Facile Benzene Reduction by a Ca²⁺/Al^I Lewis Acid/Base Combination

Abstract

Attempted synthesis of the donor–acceptor complex (BDI)Ca+←AlI(BDI) complex by reaction of (BDI)Ca+ in the form of its B(C6F5)4− salt with (BDI)AlI in benzene led to dearomatization of the solvent and formation of (BDI)Ca+(C6H6)AlIII(BDI) (BDI=CH[C(CH3)N‐Dipp]2, Dipp=2,6‐diisopropylphenyl). The C6H62− anion is strongly puckered and its boat form features four long (ca. 1.50 Å) and two short (ca. 1.34 Å) C−C bond distances. The flagpole positions of the C6H62− anion chelate an AlIII cation giving a norbornadiene‐like fragment with Al in the 7‐position. The C=C double bonds of this alumina‐norbornadiene strongly coordinate to the Ca2+ metal ion. The complex is stable in solution up to 80 °C. Several mechanisms for its formation are discussed including a highly likely frustrated Lewis pair type mechanism in which benzene is activated by the Lewis acid (BDI)Ca+ followed by nucleophilic attack by the Lewis base (BDI)AlI.

Evidence of AlIIRadical Addition to Benzene

“…The bond lengths between the aluminum atom and the carbon atom of the benzene (1.9925(9) Å) in 3 are slightly shorter than the expected Al−C single bond (2.01 Å), [24] and also than those reported for the Birch reduction by Brand et al. 2.060(3)/2.073(3) Å, [18] by Dhara et al. 2.064(3)/2.059(3) Å, [20] and also from the products resulting from the [4+2]‐cycloaddition, i.e.…”

“…The monomer and dimer species can exist in equilibrium [26] . Nonetheless, the probability of meeting a benzene solvent molecule is higher than meeting another INT1 or INT3 to dimerize, similar to the situation of a heterobimetallic Ca/Al Birch‐reduction [18] . Thus, the radical addition to a molecule of benzene is the first step of the reaction.…”

“…The groups of Power and Tokitoh have described a cis 1,4‐addition to toluene and benzene via [4+2]‐cycloaddition of a transient dialuminene compound, [15] which has been later stabilized as a dianion [16] or by coordination to Lewis bases [17] . The group of Harder has shown that the combination of the nucleophilic Al I compound ( II ) with the highly Lewis acidic (NacNac)Ca + analogue reduces benzene [18] . Notably, the isolobal Mg compound can also undergo a Birch‐type benzene reduction, when the dimerization of the intermediate Mg I radical is prevented by a super bulky spectator ligand ((HC{C(Me)N[2,6‐(3‐pentyl)‐phenyl]} 2 )) and a coordinating species, TMEDA (N,N,N′,N′‐tetramethylethylenediamine) [19] .…”

Hydroarylation of olefins catalysed by a dimeric ytterbium(II) alkyl

“…The reaction of 1 with three equivalents of phenylsilane at room temperature gives [BDI Dipp YbH] 2 (2). [BDI Dipp Yb(CH 2 CH 3 )] 2 (3) and [BDI Dipp Yb(CH 2 CH 2 CH 3 )] 2 (4) were synthesised by the addition of the appropriate alkene (ethene for 3 and propene for 4) to a degassed benzene solution of 2. All new compounds 1-4 and 6 were characterised by elemental analysis, multinuclear NMR spectroscopy and single-crystal X-ray diffraction.…”

“…In contrast, hydroarylation, the addition of an arene C-H bond across olefins, offers significant advantages over classical Friedel-Crafts alkylation as the reaction is by-product free and the olefin starting materials are generally cheaper and more readily available compared to the corresponding alkyl halide. While a number of potent nucleophilic main group species have very recently been shown to activate and functionalise aryl C-H bonds [3][4][5][6][7][8][9] , only transition metal complexes have, thus far, provided an alternative catalytic pathway to new alkylated arene products (Fig. 1b) [10][11][12][13][14] .…”