Conformational isomerism in 1,2-di-o-tolylnaphthalenes: selective rotation of the 2-aryl ring

Article abstract of DOI:10.1016/j.tet.2009.02.068

Two derivatives (X=Cl, CN) of 1,2-ditolylnaphthalene were prepared as models to investigate their conformational behavior that could involve rotation of either of the tolyl rings. The existence of anti and syn atropisomers was evident from their ¹

Full text of DOI:10.1016/j.tet.2009.02.068

Tetrahedron 65 (2009) 3664–3667
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Conformational isomerism in 1,2-di-o-tolylnaphthalenes:
selective rotation of the 2-aryl ring
*
Thian-Guan Peck, Yee-Hing Lai
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Two derivatives (X¼Cl, CN) of 1,2-ditolylnaphthalene were prepared as models to investigate their
conformational behavior that could involve rotation of either of the tolyl rings. The existence of anti and
syn atropisomers was evident from their 1H NMR spectra at room temperature indicating two pairs of
well-resolved singlets for the methyl protons. Dynamic 1H NMR studies estimated the rotation barrier to
be about 76–82 kJ molꢀ1, a value consistent with selective rotation of the 2-tolyl ring in the conformation
inter-conversion.
Received 22 November 2008
Received in revised form 6 February 2009
Accepted 26 February 2009
Available online 5 March 2009
Keywords:
Ó 2009 Published by Elsevier Ltd.
1,2-Diarylnaphthalenes
Conformational behavior
Selective rotation
Dynamic NMR
1. Introduction
only one of the tolyl rings that involves a relatively lower confor-
mational barrier (Scheme 1).
Restricted rotation (atropisomerism) of sterically hindered
groups in a large number of molecules has been studied experi-
mentally.1 There has been continued interest in the conformational
analysis of oligophenyls by various spectroscopic, analytical, and
computational studies.2 The dependence on their conformational
behavior in crystal growth,3 organic/metal interface,4 and polymer
properties5 have been reported. Earlier efforts were directed to-
ward the observation and/or isolation of the syn and anti confor-
mational isomers of ortho-disubstituted benzene derivatives or
polyarylbenzenes.6 Clough and Roberts7 successfully separated the
anti-1a and syn-1b isomers of 1,8-di-o-tolylnaphthalene 1, which
had an observed rotational barrier of 101 kJ molꢀ1, although the
half-life with respect to inter-conversion in solution was about one
day. Unique spectral and laser characteristics of 1,4-di-o-tol-
ylbenzene 2 have been observed8 but experimental identification
of its atropisomers anti-2a and anti-2b was only recently reported
in an NMR study at ꢀ150 ꢁC.9
In this work we report the conformational study of two de-
rivatives of 1,2-di-o-tolylnaphthalene 3. In principle an inter-con-
version between the two atropisomers of 1 or 2 could be achieved
by rotation of either of the two tolyl rings. In 3, there is a unique
possibility that atropisomerization engages selective rotation of
Scheme 1. Atropisomerism in selected teraryls.
2. Results and discussion
A synthetic route to 1,2-diphenylnaphthalenes using 1-benza-
* Corresponding author.
E-mail address: chmlaiyh@nus.edu.sg (Y.-H. Lai).
mido-2-naphthol 510 was reported by Mustafa and Kamel.11
0040-4020/$ – see front matter Ó 2009 Published by Elsevier Ltd.
doi:10.1016/j.tet.2009.02.068

T.-G. Peck, Y.-H. Lai / Tetrahedron 65 (2009) 3664–3667
3665
A summary of our synthetic approach to 1,2-diarylnaphthalenes 8
and 9 is illustrated in Scheme 2. Introduction of the 3,30-sub-
stituents should not affect their conformational behavior with
reference to that of the parent molecule 3. The spherical Cl and
cylindrical CN groups are, however, expected to exert appreciable
buttressing effects12 in raising their rotational barrier to be readily
estimated by dynamic 1H NMR spectroscopy. In addition the po-
larized ortho-Cl and CN functions should indirectly induce more
significant chemical shift difference in the methyl proton signals
between the anti and syn isomers.
