A facile synthesis of N-sulfonyl and N-sulfinyl aldimines under Barbier-type conditions

Article abstract of DOI:10.1021/jo800009t

(Chemical Equation Presented) A convenient synthesis of N-sulfonyl- and N-sulfinylimines by the condensation of aldehydes with sulfonyl or sulfinyl amides in the presence of benzyl bromide and zinc dust at room temperature under the Barbier-type condition

Full text of DOI:10.1021/jo800009t

procedure.7 However, due to the weak nucleophilicity of
sulfonamide, harsh acidic conditions and high temperature are
normally required in the condensation reactions.8 These condi-
tions are not compatible with the resulting imines, especially
with those derived from the aliphatic aldehydes because of their
instability and ease of enolization. Although a few existing
indirect methods can be applied to synthesize these N-sulfo-
nylimines,5 it is still desired to find an efficient procedure for
the direct condensation of aldehydes with sulfonyl amides under
mild reaction conditions. We report herein an efficient and
simple procedure for the direct condensation reactions of
aldehydes with N-sulfonyl- and N-sulfinylamides under mild
Barbier-type conditions using benzyl bromide and zinc dust to
synthesize the N-sulfonyl- and N-sulfinylimines.
A Facile Synthesis of N-Sulfonyl and N-Sulfinyl
Aldimines under Barbier-Type Conditions
Renhua Fan,* Dongming Pu, Fengqi Wen, Yang Ye, and
Xiaoli Wang
Department of Chemistry, Fudan UniVersity, 220 Handan
Road, Shanghai 200433, China
rhfan@fudan.edu.cn
ReceiVed January 2, 2008
Multicomponent reactions of aldehydes, amines, and orga-
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A convenient synthesis of N-sulfonyl- and N-sulfinylimines
by the condensation of aldehydes with sulfonyl or sulfinyl
amides in the presence of benzyl bromide and zinc dust at
room temperature under the Barbier-type conditions is
reported. The procedure is lauded by its simplicity and
adaptability to aromatic, R,ꢀ-unsaturated, and aliphatic
aldehydes.
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10.1021/jo800009t CCC: $40.75
Published on Web 03/26/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 3623–3625 3623

