Structure and stereodynamics of aryldiimino derivatives

Article abstract of DOI:10.1021/jo100106t

Aromatic diimino derivatives, having two N - CPh₂ moieties bonded at the 1,8 positions of the anthraquinone, anthracene, biphenylene, and naphthalene rings, have been investigated by variable- temperature NMR spectroscopy, X-ray diffraction, an

Full text of DOI:10.1021/jo100106t

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Structure and Stereodynamics of Aryldiimino Derivatives
Lodovico Lunazzi, Michele Mancinelli, and Andrea Mazzanti*
Department of Organic Chemistry “A. Mangini”, University of Bologna,
Viale Risorgimento 4, Bologna 40136, Italy
mazzand@ms.fci.unibo.it
Received January 22, 2010
Aromatic diimino derivatives, having two NdCPh2 moieties bonded at the 1,8 positions of the
anthraquinone, anthracene, biphenylene, and naphthalene rings, have been investigated by variable-
temperature NMR spectroscopy, X-ray diffraction, and DFT calculations. In all the compounds, the
imino substituents are essentially orthogonal to the aromatic plane and the phenyl groups bonded to
the CdN carbons are diastereotopic (i.e., cis or trans to the aromatic N-substituent of the NdC
moiety). The barriers for the cis/trans interconversion of the phenyl groups by planar nitrogen
inversion have been determined. In the cases of anthracene and anthraquinone derivatives, the
presence of syn and anti conformers was detected, and their interconversion barriers, due to the
Ar-N rotation, were measured. At very low temperature, restricted rotations of the phenyl groups,
displaying barriers in the range 4.8-6.9 kcal mol-1, were also observed for all compounds: the
barriers for the rotation of the cis were found to be larger than for the trans phenyl groups.
Introduction
however, imines are known to undergo stereodynamic pro-
cesses that involve the trigonal nitrogen atom coplanar to the
atoms to which it is bonded. NMR spectroscopy is a
convenient technique to investigate stereodynamic processes
of this type, and thus, it has been widely applied for inves-
tigating the dynamic behavior of imines.9 In principle, there
are two possible processes that might occur in imines, i.e.,
rotation about the CdN double bond and inversion of the
planar nitrogen atom (sometimes referred to as lateral shift).
There is a widespread agreement that the latter should
require less energy,9a especially in N-arylimines where the
nitrogen inversion route is facilitated by conjugation invol-
ving the nitrogen lone pair and aryl moiety.
Imines and diimine metal complexes are widely used for
enantioselective synthesis (salen complexes),1 tandem reac-
tions,2 coupling reactions,3 and olefin polymerization.4 Imi-
nes themselves (both as chiral or achiral substrates) are
employed as precursors of active species in organocatalytic
reactions like hydroxylation,5 epoxidation,6 and aziridina-
tion.7 A conformationally chiral diimine was recently obser-
ved in the solid state in its racemic form.8 In solution,
(1) McGarrigle, E. M.; Gilheany, D. G. Chem. Rev. 2005, 105, 1563–1602.
Darensbourg, D. J. Chem. Rev. 2007, 107, 2388–2410. Cozzi, P. G. Chem.
Soc. Rev. 2004, 33, 410–421. Katsuki, T. Chem. Soc. Rev. 2004, 33, 437–444.
(2) Du, H.; Ding, K. Org. Lett. 2003, 5, 1091–1093.
(3) Nelana, S. M.; Cloete, J.; Lisensky, G. C.; Nordlander, E.; Guzei,
I. A.; Mapolie, S. F.; Darkwa, J. J. Mol. Catal. A 2008, 285, 72–78.
Weissman, H.; Milstein, D. Chem. Commun. 1999, 1901–1902.
(4) Makio, H.; Fujita, T. Acc. Chem. Res. 2009, 42, 1532-1544 and
reference cited therein
When two hindered imino substituents are bonded to an
aromatic scaffold there are both conjugative and steric
effects that need to be considered. Twisting the imino group
out of the aromatic scaffold plane increases the conjugation
(5) Brodsky, B. H.; Du Bois, J. J. Am. Chem. Soc. 2005, 127, 15391–
15393.
(6) Prieur, D.; El Kazzi, A.; Kato, T.; Gornitzka, H.; Baceiredo, A. Org.
Lett. 2008, 10, 2291–2294.
€
(7) Muller, P.; Fruit, C. Chem. Rev. 2003, 103, 2905–2919.
(8) Glagovich, N. M; Reed, E. M.; Crundwell, G.; Updegraff, J. B. III;
Zeller, M.; Hunter, A. D. Acta Crystallogr. E 2005, E61, 1251–1253.
(9) (a) For a comprehensive review on this subject, see: Jennings, W. B.;
Wilson, V. E. In Acyclic Organonitrogen Stereodynamics; Lambert, J. B.,
Takeuchi, Y., Eds.; VCH Publishers: New York, 1991; Chapter 6, pp 155-243
and references cited therein. (b) Dalla Cort, A.; Gasparrini, F.; Lunazzi, L.;
Mandolini, L.; Mazzanti, A.; Pasquini, C.; Pierini, M.; Rompietti, R.; Schiaffino,
L. J. Org. Chem. 2005, 70, 8877–8883.
2572 J. Org. Chem. 2010, 75, 2572–2577
Published on Web 03/23/2010
DOI: 10.1021/jo100106t
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2010 American Chemical Society

Lunazzi et al.
JOCArticle
CHART 1
between the sp2 lone pair of the nitrogen and the aromatic
system, but at the same time, the conjugation with the CdN
π system is lost. On the other hand, the CdN π system can
develop a good conjugation with the phenyls bonded to
imine carbon. The final geometry might be therefore driven
mainly by steric factors. When the plane of the NdC moieties
is nearly orthogonal to the aromatic plane, syn and anti
forms should be, in principle, detectable. Depending on the
height of their interconversion barriers, these forms can be
considered either stereolabile conformers or configuration-
ally stable isomers. In the present paper, a number of aroma-
tic systems, bearing two equal imino substituents in two peri
positions, have been investigated (see Chart 1). The use of
different aromatic scaffolds makes it possible to modulate
the distance between the two nitrogens, and the presence of
two phenyl groups bonded to the sp2 carbons of the CdN
group makes these imino derivatives hindered enough as to
fulfill the conditions for a nonplanar structure.
FIGURE 1. (Left) Temperature dependence of the 13C signal (150.8
MHz in CD2Cl2) of one of the phenyl CH carbons of 1, showing the
effects of the sp2 N-inversion process. (Right) Computer simulation
obtained with the rate constants indicated.
TABLE 1. Experimental and DFT-Computed Barriers (kcal mol-1) of
Dynamic Processes in Compounds 1-4
N-inversion
Ar-N rotation
Ph rotation
exptla
5.8b
compd exptl computed exptl computed
computed
4.9,b 4.8c
1
2
3
4
13.4
16.2
17.3
16.6
10.6
15.1
13.7
15.5
9.2
5.8
8.1
3.9
2.7
9.8
5.4,c 4.8b 6.1,c 5.7b
5.6b 5.2,c 5.2b
6.9,c 5.4b 7.6,c 6.9b
aDue to spectral crowding these values could be determined only at
the coalescence temperature. bPair of phenyl groups trans to the aro-
matic scaffold (E). cPair of phenyl groups cis to aromatic scaffold (Z).
Results and Discussion
In the anthraquinone derivative 1 each of the four 13C lines
of the phenyl carbons (three CH and one quaternary) broad-
ens on cooling below ambient temperature and eventually
splits into two lines of equal intensity at -45 °C. In Figure 1,
the temperature dependence of one of the CH signals of the
phenyl groups is displayed as an example: the rate constants
obtained from the accompanying line shape simulation
allowed us to obtain a free energy of activation (ΔGq) of
13.4 kcal mol-1 (Table 1). This value corresponds to the
barrier required to freeze the planar inversion of the sp2
nitrogen: DFT calculations actually support this hypothesis
(the linear transitions state is calculated to be 10.6 kcal mol-1
above the ground state energy, as in Table 1). In such a situa-
tion, the two phenyl rings within each Ph2CdN moiety become,
in fact, diastereotopic in that they adopt cis and trans dis-
positions with respect to the anthraquinone moiety, thus
making anisochronous the corresponding NMR signals.