Figure 1. Anti and syn conformers of teraryls 8, 9, 10, and 11.
the structure of aziridine 7. Two major singlets (likely due to
a mixture of rotational isomers) at 2.03 and 2.32 were observed
d
for the methyl protons in its 1H NMR spectrum. Results from the
elemental analysis supported the structure of 7. Formation of
aziridine 7 should have derived from an intramolecular elimination
(dehydration) of 6, although no similar product was reported in the
preparation of 1,2-diphenylnaphthalene.11 On the other hand,
similar elimination reactions to form aziridines were observed in
trans-a a-bromoaceta-
-phenylselenonylbenzamide13 and trans-
mide,14 albeit under basic conditions. In going from 6 to 7, both SN1
and S22 mechanisms were conceivable. The former was more likely
in acidic medium in the work-up and chromatography, and par-
ticularly under the acidic conditions employed in the subsequent
intended conversion of 6 to 8.
Conversion of 6 to diarylnaphthalene 8 was carried out with zinc
dust in acetic and hydrochloric acids in the presence of a catalytic
amount of chloroplatinic acid.11 The aziridine 7 was obtained as
a side product. In the 1H NMR spectrum of 8, four singlets were
Scheme 2. Synthesis of 1,2-diarylnapthalenes 8 and 9. Reagents and conditions: (i)
Mg/BrCH2CH2Br (cat.), THF, reflux, 3 h; (ii) 5, benzene, reflux, 3 h; rt, 8 h; (iii) Zn, AcOH,
2% aq [H3O]þ2[PtCl6]2ꢀ (cat.), concd HCl, reflux, 16 h; (iv) CuCN, N-methyl-2-pyrroli-
dinone, reflux, 32 h.
Addition of the Grignard reagent of 2,6-dichlorotoluene (4) to
1,2-napthoquinone-1-benzimide (5) unexpectedly led to the iso-
lation of two products obtained in similar yields of about 20% after
chromatography. The desired product 6 was the one eluted second
from column chromatography. A rather broad melting point in the
range of 248–254 ꢁC was observed for this product even after re-
peated recrystallizations (methanol/chloroform). In its 1H NMR
spectrum (25 ꢁC) broad and unresolved signals were observed at
observed for the methyl protons at
d 1.87, 2.07, 2.15, and 2.21 in
a ratio of 3:5:3:5. That was consistent with a mixture of rotational
isomers anti-8 and syn-8 (Fig. 1), with the expected two methyl
signals from each isomer. Treatment of 8 with CuCN in refluxing N-
methyl-2-pyrrolidinone (von Braun reaction)15 gave the dinitrile 9.
In its 1H NMR spectrum, the methyl protons also appeared as two
pairs (3:2) of singlets at
d 2.44, 2.29 and 2.39, 2.08 respectively,
d
1.4–2.9 (methyl protons) and
d
7.2–7.7 (aromatic protons). Over-
indicating the existence of anti-9a and syn-9b (Fig. 1).
