Mendeleev Commun., 2016, 26, 477–479
Table 3 Recycling of Pybox ligand 1 in enantioselective addition of phenyl-
acetylene to N-benzylideneaniline.a
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Cycle
Conversionb (%)
Isolated yieldc (%)
eed (%)
1
2
3
93
91
97
83
82
88
90
90
91
aReaction conditions: N-benzylideneaniline (0.25 mmol), phenylacetylene
(0.375 mmol), Cu(MeCN)4OTf (10 mol%), Pybox 1 (10.5 mol%), CH2Cl2
(2 ml), 50°C, 7 days. bDetermined by 1H NMR analysis of the crude product.
cAfter column chromatography. dDetermined by chiral HPLC.
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the conditions of entry 6 (Table 2). To do so, after the reaction
was completed, the catalyst was separated by precipitation with
diethyl ether, redissolved in CH2Cl2 and thoroughly washed with
0.1 m aqueous solution of EDTA-Na2 to remove copper. After
drying and solvent evaporation, the ligand was reused upon
addition of a new portion of Cu(MeCN)4OTf, the solvent and
reactants (Table 3).
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The results have shown that the immobilized ligand is suf-
ficiently stable during the reaction and its isolation. So it can be
used repeatedly. The product yield and enantiomeric excess did
not change considerably in three successive cycles. In this case,
both the yields and ee values are comparable to those obtained
using Pybox ligands immobilized on insoluble supports.9,10
In conclusion, we were the first to synthesize a Pybox ligand
immobilized on a soluble support, viz., poly(ethylene glycol)
methyl ether with Mn = 5000 Da. The efficiency of the com-
plex obtained in situ from this immobilized Pybox ligand and
Cu(MeCN)4OTf in enantioselective direct addition of alkynes
to imines has been estimated. The product of addition of phenyl-
acetylene to N-benzylideneaniline has been obtained in yields
up to 88% and ee up to 92%. The ligand can be recycled at least
in three successive cycles without a decrease in the product yield
or enantioselectivity.
17 V. V. Rostovtsev, L. G. Green, V. V. Fokin and K. B. Sharpless, Angew.
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This work was supported by the Russian Science Foundation
(grant no. 14-23-00186).
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.mencom.2016.11.005.
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Received: 27th July 2016; Com. 16/5009
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