Chiral recognition of the Schiff bases by NMR spectroscopy 609
recognition of different classes of compounds.14–19 This
method does not require a series of measurements with
increasing molar ratios of the ligand to dirhodium complex
in contrast to the CLSR method. In this work the dirhodium
method was applied for chiral recognition of Schiff bases for
the first time.
Compounds 1 and 2, existing exclusively in the OH-
form, showed different behaviour from that of the other
Schiff bases studied. The most affected by complexation was
the hydrogen in the proton donor group. The υ values
of the signal were in range of 0–1.5 ppm (Table 1). The
disappearance of some of the carbon signals in the baseline
(e.g. C-2), their broadening at temperatures below 295 K and
a lack of signal dispersion in the whole range of temperatures
were observed. This indicated that the adduct formation
equilibrium is shifted towards the free ligand (Fig. 1) and that
the presence of dirhodium complex had no influence on the
position of the proton in the intramolecular hydrogen bond.
The lack of signal shifts for compound 2 suggested that large
tert-butyl substituents prevent the adduct formation between
the Schiff base and the dirhodium complex. For compound
1 it was not possible to measure deuterium isotope effects
in the presence of RhŁ because the signals were very broad
and disappearing in the baseline. For adduct of compound
2, only the measurement at 230 K gave sufficient results and
the found nCꢁDꢂ values were similar to those obtained for
the samples without the dirhodium complex (Table 1).
The other Schiff bases, being in the NH-form (5–6) or
tautomeric equilibrium (3–4 and 7–8), formed adducts with
dirhodium complex. The υ values of the signal of the
hydrogen in the proton donor group varied from ꢀ0.48 ppm
for the adduct of 3 up to 0.44 ppm for 4. At 230 K, the majority
Figure 5. nC-1(D) observed for adduct of 3 at 270 K.
7–8, they were in the range from ¾3 up to 12 ppm. These
major changes in the chemical shift observed for this position
can be associated with changes in the position of the proton
transfer equilibrium due to the presence of the dirhodium
complex. The change in the position of the equilibrium was
confirmed by changes in the values of deuterium isotope
effects (e.g. nC-2(D) for 4 at 270 K from C530 ppb to
ꢀ230 ppb) and as well as in the values of 3J(NH,H) coupling
constants (Table 1). For adducts of 3–4 and 6–8 the nC-2(D)
values were negative and changed only slightly (ca 0–40 ppb)
with lowering temperature (Table 1). This indicated the
absence of the proton transfer equilibrium and the existence
of the adducts exclusively in the NH-form (Fig. 3). Also,
the almost temperature insensitive, high 3J(NH,H) values in
range of ¾11–14 Hz confirmed the presence of the pure NH-
form. A similar change in the position of the equilibrium due
to the presence of the dirhodium complex was also observed
for secondary phosphane oxides, in which the equilibrium
was shifted towards the hydroxyphosphane form.18 Smaller
changes in the values of deuterium isotope effect were
observed for the adduct of 5.
The dispersion of the signals due to the presence of
two diastereomers was observed for all compounds studied
except 1 and 2. The ꢀ values in a range of 2.2–56 Hz
were measured for the carbons of the aliphatic chain. The
signals of the other carbons showed much smaller values
of dispersion ꢀ; up to 16 Hz for C-˛ for compound 7 at
250 K. Dispersion ꢀ was observed for a minimum of two
carbons in compound 5 and up to nine carbons in compound
7 (Table S2).
The adducts of all compounds, except 1 and 2, have
shown interesting feature. Because of the presence of four
species (deuterated and non-deuterated diastereomers of
Schiff bases), two sets of deuterium isotope effects for some
of the carbon atoms were observed in the presence of the
dirhodium complex (Fig. 5). This feature was also recorded
for the carbons remote from the chirality centre (Table S3
and 1).
The values of deuterium isotope effects were equal
or very close to each other for both diastereomers. The
small differences may be related to the conditions of the
measurements, resolution of the spectra and accuracy of
measurements of the deuterium isotope effects.
1
of the signals in the H NMR spectra were very broad and
of low intensity. The splitting of the proton donor group
signal at 230 K for compounds 3, 4, 7, 8 was connected with
the presence of two adducts: 2 : 1 and 1 : 1. On the basis of
the experiments with 2 : 1 molar ratio of compound 7 to RhŁ
both signals were assigned: the one at the lower field to
the 2 : 1 adduct and the one at the higher field to the 1 : 1
adduct. An increase in temperature caused a shift of the
adduct formation equilibrium towards the 1 : 1 adduct. The
highest values of signal dispersion ꢀ were observed for the
adducts of compounds 3–8 in the aliphatic side-chain of the
molecules (up to 30 Hz for 6 at 250 K) or methine proton (up
to 40 Hz for 4 at 230 K) close to the anisotropic groups in the
chiral Mosher acids residues (Table S1 and S2).18 The signal
dispersion (ꢀ) in range of 2.4–14 Hz was also observed
for the aromatic protons. Dispersion ꢀ was observed at
least for two protons in compound 5 and up to 5 protons in
compounds 3 and 7 (Table S1).
In 13C NMR spectra at 230 and 250 K, some signals of
the adducts were broadened and of low intensity while
some disappeared in the baseline. The major changes in the
chemical shifts observed for the C-2 signals (deshielding)
indicated that the complexation site in the Schiff bases
studied was the oxygen atom of the proton donor group
(Table 1).18 For compounds 5 and 6 existing in the NH-form,
the υ values were close to 0.5 ppm, while for the compounds
in which the proton transfer equilibrium takes place, 3–4 and
Copyright 2007 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2007; 45: 605–610
DOI: 10.1002/mrc
Rozwadowski
