Trimethylsilyl chloride promoted synthesis of α-branched amines by nucleophilic addition of organozinc halides to nitrones

Article abstract of DOI:10.1039/c2ob26202a

A general procedure for the nucleophilic addition of organozinc halides with nitrones in the presence of trimethylsilyl chloride has been developed. Trimethylsilyl chloride was found to be both an indispensable reaction promoter and a ready hydroxylamine

Full text of DOI:10.1039/c2ob26202a

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Ying Fu et al.
Trimethylsilyl chloride promoted synthesis of α-branched amines by
nucleophilic addition of organozinc halides to nitrones

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Trimethylsilyl chloride promoted synthesis of α-branched amines by
nucleophilic addition of organozinc halides to nitrones†
Ying Fu,*a Yanhua Liu,a Yaojuan Chen,a Helmut M. Hügel,b Minzhu Wang,a Danfeng Huanga and Yulai Hua
Received 24th June 2012, Accepted 31st July 2012
DOI: 10.1039/c2ob26202a
with imine derivatives such as nitrones. A few reports have
appeared in which some reactive organozinc species, such as
diorganozinc10 or diorganozinc mediated alkynylation,11 allyla-
tion,12 vinylation13 and the Reformasky reaction14 are effective.
However, to the best of our knowledge, a practical synthesis of
α-branched amine derivatives by nucleophilic addition of orga-
nozinc halides to nitrones has not been fully explored to date.
Organozinc halides are one of the most useful organometallic
reagents that can be easily prepared by direct insertion of zinc
metal into corresponding functionalized organic halides and are
therefore considered as a “real kind” of functionalized organo-
metallics.15 We were attracted by their wide application in syn-
thetic chemistry, and have reported that reaction of benzylic zinc
halides with aryl aldehydes under transition-metal-complex cata-
lysis in the presence of trimethylsilyl chloride (TMSCl) leads to
trans-stilbenes in high yield,16 whereas reaction of primary alkyl
zinc halides with aromatic aldehydes gave α,α-dialkyltoluenes.17
To broaden the utility of organozinc reagents in organic syn-
thesis, especially with the aim of exploring the utilization of
organozinc halides as a mild and selectively functionalized
nucleophile for the synthesis of α-branched amines, we report
here an interesting TMSCl promoted nucleophilic addition reac-
tion of organozinc halides with nitrones in which TMSCl was
found to be both an indispensable reaction promoter and a ready
hydroxylamine protective agent.
A general procedure for the nucleophilic addition of organo-
zinc halides with nitrones in the presence of trimethylsilyl
chloride has been developed. Trimethylsilyl chloride was
found to be both an indispensable reaction promoter and a
ready hydroxylamine protective agent in these reactions. The
produced O-(trimethylsilyl)hydroxylamines can be easily
reduced into corresponding amines just by a zinc–copper
couple in saturated aqueous NH4Cl solution.
α-Branched amines represent an important class of nitrogen con-
taining compounds, due to their attractive biological and
pharmacological activities. For example, Remacemide is used for
the treatment of acute ischemic stroke, epilepsy, Huntington’s
disease and Parkinson’s disease.1 Rivastigmine is a reversible
cholinesterase inhibitor and is used to treat Alzheimer’s disease.2
In addition, α-branched amines are highly valuable molecules in
bioorganic chemistry as they are widely found in nature, such as
2,5-dideoxy-2,5-imino-D-mannitol (DMDP) and 2,5-dideoxy-
2,5-imino-DL-glycero-D-manno-heptitol (homo DMDP), which
are found to be efficient glycoside inhibitors.3 Thus, efficient
syntheses of a library of α-branched amines are of interest in
both organic and medicinal chemistry (Fig. 1).4
The nucleophilic addition reaction of organometallic reagents
to nitrones is a convenient and reliable procedure, and hence, is
widely used in the synthesis of structurally variable α-branched
amines.5 Grignard reagents and organolithium reagents are fre-
quently deployed in these conversions due to their ready avail-
ability and high reactivity, especially when chiral nitrones6 or
chiral catalysts7 were introduced, high yields as well as high
stereoselectivities are always obtained. A large variety of natural
and semi-natural products such as pyrrolizidines and indoliz-
idines were successfully prepared via this protocol.8 However,
organozinc reagents, a kind of less reactive yet more selective
organometallic reagents,9 although extensively used in the reac-
tions with carbonyl compounds, are less explored in reactions
Initially, the reaction of benzylzinc chloride 1a and N-(4-
methoxybenzylidene)aniline oxide 2a was examined without
any catalyst. Unfortunately, no reaction occurred at room
aCollege of Chemistry and Chemical Engineering, Northwest Normal
University, Lanzhou, An’ning East Road No. 967, Gansu Province,
730070, P.R. China. E-mail: fuying@iccas.ac.cn
bHealth Innovations Research Institute and School of Applied Sciences,
RMIT University, Melbourne, 3001, Australia
†Electronic supplementary information (ESI) available: experi-
mental details, synthesis and characterisation of products. See DOI:
10.1039/c2ob26202a
Fig. 1 Drugs and natural products of α-branched amines.
