Asymmetric allylation of sulfonyl imines catalyzed by in situ generated Cu(II) complexes of chiral amino alcohol based Schiff bases

Article abstract of DOI:10.1039/c4ra10929e

A catalytic route for enantioselective synthesis of homoallyl amines through Cu(II)-Schiff base catalyzed reaction of allyltin with aryl, alkenyl-substituted N-sulfonylimines is described. The allylation reaction is promoted by a simple in situ generated Cu(II )-amino alcohol based Schiff base complex. The addition of allyltin to aldimines delivers the desired products up to 90% yield and 98% enantiomeric excess (ee). Based on experimental observations a probable mechanism was proposed for this reaction. The current methodology was extended to the synthesis of b-phenylalanine in good yield and very good enantioselectivity.

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ARTICLE TYPE
Asymmetric Allylation of Sulfonyl Imines Catalyzed by in situ
Generated Cu(II) Complexes of Chiral Amino Alcohol Based Schiff
Bases
Debashis Ghosh,a,b Prasanta Kumar Bera,a,b Manish Kumar,a,b Sayed H. R. Abdi,*a,b Noor-ul H. Khan, a,b
5
Rukhsana I. Kureshya,b and Hari C. Bajaja,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A catalytic route for enantioselective synthesis of homoallyl amines through Cu(II)-Schiff base catalyzed
reaction of allyltin with aryl, alkenyl-substituted N-sulfonylimines is described. The allylation reaction is
10 promoted by simple in situ generated Cu(II)-amino alcohol based Schiff base complex. The addition of
allyltin to aldimines delivers the desire products up to 90% yield and 98% enantiomeric excess (ee).
Based on experimental observations a probable mechanism was proposed for this reaction. Current
methodology was extended to the synthesis of -phenylalanine in good yield and very good
enantioselectivity.
base-Cu(II) complex with a suitable additive is an efficient
catalytic system for enantioselective allylation of sulfonyl imine.
15 Introduction
Asymmetric allylation of carbonyl compounds has been well
established1 and various efficient methods were reported1,2 in this
field. However, asymmetric allylation of imine,2c,3-8 particularly
Results and Discussion
Chiral Schiff base ligands (S,S)-1 to (S,R,S,R)-4 were prepared by
allylation of sulfonated imines6d-f,7a-b,8 is comparatively less 50 the condensation of aromatic bis-aldehyde with different chiral
20 explored. The allylation products i.e. homoallyl amines are
imbedded within various biologically active compounds.9 Thus
synthesis of enantio-enriched homoallyl amines is of current
research interest. Transition metal catalyzed allylation of N-
substituted imines3-7 is one of the most powerful tools for the
25 preparation of chiral homoallyl amine. However, inexpensive
copper catalyzed asymmetric allylation of imines have been
rarely explored.7 Recently A. H. Hoveyda et al., reported (2011)
enantioselective allylboration of substituted phosphinoylimines
using N-heterocyclic carbine (NHC)-Cu complex.7d Although this
30 efficient method gave excellent product yield and
enantioselectivity, this method used extremely low temperature
and required multistep synthesis of NHC-ligand. We thought of a
simple in situ generated Cu(II)-chiral Schiff base type complex
that can work under ambient reaction condition. Synthesis of the
35 ligands follow one step condensation between a bisaldehyde and
a chiral amino alcohol.10,11a The catalytic efficiency (with respect
to both yield and enantioselectivity) is heavily influenced by the
substitutions on amino alcohol part. Among various Schiff base
ligands, tert-leucinol derived Schiff base ligand was found to be
40 most suitable one in the above catalytic system. Besides this,
presence of an additive in an appropriate amount has also
influenced the enantioselectivity with a slight change in yield of
allylation product. Involvement of the additive in the catalytic
cycle was evidenced by UV-vis. as well as mass spectral studies.
45 Our present investigation revealed that a simple chiral Schiff
amino alcohols (Scheme 1) by following literature
procedure.10,11a,12 The ligand (S,S)-2' was prepared by the
reduction of (S,S)-2 with NaBH4 in MeOH (Scheme 1).
55
Scheme 1. General synthesis of chiral ligands
The required substrates 1a-1p for the synthesis of enantiorich
homoallyl sulfonamines 2a-2p can easily be prepared in a single
step by the condensation of corresponding aldehydes with
60 cyanuric chloride in good yields.11a We began our investigation
by using N-(4-chlorobenzylidene)-4-methylbenzene sulfonamide
(1b) as a representative substrate, allyltributyltin as an allylating
agent and in situ generated chiral Schiff base-Cu(II) complexes of
ligands (S,S)-1-3 and (S,R,S,R)-4 as catalysts in CH2Cl2 at RT.
65 The results are summarized in Table 1.
At first we have checked the efficiency of in situ generated L-
valinol derived ((S,S)-1) Schiff base-Cu(II) complex for the
above allylation reaction, where homoallyl sulfonamine was
obtained in moderate yield (40%) and low enantioselectivity (ee,
70 30%) (Table 1, entry 1). Then we have altered the amino alcohol
part in the Schiff base ligand by changing L-valinol with L-tert-
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leucinol ((S,S)-2) (Figure 1). This modification resulted in a
dramatic improvement of product yield (60%) and ee (64%)
Cu(OAc)2.H2O, CuOTf, Cu(OTf)2 and Cu(acac)2 (entries 5, 9-11)
and found that Cu(OAc)2.H2O was more efficient (entry 5). The
(entry 2). To get better yield as well as enantioselectivity, we 40 optimum reaction must be carried out in presence of 4 Å
have varied the structural unit of amino alcohol part as in ligands
(S,S)-3 and (S,R,S,R)-4, but the results obtained with (S,S)-2 were
better than other ligands (entries 2,4,5) used in the present study.
We have also checked the efficiency of the reduced Schiff base
ligand (S,S)-2', but we got the product in poor yield and ee with
this system (entry 3).
molecular sieves, in absence of which a large decreased in
product yield as well as ee (entry 8) was observed due to the
moisture sensitive nature of sulfonyl imine.
