New and mild allyl carbamate deprotection method catalyzed by electrogenerated nickel complexes

Article abstract of DOI:10.1016/S0040-4039(00)01223-5

A Ni(II)-catalyzed electrochemical procedure for the simple and mild deprotection of allyl carbamates to the corresponding amines is described. The amines are obtained in yields of 40-99% and the method is compatible with several functional groups. Electrolyses are carried out in DMF or in THF in single-compartment cells in the presence of a consumable zinc anode. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01223-5

Tetrahedron Letters 41 (2000) 7333±7336
New and mild allyl carbamate deprotection method catalyzed
by electrogenerated nickel complexes
Ä
Delphine Franco and Elisabet Dunach*
Laboratoire de Chimie Bioorganique, Associe au CNRS, Universite de Nice, 06108 Nice Cedex 2, France
Received 23 June 2000; accepted 18 July 2000
Abstract
A Ni(II)-catalyzed electrochemical procedure for the simple and mild deprotection of allyl carbamates to
the corresponding amines is described. The amines are obtained in yields of 40±99% and the method is
compatible with several functional groups. Electrolyses are carried out in DMF or in THF in single-
compartment cells in the presence of a consumable zinc anode. # 2000 Elsevier Science Ltd. All rights
reserved.
Keywords: allyl carbamate; deprotection; electrochemical; nickel; catalysis.
The development of mild and selective methods for the deprotection of functional groups
constitutes a signi®cant aspect in the synthesis of polyfunctional molecules. Carbamates
are widely used as protecting groups for amines in amino-acid, peptide and oligonucleotide
chemistry.1
Among the various amine protecting groups, the Boc group (t-butoxycarbonyl) has been
2
frequently used in organic synthesis for its easy removal under acidic conditions. Allyl carbamates
have also been widely used, due to the fact that allyloxycarbamoyl groups are stable under mild
acidic or basic conditions. Their selective deprotection is generally carried out in the presence of a
Pd(0) catalyst and a reducing agent.3
Allyl groups constitute useful protecting units for carboxylic acids or alcohols, and several
�6
7
6,8�10
methods, including double-bond isomerization and organometallic catalysis
for their deprotection.
are available
Electrochemistry has also been reported for the selective deprotection of several functional
groups.11 To our knowledge, the electrochemical methodology has not yet been reported for allyl
carbamate deprotection to the corresponding amines. However, electrochemistry, associated with
catalysis by metal complexes has been reported as an ecient methodology for the related
*
Corresponding author. Tel: 33-492076142; fax: 33-492076151; e-mail: dunach@unice.fr
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01223-5

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334
12
deprotection of allyl and propargyl ethers and esters in Sm(III), Ni(II),13 and Pd(II)-catalyzed
reactions.1
We present here our results on the electrochemical reductive deprotection of allyl carbamates
4,15
0
such as 1 in the presence of a catalytic amount of [Ni(bipy) ](BF ) complex, 2 (bipy=2,2 -
4 2
3
bipyridine), in a reductive decarboxylative-type process as shown in Eq. (1).
ꢀ1†
The reactions were carried out in DMF under mild and neutral conditions, at room temperature,
in a single-compartment cell ®tted with a consumable Zn rod anode. The use of consumable
anodes in selective electrosynthesis has already been reported.1
6,17
The electrolyses were run at
constant current with 10 mol% of Ni(II) complex, 2 with respect to substrates 1. The results of
the electrolyses of 1a±h are presented in Table 1.
Allyl aryl carbamates 1a±d were selectively deprotected in yields of 70±99%. The reaction
generally consumed 2±3 F/mol of 1. Esters and nitrile groups were compatible with the reaction
conditions (entries 2, 3). Interestingly, the acetophenone group of 1d (entry 4) remained unreacted
and was not reduced during electrolysis, although aryl ketones are known to be electrochemically
reduced to their pinacol adducts.18
Allylamines were found as by-products in other reported examples of allyl carbamate
deprotection,19 thus lowering the eciency of the deprotection method. It is interesting to note
that in the electrochemical procedure, no competitive allylamine formation could be observed.
Benzyl and cyclohexyl amines could be obtained in good to excellent yields from the
corresponding allyl carbamates 1e and 1f (entries 5, 6). In the case of the acetal-carbamate 1g
(entry 7), the reaction a€orded better isolated yields in THF than in DMF, due to the easier
work-up procedure in THF. In the case of THF electrolyses, bis(tri¯uoromethylsulfonimide)
^2
lithium salt was used as supporting electrolyte (10 M) instead of tetrabutylammonium
tetra¯uorobarate (used in 10^ M in DMF reactions), for its better solubility in THF. The
dimethyl acetal of 2-aminoacetaldehyde was obtained in 60% yield.
2
Protected S-(+)-amino acid 1h a€orded the corresponding deallylated dipeptide in 40% yield.
No racemization occurred upon removal of the allyloxycarbonyl group and no formation of
diketopiperazine was observed either.
In the one-compartment electrolysis, the reactions at the electrodes are, at the anode, the
oxidation of the zinc metal rod into Zn2 ions and at the cathode, we propose that Ni(0) species
+
2
0
are generated and recycled from the reduction of the Ni(II) catalyst precursor, 2.
When compared to the Pd-catalyzed methodology,
stoichiometric amount of a reducing agent such as tin hydrides.
3�6
the present method avoids the use of a
In conclusion, electrochemistry may provide an interesting and useful alternative method for
allyl carbamate deprotection under mild and catalytic conditions, the reaction taking place at
room temperature and under neutral conditions. The method is compatible with several
functional groups such as ester, nitrile, ketone or acetal. It also presents the advantage of using a
stable and easily available Ni(II) complex as the starting catalyst precursor.

