Arene C−H Activation at Aluminium(I): meta Selectivity Driven by the Electronics of S N Ar Chemistry

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.

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.

Hydrocarbon Soluble Alkali-Metal-Aluminium Hydride Surrog[ATES]

A series of group 1 hydrocarbon-soluble donor free aluminates [AM(^t BuDHP)(TMP)Al(ⁱ Bu)₂ ] (AM=Li, Na, K, Rb) have been synthesised by combining an alkali metal dihydropyridyl unit [(2-^t BuC₅ H₅ N)AM)] containing a surrogate hydride (sp³ C-H) with [(ⁱ Bu)₂ Al(TMP)]. These aluminates have been characterised by X-ray crystallography and NMR spectroscopy. While the lithium aluminate forms a monomer, the heavier alkali metal aluminates exist as polymeric chains propagated by non-covalent interactions between the alkali metal cations and the alkyldihydropyridyl units. Solvates [(THF)Li(^t BuDHP)(TMP)Al(ⁱ Bu)₂ ] and [(TMEDA)Na(^t BuDHP)(TMP)Al(ⁱ Bu)₂ ] have also been crystallographically characterised. Theoretical calculations show how the dispersion forces tend to increase on moving from Li to Rb, as opposed to the electrostatic forces of stabilization, which are orders of magnitude more significant. Having unique structural features, these bimetallic compounds can be considered as starting points for exploring unique reactivity trends as alkali-metal-aluminium hydride surrog[ATES].

Substituent Effects on Aluminyl Anions and Derived Systems: A High-Level Theory

Aluminyl anions are low-valent aluminum species bearing a lone pair of electrons and a negative charge. These systems have drawn recent synthetic interest for their nucleophilic nature, allowing for the activation of σ-bonds, and have been proposed as a pathway to hydrogen energy storage. In this research, we provide high-level ab initio geometries and energies for both the simplest aluminyl anion (AlH₂^-) and several substituted derivatives. Geometries are reported using the gold-standard CCSD(T)/aug-cc-pV(T+d)Z level of theory. Energies were extrapolated to the complete basis set limit through the focal point approach, utilizing coupled-cluster methods through perturbative quadruples and basis sets up to five-ζ quality. Geometries were rationalized using electrostatic, steric, and orbital donation effects. The donation from substituents to Al is accompanied by back-donation effects, a property traditionally thought of in transition-metal systems. Stereoelectronic effects through the secondary orbital interaction play a fundamental role in stabilizing these low-valent aluminum compounds and would likely also affect the feasibility of their use within several industrial applications. The energetic analysis of the formation of each substituted anion is rationalized as the result of three energetic schemes. The effectiveness of these schemes for determining the relative formation energies is discussed.

Seven-Membered Cyclic Potassium Diamidoalumanyls

The seven-membered cyclic potassium alumanyl species, [{SiN^Mes }AlK]₂ [{SiN^Mes }={CH₂ SiMe₂ N(Mes)}₂ ; Mes=2,4,6-Me₃ C₆ H₂ ], which adopts a dimeric structure supported by flanking K-aryl interactions, has been isolated either by direct reduction of the iodide precursor, [{SiN^Mes }AlI], or in a stepwise manner via the intermediate dialumane, [{SiN^Mes }Al]₂ . Although the intermediate dialumane has not been observed by reduction of a Dipp-substituted analogue (Dipp=2,6-i-Pr₂ C₆ H₃ ), partial oxidation of the potassium alumanyl species, [{SiN^Dipp }AlK]₂ , where {SiN^Dipp }={CH₂ SiMe₂ N(Dipp)}₂ , provided the extremely encumbered dialumane [{SiN^Dipp }Al]₂ . [{SiN^Dipp }AlK]₂ reacts with toluene by reductive activation of a methyl C(sp³ )-H bond to provide the benzyl hydridoaluminate, [{SiN^Dipp }AlH(CH₂ Ph)]K, and as a nucleophile with BPh₃ and RN=C=NR (R=i-Pr, Cy) to yield the respective Al-B- and Al-C-bonded potassium aluminaborate and alumina-amidinate products. The dimeric structure of [{SiN^Dipp }AlK]₂ can be disrupted by partial or complete sequestration of potassium. Equimolar reactions with 18-crown-6 result in the corresponding monomeric potassium alumanyl, [{SiN^Dipp }Al-K(18-cr-6)], which provides a rare example of a direct Al-K contact. In contrast, complete encapsulation of the potassium cation of [{SiN^Dipp }AlK]₂ , either by an excess of 18-cr-6 or 2,2,2-cryptand, allows the respective isolation of bright orange charge-separated species comprising the 'free' [{SiN^Dipp }Al]^- alumanyl anion. Density functional theory (DFT) calculations performed on this moiety indicate HOMO-LUMO energy gaps in the of order 200-250 kJ mol^-1 .

Factors Controlling the Aluminum(I)-meta-Selective C-H Activation in Arenes

The so far poorly understood factors controlling the complete meta-selectivity observed in the C-H activation reactions of alkylarenes promoted by aluminyl anions have been explored in detail by means of Density Functional Theory calculations. To this end, a combination of state-of-the-art computational methods, namely the activation strain model of reactivity and energy decomposition analysis, has been applied to quantitatively unveil the origin of the selectivity of the transformation as well as the influence of the associated potassium cation. It is found that the selectivity takes place during the initial nucleophilic addition step where the key LP(Al)→π*(C=C) molecular orbital interaction is more stabilizing for the meta-pathway, which results in a stronger interaction between the reactants along the entire transformation.

Dihydrogen Activation by Lithium- and Sodium-Aluminyls

To date, aluminyl anions have been exclusively isolated as their potassium salts. We report herein the synthesis of the lithium and sodium aluminyls, M₂ [Al(NON^Dipp )]₂ (M=Li, Na. NON^Dipp =[O(SiMe₂ NDipp)₂ ]^2- ; Dipp=2,6-iPr₂ C₆ H₃ ). Both compounds crystallize from non-coordinating solvent as "slipped" contacted dimeric pairs with strong M⋅⋅⋅π(aryl) interactions. Isolation from Et₂ O solution affords the monomeric ion pairs (NON^Dipp )Al-M(Et₂ O)₂ , which contain discrete Al-Li and Al-Na bonds. The ability of the full series of Li, Na and K aluminyls to activate dihydrogen is reported.