Synthesis and catalytic activity of fluorous chiral primary amine-thioureas

Article abstract of DOI:10.1039/c3nj00807j

Three enantiopure fluorous thioureas featuring a free -NH₂ group were synthesized by direct addition of aromatic isothiocyanates bearing a single n-C₈F₁₇ substituent in the ortho, meta and para position, respectively, to e

Full text of DOI:10.1039/c3nj00807j

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Synthesis and catalytic activity of fluorous chiral
primary amine-thioureas†
Simonetta Orlandi,a Gianluca Pozzi,*a Mauro Ghisettib and Maurizio Benaglia*b
Cite this: NewJ.Chem., 2013,
37, 4140
Three enantiopure fluorous thioureas featuring a free –NH2 group were synthesized by direct addition
of aromatic isothiocyanates bearing a single n-C8F17 substituent in the ortho, meta and para position,
respectively, to enantiopure (1R,2R)-1,2-diaminocyclohexane. The catalytic behavior of these bifunctional
molecules was assessed in representative Michael-type reactions. The three fluorous thioureas per-
formed similarly in all the reactions tested, thus showing that the position of the fluorous ponytail does
not have a major influence on the catalytic behavior of this class of compounds. In particular, excellent
enantioselectivities (up to 99% ee) and yields (up to 98%) were obtained for the addition of aliphatic
aldehydes to maleimides to give a-substituted succinimides. Recyclability of these primary amine-based
thioureas was found to be limited by the concurrent formation of imine-derivatives during the catalytic
process, leading to a structural modification of the organic catalyst.
Received (in Montpellier, France)
22nd July 2013,
Accepted 24th September 2013
DOI: 10.1039/c3nj00807j
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catalytic systems, where a thiourea group, acting as a Brønsted
acid, is coupled to a Lewis base in order to realize a greater
Introduction
The physico-chemical properties of organic compounds are greatly
affected by the introduction of medium-sized perfluoroalkyl chains
(C6–C10) in their molecular structure. The most apparent charac-
teristics of the resulting hybrids (fluorous compounds) are their
temperature-dependent miscibility with organic phases, hydro-
phobicity, and increased affinity for perfluorocarbons. Such
unusual properties and the ease of separation from standard
organic compounds have found practical application in catalysis.1
Thus, a broad spectrum of significant enantioselective catalytic
reactions has been accomplished in the presence of transition
metal complexes of chiral fluorous ligands.2 Fewer examples of
fluorous chiral organocatalysts have been reported, but the results
obtained so far are highly promising.2,3 Indeed, enhanced reactiv-
ities and selectivities with respect to similar non-fluorous organo-
catalysts have been observed in some instances, which can be
ascribed to both electronic and solvation effects.4 Further applica-
tions of the fluorous approach in asymmetric organocatalysis can
be envisaged, in particular in the field of reactions favored by dual
activation of the electrophilic and nucleophilic components.5
Several C–C bond formation reactions that fall into this
category are catalyzed enantioselectively by bifunctional chiral
degree of stereocontrol over the reaction by a synergic coopera-
tive effect of the two catalysts.6 The key element is the simulta-
neous presence of both hydrogen bond donor and acceptor
sites to reversibly control the non-covalent interactions. The
catalyst activity can be successfully enhanced by attaching an
electron-poor moiety such as the 3,5-bis(trifluoromethyl)phenyl
group to one end of the thiourea motif. Takemoto’s catalyst
(Fig. 1) is an outstanding, classical example of chiral bifunc-
tional thiourea of this kind.7 More recently, chiral thioureas
structurally related to Takemoto’s catalyst, featuring a free
primary amine group attached to the chiral scaffold instead
of –NMe2, have been disclosed.8 They proved to be superior
catalysts in some selected applications such as the challenging
asymmetric Michael reaction of maleimides with aldehydes.9
In common practice Takemoto’s catalyst and its analogs are
discarded after use because of difficulties related to their recovery
and recycling. Immobilization onto soluble and insoluble polymer
supports aimed at eliminating this drawback has been attempted
with variable results.7b While immobilization onto different
insoluble resins led to catalysts of poor chemical and stereo-
chemical efficiency, some better results were obtained by using
poly(ethylenglycol)-supported10a and poly(methylhydro-siloxane)-
immobilized catalysts,10b where good enantioselectivities but
modest chemical activity and recyclability were obtained. Given
the positive impact of the 3,5-bis(trifluoromethyl) substitution
pattern, fluorous functionalization should represent an obvious
alternative. Nevertheless, only two examples of fluorous chiral
bifunctional thioureas have been reported in the literature,11 and
a CNR-Istituto di Scienze e Tecnologie Molecolari, via Golgi 19, 20133 Milano, Italy.
E-mail: gianluca.pozzi@istm.cnr.it; Fax: +39 02 50314159
b
`
Dipartimento di Chimica, Universita degli Studi di Milano, via Golgi 19,
20133 Milano, Italy. E-mail: maurizio.benaglia@unimi.it; Fax: +39 02 50314159
† Electronic supplementary information (ESI) available: NMR spectra of fluorous
products, HPLC details of Michael reactions products. See DOI: 10.1039/
c3nj00807j
c
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Scheme 1 Synthesis of fluorous thioureas 1–3.
