Carbon radical addition to N-sulfonylimines mediated by triethylborane or zinc

Article abstract of DOI:10.1016/j.tet.2008.12.031

The utility of N-sulfonylimines as radical acceptors was investigated under the different reaction conditions such as the stannyl radical-mediated addition reaction, the triethylborane-mediated tin-free radical reaction, and the zinc-mediated aqueous-medi

Full text of DOI:10.1016/j.tet.2008.12.031

Tetrahedron 65 (2009) 1321–1326
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Carbon radical addition to N-sulfonylimines mediated by triethylborane or zinc
,
Masafumi Ueda a, Hideto Miyabe b, Okiko Miyata a, Takeaki Naito a
*
a Kobe Pharmaceutical University, Motoyamakita, Higashinada, Kobe 658-8558, Japan
b School of Pharmacy, Hyogo University of Health Sciences, Minatojima, Kobe 650-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The utility of N-sulfonylimines as radical acceptors was investigated under the different reaction con-
ditions such as the stannyl radical-mediated addition reaction, the triethylborane-mediated tin-free
radical reaction, and the zinc-mediated aqueous-medium radical reaction. The alkyl radical addition
reaction of N-sulfonylimines proceeded effectively without the activation by Lewis acid. These reactions
were successfully extended to one-pot reactions for preparing a wide range of amine derivatives.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 14 November 2008
Received in revised form 11 December 2008
Accepted 11 December 2008
Available online 16 December 2008
1. Introduction
strictly limited the structure of substrate, although it was the
useful tool for the synthesis of a-amino acids. Therefore, we fo-
The carbon–nitrogen double bond has attracted significant at-
tention as an excellent radical acceptor, and thus numerous useful
intramolecular carbon–carbon bond-forming reactions of imine
derivatives are available.1,2 However, until recently, the in-
termolecular radical addition to imine derivatives has not been
widely studied. Therefore, the screening of reactive imino accep-
tors has been a challenging task in synthetic organic chemis-
try.1c,3–10 In general, the efficient intermolecular radical addition to
imine derivatives required some activation for lowering LUMO
energy of C]N bond, which classified two types of method. The
Lewis or Brønsted acids were employed for increasing the re-
activity of imine derivatives toward nucleophilic carbon radical by
coordination at nitrogen atom of C]N bond. We previously
reported that an alkyl radical addition to benzaldehyde oxime
ether 1 proceeded smoothly by using BF3$OEt2 as Lewis acid, to
give the desired adducts in high yields.11 In the case of unactivated
oxime ethers, the addition of Lewis acid was found to be essential
for the successful reaction (Eq. 1). Aldimine 3 bearing the phenolic
hydroxyl group, which acted as Brønsted acid by intramolecular
hydrogen bonding interaction, exhibited good reactivity on the
radical addition reaction even in aqueous media (Eq. 2).12 As
a direct modification of substrates, the introduction of an electron-
withdrawing group such as the carbonyl or sulfonyl group5 at
iminoic carbon enhances the reactivity of C]N bond. We reported
that the triethylborane-induced radical addition to oxime ether 5
activated by an electron-withdrawing methoxycarbonyl group
proceeded smoothly even in the absence of any Lewis acid to give
the excellent yield of adduct 6 (Eq. 3).9a,b,13 However, this method
cused our attention to introduce the activating substituent on
nitrogen atom of C]N bond, from the point of view in synthetic
application. N-Sulfonylimines are well known to be as good sub-
strates for the nucleophilic addition of organometallic reagents.14
Therefore, the electron deficient imines bearing an N-sulfonyl
group are expected to exhibit the high reactivity toward nucleo-
philic alkyl radicals (Eq. 4). Our recent studies show that
N-sulfonylimines act as excellent radical acceptors.15 From a prac-
tical point of view, it is worth noting that the radical reaction of
N-sulfonylimines does not require highly purified reagents or
solvent and proceeds effectively even in the absence of Lewis acid.
More recently, the viability of N-sulfonylimines was also demon-
strated in the dialkylzinc-induced radical reactions by Tomioka’s
group.8d–f,16,17 In this paper, we present a full study of the radical
reaction of N-sulfonylimines using Et3B as a radical initiator. We
also report the one-pot reaction and zinc-mediated reaction in
aqueous media.
1) activation by coordination of Lewis acid or Brønsted acid
NHOBn
NOBn
R
LA
BF3 •OEt2, Et3B
(1)
(2)
N
CH2Cl2
98%
Ph
Et
Ph
R
1
2
O
HO
N
HO
H
R
i-PrI, Et3B
H2O-MeOH
68%
N
HN
Ph
Ph
i-Pr
3
4
* Corresponding author. Tel.: þ81 78 441 7554; fax: þ81 78 441 7556.
E-mail address: taknaito@kobepharma-u.ac.jp (T. Naito).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.12.031

1322
M. Ueda et al. / Tetrahedron 65 (2009) 1321–1326
2) activation by latent electron-withdrawing group
product 8Ad was obtained in the reaction with methyl iodide be-
cause the generation process of primary radical is less effective
(entry 5). The reaction would be initiated by the generation of alkyl
radical from alkyl iodide and stannyl radical. The alkyl radical
attacked N-sulfonylimine 7A to generate aminyl radical A stabilized
by an electron-withdrawing sulfonyl group. The aminyl radical A
was reduced with Bu3SnH to furnish 9A and regenerate stannyl
radical into propagating cycle.
