Racemization barriers of 1,1′-binaphthyl and 1,1′-binaphthalene-2,2′-diol: A DFT study

Article abstract of DOI:10.1021/jo034344u

Density functional theory has been applied to the study of various pathways and transition states for the configurational inversion of 1,1′-binaphthyl (1) and 1,1′-binaphthalene-2,2′-diol (2). The preferred pathway is found to be anti with centrosymmetric transition state. Whereas the reaction path of 1 goes downhill from transition to ground state, in the case of 2 it contains one unexpected local minimum. Very satisfactory agreement with available experimental values of activation Gibbs energies is achieved.

Full text of DOI:10.1021/jo034344u

Ra cem iza tion Ba r r ier s of 1,1′-Bin a p h th yl a n d
1,1′-Bin a p h th a len e-2,2′-d iol: A DF T Stu d y
Ludeˇk Meca,† David Rˇ eha,‡,§ and Zdeneˇk Havlas*,§,|
Institute of Chemical Technology, Technicka´ 5, 166 28 Prague 6, Czech Republic, J . Heyrovsky´ Institute of
Physical Chemistry, Dolejsˇkova 3, 182 23 Prague 8, Czech Republic, Institute of Organic Chemistry and
Biochemistry, Flemingovo na´m. 2, 166 10 Prague 6, Czech Republic, and Center for Complex Molecular
Systems and Biomolecules, 182 23 Prague 8, Czech Republic
havlas@uochb.cas.cz
Received March 14, 2003
Density functional theory has been applied to the study of various pathways and transition states
for the configurational inversion of 1,1′-binaphthyl (1) and 1,1′-binaphthalene-2,2′-diol (2). The
preferred pathway is found to be anti with centrosymmetric transition state. Whereas the reaction
path of 1 goes downhill from transition to ground state, in the case of 2 it contains one unexpected
local minimum. Very satisfactory agreement with available experimental values of activation Gibbs
energies is achieved.
In tr od u ction
1,1′-Binaphthyl (1) and its derivatives exhibit hindered
rotation about internuclear bonds, and their enantiomers
can be isolated at room temperature. Synthetically useful
1,1′-binaphthalene-2,2′-diol (2; BINOL) can be obtained
from simple derivatives of 1.
F IGURE 1. Possible racemization pathways of 1,1′-binaphthyl
(1).
TABLE 1. Ra cem iza tion of 1,1′-Bin a p h th yl (1) a n d
1,1′-Bin a p h th a len e-2,2′-d iol (2)
τ1/2
∆Gq
rac
rac
Enantiomerically pure BINOL is easily obtainable by
resolution1 of low-cost racemate and is one of the most
used chiral auxiliaries for asymmetric synthesis.2 Rota-
tional barriers of 1 and 2 have been measured3 (Table
1). The racemization barriers of 1,1′-binaphthyl deriva-
compd T (°C)
solvent
44 benzene
50 dimethylformamide 14.5
(min) (kJ /mol)
ref
1
1
2
2
68
100.7
98.5
155.5
158
3a
3b
195 naphthalene
220 diphenyl ether
270
60
3c,d
this work
† Institute of Chemical Technology.
tives have been computed using molecular mechanics4
and semiempirical methods.5
‡ J .Heyrovsky´ Institute of Physical Chemistry.
§ Center for Complex Molecular Systems and Biomolecules.
| Institute of Organic Chemistry and Biochemistry.
Racemization of binaphthyl is possible through a
passage of 2,8′- and 2′,8-substituents (anti route) or 2,2′-
and 8,8′-substituents (syn route). Cooke and Harris3b
suggested two kinds of anti pathways for racemization
(1) (a) Periasamy, M.; Venkatraman, L.; Sivakumar, S.; Sampath-
kumar, N.; Ramanathan, C. R. J . Org. Chem. 1999, 64, 7643. (b) Cai,
D.; Hughes, D. L.; Verhoeven, T. R.; Reider, P. J . Tetrahedron Lett.
1995, 36, 7991. (c) Hu, Q.-S.; Vitharana, D. R.; Pu, L. Tetrahedron:
Asymmetry 1995, 6, 2123.
(2) (a) Ojima, I. Catalytic Asymmetric Synthesis, 2nd ed.; Wiley-
VCH: New York, 2000. (b) Rosini, C.; Franzini, L.; Raffaelli, A.;
Salvadori, P. Synthesis 1992, 503. (c) Noyori, R. Asymmetric Catalysis
in Organic Synthesis; Wiley: New York, 1994.
(3) (a) Colter, A. K.; Clemens, L. M. J . Phys. Chem. 1964, 68, 651.
(b) Cooke, A. S.; Harris, M. M. J . Chem. Soc. 1963, 2365. (c) Smrcˇina,
M. Study of racemization barriers and asymmetric transformations of
selected enantiomers. PhD Dissertation (in Czech), Charles University,
Prague, 1991. (d) Kyba, E. P.; Gokel, G. W.; de J ong, F.; Koga, K.;
Sousa, L. R.; Siegel, M. G.; Kaplan, L.; Sogah, D. Y.; Cram, D. J . J .
Org. Chem. 1977, 42, 4173.
(4) (a) Dashevsky, V. G.; Kitaygorodsky, A. I. Theor. Eksp. Khim.
1967, 3, 43. (b) Gamba, A.; Rusconi, E.; Simonetta, M. Tetrahedron
1970, 26, 871. (c) Gustav, K.; Su¨hnel, J .; Wild, U. P. Chem. Phys. 1978,
31, 59. (d) Carter, R. E.; Liljefors, T. Tetrahedron 1976, 32, 2915. (e)
Liljefors, T.; Carter, R. E. Tetrahedron 1978, 34, 1611. (f) Leister, D.;
Kao, J . J . Mol. Struct. 1988, 168, 105. (g) Tsuzuki, S.; Tanabe, K.;
Nagawa, Y.; Nakanishi, H. J . Mol. Struct. 1990, 216, 279.
(5) Kranz, M.; Clark, T.; Schleyer, P. v. R. J . Org. Chem. 1993, 58,
3317.
10.1021/jo034344u CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/12/2003
J . Org. Chem. 2003, 68, 5677-5680
5677

