Selective benzylic and allylic alkylation of protic nucleophiles with sulfonamides through double Lewis acid catalyzed cleavage of sp3 carbon-nitrogen bonds

Article abstract of DOI:10.1002/chem.200801665

The acid-catalyzed benzylic and allylic alkylation of protic nucleophiles is fundamentally important for the formation of carbon-carbon and carbon-heteroatom bonds, and it is a formidable challenge for benzylic and allylic amine derivatives to be used as the alkylating agents. Herein we report a highly efficient benzylic and allylic alkylation of protic carbon and sulfur nucleophiles with sulfonamides through double Lewis acid catalyzed cleavage of sp³ carbon-nitrogen bonds at room temperature. In the presence of a catalytic amount of inexpensive ZnCl₂-TMSCl (TMSCl: chlorotrimethylsilane), 1,3-diketones, β-keto esters, β-keto amides, malononitrile, aromatic compounds, thiols, and thioacetic acid can couple with a broad range of tosylactivated benzylic and allylic amines to give diversely functionalized products in good to excellent yields and with high regioselectivity. Furthermore, the cross-coupling reaction of 1,3-dicarbonyl compounds with benzylic propargylic amine derivatives has been successfully applied to the one-step synthesis of polysubstituted furans and benzofurans.

Full text of DOI:10.1002/chem.200801665

FULL PAPER
DOI: 10.1002/chem.200801665
Selective Benzylic and Allylic Alkylation of Protic Nucleophiles with
Sulfonamides through Double Lewis Acid Catalyzed Cleavage of sp3 Carbon–
Nitrogen Bonds
Cong-Rong Liu, Man-Bo Li, Cui-Feng Yang, and Shi-Kai Tian*[a]
Abstract: The acid-catalyzed benzylic
and allylic alkylation of protic nucleo-
philes is fundamentally important for
of sp3 carbon–nitrogen bonds at room
temperature. In the presence of a cata-
lytic amount of inexpensive ZnCl2-
TMSCl (TMSCl: chlorotrimethylsi-
lane), 1,3-diketones, b-keto esters, b-
keto amides, malononitrile, aromatic
compounds, thiols, and thioacetic acid
can couple with a broad range of tosyl-
activated benzylic and allylic amines to
give diversely functionalized products
in good to excellent yields and with
high regioselectivity. Furthermore, the
cross-coupling reaction of 1,3-dicarbon-
yl compounds with benzylic propargylic
amine derivatives has been successfully
applied to the one-step synthesis of
polysubstituted furans and benzofur-
ans.
À
the formation of carbon carbon and
À
carbon heteroatom bonds, and it is a
formidable challenge for benzylic and
allylic amine derivatives to be used as
the alkylating agents. Herein we report
a highly efficient benzylic and allylic al-
kylation of protic carbon and sulfur nu-
cleophiles with sulfonamides through
double Lewis acid catalyzed cleavage
Keywords: allylic compounds · ben-
zylic compounds · carbon nucleo-
philes · oxygen heterocycles · sulfur
Introduction
results in the formation of hydrogen halides and sulfonic
acids, the strong acidity of which may lead to undesired side
reactions. One of the promising approaches to avoid this
problem is to explore benzylic and allylic amines or their de-
rivatives as alkylating agents, which can completely shut
down the formation of acidic by-products. However, it re-
mains a formidable challenge for benzylic and allylic amines
and their derivatives to be coupled with protic nucleophiles
through catalytic cleavage of sp3 carbon–nitrogen bonds
under acidic conditions.[2]
Although benzylic amine derivatives have been reported
to undergo Friedel–Crafts alkylation of aromatic com-
pounds, such an alkylation is restricted by the use of highly
activated substrates, a large excess of acids and/or elevated
temperature, and at times the formation of a multitude of
products.[3] Furthermore, the reaction conditions have not
been extended to the benzylic and allylic alkylation of other
important protic nucleophiles, such as active methylene
compounds, thiols, and thioacids. To explore benzylic and al-
lylic amines as useful alkylating agents, we planned to acti-
vate the amino groups with appropriate electron-withdraw-
ing groups (EWG) and Lewis acids (LA) to facilitate the
cleavage of benzylic and allylic sp3 carbon–nitrogen bonds
(Scheme 1). The cross-coupling reaction of benzylic and al-
The benzylic and allylic alkylation of protic nucleophiles is
fundamentally important for the formation of carbon
carbon and carbon heteroatom bonds, and benzylic and al-
À
À
lylic halides and the corresponding sulfonates are frequently
employed as the alkylating agents. In general, at least stoi-
chiometric amounts of bases are needed to activate protic
nucleophiles, such as active methylene compounds, alcohols
and thiols, to facilitate the corresponding alkylating reac-
tions. Alternatively, the alkylation of protic nucleophiles can
be catalyzed by Lewis or Brønsted acids, owing to the ready
cleavage of benzylic and allylic sp3 carbon-halogen and sp3
carbon-oxygen bonds under acidic conditions,[1] and the
scope of protic nucleophiles for the acid-catalyzed alkylation
has been successfully extended to aromatic compounds.
