In:
Chemical Science, Royal Society of Chemistry (RSC), Vol. 13, No. 25 ( 2022), p. 7566-7574
Abstract:
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 BB 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.
Type of Medium:
Online Resource
ISSN:
2041-6520
,
2041-6539
Language:
English
Publisher:
Royal Society of Chemistry (RSC)
Publication Date:
2022
detail.hit.zdb_id:
2559110-1
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