NOBn
Et3B
NHOBn
R
N
(3)
(4)
CH2Cl2
95%
MeO2C
Et
MeO2C
EWG
6
5
EWG
NHTs
NTs
N
R
This work
The development of one-pot reactions has provided the rapid
means for the preparation of complex molecules. Moreover, the
attraction to the synthetic chemists lies in the multitude of ad-
vantages that include higher yields than almost any sequential
synthesis and a single purification step. The present radical addi-
tion reaction was successfully extended to one-pot reaction
involving the formation of N-sulfonylimine (Scheme 2). Benzalde-
hyde dimethylacetal 10 was used instead of benzaldehyde. Con-
densation of dimethylacetal 10 with p-toluenesulfonamide at
150 ꢀC proceeded smoothly without any additives under the sol-
vent-free conditions to give the N-tosylimine 7A as a solid. After
CH2Cl2 was added to the same reaction vessel to dissolve 7A, RI,
Bu3SnH, and Et3B were subsequently added at ꢁ78 ꢀC and then the
reaction mixture was stirred for 8 h. When the reaction was carried
out with i-PrI, the isopropylated product 8Ab was obtained in 78%
yield after the purification. The other radical precursors such as
cyclohexyl, cyclopentyl, and sec-butyl iodides also worked well to
give 8Ac, 8Ae, and 8Af in good yields.
Ph
R
Ph
R
7
8
2. Results and discussion
2.1. Triethylborane-induced radical reaction using Bu3SnH
In order to test the reactivity of N-sulfonylimines toward nu-
cleophilic alkyl radical, we initially examined the ethyl radical ad-
dition to 7A using Et3B as an ethyl radial source (Scheme 1). When
the reaction was carried out at 25 ꢀC in the presence of Bu3SnH
(2.5 equiv), the desired product 8Aa was not obtained, but the re-
duction of C]N bond exclusively occurred to give amine 9A (Table
1, entry 1). We recently studied the Bu3SnH-promoted reduction of
the carbon–nitrogen double bond of imine derivatives.18 Based on
this result, the radical addition reaction of 7A was next performed
OMe
OMe
10
NTs
RI, Bu3SnH
Et3B
CH2Cl2
NHTs
NHTs
R
NTs
8Aa: R=Et
TsNH2
Ph
150 °C, 2 h
+
Ph
7A
8Ab: R=i-Pr
8Ac: R=c-Hexyl
8Ad: R=Me
Ph
Ph
Ph
7A
8Aa-Ad
9A
O2
Et
Bu3SnH
EtH
RI
Bu3SnI
RI, Bu3SnH
Et3B
8Ab: R=i-Pr (78%)
Et3B
NHTs
R
R
Bu3Sn
8Ac: R=c-Hexyl (77%)
8Ae: R=c-Pentyl (56%)
8Af: R=sec-Bu (43%)
Ph
7A
CH2Cl2, -78 °C
8 h
8Ab-Af
Scheme 2. One-pot reaction of imine formation and radical reaction.
NTs
Bu3SnH
8A
Ph
R
A
2.2. Tin-free radical reaction
Scheme 1. Stannyl radical-mediated radical addition to 7A.
Free radical reaction largely relied on toxic organotin chemistry.
Therefore, we have studied the tin-free radical reactions including
atom-transfer processes or single-electron transfer (SET) processes.
Based on our recent results,3b we next examined the reaction of
N-sulfonylimine 7A in the absence of Bu3SnH (Scheme 3).
at ꢁ78 ꢀC, to suppress the tin hydride-mediated reduction. As
expected, the desired adduct 8Aa was predominately obtained in
45% yield accompanied with a trace amount of amine 9A (entry 2).
This result indicates that N-sulfonylimine displays an excellent
reactivity in the intermolecular radical reaction in the absence of
Lewis acid even at ꢁ78 ꢀC.
In the absence of Bu3SnH, the reaction of 7A with an ethyl
radical was run in dichloromethane at 25 ꢀC by using Et3B (Table 2,
Good chemical yields were observed in the radical addition re-
action using secondary alkyl radical precursors such as isopropyl
and cyclohexyl iodides (entries 3 and 4), although no methylated
Et
NHTs
R
NTs
NTs
Et
RI, Et3B
8Aa: R=Et
CH2Cl2
25 °C
8Ab: R=i-Pr
8Ac: R=c-Hexyl
8Ag: R=i-Bu
Ph
7A
Ph
Ph
8Aa-Ac
11
Table 1
Stannyl radical-mediated radical addition to 7A
O2
Et
Et3B
EtI
R
Entry
RI
T (ꢀC)
Time (h)
Yield (%)
NTs
Et
7A
8Aa–Ad
9A
RI
11
Ph
R
1a
2a
3b
4b
5b
None
None
i-PrI
c-Hexyl I
MeI
25
4
8
8
8
8
Trace
45
85
50
A
ꢁ78
ꢁ78
ꢁ78
ꢁ78
Trace
Trace
Trace
Trace
Et3B
82
Et2B
Ph
n.d.c
H2O
NTs
8A
a
Reactions were carried out with Et3B (5 equiv) and Bu3SnH (2.5 equiv).
Reactions were carried out with RI (5 equiv), Et3B (2.5 equiv), and Bu3SnH
(2.5 equiv).
R
b
B
c
Complex mixture of unknown products was obtained.
Scheme 3. Et3B-mediated tin-free radical addition to 7A.

M. Ueda et al. / Tetrahedron 65 (2009) 1321–1326
1323
Table 2
OMe
NTs
Tin-free radical addition to 7A
TsNH2
150 °C, 2 h
Ph
OMe
Ph
Entry
RI
Time (h)
Product
Yield (%)
1a
2c
3c
4c
None
i-PrI
c-Hexyl I
i-BuI
2
3
3
7
8Aa
8Ab
8Ac
8Ae
84b
80
10
7A
8Aa: R=Et (72%)
8Ab: R=i-Pr (74%)
8Ac: R=c-Hexyl (59%)
8Ae: R=c-Pentyl (57%)
8Af: R=sec-Bu (48%)
55
NHTs
R
n.d.d
RI, Et3B
a
Ph
CH2Cl2, 25 °C
5 h
Reaction was carried out with Et3B (5 equivꢂ3).
