N(CH₃)₂]Ca(SCH₂Ph)(bipy) (3), calcium selenolate [CH₃C(N-2,6-ⁱPr₂C₆H₃)CHC(CH₃)NCH₂CH₂N(CH₃)₂]Ca(SeCH₂Ph)(bipy) (4), and calcium ketyl complex [CH₃C(N-2,6-ⁱPr₂C₆H₃)CHC(CH₃)NCH₂CH₂N(CH₃)₂]Ca[O-(9-C₁₃H₈˙)](bipy)·2THF (5·2THF), respectively. In addition, reactions of complex 5 with CS₂, CH₂CHCH₂Br and PhCH₂Br give the corresponding dimeric bis(thiolate) complex {[S₂CC(CMe(NAr))C(Me)NCH₂CH₂NMe₂]Ca(DME)}₂ (6), dimeric calcium bromide complex {[(9-CH₂CHCH₂-C₁₃H₈-9)-O]CaBr(THF)(bipy)}₂ (7) and {[(9-C₆H₅CH₂-C₁₃H₈-9)-O]CaBr[O-(9-C₁₃H₈)](bipy)}₂ (8). These results demonstrated that the calcium ketyl complex 5 can also be employed as a single-electron transfer reagent. All the new compounds were characterized by various spectroscopic methods, and their solid-state structures were further confirmed by single-crystal X-ray diffraction analyses.

Revisiting the Mg/TMSCl/Dipolar Solvent System for Dearomatic Silylation of Aryl Carbonyl Compounds: Substrate Scope, Transformations, and Mechanistic Studies

Dearomatic silylation of arene derivatives is an intriguing synthetic target, which represents an elegant extension of Birch reduction and produces silylated cyclohexene derivatives with great potential of further transformation. Herein, we report a systematic study on dearomatic silylation of aryl carbonyl compounds with Mg and the TMSCl/NMP adduct. The protocol displays a wide range of substrate scope, including alkyl aryl ketones, aromatic amides, benzonitriles, tert-butyl benzoates, and even 2,2'-bipyridines. Synthetic utility is demonstrated using the products as versatile substrate in various transformations. The detailed mechanism is presented with both control experimental analyses and theoretical calculations. An unusual five-coordinated silicon dianion intermediate is first proposed and described here. The selectivity is influenced by the relative rates of single electron reductions (the TMSCl/NMP adduct versus the substrate) and the steric effects.

Diverse reactivity of an Al(I)-centred anion towards ketones

The reactivity of a seven-membered cyclic potassium diamidoalumanyl toward a variety of ketone small molecules has been assessed. Whilst acetophenone generates an aluminium pinacolate derivative, reductive C-C coupling is induced between the ketyl and ortho-carbon centres of two equivalents of benzophenone. In contrast, whereas oxidative addition of an enolisable proton is observed with 2,4-dimethyl-3-pentanone, 2,2,4,4-tetramethyl-3-pentanone undergoes an unprecedented hydroalumination process, where the reducing hydride may be traced to intramolecular oxidative addition of a (sp³)C-H bond.

Diverse Reactivity of a Magnesium Silanide toward Ketones

The reactivity of a molecular magnesium silanide toward ketones displays a significant variability of outcome, resulting in adduct formation, deprotonation, dearomatisation or deoxygenation, that is dependent on the structure and...

Cobalt and Iron Stabilized Ketyl, Ketiminyl and Aldiminyl Radical Anions

Carbonyl and iminyl based radical anions are reactive intermediates in a variety of transformations in organic synthesis. Herein, the isolation of ketyl, and more importantly unprecedented ketiminyl and aldiminyl radical anions coordinated to cobalt and iron complexes is presented. Insights into the electronic structure of these unusual metal bound radical anions is provided by X-Ray diffraction analysis, NMR, IR, UV/Vis and Mössbauer spectroscopy, solid and solution state magnetometry, as well as a by a detailed computational analysis. The metal bound radical anions are very reactive and facilitate the activation of intra- and intermolecular C-H bonds.