Fig. 1 Takemoto’s catalyst and chiral bifunctional thioureas 1–4 examined in
this work.
trans-1,2-diaminocyclohexane-substituted thioureas with a free
the effects of the substitution of two trifluoromethyl groups with –NH2 group. However, this method may lead to the formation
one (or more) medium-sized fluorous substituents on the cata- of substantial amounts of the corresponding bis-thiourea deri-
lytic activity and selectivity of this class of organocatalysts still vatives. Lengthier procedures consisting of the selective protec-
need to be clarified. To gain insight into this issue, three primary tion of one of –NH2 groups of the diamine followed by reaction
amine-substituted fluorous thioureas 1–3 that differ only for the with the aromatic isothiocyanate and finally deprotection of
position of the fluorous domain (Fig. 1) have been synthesized, the amino group have been devised in order to overcome this
and their behavior as catalysts for three representative Michael- limitation.12 Alternatively, trans-1,2-diaminocyclohexane-substituted
type reactions, namely the addition of aldehydes to N-substituted thioureas can be prepared by condensation of anilines with
maleimides, the addition of nitromethane to 4-phenylbut-3-en- commercially available phenyl chlorothioformate and in situ
2-one and the addition of acetophenone to trans-b-nitrostyrene reaction of the intermediate O-phenyl arylthiocarbamate with
has been investigated. Comparison with catalyst 48d provided the unprotected diamine.13 This last method was previously
useful hints that will find application in the design of more used for the preparation of the para-substituted fluorous
effective fluorous chiral bifunctional thioureas.
thiourea 1.11b After testing all these strategies for the preparation
of 1 we found that the direct addition of fluorous isothiocyanate
8 to enantiopure (R,R)-trans-1,2-diaminocyclohexane afforded
reasonable results (65% yield with respect to 8) without the
Results and discussion
Fluorous chiral thioureas 1–3 bearing a free –NH2 group were complications associated with the selective protection–deprotection
easily obtained starting from fluorous anilines 5–7, respec- of (R,R)-trans-1,2-diaminocyclohexane or the cumbersome
tively, as shown in Scheme 1.
separation from by-products that was required in the case of
In the first step, a Cu-catalyzed coupling reaction between the O-phenyl arylthiocarbamate pathway. Therefore, the direct
n-C8F17I and the proper iodoaniline allowed the introduction of addition method was chosen for the synthesis of the ortho-
the typical perfluoroalkyl substituent n-C8F17 in the desired and meta-substituted fluorous thioureas 3 (59% yield) and 2
position of the aryl ring. Fluorous anilines 5–7 were thus (60% yield) from the corresponding fluorous isothiocyanates.
selectively obtained in good yields (68–78%) and reacted with Reaction yields were basically independent of the position of
thiophosgene under standard conditions to give aromatic the fluorous substituent and were also similar to the yield
fluorous isothiocyanates 8–10. The presence of an electron- (65%) achieved in the analogous preparation of the 3,5-bis-
withdrawing fluorous substituent in the ortho position of (trifluoromethyl)phenyl-substituted thiourea 4 (Fig. 1).
aniline 7 was found to be detrimental for this reaction, and
The catalytic behavior of fluorous thioureas 1–3 was next
the desired isothiocyanate 10 was isolated in a modest yield investigated in Michael-type reactions, such as the stereo-
(51%). In contrast, para- and meta-substitution had little effect selective addition of aldehydes to N-substituted maleimides
on the outcome of the reaction and anilines 5 and 6 gave the to give a-substituted succinimides (Scheme 2). Indeed, enantio-
corresponding isothiocyanates 8 and 9 in 92% and 83% yields, pure primary amine-thioureas structurally related to Takemoto’s
respectively.
catalyst have been already employed with success in this reac-
The direct addition of aromatic isothiocyanates to trans-1, tion, and the dual activation mode of the reaction partners has
2-diaminocyclohexane in excess is the simplest way to obtain been adequately enlightened.9,11b
c
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a solid precipitate. Nevertheless, the reaction rate was not
affected and final product, completely soluble in BTF, was
readily isolated in 90% yield and excellent ee (entry 7). Finally,
protic solvents such as CH3OH (entry 8) proved to be unsuitable,
affording the desired chiral a-substituted succinimide 16 with
low enantioselectivity, together with several by-products. The
effect of the catalyst loading was also evaluated (entries 2–4).
Lowering the catalyst loading did not affect the enantioselec-
tivities, although prolonged reaction times were required. In
particular, in the presence of 1 mol% of the catalyst with
respect to 11 the reaction did not go to completion even after
4 days (entry 4), but the product was still obtained in good yield
and excellent enantioselectivity.
Scheme 2 Addition of aldehydes to N-substituted maleimides.
The catalytic activity of thioureas 1–3 was then compared
using the optimal reaction conditions established in the first
set of experiments. As shown in Table 2, they behaved quite
similarly, both in terms of activity and enantioselectivity. Only
the reaction catalyzed by the ortho-substituted catalyst 3 (entry 4)
proceeded with a very minor reduced enantioselectivity, indicat-
ing a negligible interference of the fluorous substituent closer to
the active catalytic sites. In any case, no relevant difference was
observed in comparison to the standard thiourea catalyst 4
(entry 1). In all these experiments the succinimide product 16
was obtained in R configuration as expected from re-face
attack,9 as indication that fluorous catalysts 1–3 work according
the well-established dual activation mode of 4.