5
Table 2. Optimization study illustrating the effect of cupper salt and
45 additive used in the enantioselective allylation reaction of 1b at RTa
Additive
Yieldb
Entry CuLn
Additive
amount
ee (%)c
(%)
(mol%)
1
2
3
4
5
6
7
8
Cu(OAc)2.H2O
-
-
10
10
60
45
56
65
66
65
20
30
35
50
20
64
20
70
86
94
94
16
60
24
40
16
Cu(OAc)2.H2O
Cu(OAc)2.H2O
Cu(OAc)2.H2O
Cu(OAc)2.H2O
Cu(OAc)2.H2O
Cu(OAc)2.H2O*
Cu(OAc)2.H2O≠
CuOTf.Toluene
Cu(OTf)2
L-tert-leucine
L-Valinol
L-tert-leucinol 10
L-tert-leucinol 20
L-tert-leucinol 30
L-Tert-leucinol 20
L-tert-leucinol 20
L-tert-leucinol 20
L-tert-leucinol 20
L-tert-leucinol 20
9
10
11
10
Figure 1. Chiral ligands used in the present study
Cu(acac)2
aAs per Table 1. bIsolated yield after column chromatography. cee
determined by chiral HPLC using Daicel Chiralcel OD-H column.
*Reaction was done in the absence of ligand (S,S)-2. ≠Reaction was
50 carried out in absence of 4 Å MS.
Table 1. Screening of catalysts for asymmetric allylation reaction of
allyltin with N-(4-chlorobenzylidene)-4-methylbenzene sulfonamide (1b)
in CH2Cl2 at RTa
The effect of solvent and catalyst loading were evaluated under
so far optimized reaction condition (Table 2, entry 5) and the data
are summarized in Table 3. Catalytic runs conducted in THF,
CH3CN, CHCl3 or in Toluene were not as effective as those in
55 CH2Cl2 (entries 2, 6-10). Further, catalyst loading of 10 mol%,
was found to be optimum under the above reaction condition
(entries 1-3). Additionally, it appeared that 1:0.6 metal/ligand
ratio is the best to achieve highest enantioselectivity (entries 2, 4-
5).
15
Entry
Ligands
(S,S)-1
(S,S)-2
(S,S)-2'
(S,S)-3
(S,R,S,R)-4
Yield (%)b
ee (%)c
30
64
10
20
12
1
2
3
4
5
40
60
30
25
10
aAll the reactions were carried out by using substrate N-(4-
chlorobenzylidene)-4-methylbenzene sulfonamide (0.3 mmol),
60 Table 3. Influence of the solvent and catalyst loading on allylation
reactiona
allyltributyltin (0.45 mmol), and catalyst (10 mol%) in CH2Cl2 at RT.
bIsolated yields after column chromatography. cee determined by chiral
20 HPLC using Daicel Chiralcel OD-H column.
A good level of enantioselectivity was already achieved in
absence of an additive (Table 2, entry 1) but addition of an
additive (10 mol%) among L-tert-leucine, L-valinol and L-tert-
leucinol greatly influenced ee of the product (entries 2-4). Among
25 these L-tert-leucinol (10 mol%) had a very positive influence on
the allylation reaction. The allylation results were further
improved by increasing the amount of L-tert-leucinol to 20 mol%
(entry 5), but a further increase in its amount (30 mol%) was of
little use (entry 6). It is worth mentioning here that L-tert-leucinol
30 itself can act as ligand, therefore we used it as ligand and
conducted the allylation reaction in the absence of (S,S)-2
keeping other parameters constant. However, yield as well as ee
of the allylation product were found to be poor (entry 7) implying
that a combination of L-tert-leucinol and (S,S)-2 with copper is
Cat.
Cu(OAc)2.H2O
/(S,S)-2
Time Yield
(h)
Entry
Loading
(mol%)
5
10
15
10
10
10
10
Solvent
ee (%)c
(%)b
1
2
3
4
5
6
7
8
9
1:0.6
1:0.6
1:0.6
1:1
1:1.5
1:0.6
1:0.6
1:0.6
1:0.6
CH2Cl2 65
CH2Cl2 46
CH2Cl2 45
CH2Cl2 46
CH2Cl2 46
CHCl3 60
50
66
69
67
66
50
35
34
20
76
94
88
90
90
60
40
46
30
THF
72
10
10
CH3CN 85
Toluene 90
aAs per Table 1. bIsolated yield after column chromatography. cee
determined by chiral HPLC using Daicel Chiralcel OD-H column.
35 forming
a
highly active and enantioselective catalyst.
65
We further examined the chiral Schiff base-Cu(II) complex-
promoted allylation reaction of 1b with other allylating agents
(Table 4). Tetra-allyltin showed activity similar to the
Furthermore, it is known that different copper salts have variable
geometry and reactivity, hence we varied copper sources viz.,
2
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allyltributyltin but with significantly lower enantioselectivity 25 The desired homoallyl sulfonamines 2a-2p were obtained in
(entries 2 and 4). In a marked contrast the reaction using
allylsilanes did not proceed at all (entries 4-5). When we used
(pinacolato) allylboron as an allylating agent we got homoallyl
amine in good yield with very low enantioselectivity (entry 7).
mostly good yield with high enantioselectivities ranging from 98
to 66% enantiomeric excess (Table 5). Para- and meta-
substituted aromatic imines gave good yield (except electron
donating e.g. p-methoxy group) and exillent enantioselectivity
5
After fixing the allylting agent we optimized its amount and the 30 (entries 2-10) as compared to their ortho-counterpart (entries 11-
results showed its 1.5 equivalent (with respect to 1b) was
sufficient for getting highest product yield and enantioselectivity
(entry 2).
12). The present catalytic system works very well for bulkier
aromatic imines (like 1-naphthyl and 2-naphthyl imines; entries
13 and 14) as well as -unsaturated sulfonyl imines (e.g., trans-
cinnamyl and alpha-methyl-trans-cinnamyl imines; entries 15 and
10 Table 4. Effect of allylating agent on allylation reaction
35 16).
The present asymmetric allylation protocol was successfully
extended to the synthesis of enantioenriched -amino acid 7
(Scheme 2). -Amino acids13 are important motifs which serve as
precursors to -lactams, constituents of several medicinally
40 important compounds,14 and most importantly as monomers in
the synthesis of peptidomimetic -peptides.15 Compound 2a was
oxidized with NaIO4 in the presence of a catalytic amount of
OsO4 to form the corresponding aldehyde, which without
purification was further oxidized to furnish the desired tosyl-
45 protected -amino acid 7 with an overall yield of 60% with
retention of optical purity.