Diethylated product 11 (12% yield) was obtained.
b
c
8Aa-Af
Reactions were carried out with RI (30 equiv) and Et3B (5 equivꢂ3).
d
Adduct 8Aa was obtained in 58% yield.
Scheme 5. One-pot reaction of imine formation and tin-free radical.
entry 1). The desired adduct 8Aa was obtained in 84% yield, ac-
companied with 12% yield of the C,N-diethylated product 11, al-
though the large amount of Et3B (5 equivꢂ3) was required to
complete the reaction. Good chemical yield was also observed in
the reaction with isopropyl radical (entry 2). Moreover, the reaction
with more nucleophilic secondary isopropyl radical gave selectively
adduct 8Ab with no formation of the C,N-dialkylated products. A
slightly decrease in yield was observed in the reaction with cyclo-
hexyl radical, possibly due to the bulkiness of intermediate radical
A, which prevented Et3B to trap radical A (entry 3). The reaction
with unstable primary alkyl radical such as isobutyl radical did not
give the desired product, but significant amount of ethylated
product 8Aa was obtained because of inefficient iodine atom-
transfer process (entry 4). The present triethylborane-mediated
tin-free radical reaction has a tremendous practical advantage over
the stannyl radical-mediated reaction involving the troublesome
work-up to remove the tin residues from reaction mixture. In the
tin-free reaction conditions, Et3B played multiple role concerning
reaction pathway; thus, it acted as not only a radical initiator but
also a radical terminator to trap the intermediate radical A to give
an adduct B and a chain-propagating ethyl radical.
2.3. Zinc-induced reaction in aqueous media
Today’s environmental concerns encourage the development of
greener reaction conditions. The use of water as a solvent has re-
ceived considerable attention from economic and environmental
points of view in synthetic organic chemistry.19 Particularly, car-
bon–carbon bond-formation reaction in aqueous media is a chal-
lenging task.20 We have recently demonstrated that the radical
reactions of imines such as oxime ethers, hydrazones, and nitrones
can be performed in aqueous media by using Et3B.21 In our pre-
liminary study, the Et3B-induced ethyl radical addition reaction of
7A in aqueous media was investigated. However, the formation of
desired product 8Aa in good yield was difficult due to the com-
petitive hydrolysis of C]N bond giving TsNH2 (Scheme 6).
NTs
Et3B
H2O-THF, 25 °C
20%
NHTs
Ph
Ph
Et
7A
8Aa
Scheme 6. Et3B-mediated ethyl radical addition to 7A in aqueous media.
To demonstrate the scope of the tin-free radical reaction, we
subjected a series of N-sulfonylimines 7B–G to the ethyl radical
addition reaction (Scheme 4). The radical reaction of 7B possessing
a strong electron-withdrawing CF3 group proceeded smoothly to
give adduct 8Ba in 99% yield, although the reaction of 7E with an
electron-donating substituent gave the lower yield of product 8Ea.
It is noteworthy to mention that N-sulfonylimine 7F having phe-
nolic hydroxy group has the high reactivity, probably due to the
activation and stability by the intramolecular hydrogen-bond be-
tween a C]N bond and a phenolic 2-hydroxy group. The radical
addition to 7F proceeded effectively within 2 h only with 5 equiv of
Et3B to give the desired product 8Fa in 88% yield. The mesyl group
was also able to activate imine moiety for radical addition reaction
as shown in the reaction of 7G.
The tin-free radical reaction could be extended to the one-pot
reaction, which proceeded efficiently furnishing moderate to high
yields of desired amines (Scheme 5). The second step of tin-free
one-pot reaction was able to be carried out at room temperature.
Thus, the tin-free radical reaction is more practical than the stannyl
radical-mediated reaction conducted at ꢁ78 ꢀC.
In an attempt to further improve the yield of products in the tin-
free aqueous-medium reaction, the reaction was examined under
the metal-mediated single-electron transfer reaction condi-
tions.22,23 Metallic zinc was used as a single-electron transfer rad-
ical initiator (Scheme 7). To micro tube containing 7A, i-PrI, zinc,
and dicholoromethane as a cosolvent was added dropwise satu-
rated NH4Cl over 15 min at 25 ꢀC (Table 3, entry 1).
NHSO2R1
R2
NSO2R1
NHSO
2R1
R2I, Zn
Ar
Ar
Ar
sat. NH4Cl -CH2Cl2
25 °C, 15 min
7A-G
8Ab-Gb
9A-G
7A: Ar = Ph, R1 = Tol
7F: Ar = 2-HO-C6H4, R1 = Tol
7G: Ar = Ph, R1 = Me
RI
R
Zn
Zn
SET
ZnX
O
Zn
Zn
O
NTs
NTs
R
S
N
Tol
SET
Ph
7A
Ph
Ph
A
C
Zn
Zn
Zn
Zn
H
SO2Tol
NSO2R
NHSO2R
Ar
Et
Et3B
O
N
SET
SET
CH2Cl2, 25 °C
Ar
ZnX
O
7B: Ar = 4-CF3-C6H4, R = Tol
7C: Ar = 4-MeO2C-C6H4, R = Tol
7D: Ar = 4-Cl-C6H4, R = Tol
7E: Ar = 4-MeO-C6H4, R = Tol
7F: Ar = 2-HO-C6H4, R = Tol
7G: Ar = Ph, R = Me
8Ba (99%)
8Ca (56%)
8Da (57%)
8Ea (37%)
8Fa (88%)
8Ga (81%)
O
S
NTs
7F
H
N
Tol
H
8
9
Ph
R
Ph
B
D
Scheme 4. Ethyl radical addition to 7B–7G.
