Practical and general entry to N-tosyl aryl aldimines promoted by sulfamic acid in water and alcohol

Article abstract of DOI:10.1080/00397910601131387

A practical, indirect procedure composed of a three-component condensation using aromatic aldehydes, p-tosylamide, and sodium p-toluenesulfinate in the presence of sulfamic acid in tap water-alcohol solvents to afford amidosulfones, and the subsequent wat

Full text of DOI:10.1080/00397910601131387

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Practical and General Entry to N‐Tosyl Aryl
Aldimines Promoted by Sulfamic Acid in
Water and Alcohol
Zhenjiang Li a , Xinhua Ren a , Yuhu Shi a & Pingkai Ouyang a
a College of Life Science and Pharmaceutical Engineering, Nanjing University
of Technology, Nanjing, China
Published online: 10 Mar 2007.
To cite this article: Zhenjiang Li , Xinhua Ren , Yuhu Shi & Pingkai Ouyang (2007): Practical and General Entry
to N‐Tosyl Aryl Aldimines Promoted by Sulfamic Acid in Water and Alcohol, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 37:5, 713-724
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Synthetic Communicationsw, 37: 713–724, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910601131387
Practical and General Entry to N-Tosyl Aryl
Aldimines Promoted by Sulfamic Acid in
Water and Alcohol
Zhenjiang Li, Xinhua Ren, Yuhu Shi, and Pingkai Ouyang
College of Life Science and Pharmaceutical Engineering, Nanjing
University of Technology, Nanjing, China
Abstract: A practical, indirect procedure composed of a three-component conden-
sation using aromatic aldehydes, p-tosylamide, and sodium p-toluenesulfinate in the
presence of sulfamic acid in tap water–alcohol solvents to afford amidosulfones,
and the subsequent water two-phase basic elimination of the amidosulfones to
N-tosyl arylimines, was developed. The process has little environmental impact,
easy workup, mild reaction conditions, and good yields and is amenable to large-
scale preparations.
Keywords: aqueous solvent, condensation, sulfamic acid, sulfonamide, sulfonyl
imines
In view of the tremendous versatility of the C55N double bond of imines and
imine derivatives in their use in recent organic synthesis,[1] especially the
employment of imines in stereoselective reactions,[2] many preparative
approaches to N-substituted imines have been devised.[3] Sulfonyl imine is
one of the few kinds of imines bearing electron-withdrawing N-substitutions[4]
with appropriate electrophilicity and enough stability to undergo addition
reactions. Direct and indirect synthetic routes and various approaches from
aldehydes/ketones or their equivalents toward N-sulfonyl imines have been
Address correspondence to Zhenjiang Li, College of Life Science and Pharma-
ceutical Engineering, Nanjing University of Technology, Nanjing 210009, China.
E-mail: zjli@njut.edu.cn
713

714
Z. Li et al.
developed.[5] However, there exist some obvious drawbacks in the published
methods, such as the difficulties of heat-sensitive aldehydes to undergo direct
condensation at elevated temperature, the tendency of many carbonyls in the
strong Lewis acid or Brønsted acid catalysis to enolize, the incompatibility of
some functional groups with the harsh reaction conditions, the inconvenience
of employing unusual apparatus and using special and expensive reagents, and
especially the use of hazardous solvents and the tedious purifications. In the
course of our work on asymmetric synthesis of special amino acids,[6] we
needed to prepare arrays of N-substituted imines of diversity structures.
a-Amido sulfones[7] emerged recently as versatile and powerful precursors
to N-activated imines formed in situ or separated as stable intermediates. In
the latter category, a two-step approach[8] consisting of the preparation of ami-
dosulfones by a three-component condensation of aldehydes, amides, and
sulfinic acid, and the subsequent basic elimination of sulfinate from the inter-
mediate amidosulfones to afford N-substituted imines, appeared to be a
feasible one. Nevertheless, the scope of the preparations was limited; some
aspects of the process needed to be improved, such as the choice of more
efficient acid and use of environmentally friendly solvents. Practical,
simple, green, and large-scale preparations to N-sulfonylimines were our
main focus.
Using water or aqueous media for green, sustainable organic synthesis has
attracted much attention in the past decade.[9] The research and development
of nonhazardous alternatives to conventional organic solvents posed a
challenge for academia, industrial, and regulatory bodies.[10] Because of the
limited solubility and compatibility of organic compounds in water,[11] per-
forming organic reactions in neat water is not always feasible. Because
aqueous media has a lot to be recommended in green chemical preparations,
water-mixture solvents and aqueous two-phase systems are highly
preferred.[12] Thus employment of tap water as reaction media was another
purpose of ours.
Most recently, sulfamic acid has been widely used as a solid catalyst in
many organic reactions.[13] It has many prominent properties[14]: it is moder-
ately acidic, nonvolatile, noncorrosive, insoluble in common organic solvents,
inexpensive, and readily available and has outstanding physical stability. In
contrast to its various applications as a cost-effective Lewis acid catalyst,
sulfamic acid was rarely used as a Brønsted acid in current organic
synthesis to the best of our knowledge.
Based on the seminal work of Strating’s group[15] on preparation of ami-
dosulfones (compound 1, Scheme 1) by a three-component condensation
involving aldehydes, amide, and sodium sulfinate in acidic solvent and the
remarkable jobs of Chemla et al.[8f] and others[8a–d] on the basic elimination
of the amidosulfones 1 to afford imines 2, and in light of these considerations
on the two aspects in acid choice and solvent selection, we have first reported a
preliminary investigation on a two-step synthetic route to N-sulfonyl imines
mediated by sulfamic acid in aqueous media.[16] However, the previous