Nevertheless, the use of benzylic and allylic halides and the
corresponding sulfonates as alkylating agents in the reaction
[a] C.-R. Liu, M.-B. Li, C.-F. Yang, Prof. Dr. S.-K. Tian
Department of Chemistry
University of Science and Technology of China
Hefei, Anhui, 230026 (China)
Fax : (+86)0551-3601592
E-mail: tiansk@ustc.edu.cn
À
lylic amine derivatives with protic nucleophiles (Nu H) re-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801665.
sults in the formation of ammonia derivatives, H2N
A
Chem. Eur. J. 2009, 15, 793 – 797
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793

Table 1. Survey of catalysts and activating groups.[a]
1a–1ae
EWG
Acid
t [h]
Yield [%][b]
Scheme 1. Double activation of amino groups with EWG and LA.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1a, Ts
1a, Ts
1a, Ts
1a, Ts
1a, Ts
1a, Ts
1a, Ts
1a, Ts
1a, Ts
1a, Ts
1a, Ts
1a, Ts
1a, Ts
1a, Ts
1ab, SOPh
H2SO4
TsOH
CF3SO3H
AlCl3
24
24
24
24
24
24
24
24
24
24
24
24
24
4.5
48
48
24
24
56
<5
<5
<5
18
63
59
80
60
88
<5
9
which do not exhibit strong acidity and therefore avoid con-
siderable side reactions. Herein we report a highly efficient
benzylic and allylic alkylation of various protic nucleophiles
with sulfonamides through double Lewis acid catalyzed
cleavage of sp3 carbon–nitrogen bonds at room temperature
and its application to the one-step synthesis of polysubstitut-
ed furans and benzofurans.
FeCl3
Fe
Fe
CHTUGNTERN(NUGN SO4)3·5H2O
BiCl3
Bi2A(SO4)3
CHTUNGTRENNUNG
SnCl4·5H2O
CuCl2·2H2O
FeSO4·7H2O
FeCl2·4H2O
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
7
Results and Discussion
99
69
42
70
<5
As a strong electron-withdrawing group that can be intro-
duced easily from p-tosyl chloride, the tosyl group was first
selected to activate the amino group. In the model reaction
of N-tosyl benzhydrylamine (1a) with acetyl acetone (2a) a
number of inexpensive Lewis and Brønsted acids (5 mol%),
shown in Table 1, were examined at room temperature, but
none of them showed significant catalytic activity. Gratify-
ingly, the catalytic activity of some of the Lewis and Brønst-
ed acids were dramatically enhanced by the incorporation of
chlorotrimethylsilane (TMSCl),[4,5] which when used alone
failed to catalyze the reaction. ZnCl2-TMSCl was identified
as the catalyst system of choice judging by the yield and re-
action time. In the presence of 5 mol% of ZnCl2 and
10 mol% of TMSCl the reaction proceeded cleanly to give
substituted 1,3-diketone 3aa in 99% yield, and no inter-
1ac, PO
N
1ad, COPh
1ae, Cbz
[a] Reaction conditions: 1a–1ae (0.50 mmol), 2a (1.2 equiv), acid
(5 mol%), TMSCl (10 mol%), CH2Cl2 (0.50 mL), RT. [b] Isolated yield.
Ts=p-toluenesulfonyl. Cbz=benzyloxycarbonyl.
