How Aromatic Are the Benzene Rings in Biphenylene?
J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2905
Bruker WM 250 or at 360 MHz on a Bruker AMX 360 using CDCl3
as the solvent and either TMS as the internal standard or the CHCl3
peak at 7.24 ppm. Carbon NMR spectra were recorded at 62.9 or 90.6
MHz in CDCl3, using the solvent peak at 77.0 ppm for calibration.
Mass spectra were recorded on a Finnigan 3300 gas chromatography-
mass spectrometer using methane gas for chemical ionization (CI) or
electron impact (EI) at 70 eV. Exact mass measurements used a Perkin-
Elmer-Hitachi RMU-6E or a Kratos Concept-H instrument with
perfluorokerosene as the calibrant. Elemental analyses were carried
out by Canadian Microanalytical Services Ltd, Vancouver, BC. All
evaporations were carried out under reduced pressure on a rotary
evaporator, and all organic extracts were washed with water and dried
over anhydrous MgSO4, Na2SO4, or K2CO3 as appropriate. SiGel refers
to Merck Silica Gel, 70-230 mesh. PE refers to distilled petroleum
ether, bp 30-60 °C.
from which BLE for biphenylene ) 0.588(BLE for benzene).
Consideration of the Kekule´ structures 6A,B indicates that the
distant benzene ring can delocalize independently in both
structures 6A and 6B, which together delocalize the [14]-
Benzocyclobutadiene Adducts of Furan 3: 4a,b. Zinc dust (1 g,
large excess) was added in one portion to a solution of 1,2-
dibromobenzocyclobutene3 (0.600 g, 2.3 mmol) and the oxa[17]-
annulene2 3 (0.200 g, 0.73 mmol) in dry THF (50 mL). The mixture
was then heated to 40 °C with magnetic stirring under argon. After
12 h, the mixture was cooled to room temperature, and then inorganic
solids were removed by filtration and the filtrate was concentrated.
The crude product was chromatographed over SiGel and eluted with
9:1 pentane:ether to give a mixture of the olive green exoadducts 4a,b
(0.125 g, 46%). 1H NMR (250 MHz): δ 8.65 and 8.64 (s, 1H total,
H-6), 8.60-8.49 (m, 6H, H-1,3,4,5,15,16), 8.06-7.99 (m, 1H, H-2),
7.37-7.26 (m, 4H, H-9,10,11,12), 6.26 and 6.25 (s, 1H total 2:3 ratio,
H-14), 5.88 and 5.82, (s, 1H total 3:2 ratio, H-7), 3.60 and 3.44 (d, 1H
each, J ) 3.7 Hz, H-8,13), -4.06, -4.11 and -4.08, -4.13 (s, 6H
total 2:3 ratio, internal -CH3) 13C NMR (62.9 MHz, CD2Cl2): δ 141.6,
137.2, 136.8, 128.4, 127.3, 124.6, 124.5, 123.8, 123.3, 123.1, 122.5,
122.4, 119.6, 115.7, 80.5, 78.2, 52.6, 51.1, 31.9, 30.9, 14.5, 14.4. CI
MS, m/z 375 (MH+). Anal. Calcd for C28H22O: C, 89.81; H, 5.92.
Found: C, 89.38; H, 5.88.
annulene ring. Thus, to obtain the RE of biphenylene, one
benzene unit must be added to the BLE determined above, and
so the experimental RE of biphenylene is determined to be 1.588
benzene units (or 1.588 times the RE for benzene). Dewar1a
gives the RE of benzene as 0.869 eV and of biphenylene as
1.346 eV, and thus, biphenylene is 1.55 benzene units, in
amazingly good agreement with our experimental value.
Moyano1g reports global REPE values of 0.064â for benzene
and 0.035â for biphenylene, suggesting that on average the value
for biphenylene is 55% of the value for benzene, again agreeing
amazingly well with the experimental relative aromaticity of
55% determined above. Trinajstic’s conjugated circuit model1h
predicts 50%, while the Hess and Schaad approach1b predicts
41%. Parr’s hardness approach1f predicts 45%, and Jug’s bond-
order approach1e,i predicts 67-76%. All in not bad agreement
with our experimental approach.