Org. Biomol. Chem., 2012, 10, 7669–7672 | 7669
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Table 1 Optimization of the reaction conditionsa
Table 2 Reaction of benzylic zinc chlorides with α,N-diaryl nitronesa
Entry
Ar
R
Yieldb (%)
Entry Temp [°C] Time [h] 5 (equiv)b
3a/4a Yieldd [%]
1
2
3
4
5
6
7
8
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
4-ClC6H4 (1b)
4-ClC6H4 (1b)
4-ClC6H4 (1b)
4-ClC6H4 (1b)
4-ClC6H4 (1b)
2-ClC6H4 (1c)
2-Naphthyl (1d)
4-CH3OC6H4 (2a)
Ph (2b)
92 (4a)
90 (4b)
86 (4c)
75 (4d)
75c (4e)
67c(4f)
70 (4g)
82 (4h)
91 (4i)
80 (4j)
74 (4k)
86 (4l)
88 (4m)
83 (4n)
82 (4o)
79 (4p)
73 (4q)
62 (4r)
1
2
3
4
5
6
7
8
9
RT
60
12
6
6
—
—
—
0
65
78
92
88
86
91
—
—
4-CH3C6H4 (2c)
3,4,5-(CH3O)3C6H2 (2d)
4-HOC6H4 (2e)
3-HO-4-CH3OC6H3 (2f)
2-ClC6H4 (2g)
4-ClC6H4 (2h)
2,4-Cl2C6H3 (2i)
2-Thienyl (2j)
2-Furanyl (2k)
Ph (2b)
4-CH3OC6H4 (2a)
3,4,5-(CH3O)3C6H2 (2d)
2,4-Cl2C6H3 (2i)
2-Thienyl (2j)
1/0
1/3c
0/1
0/1
0/1
0/1
—
RT
RT
RT
RT
RT
RT
RT
TMSCl (1.0)
TMSCl (2.0)
TMSBr (2.0)
TMSI (2.0)
TMSOTf (2.0)
TBDMSCl (2.0)
TBDPMSCl (2.0)
0.5
0.5
0.5
0.5
6
9
10
11
12
13
14
15
16
17
18
6
—
a Reaction conditions: nitrone 2a (3.0 mmol) was treated with
benzylzinc chloride 1a (4.5 mmol) and organosilyl halides in THF
(15 mL) at room temperature under argon. b Amount of organosilyl
halides was based on nitrone 2a. c The ratio was determined by 1H NMR
spectra. d Isolated yields.
Ph (2b)
4-ClC6H4 (2h)
a Unless otherwise noted, the reaction were performed by employing
benzylic zinc chloride (4.5 mmol), N,α-diphenyl nitrone (3 mmol) in
THF (15 mL). b Isolated yield. c Benzylic zinc halide (2.5 equiv).
temperature. When the temperature was raised to 60 °C, the reac-
tion proceeded sluggishly and after 6 hours, the nitrone 2a had
completely consumed and the corresponding hydroxylamine 3a
was obtained in 65% yield (Table 1, entries 1 and 2).
Lewis acids can greatly enhance the reactivity of nitrones in
various kinds of reactions such as the1,3-dipolar cycloaddition18
and nucleophilic addition reactions.8b When one equivalent of
TMSCl was added, the reaction proceeded slowly at room temp-
erature and nitrone 2a disappeared in six hours. However,
hydroxylamine 3a and the O-TMS hydroxylamine ether 4a were
formed in 78% overall yield (3a/4a 1 : 3). Gratifying, when 2
equivalents of TMSCl were added, the reaction proceeded quite
quickly and cleanly. Nitrone 2a was totally consumed in only
30 minutes and the O-TMS hydroxylamine ether 4a was
obtained as the only product in 92% isolated yield (Table 1,
entries 3 and 4).