Scheme 7. Zinc-mediated alkyl radical addition to 7A, 7F, and 7G.

1324
M. Ueda et al. / Tetrahedron 65 (2009) 1321–1326
Table 3
separations were carried out on precoated silica gel plates (E. Merck
Zinc-mediated alkyl radical addition to 7A, 7F, and 7Ga
60F254). Flash column chromatography was performed using E.
Merck Kieselgel 60 (230–400 mesh). The physical and spectro-
scopic data of 8Aa,24 8Ab,25 8Ac,8b 8Ba,24 8Da,24 8Ea,24 8Ga,24 9A,26
9F,27 and 9G28 are in consistent with those reported in the
literature.
Entry
Ar
R1
R2
Product
Yield (%)
8
9A, 9F, 9G
1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Me
i-Pr
i-Pr
8Ab
8Ab
8Ac
8Ae
8Af
8Ah
8Ad
8Fb
8Gb
73
8
2b
3
No reaction
c-Hexyl
c-Pentyl
sec-Bu
tert-Bu
Me
71
64
56
66
n.d.
64
21
12
18
10
20
16
8
4
5
6
7
4.2. Triethylborane-induced radical reaction using Bu3SnH
4.2.1. Stannyl radical-mediated ethyl radical addition to 7A
at 25 ꢀC (Table 1, entry 1)
8
2-OH–C6H4
Ph
i-Pr
i-Pr
9c
37
To a solution of 7A (50 mg, 0.19 mmol) in CH2Cl2 (5 mL) were
added Bu3SnH (0.13 mL, 0.48 mmol) and Et3B (1.0 M in hexane,
0.97 mL, 0.97 mmol) under N2 atmosphere at 25 ꢀC. After being
stirred at the same temperature for 4 h, the reaction mixture was
diluted with H2O and extracted with CH2Cl2. The combined organic
layers were dried over MgSO4 and concentrated under reduced
pressure. Purification by preparative TLC (hexane/AcOEt, 2:1)
afforded impure 7A, which was re-purified by preparative TLC
(hexane/AcOEt, 4:1) to give pure 9A26 (25 mg, 50%).
a
Reactions were carried out with Zn (7 equiv), RI (5 equiv), and satd NH4Cl in
CH2Cl2.
b
Reaction was carried out in the presence of galvinoxyl free radical (5 equiv).
A small amount of MeSO2NH2 was obtained.
c
The isopropylated product 8Ab was obtained in 73% yield, along
with the reduced product 9A as a result of the zinc-induced re-
duction of C]N bond. The reaction did not take place in the pres-
ence of galvinoxyl free radical as a radical scavenger (entry 2). It
suggests that the zinc-induced reaction proceeded via radical
pathway as indicated in Scheme 7. Isopropyl radical, generated
from isopropyl iodide with zinc via single-electron transfer,
attacked the sulfonylimine 7A to form aminyl radical A, which was
reduced by zinc and then protonated to afford the desired product
8Ab. On the other hand, amine 9A was obtained via direct single-
electron transfer reduction of 7A by zinc. Not only secondary alkyl
radicals but also the bulky tert-butyl radical worked well under
similar reaction conditions (entries 3–6), although methyl radical
was not efficient (entry 7). The isopropyl radical addition to
N-sulfonylimine 7F carrying phenolic hydroxy group also proceeded
(entry 8). However, unstable N-methylsulfonylimine 7G gave a low
yield of product 8Gb, due to the competitive reduction and hy-
drolysis (entry 9). It appears that the formation of 9 depended on
the reactivity of N-sulfonylimines with alkyl radicals. Thus, the
reaction with bulky tert-butyl radical and the reaction of reactive
N-mesylimine 7G led to significant amount of reduced product 9
(entries 6 and 9). In our previous studies on indium-mediated
radical reaction in aqueous media, the alkyl radical addition to
N-sulfonylimine proceeded relatively slow in which the substrate
underwent hydrolysis as a significant side reaction. In contrast, it
should be noted that the present zinc-mediated reaction completed
within 15 min thus being able to suppress the side reaction such as
a hydrolysis.
4.2.2. Stannyl radical-mediated ethyl radical addition to 7A
at ꢁ78 ꢀC (Table 1, entry 2)
To a solution of 7A (50 mg, 0.19 mmol) in CH2Cl2 (5 mL) were
added Bu3SnH (0.13 mL, 0.48 mmol) and Et3B (1.0 M in hexane,
0.97 mL, 0.97 mmol) under N2 atmosphere at ꢁ78 ꢀC. After being
stirred at the same temperature for 8 h, the reaction mixture was
diluted with H2O and extracted with CH2Cl2. The combined organic
layers were dried over MgSO4 and concentrated under reduced
pressure. Purification by preparative TLC (hexane/AcOEt, 2:1)
afforded impure 8Aa, which was re-purified by preparative TLC
(hexane/AcOEt, 4:1) to give pure 8Aa24 (25 mg, 45%).
4.2.3. Stannyl radical-mediated alkyl radical addition to 7A
at ꢁ78 ꢀC (Table 1, entries 3–5)
To a solution of 7A (50 mg, 0.19 mmol) in CH2Cl2 (5 mL) were
added RI (0.97 mmol), Bu3SnH (0.13 mL, 0.48 mmol), and Et3B
(1.0 M in hexane, 0.48 mL, 0.48 mmol) under N2 atmosphere at
ꢁ78 ꢀC. After being stirred at the same temperature for 8 h, the
reaction mixture was diluted with H2O and extracted with CH2Cl2.