SCHEME 1
FIGURE 1. Plausible pathway for the condensation of benzaldehyde
with TsNH2 under the Barbier-type conditions.
TABLE 1. Condensation Reactions of Benzaldehyde and TsNH2
with Various Organometallic Reagentsa
TABLE 2. Condensation of Aldehyde with Amide under the
Barbier-Type Conditionsa
imineb (%)
entry
R-M
2ab (%) entry
R-M
t-BuLi
n-BuLi, ZnBr2
t-BuLi, ZnBr2
ZnBr2
2ab (%)
entry
aldehyde
amide
TsNH2
TsNH2
TsNH2
TsNH2
TsNH2
TsNH2
TsNH2
1
2
3
4
5
PhCH2Br, Zn
CH2dCHCH2Br, Zn
Et2Zn
PhCH2Br, Mg
n-BuLi
82
0
trace
trace
0
6
7
8
9
trace
11
65
0
1
2
3
4
5
6
7
8
PhCHO
2a (93)
2b (81)
2c (76)
2d (75)
2e (88)
2f (73)
2g (55)
2h (61)
2i (68)
4-CH3C6H4CHO
3-CH3C6H4CHO
4-CH3OC6H4CHO
4-(CH3)2NC6H4CHO
4-ClC6H4CHO
2-ClC6H4CHO
2-BrC6H4CHO
4-FC6H4CHO
10 ZnBr2, 4Å AMS
0
a All reactions were conducted on a 1 mmol scale. b Isolated yield
based on the benzaldehyde.
TsNH2
TsNH2
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
1-naphthyl
2-thiophene
TsNH2
TsNH2
TsNH2
TsNH2
TsNH2
TsNH2
TsNH2
2j (73)
2k (61)
2l (67)
aldehydes and amines under Barbier-type conditions.11 As a part
of our program to develop the synthetic application of sulfona-
mides,12 we tried to synthesize some homobenzyl N-sulfonyl-
amides using the same procedure. However, the reaction of
benzaldehyde and 4-methylbenzenesulfonamide under the same
conditions did not give rise to the expected benzylation product
1b but instead N-4-methylbenzenesulfonyl imine 2a in 82%
yield (Scheme 1). No homobenzyl N-sulfonylamide was ob-
tained even though the amounts of benzyl bromide and zinc
dust were increased to 3 equiv with a longer reaction time and
a higher temperature.
In order to understand the reaction pathway, several orga-
nometallic reagents were examined. When benzyl bromide was
replaced by allylic bromide, the allylation product of benzal-
dehyde was obtained as the only product (Table 1, entry 2).
Only trace or no N-4-methylbenzenesulfonyl imine 2a was
detected from the reactions of diethylzinc, benzyl Grignard
reagent, n-butyllithium, and tert-butyllithium (Table 1, entries
3–6). When the mixture of 2 equiv of ZnBr2 with n-butyllithium
was used, the expected product 2a was isolated in 11% yield,
while the butylation product of benzaldehyde was isolated as
the byproduct (Table 1, entry 7).13 The yield of 2a increased to
65% using a mixture of ZnBr2 with tert-butyllithium (Table 1,
(CH3)2CdCHCHO
(E)-CH3CHdC(CH3)CHO
CH3C(CH3)2CHO
CH3CH(CH3)CH2CHO
CH3(CH2)2CH(CH3)CHO
PhCHO
4-CH3C6H4CHO
4-CH3OC6H4CHO
4-ClC6H4CHO
2-Cl-C6H4CHO
2-BrC6H4CHO
4-FC6H4CHO
2m (66)
2n (85)c,d
2o (81)c,d
2p (80)c,d
3a (92)
3b (92)
3d (89)
3f (96)
3g (98)
3h (98)
3i (96)
3j (88)
3q (83)
3k (94)
3r (57)
3s (60)
3l (91)
3n (56)
3o (76)
3p (91)
3t (92)
3u (78)
3v (82)
2q (70)
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
t-BuSONH2
PhSONH2
t-BuSO2NH2
1-naphthyl
2-furan
2-thiophene
2-pyridine
3-pyridine
(CH3)2CdCHCHO
CH3C(CH3)2CHO
CH3CH(CH3)CH2CHO
CH3(CH2)2CH(CH3)CHO
CH3(CH2)6CHO
CH3CH2CH(CH2CH3)CHO
PhCHO
PhCHO
a Reactions were conducted on a 1 mmol scale. b Isolated yield based
on the amide. c 1 equiv of aldehyde was used. d Conversion yield based
on the amide determined from the 1H NMR of the crude products after
workup.
(10) (a) For selected recent examples, see: Sun, X. W.; Xu, M. H.; Lin, G. Q.
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4795.
entry 8). No N-4-methylbenzenesulfonyl imine 2a was formed
in the absence of organometallic reagents even in the presence
of 4 Å MS as the dehydrating reagent (Table 1, entries 9 and
10).
A plausible pathway is shown in Figure 1. PhCH2ZnBr acted
as a base to deprotonate 4-methylbenzenesulfonamide to form
a zinc species A and PhCH3. The zinc atom in A would activate
benzaldehyde as a Lewis acid, and then the intermediate A
reacted with benzaldehyde via a four-membered ring transition
state B to generate the condensation product 2a. In this
(11) Fan, R.; Pu, D.; Qin, L.; Wen, F.; Yao, G.; Wu, J. J. Org. Chem. 2007,
72, 3149.
(12) (a) Fan, R.; Pu, D.; Wen, F. J. Org. Chem. 2007, 72, 8994. (b) Fan, R.;
Wang, W.; Pu, D.; Wu, J. J. Org. Chem. 2007, 72, 5905. (c) Fan, R.; Wen, F.;
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3624 J. Org. Chem. Vol. 73, No. 9, 2008