Table 1 Optimization of reaction conditions for the asymmetric addition of
isobutyraldehyde to N-phenylmaleimide 11 catalyzed by 1
Cat. loadinga Solvent/vol. Time Conv.b Yieldc eed
Entry (mol%)
(mL)
(h)
(%)
(%)
(%)
1
2
3
4
5
6
7
8
10
10
5
DCM/10
DCM/1.5
DCM/1.5
DCM/1.5
THF/1.5
PhCH3/1.5
BTF/1.5
24
5
12
96
24
24
5
100
100
100
92
100
98
88
94
93
80
91
89
90
33
>99
>99
>99
>99
95
97
99
1
10
10
10
10
96
61
MeOH/1.5
1.5
11
a
Loading with respect to the limiting reagent. Reaction conditions:
b
The Michael addition of isobutyraldehyde to N-aryl-maleimides
12–14 (Scheme 2) bearing typical electron-donating and electron-
withdrawing substituents was run in the presence of the para-
substituted fluorous thiourea 1 under the established conditions
11 = 0.5 mmol, isobutyraldeyhde = 1 mmol, 25 1C. Evaluated by
1H-NMR analysis of the crude reaction mixture. Isolated yield. d Evaluated
c
using chiral HPLC analysis of the isolated product.
The enantioselective conjugate addition of isobutyraldehyde (Table 3, entries 1–3).
(R1 = R2 = Me) to N-phenylmaleimide 11 (X = Ph) run at 25 1C in
The catalyst tolerated well the substrate variations, as well
the presence of the para-substituted fluorous thiourea 1 was as N-benzyl substitution on the Michael acceptor partner 15
used as a model case in order to establish the optimal reaction (entry 4). Reaction yields somewhat varied with the N-substitution
solvent, reagent concentration and catalyst loading. Results are pattern, while the remarkable enantioselectivities observed were
reported in Table 1.
almost independent of these parameters. The conjugate addition
The solubility of fluorous thioureas 1–3 in most organic of cyclohexanecarboxaldehyde (entry 5) to 11 catalyzed by the
solvents (including DCM, Et2O and AcOEt) is rather low, so the para-substituted fluorous thiourea proceeded smoothly in the
reaction was first run in the presence of relatively high amounts presence of benzoic acid as the co-activating agent, in agreement
of solvent (entry 1) in order to completely dissolve the catalyst with what was reported for non-fluorous primary amine-thiourea
before addition of the reagents. Complete conversion of the catalysts based on a different chiral scaffold.9c In the case of
limiting reagent 11 was attained after 24 h, and the succinimide linear aldehydes such as n-propanal (entry 6), the introduction of
product 16 was isolated in good yield and excellent ee. Similar the fluorous tag in the catalyst structure did not modify the
results, but a faster reaction rate, were obtained when the outcome of the reaction with respect to what observed using
catalyst was suspended in a reduced volume of DCM (entry 2).
Actually, upon formation of the putative adduct between the
Table
2
Comparison of the new catalysts in the asymmetric addition of
catalyst and the reagents,9a–d the reaction mixture became
homogeneous: this also happened in other aprotic solvents
such as toluene and THF (entries 5 and 6). In those cases the
reaction was slower than in DCM and slightly reduced
enantioselectivities were observed. Quite surprisingly, trifluoro-
methylbenzene (BTF), a liquid containing both a fluorophilic
(CF3) and an organophilic (C6H5) group, exhibited a low solvent
power for the new fluorous catalysts. When the reaction was
run in such a solvent, complete solubilization of the solid
isobutyraldehyde to N-phenylmaleimide 11
Entry
Catalysta
Time (h)
Conv.b (%)
Yieldc (%)
eed (%)
1
2
3
4
4
1
2
3
5
5
5
5
100
100
100
100
91
93
93
94
>99
>99
99
98
a
Reaction conditions: 11 = 0.5 mmol, isobutyraldeyhde = 1 mmol,
catalyst = 10 mol% with respect to the limiting reagent, 25 1C in CH2Cl2.
b
c
Evaluated by 1H-NMR analysis of the crude reaction mixture. Isolated
d
catalyst took longer and it was soon followed by formation of yield. Evaluated by chiral HPLC analysis of the isolated product.
c
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Table 3 Catalytic activity of 1 in the asymmetric addition of aldehydes to
N-substituted maleimides
Entry
Maleimidea
Product
Timeb (h)
Yieldc (%)
eed (%)
1
2
3
4
5
6
12
13
14
15
11e
11
17
18
19
20
21
22
16
22
17
8
18
8
85
90
98
95
96
69
99
99
99
>99
>99
99/99 f, g
Fig. 2 Catalytically inactive imine detected in the solid material isolated in
a
recycling experiments.
Reaction conditions: N-substituted maleimide = 0.5 mmol, aldeyhde =
1 mmol, catalyst = 1 (10 mol% with respect to the limiting reagent),
25 1C in CH2Cl2. See Scheme 2 for molecular structures of N-substituted
b
maleimides and products. Quantitative conversions were obtained
the absence of a fluorous catalyst in the organic layer, while the
amount of recovered solid corresponded to the weight of the initial
catalyst. The fluorous approach is thus effective for the quick
separation of the catalyst from the organic products. However,
a second reaction run performed over 24 h with the recovered
solid material afforded 12 with 99% ee, but only in 36% yield.
