Anomalous CD/UV exciton splitting of a binaphthyl derivative: The case of 2,2′-diiodo-1,1′-binaphthalene

Article abstract of DOI:10.1021/ja000114a

The UV and CD spectra of (R)-(+)-2,2′-diiodo-1,1′-binaphthalene show an unexpectedly large value of the wavelength splitting between the two main bands, resulting from the exciton coupling of ¹B_b transitions. An hypothesis is proposed on transition moments directions, making it possible to relate quantitatively the observed splitting to the orthogonal arrangement found in the solid state and calculated for the structure in solution.

Full text of DOI:10.1021/ja000114a

J. Am. Chem. Soc. 2000, 122, 6395-6398
6395
Anomalous CD/UV Exciton Splitting of a Binaphthyl Derivative:
The Case of 2,2′-Diiodo-1,1′-binaphthalene
Lorenzo Di Bari,† Gennaro Pescitelli,† Fabio Marchetti,‡ and Piero Salvadori*,†
Contribution from the Centro di Studio del CNR per le Macromolecole Stereordinate e Otticamente AttiVe,
Dipartimento di Chimica e Chimica Industriale, UniVersita` degli Studi di Pisa, Via Risorgimento 35,
I-56126 Pisa, Italy, and Dipartimento di Ingegneria Chimica, dei Materiali, delle Materie Prime e
Metallurgia, UniVersita` degli Studi “La Sapienza” di Roma, Via del Castro Laurenziano 7,
I-00195 Roma, Italy
ReceiVed January 10, 2000
Abstract: The UV and CD spectra of (R)-(+)-2,2′-diiodo-1,1′-binaphthalene show an unexpectedly large value
of the wavelength splitting between the two main bands, resulting from the exciton coupling of 1Bb transitions.
An hypothesis is proposed on transition moments directions, making it possible to relate quantitatively the
observed splitting to the orthogonal arrangement found in the solid state and calculated for the structure in
solution.
Introduction
synthesis.9 DIBN features a large splitting of the main absorption
and dichroic bands; we show how this molecule can fit into
our model by correctly taking into account the distortion of the
transition dipole orientation brought about by the presence of
the iodine atoms.
1,1′-Binaphthyl derivatives represent an important class of
chiral auxiliaries,1 thanks to a nonplanar arrangement of the two
naphthalene moieties, which ensures a dissymmetric environ-
ment suitable for obtaining high degrees of stereoselectivity.2
Optical and chiroptical spectroscopies have been widely used
in the past as a tool for the structural investigation of these
compounds;3,4 very recently, on the basis of the exciton coupling
approach to the optical activity of dimers,5 we pointed out the
existence of a quantitative relation between the dihedral angle
θ, defined by the two naphthyl planes, and the wavelength
splitting ∆λmax between the two resolved components of the
220 nm couplet, typical of the CD spectrum of these derivatives.6
In particular, it was established that (1) unbridged derivatives
(-CH2X substituted at 2,2′, X ) H, OH, Cl), with θ ≈ 90°,
have ∆λmax < 10 nm and (2) chain-bridged derivatives (-CH2-
Y-CH2- chain at 2,2′), with θ ranging from 55° (Y ) O, NR)
to 70° (Y ) SiMe2), have ∆λmax varying from 12.5 to 14 nm.
