divergently from pyrene-4,5-diketone 1; the diketone moi-
ety is reduced and protected to facilitate the Pd-catalyzed
coupling reactions, which are known to be hindered by
the presence of the R-diketone.13,14 As shown in Scheme 1,
the 4,5-dibromo-9,10-pyrene diketone 2 was produced by
regioselective bromination of 1 according to the procedure
by Mullen et al.4 Reduction of 2 with zinc and in situ pro-
tection with the tertbutyldimethylsilyl (TBDMS) chloride
afforded the protected 4,5-dibromo isomer 3.15 Reversing
the order of reduction and bromination of 1 resulted in bis
silyl-protected 1,8-dibromo isomer 5.
Scheme 2. Synthetic Strategies Used To Access the Protected
2,7-Dibromo Isomer (11)
Scheme 1. Synthesis of 4,5-Dibromo- and 1,8-Dibromo-
Protected Diketone Isomers for Cross-Coupling
were subsequently deprotected and reoxidized to form the
final regioisomeric DꢀAꢀD chromophores 16 to 18.
Synthesis of the more elusive 2,7-dibromo isomer began
by subjecting bis-silyl 4 to the iridium-catalyzed regiose-
lective borylation developed by Marder et al.16,17 resulting
in bis-pinacol borane 6, as shown in Scheme 2. To our
knowledge, this is the first report of this reaction on a
substituted pyrene, and it is important to note that the
regioselectivity is maintained at the 2,7 positions. How-
ever, continuing with Marder’s method failed to produce
the dibrominated compound and instead resulted in the
deprotection of the TBDMS groups.18 A second route was
attempted from bis-pinacol borane 6 to form the protected
2,7-dihydroxypyrene 7, which was subsequently triflated
to give 8. However, poor yields were obtained in the
Sonogashira cross-coupling reaction between bis-triflate
8 and N,N-didodecyl-4-ethynylaniline. We then surmised
that less steric bulk might aid the cross-coupling step and
the TBDMS ether protecting groups were replaced with
methyl ethers. The route began with 4,5-dimethoxypyrene
9 (produced according to the method of Bodwell et al.19)
that was borylated to give 10 and finally brominated to
obtain the protected 2,7-dibromo isomer 11.
Scheme 3. Cross-Coupling, Deprotection, and Oxidation To
Afford Regioisomeric Pyrene DꢀA Chromophores
With the three isomers in hand, Sonogashira cross-
coupling reactions was carried out with N,N-didodecyl-4-
ethynylaniline 12 to obtain protected pyrene isomers
13ꢀ15, as shown in Scheme 3. The three protected pyrenes
Figure 1 shows the absorption spectra of diketones 16 to
18 in chloroform at 10ꢀ6 M concentrations. Diketone 16
shows absorptions starting at 500 nm, which is consistent
with a charge transfer (CT) band centered at 470 nm, as
was observed on similarly substituted pyrene DꢀA com-
(13) More, S.; Bhosale, R.; Choudhary, S.; Mateo-Alonso, A. Org.
Lett. 2012, 14, 4170–4173.
(14) Estrada, L. A.; Neckers, D. C. J. Org. Chem. 2009, 74, 8484–8487.
(15) Ciszek, J. W.; Tour, J. M. Tetrahedron Lett. 2004, 45, 2801–2803.
(16) Coventry, D. N.; Batsanov, A. S.; Goeta, A. E.; Howard,
J. A. K.; Marder, T. B.; Perutz, R. N. Chem. Commun. 2005, 2172–2174.
(17) Crawford, A. G.; Liu, Z.; Mkhalid, I. A. I.; Thibault, M.-H.;
Schwarz, N.; Alcaraz, G.; Steffen, A.; Collings, J. C.; Batsanov, A. S.;
Howard, J. A. K.; Marder, T. B. Chem.;Eur. J. 2012, 18, 5022–5035.
(18) Tan,Z.P.;Wang,L.;Wang,J.B.Chin. Chem. Lett. 2000,11, 753–756.
(19) Venkataramana, G.; Dongare, P.; Dawe, L. N.; Thompson,
D. W.; Zhao, Y.; Bodwell, G. J. Org. Lett. 2011, 13, 2240–2243.
pounds by Mullen et al.10 The CT absorption band of 17 is
€
of similar intensity as 16, but is strongly red-shifted with
a broad band centered at 570 nm and extending to over
700 nm. The CT band of 18 is the furthest red-shifted
(575 nm peak), but the least intense of the three isomers.
However, diketone 18 has an extremely strong πꢀπ*
transition at 385 nm with a molar absorptivity nearing
B
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