This loss of catalytic activity was unexpected in light of the
results reported in the literature, but it turned out to be fully
justified by mass analysis of the solid recovered from the first
reaction run. Indeed, ESI-MS in the positive mode showed three
peaks at m/z = 668 (100%), 895 (75%) and 917 (65%). The first
one corresponds to 1 ([M + H]+), whereas the other two peaks
can be attributed to the imine derivative 23 (Fig. 2) deriving
from the addition of the reaction product 16 to the primary
amine group of the catalyst. This adduct is unlikely to be fit for
dual activation of the reaction partners.
The behavior of fluorous thioureas 1–3 was further investi-
gated in two other typical Michael-type reactions where the use
of catalyst 4 was claimed to afford chiral products with high
enantioselectivities.
The addition of nitromethane to 4-phenylbut-3-en-2-one 24
(Scheme 3) was performed under conditions identical to those
reported by Duan, Wuan and coworkers for catalyst 4, that
include high catalyst loading (15 mol% with respect to 24) and
the use of CHCl3 as solvent to provide high ee associated with
reasonable yields.8k The results obtained are summarized in
Table 4.
after the indicated reaction times, as indicated by 1H-NMR analysis of
c
d
the crude reaction mixtures. Isolated yield. Evaluated by chiral
HPLC analysis of the isolated product. Reaction run in the presence
e
of PhCOOH (10 mol% loading with respect to 11) as co-catalyst.
f
Diastereomeric ratio = 1.4/1, evaluated by 1H NMR analysis of the
g
crude reaction product. ee of the major diastereomer/ee of the minor
diastereomer.
other primary amine-substituted thioureas. Indeed, compared to
branched aldehydes the reaction yield was lower and both the
a-substituted succinimide diastereoisomers were formed with
low diastereoselectivity, but excellent enantioselectivities.9
Aside from an enhanced catalytic activity that can be ascribed
to the optimized reaction conditions we employed, the results
summarized in Table 3 confirm the findings of Miura and
coworkers who recently reported the use of 1 as a catalyst in the
addition of a,a-disubstituted aliphatic aldehydes to N-substituted
maleimides.11b
The presence of a single, linear perfluoroalkyl substituent
and the relatively low fluorine content (F = 48.4%) qualify the
three primary amine-substituted thioureas 1–3 as typical light
fluorous molecules.14 According to Miura and coworkers, this
should offer the opportunity to achieve their easy separation
from reaction mixtures by selective solvent extraction.11b
We tested their procedure in the case of the addition of iso-
butyraldehyde to 11 catalyzed by 1. After completion of the
reaction performed under the optimal conditions summarized
in Table 2, both DCM and the excess of aldehyde were removed
by evaporation under reduced pressure. The residue was dis-
solved in a hexane–CHCl3 1/1 (vol/vol) mixture upon which the
undissolved fluorous material and the liquid phase containing
the organic reaction components were separated by simple
filtration. As judged from the amount of solid material recovered,
the catalyst loss (25%) was not negligible, contrary to what was
reported in the literature. Blank tests run on mixtures of product
12 and catalyst 1 confirmed that the solvent mixture hexane–
CHCl3 1/1 was able to dissolve not only 12, but also a sensible
amount of catalyst. On the other hand, a sharp reduction of the
amount of CHCl3 in the extracting solvent mixture led to solid
residues enriched in 12. The use of hexane–CHCl3 7/3 was found
to be the best compromise, leading to the complete dissolution of
12 without affecting the catalyst. The model reaction was thus run
again with fresh 1 under Miura’s conditions (high dilution, time =
24 h) and the recovery procedure was repeated using the optimal
solvent ratio. This time 19F-NMR and 1H-NMR analysis confirmed
In our hands, catalyst 4 behaved exactly as described in the
literature, showing rather slow reaction rates, but almost
complete enantioselectivity. Also the fluorous thioureas 1–3
were found to promote the enantioselective formation of the
addition product 25 in moderate yields and very good to
Scheme 3 Michael additions catalyzed by thioureas 1–4.
c
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Table 4 Comparison of the new catalysts in the asymmetric Michael addition of
nitromethane to 4-phenylbut-3-en-2-one 24
Subtle differences in the binding interactions of the ortho-
protons with Lewis bases,15 as well as modifications of the
self-assembly mode7g might be involved, in analogy with metal-
based chiral fluorous catalysts.16
Entry
Catalysta
Time (h)
Yieldb (%)
eec (%)
1
2
3
4
1
2
3
4
120
120
120
120
43
44
55
59 (57)
>99
95
98
Conclusions
99 (97)
a
The design of efficient fluorous chiral catalysts requires a
careful evaluation of the location and number of perfluoroalkyl
substituents present in the molecular structure.2 On the other
hand, the introduction of two CF3 groups in the 3,5-positions of
the aryl moiety has been recognized as a key-element for
ensuring high catalytic efficiency and selectivity of thiourea
derivatives.7,15 Other electron-withdrawing substituents, or the
presence of just one CF3 group in whatever position of the aryl
ring, are generally less effective. As a first step of a project
aimed at the development of fluorous chiral bifunctional
thioureas, it was thus crucial to verify whether this trend was
also valid when medium-sized CnF2n+1 substituents were
involved. The initial data here presented demonstrate that all
the positions of the aryl rings can be fruitfully used for the
introduction of fluorous substituents. This finding is encoura-
ging and will find applications in the design of more elaborated
fluorous chiral bifunctional thioureas. Although easily separ-
able from organic products, the fluorous thioureas here inves-
tigated cannot be efficiently reused due to the unfavorable role
of the free –NH2 group present in their structure. This is
another useful hint for the further developments in the design
of recyclable fluorous chiral thioureas. On these premises we
are currently investigating the synthesis of fluorous Takemoto’s
catalysts bearing multiple perfluoroalkyl chains on the aryl
ring, and also on the amino group of the chiral scaffold. These
features may further increase the ease of separation from
organic compounds and lead to fully recyclable chiral thiourea-
based organocatalysts.