Table 2. Catalytic alkylation of acetyl acetone with sulfonamides.[a]
1, R1
Product
t
Yield
[%][b]
1
[h]
mediate was observed by H NMR analysis of the reaction
mixture (Table 1, entry 14). It should be noted that the re-
placement of the tosyl group in substrate 1a with less elec-
tron-withdrawing groups such as SOPh, POACTHNURGTNEUNG(OPh)2, COPh
and Cbz could not activate the amino group efficiently and
therefore resulted in formation of product 3aa in much
lower yields (Table 1, entries 15–18).
1
2
3
4
1a, R=Ph
3aa
3ba
3ca
3da
3ea
3 fa
4.5
1
6
9
8
99
98
89
95
96
85
1b, R=PMP
1c, R=4-ClC6H4
1d, R=Ph
1e, R=PMP
1 f, R=4-ClC6H4
5
6[c]
30
7
1g,
1h,
3ga
7
90
A broad range of tosyl-activated benzylic and allylic
amines were examined in the ZnCl2-TMSCl-catalyzed alky-
lation of acetyl acetone (2a) at room temperature (Table 2).
The cross-coupling reaction of tosyl-activated benzylic
amines, bearing either electron-donating groups or electron-
withdrawing groups on their aromatic rings, with acetyl ace-
tone (2a) proceeded smoothly to give the corresponding
substituted 1,3-diketones in good to excellent yields
(Table 2, entries 1–8). With regard to the allylic alkylation of
acetyl acetone (2a), tosyl-activated acyclic and cyclic allylic
amines were found to serve as suitable substrates (Table 2,
entries 9–13). As demonstrated by the lack of any isomeric
products generated from the reaction with unsymmetric al-
lylic amine derivatives 1j–1l (Table 2, entries 10–12),[6] the
attack of acetyl acetone (2a) to these amine derivatives
took place only at the allylic positions in order to maintain a
8
9
10
3ha
24
73
1i, R=Ph
1j, R=Me
1k, R=nPr
3ia
3ja
3ka
5
4
4
90
86
70
11
12
1l,
3la
25
77
13
14
1m,
3ma
3aa
7
3
91
98
1af=
[a] Reaction conditions: 1 (0.50 mmol), 2a (1.2 equiv), ZnCl2 (5 mol%),
TMSCl (10 mol%), CH2Cl2 (0.50 mL), RT. [b] Isolated yield.
[c] 10 mol% of ZnCl2 and 100 mol% of TMSCl were used. PMP=4-
methACTHNUTRGNEoNUG xyphenyl.
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Chem. Eur. J. 2009, 15, 793 – 797

Allylic Alkylation of Protic Nucleophiles
FULL PAPER
Table 3. Catalytic alkylation of protic carbon nucleophiles with sulfona-
mides.[a]
maximum degree of conjugation. Furthermore, excellent se-
lectivity was observed in the application of tosyl-activated
secondary amines as benzylic and allylic alkylating agents.
For example, secondary amine derivative 1af could donate
its benzhydryl group exclusively to acetyl acetone (2a) in
the presence of the ZnCl2-TMSCl catalyst system (Table 2,
entry 14).
1
2
Product, 3
t
Yield
[h] [%][b]
The cross-coupling reaction of optically active sulfon-
amide (R)-1e (95%ee) with acetyl acetone (2a) was carried
out to afford product 3ea in nearly racemic form (Table 2,
entry 5). This result suggests that a carbocation intermediate
is generated from the sulfonamide and then couples with
the nucleophile. Nevertheless, the observed racemization
can also be ascribed, in part, to the reversible cleavage of
1
1a 2b,
3ab
1
96
2
3
1a
1a
2c, R=Me
2d, R=Ph
3ac
3ad
2
5
94
91
sp3 carbon nitrogen bonds under the reaction conditions.
4
1h 2d
3hd
9
88
À
When treated with 5 mol% of ZnCl2 and 10 mol% of
TMSCl, sulfonamide (R)-1e underwent racemization
smoothly at room temperature and its optical purity de-
creased from 95% to 11% in 24 h.
