Chiral sulfur derivatives in the allylation of acyl hydrazones: C 2-symmetric bis-sulfinamides as enhanced chiral organic promoters.

Article abstract of DOI:10.1039/c0ob00078g

Monosulfinamides and C₂-symmetric bis-sulfinamides are convenient neutral chiral promoters in the allylation of acyl hydrazones, the nature of the spacer and the substituent at the sulfinyl sulfur are key elements for the enantioselectivity of the process.

Full text of DOI:10.1039/c0ob00078g

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Chiral sulfur derivatives in the allylation of acyl hydrazones: C2-symmetric
bis-sulfinamides as enhanced chiral organic promoters.†
Inmaculada Ferna´ndez,*a Ana Alcudia,a Beatrice Gori,a Victoria Valdivia,b Roc´ıo Recio,a Mar´ıa Victoria
Garc´ıaa and Noureddine Khiar*b
Received 4th May 2010, Accepted 28th June 2010
DOI: 10.1039/c0ob00078g
Monosulfinamides and C2-symmetric bis-sulfinamides are convenient neutral chiral promoters in the
allylation of acyl hydrazones, the nature of the spacer and the substituent at the sulfinyl sulfur are key
elements for the enantioselectivity of the process.
Introduction
The preparation of chiral amines, an important structure in drugs
or drug candidates, is a standing area of interest in modern organic
synthesis.1 Even though a large variety of organometallic,2 and
organic3 catalysts were assayed in the hydrogenation of imines,
up to now the best approach is the diastereoselective addition of
organometallic reagents to sulfinamides chiral at sulfur.4 Among
the recent methods developed so far, the use of organic Lewis
bases to promote the enantioselective allylation of electrophilic
imines, hold great promises for the development of an efficient and
environmentally benign methods, for these industrially interesting
and useful important molecules.5 Within a program directed
toward the utilization of sulfur based ligand in organic and
organometallic catalysis, we have found that ferrocenyl sulfoxides
are good promoters of the allylation of hydrazones with allyl
trichlorosilane,6 extending the pioneering work of Kobayashi,
Malkov and Kocˇovsky´, and Rowlands.7 In order to determine the
mechanism of the reaction and to develop a catalytic approxima-
tion of the process, we have recently shown that C2-symmetric
ethylene-bridged bis-sulfoxides, can catalyse the reaction with
excellent enantioselectivities with only half equivalent than the
corresponding monosulfoxides.8 Based on these findings, and
taking into account that in comparison to sulfoxides ligands,
the sulfinyl oxygen in sulfinamides is presumably more Lewis
basic, in the present study we report our results on the allylation
of acyl hydrazone type I with trichloroallyl silane 1, using as
promoters a wide range of enantiomerically pure sulfinamides
and C2-symmetric bis-sulfinamides9 (Fig. 1), prepared in a very
convenient and rapid manner.
Fig. 1 Structure of monosulfinamides and C2-symmetric bis-sulfinamides
used as chiral promoters in allylation of benzoyl hydrazones.
ligands. The synthesis of S-chiral sulfinamides in optically pure
form has been carried out in high yields and enantioselectivities
by rapid condensation of LHMDS10 with diastereomerically pure
dicyclohexylidene- or diacetone-D-glucose◦alkanesulfinates 2–4,11
or (S)-menthyl p-toluenesulfinate 512 at -78 C. By simply suspend-
ing the crude reaction mixture in silica gel, followed by filtration
and crystallization, the corresponding sulfinamides 6–9 were
obtained with good yields and enantioselectivities (Scheme 1). On
the other hand the preparation of N-alkylated sulfinamides 10–
16 has been achieved by adding freshly prepared lithium benzyl-
or tert-butylamide to diastereomerically pure sulfinate esters
at -78 ◦C.13
Results and discussion
In the first part of the present study we have evaluated the
behaviour of free and alkylated monosulfinamides as potential
aDepartamento de Qu´ımica Orga´nica y Farmace´utica, Facultad de Farma-
cia, Universidad de Sevilla, 41012 Sevilla, Spain. E-mail: inmaff@us.es;
Fax: (+) 34 95 455 67 37; Tel: (+)34 95 455 59 93
bInstituto de Investigaciones Qu´ımicas, C.S.I.C-Universidad de Sevilla,
c/Ame´rico Vespucio, s/n., Isla de la Cartuja, 41092 Sevilla, , Spain. E-mail:
khiar@iiq.csic.es; Fax: (+) 34 95 446 05 65; Tel: (+) 34 95 448 95 59
† Electronic supplementary information (ESI) available: Full experimental
procedures, characterization data and copies of NMR spectra for all
compounds. See DOI: 10.1039/c0ob00078g
Scheme 1 Asymmetric synthesis of sulfinamides used in this study.
