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.
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