A Persistent Phosphanyl-Substituted Thioketyl Radical Anion

Alkali metal salts, M⁺[Ter(iPr)P−C(=S)−P(iPr)₂S]^.− (M=Na, K; 2_M; Ter=2,6‐bis‐(2,4,6‐trimethylphenyl)phenyl) containing a room‐temperature‐stable thioketyl radical anion were obtained by reduction of the thioketone precursor, Ter(iPr)P−C(=S)−P(iPr)₂S (1), with alkali metals (Na, K). Single‐crystal X‐ray studies as well as EPR spectroscopy revealed the unequivocal existence of a thioketyl radical anion in the solid state and in solution, respectively. The computed Mulliken spin density within 2_M is mainly located at the sulfur (49 %) and the carbonyl carbon (33 %) atoms. Upon adding [2.2.2]‐cryptand to the radical species 2_K to minimize the interionic interaction, an activation reaction was observed, yielding a potassium salt with a phosphanyl thioether based anion, [K(crypt)]⁺[Ter(iPr)P−C(−S‐iPr)−P(iPr)₂S]⁻ (3) as the product of an intermolecular shift of an iPr group from a second anion. The products were fully characterized and application of the radical anion as a reducing agent was demonstrated.

Photoinduced umpolung addition of carbonyl compounds with α,β-unsaturated esters enables the polysubstituted γ-lactone formation

Photoinduced reductive coupling of carbonyl compounds and α,β-unsaturated esters via ketyl radical intermediates for the synthesis of γ-lactones is described.

The Reactions of Carbon Monoxide with Silyl and Silenyl Lithium - Synthesis and Isolation of the First Stable Tetra-Silyl Di-Ketyl Biradical and 1-Silaallenolate Lithium

Reactions of carbon monoxide (CO) with tBu₂ MeSiLi and (E)-(tBu₂ MeSi)(tBuMe₂ Si)C=Si(SiMetBu₂ )Li⋅2 THF (4) were studied both experimentally and computationally. Reaction of tBu₂ MeSiLi with CO in hexane yields the first stable tetra-silyl di-ketyl biradical [(tBu₂ MeSi)₂ COLi]^. ₂ (3). Reaction of 4 with CO yields selectively and quantitatively the first reported 1-silaallenolate, (tBu₂ MeSi)(tBuMe₂ Si)C=C=Si(SiMetBu₂ )OLi⋅THF (5). Both 3 and 5 were characterized by X-ray crystallography and biradical 3 also by EPR spectroscopy. Silaallenolate 5 reacts with Me₃ SiCl to produce siloxy substituted 1-silaallene (tBu₂ MeSi)(tBuMe₂ Si)C=C=Si(SiMetBu₂ )OSiMe₃ . The reaction of 4 with CO provides a new route to 1-silaallenes. The mechanisms of the reactions of tBuMe₂ SiLi and of 4 with CO were studied by DFT calculations.

Diradical Anion of Potassium Aggregate: Reduction of Dimer Boroxide Complex

Crystalline aggregates containing metal cation clusters "wrapped" by reduced hydrocarbon anions have been presented. Initially, a dimeric complex (2) possessing a bimetallic K₂O₂ core was synthesized from the reaction between an anthracene substituted boronic acid (2-(anthracen-9-yl)phenyl)(hydroxy)(mesityl)borane (1-H) and KN(SiMe₃)₂ in THF solution. The B-O bond length (1.281(4) Å) in complex 2 is comparable to those observed in oxoboranes, indicating this may be a double bond, which is supported by its Wiberg bond order (1.9) predicted by density functional theory calculations. Subsequently, potassium aggregate complexes, diradical 3 and dihydro anion 4, were obtained through the reduction reactions of dimeric complex 2 and 1-H, respectively. They exhibit similar K₃O₂B₂ aggregate structures, but differ significantly in the geometry of the anthracene units. Complex 3 features a triplet diradical character with planar anthracene units that carry the unpaired electrons, while the anthracene groups in complex 4 display a puckered structure due to the addition of hydrogen atoms.