The combined organic layers were dried over MgSO4 and concen-
trated under reduced pressure. Purification by preparative TLC
(hexane/AcOEt, 2:1) afforded impure 8Ab–8Ac, which were re-
purified by preparative TLC (hexane/AcOEt, 4:1) to give pure
8Ab25–8Ac.8b
3. Conclusion
4.2.4. One-pot reaction for the imine formation and stannyl
radical-mediated radical addition reaction
For the first time, we have demonstrated the utility of N-
sulfonylimines as radical acceptors under three reaction
conditions. The triethylborane-induced reaction was successfully
extended to one-pot reaction for preparing a wide range of
amine derivatives. The known examples of zinc-mediated car-
bon–carbon bond-forming reactions in the aqueous media are
mainly limited to allylations of carbonyl compounds or imines;
thus, it is noteworthy that the present radical reaction is useful
for general alkylation of imines.
A
mixture of benzaldehyde dimethylacetal 10 (53 mg,
0.35 mmol) and p-TsNH2 (50 mg, 0.29 mmol) was stirred at 150 ꢀC
for 2 h and then it was cooled to room temperature to give 7A,
which was dissolved in CH2Cl2 (15 mL). RI (0.97 mmol), Bu3SnH
(0.13 mL, 0.48 mmol), and Et3B (1.0 M in hexane, 0.48 mL,
0.48 mmol) were consecutively added to the solution of 7A under
N2 atmosphere at ꢁ78 ꢀC. After being stirred at the same temper-
ature for 8 h, the reaction mixture was diluted with H2O and
extracted with CH2Cl2. The combined organic layers were dried
over MgSO4 and concentrated under reduced pressure. Purification
by preparative TLC (hexane/AcOEt, 2:1) afforded impure 8Ab–8Af,
which were re-purified by preparative TLC (hexane/AcOEt, 4:1) to
give pure 8Ab–8Af.
4. Experimental
4.1. General
Melting points are uncorrected. 1H and 13C NMR spectra were
recorded at 200, 300, or 500 MHz and at 50 or 125 MHz, re-
spectively. IR spectra were recorded using FTIR apparatus. Mass
spectra were obtained by EI or CI methods. Preparative TLC
4.2.5. N-Cyclopentyl(phenyl)methyl-4-methylbenzene-
sulfonamide (8Ae)
Colorless crystals. Mp 139–141 ꢀC (AcOEt/hexane). IR (CHCl3)
3261,1322,1164 cmꢁ1; 1H NMR (200 MHz, CDCl3)
d 7.48–7.44 (2H, m),

M. Ueda et al. / Tetrahedron 65 (2009) 1321–1326
1325
7.10–6.92 (7H, m), 5.20–5.39 (1H, m), 4.02 (1H, dd, J¼9.0, 8.0 Hz), 2.31
d, J¼8.4 Hz), 7.55 (2H, d, J¼8.4 Hz), 7.16–7.05 (4H, m), 5.96–5.86
(1H, m), 4.25 (1H, q, J¼7.2 Hz), 3.89 (3H, s), 2.33 (3H, s), 1.86–1.62
(3H, s), 2.24–2.03 (1H, m), 1.92–1.75 (1H, m), 1.65–0.95 (7H, m); 13C
NMR (50 MHz, CDCl3)
d
142.5, 140.9, 137.7, 129.0, 128.0, 126.9, 126.85,
(2H, m), 0.78 (3H, t, J¼7.2 Hz); 13C NMR (75 MHz, CDCl3)
d 166.7,
126.78, 63.01, 46.6, 29.9, 25.02, 24.97, 21.3. HRMS calcd for
C19H23NO2S (Mþ) 329.1449, found 329.1429. Anal. Calcd for
C19H23NO2S: C, 69.27;H, 7.04; N, 4.25. Found: C, 69.27; H, 7.13;N, 4.24.
146.0, 143.0, 137.5, 129.5, 129.2, 128.9, 126.9, 126.6, 59.4, 52.0, 30.3,
21.2, 10.3. HRMS calcd for C18H22NO4S (MþþH) 348.1268, found
348.1265. Anal. Calcd for C18H21NO4S: C, 62.23; H, 6.09; N, 4.03.
Found: C, 62.19; H, 6.14; N, 3.98.
4.2.6. 4-Methyl-N-(2-methyl-1-phenylbutyl)benzene-
sulfonamide (8Af)
4.3.6. N-[1-(2-Hydroxyphenyl)propyl]-4-methylbenzene-
sulfonamide (8Fa)
White solid (a 1:1 mixture of diastereomers). IR (CHCl3) 3272,
1321, 1165 cmꢁ1
;
1H NMR (200 MHz, CDCl3)
d
7.53–7.46 (2H, m),
To a solution of 7F (50 mg, 0.18 mmol) in CH2Cl2 (5 mL) was
added Et3B (1.0 M in hexane, 5 equiv) under N2 atmosphere at
25 ꢀC. After being stirred at the same temperature for 2 h, the re-
action mixture was diluted with satd NaHCO3 and extracted with
CH2Cl2. The combined organic layers were dried over MgSO4 and
concentrated under reduced pressure. Purification by preparative
TLC (hexane/AcOEt, 3:1) afforded 8Fa (45 mg, 84%) as colorless
crystals. Mp 116–118 ꢀC (AcOEt/hexane). IR (CHCl3) 3453, 3260,
7.10–6.90 (7H, m), 5.50 (1H, m), 4.19 (1/2H, dd, J¼8.6, 6.6 Hz), 4.10
(1/2H, t, J¼8.0 Hz), 2.31 (3H, s), 1.74–1.57 (2H, m), 1.26–0.65 (7H,
m); 13C NMR (50 MHz, CDCl3)
d 142.6, 142.5, 140.1, 139.6, 137.54,
137.49, 129.0, 127.8, 126.9, 126.9, 126.7, 126.6, 62.6, 62.1, 41.0, 40.6,
25.8, 25.1, 21.2, 15.4, 14.6, 11.2, 11.0. HRMS calcd for C18H23NO2S
(Mþ) 317.1448, found 317.1457.
