Evidence of AlII Radical Addition to Benzene

Electrophilic Al^III species have long dominated the aluminum reactivity towards arenes. Recently, nucleophilic low-valent Al^I aluminyl anions have showcased oxidative additions towards arenes C-C and/or C-H bonds. Herein, we communicate compelling evidence of an Al^II radical addition reaction to the benzene ring. The electron reduction of a ligand stabilized precursor with KC₈ in benzene furnishes a double addition to the benzene ring instead of a C-H bond activation, producing the corresponding cyclohexa-1,3(orl,4)-dienes as Birch-type reduction product. X-ray crystallographic analysis, EPR spectroscopy, and DFT results suggest this reactivity proceeds through a stable Al^II radical intermediate, whose stability is a consequence of a rigid scaffold in combination with strong steric protection.

Alkaline-Earth Metal Mediated Benzene-to-Biphenyl Coupling

Complex [(DIPeP BDI)Ca]2 (C6 H6 ), with a C6 H6 2- dianion bridging two Ca2+ ions, reacts with benzene to yield [(DIPeP BDI)Ca]2 (biphenyl) with a bridging biphenyl2- dianion (DIPeP BDI=HC[C(Me)N-DIPeP]2 ; DIPeP=2,6-CH(Et)2 -phenyl). The biphenyl complex was also prepared by reacting [(DIPeP BDI)Ca]2 (C6 H6 ) with biphenyl or by reduction of [(DIPeP BDI)CaI]2 with KC8 in presence of biphenyl. Benzene-benzene coupling was also observed when the deep purple product of ball-milling [(DIPP BDI)CaI(THF)]2 with K/KI was extracted with benzene (DIPP=2,6-CH(Me)2 -phenyl) giving crystalline [(DIPP BDI)Ca(THF)]2 (biphenyl) (52 % yield). Reduction of [(DIPeP BDI)SrI]2 with KC8 gave highly labile [(DIPeP BDI)Sr]2 (C6 H6 ) as a black powder (61 % yield) which reacts rapidly and selectively with benzene to [(DIPeP BDI)Sr]2 (biphenyl). DFT calculations show that the most likely route for biphenyl formation is a pathway in which the C6 H6 2- dianion attacks neutral benzene. This is facilitated by metal-benzene coordination.

Reduction of Na+ within a {Mg2 Na2 } Assembly

Ionic compounds containing sodium cations are notable for their stability and resistance to redox reactivity unless highly reducing electrical potentials are applied. Here we report that treatment of a low oxidation state {Mg₂ Na₂ } species with non-reducible organic bases induces the spontaneous and completely selective extrusion of sodium metal and oxidation of the Mg^I centers to the more conventional Mg^II state. Although these processes are also characterized by a structural reorganisation of the initially chelated diamide spectator ligand, computational quantum chemical studies indicate that intramolecular electron transfer is abetted by the frontier molecular orbitals (HOMO/LUMO) of the {Mg₂ Na₂ } ensemble, which arise exclusively from the 3s valence atomic orbitals of the constituent sodium and magnesium atoms.

C-H Activation of Inert Arenes using a Photochemically Activated Guanidinato-Magnesium(I) Compound