Reduction of Carbonyl Groups by Uranium(III) and Formation of a Stable Amide Radical Anion

Methyl benzoate, N,N-dimethylbenzamide, and benzophenone were reduced by U^III [N(SiMe₃ )₂ ]₃ resulting in uranium(IV) products. Reduction of benzophenone lead to U^IV [OC⋅Ph₂ )][N(SiMe₃ )₂ ]₃ , (1.1) which forms the dinuclear complex, [N(SiMe₃ )₂ ]₃ U^IV (OCPhPh-CPh₂ O)U^IV [N(SiMe₃ )₂ ]₃ (1.2), through coupling of the ketyl radical species upon crystallization. Reaction of N,N-dimethylbenzamide with U^III [N(SiMe₃ )₂ ]₃ resulted in U^IV [OC⋅(Ph)(NMe₂ )][N(SiMe₃ )₂ ]₃ (2), a uranium(IV) compound and the first example of a charge-separated amide radical. In the case of methyl benzoate, the reduction resulted in U^IV (OMe)[N(SiMe₃ )₂ ]₃ (3) and benzaldehyde as the reduced organic fragment. Compound 2 showed the ability to act as a uranium(III) synthon in its reactivity with trimethylsilyl azide, a reaction that yielded U^V (=NSiMe₃ )[N(SiMe₃ )₂ ]₃ . Additionally, 2 was reduced with potassium graphite resulting in [U(μ-O)[O=C(NMe₂ )(Ph)][N(SiMe₃ )₂ ]₂ ]₂ (4), a dinuclear uranium compound bridged by oxo ligands. Reduction of 2 in the presence of 15-crown-5 afforded isolation of the mono-oxo compound, [(15-crown-5)₂ K][UO[N(SiMe₃ )₂ ]₃ ] (5). The results expand the reduction capabilities of U^III complexes and demonstrate a strategy for isolating novel metal-stabilized radicals.

Tetrabutylammonium Salts of Aluminum(III) and Gallium(III) Phthalocyanine Radical Anions Bonded with Fluoren-9-olato- Anions and Indium(III) Phthalocyanine Bromide Radical Anions

Reduction of aluminum(III), gallium(III), and indium(III) phthalocyanine chlorides by sodium fluorenone ketyl in the presence of tetrabutylammonium cations yielded crystalline salts of the type (Bu₄ N⁺ )₂ [M^III (HFl-O^- )(Pc^.3- )]^.- (Br^- )⋅1.5 C₆ H₄ Cl₂ [M=Al (1), Ga (2); HFl-O^- =fluoren-9-olato^- anion; Pc=phthalocyanine] and (Bu₄ N⁺ ) [In^III Br(Pc^.3- )]^.- ⋅0.875 C₆ H₄ Cl₂ ⋅0.125 C₆ H₁₄ (3). The salts were found to contain Pc^.3- radical anions with negatively charged phthalocyanine macrocycles, as evidenced by the presence of intense bands of Pc^.3- in the near-IR region and a noticeable blueshift in both the Q and Soret bands of phthalocyanine. The metal(III) atoms coordinate HFl-O^- anions in 1 and 2 with short Al-O and Ga-O bond lengths of 1.749(2) and 1.836(6) Å, respectively. The C-O bonds [1.402(3) and 1.391(11) Å in 1 and 2, respectively] in the HFl-O^- anions are longer than the same bond in the fluorenone ketyl (1.27-1.31 Å). Salts 1-3 show effective magnetic moments of 1.72, 1.66, and 1.79 μ_B at 300 K, respectively, owing to the presence of unpaired S=1/2 spins on Pc^.3- . These spins are coupled antiferromagnetically with Weiss temperatures of -22, -14, and -30 K for 1-3, respectively. Coupling can occur in the corrugated two-dimensional phthalocyanine layers of 1 and 2 with an exchange interaction of J/k_B =-0.9 and -1.1 K, respectively, and in the π-stacking {[In^III Br(Pc^.3- )]^.- }₂ dimers of 3 with an exchange interaction of J/k_B =-10.8 K. The salts show intense electron paramagnetic resonance (EPR) signals attributed to Pc^.3- . It was found that increasing the size of the central metal atom strongly broadened these EPR signals.

