210
S. Di Dio et al. / Applied Catalysis A: General 399 (2011) 205–210
NMR spectrum of (S)-Captopril showed a quartet at 4.14–4.10 ppm
assigned to the proton of –CH in ␣-position to the carboxylic moi-
ety: because of an axial and an equatorial conformation, this proton
can couple with the two protons of –CH2 in -position to –COOH,
giving two doublets with the same coupling constant. The 1H-NMR
spectrum of the complex obtained by reacting Rh(CO)2acac with
(S)-Captopril in a molar ratio 1/2, besides a slight shift of the above
signal from 4.14–4.10 ppm to 4.18–4.15 ppm, showed also a split-
ting of this signal in two doublets with different coupling constants.
Moreover, we could observe a shift of the signal assigned to the
–CH2 in ␣-position to the –SH moiety from 2.20 to 2.10 ppm (free
thioligand) to 2.50–2.40 ppm (complex Rh/Cap). On the basis of
these results, we can assume that the shifting of (S)-Captopril peaks
is due to the proximity of the rhodium that probably forms a chelate
complex due to an interaction of the metal atom with both sulfur
and the carboxylic group.
if moderate, of both activity and enantiomeric excess after recycling
of the catalyst.
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4. Conclusive remarks
All the catalytic systems tested were active in the hydrogenation
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