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D.S. Melo et al. / Applied Catalysis A: General 411–412 (2012) 70–76
Qualitative analysis was made by GC coupled with mass
3. Results and discussion
spectrometry in
ment fitted with
a
Shimadzu GC2010/QP2010-plus instru-
Restek Rtx-5 MS capillary column
a
Hydroformylation is the first step in the HAM sequence. The
monoterpenes limonene, camphene, and -pinene have a 2,2ꢀ dis-
ubstituted encumbered double bonds, which are more difficult
hydroformylate than those of unbranched olefins. The regioselec-
tivity is induced by the substrate itself and the formyl group is
on hydroformylation of this kind of substrates strongly suggested
ligand [48]. The final step in the HAM is the hydrogenation of
the enamine (or imine) and there is evidence that this step is
more efficiently catalyzed by rhodium species with lower electron
density [53]. In cases in which neither regioselectivity nor double-
are preferred to allow for faster enamine hydrogenation [20]. In
some instances, substoichiometric amounts of phosphorus ligand
are used to allow for moderate regioselectivity and good amine
yields [33]. In the absence or at low concentration of ancillary lig-
ands such as phosphines, the double-bond isomerization may be
a strongly competitive reaction and an excess of phosphorus lig-
and is desirable to prevent the formation of [Rh(CO)3H], which is a
very active catalyst for in the double-bond isomerization. Thus, we
decided to study the effects of the kind and concentration of phos-
phine ancillaries in the HAM of monoterpenes in order to get the
best compromise for low isomerization and high imine/enamine
hydrogenation.
(30 m × 0.25 mm × 0.25 m), operating at 70 eV. The main products
were isolated by column chromatography (silica) using mixtures
of hexane and CH2Cl2 and ethanol as eluents and analyzed by
and 1H, 13C, DEPT, HMQC and COSY NMR (Bruker CXP 400, TMS,
CDCl3).
2.4. Spectroscopic data
4a (mixture of two diasteromers): Mass spectrometry: (m/z/rel.
int.): 279/0.7 (M+); 236/18.85; 182/32.29; 142/80.03; 100/100.00;
67/4.60. 13C NMR: 14.10, 16.06, 16.47, 20.81, 23.47, 25.52, 27.09,
27.62, 29.00, 29.41, 30.91, 31.02, 35.59, 35.74, 38.50, 38.75, 52.42,
53.85, 121.03, 134.00. 1H NMR: 5.36 (1H, s), 1.70–2.95 (2H, m),
1.38–1.49 (1H, m), 1.23–1.34 and 1.57–1.68 (2H, m), 1.67–1.74 and
1.29–1.48 (2H, m), 1.64 (3H, s), 1.39–1.46 (1H, m), 1.90–2.20 (2H,
m), 2.40–2.57 (2H, m), 1.36–1.47 (2H, m), 1.22–1.33 (2H, m), 0.91
(3H, t, J = 7.3 Hz).
4b (mixture of two diasteromers): Mass spectrometry: (m/z/rel.
int.): 237/0.28 (M+); 206/0.10; 178/0.50; 140/48.41; 100/100;
87/7.19. 13C NMR: 16.08, 16.49, 23.45, 25.55, 27.03, 27.62, 29.35,
30.62, 30.87, 39.98, 35.62, 35.76, 38.49, 38.74, 53.95, 57.67,
67.00, 120.98, 133.97. 1H NMR: 5.39 (1H, s), 1.73–1.89 (2H, s),
1.33–1.50 (1H, m), 1.27–1.36 and 1.58–1.68 (2H, m), 1.28–1.35
(2H, m), 1.55–1.72 and 1.66 (3H m, s), 1.35–1.49 (1H, s), 0.89
(3H, s), 1.90–2.09 (2H, m), 2.39–2.40 (2H, m), 2.46 (2H, s), 3.74
(2H, s).
4c (mixture of two diasteromers): Mass spectrometry: MS
(m/z/rel. int.): 223/0.48 (M+); 180/9.84; 126/12.22; 99/17.70;
86/79.85; 91/49.93; 70/8.05. 13C NMR: 133.84, 120.61, 29.23, 38.43,
38.34, 30.70, 37.70, 23.32, 35.22, 35.11, 16.42, 16.00, 32.78, 164.51,
61.04, 30.59, 20.23, 13.73. 1H NMR: 5.37 (1H, s), 1.63 (3H, s), 0.91
(3H, s), 7.63 (1H, s), 3.36 (2H, s), 0.91 (3H, s).
8a (mixture of two diasteromers): Mass spectrometry: (m/z/rel.
int.): 279/0.4 (M+); 236/13.00; 142/100.00; 100/97.07; 86/4.14.