4.3. Tin-free radical reaction
1455, 1316, 1159 cmꢁ1 1H NMR (200 MHz, CDCl3)
; d 7.53 (2H, d,
J¼8.2 Hz), 7.07–6.52 (6H, m), 5.91 (1H, br s), 5.62 (1H, br
4.3.1. Et3B-induced tin-free ethyl radical addition to 8A
(Table 2, entry 1)
d, J¼9.0 Hz), 4.21 (1H, m), 2.31 (3H, s), 1.91–1.74 (2H, m), 0.82 (3H, t,
J¼8.0 Hz); 13C NMR (50 MHz, CDCl3)
d 153.0, 143.0, 137.0, 129.1,
To a solution of 7A (50 mg, 0.19 mmol) in CH2Cl2 (5 mL) was
added Et3B (1.0 M in hexane, 0.95 mL, 0.95 mmol) three times in
every 1 h under N2 atmosphere at 25 ꢀC. After being further stirred
at the same temperature for 15 min, the reaction mixture was di-
luted with satd NaHCO3 and extracted with CH2Cl2. The combined
organic layers were dried over MgSO4 and concentrated under re-
duced pressure. Purification by preparative TLC (hexane/AcOEt,
5:1) afforded 8Aa (46 mg, 84%) and 11 (7.2 mg, 12%).
129.0, 128.3, 126.0, 120.2, 116.1, 59.1, 28.7, 21.4, 11.0. HRMS calcd for
C16H19NO3S (MþþH) 305.1085, found 305.1080. Anal. Calcd for
C18H21NO4S: C, 62.93; H, 6.27; N, 4.59. Found: C, 62.67; H, 6.32;
N, 4.50.
4.3.7. One-pot reaction for the imine formation and tin-free
radical addition reaction
A
mixture of benzaldehyde dimethylacetal 10 (213 mg,
1.40 mmol) and p-TsNH2 (200 mg, 1.17 mmol) was stirred at 150 ꢀC
for 2 h and then it was cooled to room temperature to give 7A,
which was dissolved in CH2Cl2 (10 mL). RI (5.7 mmol) and Et3B
(1.0 M in hexane, 0.95 mL, 0.95 mmol) were consecutively added to
the solution of 7A under N2 atmosphere at 25 ꢀC. After being stirred
at the same temperature for 2 h, Et3B (1.0 M in hexane, 0.95 mL,
0.95 mmol) was added two times in 2 h. After being further stirred
at the same temperature for 1 h, the reaction mixture was diluted
with satd NaHCO3 and extracted with CH2Cl2. The combined or-
ganic layers were dried over MgSO4 and concentrated under re-
duced pressure. Purification by column chromatography on SiO2
(hexane/AcOEt, 5:1) afforded 8Aa–8Af.
4.3.2. N-Ethyl-4-methyl-N-(1-phenylpropyl)benzene-
sulfonamide (11)
Colorless oil. IR (CHCl3) 2973, 2932, 1715, 1335, 1159 cmꢁ1 1H
;
NMR (500 MHz, CDCl3)
d
7.70 (2H, d, J¼8.5 Hz), 7.29–7.20 (7H, m),
4.89 (1H, dd, J¼9.0, 7.0 Hz), 3.18–3.06 (2H, m), 2.42 (3H, s), 2.10–
2.01 (1H, m), 1.82–1.74 (1H, m), 0.93 (1H, t, J¼7.0 Hz), 0.87 (1H, t,
J¼7.0 Hz); 13C NMR (125 MHz, CDCl3)
d 142.8, 138.8, 129.5, 128.4,
128.3,127.7,127.0, 62.2, 39.0, 24.8. HRMS calcd for C18H23NO2S (Mþ)
317.1448, found 317.1452.
4.3.3. Et3B-induced tin-free alkyl addition to 7A (Table 2,
entries 2 and 3)
To a solution of 7A (50 mg, 0.19 mmol) and RI (5.7 mmol) in
CH2Cl2 (5 mL) was added Et3B (1.0 M in hexane, 0.95 mL,
0.95 mmol) three times in every 2 h under N2 atmosphere at 25 ꢀC.
After being further stirred at the same temperature for 1 h, the
reaction mixture was diluted with satd NaHCO3 and extracted with
CH2Cl2. The combined organic layers were dried over MgSO4 and
concentrated under reduced pressure. Purification by preparative
TLC (hexane/AcOEt, 5:1) afforded 8Ab–8Ac.
4.3.8. Et3B-induced tin-free ethyl radical addition to 7A in H2O–THF
To a solution of 7A (50 mg, 0.19 mmol) in H2O–THF (5:1, 5 mL)
was added Et3B (1.0 M in hexane, 0.95 mL, 0.95 mmol) three times
in every 1 h under N2 atmosphere at 25 ꢀC. After being further
stirred at the same temperature for 15 min, the reaction mixture
was diluted with satd NaHCO3 and extracted with CH2Cl2. The
combined organic layers were dried over MgSO4 and concentrated
under reduced pressure. Purification by preparative TLC (hexane/
AcOEt, 5:1) afforded 8Aa (11 mg, 20%).