Preparation of N-Tosyl Arylimines in Water Media
715
Scheme 1.
communication was focused on green aspects of the process; several types of
aldehydes including a,b-unsaturated aliphatic aldehydes, hetero-aromatic
aldehydes, and most important, the benzaldehydes bearing electron-donating
substitutions remained elusive in the preparations. In continuation of our
work in this regard, expanded investigations on the scope and limitation of
an array of substituted benzaldehydes in different aqueous media at varied
temperatures were carried out.
The screening of an appropriate Brønsted acid was the key in this prep-
aration. Because the precedent methods in the three-component condensations
used formic acid (pKa 3.77) or acetic acid (pKa 4.76) as proton source and
solvent or cosolvent, and the acidity of the sulfinic acid is stronger (pKa
2.1) than that of the carboxylic acids, the reaction proceeded sluggishly as a
result of the low concentration of the active sulfinic acid 3 (Scheme 2) and
its partial decomposition in the case of elevated temperature. A catalytic
amount of camphorsulfonic acid (pKa 2 2.6) had been employed in one
Scheme 2. Plausible mechanism of the trapping of the intermediate imine 20 with
sulfinic acid 3 and the formation of the undesired bis-amide 5.

716
Z. Li et al.
occasion,[8e] unfortunately the intermediate 20 was protonated by the strong
acid to afford iminum ion 4, which was added preferably by sulfonylamide
to irreversibly produce the by-product bis-amide 5. Ordinarily, sulfonylimine
as intermediate 20 was formed at quite a low level in the mixture. Sulfinic acid
3 may trap it to form amidosulfone 1 within a favorable pH window. In this
context, neither carboxylic acid nor sulfonic acid was ideal for this addition
step. In searching for an appropriate acid, sulfamic acid caught our attention
because of its moderate acidity[17] (pKa 1.0) and unique structure.[18]
Although free arenesulfinic acid was used in this condensation,[8d,e] the acid
decomposed readily on storage and had to be prepared carefully on the
spot. Sulfamic acid as a moderate acid produces sulfinic acid in situ, keeps
its concentration at a reasonable level, and avoids the possible iminum ion
4 and bis-amide 5 formations.
Selection of aqueous solvents was carried out in various aqueous
systems (see Table 1). In water-miscible ones, combinations of aceto-
nitrile–water, dioxane–water, DMF–water, DMSO–water, ethanol–water,
methanol–water, and i-propanol–water were tried and showed promising
effects on the condensations. Two-phase water systems composed of
water/CH2Cl2 and water/dichloroethane were tested without success. Pure
water as solvent was excluded because most aldehydes used in the prep-
aration and all amidosulfones 7 formed were not soluble in it. Finally,
water–methanol and water–ethanol were chosen for their good perform-
ance, benign environmental influence,[12] economy, and adjustability of
solvent strength, tunable for different aldehydes. Sulfonamidosulfones 7
(see Scheme 3) precipitated from water–alcohol and pulled the conden-
sations to completion.
More than half of the preparations ran smoothly at ambient temperature,
yet several unexpected results were observed as the temperature dropped
around 08C or rose above 358C in winter or summer, Reaction of piperonal
Table 1. Three-component condensations mediated by NH2SO3Ha
Entry
Solvent (ratio by volume)
Time (h)
Yield (wt %)
1
2
3
4
5
6
7
8
9
CH3CN–H2O (1 : 1)
Dioxane–H2O (1 : 1)
DMF–H2O (1 : 1)
DMSO–H2O (1 : 1)
EtOH–H2O (1 : 1)
MeOH–H2O (1 : 1)
i-PrOH–H2O (1 : 1)
H2O/CH2Cl2 (1 : 1)
H2O/DCE (1 : 1)
24
24
24
24
12
12
12
12
12
35
47
29
54
69
74
65
—
—
aBenzaldehyde (1 mmol), TsNH2 (1 equiv), TolSO2Na (1.2 equiv), NH2SO3H
(2 equiv), in 5 mL of solvent, at rt (ca. 208C), for 12 h.

