Selective catalytic monoreduction of phthalimides and imidazolidine-2,4- diones

Article abstract of DOI:10.1002/anie.201104226

Fluoride's new role: Selective and efficient monoreductions of imides can be achieved with polymethylhydrosiloxane (PMHS) and tetra-n-butylammonium fluoride (TBAF) as catalyst (see scheme). The system is characterized by good chemoselectivity, operational

Full text of DOI:10.1002/anie.201104226

Communications
DOI: 10.1002/anie.201104226
Chemoselective Reduction
Selective Catalytic Monoreduction of Phthalimides and Imidazolidine-
2,4-diones**
Shoubhik Das, Daniele Addis, Leif R. Knçpke, Ursula Bentrup, Kathrin Junge,
Angelika Brꢀckner, and Matthias Beller*
Isoindolinones represent an attractive target for organic
synthesis and a valuable scaffold for medicinal chemistry
because of their widespread and diverse biological activities
(Scheme 1).[1–3] Despite many known methods, there is an
ongoing interest in the development of convenient and
general protocols for the synthesis of these compounds.
example, LiAlH4, leads to complete reduction to give
pyrrolidines.[7] Unfortunately, more benign catalytic reduc-
tions that use heterogeneous catalysts (Raney nickel) need
drastic reaction conditions and are not applicable in the
presence of other sensitive functional groups. So far, only few
organometallic complexes were explored for the reduction of
imides. In this respect, Patton and Drago[8] reported the
hydrogenation of N-methylsuccinimide at 1008C in the
presence of water-soluble ruthenium catalysts. Nevertheless,
the yield of N-methylpyrrolidone was low and no general
substrate scope was shown. More recently, Bruneau, Dixneuf,
and co-workers developed an elegant hydrogenation of
aromatic imides by using either [Ru4H6(p-cymene)4]Cl2 or
[RuCl2(p-cymene)]2 in water.[9] However, the arene is reduced
in addition to the imide group under this conditions. Notably,
Ikariya and co-workers,[10a] as well as Bergens and co-work-
ers[10b] reported the hydrogenation of imides to w-hydroxy
carboxamides by also applying ruthenium catalysts. It should
be noted that all of these methods are associated either with
expensive transition-metal catalysts or multistep procedures.
Furthermore, there is no general methodology known that
allows the selective reduction of imides in the presence of
other reducible groups.
Scheme 1. Selected examples of therapeutically important isoindoli-
none derivatives.
Hence, in recent years, novel approaches such as rhenium-
catalyzed reactions of aromatic aldimines with isocyanates,[4a]
acid-catalyzed aza-Nazarov reactions of N-acyliminium
ions,[4b] palladium-catalyzed carbonylations of benzylic ami-
nes,[4c] and iodoaminations of a-substituted 2-vinylbenzami-
des[4d] have been developed. In spite of all these achieve-
ments, the selective monoreduction of readily available
phthalimides represents the most straightforward and effi-
cient route to this class of compounds. Known reductions of
phthalimides make use of stoichiometric amounts of tin[5] or
zinc[6] in the presence of an acid. On the other hand, the
application of more common organometallic hydrides, for
We recently became interested in the catalytic reduction
of carboxylic acid derivatives, especially amides and
nitriles.[11] Complementary to catalytic hydrogenations,
hydrosilylations that use silanes offer a unique control of
chemoselectivity.[12] For example, selective iron- and zinc-
catalyzed hydrosilylations of tertiary as well as secondary
amides are possible in the presence of various other func-
tional groups.[13] Based on this work, we demonstrate herein
that selective reduction of imides is achieved with inexpensive
polymethylhydrosiloxane (PMHS) in the presence of catalytic
amounts of fluoride ions.
The reaction of N-methylphthalimide (1a) with different
silanes in THF was initially investigated as a model system to
identify and optimize the critical reaction parameters
(Table 1). As expected, the reaction did not occur in the
absence of any catalyst (Table 1, entry 1). Also, when using
our previously reported zinc[13a] and iron[13b–c] catalysts, no
conversion was observed. However, use of PhSiH3 in the
presence of 5 mol% simple tetra-n-butylammonium fluoride
(TBAF)[14] gave N-methylisoindolinone (2a) in 40% yield
(Table 1, entry 2). When Ph2SiH2 was used, an excellent yield
(80%) of 2a was obtained (Table 1, entry 3). Other fluoride
sources, such as CsF, KF, and TBAF hydrate showed less
activity, whereas the corresponding bromide (TBABr) was
completely inactive (Table 1, entries 8–11).