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Table 1 Enantioselective allylation of 17 using sulfinamides 6–16.a
Entry
L*
R
R¢
X
Y
ee.L*C (%)
(h)
Yield (%)b
ee 18c (conf)
∑∑
∑∑
∑∑
1
2
3
4
6
7
8
9
Et
H
H
H
H
O
O
99.9
96.0
94.0
99.9
0.5
0.5
0.6
0.7
50
60
80
100
7 (S)
p-Tol
i-Pr
t-Bu
74 (S)
56 (S)
82 (R)
O
∑∑
O
∑∑
∑∑
∑∑
∑∑
∑∑
5
6
7
8
9
10
11
10
11
12
13
14
15
16
Et
t-Bu
Bn
t-Bu
Bn
t-Bu
Bn
t-Bu
O
O
O
O
96.0
98.0
98.0
100
—
90.0
—
0.5
0.5
0.5
1.0
1.5
0.5
24
73
81
91
76
92
90
84
30 (S)
6 (S)
p-Tol
p-Tol
i-Pr
i-Pr
t-Bu
t-Bu
0 (S)
72 (S)
70 (S)
80 (R)
84 (R)
O
∑∑
O
O
∑∑
a Reactions were conducted in presence of 3.0 equiv. of L* and 0.5 equiv. of 2-methyl-2-butene in order to suppress ligand racemization. [L*] = 0.46 M.
b Isolated yield. c Enantiomeric excesses were determined by HPLC (chiralpak column).
Using the best conditions previously reported for the reaction
of allylation,8 we underwent a comparative study of the effect of
the substituent at the sulfinyl sulfur and the amidic nitrogen on
the stereoselectivity, the results are reported in Table 1. As already
observed in previous studies, the concentration of the ligand in the
reaction is critical, being 3 equivalents of the ligand in a 0.46 M
solution, the best concentration for a good enantioselectivity. Any
decrease in this concentration or in the number of equivalents of
ligand led to a enantioselectivity drop.
nitrogen and the enantioselectivity of the process (compare for
instance in Table 1, entries 1 with 5, and 2 with 7).
We have recently presented compelling data indicating a dual
pathway in the allylation of benzoyl hydrazones with allyl
trichlorosilane using sulfoxide as organic ligands. The results
summarized in Table 1 point out a similar behavior to that of
sulfinamides ligands. Taking into account that a non-linear effect
(NLE) is an important aspect in mechanistic studies,14 we decided
to study the influence of the enantiopurity of the sulfinamide
ligands on enantioselectivity of the allylated product. While non
impressive, the reaction exhibits a small negative NLE with regard
to enantiopurity of (S)-N-benzyl isopropylsulfinamide 13 used
as ligand, as is shown in Fig. 2. This proves that other species
From the analysis of the results summarized in Table 1, some
general conclusions can be drawn. The first one is that simple
sulfinamides 6–9 are able to catalyze the reaction affording
the allylated product is in moderate to high chemical yields
(Table 1, entries 1-4). Interestingly, the reaction takes place in
short reaction times and the catalysts can be recovered from the
crude reaction mixtures by column chromatography. The second
observation is the clear significance of the substituent at the sulfinyl
sulfur on the enantioselectivity of the process. Surprisingly, the
influence the size difference between the substituents at the sulfinyl
sulfur on the enantioselectivity is the opposite to that observed in
the case of sulfoxides. Thus, while in the case of using sulfoxides as
organic ligands, the highest ees were obtained with those having
small sulfur substituents, in the case of sulfinamides sterically less
demanding ethylsulfinamides 6 and 10 (Table 1, entries 1), afforded
the allylated hydrazine with the lowest ees. On the contrary, the
highest enantioselectivities (80–84% ee) were obtained with the
bulkier tert-butylsulfinamides 9 (Table 1, entries 4). The same
behaviour is maintained in the case N-substituted sulfinamides 10–
16 (Table 1, entries 5–11), where sterically hindered sulfinamides
15 and 16 afforded the best enantioselectivities (Table 1, entries
10 and 11). On the other hand, there is no coherent relationship
between the size or the electronic nature of the substituent on the
Fig. 2 Graph of allylated product 18 enantioselectivity vs. ligand 13
enantiopurity.