Sodium-Ketyl Radical Anions by Reverse Pinacol Reaction and Their Coupling with Iodoarenes

Transition-metal-free C-C bond formation between aryl iodides and pinacols is presented. Pinacols are readily transformed by deprotonation with NaH to their Na-pinacolates. C-C-bond homolysis at room temperature generates in situ the corresponding Na-ketyl radical anions which react with various aryl iodides in a homolytic aromatic substitution reaction to form tertiary Na-alcoholates. Protonation eventually provides tertiary alcohols in an unprecedented route.

Isolation of a uranium(iii) benzophenone ketyl radical that displays redox-active ligand behaviour

The first uranium(iii) charge separated ketyl radical complex, Tp*₂U(OC·Ph₂), has been isolated and acts as a potent two-electron reductant with reducing equivalents derived from both uranium and the redox-active benzophenone.

Reaction of benzopinacol with non-ionic bases: reversing the pinacol coupling

The reaction of benzopinacol with the non-ionic bases butyllithium and phosphazene P4 leads to the formation of the corresponding ketyl radical anions, which have been characterized by EPR/ENDOR spectroscopy. This conversion has a high efficiency. Such a reversed pinacol reaction can be used for a controlled release of ketyl radicals. Moreover, the nature of the base has a marked effect on the association of the ketyl radical anion and the counterions. This illustrates the importance of ion pairing for reductive coupling.

Synthesis, Characterization, and Reactions of Isolable (β-Diketiminato)Nb(III) Imido Complexes