On further lowering the temperature the effects of a second
process are observed, apparently affecting all of the signals
but more conveniently followed by monitoring the 1H spec-
trum of the anthraquinone moiety. As shown in Figure 2, the
doublet (J = 8.1 Hz) of the hydrogens in positions 2,7
(assigned by 2D-HMBC) broadens below -50 °C and splits
into two doublets with a 94:6 ratio below -100 °C, the
corresponding barrier being 9.2 kcal mol-1 (Table 1). This
process is due to the restricted rotation about the N-anthra-
quinone bond which gives rise to conformers with the
Ph2CdN moieties nearly orthogonal to the anthraquinone,
corresponding to the syn and anti structures of Figure 3. The
DFT calculations indicate, likewise, that the barrier for the
Ar-N rotation of 1 is expected to be lower than that for the
planar nitrogen inversion, the computed values being 8.1 and
10.6 kcal mol-1, respectively (Table 1).
The same calculations predict that the syn conformer
should be more stable than the anti by 0.7 kcal mol-1
(Figure 3). Such an assignment is further supported by
X-ray diffraction of 1 that shows how the imino moieties
are nearly orthogonal to the anthraquinone ring and that the
syn structure is actually adopted: the computed geometry for
the syn conformer is very close to that obtained experimen-
tally (Figure 3), and this shows that the computational tool
employed is suitable to tackle this problem.
It is somewhat surprising that the apparently more hin-
dered syn conformer is more stable than the less hindered
anti. This depends on a most delicate balance between a
variety of attractive and repulsing interactions that cannot
be rationalized by means of a simple, intuitive argument:
this complex situation is satisfactorily accounted for solely
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1
FIGURE 2. (Left) Temperature dependence of the H signal (600
MHz in CD2Cl2) of the hydrogens in positions 2,7 of the anthra-
quinone moiety of 1, showing the effects of the interconversion
process between the syn and anti conformers by Ar-N rotation.
The signal of the latter (6%) has been increased 10 times in the inset
of the -102 °C trace. (Right) Computer line shape simulation obtai-
ned with the rate constants indicated, corresponding to the syn/anti
exchange.
FIGURE 3. (Top) X-ray diffraction structure of 1. Underneath are
shown the two possible pathways with the corresponding transition
states connecting the syn and anti conformers (the theoretically
calculated energy values are in kcal mol-1 and the square brackets
indicate the transition states).
0.11 and 0.03 ppm, respectively. This feature, on the contrary,
does not occur in the downfield minor signals of the anti
conformer (7.97 for H4,5 and 6.74 ppm for H2,7) that remain
single signals (Figure 4). This indicates that the syn conformer
comprises two unequally populated forms whereas the anti
conformer does not. In addition, the signal due to the four
ortho hydrogens of the pair of diastereotopic phenyl groups in a
trans relationship to anthraquinone11 splits, at -120 °C, owing
to the presence of the syn and anti conformers. However, when
cooled to -155 °C, the additional 55:45 splitting mentioned
above for the syn anthraquinone signals is not observed, being
obscured by a line broadening that eventually splits both the
syn and anti ortho signals of the phenyl in the trans position
into two equally intense signals (corresponding each to two
hydrogens), with a very large separation (approximately
0.7 ppm).12 Since such an effect is detected for the trans phenyl
signals but not for the anthraquinone signals it is attributable to
by appropriate DFT computations. In order to explore the
possible existence of motions with even smaller barriers, a
solvent which could reach temperatures much lower than
-100 °C (as, for instance, CDFCl2) had to be used. As
expected on the basis of the previous investigations in CD2-
1
Cl2, the H spectrum in CDFCl2 also exhibits, at -40 °C,
diastereotopic phenyl groups owing to the restricted sp2
N-inversion.
On further lowering the temperature, the anthraquinone
signals of the hydrogens in positions 2,7 and 4,5 broaden and
eventually split at -120 °C into two unequally intense signals,
due, as previously mentioned, to the restricted N-anthraqui-
none rotation generating syn and anti conformers.
Contrary to that observed in CD2Cl2 (where the corre-
sponding ratio was 94:6), here the ratio between the upfield
and downfield doublets is about 65:35 (Figure 4). This
change of proportions agrees with the assignment of the
more intense signal to the syn conformer. The latter, in fact,
has a computed dipole moment (3.76 D) larger than the anti
(3.30 D) and thus is expected to reduce its proportion (in the
present case from 94% to 65%) in a solvent like CHFCl2
(dipole moment10a 1.34 D), which is less polar than CH2Cl2
(dipole moment10b 1.60 D).
