Unprecedented direct conversion of N-N and N=N bonds to N-(tert-butyloxy)carbamates

Article abstract of DOI:10.1055/s-2001-17439

For the first time a one step direct conversion of aromatic hydrazines and azo compounds to N-(tert-butoxycarbonyl) amines is achieved via reductive cleavage of N-N and N=N bonds using the inexpensive and safe hydride source polymethylhydrosiloxane (PMHS) and di-tert-butyl dicarbonate [(Boc)₂O] in the presence of a catalytic amount of 10% Pd-C.

Full text of DOI:10.1055/s-2001-17439

LETTER
1561
Unprecedented Direct Conversion of N-N and N=N bonds to N-(tert-Butyloxy)-
1
carbamates
D
S
irec
t
Conve
r
.
sion of N-
N
Ca nd N=N bonds
t
h
o
N
-(tert-Bu
a
tyloxy)-car
n
bamates drasekhar,* Ch. Raji Reddy, R. Jagadeeshwar Rao
Indian Institute of Chemical Technology, Hyderabad, 500007, India
Fax +91(40)7170512; E-mail: srivaric@iict.ap.nic.in
Received 13 August 2001
R-NH-NH2
Abstract: For the first time a one step direct conversion of aromatic
hydrazines and azo compounds to N-(tert-butoxycarbonyl) amines
is achieved via reductive cleavage of N-N and N=N bonds using the
inexpensive and safe hydride source polymethylhydrosiloxane
PMHS/Pd-C
or
R-NH-Boc
R-N=N-R (Boc)
O, ethanol, r.t.
2
Me
(
PMHS) and di-tert-butyl dicarbonate [(Boc) O] in the presence of
R=aryl
2
(
Si
O
)n
a catalytic amount of 10% Pd-C.
H
Key words: polymethylhydrosiloxane, hydrazines, azo com-
pounds, tert-butyl carbamates, one-pot reaction
PMHS
Scheme
ly as a safe, economic reagent as exemplified in a recent
review.15
Amines, both in protected as well as unprotected forms
are useful intermediates in basic as well as applied re-
search. This class has gained more prominence since the
2
In an initial study, phenylydrazine (entry 1) was treated
with polymethylhydrosiloxane, Pd-C, di-tert-butyl dicar-
bonate and ethanol as a solvent, to isolate tert-butoxycar-
bonyl aniline in 85% yield. In the second instance
azobenzene was treated under identical conditions to yield
Boc-protected aniline in almost similar yield (entry 2).
The halo aromatics (entries 5 and 6) were also stable un-
der these conditions. Also carboxylic acid and ester sub-
strates (entries 11 to 14) underwent smooth
chemoselective reductive cleavage of N-N and N=N
bonds to furnish the corresponding Boc-protected amines
in good yields. Other substrates (entries 3, 4 and 7 to 10)
were equally well behaved under the reaction conditions.
advent of the combinatorial chemistry, as amines undergo
3
diverse reactions with ease. The most common approach-
es to amines involve reduction of nitro compounds, imi-
nes, azides and oximes besides hydrazines and azo
4
compounds. There are few general methods to reduce the
nitrogen-nitrogen bond in hydrazines and the conditions
required to cleave this bond depend markedly on the sub-
5
stituents attached to it. However, substituted hydrazines
can be reduced to the corresponding amines by catalytic
6
7,8
hydrogenation or using metal in a protic solvent. In the
first case usually it is necessary to work under pressure
and acidic medium and in the second case, liquid ammo-
nia is the most common solvent. The most important
methods to reduce azo compounds lead to the correspond-
In conclusion, we have developed a convenient method
for direct conversion of hydrazines and azo compounds to
more useful Boc-protected amines. We believe that the re-
agent system described here may have tremendous poten-
tial use in organic synthesis due to its high
chemoselectivity, efficiency, economy, simplicity and
safety.
9
ing hydrazo derivatives. They include diimide, complex
metal hydrides or cobalt boride in the presence of hydra-
1
0
11,12
zine, as well as metal mediated procedures. In addi-
tion to these NiCl H O-Li-DTBB (cat.) combination has
2
2
been used for the reduction of hydrazines and azo com-
13
pounds to amines. Surprisingly, no report is documented
in the literature for direct conversion of hydrazines and
azo compounds to Boc-protected amines, which may be
desirable since, amines are often protected as Boc deriva-
tives in the course of synthesis or in the case of isolation.
Therefore one-pot conversion of hydrazines and azo com-
pounds to N-(tert-butoxy carbonyl) amines is certainly a
Typical experimental procedure
To a stirred solution of substrate (1 mmol) in EtOH (10 mL) was
added PMHS (0.180g, 3 mmols) and 10% Pd-C (~15 mg). Di-tert-
1
6
butyl dicarbonate (0.240g, 1.1 mmol) was added and the mixture
stirred for the given time (see Table) at room temperature. After the
completion of the reaction, the reaction mixture was filtered and the
14
useful transformation in organic synthesis. Our efforts in
this direction have resulted in identifying polymethylhy- filtrate was evaporated in vacuo. The residue was purified by chro-
drosiloxane under Pd-C catalysis as an efficient reagent matography on a silica gel to give the corresponding NH-Boc prod-
ucts in excellent yields (see Table).
(
Scheme). PMHS is already gaining importance especial-
Synlett 2001, No. 10, 28 09 2001. Article Identifier:
437-2096,E;2001,0,10,1561,1562,ftx,en;D10901st.pdf.
Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214
1
©

1
562
S. Chandrasekhar et al.
LETTER
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aimes
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(
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1
Time Yield
Product
(
h)
(%)a
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NHBoc
3
5
85
NH-NH2
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(
(
2
80
NHBoc
3
4
H3C
H3C
NH-NH2
N=N
3
5
78
80
CH3
CH3
(
(
(
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80
90
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Cl
X
Cl
X
X
Cl
Cl
X=F, Cl
NHBoc
7
8
HO
HO
NH-NH2
N=N
3.5
5
81
80
OH
OH
(
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EtO
EtO
NH-NH2
N=N
9
0
3
5
78
76
1
321. (c) Methoden der Organischen Chemie (Houben-
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1
OEt
OEt
4
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Y
(
1
1
1
2
NH-NH2
4
6
78
76
(
(
b) Akiba, M.; Cava, M. P. Synth. Commun. 1984, 14, 1119.
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Y
Y
Y=COOH
NHBoc
Z
Z
NH-NH2
1
3
3
5
80
79
(
12) (a) Methoden der Organischen Chemie (Houben-Weyl),
4
(1d); Bracht, J., Ed.; Thieme: Stuttgart, 1981, 575.
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Z
(b) Adam, W.; Moorthy, J. N.; Nau, W. M.; Scaiamo, J. C. J.
Z
1
4
Am. Chem. Soc. 1997, 119, 6749.
Z=COOMe
(
13) Alonso, F.; Radivoy, G.; Yus, M. Tetrahedron 2000, 56,
8
673.
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a
1
All yields refer to pure isolated products, characterised by H NMR,
(
mass and IR.
2
Ahmed, M. Synlett 2000, 1655. (c) Chandrasekhar, S.;
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Acknowledgements
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CRR thank CSIR and RJR thank UGC, New Delhi for financial as-
sistance.
(
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2
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Synlett 2001, No. 10, 1561–1562 ISSN 0936-5214 © Thieme Stuttgart · New York