Reaction conditions: 24 = 0.5 mmol, CH3NO2 = 1 mmol, catalyst =
b
15 mol% with respect to the limiting reagent, 25 1C in CHCl3. Isolated
yield (values in parentheses = ref. 8m). Evaluated by chiral HPLC
analysis of the isolated product. In all cases (R)-25 was obtained as
the major product.
c
excellent ee. Catalyst 3 (entry 3) featuring the ortho-fluorous
substituent performed analogously to 4, whereas both 1 (entry 1)
and 2 (entry 3) proved to be only slightly less active than the
other two catalysts.
Compared to other Michael-type reactions, the enantio-
selective addition of ketones to trans-b-nitrostyrene derivatives
catalyzed by chiral primary amine-based thioureas often pre-
sents slow reactions rates and low selectivities. A few thioureas
where the amine moiety is different from trans-1,2-diamino-
cyclohexane have been specifically developed to overcome these
drawbacks.8b,c,j However, in a recent paper the use of 4 com-
bined to p-nitrobenzoic acid as a co-catalyst was claimed to
provide the addition products with excellent yields and ees.8e
Therefore we decided to test thioureas 1–3 as catalysts for the
addition of acetophenone to trans-b-nitrostyrene 26 (Scheme 3)
under the reported optimal conditions. To our surprise, the use
of the three fluorous catalysts afforded product 27 in very low
yields (o10%). This was also the case when the reaction was
repeated in the presence of catalyst 4, which gave product 27 in
9% yield after 72 h, far from the impressive 92% yield reported
in the literature.
More reliable results (Table 5) were obtained when the
reaction was run according to a protocol devised by Huang
and Jacobsen.8b In particular, by using PhCOOH as a co-catalyst
it was possible to evidence the similar catalytic behavior of 4
and its fluorous analogs 1–3, and also to confirm that the
presence of fluorous substituents on the aryl ring has detect-
able positive effects on the enantioselectivity. We have no clear
explanation yet either for this trend or for the small variations
observed within this series of catalysts in the model reactions.
Experimental
General
Commercially available reagents and anhydrous solvents
(DMSO, DCM, THF) were used as received. Thiourea 4 was
prepared as described in the literature.8d Reactions were mon-
itored by TLC on silica gel 60 F254. Column chromatography
was carried out on silica gel SI 60 (Merck, Germany), 0.063–
0.200 mm (normal) or 0.040–0.063 mm (flash). Melting points
Table 5 Comparison of the new catalysts in the asymmetric Michael addition of were determined using a capillary melting point apparatus
Bu¨chi SMP-20. 1H NMR (300 MHz), 13C NMR (75 MHz) and
acetophenone to trans-b-nitrostyrene 26
19F NMR (282 MHz) spectra were recorded on a Bruker AC 300
Entry
Catalysta
Time (h)
Yieldb (%)
eec (%)
spectrometer. 1H NMR (400 MHz) and 13C NMR APT (100 MHz)
spectra were recorded on a Bruker Avance 400 spectrometer.
Chemical shifts were calibrated using tetramethylsilane for
1H (d = 0), CFCl3 for 19F, and residual solvent peaks for 13C.
Electrospray ionization (ESI) mass spectra were obtained using
a Finnigan LCQ Advantage Thermo spectrometer. HPLC ana-
lyses were performed on an Agilent 1100 instrument. Chiral
stationary phases and elution conditions are given in the ESI.†
1
2
3
4
1
2
3
4
48
48
48
48
15
13
18
18
85
89
91
82
a
Reaction conditions: 26 = 0.5 mmol, acetophenone = 2.5 mmol,
PhCOOH = 0.05 mmol, catalyst = 10 mol% with respect to the limiting
b
c
reagent, 25 1C in toluene. Isolated yield. Evaluated by chiral HPLC
analysis of the isolated product. In all cases (S)-27 was obtained as the
major product.
c
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Optical rotatory power of chiral compounds was measured at triethylamine (0.85 mL, 6.1 mmol) and anhydrous THF (40 mL).
25 1C with a Perkin Elmer 241 polarimeter at l = 589 nm.