With the preliminary success of the effect of trimethylsilyl
chloride, several other organosilyl reagents were then screened.
TMSBr, TMSI and TMSOTf worked in the same way in this
reaction system. They all gave the desired silylated hydroxyl-
amine derivatives 4a cleanly with similar yield compared to
TMSCl (entries 5–7). However, the widely used hydroxyl group
protection reagents, tert-butyldimethylsilyl chloride (TBDMSCl)
and tert-butyldiphenylsilyl chloride (TBDPSCl) were ineffective
(entries 8 and 9). When considering the price and availability of
these trimethylsilyl reagents, TMSCl is the best choice and was
selected in our following studies.
benzylic zinc halide to give the corresponding O-silylated
hydroxylamines in moderate to high yields. Aromatic hetero-
cyclic aldehyde derived nitrones such as 2-thienyl and 2-furanyl
substituted nitrones all reacted smoothly, with the corresponding
silylated hydroxylamine derivatives obtained in good yield
(Table 2, entries 10, 11 and 16). Interestingly, hydroxyl substi-
tuents on the benzylidine ring of the nitrones do not require
further protection as the desired O-silylated hydroxylamine
ethers were obtained in moderate yield when a 2.5 molar ratio of
benzylic zinc chloride was added (Table 2, entries 5 and 6).
Encouraged by our success with benzylic zinc chlorides, we
then investigated other organozinc reagents (Table 3). To our
delight, other organozinc reagents (R1ZnBr) selected, such as
R1 = aryl (entries 1–6), thienyl (entries 7–9) and alkyl zinc
bromides (entries 10–15) all reacted smoothly with α,N-diaryl-
nitrones and gave the desired products in good yields. Both
primary and secondary alkyl zinc bromide can be applied here
(entries 10–15). Halogens such as chloro, fluoro are tolerated in
both organozinc halides and nitrones. However, nitrones bearing
hydroxyl groups were unreactive to aryl and alkyl zinc reagents
this time (entries 16 and 17). This is probably due to the strong
electron donating effects of the phenolate of the hydroxylated
nitrone19 which significantly reduced the electrophilicity of these
nitrones so that only the more reactive organozinc species such
as benzylic zinc halide could be used successfully.
Attracted by this unexpected reactivity, we then investigated
the efficacy of TMSCl on reactions of various benzylic zinc
halides with α,N-diaryl nitrones. Different kinds of α,N-diaryl
nitrones bearing both electron donating and electron withdrawing
groups were subjected to the optimized reaction conditions
(Table 2). The results showed that the reaction has broad applica-
bility. N-Phenyl nitrones bearing both electron-donating and
electron-withdrawing groups on the C-phenyl rings reacted with
Although Grignard reagents can react readily with nitrones
and give hydroxylamines in high yield,20 our TMSCl promoted
organozinc reagent method has some significant advantages. The
organozinc reagent can be highly functionalized whereas func-
tionalized Grignard reagents are not readily utilized. The most
practical advantage of our protocol is that the reaction of
7670 | Org. Biomol. Chem., 2012, 10, 7669–7672
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Table 3 Reaction of various organozinc bromides with nitronesa
Scheme 1 Reduction of O-TMS hydroxylamine ethers to amines.
