CHEMCATCHEM
FULL PAPERS
tions requires a balance of equi-
librium and a suitable sequence
of reversible and irreversible
steps.[37]
Table 2. MOF–[LM]-catalyzed cascade reaction of aldehydes and ethyl nitroacetate: olefination[a]–alkene hydro-
genation–nitro hydrogenation reactions.[b]
Entry Catalyst
Reaction
mode
R
Conv.[c]
[%]
t
Selectivity [%]
[h] Product B Product C
The development of catalysts
for one-pot multistep reactions
is important, as chemists aim at
minimizing the use of reagents
and solvents as well as at reduc-
ing intermediate separation and
purification steps.[12, 38] An exam-
ple of a multistep sequential
process using differentiated bi-
functional acid–metal catalysts
was the cyclization of citronellal
followed by hydrogenation to
yield menthol in a one-pot syn-
thesis[39] or the acetal hydrolysis
followed by condensation and
subsequent hydrogenation.[40] As
1
2
3
4
5
6
7
8
UiO-67–NH2 +UiO-67–NH2–[L3Rh]
two step[d] Ph
100
100
100
100
100
100
100
100
100
48
30 100
5
95
–
60
90
3
80
40
5
–
–
–
50
UiO-67–NH2–[L3Rh]
UiO-67–NH2–[L3Rh]
UiO-67–NH2–[L3Rh]
UiO-67–NH2–[L3Rh]
UiO-67–NH2–[L3Rh]
UiO-67–NH2–[L3Rh]+DIOP
one pot[e]
one pot[e]
one pot[e]
one pot[f]
one pot[f]
one pot[e]
Ph
Ph
Ph
Ph
Ph
Ph
48
72
44
72
48
48
34 100
48 100
44
24
40
10
97
20
60
95
UiO-66-NH2–L3+UiO-66–NH2–[L3Rh] two step[d] Ph
9
UiO-66–NH2–[L3Rh]
UiO-66–NH2–[L3Rh]
UiO-66–NH2–[L3Rh]
UiO-66–NH2–[L2Ir]
one pot[e]
one pot[e]
one pot[e]
one pot[f]
Ph
10
11
12
4Me-Ph 100
4F-Ph
Ph
100
30
98
50
[a] T=110 8C; solvent toluene (1 mL); [b] All reactions were performed in toluene with 1 mol% of the catalyst
(containing Rh); [c] Determined from GC and 1H NMR analysis by using ethyl nitroacetate; [d] Two-step reac-
tion: glass reactor + Autoclave Engineers; [e] One-pot reaction: glass reactor with manometer; [f] Autoclave
Engineers device. Decomposition of catalyst was observed.
mentioned above, a-nitrocinnamates and acrylates are versa-
tile building blocks in organic synthesis owing to the possibili-
ty of converting either the nitro or the ester group into other
functional groups. To examine the catalytic ability of UiO-67–
NH2 derivatives as multifunctional catalysts, we chose as
a model the reaction between an aldehyde and ethyl nitroace-
tate that yields product A (ethyl 2-nitro-3-phenylacrylate),
which, in the presence of hydrogen, gives product B (ethyl 2-
nitro-3-phenylpropanoate) and/or product C (ethyl 2-amino-3-
phenylpropanoate)—the metal-catalyzed reaction (Scheme 3).
We have obtained the final products B and C by using either
a two-step reaction or a one-pot reaction.
We found that the one-pot reaction of benzaldehyde and
ethyl nitroacetate in the presence of a catalytic amount of
UiO-67–NH2–[LRh] derivative in toluene at 1008C yielded prod-
uct A, with 100% conversion after 24 h. Then, hydrogen (5 bar)
was incorporated into the reaction media and the hydrogena-
tion of the resulting double bond was completed after 6 h. If
desired, the process can proceed, and finally, the reduction of
NO2 occurs, which yields the corresponding benzyl amino de-
rivative (Scheme 3 and Table 2, entries 4 and 6). Next, the UiO-
66–NH2–[LRh]-catalyzed reaction can selectively lead to the re-
duction of the double bond, which yields the corresponding
ethyl 2-nitro-3-phenylpropanoate or 2-nitro-3-(p-tolyl)propa-
noate if the substrate is 4-methylbenzaldehyde (Table 2, en-
tries 9–11). The same reaction catalyzed by UiO-66–NH2–[LIr]
gave only 30% yield (product selectivity=50%) and the Ir cat-
alyst decomposes under reaction conditions (Table 2, entry 12).
Notably, the reaction of benzaldehyde and ethyl nitroacetate
in the presence of UiO-66–NH2–[L3Rh] with 0.11 mol% of the
catalyst (containing Rh) gives selectively the corresponding hy-
drogenated product in 97% yield after 44 h (total condensa-
tion–alkene hydrogenation reactions). If the reaction continues,
the hydrogenation of the nitro group can be achieved after an
additional 24 h.
To obtain these products, in the first case, the UiO-67–NH2–L
catalyst was suspended in toluene and then benzaldehyde and
ethyl nitroacetate were added. The condensation product of
these reactants, product A, formed, and the aldehyde was
completely consumed within 24 h (Table 2). The product was
isolated through filtration and added to a suspension of UiO-
67–NH2–[LRh] in toluene; the reaction mixture was then trans-
ferred to an autoclave (heated to 808C under 6 bar hydrogen
for 10 h; 1 bar=0.1 MPa), which yielded the corresponding
benzyl amine (product C in Scheme 3; Table 2, entry 1) as the
sole product. If the reaction was performed in the presence of
UiO-66–NH2–L3+UiO-66–NH2–[L3Rh], then only product B was
formed (without the isolation of the corresponding acrylate) in
high yield (Table 2, entries 8 and 9).
To study the effect of the chiral amino group on asymmetric
induction in cascade reactions, we measured the optical rota-
tion of product C obtained through the cascade reaction but
we have not observed enantioselectivity in this reac-
tion, even in the presence of a chiral diphosphine,
such as DIOP [DIOP=2,3-(isopropylidenedioxy)-2,3-di-
hydroxy-1,4-bis(diphenylphosphanyl)butane)]
(Table 2, entry 7).
Control experiments confirmed that the intact UiO-
66–NH2–[LRh] catalyst was responsible for the ob-
served catalytic activity, because once the solid cata-
lyst was removed through filtration at low conversion
the reaction stopped (hot filtration experiment; Fig-
ure S29). Moreover, no evidence was found for the
Scheme 3. M-Zr-MOF-catalyzed cascade condensation–olefin-nitro hydrogenation
reaction. R=Ar; M=Rh, Ir.
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ChemCatChem 0000, 00, 1 – 10
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Rasero-Almansa, Antonia M.