A useful quantitative distinction between the two classes of
compounds is then possible, in addition to the evidence of the
splitting of the main UV band,3,7 so far observed only for
bridged derivatives.3,4,6,8
Results
The UV and CD spectra of (R)-(+)-2,2′-diiodo-1,1′-binaph-
thalene (DIBN, 1) are reported in Figure 1. In the UV spectrum
the strongest band is split into two components, with maxima
at 226 and 243 nm, of similar intensities, while the CD couplet,
with zero-point at 231 nm, has the unpredictably6 large ∆λmax
value of 19 nm. As for the conformation adopted by 1, the
following was observed:
(1) The absolute configuration of (R)-(+)-1, established by
chemical correlation with (R)-2,2′-diamino-1,1′-binaphthalene,
along with the negative sign of the couplet, indicates safely that
θ does not exceed the critical value of 110°.4
(2) The molecular structure of (R)-(+)-1 in the solid state,
shown in Figure 2, possesses a pseudo-C2 axis relating the two
naphthyl moieties, which are rotated around the C(1)-C(11)
bond by an angle θ ) 91.5°. The two iodine atoms of the
molecule are removed from each other by 4.22 Å; a I‚‚‚I distance
slightly longer (4.33 Å) separates the iodine atoms of different
molecules in the crystal packing. The C-I distances, 2.110 Å
mean value, are in keeping with the value 2.09(1) Å averaged
on 51 different aryl iodides.10
Here we focus our attention on a first system bearing two
auxochromic groups at 2 and 2′, 2,2′-diiodo-1,1′-binaphthalene
(DIBN), which recently found application in an enantioselective
* Address correspondence to this author.
† Universita` degli Studi di Pisa.
(3) The MNDO-PM311 optimized geometry for 1 has θ )
90° and a quite sharp potential energy well for the torsional
‡ Universita` degli Studi “La Sapienza” di Roma.
(1) (a) Rosini, C.; Franzini, L.; Raffaelli, A.; Salvadori, P. Synthesis 1992,
503. (b) Pu, L. Chem. ReV. 1998, 98, 2405.
(2) Whitesell, J. K. Chem. ReV. 1989, 89, 1581.
(8) (a) Rosini, C.; Tanturli, R.; Pertici, P.; Salvadori, P. Tetrahedron:
Asymm. 1996, 7, 2971. (b) Meyers, A. I.; Nguyen, T.; Stoianova, D.;
Sreerama, N.; Woody, R. W.; Koslowski, A.; Fleischhauer, J. Chirality 1997,
9, 431. (c) Stara´, I. G.; Stary´, I.; Tichy´, M.; Za´vada, J.; Fiedler, P. J. Org.
Chem. 1994, 59, 1326. (d) Noyori, R.; Sano, N.; Murata, S.; Okamoto, Y.;
Yuki, H.; Ito, T. Tetrahedron Lett. 1982, 23, 2969.
(9) Ochiai, M.; Kitagawa, Y.; Takayama, N.; Takaoka, Y.; Shiro, M. J.
Am. Chem. Soc. 1999, 121, 9233.
(10) Allen, F. H.; Kennard, O.: Watson, D. G.; Brammer, L.; Orpen, A.
G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.
(3) (a) Mislow, K.; Glass, M. A. W.; O’Brien, R. E.; Rutkin, P.;
Steinberg, D. H.; Weiss, J.; Djerassi, C. J. Am. Chem. Soc. 1962, 84, 1455.
(b) Mislow, K.; Bunnenberg, E.; Records, R.; Wellman, K.; Djerassi, C. J.
Am. Chem. Soc. 1963, 85, 1342.
(4) Mason, S. F.; Seal, R. H.; Roberts, D. R. Tetrahedron 1974, 30, 1671.
(5) Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy-Exciton
Coupling in Organic Stereochemistry; Oxford University Press: Oxford,
1983.
(6) Di Bari, L.; Pescitelli, G.; Salvadori, P. J. Am. Chem. Soc. 1999,
121, 7998.
(11) (a) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209. (b) Stewart,
J. J. P. J. Comput. Chem. 1989, 10, 221.
(7) Rosini, C.; Rosati, I.; Spada, G. P. Chirality 1995, 7, 353.
10.1021/ja000114a CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/24/2000

6396 J. Am. Chem. Soc., Vol. 122, No. 27, 2000
Di Bari et al.