The stirred solution was cooled to 0 1C and thiophosgene
(0.17 mL, 2.2 mmol) was then carefully added via syringe over
10 min. The reaction was allowed to warm to room temperature
Synthesis of fluorous anilines 5–7: general procedure
A flame-dried Schlenk tube was charged with the proper and stirred for further 12 h. After this water (5 mL) was added.
iodoaniline (1.64 g, 7.5 mmol), copper powder (1.43 g, 22 mmol) The layers were separated and the aqueous layer was extracted
and anhydrous DMSO (8 mL), evacuated, and backfilled with with Et2O (2 Â 15 mL). The combined organic layers were
nitrogen. The vigorously stirred suspension was heated to 125 1C, washed with water, brine, and dried over MgSO4. The residue
then n-perfluorooctyl iodide (2.22 mL, 8.3 mmol) was added using obtained after concentration in vacuo was purified by column
a syringe over 5 min. After 12 h the reaction mixture was cooled to chromatography (SiO2, hexane) to give the corresponding fluorous
RT, quenched with water (10 mL), diluted with Et2O (20 mL) and isothiocyanate.
filtered through a Celites plug. The solid was discarded and the
liquid layers were separated. The aqueous phase was extracted
Isothiocyanato-4-(n-perfluorooctyl)benzene (8)
with Et2O (2 Â 15 mL). The combined organic layers were washed From fluorous aniline 5. White solid (1.02 g, 92.2% yield), m.p.
with brine and dried over anhydrous MgSO4. The residue 54–55 1C. 1H NMR (300 MHz, CDCl3): d = 7.59 (d, J = 8.5 Hz, 1H),
obtained after concentration in vacuo was purified by column 7.34 (d, J = 8.5 Hz, 1H) ppm. 13C NMR APT (100 MHz, CDCl3):
chromatography (SiO2, hexane–AcOEt 4 : 1) to give the corre- d = 138.7, 135.6, 128.4 (t, JCF = 6.8 Hz), 127.5 (t, JCF = 25 Hz),
sponding fluorous aniline.
125.9, 119–105 (m, C8F17) ppm. 19F NMR (282 MHz, CDCl3):
d = À81.7 (t, J = 10.1 Hz, 3F), À111.7 (t, J = 14.4 Hz, 2F), À122.1
(br s, 2F), À122.8 (m, 6F), À123.6 (br s, 2F), À127.0 (br s, 2F) ppm.
4-(n-Perfluorooctyl)aniline (5)
From 4-iodoaniline. White solid (2.97 g, 77.5% yield), m.p. 41– C15H4F17NS (553.24): calcd C 32.56, H 0.73, N 2.53; found C 32.45,
1
43 1C. H NMR (300 MHz, CDCl3): d = 7.34 (d, J = 8.5 Hz, 2H), H 0.69, N 2.48.
6.71 (d, J = 8.5 Hz, 2H), 3.96 (br s, 2H) ppm. 13C NMR (75 MHz,
Isothiocyanato-3-(n-perfluorooctyl)benzene (9)
CDCl3): d = 149.8, 128.4 (t, JCF = 5.8 Hz), 118.3 (t, JCF = 25 Hz),
120–105 (m, C8F17), 114.4 ppm. 19F NMR (282 MHz, CDCl3): From fluorous aniline 6. White solid (0.91 g, 82.7% yield), m.p.
d = À81.7 (t, J = 10.2 Hz, 3F), À111.7 (t, J = 14.5 Hz, 2F), À122.2 33–34 1C. 1H NMR (300 MHz, CDCl3): d = 7.54–7.48 (m, 2H),
(br s, 2F), À122.9 (br s, 6F), À123.6 (br s, 2F), À127.0 (br s, 2F) ppm. 7.45–7.43 (m, 2H) ppm. 13C NMR APT (100 MHz, CDCl3):
C14H6F17N (511.18): calcd C 32.89, H 1.18, N 2.74; found d = 138.1, 132.5, 130.8 (t, JCF = 24 Hz), 130.1, 129.2, 125.4
C 32.81, H 1.23, N 2.54.
(t, JCF = 6.4 Hz), 124.1, (t, JCF = 6.4 Hz), 119–105 (m, C8F17) ppm.
19F NMR (282 MHz, CDCl3): d = À81.7 (t, J = 9.9 Hz, 3F), À111.8
(t, J = 14.1 Hz, 2F), À122.1 (br s, 2F), À122.6 (m, 6F), À123.6
3-(n-Perfluorooctyl)aniline (6)
From 3-iodoaniline. Waxy off-white solid (3.00 g, 78.3% yield), (br s, 2F), À127.0 (br s, 2F) ppm. C15H4F17NS (553.24): calcd
m.p. 28–30 1C. 1H NMR (300 MHz, CDCl3): d = 7.30–7.22 C 32.56, H 0.73, N 2.53; found C 32.51, H 0.70, N 2.62.
(m, 1H), 6.96 (d, J = 7.8 Hz, 1H), 6.88–6.81 (m, 2H), 3.82 (br s,
2H) ppm. 13C NMR APT (100 MHz, CDCl3): d = 146.6, 130.0
Isothiocyanato-2-(n-perfluorooctyl)benzene (10)
(t, JCF = 24 Hz), 120–105 (m, C8F17), 129.6, 118.2, 116.7 (t, JCF
=
From fluorous aniline 7. Brown thick oil (0.55 g, 50.7% yield).