Meca et al.
TABLE 2. Exp er im en ta l a n d Ca lcu la ted C-C Bon d
Len gth s (p m ) of 1
bond
X-ray6
X-ray7
calcd
C(1)-C(1′)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(10)
C(5)-C(10)
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
C(8)-C(9)
C(9)-C(10)
C(1)-C(9)
147.5
136.5
140.4
136.0
141.3
141.4
134.6
140.4
135.9
141.3
141.6
143.3
148.6
137.7
140.9
135.8
142.1
142.1
137.0
139.1
136.1
142.6
141.2
143.5
149.8
138.3
141.5
137.4
142.0
142.2
137.6
141.6
137.6
142.3
143.5
143.5
of substituted binaphthyls“mesoid” (anti-Ci in Figure 1)
and “racemoid” (anti-C2 in Figure 1). Carter and Liljefors4d
added syn analogues of these routes (syn-C2 and syn-Cs
in Figure 1). They investigated all four racemization
pathways of 1 and showed anti-Ci to be the preferred
route with one Ci-symmetric local minimum. Tsuzuki et
al.4g found this anti-Ci pathway to have no local minimum
but Ci-symmetric saddle point by MM2′ and the same
result we report for 1 and 2 in this paper. Kranz et al.5
excluded anti-Ci route from consideration and investi-
gated syn-C2 and anti-C2 pathways of 1 by MNDO, AM1
and PM3. They preferred slightly the anti-C2 over syn-
C2 route in conclusion. To the best of our knowledge
racemization of 2 was not computationally investigated
before.
Resu lts a n d Discu ssion
1,1′-Bin a p h th yl (1). Gr ou n d -Sta te Con for m a tion .
Calculated bond lengths of 1 are compared to those from
X-ray analyses (Table 2). Deviations from experimental
values do not exceed 2 pm. X-ray structures of racemate
and pure enantiomer of 1 were determined having C(2)-
C(1)-C(1′)-C(2′) dihedral angles 68°6,7 and 103°,7 re-
spectively. Our calculated C2-symmetric structure has an
energy minimum at 75°.
Tr a n sition Str u ctu r es. Four types of racemization
pathways, which were taken in consideration, are sche-
matically drawn in Figure 1. However, we were able to
locate only two transition structuressanti-Ci and syn-
C2 (Figure 2). Table 3 shows relative energies and
selected geometrical parameters of these transition states.
Our anti-Ci-TS energy (96 kJ /mol) agrees almost exactly
with the experimental values, whereas semiempirical
AM1 and PM3 underestimate8 (84 and 75 kJ /mol) and
ab initio Hartree-Fock (HF) overestimates8 (127 kJ /mol)
the value of the racemization barrier. IRC calculation of
anti-Ci-TS (Figure 3) shows simple downhill path to the
ground state. During racemization starting from one GS
enantiomer, atoms H(2) and H(8′) initially pass each
other with closest contact of 155 pm, then their non-
bonded contact reaches 179 pm in the TS, and finally
atoms H(2′) and H(8) pass each other to give the opposite
GS enantiomer.
F IGURE 2. Computed transition structures of 1.
Bin a p h th a len e-2,2′-d iol (2). Ground state conforma-
tion. Calculated bond lengths of 2 are compared to those
from X-ray analyses in Table 4. Deviations from the
experimental values do not exceed 3 pm. Experimental
C(2)-C(1)-C(1′)-C(2′) dihedral angles are 88° for racemate
and 77° for pure enantiomer of 2.9 Our calculated C2-
symmetric structure has an energy minimum at 95°.
Transition structures. Again, all four types of racem-
ization pathways (Figure 1) were investigated. However,
we were able to find only three transition structures -
anti-Ci, syn-C2 and anti-C2 (Figure 4). Their relative
energies and important structural parameters are shown
in Table 5. Like 1, the lowest in energy is the anti-Ci-TS
(158 kJ /mol agrees excellently with the experimental
values; AM1 and PM3 underestimate8 - 129 and 144 kJ /
mol - and HF highly overestimates8 - 210 kJ /mol - the
height of the rotational barrier). This centrosymmetric
transition structure has both hydroxy groups in syn
conformation (!), having short H(2)-C(8′) nonbonded
(6) Kerr, K. A.; Robertson, J . M. J . Chem. Soc. B 1969, 1146.
(7) Kress, R. B.; Duesler, E. N.; Etter, M. C.; Paul, I. C.; Curtin, D.
Y. J . Am. Chem. Soc. 1980, 102, 7709.
(8) Meca, L.; Rˇ eha, D.; Havlas, Z. Unpublished results. AM1, PM3
and HF computed geometries are similar to the reported B3LYP
geometries. For HF calculations 6-31G(d, p) basis sets were employed.
(9) Toda, F.; Tanaka, K.; Miyamoto, H.; Koshima, H.; Miyahara, I.;
Hirotsu, K. J . Chem. Soc., Perkin Trans. 2 1997, 1877.
5678 J . Org. Chem., Vol. 68, No. 14, 2003