Preparation of N-Tosyl Arylimines in Water Media
717
Scheme 3.
was an extreme example, which appeared whimsical, and the condensation
reactions fluctuated abruptly in yields. Thus the influence of temperature on
the first condensation step was investigated (Table 2). For aldehydes with
electron-withdrawing substitutions on the phenyl group, the condensations
proceeded faster (entries 22–24) even at lower temperature (entry 25) in com-
parison with benzaldehyde (entries 1–4) as control. On the contrary, electron-
donating groups made the condensation slower (entries 11 and 13), but longer
reaction time and lower temperature resulted in good yield (entries 12, 14, and
15). The compositions of the water–alcohol were adjusted in such a manner as
to keep the aldehyde, amide, sodium sulfinate, and sulfamic acid totally
dissolved and a clear solution formed. As the condensation proceeded, the sul-
fonamidosulfones 7 precipitated as white particles. Pure 7 was obtained by
simple filtration. Sulfamic acid remaining in the filtrate can be recovered
for large-scale preparations. Sulfonamidosulfones 7 were eliminated in the
basic two-phase water systems to afford the imines 8 quantitatively, which
remained in the lower organic phase in pure form. The eliminated sulfinate
went to the upper water phase and was removed with the basic water. Thus
the products of the two steps separated simultaneously as the reactions
proceeded, making the preparation quite efficient and the separations remark-
ably simple.
In conclusion, a practical, mild, convenient, and green synthesis has been
developed for the preparation of N-tosyl imines of substituted benzaldehydes
through a two-step approach (Scheme 3). The first step was sulfamic acid–
mediated three-component condensation in tap water–alcohol solvent, and
the second step was basic elimination in a two-phase tap water media.
Electron-withdrawing substitution(s) on the benzaldehydes facilitated the con-
densations. For benzaldehydes with electron-donating substitution(s), lower
temperatures and longer times gave acceptable condensation yields. Higher
temperatures worsen the partial decomposition of the sulfonamidosulfones,
which compromised the faster reactions. Methanol–water and ethanol–
water were the preferred solvents with benign environmental influence and
efficient performance. Sulfamic acid was ideal[19] for the three-component
condensations.

Table 2. Preparation of tosylimines by three-component condensations mediated by NH2SO3H and subsequent basic elimination
Entry
Aldehyde
Iminea
Ar
Solvent (ratio by volume)
Tempb (8C)
Timeb (h)
Yieldc (wt %)
1d
2
6a
8a[5e]
C6H5
MeOH–H2O (1:1)
EtOH–H2O (1:1)
MeOH–H2O (1:1)
EtOH–H2O (1:1)
MeOH–H2O (1:1)
20
0
12
74
69
72
65
68
68
37
44
68
70
47
56
31
66
74
65
64
76
75
47
50
81
71
68
67
3
4
5
6b
8b[5ll]
4-MeC6H4
6
20
7
6c
6d
6e
8c[5ll]
8d[5e]
8e[5dd]
2-HOC6H4
4-HOC6H4
4-MeOC6H4
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
0
20
0
6f
8f[5d]
3,4-(MeO)2C6H3
Piperonyl
EtOH–H2O (4:3)
EtOH–H2O (2:1)
6g
8g[5gg]
20
0
48
12
210
20
0
6h
6i
8h[5gg]
8i[5e]
2-ClC6H4
4-ClC6H4
2,4-Cl2C6H3
MeOH–H2O (4 : 3)
EtOH–H2O (4 : 3)
MeOH–H2O (1 : 1)
20
0
6j
8j[5ll]
20
35
20
6
6k
6l
6m
8k[5e]
8l[5dd]
8m[5m]
4-CF3C6H4
4-NO2C6H4
3-NO2C6H4
210
aAll imines gave satisfactory spectroscopic data that agreed with that of the literature.[5]
bOf the first three-component condensation step.
cBased on 6, overall separated yield of 8.
dPreparation of 8a was scaled up to 3 L (1 mol of 6a); overall separated yield was 75%.