Attempted Dehydration of the Exo Adducts 4a,b. As detailed in
the following table, an acid (0.1 mmol) in solvent (1 mL) was added
to exo adducts 4a/4b (10 mg) at the stated temperature. The runs were
conducted until some change in the starting material occurred (moni-
tored by TLC). All the attempted reactions led to extensive decomposi-
tion of the starting material and only trace formation of product 2, which
was identified from its internal methyl signals at δ -2.77.
While 13C shifts do not give data that can be used quite as
reliably as protons, the same trends are evident. The bridge
carbons for 7, 2, and 6 appear at δ 30.0, 33.6, and 33.9 and
35.5 and 36.0, respectively, while the internal methyl carbons
appear at δ 14.0, 16.1, and 16.2 and 17.0 and 17.7, respectively,
which using the RA equation derived above (for protons) yields
about 65% as the relative aromaticity of biphenylene relative
to benzene based on carbon shift data. This is fairly good
agreement considering the ring current effect for carbon is the
same magnitude as for protons, yet in general, shifts for carbon
are of much wider range.
Table 1
no.
temp, °C
solvent
acid
1
2
3
4
5
6
7
8
9
0-25
0-25
AcOH
benzene
none
benzene
AcOH
THF
THF
Et2O
benzene
CH2Cl2
benzene
36%HCl
HCl (gas)
H2F2 (liquid)
47% HI
HCl (gas)
TiCl4
SnCl4
BF3‚Et2O
Al2O3
-50-25
-10-25
-10-25
-10-50
-10-50
0-25
Conclusions
80
The aromaticity of biphenylene relative to benzene has been
determined to be about 55% based on the relative bond fixing
ability of the two aromatic systems. This agrees quite well with
their relative global resonance energy per electron predictions.
The Dewar resonance energy of biphenylene is estimated from
chemical shift data to be 1.59 times the DRE of benzene, while
Dewar’s calculations predict 1.55. This suggests that this
method of estimating Dewar resonance energies is valid even
though biphenylene contains [4n] circuits.
10
11
25-55
Nafion-H
Nafion-H, light
25
1,1,2,2-Tetrabromobenzocyclobutene. The mixture of 1,2-dibro-
mobenzocyclobutenes obtained by the procedure of Cava and co-
workers3 was purified (this is essential) according to the procedure of
Barton and co-workers.6 NBS (17.80 g, 0.1 mol) and propylene
carbonate (10 mL) were added to the dibromide thus obtained (5.62 g,
20 mmol) in CCl4 (100 mL), and then AIBN (∼10 mg, catalyst) was
added to the mixture, which was then refluxed with good stirring and
irradiation with two 250 W sunlamps for 12 h. The mixture was cooled
to 25 °C and filtered under suction. The residue was washed with
dichloromethane (50 mL), and the combined filtrates were washed well
with water, 10% sodium bisulfite, then saturated aqueous NaCl, and
then were dried and evaporated to leave an orange oil. Trituration of
this oil with pentane gave white cubic crystals of product, 5.44 g (60%),
Experimental Section
Melting points were determined on a Reichert 7905 melting point
apparatus integrated to a chrome-alumel thermocouple. Infrared
spectra, major peaks only, calibrated with polystyrene, were recorded
on a Bruker IFS25 FT-IR or on a Perkin-Elmer 283 spectrometer as
KBr disks unless otherwise stated. Ultraviolet-visible spectra were
recorded on a Cary 5 or a Perkin-Elmer Lambda-4B spectrometer in
cyclohexane. Proton NMR spectra were recorded at 250 MHz on a
(6) Barton, J. W.; Shepherd, M. K.; Willis, R. J. J. Chem. Soc., Perkin
Trans. 1 1986, 967-971.