[*] S. Das, Dr. D. Addis, L. R. Knçpke, Dr. U. Bentrup, Dr. K. Junge,
Prof. Dr. A. Brꢀckner, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e.V. (LIKAT)
an der Universitꢁt Rostock
Albert-Einstein-Straße 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
Homepage: http://www.catalysis.de
[**] This work was funded by the State of Mecklenburg-Western
Pomerania, the BMBF, and the DFG (Leibniz Prize). We thank Dr. W.
Baumann, Dr. C. Fischer, S. Buchholz, S. Schareina, A. Kammer, A.
Koch, B. Wendt, S. Rossmeisl, and K. Mevius (all at the LIKAT) for
their excellent technical and analytical support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104226.
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Angew. Chem. Int. Ed. 2011, 50, 9180 –9184

Table 1: Optimization of the conditions for the monoreduction of
imides.[a]
Entry
Catalyst
(mol%)
Silane
(equiv)
Solvent
Yield[b]
[%]
1
2
3
4
–
PhSiH3 (2)
PhSiH3 (2)
Ph2SiH2 (3)
(EtO)2MeSiH (3)
PMHS (5)
Et3SiH(3)
TMDS (3)
PMHS (5)
PMHS (5)
PMHS (5)
PMHS (5)
PMHS (5)
PMHS (5)
PMHS (5)
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
Toluene
DME
CH2Cl2
–
40
80
69
85
2
TBAF (5)
TBAF (5)
TBAF (5)
TBAF (5)
TBAF (5)
TBAF (5)
TBABr (5)
CsF (10)
KOtBu (20)
TBAF hydrate (3)
TBAF (5)
TBAF (5)
TBAF (5)
5[c]
6
7
8
9
10
11
12
13
14
35
0
35
25
75
78
70
18
Scheme 2. Fluoride-catalyzed reduction of aromatic imides 1a–1k and
substrate 1l. Yields of isolated products 2 are given in brackets.
Substrate 1k and 1l react at 658C.
[a] Reaction conditions: 1a (1.0 mmol), catalyst, silane, solvent (3 mL),
RT, 24 h. [b] Determined by gas chromatography with hexadecane as an
internal standard. [c] 1 mmol PMHS=0.06 mL. Entry in bold represents
the optimized reaction conditions.
used as substrate, a chemoselective reduction of the carbonyl
group took place in the presence of the thiocarbonyl group.
To obtain more information on the mechanism of this
catalytic reduction, simultaneous in situ ATR-FTIR and UV/
Vis spectroscopic measurements were carried out to study the
model reaction between 1a and Ph2SiH2 with TBAF as
catalyst. The obtained in situ ATR-FTIR and UV/Vis spectra
are shown in Figures 1 and 2. Besides the bands of the solvent
To our delight, variation of different silanes demonstrated
that less expensive and nontoxic PMHS[15] is a more suitable
hydride source than other more special silanes. Hence, the
reduction of N-methylphthalimide (1a) at room temperature
in the presence of five equivalents of PMHS gave 2a in 85%
yield (Table 1, entry 5). Notably, no further reduction was
observed. Expediently, PMHS can be easily separated from
the reaction mixture because of its polymeric nature. Among
other solvents, the reaction works well in THF, toluene, and
DME (Table 1, entries 12–14).
Once suitable reaction conditions for the model system
were identified, the scope and limitations of this novel
fluoride-catalyzed reduction of imides with PMHS were
explored. As shown in Scheme 2, 12 different N-substituted
phthalimides (1) are smoothly reduced to the corresponding
isoindolinones (2).
