Alkaline‐Earth Metal Mediated Benzene‐to‐Biphenyl Coupling

Abstract

Complex [( DIPeP BDI)Ca] 2 (C 6 H 6 ), with a C 6 H 6 2À dianion bridging two Ca 2 + ions, reacts with benzene to yield [( DIPeP BDI)Ca] 2 (biphenyl) with a bridging biphenyl 2À 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 (C 6 H 6 ) with biphenyl or by reduction of [( DIPeP BDI)CaI] 2 with KC 8 in presence of biphenyl. Benzene-benzene coupling was also observed when the deep purple product of ballmilling [( 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 KC 8 gave highly labile [( DIPeP BDI)Sr] 2 (C 6 H 6 ) 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 C 6 H 6 2À dianion attacks neutral benzene. This is facilitated by metal-benzene coordination.

On the existence of low-valent magnesium–calcium complexes

“…reacted with benzene by dehydrogenative C-C coupling to give a biphenyl 2− ion. 20 For the Mg/Ca combination, we found no indication of biphenyl formation. These observations demonstrate the importance of solvent effects in salt-metathesis reactions.…”

“…In contrast, recently isolated Ca and Sr complexes with a μ 6 -bridging C 6 H 6 2− ion ( V ) reacted with benzene by dehydrogenative C–C coupling to give a biphenyl 2− ion. 20 For the Mg/Ca combination, we found no indication of biphenyl formation.…”

“…Previous attempts to prepare (BDI)Ca-Ca(BDI) or (BDI)Sr-Sr(BDI) complexes in aromatic solvents led to solvent reduction and formation of complexes with bridging C 6 H 6 2− dianions (V). 15,20 The extreme reducing power of (BDI)Ca-Ca(BDI) complexes was further demonstrated by isolation of the N 2activated complex VI when alkanes were used as the solvent and N 2 as an "inert" gas. 15 Similar N 2 reduction to N 2 2− was found in an attempt to isolate a mixed-metal Ca I /K I complex.…”

A fruitful century for the scalable synthesis and reactions of biphenyl derivatives: applications and biological aspects

“…Similarly, the direct dehydrogenative coupling of benzene through low-valent alkaline-earth metals (Ae) intermediates was achieved via formation of metal (Ae) complex with a C 6 H 5 2− dianion which acts as reducing agent to bulky β-diketiminate ligand BDI 80A to yield [( DIPeP BDI)Ca] 2 (biphenyl) 81 as dianionic N , C -chelating ligand, which rapidly decomposed to afford biphenyl scaffold 2 (Scheme 35). 100…”

A Blueprint for the Stabilization of Sub‐Valent Alkaline Earth Complexes**

“…[{Mg(BDI Dipp )} 2 ] ( A ); BDI Dipp ={[(Dipp)NC(Me)] 2 CH} − ; Dipp=2,6‐diisopropylphenyl) (Figure 1). [2] Since this initial discovery, Mg(I) dimers have been used as bespoke reducing agents towards an array of organic and inorganic substrates [3–7] . Mg(I) dimers are usually obtained via reduction of heteroleptic Mg(II) halide precursors with alkali metals [8,9] .…”