(11) These signals are due to the four ortho hydrogens of the pair of
phenyl groups (indicated as blue in Figure 4) in a trans relationship to the
anthraquinone moiety. This assignment was ascertained on the basis of a
NOE experiment at -50 °C (where the sp2 N-inversion is frozen) by
irradiating the antraquinone signals of the hydrogens in position 2,7
(indicated as red in Figure 4). An enhancement was observed for the signals
of the diastereotopic pair of phenyl groups at 7.7-7.3 ppm, indicating that
these signals correspond to the pair of phenyl groups cis to the anthraquinone
moiety. As a consequence, the dynamic effects we observed involve the ortho
signals of the pair of diastereotopic phenyls trans to anthraquinone.
(12) In the -155 °C spectrum the ortho signals corresponding to two
hydrogens are visible only on the left side of the spectrum (at about 8.1 ppm,
as in Figure 4), whereas the corresponding companion on the right side is
overlapped by a number of other signals. Thus, the mentioned separation of
approximately 0.7 ppm of the diastereotopic ortho signals was obtained by
doubling the separation between the averaged signals due to four hydrogens
(see the spectrum at -120 °C) and the left side signal due to two hydrogens
(see the spectrum at -155 °C).
When the temperature is further lowered, the upfield major
signals of the syn conformer (7.86 ppm for H4,5 and 6.65 ppm
for H2,7) broaden below -130 °C and split, at -155 °C, into
two signals in a 55:45 proportion, the separations being
(10) (a)Fuoss, R. M.J. Am. Chem. Soc. 1938, 60, 1633–1637. (b) Richardi, J.;
Fries, P. H.; Krienke, H. J. Phys. Chem. B 1998, 102, 5196–5201.
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FIGURE 5. DFT-computed structures of the two possible confor-
mations available to the conformer syn of 1. The relative energies of
the two minima are in kcal mol-1 (the structure with E = 0.0 is the
same as that with E = 0.0 of Figure 3).
of phenyl groups in the position trans to the anthraquinone
moiety14 (∼5.8 kcal mol-1, as in Table 1).
The anthracene derivative 2 displays two diastereotopic
phenyl groups (cis and trans to the anthracene ring) even at
ambient temperature since in the 1H spectrum two signals for
the ortho, meta, and para hydrogens are visible. At higher
temperatures these signals broaden and coalesce, eventually
yielding averaged signals at about þ130 °C (Figure S1,
Supporting Information) since the two phenyl groups
bonded to the CdN moiety become equivalent due to the
fast sp2 nitrogen inversion. By line shape simulation it was
found that the barrier is 16.2 kcal mol-1, the calculated value
for this type of process being 15.1 kcal mol-1 according to
DFT computations. The same calculations also predict that
both the syn and anti conformers should be populated,
1
FIGURE 4. (Top) Portion of the H spectrum of 1 (600 MHz in
CDFCl2 at -120 °C) showing the anthraquinone signals of H2,7 and
having a computed energy difference of 0.26 kcal mol-1
,
H
4,5 (in red) as well as the signal of the four ortho hydrogens of the
and their interconversion barrier is calculated to be quite low
(3.9 kcal mol-1).
pair of diastereotopic phenyl groups trans to the anthraquinione
moiety (in blue). These signals are all split in a 65:35 ratio corre-
sponding to syn (s, major) and anti (a, minor) conformer. (Bottom)
The same spectrum at -155 °C showing additional splitting (55:45
ratio) of the anthraquinone signals in the case of the syn conformer
(the signals of the anti do not split). Also shown are the signals of the
four ortho hydrogens of the phenyls trans to anthraquinone (both
syn and anti at -120 °C) that split, at -155 °C, into two 1:1 signals
(only the left signals of two hydrogens are displayed) due to
restricted phenyl rotation.
