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

The classical route to the PMe 3 -stabilised polycyclic aromatic hydrocarbon (PAH)-substituted diborenes B 2 Ar 2 (PMe 3 ) 2 (Ar = 9-phenanthryl 7-Phen; Ar = 1-pyrenyl 7-Pyr) via the corresponding 1,2-diaryl-1,2-dimethoxydiborane(4) precursors, B 2 Ar 2 (OMe) 2 , is marred by the systematic decomposition of the latter to BAr(OMe) 2 during reaction workup. Calculations suggest this results from the absence of a second ortho -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, B 2 Ar 2 Cl 2 (PMe 3 ) 2 , obtained from the SMe 2 adducts, which are synthesised by direct NMe 2 -Cl exchange at B 2 Ar 2 (NMe 2 ) 2 with (Me 2 S)BCl 3 . The low-lying π* molecular orbitals (MOs) located on the PAH substituents of 7-Phen and 7-Pyr intercalate between the B-B-based π and π* MOs, leading to a relatively small HOMO-LUMO gap of 3.20 and 2.72 eV, respectively. Under vacuum or at high temperature 7-Phen and 7-Pyr undergo intramolecular hydroarylation of the B[double bond, length as m-dash]B bond to yield 1,2-dihydronaphtho[1,8- cd ][1,2]diborole derivatives. Hydrogenation of 7-Phen, 7-Pyr and their 9-anthryl and mesityl analogues III and II, respectively, results in all cases in splitting of the B-B bond and isolation of the monoboranes (Me 3 P)BArH 2 . NMR-spectroscopic monitoring of the reactions, solid-state structures of isolated reaction intermediates and computational mechanistic analyses show that the hydrogenation of the three PAH-substituted diborenes proceeds via a different pathway to that of the dimesityldiborene. Rather than occurring exclusively at the B-B bond, hydrogenation of 7-Ar and III proceeds via a hydroarylated intermediate, which undergoes one B-B bond-centered H 2 addition, followed by hydrogenation of the endocyclic B-C bond resulting from hydroarylation, making the latter effectively reversible.