4.3.4. Et3B-induced tin-free ethyl radical addition to 7B–7E and 7G
To a solution of 7B–7G (50 mg) in CH2Cl2 (5 mL) was added Et3B
(1.0 M in hexane, 5 equiv) three times in every 1 h under N2 at-
mosphere at 25 ꢀC. After being further stirred at the same tem-
perature overnight, the reaction mixture was diluted with satd
NaHCO3 and extracted with CH2Cl2. The combined organic layers
were dried over MgSO4 and concentrated under reduced pressure.
Purification by preparative TLC (hexane/AcOEt, 3:1) afforded 8Ba–
8Ea, and 8Ga.24
4.4. Zinc-induced reaction in aqueous media
4.4.1. General procedure for zinc-meditated alkyl radical addition
To
a micro tube containing N-sulfonylimine (50 mg), RI
(5 equiv), zinc dust (7 equiv), and CH2Cl2 (0.1 mL) was added
dropwise satd NH4Cl (0.4 mL) at 25 ꢀC over 15 min. After being
stirred at the same temperature for 15 min, the reaction mixture
was diluted with satd NH4Cl and then extracted with CH2Cl2. The
combined organic layers were washed with brine, dried over
MgSO4, and concentrated at reduced pressure. Purification of the
residue by preparative TLC (hexane/AcOEt, 3:1) afforded 8Ab–8Ah,
8Fb, or 8Gb and 9A, 9F,27 or 9G.28
4.3.5. 4-(1-{[(4-Methylphenyl)sulfonyl]amino}propyl)benzoic acid
methyl ester (8Ca)
Colorless crystals. Mp 132–133.5 ꢀC (AcOEt/hexane). IR (CHCl3)
3266, 1721, 1286, 1160 cmꢁ1; 1H NMR (300 MHz, CDCl3)
d 7.43 (2H,

1326
M. Ueda et al. / Tetrahedron 65 (2009) 1321–1326
M. P.; Feray, L.; Nouguier, R.; Perfetti, P. J. Org. Chem. 1999, 64, 9189; (d)
Cougnon, F.; Feray, L.; Bazin, S.; Bertrand, M. P. Tetrahedron 2007, 63, 11959.
7. (a) Friestad, G. K.; Ji, A. Org. Lett. 2008, 10, 2311; (b) Friestad, G. K.; Draghici, C.;
Soukri, M.; Qin, J. J. Org. Chem. 2005, 70, 6330; (c) Friestad, G. K.; Shen, Y.;
Ruggles, E. L. Angew. Chem., Int. Ed. 2003, 42, 5061; (d) Friestad, G. K.; Qin, J.
J. Am. Chem. Soc. 2001, 123, 9922; (e) Friestad, G. K.; Qin, J. J. Am. Chem.
Soc. 2000, 122, 8329.
8. (a) Yamada, K.; Nakano, M.; Maekawa, M.; Akindele, T.; Tomioka, K. Org. Lett.
2008, 10, 3805; (b) Yamada, K.; Yamamoto, Y.; Maekawa, M.; Akindele, T.;
Umeki, H.; Tomioka, K. Org. Lett. 2006, 8, 87; (c) Yamada, K.; Yamamoto, Y.;
Maekawa, M.; Tomioka, K. J. Org. Chem. 2004, 69, 1531; (d) Yamada, K.;
Yamamoto, Y.; Maekawa, M.; Chen, J.; Tomioka, K. Tetrahedron Lett. 2004, 45,
6595; (e) Yamada, K.; Yamamoto, Y.; Tomioka, K. Org. Lett. 2003, 5, 1797; (f)
Yamada, K.; Fujihara, H.; Yamamoto, Y.; Miwa, Y.; Taga, T.; Tomioka, K. Org.
Lett. 2002, 4, 3509.
4.4.2. N-(2,2-Dimethyl-1-phenylpropyl)-4-methylbenzene-
sulfonamide (8Ah)
White solid. IR (neat) 3274, 2973, 1326, 1163 cmꢁ1 1H NMR
;
(200 MHz, CDCl3) d 7.44–7.40 (2H, m), 7.06–6.90 (7H, m), 5.36–5.31
(1H, m), 4.03 (1H, m, J¼9.2 Hz), 2.28 (3H, s), 0.89 (9H, s); 13C NMR
(50 MHz, CDCl3) d 142.5, 138.2, 137.1, 128.9, 127.9, 127.4, 126.9, 126.6,
66.8, 35.1, 26.5, 21.2. HRMS calcd for C18H23NO2S (Mþ) 317.1448,
found 317.1446.
4.4.3. N-[1-(2-Hydoroxyphenyl)-2-methylpropyl]-4-methyl-
benzenesulfonamide (8Fb)
Colorless crystals. Mp 179–181 ꢀC (AcOEt/hexane). IR (neat)
9. (a) Ueda, M.; Ono, A.; Nakao, D.; Miyata, O.; Naito, T. Tetrahedron Lett. 2007, 48,
841; (b) Miyabe, H.; Nishimura, A.; Fujishima, Y.; Naito, T. Tetrahedron 2003, 59,
1901; (c) Miyabe, H.; Konishi, C.; Naito, T. Org. Lett. 2000, 2, 1443.
