50
N.M.R. Martins et al. / Catalysis Communications 87 (2016) 49–52
Scheme 1. Synthesis of 1.
polymer as a homogeneous catalyst for the Henry reaction in aqueous
medium.
(entries 2–11). The best results were obtained when 1 was used as a cat-
alyst in water (entry 11), and thus water was chosen as the sole solvent
for further studies. The variation of the catalyst amount (entries 11–14)
allowed to conclude that 6 mol% loading is optimal for the given reac-
tion conditions and thus this amount was applied for the next steps.
The effect of temperature on the performance of this protocol was
evaluated over a temperature range of 20–80 °C (entries 13, 15–17),
where 40 °C was found to be optimum to produce β-nitroaldol in
80.1% yield after 24 h (entry 15). More interestingly, an increase in the
temperature from 40 °C to 80 °C in the Henry reaction leads to a de-
crease of the product yield (entries 15–17). This may be due to the dis-
2
. Results and discussion
2
3
.1. Synthesis and characterization of H L and 1
5
-(2-(4,4-dimethyl-2,6-
dioxocyclohexylidene)hydrazinyl)isophthalic acid (H
3
L) (Scheme 1)
was synthesized by the Japp-Klingemann reaction [15] of 3,5-
dicarboxybenzenediazonium chloride and 5,5-dimethylcyclohexane-
1
2+
2−
1
,3-dione in basic medium. The H NMR spectrum of H
3
L in DMSO-d
6
sociation of 1 to Zn and HL . The increase of reaction time up to 3 h
(within the 0.25–15 h range) led to higher conversions (entries 15 and
18–22). Under such optimized conditions, a product yield of 80% with
shows the resonance for the NH group adjacent to the aryl unit at δ
4.59 (Fig. S1 in Electronic Supplementary information). Moreover,
1
1
3
the presence, in the C NMR spectrum, of two distinct resonances (at
92.86 and 196.86 ppm, Fig. S2 in ESI) for the carbonyl groups, indicate
3
syn:anti molar ratio of 76:24 was obtained (entry 20), whereas H L
1
shows low activity (entry 23). The increase reaction time from 3 h to
24 h is practically not affect the yield of β-nitroaldol and its syn:anti
ratio (entries 15, 20–22).
that one of these groups works as acceptor of the proton of the
hydrazone NH moiety. This is in agreement with the IR data which
clearly reveals the stretching bands for ν(C_O) and ν(C_O⋯H) at
1
The electronic properties of the aldehydes influence the yields and
diastereoselectivities of the reaction catalysed by 1 (Table 2). Thus,
aryl aldehydes bearing an electron-withdrawing group (nitro or
bromo) lead to higher yields of product as compared to those having
an electron-donating moiety (entries 1 and 2 vs. 4, Table 2). Moreover,
the steric effect is also perceived when para-, meta- and ortho-methyl-
678 and 1619 cm− , respectively, the latter being shifted on account
1
of the H-bond. Elemental analysis and the ESI-MS peak at 333.2
+
[
Mr + H] support the formulation of H
The reaction of Zn(NO ·6H O with H
water (1:1) at 75 °C for 48 h leads to the new Zn(II) coordination poly-
3
L.
)
3 2
2
3
L in a mixture of DMF and
1
3
mer [Zn(μ -HL)(H
2
O)
2
]
n
∙nH
2
O (1) (Scheme 1). In the H NMR spectrum
L (detected at δ 13.37 for H L,
of 1 the original carboxylic protons of H
3
3
Fig. S3 in ESI) have disappeared (due to coordination) and the
hydrazone proton is shifted downfield (14.89 ppm) relative to the
value shown for H
atom is also shifted from 166.01 (in H
ESI). Several bands in the IR spectrum of 1, viz. 3421 and 3080 (br)
ν(H O), 2953 ν(NH), 1625, 1581 and 1550 ν(C_O), 1508 ν(C_N)
3
L (see above). Due to coordination carboxylic carbon
3
L) to 170.16 ppm in 1 (Fig. S4 in
2
−
1
cm , are significantly shifted in comparison with the corresponding
ones (see ESI file) of the pro-ligand. The ESI-MS peak observed at
+
3
96.6 [Mr–3H
2
O + H] (Fig. S5) accounts for the existence of a mono-
meric unit in solution. The elemental analyses are consistent with the
proposed formulation, which is proved by single crystal X-ray crystal-
lography (Fig. 1, Fig. S6, Tables S1–S3, for the discussion of structure of
1
see the Electronic Supplementary information).
2
.2. Catalytic activity of 1 in the Henry reaction
The catalytic activity of 1 towards the Henry reaction between benz-
aldehyde and nitroethane (Scheme 2) was studied, and the influence of
varying the solvent, catalyst amount, time and temperature was evalu-
ated (Table 1). No reaction was observed between benzaldehyde and
nitroethane in water at room temperature (entry 1), while in the pres-
Fig. 1. The molecular structure of 1 with partial atom numbering scheme. H-bond
interactions are drawn in dashed light-blue colour. Symmetry codes to generate
equivalent atoms: i) 1 − x, y, 1/2 − z; ii) -1/2 + x, 1/2 − y, −1/2 + z; iii) −1/2 + x, 1/
ence of zinc salts the low yields of 14% [with Zn(NO
with Zn(OAc) , entry 3] were obtained. The catalytic activity of H
were evaluated in the presence and absence of various solvents
3 2
) , entry 2] and 35%
[
2
3
L and
2
− y, 1/2 + z; iv) 1/2 − x, 1/2 − y, 1 − z; v) 1/2 − x, 1/2 − y, 1 − z; vi) 1 − x, 1 − y,
1
1 − z; vii) 1/2 + x, 1/2 − y, −1/2 + z.
Martins, Nuno M.R.