We have investigated both the chemical reduction of (BDI)Nb(V) imido complexes (BDI = HC[C(Me)NAr](2); Ar = 2,6-(i)Pr(2)-C(6)H(3)) to the formal Nb(III) oxidation state and the ability of these Nb(III) complexes to behave as two-electron reductants. The reduction of the Nb(V) species was found to depend heavily on the nature of available supporting ligands, but the chemistry of the reduced compounds proceeded cleanly with a number of unsaturated organic reagents. Accordingly, the novel Nb(V) bis(imido) complexes supported by the monoazabutadiene (mad) ligand (mad)Nb(N(t)Bu)(NAr)(L') (L' = py, thf) were formed by either KC(8) reduction of (BDI)Nb(N(t)Bu)Cl(2)(py) in the absence of strong π-acids or by H(2) reduction of the Nb(V) dimethyl complex (BDI)Nb(N(t)Bu)Me(2) in THF. These products are likely formed though an intramolecular, 2 e(-) reductive C-N bond cleavage, as has been observed previously for related Group 4 systems, suggesting that transient Nb(III) intermediates were present in both cases. In the presence of 1,2-bis(dimethylphosphino)ethane (dmpe), KC(8) reduction of (BDI)Nb(N(t)Bu)Cl(2)(py) was arrested at the Nb(IV) oxidation state to give (BDI)Nb(N(t)Bu)Cl(dmpe), which was characterized by solution-state EPR spectroscopy as a Nb-centered paramagnet with strong coupling to the two equivalent phosphorus nuclei (A(iso){(93)Nb} = 120.5×10(-4) cm(-1), A(iso){(31)P} = 31.0×10(-4) cm(-1), g(iso) = 1.9815). When strong π-acids were used to intercept the thermally unstable Nb(III) complex (BDI)Nb(N(t)Bu)(py) prior to reductive cleavage of the ligand C-N bond, the thermally stable Nb(III) species (BDI)Nb(N(t)Bu)(CX)(2)(L″) (X = O, L″ = py; X = NXyl, L″ = CNXyl; Xyl = 2,6-Me(2)-C(6)H(3)) were obtained in good yields. The Nb(III) complexes (BDI)Nb(N(t)Bu)py, (BDI)Nb(N(t)Bu)(CO)(2)(py) and (BDI)Nb(N(t)Bu)(CO)(2) were subsequently investigated for their ability to serve as two-electron reducing reagents for both metal-ligand multiple bond formation and for the reduction of organic π-systems. The reduction of mesityl azide by (BDI)Nb(N(t)Bu)(py) and diphenylsulfoxide by (BDI)Nb(N(t)Bu)(CO)(2) led to the monomeric bis(imido) and dimeric oxo complexes (BDI)Nb(N(t)Bu)(NMes)(py) and [(BDI)Nb(N(t)Bu)](2)(μ2-O)(2), respectively. MeLi addition to (BDI)Nb(N(t)Bu)(CO)(2)(py) resulted in the formation of a Nb-acylate via methide addition to one of the carbonyl carbons. The acylate product was revealed to have a short Nb-C(acylate) bond distance (2.059(4) Å), consistent with multiple Nb-C bond character resulting from Nb(III) back-bonding into the acylate carbon. The interaction of (BDI)Nb(N(t)Bu)(CO)(2) with two equivalents of 4,4'-dichlorobenzophenone resulted in the clean, quantitative formation of the corresponding pinacol coupling product, but introduction of the ketone in 1: 1 molar ratios resulted in mixtures of the pinacol product and the starting material, suggesting that ketone coordination to the Nb(III) complex may be reversible. Relatedly, addition of 1-phenyl-1-propyne to (BDI)Nb(N(t)Bu)(CO)(2) formed a thermally unstable 1: 1 Nb/alkyne complex, as characterized by NMR and IR spectroscopies; reaction of this species with HCl/MeOH yielded a 2: 1 mixture of 1-phenyl-1-propene and the free alkyne, suggesting a high degree of covalency in the Nb-C bonds.

A Well-Defined Hydrocarbon-Soluble Calcium Hydroxide: Synthesis, Structure, and Reactivity

Controlled hydrolysis of a (beta-diketiminate)calcium-amide gave a heteroleptic (beta-diketiminate)calcium-hydroxide complex that is remarkably stable against ligand exchange and formation of Ca(OH)2. The structure of this dimeric complex shows OH- units that symmetrically bridge the Ca2+ ions. This hydrocarbon-soluble calcium hydroxide reacted rapidly with CO2 to produce a gel from which amorphous CaCO3 slowly separated. This reaction behavior allows for sol-gel coating with CaCO3 from an organic solvent. Reaction with benzophenone did not lead to nucleophilic attack of OH- to the carbonyl but gave a red benzophenone adduct instead.

Sodium compounds of the benzophenone dianion (diphenyloxidomethanide)

The benzophenone dianion [diphenyloxidomethanide, (Ph(2)CO)(2-)], which occurs in the well known deeply violet sodium/benzophenone tetrahydrofuran solutions, was crystallised with sodium cations in form of the two polymeric chain compounds [Na(2)(Ph(2)CO)(tetraglyme)](infinity) and [Na(2)(Ph(2)CO)(thf)(2)](infinity). It was found to aggregate with its conjugated acid, the alcoholate (Ph(2)CHO)(-), around a central unit of sodium hydroxide, resulting in the mixed cage compound [Na(13)(Ph(2)CO)(4)(Ph(2)CHO)(4)(OH)(mtbe)(4)].mtbe. The structural parameters of the benzophenone dianion indicate that a considerable amount of its negative charge is located within the phenyl rings, rather than on the formally anionic benzylic carbon atom. The topological analysis of the electron density of the monomeric model structure [Na(2)(Ph(2)CO)] reveals an even positive charge for this particular atom, hence (Ph(2)CO)(2-) is, despite its usual representation, not a vicinal dianion.