5[c] 1a 2e,
3ae 48 62
3af 24 80
3ag 0.5 80
The scope of protic carbon nucleophiles is very general in
the cross-coupling reaction with tosyl-activated benzylic and
allylic amines (Table 3). As indicated by reaction time, yield
and the amount of catalyst, 1,3-diketones and b-keto esters
showed better reactivity than b-keto amides and malononi-
trile in the benzylic alkylation of active methylene com-
pounds with tosyl-activated amines (Table 3, entries 1–6).
Appropriately activated arenes could undergo Friedel–
Crafts alkylation with tosyl-activated benzylic and allylic
amines to give the corresponding products in very good to
excellent yields[7] and with greater than 20:1 regioselectivity
(Table 3, entries 7–14).[6] Owing to their electron-donating
nature, hydroxy, methoxy, and methylmercapto not only ac-
tivate arenes in the Friedel–Crafts reaction, but also domi-
nate the regioselectivity thereof. As expected, the para-posi-
tion of aromatic ring in the nucleophile, if available, was al-
kylated predominantly by a tosyl-activated amine. Alterna-
tively, the ortho-position was the choice if the para-position
was occupied previously by another group.
6
1a 2 f,
(NC)2CH
7
8
1a
1a
2g, R=H
2h, R=Me
3ah
1
92
9
1h 2h
3hh
3
89
10
11
1j 2h
1af 2h
3jh 1.5 91
3ah 93
1
12
1a 2i, PhSMe
3ai 20 82
13
14
1a
1a
2j, R=H
2k, R=Me
3aj 0.5 82
3ak
1
90
The cross-coupling reaction of 1,3-dicarbonyl compounds
with benzylic propargylic amine derivatives was successfully
applied to the synthesis of tetrasubstituted furans,[8] which
are key moieties in many natural products and pharmaceuti-
cals.[9] The ZnCl2-TMSCl-catalyzed cross-coupling reaction
of acetyl acetone (2a) with sulfonamide 1n proceeded
smoothly at room temperature to give b-alkynyl diketone
3na, which was subsequently converted to tetrasubstituted
furan 4na through cycloisomerization in the presence of
10 mol% of ZnCl2 and 50 mol% of TMSCl at 708C
(Scheme 2).[10] Tentatively, the intramolecular nucleophilic
[a] Reaction conditions: 1 (0.50 mmol), 2 (1.2 equiv), ZnCl2 (5 mol%),
TMSCl (10 mol%), CH2Cl2 (0.50 mL), RT. [b] Isolated yield.
[c] 10 mol% of ZnCl2 and 50 mol% of TMSCl were used.
More excitingly, the direct treatment of sulfonamide 1n
and acetyl acetone (2a) with 10 mol% of ZnCl2 and
50 mol% of TMSCl at 708C resulted in formation of tetra-
substituted furan 4na in good yield (Table 4, entry 1). Fur-
thermore, this one-step procedure for the cross-coupling re-
action of 1,3-dicarbonyl compounds with benzylic propargyl-
ic amine derivatives has proved to be useful for the con-
struction of several other tetrasubstituted furans (Table 4,
entries 2–4).
Similarly, the cross-coupling reaction of para-substituted
phenols with benzylic propargylic amine derivatives was suc-
cessfully applied to the one-step synthesis of polysubstituted
benzofurans, such as compound 4nj (Scheme 3). As indicat-
ed by the appearance of intermediate 3nj at an early stage
À
attack of the enolic form of 1,3-dicarbonyl group to carbon
carbon triple bond (5-exo-dig cyclization), which is activated
by a Lewis acid (LA, ZnCl2-TMSCl) through p-coordination
(A), followed by the loss of proton to generate cyclic inter-
mediate B. Protonolysis of intermediate B regenerates the
Lewis acid catalyst and at the same time releases intermedi-
ate C, which subsequently undergoes the acid-catalyzed aro-
matization to give the final product 4na.
Chem. Eur. J. 2009, 15, 793 – 797
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S.-K. Tian et al.
Table 5. Catalytic alkylation of protic sulfur nucleophiles with sulfonami-
des.[a]
1
5
Product, 6
t
Yield
[h] [%][b]
1
1a 5a,
1a 5b,
6a
1
99
2
3
6b
6c
1
1
98
92
1a 5c, PhCH2SH
4
5
6
1a 5d,
6d 1.5
99
93
94
Scheme 2. Cycloisomerization of b-alkynyl diketone 3na to generate
furan 4na.