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Table 1
Electroreduction of allyl carbamates 1 catalyzed by complex 2
a)

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336
References
1
2
3
. Kocienski, P. J. In Protecting Groups; Thieme Verlag, Stuttgart, 1994.
. Subhas Bose, D.; Lakshminarayana, V. Synthesis 1999, 1, 66±68, and references cited therein.
. (a) Merzouk, A.; Guibe
3509±13555. (c) Guibe, F. Tetrahedron 1998, 54, 2967±3042.
. Kimbonguila, A. M.; Merzouk, A.; Guibe, F.; Lo€et, A. Tetrahedron 1999, 55, 6931±6944.
. Boos, E. C.; Barnabe, P.; Hiemstra, H.; Speckamp, W. N.; Kaptein, B.; Boesten, W. J. J. Org. Chem. 1995, 60,
733±1740.
. Zhang, H. X.; Guibe
, F.; Lo€et, F. Tetrahedron Lett. 1992, 33, 477±480. (b) Guibe, F. Tetrahedron 1997, 53,
 Â
1
Â
4
5
Â
Â
1
6
7
8
9
Â
, F.; Balavoine, G. Tetrahedron Lett. 1988, 29, 619±622.
. Gigg, J.; Gigg, R. J. Chem. Soc. C. 1966, 82±86.
. Mereyala, H. B.; Guntha, S. Tetrahedron Lett. 1993, 34, 6929±6930.
. Beugelmans, R.; Bourdet, S.; Bigot, A.; Zhu, J. Tetrahedron Lett. 1994, 35, 4349±4350.
1
1
1
1
0. Taniguchi, T.; Ogasawara, K. Tetrahedron Lett. 1998, 39, 4679±4682.
1. Montenegro, M. I. Electrochim. Acta 1986, 31, 607±620.
2. Hebri, H.; Dun
Ä
3. (a) Olivero, S.; Dun
ach, E.; Pe
Ä
Â
richon, J. Tetrahedron Lett. 1993, 34, 475±78.
ach, E. J. Chem. Soc. Chem. Commun. 1995, 2497±98. (b) Olivero, S.; Dunach, E. Tetrahedron
Ä
Lett. 1997, 38, 6193±97. (c) Yasuhara, A.; Kasano, A.; Sakamoto, T. J. Org. Chem. 1999, 64, 4211±4213.
4. Torii, S.; Tanaka, H.; Katoh, T.; Morisaki, K. Tetrahedron Lett. 1984, 25, 3207±3208.
1
1
5. Franco, D.; Panyella, D.; Rocamora, M.; Gomez, M.; Clinet, J. C.; Muller, G.; Dun
999, 40, 5685±5688.
6. Chaussard, J.; Folest, J. C.; Nedelec, J. Y.; Perichon, J.; Sibille, S.; Troupel, M. Synthesis 1990, 369±387.
  Â
Ä
ach, E. Tetrahedron Lett.
1
1
1
1
1
2
7. Silvestri, G.; Gambino, S.; Filardo, G.; Gullota, A. Angew. Chem. Int. Ed. Engl. 1984, 23, 979±980.
8. Fournier, F.; Fournier, M. Can. J. Chem. 1986, 64, 881±890.
9. Kinoshita, H.; Inomata, K.; Kameda, T.; Kotake, H. Chem. Lett. 1985, 515±517.
Ä
0. Derien, S.; Dunach, E.; Perichon, J. J. Am. Chem. Soc. 1991, 113, 8447±8454.
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