(THF), 1a showed characteristic bands at 1773, 1719, and
À1
=
=
1380 cm , which are assigned to nsC O, nasC O, and dsCH3,
respectively (Figure 1).[16,17] These band positions were not
affected by addition of TBAF. Addition of one equivalent of
Ph2SiH2 led to a new dsCH3 band at 1394 cmÀ1. The additional
bands at 1126 and 1117 cmÀ1 can be assigned to nSi Ph.
[17]
À
À
The observed splitting of the nSi Ph band is surprising
because the spectrum of Ph2SiH2 in THF contains only one
band at 1122 cmÀ1, the position of which shifts to 1120 cmÀ1 by
adding TBAF (see also Figure S1 in the Supporting Informa-
=
tion). After 10 minutes reaction time, the nasC O band
Notably, we did not observe further reduction toward the
corresponding isoindolines in any of the cases. From a
synthetic point of view, it is important that a range of
functional groups, including halides, ethers, acetals, epoxides,
and internal and terminal alkynes, are well-tolerated in this
catalytic protocol (Scheme 2). To the best of our knowledge,
similar functional-group tolerance has not been shown in
imide reductions. While most reactions proceeded smoothly
at room temperature, a slightly higher temperature (658C)
was needed in case of N-methyl-4-bromophthalimide (1k),
and the isoindolinone was obtained as a 1:1 mixture of two
regioisomers.
Furthermore, the reaction was easily scaled up to a
100 mmol scale, and 12 g of 2a were isolated straightfor-
wardly from the reaction of 1a. Interestingly, when 1l was
Figure 1. In situ ATR-FTIR spectra recorded during reaction of 1a and
Ph2SiH2 with TBAF as catalyst at room temperature.
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Communications
Figure 2. In situ UV/Vis spectra recorded during reaction of 1a and
Ph2SiH2 with TBAF as catalyst at room temperature.
showed a slight shift from 1719 to 1715 cmÀ1, while the nsC O
band at 1773 cmÀ1 and the dsCH3 band at 1380 cmÀ1 dis-
appeared completely. In turn, the dsCH3 band at 1394 cmÀ1
became dominant.
=
Scheme 3. Proposed reaction mechanism for the monoreduction of
imides.
The UV/Vis spectrum of the reactant 1a in THF did not
change after the addition of TBAF, and was characterized by
bands at 265, 302, and 324 nm; these bands result from the
conjugated interaction of the two carbonyl chromophores
with the electron pair of the nitrogen atom (Figure 2). The
addition of one equivalent of Ph2SiH2 did not cause significant
changes within the first 5 minutes of the reaction. However,
after 10 minutes the band at 324 nm vanished, thus indicating
the reduction of one carbonyl group. Simultaneously, a new
band at 370 nm started to appear, which resulted from
product 2a as confirmed by a separate measurement of the
UV/Vis spectrum of the product in THF.
species to transfer the second hydride atom and to form
À À
Ph2HSi O SiHPh2. As can be seen from the UV/Vis spectra
in Figure 2, an additional band appeared at 536 nm and
increased in intensity as the amount of product increased. The
appearance of absorption bands at such high wavelengths
points to the formation of small amounts of molecules with
highly conjugated chromophores as side products.
The proposed mechanism, which is based on the in situ
spectroscopic measurements, is illustrated in Scheme 3. This
mechanistic proposal is also supported by the reaction of 1a
with Ph2SiD2, the deuterium atoms of which were incorpo-
rated in the product 2a (Scheme 3).
The results of the in situ measurements suggest that, in the
first step of the catalytic cycle, the fluoride ion is coordinated
À
to the silicon atom of Ph2SiH2 (band shift of nSi Ph as
Fluoride-activated silanes are clearly unable to reduce the
corresponding isoindolinones further to the corresponding
isoindolines. However, by simply adding an Fe3(CO)12 catalyst
system in the presence of PMHS,[19] complete reduction took
place. By using this dual catalyst system, N-benzylisoindoline
(3e) was obtained from N-benzylphthalimide (1e) in 55%
yield of isolated product without optimization (Scheme 4).
Selective reduction of one or both carbonyl groups of the
imide is possible by simple variation of the catalyst system.