1h 5c
6e
6 f
2
2
Table 4. One-step synthesis of tetrasubstituted furans.
1j
5c
7
8
1a 5e, AcSH
6g 0.5
91
93
1e 5e
6h
2
1, R1
2, R2, R3
4
Isolated Yield [%]
9
1g 5e
6i
6j
1
2
83
1
2
3
4
1n, Ph
1n
1n
2a, Me, Me
2l, Ph, Ph
2c, OEt, Me
2a
4na
4nl
4nc
4oa
65
63
47
61
10 1h 5e
11 1j 5e
56
96
1o, nBu
6k 0.5
12 1n 5e
6l
1
81
[a] Reaction conditions: 1 (0.50 mmol), 5 (1.2 equiv), ZnCl2 (5 mol%),
TMSCl (10 mol%), CH2Cl2 (0.50 mL), RT. [b] Isolated yield. Ac=acetyl.
was obtained from the reaction of 4-chlorothiophenol (5b)
with N-tosyl benzhydrylamine (1a) in excellent yield
(Table 5, entry 2). The overwhelming selectivity of S-alkyla-
tion over C-alkylation in the reaction of mercapto-substitut-
ed arenes with sulfonamides prompted us to investigate
more protic sulfur nucleophiles to synthesize diverse sulfur-
containing compounds. As expected, aliphatic thiols were
found to serve as excellent substrates in the cross-coupling
reaction with tosyl-activated benzylic and allylic amines
(Table 5, entries 3–6). Further investigation showed that
readily available thioacetic acid reacted well with a range of
tosyl-activated benzylic and allylic amines to afford the cor-
responding thioesters in good to excellent yields (Table 5,
entries 7–12).
Scheme 3. One-step synthesis of benzofuran 4nj.
of the reaction, benzofuran 4nj should be formed in a reac-
tion pathway that is analogous to that for tetrasubstituted
furans.[11]
To expand the scope of protic nucleophiles, we found that
mercapto-substituted arenes were also able to couple with
tosyl-activated benzylic and allylic amines, but the reaction
gave the corresponding thioethers instead of C-alkylation
products. For example, the cross-coupling reaction of N-
tosyl benzhydrylamine (1a) with 2-methylthiophenol (5a) in
the presence of 5 mol% of ZnCl2 and 10 mol% of TMSCl
proceeded smoothly at room temperature to give thioether
6a in 99% yield (Table 5, entry 1). Likewise, thioether 6b
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Allylic Alkylation of Protic Nucleophiles
FULL PAPER
Conclusion
[1] For a review, see: G. A. Olah, R. Krishnamurti, G. K. Surya in Com-
prehensive Organic Synthesis, (Eds.: B. M. Trost, I. Fleming), Perga-
mon Press, Oxford, 1991, Vol. 3, pp. 293–339.
We have developed, for the first time, a highly efficient ben-
zylic and allylic alkylation of protic carbon and sulfur nucle-
ophiles with sulfonamides through double Lewis acid cata-
lyzed cleavage of sp3 carbon–nitrogen bonds at room tem-
perature. In the presence of a catalytic amount of inexpen-
sive ZnCl2-TMSCl, 1,3-diketones, b-keto esters, b-keto
amides, malononitrile, aromatic compounds, thiols, and thio-
acetic acid are able to couple with a broad range of tosyl-ac-
tivated benzylic and allylic amines to give diversely func-
tionalized products in good to excellent yields and with high
regioselectivity. Furthermore, the cross-coupling reaction of
1,3-dicarbonyl compounds with benzylic propargylic amine
derivatives has been successfully applied to the one-step
synthesis of polysubstituted furans and benzofurans.
[2] Although benzylic and allylic sp3 carbon nitrogen bonds can be
À
cleaved by certain metals and strong bases, benzylic and allylic
amine derivatives have hardly been exploited as useful alkylating
agents. For reviews, see: a) T. W. Greene, P. G. M. Wuts, Protective
Groups in Organic Synthesis, 3rd ed., Wiley: New York, 1999; b) S.
Escoubet, S. Gastaldi, M. Bertrand, Eur. J. Org. Chem. 2005, 3855–
3873.