lapping AB quartets of the vinyl protons appeared at
a major AB quartet (J 9.6 Hz) observed at
d
6.3–7.0, with
The existence of anti and syn isomers of 1,2-diarylbenzenes 1012a
was evident from 1H NMR studies with barriers to rotation of 62–
75 kJ molꢀ1. Rotational barriers in 9,10-diarylphenanthrenes 11,16
however, were found to involve much larger barriers in the range of
115–158 kJ molꢀ1. Their conformational behavior in 11 could not be
studied16a by dynamic 1H NMR spectroscopy and experimental
data were estimated by a gas chromatographic method.16b,c Dy-
namic 1H NMR (300 MHz) studies of 8 and 9 at high temperatures
were attempted using deuterated nitrobenzene as the solvent. A
solvent shift of the methyl signals was observed accompanied by
relatively poorer resolution (compared to that in CDCl3) of these
signals. The observed spectra, however, still showed four well-re-
solved methyl signals appropriate for conformational studies based
on the coalescence temperature method17 to estimate the free
d
6.45 and 6.85. At ꢀ20 ꢁC,
up to six singlets were observed for the methyl protons but the
region for vinyl protons became more complicated. Most of these
signals coalesced at ca. 70 ꢁC. At 120 ꢁC two broad singlets at
d
2.06
and 2.11 were observed for the methyl protons and a major AB
quartet (J 9.8 Hz) appeared at 6.54 and 6.90. Consecutive anti and/
d
or syn addition of Grignard reagent of 4 to 5 would be expected to
afford a mixture of up to four stereoisomers of 6. Restricted rotation
of the aryl rings would further result in different conformers. Steric
consideration would expect preferred anti addition of Grignard
reagents of 4 to 5 to give mainly trans-6. The presence of rotational
isomers was believed to be responsible for the broad melting point
and 1H NMR signals observed. At the higher temperature limit,
rapid rotation of the aryl rings within the NMR time scale would
result in simpler averaged signals, e.g., two singlets for the methyl
protons in trans-6.
The component eluted first from column chromatography
showed a molecular ion at m/z 495 with a correct isotope pattern
for two chlorine atoms. The base peak was observed at m/z 105
corresponding to the C6H5CO fragment. Its IR spectrum indicated
the absence of O–H group but the C]O absorption was clearly
observed at 1635 cmꢀ1. The above spectral data are consistent with
energy of activation (
D
Gzc¼4.57Tc[9.97þlog10(Tc/Dn)]).18 An advan-
tage of studying the diarylnaphthalenes 8 and 9, compared with the
highly symmetrical 10 and 11, is that coalescences of two pairs of
methyl signals could be used to give a more accurate estimate of the
energy barrier.
Another interesting feature in the rotational isomerism of 1,2-
diarylnaphthalenes is the four possible transition states Ia–Id
(Fig. 2) versus the respective sets of two in the 1,2-diarylbenzenes
(IIa and IIb) and 9,10-diarylphenanthrenes (IIIa and IIIb). In any of

3666
T.-G. Peck, Y.-H. Lai / Tetrahedron 65 (2009) 3664–3667
Table 1
Comparison of dynamic 1H NMR data for the barrier to rotation in 1,2-diary-
lbenzenes and 1,2-diarylnaphthalenes
Cpd
Dn/Hz
kc/Hz
Tc/ꢁC
D
Gzc/kJ molꢀ1
8a
17.9
60.1
15.2
62.4
7.0
39.8
133.5
33.8
138.6
15.6
110
122
78
98
58
82.2
81.0
75.6
75.7
74.0
70.9
9a
10ab
10bb
5.6
12.5
42
a
Our work; 1H NMR spectra determined on a 300 MHz spectrometer.
Reference 16; 1H NMR spectra determined on a 90 MHz spectrometer.
b
The relevant data from the dynamic NMR studies of 8 and 9 are
summarized in Table 1. For 8 or 9, values of the transition state free
energy at coalescence,
D
Gzc, calculated from coalescences of the two
pairs of methyl signals agree very well to give a good estimate of
the rotational barrier for the aryl rings. In going from the dinitrile 9
(
D
Gzc¼ca. 75.7 kJ molꢀ1) to dichloride 8 (
D
Gzc¼ca. 81.6 kJ molꢀ1) an
increase in energy barrier was observed. This is consistent with an
increase in steric demand19 going from the cylindrical cyano group
to the spherical chlorine atom. A greater steric demand (buttressing
effect) of the substituent 30-X would thrust the 20-methyl group
closer to the p-cloud of the opposite aryl ring in Ia or H3 in Ib, thus
increasing the energy barrier for isomerization.