UV irradiation of solutions of a guanidinate coordinated dimagnesium(I) compound, [{(Priso)Mg}₂ ] 3 (Priso=[(DipN)₂ CNPrⁱ ₂ ]^- , Dip=2,6-diisopropylphenyl), in either benzene, toluene, the three isomers of xylene, or mesitylene, leads to facile activation of an aromatic C-H bond of the solvent in all cases, and formation of aryl/hydride bridged magnesium(II) products, [{(Priso)Mg}₂ (μ-H)(μ-Ar)] 4-9. In contrast to similar reactions reported for β-diketiminate coordinated counterparts of 3, these C-H activations proceed with little regioselectivity, though they are considerably faster. Reaction of 3 with an excess of the pyridine, p-NC₅ H₄ Bu^t (py^But ), gave [(Priso)Mg(py^But H)(py^But )₂ ] 10, presumably via reduction of the pyridine to yield a radical intermediate, [(Priso)Mg(py^But ⋅)(py^But )₂ ] 11, which then abstracts a proton from the reaction solvent or a reactant. DFT calculations suggest two possible pathways to the observed arene C-H activations. One of these involves photochemical cleavage of the Mg-Mg bond of 3, generating magnesium(I) doublet radicals, (Priso)Mg⋅. These then doubly reduce the arene substrate to give "Birch-like" products, which subsequently rearrange via C-H activation of the arene. Circumstantial evidence for the photochemical generation of transient magnesium radical species includes the fact that irradiation of a cyclohexane solution of 3 leads to an intramolecular aliphatic C-H activation process and formation of an alkyl-bridged magnesium(II) species, [{Mg(μ-Priso^-H )}₂ ] 12. Furthermore, irradiation of a 1 : 1 mixture of 3 and the β-diketiminato dimagnesium(I) compound, [{(^Dip Nacnac)Mg}₂ ] (^Dip Nacnac=[HC(MeCNDip)₂ ]^- ), effects a "scrambling" reaction, and the near quantitative formation of an unsymmetrical dimagnesium(I) compound, [(Priso)Mg-Mg(^Dip Nacnac)] 13. Finally, the EPR spectrum (77 K) of a glassed solution of UV irradiated 3 is dominated by a broad featureless signal, indicating the presence of a doublet radical species.

Access to a Labile Monomeric Magnesium Radical by Ball-Milling

In order to isolate a monometallic Mg radical, the precursor (Am)MgI⋅(CAAC) (1) was prepared (Am=tBuC(N‐DIPP)₂, DIPP=2,6‐diisopropylphenyl, CAAC=cyclic (alkyl)(amino)carbene). Reduction of a solution of 1 in toluene with the reducing agent K/KI led to formation of a deep purple complex that rapidly decomposed. Ball‐milling of 1 with K/KI gave the low‐valent Mg^I complex (Am)Mg⋅(CAAC) (2) which after rapid extraction with pentane and crystallization was isolated in 15 % yield. Although a benzene solution of 2 decomposes rapidly to give Mg(Am)₂ (3) and unidentified products, the radical is stable in the solid state. Its crystal structure shows planar trigonal coordination at Mg. The extremely short Mg−C distance of 2.056(2) Å indicates strong Mg−CAAC bonding. Calculations and EPR measurements show that most of the spin density is in a π* orbital located at the C−N bond in CAAC, leading to significant C−N bond elongation. This is supported by calculated NPA charges in 2: Mg +1.73, CAAC −0.82. Similar metal‐to‐CAAC charge transfer was calculated for M⁰(CAAC)₂ and [M^I(CAAC)₂⁺] (M=Be, Mg, Ca) complexes in which the metal charges range from +1.50 to +1.70. Although the spin density of the radical is mainly located at the CAAC ligand, complex 2 reacts as a low‐valent Mg^I complex: reaction with a I₂ solution in toluene gave (Am)MgI⋅(CAAC) (1) as the major product.

Grignard reagent formation via C-F bond activation: a centenary perspective

Examples of Grignard reagents obtained by C-F bond activation with magnesium have kept appearing in the literature over the last century. Due to the high bond dissociation energy of the C-F bond, a lot of effort has been invested in the preparation of highly active forms of magnesium for this purpose. Originally, magnesium activation was achieved by the application of additives, notably iodine. Later work focused on the generation of highly active magnesium powder by reduction of magnesium salts with alkali metals ("Rieke magnesium"). Modern approaches to the problem involve the application of Mg(I)-Mg(I) dimers and C-F bond activation performed by a transition metal catalyst followed by transmetallation with a magnesium salt. The purpose of this article is to provide an overview of fluoro-Grignard reagent preparation approaches reported to date.

Cationic Heterobimetallic Mg(Zn)/Al(Ga) Combinations for Cooperative C-F Bond Cleavage