[3] a) J. Esquivias, R. G. Arrayas, J. C. Carretero, Angew. Chem. 2006,
118, 645–649; Angew. Chem. Int. Ed. 2006, 45, 629–633; b) S. Shira-
kawa, S. Kobayashi, Org. Lett. 2006, 8, 4939–4942; c) M. R. Seong,
H. J. Lee, J. N. Kim, Tetrahedron Lett. 1998, 39, 6219–6222; d) H. J.
Lee, M. R. Seong, J. N. Kim, Tetrahedron Lett. 1998, 39, 6223–6226.
[4] For studies on the enhanced catalytic activity of metal Lewis acids
with silicon Lewis acids in the cleavage of sp3 carbon halogen and
À
sp3 carbon oxygen bonds, see: a) Y. Onishi, T. Ito, M. Yasuda, A.
À
Baba, Tetrahedron 2002, 58, 8227–8235; b) T. Saito, Y. Nishimoto,
M. Yasuda, A. Baba, J. Org. Chem. 2006, 71, 8516–8522, and refer-
ences therein.
[5] We recently found that a few Lewis acids in combination with
TMSCl were effective in catalyzing the nucleophilic additions to
imines. See: a) C.-R. Liu, M.-B. Li, C.-F. Yang, S.-K. Tian, Chem.
Commun. 2008, 1249–1251; b) B.-L. Yang, S.-K. Tian, Eur. J. Org.
Chem. 2007, 4646–4650; c) Q.-Y. Song, B.-L. Yang, S.-K. Tian, J.
Org. Chem. 2007, 72, 5407–5410.
[6] Observed by H NMR spectral analysis.
[7] For the reaction of dibenzylic amine derivatives with highly activat-
ed arenes or heteroarenes, see ref. [3a and 3b].
Experimental Section
General procedure for the catalytic alkylation of protic nucleophiles with
sulfonamides: To a solution of sulfonamide 1 (0.50 mmol) in dichloro-
ACHTUNGTRENNUNGmethane (0.50 mL) were added protic nucleophile 2 or 5 (0.60 mmol),
TMSCl (5.4 mg, 6.3 mL, 0.050 mmol), and ZnCl2 (3.4 mg, 5 mol%). The
resulting mixture was stirred at room temperature until no further trans-
formation was observed by using thin layer chromatography (TLC) anal-
ysis, and then purified by flash column chromatography on silica gel, elut-
ing with petroleum ether/EtOAc (200:1 to 10:1), to give product 3 or 6
(Tables 2, 3 and 5).
1
[8] For recent reviews, see: a) S. F. Kirsch, Org. Biomol. Chem. 2006, 4,
2076–2080; b) R. C. D. Brown, Angew. Chem. 2005, 117, 872–874;
Angew. Chem. Int. Ed. 2005, 44, 850–852.
General procedure for the one-step synthesis of tetrasubstituted furans:
To a solution of sulfonamide 1n (or 1o, 0.50 mmol) in 1,2-dichloroethane
(0.50 mL) were added protic carbon nucleophile 2a (2c or 2l,
0.60 mmol), TMSCl (27.2 mg, 31.5 mL, 0.25 mmol), and ZnCl2 (6.8 mg,
10 mol%). The resulting mixture was stirred at 708C for 24 h, cooled to
room temperature, and purified by flash column chromatography on
silica gel, eluting with petroleum ether/EtOAc (200:1 to 50:1), to give
product 4 (Table 4).
[9] For a review, see: B. A. Keay, P. W. Dibble in Comprehensive Heter-
ocyclic Chemistry II, (Eds.: A. R. Katritzky, C. W. Rees, E. F. V.
Scriven), Pergamon Press, Oxford, 1996, Vol. 2, pp. 395–436.
[10] Neither ZnCl2 alone nor TMSCl alone was able to catalyze the cy-
cloisomerization of b-alkynyl diketone 3na.
[11] The treatment of compound 3nj with 10 mol% of ZnCl2 and
50 mol% of TMSCl at 708C resulted in formation of benzofuran
4nj in 75% yield.
Received: August 12, 2008
Published online: November 26, 2008
Acknowledgement
We are grateful for the financial support from the National Natural Sci-
ence Foundation of China (20672105), the Chinese Academy of Sciences,
and the University of Science and Technology of China.
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