TABLE 3. One-Pot Condensation and Allylation Reactionsa b
,
formations of 4n, 4o, 4p, and 4t in good yields also verified
the formations of the unstable aliphatic N-Ts imines 2n, 2o,
2p, and 2t.
The basic zinc salts formed in the condensation reaction could
act as a base and a Lewis acid to promote the subsequent one-
pot nucleophilic addition. When diethyl phosphate was used as
the nucleophile, no other base or Lewis acid was required in
the addition step (1).16
entry
aldehyde
4c (%)
1
2
3
4
5
6
7
8
9
PhCHO
4a (91)
4d (82)
4i (81)
4l (88)
4m (87)
4n (81)
4o (76)
4p (71)
4t (72)
4-CH3OC6H4CHO
4-FC6H4CHO
(CH3)2CdCHCHO
(E)-CH3CHdC(CH3)CHO
CH3C(CH3)2CHO
CH3CH(CH3)CH2CHO
CH3(CH2)2CH(CH3)CHO
CH3(CH2)6CHO
(1)
a 1.1 equiv of aldehyde was used. b Reactions were conducted on a 1
mmol scale. c Isolated yield based on the amide.
In summary, we have developed an efficient and simple
method for the synthesis of N-sulfonyl- and N-sulfinylimines
by the condensation of aldehydes with sulfonyl- and sulfinyl-
amides in the presence of benzyl bromide and zinc dust at room
temperature under Barbier-type conditions. The potential of this
reaction system can be evaluated by its adaptability to a wide
variety of aldehydes and the mild reaction conditions. In
addition, the one-pot procedure for the condensations and the
following nucleophilic additions offers manipulability by avoid-
ing the isolation of the unstable imine intermediates. The further
development of asymmetric reactions is ongoing and will be
reported in due course.
condensation reaction, PhCH2ZnBr worked not only as the
dehydrating agent but as a Lewis acid to promote the reaction.
Optimization of the reaction conditions showed that no further
activation of the zinc powder was necessary, and the best ratio
among benzaldehyde, TsNH2, benzyl bromide, and zinc was
1.5:1:2:3, in which the yield of 2a increased to 93%. As shown
in Table 2, entries 1–16, the scope of the condensation reactions
of aldehydes with 4-methylbenzenesulfonamide was explored.
It turned out that our proposed reaction system could adapt to
various aldehydes. For aromatic aldehydes containing either
electron-withdrawing or -donating substituents, or the hetaryl
aldehydes (Table 2, entries 1–11), the reactions proceeded
smoothly and gave rise to the aromatic N-Ts imines in moderate
to high yields, comparable with the reactions of other methods.
Because of the mild condensation reaction conditions, R,ꢀ-
unsaturated aldehydes and nonenolizable or enolizable aliphatic
aldehydes were also good candidates for the reactions (Table
2, entries 12–16) despite their low reactivity and inherent
instability. This protocol could also be applied in the synthesis
of N-tert-butanesulfinylimine,14 N-benzenesulfinylimine,15 and
N-tert-butanesulfonylimine5b (Table 2, entries 17–36). The
procedure was effective for a wide range of aldehydes, including
electron-rich aldehydes and unreactive sterically demanding
aldehydes.
After the reaction, benzyl bromide was converted into toluene,
and the zinc salt formed in the reaction could be removed by a
simple wash with brine. The resulting stable imines could be
purified by recrystallization or column chromatography. Ali-
phatic N-Ts imines 2n, 2o, 2p, and 2t were not purified because
of their instability. Further investigation indicated that the
condensation reactions and allylation of the resulting N-Ts
imines could be carried out in one pot (Table 3). Because of
the excess of zinc dust in the condensation reactions, the addition
of allylic bromide was enough to make the allylation reactions
occur. Homoallylic N-sulfonylamides 4 derived from various
aldehydes were obtained from the one-pot reactions. The
Experimental Section
General Procedure for the Condensation of Aldehyde with
Amide under the Barbier-Type Conditions. A Schlenk tube was
charged with zinc dust (195 mg, 3 mmol), evacuated, and backfilled
with argon. Benzyl bromide (238 µL, 2 mmol) and THF (2 mL)
were added. After the mixture was stirred at room temperature for
15 min, amide (1 mmol) and aldehyde (1.5 mmol) were successively
added. The reaction mixture was stirred at room temperature until
the amide disappeared as monitored by TLC. The mixture was
quenched with brine and extracted with ethyl acetate (100 mL ×
3). The organic layer was dried over Na2SO4 and concentrated in
vacuo. The residue was purified by column chromatography on
silica gel or recrystallization to provide the desired product.
N-Benzylidene-4-methylbenzenesulfonamide 2a: 1H NMR (400
MHz, CDCl3) δ 9.04 (s, 1H), 7.88–7.94 (m, 4H), 7.62 (t, J ) 7.5
Hz, 1H), 7.49 (t, J ) 7.6 Hz, 2H), 7.35 (d, J ) 8.2 Hz, 2H), 2.44
(s, 3H). The spectral data are consistent with those reported in the
literature.5a
Acknowledgment. Financial support from the National
Natural Science Foundation of China (20702006), the Shanghai
Rising-Star program (07QA14007), and Fudan University is
gratefully acknowledged.
Supporting Information Available: Experimental proce-
dures, characterization data, and copies of 1H and 13C NMR of
new compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
JO800009T
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