Table 1. Frequencies in kK (1 kK ) 103 cm-1) of Extrema of
Spectra Reported in Figure 1 Resolved through the Second
Derivativea
νext (kK)
UV DIBN
CD (R)-DIBN
40.9 (244 nm)
UV Np-I
41.1 (243 nm)
42.1
43.1
44.3
45.3
46.7
44.2 (226 nm)
44.5 (225 nm)
46.8
49.1
a Bold text: main components of the couplet (∆λmax ) 18 ( 1 nm,
∆ν ≈ 3.3 kK). Light-face text: principal vibrational sequence (∆ν ≈
2.3 kK). Italic text: secondary vibrational sequence (∆ν ≈ 1.0 kK).
wavelength range, and the compound had to be synthesized.
The UV spectrum of 2-iodonaphthalene (Figure 1, upper part,
dashed line) shows itself an evident splitting of the band between
200 and 250 nm, in contrast to the spectrum of naphthalene,
where only a shoulder is detectable on the high energy side of
1
the Bb band at 220 nm.13 Despite some older hypotheses on
the direct role of nfσ* transitions localized on iodine atoms,14
subsequent findings demonstrated that such contributions are
negligible both in the absorption and in the circular dichroism
of aryl iodides.15 The origin of the splitting in the UV spectrum
of 2-iodonaphthalene has then to be found in a vibrational
structure. Second derivative analysis, in fact, resolves the
spectrum into at least five bands, which can be arranged
according to the tentative scheme of Table 1, i.e., two different
vibrational progressions with spacings of about 2300 and 1000
cm-1. A strong confirmation to this interpretation comes from
the CD spectrum of (R)-(+)-1, where a progression with ∆ν )
2300 cm-1 (corresponding to ∆λ ≈ 11 nm) is likewise evident;
indeed, this represents a case where the vibrational structuring
of an exciton CD couplet strikingly appears, even at room
temperature;16 in the absorption, the summation, rather than the
mutual cancellation, of the components, only leads to an
unfeatured broadening of the peaks.
Figure 1. UV (top) and CD (bottom) spectra of (R)-(+)-2,2′-diiodo-
1,1′-binaphthalene (1), 0.672 mM in CH3CN, and UV spectrum (top,
dashed line) of 2-iodonaphthalene, 1.44 mM in CH3CN.
The splitting between the two strongest components in the
UV and in the CD spectrum of 1 is, however, sufficiently larger
in frequency (∆λmax ) 18 nm, ∆νmax ≈ 3300 cm-1) than the
vibrational separation, and it can be entirely ascribed to an
exciton-type mechanism:5 the very good agreement between the
two wavelength splittings on the value ∆λmax ) 18 ( 1 nm
supports this view and, as a consequence, the assignment of
the two extrema at 225 and 244 nm in the CD spectrum to 0-0
components; the unexpectedly large value of ∆λmax, however,
still remains to be interpreted.
In the formulation of the quantitative relation between θ and
∆λmax,6 we used a model for 1,1′-binaphthyl derivatives describ-
ing πfπ* Bb transitions, which mainly give rise to the 220
Figure 2. ORTEP view of 2,2′-diiodo-1,1′-binaphthalene (1).
1
nm couplet, by means of transition dipole moments of defined
position and direction, as prescribed by the coupled dipoles
approach;5 discarding the effects of substituents, we placed such
moments in the center of each aromatic plane with a polarization
parallel to the naphthalene long axis, as in the parent chro-
mophore.17 However, in the present case, the â-substitution with
mode of rotation about θ, as a consequence to the steric
hindrance of iodine atoms.12
These observations strongly support a quasiorthogonal ar-
rangment for 1, with θ ≈ 90°. The anomalous appearance of
UV and CD spectra of 1, which would be in accordance with
a considerably smaller θ, must then be explained.
(13) Jaffe´, H. H.; Orchin, M. Theory and Applications of UltraViolet
Spectroscopy; Wiley: New York, 1962.
(14) (a) Dunn, T. M.; Iredale, T. J. Chem. Soc. 1952, 1592. (b) Beringer,
F. M.; Lillien, I. J. Am. Chem. Soc. 1960, 82, 5135.
(15) (a) Chaudri, B. A.; Goodwin, D. G.; Hudson, H. R.; Bartlett, L.;
Scopes, P. M. J. Chem. Soc. (C) 1970, 1329. (b) Cookson, R. C.; Coxon,
J. M. J. Chem. Soc. (C) 1971, 1466.
(16) (a) Weigang, O. E., Jr. J. Chem. Phys. 1965, 43, 71. (b) Langeveld-
Voss, B. M. W.; Beljonne, D.; Shuai, Z.; Janssen, R. A. J.; Meskers, S. C.