For this reason, we could observe separate NMR signals
for the syn and anti conformers of 2 only below -160 °C,
where the ratio between the major and minor signals is about
86:14 at -168 °C (Figure 6). X-ray diffraction (Figure S2,
Supporting Information) shows that the structure of 2 in the
crystal is syn (as observed in 1), whereas computations
suggest that the anti should be slightly more stable (by 0.26
kcal mol-1). This discrepancy could be a consequence of the
solvent effect, and therefore, we repeated the calculations by
taking into account the effect of the solvent (CH3CN) from
which the compound had been crystallized. In this case, the
computed energy difference between the two forms is essen-
tially zero, thus proving that the solvent does play a role in
determining the conformer proportions (furthermore, the
computed energy differences are too small to allow a reliable
assignment solely on theoretical grounds). Thus, the X-ray
identification of a syn structure of 2 seems to be the most
plausible assignment also for the more populated of the two
conformers observed in solution. In addition, it is worth
mentioning that a restricted rotation of the phenyl groups of
the major conformer of 2 was observed, since the corre-
sponding ortho and meta hydrogen signals (for both the
pairs of phenyls cis and trans to anthracene) split into two,
with a 1:1 ratio below -165 °C.15 The two barriers for this
the restricted phenyl rotation that makes the ortho hydrogens
diastereotopic in both the syn and anti conformers. The frozen
phenyl rotation also accounts for the two unequally populated
forms observed for the syn conformer at -155 °C: these forms,
in fact, are due to the two different dispositions that can be
adopted by the frozen phenyl groups. Indeed, DFT computa-
tions predict that there are two such forms, corresponding to
the two energy minima displayed in Figure 5. This feature is
observed in the syn but not in the anti conformer: computations
indicate in fact that the anti has only one minimum of energy
(i.e., that reported in Figure 3): this agrees well with the experi-
mental observation. Owing to the complex spectral appearance,
due to the large number of overlapping lines, a quantitative line
shape determination of the phenyl rotation rate was impossible
and an approximate estimate of the barrier,13 based on the
coalescence temperature, could be obtained solely for the pair
(14) The signals of the phenyl groups cis to anthraquinone are, in fact,
overlapped by other spectral lines.
(15) The barriers were estimated at the coalescence temperature of the
signals of the major conformer. In the coalescence region the signals of the
minor conformer were not sufficiently intense to be observed.
(13) Although in principle two different barriers should be observed for
the phenyl rotation in the syn and anti conformers, this difference (which in
any case is likely to be small) could not be appreciated since these low-
temperature spectra did not exhibit sufficient details for this purpose.
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The experimental barrier for the planar nitrogen inversion,
as obtained from the line shape analysis, is 17.3 kcal mol-1
(Table 1). According to calculations, the anti conformer of 3
is more stable than the syn by 0.92 kcal mol-1, in agreement
with the X-ray structure that, contrary to the case of
compounds 1 and 2, shows that the structure anti is adopted
in the crystalline state (Figure S2, Supporting Information).
The barrier calculated by DFT computations for the
syn/anti interconversion due to Ar-N rotation is too low
(2.7 kcal mol-1) to allow an experimental determination by
variable-temperature NMR, and indeed, the spectra of 3 do
not display the effects of this process even at -165 °C. On the
other hand, the restricted phenyl rotation, which renders
diastereotopic the corresponding ortho hydrogens, was obser-
ved, as in the case of 1 and 2.
It should be noted that the rotation barriers for the trans
and the cis phenyl rings are both calculated to be higher than
that for the Ar-N rotation (5.2 and 5.2 vs 2.7 kcal mol-1, as
in Table 1). The rotational process of the trans phenyl is
visible even in the presence of a fast Ar-N rotation, whereas
this is not the case for the rotation of the cis phenyl group.
For this reason, the barrier for the phenyl rotational process
could be determined solely for the phenyl groups trans to
biphenylene (∼5.6 kcal mol-1 at the coalescence tempera-
ture, as in Table 1).
In addition, compound 4 displays at ambient temperature
diastereotopic cis and trans phenyl groups, but in this case,
they belong solely to a single conformer. There is no evi-
dence, in fact, of the existence of a second conformer, even at
very low temperature (see, for instance, the naphthalene
signal of the hydrogens in position 2,7 in Figure S4 of the
Supporting Information). The anti structure was assigned to
the observed conformer because DFT calculations predict
the syn to be less stable than the anti by as much as 5.4 kcal
mol-1. Due to the proximity of the R positions 1,8 of
naphthalene, the NdCPh2 groups bonded to these positions
are very crowded in the syn situation, hence accounting for
the much lower stability of this conformer with respect to the
anti. Such a theoretical assignment is further supported by
the X-ray diffraction which shows that in the solid state the
molecule adopt indeed the anti structure (Figure S5, Sup-
porting Information).
