Dehydrohalogenation of halobenzenes and C(sp3)-X (X = F, OPh) bond activation by a molecular calcium hydride

Abstract

The molecular calcium hydride, [(BDI)CaH] 2 (BDI = HC{(Me)CN-2,6-i-Pr 2 C 6 H 3 } 2 ), effects the slow hydrodehalogenation of C 6 H 5 X (X = I, Br) to provide benzene and the respective dimeric calcium halides, [(BDI)CaX] 2 . Although a similar reaction with fluorobenzene was non-discriminating, the analogous hydrogenation of chlorobenzene was observed, albeit this process yields the calcium hydride-chloride as the alkaline earth-containing product. Assessment of the bromide- and chloride-based processes by density functional theory (DFT) calculations, imply that the reactions take place with the retention of the dimeric calcium structures throughout. Both systems invoke an S N Ar-type displacement of the halide, via barriers (in the range 32–34 kcal mol −1 for C 6 H 5 Br and 31.1–32.9 kcal mol −1 for C 6 H 5 Cl), which vary only marginally during the transformation of the initial hydride and halide-hydride intermediates. The isolation of the calcium hydride-chloride is ascribed, therefore, to its more rapid crystallisation and depletion from solution. Also reported is the reactivity of [(BDI)CaH] 2 with α,α,α-trifluorotoluene and anisole, which yield the corresponding dicalcium hydride-fluoride and phenoxide derivatives, respectively, rather than the products of directed ortho metalation.