The conformational energy barriers observed for 8 and 9 are
similar to those reported for 10a and 10b (Table 1) but much lower
than those for 11 (>110 kJ molꢀ1). This is clearly consistent with the
qualitative prediction discussed earlier that rotational isomerization
in 8 and 9 would likely involve the transition states Ia and/or Ib,
which are similar to IIa and IIb, respectively (Fig. 2). In addition to the
buttressing effect of the substituent 30-X, Ia would also experience
a tunneling buttressing effect involving the H5/H4/H3/H60 in-
teraction while an H5/H4/H3/20-Me interaction would exist in
Ib (Fig. 2). These steric interactions are believed to account for the
relatively higher rotational energy barriers in 8 and 9 compared to
those for 10a and 10b, respectively (Table 1).
Figure 2. Transition states in the rotational isomerism in ortho-diarylbenzenoids.
these transition states, one aryl ring is expected to be near per-
pendicular and another near planar to the reference central ben-
zenoid moiety. The rotational barriers for the aryl rings in 10 and 11
are <7516 and >11017 kJ molꢀ1, respectively. Such a large difference
in conformational barrier is clearly explained when the ‘bay region’
steric interactions are considered as in IIa versus IIIa or IIb versus
IIIb. These are qualitatively consistent with an expected increase in
the relative measures of steric effects observed going from triphe-
nylene to dibenzochrysene. Among the possible transition states
Ia–Id (Fig. 1), the pair of Ia and Ib would then be expected to be
relatively more stable than Ic and Id on the basis of steric consid-
erations. The rotational barriers of the aryl rings in 8 and 9 should
then be similar to those observed for 10a and 10b but much lower
than those for 11a and 11b.
3. Conclusion
The dynamic 1H NMR studies of 8 and 9 provided good evidence
that they behave differently from the conformational isomerization
in 1,8-di-o-tolylnaphthalene 1, 1,4-di-o-tolylbenzene 2, and 1,2-di-
o-tolylbenzenes 10 in that 8 and 9 could involve rotation of either of
their tolyl rings. In conclusion, the conformational isomerization of
anti and syn atropisomers of 1,2-di-o-tolylnaphthalene 3 is thus
expected to proceed selectively via rotation of only the 2-tolyl ring.
In the 1H NMR spectrum (300 MHz; C6D5NO2) of a mixture of
anti-8 and syn-8, two pairs of methyl signals were observed at
d
2.44, 2.29 and 2.39, 2.08, respectively, in a ratio of ca. 1.4:1.0. All
4. Experimental section
four methyl signals broadened as the temperature was raised and
the two singlets at
d
2.44 and 2.39 (Dn¼15.2 Hz) first collapsed at
4.1. Benzamide 6 and aziridine 7
78 ꢁC followed by those at
d
2.29 and 2.08 (Dn¼62.4 Hz) at 98 ꢁC. At
120 ꢁC, two singlets were observed at the averaged chemical shifts
A solution of 2,6-dichlorotoluene (9.24 g, 57.4 mmol) in dry THF
(50 mL) was added dropwise to a mixture of magnesium turnings
(1.40 g, 57.4 mmol) and catalystic amount of 1,2-dibromoethane
under nitrogen atmosphere. The mixture was refluxed gently for
about 3 h until all magnesium dissolved. The Grignard reagent
formed was cooled and dry benzene (50 mL) was added. 1,2-
Naphthoquinone-1-benzimide (5) (1.5 g, 5.74 mmol) was then
added and the mixture was further refluxed for another 3 h. After
additional stirring for another 8 h at room temperature, the re-
action mixture was cooled in an ice-bath and an aqueous solution
of ammonium chloride was added slowly with stirring. The organic
layer was separated and the aqueous layer was extracted three
times with dichloromethane. The combined organic extracts were
washed, dried and evaporated to yield the crude product, which
of
d 2.42 and 2.19, respectively. This phenomenon is typical of
a dynamic uncoupled AB system. When the sample was cooled, the
original spectrum at room temperature was again observed. This is
consistent with a true conformational inter-conversion between
the rotational isomers anti-8 and syn-8. In the 1H NMR spectrum
(300 MHz; C6D5NO2) of a mixture of anti-9 and syn-9, two pairs of
methyl signals were observed at
d
2.21, 2.07 (Dn¼17.9 Hz) and 2.15,
1.87 (Dn¼60.1 Hz), respectively, also in a ratio of ca. 1.4:1.0. The
relatively more deshielded methyl signals in 9 are likely due to the
anisotropic effect of the cyano group. As a sample of anti-9 and syn-
9 was warmed, a similar coalescence phenomenon of the methyl
signals was observed with Tc values at 110 ꢁC and 122 ꢁC,
respectively.