13C NMR: 14.10, 20.00, 20.81, 21.53, 23.51, 24.13, 24.70, 24.92,
27.95, 28.00, 28.98, 29.98, 32.47, 35.90, 37.02, 40.52, 41.16, 43.01,
48.65, 49.04, 49.32, 52.63, 53.84, 54.57. 1H NMR: 1.61–1.70 (1H, m),
2.04–2.16 (1H, m), 1.50–1.58 and 1.15–1.23 (2H, m), 1.20–1.38 (2H,
m), 1.70–1.78 (1H, m), 1.54–1.68 and 1.09, 1.20 (2H, m), 0.81 and
0.89 (3H, s) 0.92 (3H, s), 1.54–1.63 and 1.21–1.30 (2H, m), 2.36–2.50
(2H, m), 2.40–2.47 (2H, m), 1.37–1.52 (2H, m), 1.22–1.40 (2H, m),
0.92 (3H, t, J = 7.18 Hz).
8b (mixture of two diasteromers): Mass spectrometry: (m/z/rel.
int.): 237/0.42 (M+); 208/0.04; 100/100; 79/1.36. 13C NMR: 19.94,
21.50, 23.42, 24.09, 24.64, 24.87, 27.73, 27.91, 29.95, 32.43, 35.88,
37.00, 37.12, 40.53, 41.08, 42.92, 48.70, 48.95, 49.21, 52.77, 53.98,
59.21, 59.82, 67.00. 1H NMR: 1.69–1.77 (1H, m), 2.04–2.13 (1H, m),
1.49–1.58 and 1.01–1.09 (2H, m), 1.20–1.38 (2H, m), 1.28–1.37 (1H,
m), 1–12–1.20 and 1.55–1.64 (2H, m), 0.92 and 0.81 (3H, s), 0.89 and
0.96 (3H, s), 1.20–1.28 and 1.42–1.62 (2H, m), 2.21–2.37 (2H, m),
2.45 (2H, s) 3.72 (2H, t, J = 4.44 Hz).
8c (mixture of two diasteromers): Mass spectrometry: (m/z/rel.
int.): 223/4.14 (M+); 180/6.66; 149/0.74; 86/100; 67/4.58.
13 (mixture of two diasteromers): Mass spectrometry (m/z/rel.
int.): 279/1.09 (M+); 236/5.63; 210/5.30; 142/100; 143/10.74;
100/91.39; 101/6.61. 13C NMR: 39.41, 41.04, 33.18, 24.63, 23.52,
45.93, 22.64, 20.13, 26.86, 33.51, 52.68, 53.88, 29.02, 20.80, 14.05.
1H NMR: 1.29–1.38 (1H, m), 1.36–1.45 (1H, m), 1.94–2.05 (2H,
m), 1.66–1.77 (2H, m), 1.66–1.72 (1H, m), 1.16–1.30 (2H, m), 0.81
(3H, s), 1.19 (3H, s), 1.85–1.90 and 1.28–1.35 (2H, m), 2.40 (2H, t,
J = 7.46 Hz), 2.40 (4H, t, J = 7.46 Hz), 1.40–1.53 (2H, m), 1.27–1.40
(2H, m), 0.91 (6H, t, J = 7.28 Hz).
The
main
intermediates
and
products
for
the
hydroaminomethylation (HAM) of limonene (1) are shown in
Scheme 1. In order to verify the effect of the phosphorus ancil-
lary in the HAM of 1 with di-n-butylamine, we employed a
catalyst precursor without pre-coordinated phosphorus ligand
([Rh(cod)(-OMe)]2) and varied the P/Rh molar ratio by the addi-
tion of appropriate quantities of triphenylphosphine, as shown in
Table 1, entries 1–5.
It is noteworthy that, in the absence of PPh3 (entry 1), 1 is
rapidly consumed and so are the intermediates 2 and 3a, but a
fair amount of limonene double-bond isomers and other uniden-
tified products referred to as “others” in the table are also formed.
Increasing the PPh3/Rh ratio (entry 2 to 5), the side products are
reduced, but at a PPh3/Rh of 5 (entry 4) or greater (entry 5), the
rate of enamine hydrogenation. Even after 22 h (entry 4) signifi-
cant amounts of 2 and 3a are still present in the reaction mixture.
For the triphenylphosphine-promoted rhodium hydroformylation,
the catalytically active species are [Rh(PPh3)m(CO)n(H)] (m = 0, 1,
2; n = 1, 2, 3; m + n = 3) [54]. The increase in PPh3/Rh ratio reduces
(m = 0), which is also very active for double-bond isomerization,
but increases the concentration of species containing two ligands
species are not good catalysts for enamine hydrogenation of 3a
either, as this enamine accumulates at higher PPh3/Rh ratios. Based
on DFT calculations and studying phosphines with different dona-
tion ability, Clarke et al. [53] suggested that the rate-determining
step on enamine hydrogenation is the reductive elimination. Of
course a rhodium species containing two phosphine ligands will
be more electron-rich than a rhodium species containing only one
phosphine ligand.
Thus, the PPh3/Rh ratio of 2.5 (entry 3) gives a good com-
promise for low double-bond isomerization and good activity for