J.; Meijer, E. W.; Bre´das, J.-L. AdV. Mater. 1998, 10, 1343.
(17) Bree, A.; Thirunamachandran, T. Mol. Phys. 1962, 5, 397.
Discussion
In the exciton coupling approach,5 a spectroscopic description
of the isolated chromophores present on the molecule is
necessary. The absorption spectrum of 2-iodonaphthalene is
apparently not available in the literature with a sufficient
(12) PM3 calculations were executed with Gaussian 94, Gaussian Inc.,
Pittsburgh, PA.

CD/UV Exciton Splitting of a Binaphthyl DeriVatiVe
J. Am. Chem. Soc., Vol. 122, No. 27, 2000 6397
Table 2. Selected Crystal Data and Structure Refinement for
2,2′-Diiodo-1,1′-binaphthalene (1)
empirical formula
formula weight
temperature
crystal system
space group
C20H12I2
506.10
293(2) K
orthorhombic
P212121
unit cell dimensions
a ) 10.439(1) Å
b ) 12.054(1) Å
c ) 13.318(3) Å
1675.8(4) Å3, 4
1.057
R1 ) 0.0282, wR2 ) 0.0623
R1 ) 0.0364, wR2 ) 0.0659
Figure 3. Geometrical model for the 1,1′-binaphthyl derivative showing
1
deviation R of Bb transition dipole moments from directions parallel
volume, Z
to the naphthalene long axes (dashed lines).
goodness-of-fit on F2
final R indices [I > 2σ (I )]
R indices (all data)
iodine would account for a substantial deviation of 1Bb moments
toward the direction of the C-I bond. A direct access to the
variable R (see Figure 3) is not available: the CNDO/S-CI
method, commonly employed in calculations of transition
moments polarization, is not parametrized for iodine; for a
matter of comparison, it calculates 0 < R < 6° for â-substituted
naphthalene with X ) OH, CH3, CH2OH.18 As a consequence,
we must resort to the qualitative description afforded by Platt’s
spectroscopic moments,19 and the subsequent sum vector rule
for the assessment of transition moment directions, which is
indeed in keeping with the proposed picture: iodine has the
very large moment value of 21 (cm‚mol/L)-1/2, to be compared
to 7 for CH3 and 3 for CH2Cl; a similar, though less pronounced,
trend is observed for Petruska’s q-factors.20
model usually adopted in calculations of optical activity of this
class of compounds,4,6 introducing a substantial deviation of
1Bb transition dipole moments toward C-2, justified by the
presence of the auxochromic substituent. This allowed us to
explain quantitatively the large value of the splitting on the basis
of the standard hypotheses of the exciton model for dimers. It
was also demonstrated that the vibronic structure, apparent in
the UV spectrum of the isolated chromophore, negligibly affects
the exciton splitting.
As a complementary remark, we want to point out that also
in the case of (R)-(+)-2,2′-diiodo-1,1′-binaphthalene, a deriva-
tive with the most bulky substituents at 2 and 2′, the dihedral
angle θ is far from the critical value of 110° where the sign of
the couplet theoretically inverts,4 in accordance with every other
2,2′-substituted 1,1′-binaphthyl derivative so far known,6 with
the only possible exception being -CHBr2 disubstituted.23
The final question is: Can the transition dipole moment
deviation account for the already discussed anomalies in the
spectra of (R)-(+)-1? To answer that, we made the two following
ultimate observations:
(1) Making use of the formula ∆λmax ) (2/hc)λ0 V12,6 (where
2
λ0 ) 236 nm, from the spectrum of 2-iodonaphthalene) and
evaluating the Coulombic coupling potential V12 for the model
Experimental Section
Melting points were taken with a Kofler apparatus and are not
in Figure 3, a value of 18 nm is calculated for theoretical ∆λmax
,
1
corrected. H NMR spectra were recorded with a Varian VXR 300
with θ ) 90°, if R has a Value of about 15°.
