Conjugate addition to diethyl azodicarboxylate under organic- perfluorinated biphasic homogeneous catalysis by nickel(II) species

Article abstract of DOI:10.1016/S0040-4039(00)00549-9

The nickel(II) complex of the Schiff base of salicylaldehyde and 4- perfluorodecylaniline catalyzes the conjugate addition of β-diketones to electron-deficient substrates. Electrophilic amination with azodicarboxylate affords quaternary centers. The catal

Full text of DOI:10.1016/S0040-4039(00)00549-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 4093–4095
Conjugate addition to diethyl azodicarboxylate under
organic-perfluorinated biphasic homogeneous catalysis by
nickel(II) species
∗
Miriam Meseguer, Marcial Moreno-Mañas and Adelina Vallribera
Department of Chemistry, Universitat Autònoma de Barcelona, Cerdanyola, 08193- Barcelona, Spain
Received 6 March 2000; accepted 31 March 2000
Abstract
The nickel(II) complex of the Schiff base of salicylaldehyde and 4-perfluorodecylaniline catalyzes the conjugate
addition of β-diketones to electron-deficient substrates. Electrophilic amination with azodicarboxylate affords
quaternary centers. The catalyst is recovered in solution and reutilized. Isolation of reaction products is very simple
and does not require chromatography. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Michael reactions; nickel(II); catalysis; amination.
Electrophilic amination is an important synthetic operation, in particular in the preparation of amino
1
acids. Conjugate addition to azodicarboxylates is one of the preferred methods of electrophilic ami-
nation. Transition metal-catalyzed conjugate additions are attractive since, avoiding the classical basic
2
catalysis, secondary reactions are minimized. Twenty years ago Nelson et al. reported the conjugate
addition to electron-deficient compounds catalyzed by the Ni(II) complex of acetylacetone.3 We found
,4
that the Ni(II) complex of salicylaldehyde is an even better catalyst since the complex of acetylacetone
5
reacts with electrophiles, whereas the complex of salicylaldehyde does not. However, after-work up
salicyladehyde contaminates the final product, column chromatography being required. In the course
of our research we needed an expeditious separation of the final products and we were attracted by
the biphasic homogeneous catalysis with recovery of the catalyst in perfluorinated (or polyfluorinated)
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solvents. Recovery of metal catalysts by this technique has been applied to metal-catalyzed cyclopro-
7
8
panations (Rh), radical additions of trichloromethyl groups to olefins (Cu), Heck and cross-coupling
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10
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reaction (Pd), allylation of nucleophiles (Tsuji–Trost reaction, Pd), epoxidation of olefins (Mn, Ru),
hydroboration (Rh), hydrogenation (Rh), ethylene oligomerization (Ni), and in reactions catalyzed
by Lewis acids (Yb).
Complex 3 was prepared by the sequence outlined in Scheme 1. Perfluoroalkylation of 4-iodoaniline
by the general method of McLoughlin and Thrower afforded 1, which was converted into 3 by standard
1
2
13
14
1
5
16
∗
Corresponding author.
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00549-9
tetl 16828

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094
procedures. Reaction of acetylacetone (4a) with diethyl azodicarboxylate (5) was performed in a mixture
of toluene and 1-bromoperfluorooctane (1:1) (Scheme 2). These solvents are miscible at 70–80°C, but
9
a
form two layers at room temperature. The final product 6a was recovered by precipitation from the
1
5
toluene phase and was pure according to H NMR and mp criteria, further purification not being
1
6
required. The reaction was performed five times with the indicated yields. Clearly, four runs are
at present the limit. The catalyst was also efficient in generating 6b, possessing a quaternary center.
Compound 6b is an oil, and isolation from excess 4b was achieved by distillation of 4b. The residue
1
was pure 6b according to H NMR and GC criteria. Again, the reaction was performed four times and
only a noticeable drop in yield was observed in the fifth run. The lower yield observed in the first run is
attributed to saturation of the prefluorinated phase with 6b.17
Scheme 1. Preparation of 3: (i) I-C10
F21, Cu, DMSO, 130°C (Ref. 16); (ii) salicylaldehyde, toluene, azeotropic removal of
water; (iii) Ni(AcO) , MeOH/water, 100°C
2
8
Scheme 2. Preparation of compounds 6: toluene (15 mL)/BrC F17 (15 mL), 3 (2% molar, [3]=0.002 M), [5]=0.1 M, molar ratio
4
/5=2
In summary, nickel(II) complex 3 holds promise as an efficient catalyst in conjugate additions
performed in neutral media, permitting easy isolation of final products.
Acknowledgements
Financial support from DGESIC (Projects PB93-0896 and PB98-0902) and CIRIT (Project SGR98-
0056) is gratefully acknowledged.
References
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7. Experimental procedure: A solution of diketone 4 (6 mmol) in toluene (15 mL) was added to a stirred solution of 3 (0.08 g,
0
.06 mmol (2%)) in 1-bromoperfluorooctane maintained at 60°C. The mixture was heated and stirred until one phase was
formed. Then 5 (0.50 g, 3.0 mmol) was added, the mixture was refluxed for 3 days and then cooled at room temperature.
5
The two phases were separated and the lower one was added to the second run. Compound 6a (mp 122–125°C, lit. mp
1
14–116°C) precipitated out from the toluene in the indicated yields (Scheme 2). For 6b the toluene was evaporated and
excess 3-methylacetylacetone was distilled off (bp 150°C/14 mmHg). The residue was pure 6b. See Ref. 5 for spectral data
of 6.