FIGURE 6. (Left) Low temperature 1H signals of H9 and H10
of the anthracene moiety of 2 (600 MHz in CDFCl2/CBrF3, 3:1)
The arrows indicate the corresponding signals of the minor con-
former (14%). (Right) Simulation obtained with the rate constants
reported.
rotation process are slightly different, that involving the
phenyl trans (∼4.8 kcal mol-1) being lower than that invol-
ving the phenyl cis (∼5.4 kcal mol-1), probably because the
latter experiences a slightly larger steric hindrance. Since the
latter barrier is almost equal, within the errors, to that of the
Ar-N rotation (i.e., the syn/anti interconversion, 5.4 and
5.8 kcal mol-1, respectively), it cannot be excluded that the
cis phenyl signals actually monitor the same process dis-
played by the anthracene signals. Indeed, the calculated
energy for the cis-phenyl rotation is higher than that of the
Ar-N rotation (6.1 vs 3.9 kcal mol-1, as in Table 1). If this is
the case, the diastereotopicity of the ortho signals of the cis
phenyl ring can be observed only when the Ar-N rotation is
also frozen.16
At higher temperatures, the fast N-inversion process is
expected to eliminate the diastereotopicity of the phenyl
groups. For instance, the two ortho 1H signals of the phenyl
groups coalesce into a single signal at þ65 °C (Figure S6,
Supporting Information), the corresponding barrier being
Once again, the NMR spectrum of biphenylene derivative
3 shows that the two phenyl groups bonded to the NdC
moiety are diastereotopic at ambient temperature, in that
16.6 kcal mol-1
.
In the low-temperature NMR spectra of 4 the effects of the
restricted rotation of the phenyl groups, both in the cis and
trans position, were observed, with experimental barriers
lower than that predicted by calculations for the Ar-N
rotation. In Figure S4 of the Supporting Information, the
signal at 7.47 ppm (for the four ortho hydrogens of the two
phenyls cis) splits at -141 °C into two equal signals, with a
separation of 265 Hz at 600 MHz (the coalescence tempera-
ture is about -120 °C). The signal at 7.31 ppm (corres-
ponding to the four ortho hydrogens of the two phenyls
trans) similarly splits at -162 °C with a separation of 670 Hz
at 600 MHz (the coalescence temperature is about -145 °C).
From these data an approximate estimate of the two barriers
could be determined (∼6.9 and ∼5.4 kcal mol-1, respectively, as
1
they display three pairs of H NMR signals for the ortho,
meta, and para hydrogens. These phenyls exchange their
positions increasingly fast on warming, eventually yielding
three averaged signals for the ortho, meta, and para hydro-
gens above þ100 °C (Figure S3, Supporting Information).
(16) Such a situation, for instance, has been encountered in some aliphatic
amines where the larger C-N rotation barrier is NMR invisible until the
N-inversion, which has a lower barrier, is also frozen; see: Jackson, W. R.;
Jennings, W. B. Tetrahedron Lett. 1974, 15, 1837–1838. See also: Anderson,
J. E.; Casarini, D.; Ijeh, A. J.; Lunazzi, L. J. Am. Chem. Soc. 1997, 119, 8050–
8057. Casarini, D.; Lunazzi, L.; Mazzanti, A.; Foresti, E. J. Org. Chem. 1998,
ꢀ
~
63, 4746–4754. Lunazzi, L.; Mazzanti, A.; Munoz Alvarez, A. J. Org. Chem.
2000, 65, 3200–3206. Jewett, J. G.; Breeyear, J. J; Brown, J. H.; Bushweller,
C. H. J. Am. Chem. Soc. 2000, 122, 308–323.
2576 J. Org. Chem. Vol. 75, No. 8, 2010

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NMR Spectroscopy. The spectra were recorded at 600 MHz for
in Table 1). As conceivable, the rotation barrier of the trans
phenyls is lower than that of the cis, presumably because the
latter are more hindered, owing to the proximity to the naph-
thalene ring.