spectrometer at 300 MHz in CDCl3 solution. Chemical shift values are
expressed in ppm relative to tetramethylsilane. Abbreviations are as
follows: s, singlet; d, doublet; t, triplet; m, multiplet. Mass spectra
were obtained on a VG 7070E electron impact spectrometer with direct
introduction of the sample. Optical rotatory power was measured with
a Perkin-Elmer 142 spectropolarimeter with a 1 dm cell. UV and CD
spectra were recorded using a UV-Vis Varian CARY 4E spectrometer
and a Jasco J600 spectropolarimeter, respectively, with 0.01 or 0.02
cm cells in CH3CN. The CD spectrum was recorded at room
temperature with the following conditions: speed 10 nm/min, response
2 s, and bandwidth 1.0 nm. Enantiomeric excess was measured by
HPLC on a Daicel CHIRALCEL OD column using a Jasco PU-980
chromatograph equipped with a Jasco UV-975 detector, with the
following conditions: eluent hexane/2-propanol/formic acid 90:10:1,
flow 0.5 mL/min, UV detector at λ ) 280 nm.
(2) Calculating UV and CD spectra of a generic 1,1′-
binaphthyl derivative by means of the classical physics DeVoe’s
coupled oscillators approach,21 with θ ) 90° and R ) 15°, and
even using the large experimental bandwidth of 2-iodonaph-
thalene (∆ν˜1/2 ≈ 6 kK), one obtains an appreciable splitting of
the UV main band, with two components of equal intensities
and a wavelength separation ∆λmax of 18 nm.
We do not discuss here the role possibly played by transition
dipoles positions, which in principle can be displaced from the
center of each naphthalene. The effect of transition moments
locations on exciton-type calculations is one of the most
involved theoretical questions, yet not adequately debated in
the literature.22
X-ray diffraction measurements were executed by means of a
Siemens P4 diffractometer equipped with graphite monochromated Mo
KR radiation (λ ) 0.71073 Å), operating at room temperature (T )
293 K). The intensity data collection was obtained with a ω/2θ scan
mode, collecting a redundant set of data to check the diffraction
symmetry and the reliability of the absorption correction procedure.
Intensities were corrected for absorption by means of a ψ-scan method.
The data reduction was done with the SHELXTL package24 and
structure solutions and refinements, based on full-matrix least-squares
on F2, were done with SHELX97 program.25
Conclusions
In this paper we reported for the first time the UV spectrum
of an unbridged 1,1′-binaphthyl derivative characterized by a
marked splitting of the 220 nm band. We modified the geometric
(18) CNDO-CI calculations were executed with a CNDO/M program,
according to Del Bene and Jaffe´’s formulation (Del Bene, J.; Jaffe´, H. H.
J. Phys. Chem. 1968, 49, 1221), with Mataga approximation of two-electron
repulsion integrals. Up to 60 singly excited states, with maximum energy
values of 7.0 eV, were included in the CI. The initial geometry was
calculated as in ref 12.
Pale yellow prismatic crystals of 2,2′-diiodo-1,1′-binaphthalene,
obtained by recrystallization from 80% hexane/20% xylene, were found
to have the lattice parameters listed in Table 2. The unit cell parameters
(19) Platt, J. R. J. Chem. Phys. 1951, 19, 263.
(20) Petruska, J. J. Chem. Phys. 1961, 34, 1120.
(21) (a) DeVoe, H. J. Chem. Phys. 1964, 41, 393. (b) DeVoe, H. J. Chem.
Phys. 1965, 43, 3199.
(22) For a few examples of discussion concerning diaryl derivatives
see: (a) Mason, S. F. J. Chem. Soc., Chem. Commun. 1973, 239. (b) Tanaka,
J.; Ozeki-Minakata, K.; Ogura, F.; Nakagawa, M. Spectrochim. Acta 1973,
29A, 897. (c) Gottarelli, G.; Proni, G.; Spada, G. P.; Fabbri, D.; Gladiali,
S.; Rosini, C. J. Org. Chem. 1996, 61, 2013.