Finally, we performed some reduction reactions of other
imide derivatives. Because of the chemoselective transforma-
tion of 1l shown in Scheme 2, we focused our efforts on
imidazolidine-2,4-diones (4; Scheme 5). By using the above-
mentioned optimized conditions, smooth reductions occurred
detected by ATR-FTIR) to form a reactive pentacoordinated
silicon species A (Scheme 3), which immediately attacks the
carbonyl group of 1a to give a hexacoordinated silicon species
within which the hydride transfer can take place.[18] Con-
sumption of the respective bands in the ATR-FTIR as well as
in the UV/Vis spectra confirmed this assumption. Changes of
the coordination sphere of silicon are additionally indicated
À
by the splitting of the nSi Ph band.
During the course of the reaction, the formation of
product 2a is observed by an increase in the intensity of the
respective bands in the ATR-FTIR (1700 cmÀ1) as well as in
the UV/Vis (370 nm) spectra. The addition of a second
equivalent of Ph2SiH2 after 55 minutes caused a distinct
increase in the amount of product formed (see Figure S2 in
the Supporting Information). As shown in the ATR-FTIR
À1
=
spectra, the intensities of the nC O band at 1700 cm
(product 2a) and the nSi Ph band at 1126/1117 cmÀ1
increased simultaneously (Figure 1). Considering the fact
À
À À
À À
that both Si O Si and Si O Ph vibrations are observed in
this region,[17] the latter effect is obviously caused by the
À À
formation of Ph2HSi O SiHPh2. This result implies the
interaction of another activated pentacoordinated silicon
Scheme 4. One-pot reduction of N-benzylphthalimide to N-benzylisoin-
doline.
9182
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Angew. Chem. Int. Ed. 2011, 50, 9180 –9184

683 (s), 669 (w), 587 (s), 528 (s), 491 (s), 417 ppm (s); MS (EI): m/z
(rel. int.) 147. HRMS (EI, m/z) calculated for C9H9ON, 148.07569;
found 148.07568.
Received: June 19, 2011
Published online: August 24, 2011
Keywords: amides · amines · fluorides ·
.
homogeneous catalysis · reduction
Scheme 5. Fluoride-catalyzed reduction of imidazolidine-2,4-diones 4.
Yields of isolated products 5 are given in brackets.
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at 658C. Surprisingly, the obtained products were not the
simple deoxygenated compounds; instead formation of new
double bonds in place of the amide bonds occurred (5;
Scheme 5). To the best of our knowledge, this kind of
transformation has not been observed previously for imida-
zolidinone derivatives. The mechanism shown in Scheme 3
can well explain the formation of the double bond in case of
imidazolinone derivatives. Because of the presence of hydro-
gen atoms in a-position to the carbonyl group, elimination of
water and formation of a double bond from species B is
preferred; the absence of a hydrogen atom on the a-carbon
atom in phthalimide derivatives leads to the selective mono-
reduction of imides. Interestingly, the reaction with secondary
amides of imidazolidine-2,4-dione derivatives did not work at
all.
In summary, we have developed novel chemoselective
reduction reactions of phthalimides and imidazolidine-2,4-
diones. By using low-cost polymethylhydrosiloxane in the
presence of readily available fluoride ions as catalyst, good to
excellent yields are obtained for different imides. Notable
features of our novel protocols are the chemoselectivity,
operational simplicity, and safe and mild reaction conditions.
In situ spectroscopic investigations allowed for a concise
mechanistic proposal. In addition, by combining fluoride- and
iron-catalyzed hydrosilylations, full reduction of phthalimides
to isoindolines is also possible.
Experimental Section
General procedure for the reduction of imides: A dried 10 mL
Schlenk tube containing a stirrer bar was charged with TBAF (1m
solution in THF, 50 mL, 5 mol%) and the imide (1 mmol). Dry THF
(3 mL) and PMHS (300 mL, 5 mmol) were added sequentially after
purging the Schlenk tube with argon. The mixture was stirred at room
temperature and monitored by thin-layer chromatography. After
complete disappearance of the substrates, the reaction mixture was
filtered through Celite and washed with ethyl acetate. The combined
fractions were concentrated under reduced pressure. The residue was
then purified by column chromatography on silica gel by using a
mixture of ethyl acetate and hexane as the eluent.
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