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the structures of 2a and 3a were elucidated unambiguously by
X-ray diffraction analysis (Figure 1).[7,8] As expected, 2a has
a highly planar geometry, with a mean plane deviation of
0.141 ꢀ. On the other hand, 3a can be regarded as an
produced 2b and 3b in 4% and 59% yields, respectively
(entry 8).[12] Oxidation of bis(3,5-ditrifluoromethylphenyl)-
anthracene 4c under conditions A provided pyrazine-fused
dimer 2c in 57% yield (entry 9). In the case of 1c, no pyrrole-
fused dimer 3c was obtained, even under conditions B,
probably because of the steric hindrance of the aryl groups.
These results suggest the versatility of the pyrazine-fusion
reaction for various aminoanthracenes, while silylethynyl
groups appear to be optimal to form aza[7]helicenes 3.
We propose a possible mechanism for the formation of
azahelicene 3a in Scheme 2. The reaction begins with
oxidation of the aminoanthracene 1 with DDQ to produce
the plausible aminyl radical intermediate I.[13] Two radical
Figure 1. X-ray crystal structures of 2a and 3a. a) Top view and b) side
view of 2a, and c) top view and d) side view of 3a. The thermal
ellipsoids are scaled at the 50% probability level. The hydrogen atoms,
alkynyl groups in (a) and (b), and silyl groups in (c) and (d) are
omitted for clarity.
aza[7]helicene because of its helical conformation. The
ethynyl groups are distorted as a result of steric repulsion
between the anthracene core and the bulky silyl groups. The
two naphthalene subunits (AB and CD in Figure 1c) are
twisted by 46.58. The synthesis of helicenes by intermolecular
homocoupling reaction in a single step is rare.[9–11]
Scheme 2. Possible mechanism for the formation of 3a.
À
intermediates I then couple to form a C C single bond.
Tautomerization at one side regenerates a benzene ring and
one NH2 group, which then undergoes nucleophilic addition
to the imine moiety on the other side to form a five-
membered ring. Finally, elimination of ammonia produces the
pyrrole ring. The key intermediate II may allow the formation
of the distorted structure of 3a.[14] Elimination of gaseous
ammonia and aromatic stabilization as a result of the
formation of a benzene ring could be the driving force of
the reaction.
Interestingly, the reaction was significantly dependent on
the solvent. In other solvents, such as dichloromethane, THF,
and DMF, the yields were substantially lower, and the
products were not formed in toluene. We found that the
reaction in CHCl3 with 0.5% 2-methyl-2-butene as a stabilizer
predominantly produced 2a (Table 1, entry 2). The chloro-
form used in entry 1 contained 0.5% ethanol instead of 2-
methyl-2-butene, while a trace of hydrogen chloride may also
be present in chloroform. We anticipated that the product
distribution could be influenced by the presence of acid or
ethanol as an additive. To eliminate other factors we chose
distilled dichloromethane as a standard solvent. Eventually,
the addition of 0.5% (v/v) trifluoroacetic acid (TFA, con-
ditions A) resulted in the exclusive formation of 2a in 87%
yield. On the other hand, the reaction in dichloromethane
with 5% ethanol (conditions B) predominantly afforded 3a in
68% yield along with 2a in 9% yield. The selective formation
of 3a over 2a is rather remarkable, since 3a is clearly
thermodynamically unfavorable compared to 2a. In both
cases, the total yields of the reaction were considerably
increased in the presence of additives, thus indicating that the
additives not only accelerate the reaction but also control the
product distribution.
Figure 2 shows the UV/Vis absorption and emission
spectra of 2a and 3a in dichloromethane. The lowest energy
bands of both 2a and 3a exhibit bathochromic shifts
compared to those of 9,10-bis(triisopropylsilylethynyl)anthra-
cene (4), thus indicating effective p conjugation between the
two anthracene moieties. The fluorescence of 2a and 3a also
had moderate quantum yields (Ff = 0.45 for 2a and Ff = 0.36
for 3a). The Stokes shift of 3a (Dn = 2220 cmÀ1) was larger
than that of 2a (Dn = 473 cmÀ1), which reflects the distorted
conformation of 3a.
To investigate the properties of 3a as a helicene, we next
carried out its optical resolution, which could be perfectly
separated on a HPLC column with a chiral stationary phase to
afford (P)-(+)-3a and (M)-(À)-3a (see Figure S23 in the
Supporting Information). The helical chirality of these
enantiomers was stable and no racemization was observed
by HPLC analysis on a chiral stationary phase after three
months. Figure 2c shows the circular dichromism (CD)
spectra of 3a in dichloromethane. The spectra of (P)-3a and
We next examined the scope of the substrates. The
oxidation of bis(triethylsilylethynyl)anthracene 1b under
conditions A afforded pyrazine-fused dimer 2b in 52%
yield (Table 1, entry 7). On the other hand, conditions B
2
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!
Goto, Kiyohiko