(23) Gottarelli, G.; Spada, G. P.; Bartsch, R.; Solladie´, G.; Zimmermann,
R. J. Org. Chem. 1986, 51, 589.
(24) Sheldrick, G. M. SHELXTL-Plus, Relat. 5.03, Siemens Analytical
X-ray Instruments Inc., Madison, WI, 1995.
(25) Sheldrick, G. M. SHELXS-97, Program for Solution and Refinement
of Crystal Structures from Diffraction Data, Universita¨t Go¨ttingen, 1997.

6398 J. Am. Chem. Soc., Vol. 122, No. 27, 2000
Di Bari et al.
were calculated from the setting angles of 36 reflections having 5.0°
< θ < 12.5°. The structure was solved by direct methods and completed
by standard Fourier methods. The hydrogen atoms were introduced in
calculated positions and were let ride on the connected carbon atoms.
The final refinement cycle gave the reliability factors listed at the bottom
of Table 2.
(R)-(+)-1,1′-Binaphthyl-2,2′-dicarboxylic acid, obtained by reac-
tion of 1 with (a) 2.2 equiv of n-butyllithium, 1.6 M in hexane, in
THF at -78 °C for 4 h, under argon atmosphere and (b) gaseous CO2
at -78 °C for 12 h and at -40 °C for 2 h with standard workup and
recrystallization from toluene, showed molecular ion peak at m/z 314
and ee(HPLC) ) 97%.
(R)-(+)-2,2′-diamino-1,1′-binaphthalene, ee g 99.5%, and 2-bromo-
naphthalene were purchased from Aldrich. Acetonitrile for spectroscopy
was purchased from Fluka and not further purified. THF was refluxed
several hours on Na/K alloy and distilled immediately before use. The
remaining reagents and solvents were commercial products and were
purified according to standard procedures.
2-Iodonaphthalene was prepared by transhalogenation from 2-bro-
monaphthalene, according to the standard procedure,27 and purified by
recrystallization from absolute ethanol. Mp ) 50-52 °C. 1H NMR: δ
7.45-7.50 (2H, m, H6 and H7), 7.56 (1H, d, J ) 8.5 Hz, H3), 7.70
(1H, d, J ) 8.5 Hz, H4), 7.70 (1H, m, H5 or H8), 7.76-7.79 (1H, m,
H5 or H8), 8.23 (1H, s, H1).
(R)-(+)-DIBN (1) was prepared from (R)-(+)-2,2′-diamino-1,1′-
binaphthalene and purified according to the published pro-
cedure.26 Mp ) 220-222 °C. [R]D +15.9 (c 2.16, py) [lit.26 [R]D
23
23
Supporting Information Available: Table of crystal data,
structure solution and refinement, and bond lengths and angles
for 2,2′-diiodo-1,1′-binaphthalene (1) (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
1
+16.2 ( 0.2 (c 1.725, py)]. H NMR: δ 7.05 (1H, d, J ) 8.1 Hz,
H8,8′), 7.27 (1H, t, J ) 8.0 Hz, H7,7′), 7.49 (1H, t, J ) 8.1 Hz, H6,6′),
7.70 (1H, d, J ) 8.7 Hz, H4,4′), 7.91 (1H, d, J ) 8.1 Hz, H5,5′), 8.04
(1H, d, J ) 8.7 Hz, H3,3′). MS, m/z (rel intensity): 506 (M+, 100%),
379 ([M - I]+, 21%), 252 ([M - 2I]+, 100%), 126 (C10H6+, 86%).
JA000114A
(26) Brown, K. J.; Berry, M. S.; Murdoch, J. R. J. Org. Chem. 1985,
50, 4345.
(27) Bachmann, W. E.; Kloetzel, M. C. J. Org. Chem. 1938, 3, 55.