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

The molecular calcium hydride, [(BDI)CaH]2(BDI¼HC{(Me)CN-2,6-i-Pr2C6H3}2), effects the slowhydrodehalogenation of C6H5X(X¼I, Br) to provide benzene and the respective dimeric calcium halides,[(BDI)CaX]2. Although a similar reaction withfluorobenzene was non-discriminating, the analogoushydrogenation of chlorobenzene was observed, albeit this process yields the calcium hydride-chloride asthe alkaline earth-containing product. Assessment of the bromide- and chloride-based processes bydensity functional theory (DFT) calculations, imply that the reactions take place with the retention of thedimeric calcium structures throughout. Both systems invoke an SNAr-type displacement of the halide, viabarriers (in the range 32e34 kcal mol1for C6H5Br and 31.1e32.9 kcal mol1for C6H5Cl), which vary onlymarginally during the transformation of the initial hydride and halide-hydride intermediates. Theisolation of the calcium hydride-chloride is ascribed, therefore, to its more rapid crystallisation anddepletion from solution. Also reported is the reactivity of[ (BDI)CaH]2 witha,a,a-trifluorotoluene andanisole, which yield the corresponding dicalcium hydride-fluoride and phenoxide derivatives, respec-tively, rather than the products of directed ortho metalation.