Amount of allylating
Entry Allylating agents
Yield (%)b ee (%)c
agents (equiv.)
1
2
3
1
50
66
66
90
94
92
1.5
2
4
1.5
68
50
5
6
1.5
1.5
20
10
-
<10
7
1.5
70
20
aAs per Table 1. bIsolated yield after column chromatography. cee
determined by chiral HPLC using Daicel Chiralcel OD-H column
Scheme 2. Synthesis of enantioriched -amino acid 7
The optimal condition established for the enantioselective
15 allylation of N-(4-chlorobenzylidene)-4-methylbenzene
Mechanism
sulfonamide 1b (Table 4, entry 2) was applied to other aromatic
as well as -unsaturated sulfonyl imines.
50 To support the mechanism as given in Scheme 3 for the catalytic
allylation reaction, a stepwise UV-vis. spectral study was carried
out with N-(4-chlorobenzylidene)-4-methylbenzene sulfonamide
(1b) and allyltributyltin in CHCl3 as solvent at RT (Fig. 2A, 2B &
2C).
Table 5. Chiral Cu (II)-Schiff base catalysed enantioselective allylation of
sulfonyl iminea
0.5
0.4
0.3
0.2
0.1
0.0
Cu2-(S,S)-2+L-tert-leucinol+1b+Allytin
Cu2-(S,S)-2+L-tert-leucinol+1b
Cu2-(S,S)-2+L-tert-leucinol
Complex Cu2-(S,S)-2
20
Ligand (S,S)-2
Entry Substrate
R1/R2
C6H5/Me (1a)
p-ClC6H4/Me (1b)
p-ClC6H4/H (1c)
p-ClC6H4/NO2 (1d)
p-BrC6H4/H (1e)
p-FC6H4/Me (1f)
Yield (%)b ee (%)c
1
Ph
2a (60)
2b (66)
2c (58)
2d (68)
2e (60)
2f (69)
2g (67)
2h (65)
2i (30)
2j (64)
2k (70)
2l (65)
80
94
90
78
98
90
80
76
84
82
66
78
2
3
4
5
p-ClC6H4
p-ClC6H4
p-ClC6H4
p-BrC6H4
p-FC6H4
Isobestic point
Isobestic points
6
7
8
9
p-NO2C6H4 p-NO2C6H4/Me (1g)
p-CF3C6H4 p-CF3C6H4/Me (1h)
p-MeOC6H4 p-MeOC6H4/H (1i)
10
11
12
13
14
m-ClC6H4
o-ClC6H4
o-FC6H4
1-naphthyl
2-naphthyl
trans-
cinnamyl
alpha-Methyl-
trans-
m-ClC6H4/Me (1j)
o-ClC6H4/Me (1k)
o-FC6H4/Me (1l)
1-naphthyl/Me (1m)
2-naphthyl/Me (1n)
300
400
500
2m (90) 92
Wavelength(nm)
55
2n (84)
94
Figure 2A Uv-vis spectra of ligand, in situ generate complex and reaction
mixture after sequential addition of additive, substrate and allyltributyltin.
15
(E)-C6H5CH=CH/Me (1o) 2o (76)
86
(E)-C6H5CH=C(CH3)/Me
Initially, in situ formation of the complex was confirmed by UV-
vis. (strong blue shift i.e. from 438 nm to 412 nm, Figure 2A) as
16
2p (75)
(1p)
94
cinnamyl
60 well as ESI-MS spectral analysis (Figure 3). After the addition of
additive (L-tert-leucinol) to the complex solution, the intensity of
LMCT band at ~410 nm has increase significantly whereas d-d
band was red shifted (from 608 nm to 621 nm) with isobestic
aAll the reactions were carried out by using substrates 1a-1o (0.3 mmol),
allyltributyltin (0.45 mmol), and catalyst (10 mol%) in CH2Cl2 at RT.
bIsolated yields after column chromatography. cee determined by chiral
HPLC using OD-H, AD-H, IA chiral column.
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points at ~348 nm and ~438 nm suggests the formation of
intermediate I-1 (Scheme 3).
To further confirm this interaction additional experiment on
varying the amount of tert-leucinol to the in situ generated copper
complex was carried out and was monitored on UV-vis and the
spectra (Figure 2C) that clearly show isobestic point at ~441 nm,
20 and thereby additionally supports the formation intermediate I-1
during the catalytic cycle.
0.03
Cu2-(S,S)-2
Cu2-(S,S)-2+L-tert-leucinol
Cu2-(S,S)-2+L-tert-leucinol+1b
CAT ADD MEOH
100
1
(0.017)
1: TOF MS ES+
342
Cu2-(S,S)-2+L-tert-leucinol+1b+Allyltin
644.62
646.62
0.02
[I-1+H+]
0.01
0.00
565.44
563.44
647.61
587.44
500
600
700
800
559.49
648.62
623.43
585.44
Wavelength (nm)
567.44
589.46
625.44
621.44
601.44
599.45
581.48
583.51
643.45
649.60
609.49
611.50
603.44
568.45
569.44
627.45
590.46
604.45
Figure 2B Uv-vis spectra (for d-d transition) of in situ generate complex
5 and reaction mixture after sequential addition of additive, substrate and
allyltributyltin.
550.53
639.50641.49
635.58
558.50
580.50
650.58
655.51
613.51
630.58
571.49
597.47
595 600
659.58
555.47
614.51
0
m/z
660
550 555
560
565
570
575
580
585
590
605
610
615 620
625
630
635
640
645
650
655
Figure 4 ESI-MS spectum of the in situ generated complex after
interaction with additive was recorded in methanol.
0.30
25 This intermediate I-1 was further confirmed by ESI-MS spectral
analysis where new molecular mass peak equivalent to [I-1+H+]
species (Figure 4) is clearly visible. After the addition of
substrate to the above solution, LMCT band maxima slightly
diminished with isobestic points at ~348 nm and ~438 nm. A
30 significant red shift in the d-d band (from 621 nm to 631 nm)
confirms the direct co-ordination of the substrate to the vacant d
orbital of the copper through the lone pair of the nitrogen atom of
the N-tosylimine. The generation of the isobestic points may be
due to the change in geometry of the copper complex and this can
35 be attributed to the attachment of the L-tert-leucinol and the
substrate to the complex to form an intermediate I-2 (Scheme 3).
