Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
mol·L–1 in KOH at 118 °C for 23 h.[22] A later study reported Conclusions
for the base-induced racemization, that the enantiomeric excess
The naphthyl-paneled cavities formed in the hydro-
of (–)-BINOL decreased from 99% to 0% in boiling 5% aque-
2–
ous NaOH over 12 h.[
23]
gen-bonded (rac-BINOLAT )(rac-BINOL)
strands act as
2
The estimated experimental enantio-
2+
–1
host for metal complex counter cations: [Cu(NH
3
)
5
]
in
merization barriers from these conditions are 135.3 kJ·mol
2
+
2–
the structure of [Cu(NH ) ] (rac-BINOLAT )(rac-BINOL)
–
1
[24]
3 5
2+
2
and 122.3 kJ·mol , respectively. BINOL does not show any
2+
2–
2
or {Cu (NH ) } in [Cu (NH ) (μ-rac-BINOLAT -κ O,OЈ:
κO)] (rac-BINOL) . While the [Cu(NH ) ] -guest cation is
3
2
3 2
sign of racemization on heating in neutral water at 100 °C for
2
+
2
2
3 5
2
4 h.
The enantiomerization transition barrier (corrected for zero-
point energy) at the B3LYP/6-31G(d) level of theory was cal-
accommodated through second-sphere N–H···O hydrogen
2+
bonding, the {Cu (NH ) } moiety is coordinated by oxide
3
2
atoms from BINOLAT. At the same time the reaction condi-
tions in methanol/aqueous ammonia induce an in-situ racemi-
zation when R- or S-BINOL are used as reagents. The products
contain a racemic BINOL mixture and are identically obtained
from rac-BINOL. Noteworthy, the quantitative BINOL race-
mization takes place at room temperature in the presence of
–
1
culated for neutral BINOL as 158.4 kJ·mol
for the
1–
–1
monoanion BINOL as 167.0 kJ·mol and for the dianion BI-
NOL (denoted as BINOLAT2– in this work) as 113.4 kJ/mol.
2
–
–
1
The later value corresponds to about 120 kJ·mol at 100 °C.
Following the process of racemization experimentally by enan-
tioselective chromatography gave a barrier for the dianion of
2+
2+
2+
2+
+
Cu (but not with Ni , Zn , Cd or Ag ). Such a facile
induced racemization could allow the recycling of an unwanted
enantiomer of BINOL or its derivatives.
–
1
136.2Ϯ0.5 kJ·mol at 385.15 K (112 °C) in butan-1-ol and of
–1
1
30–134 kJ·mol in water at 372.25 K (100 °C).[24]
Typically, a racemization of an enantiomeric compound
seems to be disadvantageous, but in some cases induced race- Supporting Information (see footnote on the first page of this article):
mizations can be turned into an advantage. This is the case for Synthesis and analysis, crystal data and refinement details, crystal
photographs, DTG, TG, DTA and MS-trend-scan curve for 2, lists with
bond lengths and angles and hydrogen-bonding interactions, racemiza-
tion of R-BINOL.
processes that utilize the separation of a desirable enantiomer
from a racemic mixture through resolution. Such a separation
inherently yields about 50% of the undesirable enantiomer and
only 50% for the wanted one. Hence, recycling of the undesir-
Crystallographic data (excluding structure factors) for the structures in
able isomer via a racemization is then economically advan- this paper have been deposited with the Cambridge Crystallographic
tageous in order to enrich the desired product step-by-step. Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
Controlled racemization requires appropriate techniques which
must be available.[25,26]
Subsequent to our above findings on the racemization of
numbers CCDC-1975329, CCDC-1975330, CCDC-1975331, and
CCDC-1975332 (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.
ac.uk, http://www.ccdc.cam.ac.uk).
1,1Ј-bi-2-naphthol by copper(II) under ammoniacal conditions,
we determined the rate constant and activation energies for the
process. The change of the optical activity of a methanol R-
BINOL solution (30.0 mg, 0.105 mmol in 1.5 mL) upon ad-
dition of copper(II) nitrate (2.54 mg, 0.0105 mmol) in aqueous
ammonia (1.5 mL, 26%) was measured at different tempera-
Keywords: 1,1Ј-Binaphthalene-2,2Ј-diol; 1,1Ј-Bi-2-baphthol;
BINOL; Racemization; Copper
tures (see Supporting Information for details). The changes of References
the optical activity α gave the rate constant k at different tem-
1
[
1] a) J.-Z. Wang, J. Zhou, C. Xu, H. Sun, L. Kürti, Q.-L. Xu, J. Am.
peratures from which the Arrhenius plot of lnk vs. reciprocal
1
Chem. Soc. 2016, 138, 5202–5205; b) Y.-H. Chen, D.-J. Cheng,
J. Zhang, Y. Wang, X.-Y. Liu, B. Tan, J. Am. Chem. Soc. 2015,
absolute temperature (1/T) (Figure S9, Supporting Infor-
–
1
1
37, 15062–15065; c) H. Wang, Chirality 2010, 22, 827–837.
2] Y. Chen, S. Yekta, A. K. Yudin, Chem. Rev. 2003, 103, 3155–
211.
mation) yielded an activation energy of 99 kJ·mol . This value
[
[
[
is smaller than what was experimentally found and calculated
3
[
24]
for the dianion in basic solution alone.
Thus, an additional
3] a) J. M. Brunel, Chem. Rev. 2005, 105, 857–898; b) J. M. Brunel,
Chem. Rev. 2007, 107, PR1–PR45.
4] L. Pu, Chem. Rev. 1998, 98, 2405–2494.
2
+
role of the Cu ion can be assumed, possibly through a weak
coordination which aids the racemizing rotation about the C1–
C1’ bond. Since Cu is known for not being redox-inert in
many reactions, a radical process may be involved in the race-
2
+
[5] D. Braga, Chem. Commun. 2003, 2751–2754.
[
6] D. Braga, G. R. Desiraju, J. S. Miller, A. G. Orpen, S. L. Price,
CrystEngComm 2002, 4, 500–509.
mization. A brief density functional theoretical modeling at- [7] F. Grepioni, S. Gladiali, L. Scaccianoce, P. Ribeiro, D. Braga,
2
–
New J. Chem. 2001, 25, 690–695.
tempt of a reaction coordinate in which BINOLAT is coordi-
2
+
2+
2+
[8] J. Pauls, S. Chitsaz, B. Neumüller, Z. Anorg. Allg. Chem. 2000,
nated by a metal atom (precisely M = Cu , Ni and Zn )
and the metal atom is additionally coordinated by two ammine
626, 2028–2034.
[
9] L. R. Nassimbeni, H. Su, New J. Chem. 2002, 26, 989–995.
ligands (either planar or tetrahedral) did not give a transition [10] D. J. Liu, Z. X. Shang, R. Wang, J. G. Qin, Chin. Chem. Lett.
003, 14, 375–378.
[11] B.-M. Ji, H.-Z. Chen, C.-X. Du, K.-L. Ding, Chin. J. Chem. 2004,
2, 1045–1051.
2
state along proposed reaction coordinate (see Supporting Infor-
2+
mation for details). There was no difference for (active) Cu
2
2
+
2+
vs. non-active Ni and Zn along the investigated reaction
coordinate.
[
12] F. Toda, M. Senzaki, R. Kuroda, Chem. Commun. 2002, 1788–
1789.
Z. Anorg. Allg. Chem. 2020, 1–7
www.zaac.wiley-vch.de
5
© 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim