Synthesis and hydrogenation of polycyclic aromatic hydrocarbon-substituted diborenes uncatalysed hydrogenative B-C bond cleavage.

The classical route to the PMe-stabilised polycyclic aromatic hydrocarbon (PAH)-substituted diborenes BAr(PMe) (Ar = 9-phenanthryl 7-Phen; Ar = 1-pyrenyl 7-Pyr) the corresponding 1,2-diaryl-1,2-dimethoxydiborane(4) precursors, BAr(OMe), is marred by the systematic decomposition of the latter to BAr(OMe) during reaction workup. Calculations suggest this results from the absence of a second -substituent on the boron-bound aryl rings, which enables their free rotation and exposes the B-B bond to nucleophilic attack. 7-Phen and 7-Pyr are obtained by the reduction of the corresponding 1,2-diaryl-1,2-dichlorodiborane precursors, BArCl(PMe), obtained from the SMe adducts, which are synthesised by direct NMe-Cl exchange at BAr(NMe) with (MeS)BCl.