(a)
(b)
(c)
(d)
(e)
(f)
(g)
0.25
0.20
0.15
Isobestic point
0.10
0.05
0.00
-0.05
400
500
Wavelength (nm)
Figure 2C Uv-vis spectra of ligand, in situ generate complex and the
reaction mixture with varing concentration of tert-leucinol: (a) ligand; (b)
10 in situ generate complex; (c) complex:additive (1:0.2); (d)
complex:additive
(1:0.4);
(e)
complex:additive
(1:0.6);
(f)
complex:additive (1:0.8); (g) complex:additive (1:1).
CU COM B
100
1
(0.019)
1: TOF MS ES+
4.74
588.44
[Cu2-(S,S)-2+2H+]
590.41
565.29
563.32
591.42
649.32
537.28
651.39
588.25
567.28
511.27
559.28
557.37
535.29
653.43
654.35
547.33
519.26
541.33
610.44
610.27
568.29
571.53
526.40
556.47
530.26
587.26
626.36
624.42
637.39
627.46
591.90
610.83
579.22
600.39
639.26
602.28
0
m/z
510 515 520 525 530 535 540 545 550 555 560 565 570 575 580 585 590 595 600 605 610 615 620 625 630 635 640 645 650 655 660
Figure 3 ESI-MS spectum of the in situ generated complex was taken in
15 methanol.
Scheme 3 Proposed catalytic cycle.
4
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Typical procedure for preparation of ligand (S,S)-2'
The intermediate I-2 was further confirmed by ESI-MS spectral
analysis where new molecular mass peak corresponding to [I-
2+CH3OH+H+] species (see Figure
in Supporting
Information).On further addition of allyltributyltin to the reaction
mixture there was no observable change in the spectrum. The
addition of allyltributyltin follows product formation and
regenerates the intermediate I-1 for the next cycle.
To the solution of (S,S)-2 (405 mg, 1 mmol) in 20 mL dry
55 methanol NaBH4 (303 mg, 4x2 mmol) was added portion wise in
four equal parts. The reaction was monitored by TLC. After
completion of the reaction (10 h), solvent was completely
removed under reduced pressure. Then the reaction mass was
washed by water and extracted with CH2Cl2 and dried by
60 Na2SO4. Further purification was done by flash column
chromatography (EtOAc/hexane = 1:4) using silica gel (100-200
mesh). White solid. yield: 90%; m. p.: 150-152 oC. 1H NMR
(CDCl3, 500 MHz): δ = 0.91 (s, 18H), 1.19 (s, 9H), 2.35 (m, 2H),
3.52-3.78 (m, 4H), 3.89-4.03 (m, 4H), 4.67 (br, 4H), 6.97 (s, 2H).
65 13C NMR (CDCl3, 125 MHz): δ = 27.24, 29.63, 31.52, 33.99,
52.14, 60.59, 67.65, 123.39, 125.58, 141.46, 154.42. TOF–MS
(ESI+) calcd [M + H+] for (C24H44N2O3) 409.34, Found: 409.66
1
5
Conclusion
In conclusion, a new copper based catalytic protocol was
10 developed for asymmetric allylation reaction of aryl, alkenyl-
substituted N-sulfonylimines using allyltributyltin as allylating
agent. More importantly, the present catalytic system is simple
and works under ordinary reaction condition giving high yields
(up to 90%) of the homoallyl amines with excellent
15 enantioselectivities (ee up to 98%) as compared to the other
reports. Additionally, the current methodology was extended to
the synthesis of -phenylalanine in good and very good
enantioselectivity.
General procedure for Cu(II)-Schiff base complex catalyzed
asymmetric allylation reaction of tosylimines 1a-1o
70 To a mixture of Cu(OAc)2.H2O (0.03 mmol), ligand (S,S-2)
(0.018 mmol) and 60 mg of powdered activated 4 Å MS in a
nitrogen-filled 5 mL reactor, dry DCM (2 mL) was added. After
Acknowledgements
o
the mixture was stirred at 27 C for 2 h, L-tert-leucinol (0.06
mmol) was added and again stirred for 30 minutes. To it
75 sulfonimines 1a-1o (0.3 mmol) followed by allyltributyltin (0.45
mmol) were added and the resulting mixture was stirred at same
temperature until the reaction was completed as indicated by
TLC. After completion of the reaction, the solvent was removed
under vacuum and the residue thus obtained was purified by silica
80 gel flash column chromatography with ethyl acetate/hexane as
eluent. Products were confirmed by NMR and LCMS data
corresponded to those published.
20 CSIR-CSMCRI registration no is 122/2014. Authors are thankful
to CSIR and CSIR-Indus Magic Project CSC0123 for financial
support. Debashis Ghosh is thankful to AcSIR for Ph. D
enrolment. Analytical Discipline and Centralized Instrumental
Facility is gratefully acknowledged for providing instrumental
25 facilities.
Experimental Section
N-(1-Phenylbut-3-enyl)-4-methylbenzenesulfonamide 2a6f
The product was isolated as a white solid (yield 57 mg, 60%)
85 after purification by silica gel chromatography (Hexane/EtOAc =
90:10). Optical Rotation: [α]27D = -71.5 (c 0.5, CH2Cl2).6f 1HNMR
(CDCl3, 200 MHz): δ = 2.37-2.45 (m, 5H), 4.37-4.39 (m, 1H),
4.89-5.09 (m, 3H), 5.45-5.53 (m, 1H), 7.09-7.17 (m, 7H), 7.53-
7.57 (d, J = 7 Hz, 2H). 13C NMR (CDCl3, 125 MHz): δ = 23.06,
90 43.47, 58.66, 120.96, 128.15, 128.75, 128.99, 129.97, 130.92,
134.66, 139.03, 141.91, 144.72. TOF–MS (ESI+) calcd [M +
Na+] for (C17H19NaNO2S) 324.10, Found: 324.40. HPLC (Daicel
Chiralcel OD-H, hexanes/2-propanol = 95:5, flow rate = 0.8
mL/min, λ = 230 nm) tmajor(S) = 19.34 min, tminor(R) = 14.31 min.
Different aldehydes and reagents were used as received. The
imine substrates (1a-1o) were prepared following literature
procedure.11a All the solvents were dried using standard
30 procedures,16 distilled and stored under activated molecular
sieves. NMR spectra were obtained with a Bruker F113V spec-
trometer (200 and 500 MHz) and are referenced internally with
tetra-methylsilane (TMS). Splitting patterns were reported as s,
singlet; d, doublet; dd, doublet of doublet; t, triplet; q, quartet; m,
35 multiplet; br, broad. Enantiomeric ratio (er) values were
determined by HPLC (Shimadzu SCL-10AVP) using Daicel
Chirl-pak OD-H, AS-H, IA, IB and IC chiral columns with 2-
propanol/hexaneas eluent. For the product purification flash
chromatography was performed using silica gel 100–200 mesh.
95 N-(1-(4-Chlorophenyl)but-3-enyl)-4-
40 General procedure for preparation of catalysts (S,S)-1 to
(S,R,S,R)-411a
methylbenzenesulfonamide 2b6f
The product was isolated as a white solid (yield 69 mg, 66%)
after purification by silica gel chromatography (Hexane/EtOAc =
To a solution of 4-tert-Butyl-2,6-diformylphenol (300 mg; 1.45
mmol) in dry toluene chiral (S)-tert-leucinol (341 mg; 2.91 mmol)
was added and the reaction was vigorously stirred for 2 days
45 under reflux condition. In two days yellow coloured precipitate of
desired product was formed in the reaction mixture which was
filtered, washed twice with cold methanol and dried to get desired
ligand (S,S)-2 in sufficient purity (yield, 515 mg; 88%) and was
fully characterised by NMR and elemental analysis before its use
50 in catalytic reaction. All other Schiff base ligands (S,S)-1 to
(S,R,S,R)-4 were prepared following the above reaction
procedure.
88:12). Optical Rotation: [α]27 = -85.4 (c 0.5, CH2Cl2).6f 1H
D
100 NMR (CDCl3, 200 MHz): δ = 2.39-2.44 (m, 5H), 4.29-4.39 (dd, J
= 6.6 and 13.2 Hz, 1H), 5.01-5.09 (m, 3H), 5.38-5.59 (m, 1H),
6.98-7.17 (m, 6H), 7.51-7.55 (d, J = 8 Hz, 2H). 13C NMR (CDCl3,
125 MHz): δ = 21.44, 41.71, 56.56, 119.69, 127.11, 128, 128.42,
129.34, 132.61, 138.82, 143.37. TOF–MS (ESI+) calcd [M +
105 Na+] for (C17H18ClNaNO2S) 358.06, Found: 358.23. HPLC
(Daicel Chiralcel OD-H, hexanes/2-propanol = 95:5, flow rate =
0.8 mL/min, λ = 230 nm) tmajor = 22.5 min, tminor = 18.4 min.
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N-(1-(4-Chlorophenyl)but-3-enyl)benzenesulfonamide 2c
The product was isolated as a white solid (yield 58.7 mg, 58%) 60 (CDCl3, 200 MHz): δ = 2.38-2.46 (m, 5H), 4.41-4.51 (dd, J = 6.4
88:12). Optical Rotation: [α]27D = -59.4 (c 0.5, CH2Cl2). 1H NMR
after purification by silica gel chromatography (Hexane/EtOAc =
88:12). Optical Rotation: [α]27D = -80.8 (c 0.5, CH2Cl2). 1H NMR
(CDCl3, 500 MHz): δ = 2.40-2.43 (t, J = 7 Hz, 2H), 4.37-4.41
(dd, J = 7 and 13.5 Hz, 1H), 4.87-4.88 (d, J = 6 Hz, 1H), 5.06-5.1
and 12.6 Hz, 1H), 5.04-5.15 (m, 3H), 5.36-5.56 (m, 1H), 7.15-
7.32 (m, 4H), 7.53-7.57 (d, J = 8 Hz, 2H), 8.03-8.07 (d, J = 8.6
Hz, 2H). 13C NMR (CDCl3, 25 MHz): δ = 21.45, 41.59, 56.38,
120.49, 123.55, 127.14, 127.56, 129.48, 131.86, 143.82, 147.9.
5
(m, 2H), 5.44-5.51 (m, 1H), 6.99-7.02 (m, 2H), 7.12-7.14 (m, 65 HPLC (Daicel Chiralcel AD-H, hexanes/2-propanol = 95:5, flow
2H), 7.35-7.38 (m, 2H), 7.48-7.51 (m, 1H), 7.64-7.65 (d, J = 7.5
Hz, 2H). 13C NMR (CDCl3, 125 MHz): δ = 41.75, 56.42, 119.88,
rate = 1 mL/min, λ = 254 nm) tmajor = 22.5 min, tminor = 23.8 min.
N-(1-(4-Trifluoromethylphenyl)but-3-enyl)-4-
methylbenzenesulfonamide 2h6f
10 127.06, 127.97, 128.5, 128.78, 132.49, 132.53, 138.7, 139.49.
TOF–MS (ESI+) calcd [M + Na+] for (C16H16ClNaNO2S) 344.05,
The product was isolated as a white solid (yield 75 mg, 65%)
Found: 344.23. HPLC (Daicel Chiralcel OD-H, hexanes/2- 70 after purification by silica gel chromatography (Hexane/EtOAc =
1
propanol = 95:5, flow rate = 0.8 mL/min, λ = 230 nm) tmajor
=
82:18). H NMR (CDCl3, 500 MHz): δ = 2.38 (s, 3H), 2.41-2.43
(m, 2H), 4.31-4.4 (dd, J = 6.4 and 13 Hz, 1H), 4.9-4.93 (d, J = 6
Hz, 2H), 5.01-5.09 (m, 2H), 5.39-5.59 (m, 1H), 6.81-6.89 (m,
2H), 7.01-7.17 (m, 4H), 7.51-7.55 (d, J = 8.4 Hz, 2H). 13C NMR
21.75 min, tminor = 18.4 min.
15 N-(1-(4-Chlorophenyl)but-3-enyl)-4-nitrobenzenesulfonamide
2d
The product was isolated as a white solid (yield 78 mg, 68%) 75 (CDCl3, 125 MHz): δ = 22.69, 41.66, 56.56, 20.14, 125.23,
after purification by silica gel chromatography (Hexane/EtOAc =
85:15). Optical Rotation: [α]27D = -69.4 (c 0.5, CH2Cl2). 1H NMR
20 (CDCl3, 200 MHz): δ = 2.43-2.49 (t, J = 7 Hz, 2H), 4.44-4.53
(dd, J = 6.4 and 13 Hz, 1H), 5.05-5.17 (m, 3H), 5.44-5.65 (m,
127.05, 127.12, 129.32, 132.31, 143.46. HPLC (Daicel Chiralcel
IA, hexanes/2-propanol = 95:5, flow rate = 0.8 mL/min, λ = 220
nm) tmajor = 21.75 min, tminor = 20 min.
N-(1-(4-Methoxyphenyl)but-3-enyl)benzenesulfonamide 2i
1H), 6.97-7.16 (m, 4H), 7.74-7.78 (d, J = 9 Hz, 2H), 8.16-8.2 (d, 80 The product was isolated as a white solid (yield 28 mg, 30%)
J = 9 Hz, 2H). 13C NMR (CDCl3, 125 MHz): δ = 41.66, 56.78,
after purification by silica gel chromatography (Hexane/EtOAc =
88:12). H NMR (CDCl3, 200 MHz): δ = 2.41-2.48 (t, J = 7 Hz,
2H), 2.75 (s, 3H), 4.31-4.4 (dd, J = 6.6 and 13.2 Hz, 1H), 4.8-
4.83 (d, J = 5 Hz, 2H), 5.02-5.09 (m, 2H), 5.42-5.62 (m, 1H),
1
120.31, 123.9, 128.05, 128.26, 128.67, 132.20, 133.85, 138.01,
25 146.23, 149.73. TOF–MS (ESI-) calcd [M
-
H+] for
(C16H14ClN2O4S) 365.04, Found: 364.82. HPLC (Daicel
Chiralcel OD-H, hexanes/2-propanol = 95:5, flow rate = 0.8 85 6.67-6.99 (m, 4H), 7.26-7.47 (m, 3H), 7.64-7.68 (d, J = 7.2 Hz,
mL/min, λ = 254 nm) tmajor = 17.78 min, tminor = 14.55 min.
N-(1-(4-Bromophenyl)but-3-enyl)benzenesulfonamide 2e
30 The product was isolated as a white solid (yield 71 mg, 60%)
after purification by silica gel chromatography (Hexane/EtOAc =
2H). 13C NMR (CDCl3, 125 MHz): δ = 41.82, 55.21, 56.75,
113.71, 119.09, 127.05, 127.71, 128.66, 132.23, 133.2, 140.49,
158.82. HPLC (Daicel Chiralcel ODH, hexanes/2-propanol =
95:5, flow rate = 0.8 mL/min, λ = 254 nm) tmajor = 28.2 min, tminor
84:16). Optical Rotation: [α]27D = -70.7 (c 0.5, CH2Cl2). 1H NMR 90 = 24.59 min.
(CDCl3, 200 MHz): δ = 2.38-2.44 (t, J = 7 Hz, 2H), 4.32-4.42
(dd, J = 6.4 and 13 Hz, 1H), 4.97-5.11 (m, 3H), 5.42-5.59 (m,
35 1H), 6.93-6.97 (d, J = 8.4 Hz, 2H), 7.3-7.54 (m, 4H), 7.63-7.66
(d, J = 7.4 Hz, 2H). 13C NMR (CDCl3, 125 MHz): δ = 41.68,
N-(1-(3-Chlorophenyl)but-3-enyl)-4-
methylbenzenesulfonamide 2j6f
The product was isolated as a viscose liquid (yield 68 mg, 64%)
after purification by silica gel chromatography (Hexane/EtOAc =
56.5, 119.87, 127.04, 128.32, 128.79, 131.44, 132.48, 139.22, 95 84:16). Optical Rotation: [α]27D = -50.1 (c 1, CH2Cl2). H NMR
1
140.2. TOF–MS (ESI+) calcd [M + Na+] for (C16H16BrNaNO2S)
388, Found: 388.13. HPLC (Daicel Chiralcel OD-H, hexanes/2-
(CDCl3, 200 MHz): δ = 2.38-2.44 (m, 5H), 4.31-4.41 (dd, J = 6.6
and 13 Hz, 1H), 4.96-5.12 (m, 3H), 5,4-5.57 (m, 1H), 6.95-7.26
(m, 6H), 7.51-7.55 (d, J = 8.6 Hz, 2H). 13C NMR (CDCl3, 125
MHz): δ = 21.44, 41.71, 56.55, 119.82, 124.84, 126.80, 127.08,
40 propanol = 95:5, flow rate = 0.8 mL/min, λ = 230 nm) tmajor
=
23.6 min, tminor = 20 min.
N-(1-(4-Fluorophenyl)but-3-enyl)-4-ethylbenzenesulfonamide 100 127.47, 129.35, 129.61, 132.53, 134.21, 137.13, 142.23, 143.41.
2f6f
HPLC (Daicel Chiralcel OD-H, hexanes/2-propanol = 95:5, flow
rate = 0.8 mL/min, λ = 220 nm) tmajor = 18.79 min, tminor = 17.14
min.
The product was isolated as a white solid (yield 69 mg, 69%)
45 after purification by silica gel chromatography (Hexane/EtOAc =
85:15). Optical Rotation: [α]27D = -71.5 (c 0.5, CH2Cl2). 1H NMR
N-(1-(2-Chlorophenyl)but-3-enyl)-4-
(CDCl3, 500 MHz): δ = 2.38 (s, 3H), 2.39-2.43 (m, 2H), 4.34- 105 methylbenzenesulfonamide 2k6f
4.38 (dd, J = 6.5 and 13 Hz, 1H), 5-5.06 (m, 3H), 5.44-5.53 (m,
1H), 6.83-6.87 (m, 2H), 7.03-7.07 (m, 2H), 7.15-7.16 (d, J = 8
50 Hz, 2H), 7.53-7.54 (d, J = 8.5 Hz, 2H). 13C NMR (CDCl3, 125
MHz): δ = 21.44, 41.9. 56.4, 115.07, 115.24, 119.6, 127.12,
The product was isolated as a white solid (yield 74 mg, 70%)
after purification by silica gel chromatography (Hexane/EtOAc =
85:15). Optical Rotation: [α]27D = -46.1 (c 0.5, CH2Cl2). 1H NMR
(CDCl3, 200 MHz): δ = 2.36 (s, 3H), 2.42-2.48 (m, 2H), 4.74-
128.2, 128.26, 129.33, 132.78, 136.12, 137.32, 143.28, 162.95. 110 4.84 (dd, J = 6.8 and 13.4 Hz, 1H), 5.02-5.14 (m, 3H), 5.38-5.58
HPLC (Daicel Chiralcel OD-H, hexanes/2-propanol = 95:5, flow
rate = 1 mL/min, λ = 220 nm) tmajor = 17.38 min, tminor = 14.4 min.
55 N-(1-(4-Nitrophenyl)but-3-enyl)-4-methylbenzenesulfonamide
2g
(m, 1H), 7,06-7.23 (m, 6H), 7.57-7.21 (d, J = 8 Hz, 2H). 13C
NMR (CDCl3, 125 MHz): δ = 21.44, 40.23, 54.07, 119.53,
126.76, 127.15, 128.37, 128.42, 129.31, 132.72, 136.78, 137.78,
143.74. HPLC (Daicel Chiralcel OD-H, hexanes/2-propanol =
The product was isolated as a white solid (yield 73 mg, 67%) 115 95:5, flow rate = 0.8 mL/min, λ = 230 nm) tmajor = 21.1 min, tminor
after purification by silica gel chromatography (Hexane/EtOAc = = 15.1 min.
6
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RSC Advances
N-(1-(2-Fluorophenyl)but-3-enyl)-4-
methylbenzenesulfonamide 2l
The product was isolated as a white solid (yield 65 mg, 65%)
after purification by silica gel chromatography (Hexane/EtOAc =
86:14). Optical Rotation: [α]27D = -48.6 (c 0.5, CH2Cl2). 1H NMR
(CDCl3, 500 MHz): δ = 2.34 (s, 3H), 2.43-2.51 (m, 2H), 4.53-
326.23. HPLC (Daicel Chiralcel OD-H, hexanes/2-propanol =
60 96:4, flow rate = 0.8 mL/min, λ = 254 nm) tmajor = 24.5 min, tminor
= 26.05 min.
(E)-N-(2-methyl-1-phenylhexa-1,5-dien-3-yl)-4-methyl-
benzenesulfonamide 2p
5
The product was isolated as a white solid (yield 76 mg, 75%)
4.64 (dd, J = 7.5 and 14.5 Hz, 1H), 5-5.06 (m, 3H), 5.47-5.55 (m, 65 after purification by silica gel chromatography (Hexane/EtOAc =
1H), 6.82-6.86 (m, 1H), 6.93-6.96 (m, 1H), 7.06-7.15 (m, 4H),
89:11). Optical Rotation: [α]27D = -94.5 (c 0.5, CH2Cl2). 1H NMR
(CDCl3, 500 MHz): δ = 1.56 (s, 3H), 2.28-2.35 (m, 2H), 2.37 (s,
3H), 3.91-3.95 (dd, J = 6.5 and 13.5 Hz, 1H), 4.62-4.63 (d, J = 6
Hz, 1H), 5.09-5.12 (m, 2H), 5.59-5.67 (m, 1H), 6.25 (s, 1H),
7.54-7.56 (d, J = 8.5 Hz, 2H). 13C NMR (CDCl3, 125 MHz): δ =
10 21.42, 40.65, 52.92, 115.4, 115.57, 119.26, 123.99, 127.02,
127.33, 127.43, 128.72, 128.95, 129.01, 129.28, 132.87, 137.12,
143.13, 158.97, 160.92. HPLC (Daicel Chiralcel OD-H, 70 7.01-7.03 (d, J = 7.5 Hz, 1H), 7.18-7.22 (m, 3H), 7.28-7.29 (m,
hexanes/2-propanol = 95:5, flow rate = 1 mL/min, λ = 220 nm)
tmajor = 13.5 min, tminor = 10.63 min.
15 N-(1-(1-Naphthyl)but-3-enyl)-4-methylbenzenesulfonamide
2m
2H), 7.71-7.72 (d, J = 8.5 Hz, 1H). 13C NMR (CDCl3, 125 MHz):
δ = 21.45, 38.5, 60.77, 118.92, 126.56, 127.41,127.95, 128.2,
128.83, 129.39, 133.24, 135.38, 136.94, 137.65, 143.22. TOF–
MS (ESI-) calcd [M - H+] for (C20H23NO2S) 340.14, Found:
The product was isolated as a white solid (yield 95 mg, 90%) 75 340.35. HPLC (Daicel Chiralcel OD-H, hexanes/2-propanol =
after purification by silica gel chromatography (Hexane/EtOAc =
90:10). Optical Rotation: [α]27D = -80.4 (c 0.5, CH2Cl2). 1H NMR
20 (CDCl3, 500 MHz): δ = 2.29 (s,3H), 2.58-2.68 (m, 2H), 4.99-5.01
(d, J = 6.5 Hz, 1H), 5.08-5.13 (m, 2H), 5.2-5.24 (dd, J = 6.5 and
96:4, flow rate = 0.8 mL/min, λ = 254 nm) tmajor = 18.96 min,
tminor = 20.33 min.