2
7
Entry R1
R2
R3 Yieldb (%)
1
2
Ph(7a)
Ph(7a)
Ph(2b)
Bn 73 (6a)
3,4,5-(CH3O)3C6H2 (2d) Ph 91 (6b)
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Ph(7a)
Ph(7a)
2-ClC6H4(2g)
4-FC6H4(2l)
Ph(2b)
Ph 71 (6c)
Ph 84 (6d)
Ph 86 (6c)
Ph 73 (6e)
Ph 76 (6f)
Ph 77 (6g)
Ph 82 (6h)
Ph 77 (6i)
Ph 75 (6j)
Ph 79 (6k)
Ph 75 (6l)
Ph 65 (6m)
Ph 75 (6n)
Ph NRc
2-ClC6H4(7b)
1-Naphthyl(7c) 4-CH3C6H4(2c)
3-Thienyl(7d)
3-Thienyl(7d)
3-Thienyl(7d)
n-Hexyl(7e)
n-Hexyl(7e)
n-Hexyl(7e)
n-Hexyl(7e)
s-Butyl(7f)
s-Butyl(7f)
ph(7a)
2-ClC6H4(2g)
4-FC6H4(2l)
2,4-Cl2C6H3(2i)
4-CH3OC6H4(2a)
3,4,5-(CH3O)3C6H2(2d)
2-ClC6H4(2g)
2-Thienyl(2j)
4-CH3C6H4(2c)
2-CH3OC6H4(2n)
3-HO-4-CH3OC6H3(2f)
4-HOC6H4(2e)
Scheme 2 Synthesis of α-branched polyhydroxylated pyrrolidine 12.
n-Hexyl(7e)
Ph NRc
a Unless otherwise noted, the reaction was performed by employing
organozinc halide (4.5 mmol), N,α-diphenyl nitrone (3 mmol) in THF
(15 mL). b Isolated yield. c NR = No reaction.
2 hours in saturated aqueous NH4Cl solution and with zinc metal
in the presence of catalytic amount of Cu(OAc)2, 10 was com-
pletely reduced to the pyrrolidine 11 in 91% yield. Deprotection
of 11 by catalytic hydrogenation in acid solution gave the
α-ethyl polyhydroxylated pyrrolidine 12 in quantitative yield
which is a structural analogue of 2,5-dideoxy-2,5-imino-DL-
glycero-D-manno-heptitol (homo DMDP), a natural selective
inhibitor of β-glucosidases.3b
In conclusion, we have successfully developed a new method
for the synthesis of substituted O-trimethylsilyl hydroxylamine
ethers via a TMSCl-catalyzed nucleophilic addition of organo-
zinc halides to nitrones. To the best of our knowledge, the
present reaction is the first example of a TMSCl catalyzed
nucleophilic addition reaction of organozinc reagents with
nitrones whereby the TMSCl acts both as a reaction promoter
and a good protection reagent for the sensitive N-hydroxylamine
group. Furthermore, the O-TMS protected hydroxylamine ethers
can be easily reduced by zinc to the corresponding amines. This
protocol offers a practically useful method in natural product
synthesis.
Grignard reagents with nitrones leads only to hydroxylamines
under the same reaction condition and since the hydroxylamine
derivatives, especially N-substituted aromatic hydroxylamines
are very air sensitive. They decompose readily20 and usually
cannot be isolated in a pure form by quickly polymerizing after
column chromatography, thus, the synthesis, purification and
characterization of hydroxylamines proved challenging. Even
when these compounds were kept under nitrogen in a deep
freezer for several weeks, they were found to have decomposed
1
as indicated by H NMR analysis. However, in our protocol, the
O-TMS hydroxylamine ethers are stable to air and can be kept in
a refrigerator for months without any decomposition. We con-
sider our protocol whereby TMSCl serves as a reaction promoter
and a ready N-hydroxylamine protecting reagent is a significant
improvement on the reaction of any other previously reported
Grignard and diorganozinc methods.
The O-trimethylsilyl hydroxylamine ethers can be easily con-
verted into corresponding amines by simply stirring the ethers
with a zinc–copper couple in a saturated aqueous NH4Cl at
50 °C for 1 hour. As illustrated in Scheme 1, the substrates 4a
and 4d were reduced using this protocol to the corresponding
amines in 77% and 72% isolated yields respectively.
To test the application of our TMSCl promoted organozinc
protocol for natural product synthesis, we prepared a cyclic
nitrone 9 from D-mannose21 (Scheme 2). Gratifyingly, 9 reacted
with vinylzinc chloride in the presence of two equivalents of
TMSCl at room temperature to furnish the desired O-TMS-
hydroxylamine ether 10 in 84% yield stereospecifically. Upon
further stirring the reaction mixture at room temperature for
Acknowledgements
The authors are grateful for financial support from the National
Natural Science Foundation of China (No. 20962017) and
the Natural Science Foundation of Gansu Province, China
(No. 1107RJZA263).
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7672 | Org. Biomol. Chem., 2012, 10, 7669–7672
This journal is © The Royal Society of Chemistry 2012