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Table 2 Enantioselective allylation of 17 using sulfinamides 24–28
Entrya
L*
Time/h
Yield (%)b
18S:18Rc (%)
%e.e. 18 (conf.)
1
2
3
4
5
6
24
25
26
27
28
28d
1
63
67
49
50
70
50
54 : 47
73 : 27
13 : 87
75 : 25
5 : 95
7 (S)
72
36
1
72
36
46 (S)
74 (S)
50 (S)
90 (R)
70 (R)
15 : 85
a Reactions were conducted in presence of 0.5 equivalent of 2-methyl-2-butene in order to suppress ligand racemisation. b Isolated yield. c Enantiomeric
excesses were determined by chiral HPLC analysis using Daicel chiralpack AD-column. d Reaction conducted in the presence of 0.5 equivalent of ligand.
Scheme 2 Synthesis of C2-Symmetric bis-sulfinamides 24–28. Reagents and Conditions: a) RS(O)NH2 (7–9), Ti(OEt)4, THF [19 (77%); 20 (60%); 21
(72%); 22 (64%); 23 (60%)]. b) NaBH4, MeOH [24 (91%); 25 (85%); 26 (85%); 27 (82%); 28 (79%)].
beside a simple monomeric species are present under the reaction
conditions, producing different chiral entities.
afforded the corresponding bis-sulfinylimines 19–23 in moderate
to good yields, Scheme 2. The reduction of the iminic function
with sodium borohydride in methanol yields the corresponding
bidentate neutral ligands 24–28 in excellent yields, Scheme 2.15
The ability of the obtained bisulfinamides to act as chiral Lewis
base was assayed in the allylation of benzoyl hydrazone 7, and the
results are compiled in Table 2.
Based on these precedents, we decided to assay in the same
reaction a range of differently substituted C2-symmetric bis-
sulfinamides with two different spacers between the sulfinyl sulfurs.
Condensation of sulfinamides 7–9 with isophthalaldehyde or with
adipaldehyde using Ti(OEt)4 as dehydrating reagent in THF,
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Table 3 Asymmetric allylation of hydrazones 17, 29–32 using bissulfinamide 28 as ligand.a
Entry
Hydrazone (R, R¢)
Hydrazine
Yieldb (%)
[L*]/M
S:Rc(%)
% ee
1
2
3
4
5
6
29 (Ac, Ph)
—
18
18
33
34
35
—
70
77
78
70
11
0.20
0.42
0.20
0.20
0.15
0.20
—
—
90
86
66
64
24
17 (Bz, iPr)
5 : 95
7 : 93
17 : 83
82 : 18
62 : 38
17 (Bz, iPr)
30 (Bz, cHex)
31 (Bz, Ph)
32 (Bz, pClC6H4)
a Reactions were conducted in presence of 0.5 equivalent of 2-methyl-2-butene in order to suppress ligand racemisation. b Isolated yield. c Enantiomeric
excesses were determined by chiral HPLC analysis using Daicel chiralpak column.