6.7 Hz), 112.9 (t, JCF = 6.7 Hz) ppm. 19F NMR (282 MHz, CDCl3): 1H NMR (300 MHz, CDCl3): d = 7.60–7.56 (m, 2H), 7.44–7.39
d = À81.7 (t, J = 10.3 Hz, 3F), À111.7 (t, J = 14.2 Hz, 2 F), À122.2 (m, 1H) ppm. 13C NMR APT (100 MHz, CDCl3): d = 136.7, 133.3,
(br s, 2F), À122.8 (br s, 6F), À123.6 (br s, 2F), À127.0 (br s, 2F) ppm. 130.2, 129.2, 129.1 (t, JCF = 7.9 Hz), 126.9, 123.6 (t, JCF = 23 Hz).
C14H6F17N (511.18): calcd C 32.89, H 1.18, N 2.74; found 119–108 (m, C8F17) ppm. 19F NMR (282 MHz, CDCl3): d = À81.8
C 32.76, H 1.21, N, 2.76.
(t, J = 9.9 Hz, 3F), À110.0 (t, J = 14.1 Hz, 2F), À122.2 (m, 4F),
À122.7 (br s, 4F), À123.6 (br s, 2F), À127.0 (br s, 2F) ppm.
2-(n-Perfluorooctyl)aniline (7)
C
15H4F17NS (553.24): calcd C 32.56, H 0.73, N, 2.53; found
From 2-iodoaniline. White solid (2.63 g, 68.5% yield), m.p. 44– C 32.78, H 0.74, N, 2.61.
1
45 1C. H NMR (300 MHz, CDCl3): d = 7.37–7.32 (m, 2H), 6.80
Synthesis of fluorous thioureas 1–3: general procedure
A flame-dried two-neck round bottom flask was charged under
(t, J = 7.6 Hz, 1H), 6.71 (d, J = 8.5 Hz, 1H), 4.21 (br s, 2H) ppm.
13C NMR (75 MHz, CDCl3): d = 146.1, 133.2, 129.1 (t, JCF
=
8.3 Hz), 117.9, 121–105 (m, C8F17, C1, C2) ppm. 19F NMR (282 MHz, nitrogen with (1R,2R)-1,2-diaminocyclohexane (138 mg,
CDCl3): d = À81.7 (t, J = 9.8 Hz, 3F), À109.4 (t, J = 14.8 Hz, 2F), 1.20 mmol) and anhydrous DCM (4 mL). To the stirred solution
À122.3 (m, 8F), À123.6 (br s, 2F), À127.0 (br s, 2F) ppm. cooled to 0 1C the proper fluorous isothiocyanate (553 mg,
C
14H6F17N (511.18): calcd C 32.89, H 1.18, N 2.74; found 1.00 mmol) was then added. The reaction was allowed to warm
C 32.87, H 1.20, N 2.67.
to room temperature and stirred for further 12 h. After this
water the reaction mixture was diluted with DCM (20 ml)
and the resulting solution was washed with water and dried
Synthesis of fluorous isothiocyanates 8–10: general procedure
A flame-dried two-neck round bottom flask was charged under over MgSO4. The solvent was removed under reduced pressure
nitrogen with the proper fluorous aniline (1.02 g, 2.0 mmol), and the residue was purified by column chromatography
c
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(SiO2, DCM–MeOH 95 : 5 to 9 : 1) to give the corresponding (0.5 mmol) and DCM (1.5 mL). The fluorous thiourea catalyst
fluorous thiourea.
(0.05 mmol) was added to the solution and the resulting mixture
was stirred for 10 min prior to the addition of neat aldehyde
(1 mmol), upon which a clear solution was obtained. The progress
of the reaction was followed by TLC. Stirring was continued for
1-[(1R,2R)-2-Aminocyclohexyl]-3-[4-(n-perfluorooctyl)phenyl]-
thiourea (1)
From fluorous isothiocyanate 8. White solid (434 mg, 65.1% the time indicated in Tables 1–3, then the reaction mixture was
yield), m.p. 187–188 1C. [a]2D5 = +67.9 (c = 0.43 in CH2Cl2). evaporated under reduced pressure to eliminate all volatiles. The
1H NMR (400 MHz, CD3OD): d = 7.75 (d, J = 8.6 Hz, 2H), 7.57 residue was directly purified by flash column chromatography
(d, J = 8.6 Hz, 2H), 4.25 (br s, 1H), 2.60 (m, 1H), 2.10 (m, 1H), (SiO2, hexane–AcOEt 2/1) affording the desired a-substituted
1.99 (m, 1H), 1.76 (m, 2H), 1.42–1.20 (m, 4H) ppm. 13C NMR succinimide. Structural characterization by NMR analysis and
APT (100 MHz, CD3OD): d = 184.0, 145.4, 129.3 (t, JCF = 6.5 Hz), determination of the enantiomeric excess by chiral HPLC (ESI†)
125.9 (t, JCF = 23 Hz), 124.7, 58.5, 57.2, 33.2, 32.8, 26.5, analysis were performed on samples of the isolated product.
26.0 ppm. 19F NMR (282 MHz, CDCl3): d = À80.9 (t, J =
Michael addition of nitromethane to 4-phenylbut-3-en-2-one:
typical procedure
9.6 Hz, 3F), À109.6 (t, J = 14.8 Hz, 2F), À120.7 (br s, 2F),
À121.3 (m, 6F), À122.26 (br s, 2F), À125.8 (br s, 2F) ppm. MS
(ESI+): calcd for C21H19F17N3S [M + H]+ 668.1; found 668.2.