Low-valent (Me BDI)Al and (Me BDI)Ga and highly Lewis acidic cations in [(tBu BDI)M+ ⋅C6 H6 ][(B(C6 F5 )4 - ] (M=Mg or Zn, Me BDI=HC[C(Me)N-DIPP]2 , tBu BDI=HC[C(tBu)N-DIPP]2 , DIPP=2,6-diisopropylphenyl) react to heterobimetallic cations [(tBu BDI)Mg-Al(Me BDI)+ ], [(tBu BDI)Mg-Ga(Me BDI)+ ] and [(tBu BDI)Zn-Ga(Me BDI)+ ]. These cations feature long Mg-Al (or Ga) bonds while the Zn-Ga bond is short. The [(tBu BDI)Zn-Al(Me BDI)+ ] cation was not formed. Combined AIM and charge calculations suggest that the metal-metal bonds to Zn are considerably more covalent, whereas those to Mg should be described as weak AlI (or GaI )→Mg2+ donor bonds. Failure to isolate the Zn-Al combination originates from cleavage of the C-F bond in the solvent fluorobenzene to give (tBu BDI)ZnPh and (Me BDI)AlF+ which is extremely Lewis acidic and was not observed, but (Me BDI)Al(F)-(μ-F)-(F)Al(Me BDI)+ was verified by X-ray diffraction. DFT calculations show that the remarkably facile C-F bond cleavage follows a dearomatization/rearomatization route.

Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation

Strongly Lewis acidic cationic aluminium complexes, stabilized by β–diketiminate (BDI) ligands and free of Lewis bases, have been prepared as their B(C₆F₅)₄⁻ salts and were investigated for catalytic activity in imine hydrogenation. The backbone (R1) and N (R2) substituents on the ^R1,R2BDI ligand (^R1,R2BDI=HC[C(R1)N(R2)]₂) influence sterics and Lewis acidity. Ligand bulk increases along the row ^Me,DIPPBDI<^Me,DIPePBDI≈^tBu,DIPPBDI<^tBu,DIPePBDI; DIPP=2,6‐C(H)Me₂‐phenyl, DIPeP=2,6‐C(H)Et₂‐phenyl. The Gutmann‐Beckett test showed acceptor numbers of: (^tBu,DIPPBDI)AlMe⁺ 85.6, (^tBu,DIPePBDI)AlMe⁺ 85.9, (^Me,DIPPBDI)AlMe⁺ 89.7, (^Me,DIPePBDI)AlMe⁺ 90.8, (^Me,DIPPBDI)AlH⁺ 95.3. Steric and electronic factors need to be balanced for catalytic activity in imine hydrogenation. Open, highly Lewis acidic, cations strongly coordinate imine rendering it inactive as a Frustrated Lewis Pair (FLP). The bulkiest cations do not coordinate imine but its combination is also not an active catalyst. The cation (^tBu,DIPPBDI)AlMe⁺ shows the best catalytic activity for various imines and is also an active catalyst for the Tishchenko reaction of benzaldehyde to benzylbenzoate. DFT calculations on the mechanism of imine hydrogenation catalysed by cationic Al complexes reveal two interconnected catalytic cycles operating in concert. Hydrogen is activated either by FLP reactivity of an Al⋅⋅⋅imine couple or, after formation of significant quantities of amine, by reaction with an Al⋅⋅⋅amine couple. The latter autocatalytic Al⋅⋅⋅amine cycle is energetically favoured.

Nucleophilic Aromatic Substitution at Benzene with Powerful Strontium Hydride and Alkyl Complexes

Key to the isolation of the first alkyl strontium complex was the synthesis of a strontium hydride complex that is stable towards ligand exchange reactions. This goal was achieved by using the super bulky β-diketiminate ligand ^DIPeP BDI (CH[C(Me)N-DIPeP]₂ , DIPeP=2,6-diisopentylphenyl). Reaction of ^DIPeP BDI-H with Sr[N(SiMe₃ )₂ ]₂ gave (^DIPeP BDI)SrN(SiMe₃ )₂ , which was converted with PhSiH₃ into [(^DIPeP BDI)SrH]₂ . Dissolved in C₆ D₆ , the strontium hydride complex is stable up to 70 °C. At 60 °C, H-D isotope exchange gave full conversion into [(^DIPeP BDI)SrD]₂ and C₆ D₅ H. Since H-D exchange with D₂ is facile, the strontium hydride complex served as a catalyst for the deuteration of C₆ H₆ by D₂ . Reaction of [(^DIPeP BDI)SrH]₂ with ethylene gave [(^DIPeP BDI)SrEt]₂ . The high reactivity of this alkyl strontium complex is demonstrated by facile ethylene polymerization and nucleophilic aromatic substitution with C₆ D₆ , giving alkylated aromatic products and [(^DIPeP BDI)SrD]₂ .