Racemization Barriers of 1,1′-Binaphthyl and 1,1′-Binaphthalene-2,2′-diol
TABLE 3. Rela tive En er gies a n d Geom etr ica l P a r a m eter s of Sta tion a r y P oin ts of 1
distance (pm)
structure
rel energy (kJ /mol)
angle C(2)-C(1)-C(1′)-C(2′) (deg)
C(1)-C(1′)
H(2)-H(8′)
H(2)-H(2′)
H(8)-H(8′)
GS
anti-Ci-TS
syn-C2-TS
0
75.0
180
26.4
150
150
153
373
179
313
325
96.1
133.0
201
163
F IGURE 3. B3LYP 6-31G racemization path of 1.
TABLE 4. Exp er im en ta l a n d Ca lcu la ted C-C a n d C-O
Bon d Len gth s (p m ) of 2
bond
X-ray rac9
X-ray enant9
calcd
C(1)-C(1′)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(10)
C(5)-C(10)
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
C(8)-C(9)
C(9)-C(10)
C(1)-C(9)
C(2)-O(1)
148.9
138.8
141.5
135.3
141.0
141.9
135.4
139.9
136.3
141.5
142.9
141.9
136.8
150.1
137.5
141.7
135.4
140.7
141.9
134.9
141.1
135.4
141.7
142.5
142.7
136.1
149.8
139.2
141.9
137.1
142.2
141.9
137.7
141.5
137.8
142.2
143.4
143.7
135.9
distance (209 pm). IRC calculation startin from the anti-
Ci-TS (Figure 5) exhibits one local minimum (M) between
TS and GS, which differs from GS in one H(2)-O(2)-
C(2)-C(1) dihedral angle and is by 18 kJ /mol above the
GS. The shortest nonbonded contact between H(8′) and
O(2) during TSfM transformation is 190 pm (234 pm
for TS itself).
Con clu sion s
We have shown that density functional computations
are excellent for calculating the racemization barriers of
binaphthyls. The preferred racemization route is anti-Ci
for both investigated compounds. Nevertheless, syn-C2
racemization path might be important for other 2,2′-
disubstituted binaphthyls (syn-C2 is only 17 kJ /mol over
preferred anti-Ci for BINOL 2). Racemization of both
F IGURE 4. Computed transition structures of 2.
investigated binaphthyl compounds does not affect the
internuclear C(1)-C(1′) bond length. Energy increase of
TS is mainly caused by nonplanar geometries of aromatic
rings.
J . Org. Chem, Vol. 68, No. 14, 2003 5679