A 5 mL jacketed reactor equipped with a stirring bar and
C
21H18F17N3S (667.42): calcd C 37.79, H 2.72, N, 6.30; found
thermostatted at 25 1C with circulating H2O from a Julabo F10
Cryostat was charged with CH3NO2 (1 mmol), a,b-enone
(0.5 mmol) and CHCl3 (1.5 mL). The fluorous thiourea catalyst
(0.075 mmol) was added to the solution and the resulting mixture
was stirred for 5 d. After evaporation under reduced pressure, the
residue was directly purified by flash column chromatography
(SiO2, hexane–AcOEt 4/1) affording 5-nitro-4-(R)-phenylpentan-
2-one. Structural characterization by NMR analysis and determi-
nation of the enantiomeric excess by chiral HPLC analysis (ESI†)
were performed on samples of the isolated product.
C 37.66, H 2.69, N 6.01.
1-[(1R,2R)-2-Aminocyclohexyl]-3-[3-(n-perfluorooctyl)phenyl]-
thiourea (2)
From fluorous isothiocyanate 9. White solid (403 mg, 60.4%
yield), m.p. 151–152 1C. [a]2D5 = +44.3 (c = 0.42 in CH2Cl2).
1H NMR (400 MHz, CD3OD): d = 7.93 (s, 1H), 7.74 (d, J =
7.9 Hz, 1H), 7.54 (t, J = 7.9 Hz, 1H), 7.40 (d, J = 7.9 Hz, 1H), 4.42
(br s, 1H), 2.80 (m, 1H), 2.11–2.03 (m, 2H), 1.80 (m, 2H), 1.45–
1.30 (m, 4H) ppm. 13C NMR APT (100 MHz, CD3OD): d = 183.5,
141.5, 130.4, 130.0 (t, JCF = 24 Hz), 128.5 (t, JCF = 4.5 Hz), 123.9
(t, JCF = 6.5 Hz), 123.2, 58.5, 56.3 32.7, 32.5, 25.8, 25.3 ppm.
19F NMR (282 MHz, CD3OD): d = À79.3 (t, J = 11.3 Hz, 3F),
À108.5 (t, J = 14.1 Hz, 2F), À119.2 (br s, 2F), À119.8 (m, 6F),
À120.7 (br s, 2F), À124.2 (br s, 2F) ppm. MS (ESI+): calcd for
Michael addition of acetophenone to trans-b-nitrostyrene:
typical procedure
A 2 mL jacketed reactor provided with a stirring bar and thermo-
stated at 25 1C with circulating H2O from a Julabo F10 Cryostat was
charged with b-nitrostyrene (0.5 mmol), PhCOOH (0.05 mmol) and a
fluorous thiourea catalyst (0.05 mmol). Toluene (25 mL) was added
under nitrogen, followed by the addition of acetophenone
(2.5 mmol). The resulting mixture was stirred for 48 h. After
evaporation under reduced pressure, the residue was directly pur-
ified by flash column chromatography on SiO2 then it was applied to
silica gel and the product isolated by flash column chromatography
(SiO2, hexane–Et2O 2/1). Structural characterization by NMR analysis
and determination of the enantiomeric excess by chiral HPLC
analysis (ESI†) were performed on samples of the isolated product.
C
21H19F17N3S [M + H]+ 668.1; found 668.2. C21H18F17N3S
(667.42): calcd C 37.79, H 2.72, N, 6.30; found C 37.70,
H 2.62, N 6.11.
1-[(1R,2R)-2-Aminocyclohexyl]-3-[2-(n-perfluorooctyl)phenyl]-
thiourea (3)
From fluorous isothiocyanate 10. White solid (394 mg, 59.1%
yield), m.p. 62–64 1C. [a]2D5 = +42.0 (c = 0.42 in CH2Cl2). 1H NMR
(400 MHz, CD3OD): d = 7.71–7.64 (m, 3H), 7.51–7.47 (m, 1H),
4.27 (br s, 1H), 2.61 (m, 1H), 2.01 (m, 2H), 1.76 (m, 2H), 1.40–
1.21 (m, 4H) ppm. 13C NMR APT (100 MHz, CD3OD): d = 184.7,
139.0, 133.9, 133.5, 129.7 (t, JCF = 8.5 Hz), 128.3, 61.4, 56.0, 34.6,
32.5, 26.0, 25.8 ppm. 19F NMR (282 MHz, CDCl3): d = À81.2
(t, J = 9.6 Hz, 3F), À107.3 (t, J = 13.6 Hz, 2F), À121.2 to À122.2
(m, 8F), À123.1 (br s, 2F), À126.5 (br s, 2F) ppm. MS (ESI+):
Acknowledgements
MB thanks MIUR (Rome) within the national project ‘‘Nuovi metodi
catalitici stereoselettivi e sintesi stereoselettiva di molecole funzio-
nali’’, and the European COST Action CM0905-Organocatalysis.
calcd for
21H18F17N3S (667.42): calcd C 37.79, H 2.72, N, 6.30; found
C 37.51, H 2.81, N, 6.14.
C21H19F17N3S [M +
H]+ 668.1; found 668.2.
C
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A 5 mL jacketed reactor equipped with a stirring bar and
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2 G. Pozzi, in Enantioselective Homogeneous Supported Catalysis,
ˇ
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c
4146 New J. Chem., 2013, 37, 4140--4147
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