Journal of the American Chemical Society
Article
Except DBTPD, other cyclophanes were used as precursors for
(attempted) syntheses of [8]carbon nanobelt and cycloarenes,
including [n]circulenes and Kekulene, and their structures and
properties have barely been explored. DBTPD with a quasi-
planar structure, accompanied by some distortion in the central
ring, is different from the aforementioned cyclophanes. PNPD8
and PPD9 were assigned to be tub-shaped (syn-conformers)
based on theoretical investigations or chemical shifts in the 1H
NMR spectra, whereas NPD,7c BPPD,11 and DBAPD12 were
proposed to adopt step-like anti-conformers. Nonetheless,
none of their structures were proved experimentally.
mide (NBS) and the subsequent base-mediated intramolecular
couplings of benzylic (dibromo)methine (ArCHBr2) and
bromomethene (ArCH2Br) of the thus generated hexabro-
mo-substituted intermediate.15 Debromination of 3 through
successive treatment with n-butyllithium and methanol
produced 2,11-di-tert-butylpicene (4).
Next, how to effectively construct a cyclophane is equally
important. NPD,7 PNPD,8 BPPD,11 and DBAPD12 were
synthesized from the corresponding dithiacyclophanes using a
conventional route, which, regardless of chemical yields,
required many essential synthetic steps, including methylation,
oxidation, Stevens rearrangement, oxidation, and finally 1,2-
Hofmann elimination or pyrolysis. Normally, PPD was
achieved in 3−6 mg with small yields of 10−15% by
photocylization9a or nickel-catalyzed cyclization9b of halo-
substituted [2.2.2.2]paracyclophanetetraenes that were also
acquired inefficiently. The aforementioned synthetic ap-
proaches thus may not be suitable for picenophanes.
Alternatively, a new and efficient method was developed
herein where flexible acetyl groups were used as linkers for
connecting two picenyl moieties. Accordingly, palladium-
catalyzed Heck reaction of 3 with n-butyl vinyl ether was
conducted, which was followed by hydrolysis under acidic
conditions to obtain 5,8-diacetyl-2,11-di-tert-butylpicene (5).16
Picenophane 6 was produced through palladium-catalyzed α-
arylation17 of diketone 5 with an equal amount of 3. cis-
Ethylene linkers in 8 and ethano bridges in 9 were prepared by
reducing carbonyl groups of 6 and through the consecutive
dehydration and dehydroxylation of the resulting compound,
respectively. It needs to be emphasized that, with enough
dibromopicene 3 in hand, the title compound 8 was
accomplished on a semigram scale (415 mg). To study the
molecular dynamics of [2,2](5,8)picenophanediene experi-
mentally, isopropyl-substituted derivative 10 was prepared
from 5 and 5,8-dibromo-2-tert-butyl-11-isopropylpicene (11)
using the aforementioned synthetic method (see Scheme S1 in
the Supporting Information).
RESULTS AND DISCUSSION
■
Synthesis. In this study, a new series of picenophanes13
were synthesized, and their structures, molecular dynamics,
and photophysical properties were explored. Despite several
synthetic methods for preparations of picene14a,e and 5,8-
dihalopicene14e,f being reported, efficient and hence gram-scale
syntheses are still a great challenge.14g Picenophanes may also
suffer from low solubility in common organic solvents.
Introduction of additional alkyl chains should improve the
issue. Therefore, new synthetic approaches to di-tert-butyl-
substituted picene and several picenophanes were investigated
herein (Scheme 1). The Suzuki reaction of 4-bromo-2,3-
dimethylphenyl triflate (1) with 2,4,6-tris(5-tert-butyl-2-
methylphenyl)boroxin yielded 1,4-bis(5-tert-butyl-2-methyl-
phenyl)-2,3-dimethylbenzene (2), which was converted into
5,8-dibromo-2,11-di-tert-butylpicene (3) on a multigram scale
(2.32 g) through radical bromination with N-bromosuccini-
Scheme 1. Syntheses of Picenes and Picenophanes
Structural Analysis. Crystals of 6 and 8 were prepared
through the slow diffusion of methanol in dichloromethane
and chloroform solutions, respectively, at room temperature,
and their diffraction data were collected at 100 K. The notable
features of tub-shaped picenophane are mainly related to the
arrangement of two picenyl moieties, as verified by the
following structural parameters (see Figure 2a): (1) the
dihedral angle between the mean square planes (θ1) of the two
picenyl moieties; (2) degree of staggering, determined using
the torsion angle of a bridge (θ2); (3) opening (d1) and
shrinking (d2) of the tub, as estimated using the shortest
interplanar distances between any two peripheral carbon atoms
(C1, C2, C11, C12, C13, and C14) and inner carbon atoms
(C5, C6, C7, and C8), respectively; and 4) the average of the
distance (d3) between the internal hydrogen atoms (H6···H6′
and H7···H7′).
Compound 6 crystallized in the monoclinic P21/c space
group, with oxygen atoms and one tert-butyl group being
crystallographically disordered (Figure 2b). Moreover, 6 with
flexible linkers was expected to exhibit two conformers anti-6
and syn-6. However, only syn-6 was obtained in the solid state,
and the values of its structural parameters θ1, θ2, d1, d2, and d3
were determined to be 103.2°, 83.1°, 9.092(3) Å, 3.345(3) Å,
and 2.38 Å, respectively. According to the dihedral angle
between the mean square planes of the first and fifth six-
membered rings, one of the picenyl planes exhibited a higher
B
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX