Direct synthesis of N-sulfinyl- and N-sulfonylimines via copper/l-proline-catalyzed aerobic oxidative cascade reaction of alcohols with sulfinamides or sulfonamides

Article abstract of DOI:10.1039/c6ra26490e

An efficient one-pot synthetic method of N-sulfinyl- and N-sulfonylimines by the condensation of alcohols with sulfinamides or sulfonamides under mild and green conditions has been developed using a combination of CuI, l-proline and TEMPO. This system shows excellent functional group compatibility for a wide range of substrates and affords the corresponding products in good to excellent yields.

Full text of DOI:10.1039/c6ra26490e

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Direct synthesis of N-sulfinyl- and N-sulfonylimines
via copper/L-proline-catalyzed aerobic oxidative
cascade reaction of alcohols with sulfinamides or
sulfonamides†
Cite this: RSC Adv., 2017, 7, 9431
*
*
Guofu Zhang, Shengjun Xu, Xiaoqiang Xie, Chengrong Ding and Shang Shan
Received 8th November 2016
Accepted 26th January 2017
An efficient one-pot synthetic method of N-sulfinyl- and N-sulfonylimines by the condensation of alcohols
with sulfinamides or sulfonamides under mild and green conditions has been developed using
a combination of CuI, L-proline and TEMPO. This system shows excellent functional group compatibility
for a wide range of substrates and affords the corresponding products in good to excellent yields.
DOI: 10.1039/c6ra26490e
rsc.li/rsc-advances
N-Sulnyl- and N-sulfonylimines are versatile intermediates in with an organometallic reagent (DIBAL, MeLi) and menthyl
organic synthesis.1 As active substrates, they can undergo sulnate;5 (2) asymmetric oxidation of sulfenimines6 and (3)
various nucleophilic addition reactions,2 radical reactions,2j and condensation of sulnamides or sulfonamides with aldehydes
hetero-Diels–Alder reactions3 to afford the expected N-sulnyl- (Scheme 2a). The last method seems to be the most common
and N-sulfonylamide derivatives which are a class of important and useful because the pure starting materials are now
structure motifs prevalent in drugs, such as potent throm- commercially available. Therefore, in the past decades, much
boxane receptor antagonists (A),4a inhibitors of Mycobacterium attention have been paid to the direct condensation of
tuberculosis (B),4b metalloprotease inhibitors (C)4c and potential
antitrypanosomal agents (D)4d (Scheme 1).
Due to the wide range of synthetic utility of these N-sulnyl-
and N-sulfonylaldimines, numerous synthetic methods have
been developed. The main methods for the preparation of N-
sulnyl- and N-sulfonylimines include: (1) reaction of nitriles
Scheme 1 Biologically active sulfonamides scaffold.
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou
310014, People's Republic of China. E-mail: dingcr@zjut.edu.cn; shans2001@163.
com; Fax: +86-571-88320147; Tel: +86-571-88320147
† Electronic supplementary information (ESI) available: Detailed experimental
procedures, the optimization of copper-catalyzed oxidative cascade reaction and
NMR data for products. See DOI: 10.1039/c6ra26490e
Scheme
sulfonylimines.
2 Methods for the preparation of N-sulfinyl- and N-
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sulnamides or sulfonamides with aldehydes for the synthesis conditions with ambient air as the oxidant. Therefore, we
of N-sulnyl- and N-sulfonylimines [Scheme 2a, eqn (3)].5e,7–18 sought to utilize the combination of copper salt, ligand and
Even though these reported protocols could be easier to obtain TEMPO as efficient catalysts for oxidation of alcohols to alde-
N-sulnyl- or N-sulfonylimines, excess Lewis acid or stoichio- hydes followed by condensation with suln- or sulfonamides.
metric base had to be used, which generated a large amount of Herein, we reported a copper-catalyzed one-pot multi-step
environmentally unfriendly metallic waste. On the other hand, reaction system for synthesis of N-sulnyl- and N-sulfonyli-
aldehydes can be obtained from the oxidation of alcohols.19 mines from alcohols with suln- or sulfonamides under air
Therefore, the method that utilizing alcohol as one of the (Scheme 2c).
staring materials to afford the N-sulnyl- or N-sulfonylimine
Initially, benzalcohol and 4-toluenesulnamide were
was attractive. To the best of our knowledge, there was only one selected as the model substrates to determine the optimal
report on the formation of N-sulfonyl-imines starting directly conditions. First, the effect of solvent on this oxidative cascade
from alcohols, which involved saccharin–lithium bromide- transformation was examined (Table 1, entries 1–6). Good
catalyzed oxidation of alcohols to aldehydes/ketones with conversion of N-sulnylimine was obtained, when the reaction
chloramine-T followed by their condensation to afford N-tosy- was performed in CH2Cl2 or CH3OH with 5 mol% CuI and 5
limines (Scheme 2b).20
mol% L-proline in the presence of 1.0 equiv. K2CO3 under air at
copper/TEMPO-catalyzed 60 ꢀC (Table 1, entries 1 and 4). The employment of THF, DMSO
Recently, Stahl19a,b reported
a
alcohols oxidation to desired aldehydes efficiently under mild or DMF led to moderate conversions of the substrates (entries 2,
3, 6). To our delight, when switching the solvent to toluene, the
conversion reached to 88% (entry 5). Aer then, the different
copper salts were screened. As a result, the catalytic efficiency of
Cu(I) salts were better than Cu(II) salts in this catalytic system
Table 1 The optimization of copper-catalyzed oxidative cascade
reaction between alcohol and 4-toluenesulfinamidea
Table 2 Scope of N-p-tolylsulfinyl aldimines formationa
Entry
Solvent
Cu salt
Ligand
Conv.b (%)
1
2
3
4
5
6
7
8
CH2Cl2
THF
DMSO
CH3OH
Toluene
DMF
CuI
CuI
CuI
CuI
CuI
CuI
CuCl
CuBr
CuCl2
CuBr2
CuSO4
Cu(OAc)2
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
—
CuI
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Proline
L-Valine
b-Alanine
L-Histidine
Sarcosine
Glycine
Phenprobamate
Pyrrolidine
L-Proline
L-Proline
L-Proline
L-Proline
—
81
76
51
83
88
54
82
83
73
74
45
68
Trace
Trace
74
76
8
77
40
61
57
68
46
69
Entry
Product
Yieldb (%)
1
2
3
4
5
6
7
8
R ¼ H
94
90
91
91
89
89
92
91
93
93
88
85
86
90
88
93
R ¼ 3,4-Me
R ¼ p-OMe
R ¼ 3,4,5-OMe
R ¼ p-SMe
R ¼ p-Cl
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
9
10
11
12
13c
14d
15e
16f
17g
18
19
20
21
22
23
24
25h
26i
27j,h
28h
29h
R ¼ p-F
R ¼ p-Br
9
R ¼ p-NO2
R ¼ p-CF3
R ¼ o-Br
10
11
12
13
14
15
16
R ¼ o-I
R ¼ o-Cl
R ¼ 2,4-Cl
R ¼ o-Me
R ¼ m-NO2
>99 (94k)
31
11
10
20
a
Reaction conditions: benzalcohol (1.0 mmol), 4-toluenesulnamide
(1.0 mmol), copper salt (5.0 mol%), TEMPO (5.0 mol%), ligand (5.0
mol%), K2CO3 (1.0 mmol), solvent (4 mL), 4 A MS (700 mg), 60 ꢀC,
˚
a
b
c
d
e
f
12 h. Determined by HPLC. 120 ꢀC. 100 ꢀC. 80 ꢀC. 40 ꢀC.
Reaction conditions: substrates (1.0 mmol), CuI (5 mol%), L-proline (5
g
h
i
j
25 ꢀC. K2CO3 (0.5 equiv.). K2CO3 was omitted. TEMPO was mol%), TEMPO (5 mol%), K2CO3 (0.5 equiv.), toluene (4.0 mL), 4 A MS
˚
omitted. k Isolated yields.
(700 mg), 60 ꢀC, 12 h. Isolated yields.
b
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Table 3 Scope of N-sulfinyl aldimines formationa
(entries 5, 7–12), and CuI showed the better catalytic efficiency
than other Cu(I) salts with the 88% conversion (entries 5, 7 and
8). Furthermore, the temperature also played a decisive role in
this system. As shown in Table 1 entries 13 to 17, trace
conversion of N-sulnylimine was obtained at 120 ꢀC or 100 ꢀC,
ꢀ
ꢀ
ꢀ
and the conversions at 80 C, 40 C and 25 C were only 74%,
76% and 8%, respectively. Further investigation revealed that
the ligand played a critical role in this copper-catalyzed trans-
formation. Among the examined ligands such as L-proline,
L-valine, b-alanine, L-histidine, sarcosine, glycine, phenproba-
mate and pyrrolidine, L-proline was the best (Table 1, entries 5,
18–24). Finally, control experiments showed that when CuI,
TEMPO, L-proline or K2CO3 was omitted most substrates were
recovered (entries 26–29), and quantitative conversion was ob-
tained when 0.5 equiv. K2CO3 was used under the reaction
conditions (Table 1, entry 25). Thus, the optimized reaction
conditions (entry 25): substrates (1.0 mmol), TEMPO (5 mol%),
Product
Yieldb (%)
(1) R ¼ 3,4,5-OMe
(2) R ¼ o-I
86
80
78
96
95
97
92
95
(3) R ¼ o-OMe
(4) R ¼ p-Br
(5) R ¼ p-Cl
(6) R ¼ p-NO2
(7) R ¼ H
(8) R ¼ p-NO2
(9) R ¼ H
91
92
89
(10) R ¼ p-Cl
(11) R ¼ 3,4,5-OMe
(12) R ¼ o-NO2
(13) R ¼ 3,4-Me
(14) R ¼ m-OMe
(15) R ¼ p-F
ꢀ
K2CO3 (0.5 equiv.) at 60 C with CuI (5 mol%) as catalyst and
95
89c
90d
91e
L-proline (5 mol%) as ligand were found.
With the optimized conditions in hand, various aromatic
alcohols were subjected to the standard reaction conditions. As
shown in Table 2, various aromatic alcohols and 4-toluene-
sulnamide were efficiently oxidative condensed into the cor-
responding N-sulnylimines. The reaction was not only highly
efficient but also showed excellent functional groups compati-
bility. A wide range of aromatic alcohols bearing electron-donating
groups such as methyl and methoxy, or electron-withdrawing
groups including halogen, nitro and triuoromethyl substituents
were converted into their corresponding N-sulnylimines with
good to excellent isolated yields (entries 1–16, 18, 21). Surprisingly,
the efficient transformation of p-methylthiobenzyl alcohol and
4-toluenesulnamide into the desired product was observed
without transformation to sulfoxide or sulfone (entry 5). In addi-
tion, it was worth noting that the sterically hindered alcohols also
provided the corresponding N-sulnylimines in 85–90% yields
(entries 11–15). Gratifyingly, heteroaryl alcohols such as 2-thienyl
and 2-furyl methanol were also well tolerated to give the desire
products in 88–90% yields (entries 19–20). Unsaturated alcohol
(entries 17) also reacted to form the imine in 93% isolated yield.
Unfortunately, less active aliphatic alcohols and secondary alco-
hols were not suitable in the reaction.
a
Reaction conditions: substrates (1.0 mmol), CuI (5 mol%), L-proline (5
˚
mol%), TEMPO (5 mol%), K2CO3 (0.5 equiv.), toluene (4.0 mL), 4 A MS
(700 mg), 60 ꢀC. b Isolated yields. c Reaction time 15 h. d Reaction time
e
14 h. Reaction time 13 h.
Finally, we turned our efforts to expand the scope of
sulfonamides (Table 4). Although the strong electron-
withdrawing character of the sulfonyl group leads to very low
nucleophilicity of the RSO2NH2 nitrogen (much lower than of
Table 4 Scope of N-sulfonyl aldimines formationa
Product
Yieldb (%)
Next, the compatibility of a variety of other sulnamides on this
oxidative cascade reaction was examined, including tert-butane-
sulnamide, bezenesulnamide and 4-chloro-bezenesulnamide,
shown in Table 3. Good to excellent isolated yields were obtained
for aromatic alcohols with electron-withdrawing (products 2, 4–6,
8, 10, 12, 15) and electron-donating (products 1, 3, 11, 13, 14)
substituents. For aromatic alcohols bearing either ortho- (products
2, 3, 12) or meta- (product 14) substituents, the reactions pro-
ceeded smoothly and afforded N-sulnylimines in good yields. In
addition, allyl alcohols could also well tolerated under the optimal
conditions in good isolated yields (entries 7, 8). However less active
aliphatic alcohols and secondary alcohols could not afford the
target products. In general, aromatic alcohols bearing electron-
withdrawing substituents condensed more effectively with sul-
namides than those bearing electron-donating substituents
(product 6 vs. 1, product 12 vs. 11, product 15 vs. 13).
(1) R ¼ H
93
91
89c
76
78
81
80
(2) R ¼ p-Br
(3) R ¼ 3,4,5-OMe
(4) R ¼ H
(5) R ¼ o-Me
(6) R ¼ m-OMe
(7) R ¼ p-NO2
(8) R ¼ H
87
(9) R ¼ p-NO2
(10) R ¼ p-F
90
89c
a
Reaction conditions: substrates (1.0 mmol), CuI (5 mol%), L-proline (5
˚
mol%), TEMPO (5 mol%), K2CO3 (0.5 equiv.), toluene (4.0 mL), 4 A MS
(700 mg), 60 ꢀC. b Isolated yields. c 1.2 equiv. of alcohol was used.
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the insertion of a sulnamide or sulfonamide to afford species
F. Liberation of water from the F gave the product G.
Conclusions
In summary, an efficient and mild copper-catalyzed aerobic
oxidative cascade system for the synthesis of N-sulnyl- and N-
sulfonylimines directly from aryl or allyl alcohols with sulna-
mides or sulfonamides in one pot has been successfully devel-
oped. Under the optimized conditions, a wide range of
arylalcohols and various sulnamides (including chiral tert-
sulnamide) or sulfonamides were smoothly condensed into
corresponding N-sulnyl- or N-sulfonylimines with good to
excellent isolated yields. Less active aliphatic alcohols and
secondary alcohols were not suitable in the oxidative conden-
sation system, unfortunately. What's more, this is the rst
example of copper-catalyzed aerobic oxidative cascade conden-
sation for the formation of N-sulnyl- and N-sulfonyl-imines
from sulnamides or sulfonamides with alcohols.
Scheme 3 Oxidative condensation of 4-nitrobenzyl alcohol and (R)-
(+)-2-methyl-2-propanesulfinamide.
Acknowledgements
We acknowledge nancial support from the National Natural
Science Foundation of China (no. 20702051), the Natural
Science Foundation of Zhejiang Province (LY13B020017) and
the Key Innovation Team of Science and Technology in Zhejiang
Province (no. 2010R50018).
Scheme 4 A plausible catalytic cycle for the aerobic oxidative cascade
condensation.
RSONH2), prolonging the reaction time to 24 h also obtained
satisfying results. Here we investigated the oxidative cascade
reaction of aryl alcohols with various kinds of sulfonamides.
Electron-donating and -withdrawing substituents at the ortho-,
para- and meta- positions of the aryl alcohol partners were well
tolerated, providing the desired products in good yields.
Surprisingly, the 2-pyridinesulfonamide was also toleranted in
this system, giving the corresponding products in good isolated
yields (Table 4, products 4–7). Beyond that, branched alkyl
sulfonamides also underwent oxidative cascade condensation
successfully to give the corresponding products in 87–90%
isolated yields, such as tert-butylsulfonamide and cyclopropyl-
sulfonamide (products 8–10).
Given the synthetic importance of chiral sulnamides in
organic synthesis, we checked whether racemization occurred
in the reaction using pure stereochemical tert-butylsuln-amide
as one of starting materials (Scheme 3). Satisfyingly, the imine
product was obtained in 97% yield with 93% e.e. It was obvious
that the chiral sulnamide was also suitable in our system.
Based on the above promising results and related published
research studies,17,19r,21 a possible mechanism was depicted in
Scheme 4. The reaction of L-proline, K2CO3 and CuI gave the
complex A, which reacted with O2 to afford a Cu(II)-superoxide
species B. Species B abstracted a hydrogen from TEMPOH to
form species C. Subsequent reaction of the species C with
alcohol released H2O2 and afforded species D. b-Hydrogen
elimination of the alkoxo moiety in D would occur to give
a carbonyl product and TEMPOH, and regenerate A, followed by
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