Firstly, regarding the enantioselectivity of the process, the same
behaviour previously observed with the monosulfinamide is con-
served in the case of bis-sulfinamides. Less sterically demanding
ligands, lead to the allylated product with low ees, while enhancing
the steric hindrance of the sulfinamide induces an increase in the
enantioselectivity. This trend is observed with both kind of ligands,
those with an aromatic spacer (Table 2, compare entries 1, 2 and 3),
Fig. 3 Possible dicoordinate transition states for the organocatalytic
allylation of hydrazones with sulfinamides.
and those with the aliphatic one (Table 2, compare entries 4, and
5). Secondly, the more flexible ligands, namely the bis-sulfinamides
with the aliphatic spacer, 27 and 28, give better results than the
aromatic analogs (Table 2, compare entries 3 and 5). Finally, all the
bis-sulfinamides gave the allylated product in moderate to good
yields and in the case of bis-sulfinamide 28 with an excellent 90%
ee (Table 2, entry 5) using only one equivalent of ligand, in contrast
to monosulfinamides where 3 equivalents were necessary.
of bis-sulfinamides with the aliphatic spacer. The observed (-)-
NLE point out in the same direction, with several molecules of
the chiral ligand involved in the intermediate transition state.
Conclusion
To determine the scope of the reaction, other hydrazones 29–
3216 were synthesized following the literature procedure, and tested
in the allylation reaction using the best ligand 28, under the
optimized reaction conditions, and the results obtained are shown
in Table 3. As can be seen from Table 3, both aliphatic and
aromatic hydrazones provided good to high enantioselectivities,
obtaining the best result in the case of the isopropilderivative 17.
Interestingly, the allylation of the p-chlorophenyl hydrazone 32
gave the corresponding hydrazine 35 in a very low chemical yield
and poor stereoselectivity. The acetyl derivative 29 did not react,
probably due to stereoelectronic factors.
Taken all together, the results obtained so far can give some
insights on the mechanism of the organocatalytic allylation of
hydrazones with neutral sulfinamide, unknown for the moment.
Firstly, the erosion of the enantioselectivity upon reducing the
catalyst loading and upon diluting the reaction medium, suggest
the possibility that the reaction could proceed through a dico-
ordinate transition state, as indicated in Fig. 3. Dicoordinated
intermediate can be formed intramolecularly specially in the case
In summary, we have demonstrated that bis-sulfinamides are
effective promoters for the allylation of benzoyl hydrazones with
allyl trichlorosilane, in highly concentrated conditions. These
preliminary studies done in order to rationalize the mechanism
of the allylation of benzoyl hydrazones with trichloroallyl silane
using chiral sulfur ligands, confirm that a dicoordinate transition
is likely to be responsible for the high enantioselectivity observed
especially in the case of C2-symmetric ligands.
Experimental
All reactions were run under an atmosphere of dry argon using
oven-dried glassware and freshly distilled and dried solvents over
activated molecular sieves. TLC was performed on Silica Gel
GF254 (Merck) with detection by charring with phosphomolybdic
acid/EtOH. For flash chromatography, silica Gel (Merck 230–400
mesh) was used. Columns were eluted with positive air pressure.
Chromatographic eluents are given as volume to volume ratios
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(v/v). NMR spectra were recorded with a Bruker AMX500
(1H, 500 MHz) and Bruker Avance DRX500 (1H, 500 MHz)
spectrometers. Chemical shifts are reported in ppm, and coupling
constants are reported in Hz. Routine spectra were referenced
to the residual proton or carbon signals of the solvent. High-
resolution mass spectra were recorded on a Kratos MS-80RFA
241-MC apparatus. Optical rotations were determined with a
Perkin-Elmer 341 polarimeter. The organic extracts were dried
over anhydrous sodium sulfate and concentrated in vacuo.
Sulfinyl chlorides were obtained by the method reported by
Hermann.17 Optically pure alkanesulfinates were prepared as
previously described following DAG methodology.11
General procedure for the synthesis of C2-symmetric
bis-sulfinamides, 24–28.
To a solution of the corresponding C2-symmetric bis-sulfinylimine,
19–23, in MeOH is added at 0 ◦C, sodium borohydride (2 eq.).
After stirring for 30 min, acetone is added and the reaction is
stirred another 5 min. The solvent is removed in vacuo and the
residue is purified by flash chromatography.
(R,R)-N,N¢-Bis-(tert-butylsulfinyl)-1,6-hexanediamine, 28.