Meca et al.
TABLE 5. Rela tive En er gies a n d Geom etr ica l P a r a m eter s of Sta tion a r y P oin ts of 2
distance (pm)
H(2)-H(8′) H(2)-H(2′)
structure
rel energy (kJ /mol)
angle C(2)-C(1)-C(1′)-C(2′) (deg)
C(1)-C(1′)
H(8)-H(8′)
GS
0
94.8
180
25.6
150
149
150
152
327
218
336
375
anti-Ci-TS
syn-C2-TS
anti-C2-TS
158.3
175.3
249.4
dist C(8)-H(2′) 209
255
202
172.7
dis. O(1)-H(8′) 201
-2
order rate equation: k ) ∑ ti ∑ ti ln(ee0/eei) with the
ee0 at the time t)0 and eei at ti, respectively), and
∆Gqrac)(158(1) kJ /mol.
The density functional calculations of B3LYP quality
with basis sets 6-31G(d,p) were carried out using the
GAUSSIAN 9810 program suite. Transition structure
optimizations have been done using the STQN procedure
without any geometry constraints. Second derivative
(frequency) calculations proved the nature of stationary
points (exactly 1 negative eigenvalue). The reaction path
was determined by IRC calculations (at somewhat smaller
6-31G basis sets) for selected structures.
Ack n ow led gm en t. We thank Dr. J aroslav Pecka for
helpful discussions about IRC computations and Dr.
Sˇteˇpa´n Vyskocˇil for providing a liquid chromatograph
with a chiral column for measuring BINOL racemiza-
tion parameters. This project, LN00A0032 (Center for
Complex Molecular Systems and Biomolecules), was
supported by the Ministry of Education of the Czech
Republic.
F IGURE 5. B3LYP 6-31G racemization path of 2.
Su p p or tin g In for m a tion Ava ila ble: Cartesian coordi-
nates and electronic energies of all stationary points. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Exp er im en ta l Section
Racemization barrier of BINOL was determined as
follows:
Solution of 25 mg (S)-BINOL in 500 mg diphenyl ether
in a test tube was placed to an oil bath at (220(1) °C.
Samples were analyzed at Chiralpak AD column (250 ×
4.6 mm, solvent 5% 2-propanol in hexane, flow 0.5 mL/
min, detection UV 254 nm) to give following results (ret.
times of enantiomers: tR)29 min, tS)31 min):
J O034344U
(10) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J . B.; Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.; Liashenko,
A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J .;
Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez,
C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen, W.; Wong, M.
W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.; and Pople, J . A.
Gaussian 98 (Revision A.6); Gaussian, Inc., Pittsburgh, PA, 1998.
time (min)
ee (%)
0
94
40
55.5
80
36
120
24.5
from which k)1.93 × 10-4 s-1 (calculated by the first-
5680 J . Org. Chem., Vol. 68, No. 14, 2003