3-(4-methylphenylsulfonamido)-3-phenylpropanoic acid11a
The compound 2a (0.12 mmol, 37 mg) was dissolved in
13 Hz, 1H), 5.5-5.58 (m, 1H), 6.99-7 (d, J = 8 Hz, 2H), 7.27-7.3 80 THF/H2O (320/160 μL) and the reaction vial was covered with an
(m, 2H), 7.45-7.48 (m, 4H), 7.66-7.68 (m, 1H), 7.79-7.81 (m,
1H), 7.89-7.91 (m, 1H). 13C NMR (CDCl3, 125 MHz): δ = 21.37,
25 41.16, 53.17, 119.44, 122.37, 124.19, 125.04, 125.54, 126.24,
127.06, 127.96, 128.91, 129.12, 133.11, 143.03. TOF–MS (ESI-)
aluminium foil. OsO4 (0.05 mmol, 2.5 wt% in t-BuOH, 81μL)
was then added followed by a portion wise addition of NaIO4 (0.3
mmol, 64 mg) over a period of 15 min. After stirring for 4 h at
room temperature, the reaction mixture was filtered and the
calcd [M - H+] for (C21H20NO2S) 350.13, Found: 350.19. HPLC 85 residue was washed with ethyl acetate. The organic solvents were
(Daicel Chiralcel OD-H, hexanes/2-propanol = 95:5, flow rate =
0.8 mL/min, λ = 230 nm) tmajor = 23.1 min, tminor = 19.9 min.
30 N-(1-(2-Naphthyl)but-3-enyl)-4-methylbenzenesulfonamide
2n
then evaporated and EtOAc and sat. NH4Cl were added to the
reaction mixture. The organic layer was extracted and
concentrated to yield the crude aldehyde which was directly used
for the next step.
The product was isolated as a white solid (yield 88.2 mg, 84%)
after purification by silica gel chromatography (Hexane/EtOAc =
88:12). Optical Rotation: [α]27D = -89.1 (c 0.5, CH2Cl2). 1H NMR
35 (CDCl3, 500 MHz): δ = 2.2 (s, 3H), 2.49-2.6 (m, 2H), 4.53-4.57
(dd, J = 7 and 13.5 Hz, 1H), 5.06-5.1 (m, 3H), 5.47-5.58 (m, 1H),
90
The crude aldehyde was dissolved in tert-butanol (700 μL) and
2-methyl-2-butene (0.96 mmol, 100 μL) was added. A solution of
NaClO2 (0.90 mmol, 82 mg) and NaH2PO4 (0.78 mmol, 93 mg) in
H2O (300 μL) was then slowly added. After stirring the reaction
for 12 h at room temperature, conc. HCl (50 μL) was added drop
6.96-6.97 (d, J = 8 Hz, 2H), 7.18-7.2 (m, 1H), 7.42-7.43 (m, 3H), 95 wise and the reaction mixture was extracted with EtOAc. The
7.49-7.51 (d, J = 8 Hz, 2H), 7.63-7.65 (m, 2H), 7.74-7.76 (m,
1H). 13C NMR (CDCl3, 125 MHz): δ = 21.25, 41.67, 57.30,
40 119.37, 124.22, 125.82, 125.89, 126.07, 127.09, 127.47, 127.78,
128.25, 129.13, 132.64, 132.94, 133, 137.31, 143.08. TOF–MS
organic layer was concentrated to yield the crude product which
was then purified by column chromatography (1:5, MeOH:DCM
as the eluent) to give the desired compound 7 as a white solid.
Optical Rotation: [α]27 = -30.5 (c 0.3, CH2Cl2). 1H NMR
D
(ESI-) calcd [M - H+] for (C21H20NO2S) 350.13, Found: 350.20. 100 (CDCl3, 500 MHz): δ = 2.36 (s, 3H), 2.78-2.93 (m, 2H), 4.7-4.74
HPLC (Daicel Chiralcel OD-H, hexanes/2-propanol = 95:5, flow
rate = 0.8 mL/min, λ = 220 nm) tmajor = 27.9 min, tminor = 20.8
45 min.
(m, 1H), 5.89-5.9 (d, J = 8 Hz, 1H), 7.09-7.18 (m, 7H), 7.57-7.58
(m, 2H). 13C NMR (CDCl3, 125 MHz): δ = 21.46, 40.87, 53.99,
126.44, 127.06, 127.84, 128.6, 129.47, 137.05, 138.95, 143.39,
175.22.
(E)-N-(1-Phenylhexa-1,5-diene-3-yl)-4-
methylbenzenesulfonamide 2o6f
The product was isolated as a white solid (yield 74 mg, 76%)
after purification by silica gel chromatography (Hexane/EtOAc =
50 90:10). Optical Rotation: [α]27D = -90.5 (c 0.6, CH2Cl2). 1H NMR
(CDCl3, 500 MHz): δ = 2.32-2.43 (m, 5H), 4.03-4.04 (m, 1H),
4.85-4.86 (d, J = 6.5 Hz, 1H), 5.05-5.11 (m, 2H), 5.61-5.69 (m,
1H), 5.78-5.62 (m, 1H), 6.27-6.3 (m, 1H), 7.12-7.25 (m, 7H),
7.72-7.74 (d, J = 8 Hz, 2H). 13C NMR (CDCl3, 125 MHz): δ =
55 21.36, 40.14, 55.18, 119.33, 126.34, 127.29, 127.66,127.92,
128.04, 128.10,m 128.28, 128.35, 128.6, 128.71, 128.93, 129.47,
129.69, 129.78, 131.53, 132.77, 136.14, 137.91, 143.28. TOF–
MS (ESI-) calcd [M - H+] for (C19H20NO2S) 326.13, Found:
105 Notes and references
a Discipline of Inorganic Materials and Catalysis, CSIR- Central Salt and
Marine Chemicals Research Institute (CSIR-CSMCRI), G. B. Marg,
Bhavnagar- 364 002, Gujarat, India. Fax: (+91) 0278-2566970. E-mail:
shrabdi@csmcri.org
110 b Academy of Scientific and Innovative Research, CSIR-Central Salt and
Marine Chemicals Research Institute (CSIR-CSMCRI), G. B. Marg,
Bhavnagar- 364 002, Gujarat, India
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
115 DOI: 10.1039/b000000x/
1
(a) K. Furuta, M. Mouri and H. Yamamoto, Synlett., 1991,
561-562; (b) G. E. Keck, K. H. Tarbet and L. S. Geraci, J. Am.
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DOI: 10.1039/C4RA10929E
RSC Advances
Page 8 of 8
Chem. Soc., 1993, 115, 8467; (c) C. M. Yu, S. K. Yoon, H. S.
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