T.-G. Peck, Y.-H. Lai / Tetrahedron 65 (2009) 3664–3667
3667
was chromatographed on silica gel using a mixture of dichloro-
methane and hexane (3:1) as the eluant. Eluted first was the azir-
idine derivative 7 (0.56 g, 20%), mp 130–136 ꢁC. 1H NMR (300 MHz,
dichloromethane (1:1) as the eluant, the fractions found to contain
the product were combined and evaporated to give 9, 2.55 g (54%).
A sample recrystallized from cyclohexane/benzene afforded color-
CDCl3)
d
7.0–8.0 (m, 16H, ArH and CH]CH), 6.70 (br d, J 8 Hz, 1H,
less crystals of 9, mp 170–172 ꢁC. 1H NMR
d 7.1–8.2 (m, 12H, ArH),
CH]CH), 2.32, 2.03 (s, total 6H, CH3); IR (KBr) 3235 (br),1640,1505,
1470, 1425, 1372, 1295 (br), 1270 (br), 1000, 780, 748, 708 cmꢀ1; MS
m/z 495 (Mꢂþ, 65),105 (100), 77 (60). Anal. Calcd for C31H23ONCl2: C,
75.00; H, 4.67; N, 2.82%. Found: C, 74.27; H, 4.46; N, 3.26%. Mr calcd
for C31H23ONCl2: 495.1156; found (MS): 495.1156. Eluted next was
the desired product 6, 0.68 g (23%). Recrystallization of a sample
from chloroform/benzene gave colorless crystals of 6, mp 248–
254 ꢁC. 1H NMR (300 MHz, DMSO-d6/CDCl3), refer to discussion in
the text; IR (KBr) 3450, 3400, 3140 (br), 2920, 2840, 1700, 1500,
1480, 1420, 1300, 1285, 1100, 1050, 1020, 1000, 800, 760, 730, 710,
625 cmꢀ1; MS m/z 513 (Mꢂþ, 1), 496 (1), 408 (52), 345 (28), 254 (10),
153 (38),105 (100), 77 (60), 28 (69). Anal. Calcd for C31H25O2NCl2: C,
72.38; H, 4.90; N, 2.72%. Found: C, 72.37; H, 4.74; N, 2.78%.
2.44, 2.29 (s, total 2H, CH3), 2.39, 2.08 (s, total 2H, CH3); IR (KBr)
2210, 1430, 1375, 1260, 1230, 820, 800, 740, 720 cmꢀ1; MS m/z 358
(Mꢂþ, 100), 343 (30), 240 (22). Anal. Calcd for C26H18N2: C, 87.12; H,
5.06; N, 7.82%. Found: C, 87.07; H, 5.10; N, 7.82%.
Acknowledgements
This work was supported by the Singapore Ministry of Educa-
tion AcRF Tier 1 Grant R-143-000-248-112. The authors thank the
staff at the Chemical, Molecular and Materials Analysis Center,
Department of Chemistry, National University of Singapore, for
their technical assistance.
References and notes
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