1
1H and 150.8 MHz for 13C. The assignments of the H and 13C
signals were obtained by bidimensional experiments (edited-
gHSQC21 and gHMBC22 sequences). The NOE experiments on
3 were obtained by means of the DPFGSE-NOE23 sequence. To
selectively irradiate the desired signal, a 50 Hz wide shaped pulse
was calculated with a refocusing-SNOB shape24 and a pulse width
of 37 ms. The mixing time was set to 1.5 s. Temperature calibrations
were performed before the experiments, using a Cu/Ni thermo-
couple immersed in a dummy sample tube filled with 1,1,2,2-
tetrachloroethane for the high temperature or isopentane for the
low temperature range, and under conditions as nearly identical as
possible. The uncertainty in the temperatures was estimated from
the calibration curve to be (1 °C. The line shape simulations were
performed by means of a PC version of the QCPE program
DNMR 6 no. 633, Indiana University, Bloomington, IN.
Calculations. Geometry optimizations were carried out at the
B3LYP/6-31G(d)25 level by means of the Gaussian 03 series of
programs26 (see the Supporting Information): the standard Berny
algorithm in redundant internal coordinates and default criteria of
convergence were employed. The energies reported in Table 1 are not
ZPE corrected; full thermochemistry-corrected data are reported in
the Supporting Information. Harmonic vibrational frequencies were
calculated for all the stationary points. For each optimized ground
state the frequency analysis showed the absence of imaginary
frequencies, whereas each transition state showed a single imaginary
frequency. Visual inspection of the corresponding normal mode27
was used to confirm that the correct transition state had been found.
Conclusions
It has been shown that 1,8 aryldiimino derivatives of
antraquinone, anthracene, biphenylene, and naphthalene
are not planar, and for this reason they can originate syn
and anti conformers. The interconversion barriers involving
N-inversion and Ar-N bond rotation have been determined
by variable-temperature NMR spectroscopy. DFT calcula-
tions indicate that the preferred conformer switches from syn
to anti on reducing the distance between the nitrogen atoms,
a result supported by X-ray diffraction analysis.
Experimental Section
Materials. 1,8-Dichloroanthraquinone, diphenylmethanimine,
and Xantphos were commercially available. 1,8-dibromoanthra-
cene,17 1,8-dibromobiphenylene,18 and 1,8-diiodonaphthalene19
were prepared following known procedures.
General Procedure for 1-4.20. To a solution of the appro-
priate 1,8-dihalogen compound (10 mmol, in 20 mL dry toluene)
were added Pd2(dba)3 (2 mol %, 192 mg), Xantphos (4 mol %,
231 mg), and NaO-t-Bu (30 mmol, 2.805 g) at room tempera-
ture, and the solution was stirred for 30 min. Then diphenyl-
methanimine (23 mmol, 4.168 g, 3.87 mL) was added and the
solution heated at 100 °C for 24 h. After the solution was cooled
at room temperature, the organic layer was extracted with EtO2
and dried with Na2SO4. The solvent was evaporated, and the
crude products were prepurified by chromatography on silica
gel (hexane/Et2O mixture 10:1 v/v) to obtain a mixture contain-
ing mainly the 1-chloro,8-(diphenylmethyleneamino) derivative
and the target product. The mixture was separated by prepara-
tive HPLC (see the Supporting Information for details). The
isolated 1-chloro-8-(diphenylmethyleneamino) intermediates
were then reacted as above to yield a second batch of the
products (total yields: 56% for 1, 62% for 2, 58% for 3, 18%
for 4). Crystals suitable for X-ray diffraction analyses were
obtained by slow evaporation of acetonitrile solutions. Spectro-
scopic and analytical data are reported in the Supporting
Information.
Acknowledgment. L.L. and A.M. received financial sup-
port from the University of Bologna (RFO) and from
MIUR, Rome (PRIN national project “Stereoselection in
Organic Synthesis, Methodologies and Applications”). Dr.
M. Di Mattia, Sigma-Tau, Rome, is gratefully acknowl-
edged for the acquisition of the ESI-TOF mass spectra.
Supporting Information Available: High-temperature VT
spectra of 2-4; low-temperature VT spectra of 4; spectroscopic
1
and analytical data, X-ray structures, H, 13C NMR spectra,
computational data of 1-4. This material is available free of
charge via the Internet at http://pubs.acs.org.
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