10. For other examples, see: (a) Risberg, E.; Fischer, A.; Somfai, P. Chem. Commun.
2004, 2088; (b) Singh, N.; Anand, R. D.; Trehan, S. Tetrahedron Lett. 2004, 45,
3465, 3285,1599,1312,1163 cmꢁ1; 1H NMR (200 MHz, CDCl3)
d 7.47
(2H, d, J¼8.2 Hz), 7.01–6.49 (6H, m), 5.70–5.56 (1H, m), 5.54–5.38
(1H, m), 3.92 (1H, t, J¼8.8 Hz), 2.28 (3H, s), 2.20–2.03 (1H, m), 1.05
(3H, d, J¼7.0 Hz), 0.68 (3H, d, J¼7.0 Hz); 13C NMR (50 MHz, CDCl3)
´
2911; (c) Fernandez, M.; Alonso, R. Org. Lett. 2003, 5, 2461; (d) Torrente, S.;
Alonso, R. Org. Lett. 2001, 3, 1985; (e) Russell, G. A.; Wang, L.; Rajaratnam, R.
J. Org. Chem. 1996, 61, 8988.
d
152.5, 142.8, 130.0, 129.0, 128.1, 126.9, 125.8, 120.3, 115.9, 63.8,
32.7, 21.3, 20.0, 19.8. HRMS calcd for C17H21NO3S (Mþ) 319.1241,
found 319.1214. Anal. Calcd for C17H21NO3S: C, 63.92; H, 6.63; N,
4.39. Found: C, 63.70; H, 6.55; N, 4.33.
11. (a) Miyabe, H.; Fujii, K.; Naito, T. Org. Lett. 1999, 1, 569; (b) Miyabe, H.; Fujii, K.;
Naito, T. Org. Biomol. Chem. 2003, 1, 381; (c) Miyabe, H.; Yamakawa, K.; Yosh-
ioka, N.; Naito, T. Tetrahedron 1999, 55, 11209; (d) Miyabe, H.; Shibata, R.;
Sangawa, M.; Ushiro, C.; Naito, T. Tetrahedron 1998, 54, 11431; (e) Miyabe, H.;
Shibata, R.; Ushiro, C.; Naito, T. Tetrahedron Lett. 1998, 39, 631.
12. Miyabe, H.; Yamaoka, Y.; Takemoto, Y. Synlett 2004, 2597.
13. (a) Ueda, M.; Miyabe, H.; Sugino, H.; Naito, T. Org. Biomol. Chem. 2005, 3, 1124;
(b) Miyabe, H.; Ueda, M.; Nishimura, A.; Naito, T. Tetrahedron 2004, 60, 4227; (c)
McNabb, S.; Ueda, M.; Naito, T. Org. Lett. 2004, 6, 1911; (d) Ueda, M.; Miyabe, H.;
Nishimura, A.; Sugino, H.; Naito, T. Tetrahedron: Asymmetry 2003, 14, 2857; (e)
Miyabe, H.; Ueda, M.; Nishimura, A.; Naito, T. Org. Lett. 2002, 4, 131; (f) Miyabe,
H.; Ushiro, C.; Ueda, M.; Yamakawa, K.; Naito, T. J. Org. Chem. 2000, 65, 176; (g)
Miyabe, H.; Ueda, M.; Yoshioka, N.; Naito, T. Synlett 1999, 465; (h) Miyabe, H.;
Yoshioka, N.; Ueda, M.; Naito, T. J. Chem. Soc., Perkin Trans. 1 1998, 3659; (i)
Miyabe, H.; Fujishima, Y.; Naito, T. J. Org. Chem. 1999, 64, 2174; (j) Miyabe, H.;
Ushiro, C.; Naito, T. Chem. Commun. 1997, 1789.
4.4.4. N-(2-Methyl-1-phenylpropyl)methanesulfonamide (8Gb)
Colorless oil. IR (neat) 3022, 1602, 1331, 1152 cmꢁ1 1H NMR
;
(200 MHz, CDCl3) d 7.40–7.24 (5H, m), 5.28–5.15 (1H, m), 4.16 (1H, t,
J¼8.2 Hz), 2.53 (3H, s), 2.06–1.88 (1H, m),1.04 (3H, d, J¼7.0 Hz), 0.83
(3H, d, J¼7.0 Hz); 13C NMR (50 MHz, CDCl3)
d 140.7, 128.5, 127.5,
127.5, 127.0, 64.1, 41.2, 34.2, 19.5, 19.0. HRMS calcd for C11H17NO2S
(Mþ) 227.0979, found 227.1001.
Acknowledgements
14. For review, see: (a) Bloch, R. Chem. Rev. 1998, 98, 1407; (b) Kobayashi, S.;
Ishitani, H. Chem. Rev. 1998, 99, 1069.
15. Miyabe, H.; Ueda, M.; Naito, T. Chem. Commun. 2000, 2059.
16. For review, see: Yamada, K.; Yamamoto, Y.; Tomioka, K. J. Synth. Org. Chem. Jpn.
2004, 62, 1158.
17. Prof. Tomioka also reported the diastereoselective radical addition to N-sulfi-
nylimines. Akindele, T.; Yamamoto, Y.; Maekawa, M.; Umeki, H.; Yamada, K.;
Tomioka, K. Org. Lett. 2006, 8, 5729.
We wish to thank Grant-in Aid for Scientific Research (B) (T.N.),
for Scientific Research (C) (H.M. and O.M.) and for Young Scientists
(B) (M.U.) from the Ministry of Education, Culture, Sports, Science
and Technology of Japan, and also the Science Research Promotion
Fund of the Japan Private School Promotion Foundation.
18. Ueda, M.; Miyabe, H.; Namba, M.; Nakabayashi, T.; Naito, T. Tetrahedron Lett.
2002, 43, 4369.
´
19. Garner, P. P.; Parker, D. T.; Gajewski, J. J.; Lubineau, A.; Ange, J.; Queneau, Y.;
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