Preparation of N-Tosyl Arylimines in Water Media
719
EXPERIMENTAL
All reactants and solvents are commercially available and were used as
received without purification. Melting points were determined using a micro-
scope hot-stage apparatus without correction. IR spectra were recorded for
1
KBr pellets on a Thermo Nicolet Nexus 870 spectrometer. H NMR and
13C NMR spectra were recorded at 500 and 125.5 MHz, respectively, in
CDCl3, on a Bruker DRX-500 instrument. Chemical shifts were expressed
1
in parts per million (ppm) using TMS as internal standard for H NMR;
for 13C NMR spectra, CDCl3 (d ¼ 77.0 ppm) was used. High-resolution
electrospray mass spectra were obtained on a Mariner ESI-TOF spectrometer.
Preparation of Sulfonamidosulfones 7; General Procedure
To a stirred mixture of TsNH2 (342 mg, 2 mmol), TolSO2Na (428 mg,
2.4 mmol), and H2NSO3H (388 mg, 4 mmol) in tap water–alcohol (10 mL,
compositions as in Table 2), the corresponding aldehyde 6 (2 mmol) was
added in one portion. A clear solution formed within 15 to 30 min. The
liquid turned milky, and a white precipitate accumulated. As the reaction
completed, the white precipitate product was collected by filtration and
washed with water (2 ꢀ 3 mL) and hexane (3 mL). Sulfonamidosulfone 7
was obtained as a white solid. The wet sulfonamidosulfones 7 were
subjected to the next elimination step immediately because of their instability
on drying or further purification.
Preparation of N-Tosyl Imines 8; Typical Procedure
The sulfonamidosulfone 7 as wet filtrate cake was dispersed into CH2Cl2 (15
mL) (some entries partially dissolved), then saturated aqueous NaHCO3
(15 mL) was added, and the two-phase liquid was stirred for 2 h at rt. The
organic phase was separated, and the aqueous phase was extracted with
CH2Cl2 (2 ꢀ 5 mL). The combined organic phase was dried (Na2SO4), and
solvent was vacuum evaporated. The residue was pure imine 8.
Data
N-Benzylidene-4-methylbenzenesulfonamide (8a)[5e]
1
White solid; mp 113–1148C. IR: 3261, 1652, 1326, 1170 cm21. H NMR:
d ¼ 2.45 (s, 3 H), 7.36 (d, J ¼ 8.0 Hz, 2 H), 7.50 (t, J ¼ 7.5 Hz, 2 H), 7.64
(t, J ¼ 7.5 Hz, 1 H), 7.90–7.95 (q, 4 H), 9.05 (s, 1 H). 13C NMR: d ¼ 21.6,

720
Z. Li et al.
128.2, 128.9, 129.2, 130.0, 131.1, 133.8, 135.9, 143.4, 192.4. HRMS: m/z
[M þ H]þ calcd. for C14H14NO2Sþ: 260.0745; found 260.0739.
N-(Benzo[d][1,3]dioxol-5-ylmethylene)-4-methylbenzenesulfonamide
(8g)[5gg]
1
White solid; mp 119–1208C. IR: 3327, 1658, 1334, 1171 cm21. H NMR:
d ¼ 2.43 (s, 1 H), 6.06 (s, 2 H), 6.98 (d, J ¼ 8.2 Hz, 1 H), 7.10 (m, 1 H),
7.22 (d, J ¼ 8.5 Hz, 2 H), 7.58 (d, J ¼ 1.8 Hz, 1 H), 7.63 (d, J ¼ 8.5 Hz, 2
H), 9.00 (s, 1 H).
4-Methyl-N-(3-nitrobenzylidene)benzenesulfonamide (8m)[5m]
1
White solid; mp 141–1428C. IR: 3247, 1675, 1317, 1128 cm21. H NMR:
d ¼ 2.48 (s, 3H), 7.41 (d, J ¼ 8.0 Hz, 2 H), 7.74 (t, J ¼ 8.0 Hz, 1 H), 7.93
(d, J ¼ 8.0 Hz, 2 H), 8.27 (d, J ¼ 7.5 Hz, 2 H), 8.47 (d, J ¼ 7.5 Hz, 1 H),
8.79 (s, 1 H), 9.13 (s, 1 H).
ACKNOWLEDGMENT
This work was supported by the Major Basic Research and Development
Program of China (Grant No. 2003CB716004).
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