CATALYTIC BORYLATION OF C-H BONDS: A ROUTE TO PHOTOPHYSICALLY INTERESTING PYRENE DERIVATIVES

Abstract

Pyrene derivatives have found use in a wide range of applications that make use of the unique structural, optical and charge-transfer properties of pyrene. Nearly all pyrene derivatives are substituted at the 1-, 3-, 6- and 8-positions, i.e. the sites of electrophilic aromatic substitution. In contrast, derivatives substituted at the 2- and 2,7-postions of pyrene are rare, as their syntheses involve laborious multistep processes. Such derivatives are of interest because they retain the long axis of symmetry and display unusual photophysical properties.
Using the regiospecific direct C-H borylation of pyrene with an Ir-based catalyst, prepared in situ via reaction of [Ir(OMe)COD]2 with 4,4’-di-tert-butyl-2,2’-bipyridine, 2,7-bis(Bpin)pyrene 1 and 2-(Bpin)pyrene 2 (pin = OCMe2CMe2O) were synthesized. Straightforward derivatization strategies, converted 1 and 2 into nominally nucleophilic and electrophilic pyrene derivatives, which were further utilized in Suzuki-Miyaura, Sonogashira, Buchwald-Hartwig and Negishi cross-coupling reactions. Using this methodology, a library of 2- and 2,7-substituted pyrenes bearing donor and acceptor groups, including aryl, ethynyl, arylethynyl, alkyl, hydroxy, alkoxy, diarylamino, carboxylic acid and diarylboryl derivatives, was prepared.
The solid-state structures of several derivatives obtained by single-crystal X-ray diffraction revealed a diverse range of structures and packing modes.
The influence of the substitution position and the nature of the substituent upon pyrene was investigated in a detailed photophysical study. The existence of nodal planes passing through the 2- and 7-positions, perpendicular to the molecular plane, in both the HOMO and LUMO of pyrene accounts for the differing photophysical behaviour of derivatives substituted at 2- and 2,7-positions from those substituted at the 1-position.

Finally, the applicability of iridium-catalysed C-H borylation and metal-catalyzed cross-coupling is further demonstrated on a range of interesting substrates.