◦
1
79% Yield, white solid. M.p.: 79–82 C. H-NMR (500 MHz,
CDCl3) d: 3.25–3.06 (m, 6H), 1.59 (m, 4H), 1.35 (m, 4H), 1.20
(s, 18H). 13C-NMR (125 MHz, CDCl3) d: 55.6, 45.6, 30.9, 26.3,
22.6. HRMS Calc. for C14H33N2O2S2(M+H)+: 325.2064. Found:
325.1985.
Menthyl p-toluenesulfinate was prepared as described by Sol-
ladie`12 and used as starting material for the synthesis of N-alkyl-
p-toluenesulfinamides 11 and 12 as previously described.13
Enantioselective allylation of N-(benzoyl)isobutylhydrazone.
General method:. (Kobayashi conditions) To a solution of N-
(benzoyl)isobutylhydrazone 17 (20.5 mg, 0.108 mmol), the chiral
sulfinamide 6–16 (0.324 mmol) or chiral C2-symmetric bissulfi-
namide 24–28 (0.108 mmol), and 2-methyl-2-butene (27 mL, 0.054
mmol) in dichloromethane (0.7 mL) was added allyltrichlorosilane
2 (23 mL, 0.162 mmol) at -78 ◦C. After stirred at -78 ◦C for
the time indicated in Tables 1 and 2, the reaction was quenched
by adding saturated aqueous NaHCO3 (1 mL). After warmed
to room temperature, saturated NaCl aqueous solution was
added, and the mixture was extracted with dichloromethane (three
times). The combined organic layers were dried over Na2SO4,
filtered and concentrated in vacuo. The residue was purified
by column chromatography (AcOEt: hexanes, 1 : 4) to afford
the corresponding N¢-(1-isopropylbut-3-enyl)benzohydrazide 18
in high chemical yields as a white solid, mp 73–74 ◦C.
General procedure for the synthesis of N-alkyl alkanesulfinamides,
10, 13–16.
To a solution of the corresponding amine (2.2 eq.) in THF at
-78 ◦C, n-BuLi (in hexane, 2.0 eq.) was added. The solution was
stirred at -78 ◦C for 30 min. and then it was added on a solution
of the corresponding alkanesulfinate 2–4 (1 eq.) in THF, and it
was stirred until all the starting material is consumed (observed by
TLC, AcOEt–CH2Cl2, 1 : 4), from 0.5 to 1 h. Then it was quenched
with saturated NH4Cl aqueous solution, extracted with AcOEt,
washed with saturated NaHCO3 aqueous solution and brine. The
organic layer was dried over Na2SO4 and the solvent evaporated.
The residue was purified by flash chromatography.
(R)-N-tert-Butyl tert-butylsulfinamide, 16.
Prepared from tert-butylamine and (R)-DAG tert-butylsulfinate
4. Time of reaction: 45 min. Purified by cc: AcOEt : Hexane, 1 : 1,
Acknowledgements
◦
to AcOEt. 50% Yield, white solid. M.p. 79–81 C. [a]2D0: -38 (c
We thank the DGICyT for financial support (grant No.
CTQ2010-21755-C02-01 and CTQ2010-21755-C02-02) and The
Junta de Andaluc´ıa (grant P06-FQM-01852 and P07-FQM-2774),
B. Gori, M. V. Garc´ıa, V. Valdivia and R. Recio thank respec-
tively, Fundacio´n Ramo´n Areces, CDCH Universidad Central
de Venezuela, CSIC and the Junta de Andalucia for predoctoral
grants.
2.0, CHCl3). 1H-NMR (500 MHz, CDCl3) d: 3.01 (brs, 1H), 1.31
(s, 9H), 1.20 (s, 9H). 13C-NMR (125 MHz, CDCl3) d: 55.1, 53.1,
31.0, 22.4. HRMS Calc. for C8H20NOS (M+H)+: 178.1266, Found
178.1265.
General procedure for the synthesis of C2-symmetric
bis-(sulfinyl)hexanediimines, 22, 23.
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The Royal Society of Chemistry 2010
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