An asymmetric catalytic carbon-carbon bond formation in a fluorous biphasic system based on perfluoroalkyl-BINOL

Article abstract of DOI:10.1016/S0040-4039(00)01519-7

Asymmetric diethylzinc addition to arylaldehydes in a fluorous biphasic system catalyzed by perfluoroalkyl-BINOL has been accomplished. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01519-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8813–8816
An asymmetric catalytic carbonꢀcarbon bond formation in
a fluorous biphasic system based on perfluoroalkyl-BINOL
Yuan Tian and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, Hong Kong
Received 8 June 2000; revised 24 August 2000; accepted 7 September 2000
Abstract
Asymmetric diethylzinc addition to arylaldehydes in a fluorous biphasic system catalyzed by
perfluoroalkyl-BINOL has been accomplished. © 2000 Elsevier Science Ltd. All rights reserved.
Homogeneous asymmetric catalysis has been applied in countless bench-scale and commodity
chemical transformations. However, for chiral catalysts, which are often expensive and not easy
to prepare, there remains a need for improving their recovery methodologies. In 1994, a new
1
type of two-phase catalytic system—fluorous biphasic catalysis (FBC)—was developed based on
the temperature-dependent phase separation of common organic and perfluorous solvents. A
prerequisite is the use of a perfluorinated catalyst that shows preferential solubility in the
perfluorous phase. Nowadays an increasing number of academic and industrial groups are
2
working on the design and application of perfluorinated catalysts. To our knowledge, however,
3
only a few papers have been reported describing asymmetric fluorous biphasic catalysis.
In our previous work, we had synthesized perfluoroalkyl-BINOLs and examined the catalytic
4
reactivities of BINOL-metal complexes. Herein, we report the asymmetric carbonꢀcarbon bond
formation in a fluorous biphasic system catalyzed by a titianium perfluoroalkyl-BINOL
complex.
A highly perfluorinated BINOL derivative, 4,4%,6,6%-tetraperfluorooctyl-BINOL 3 which con-
tains 32 perfluorocarbons was designed for asymmetric FBC. Bromination of 4,4%-dibromo-
5
6
BINOL 1 at −78°C gave tetrabromide 2 in good yield, which underwent a copper-mediated
7
cross-coupling reaction with perfluorooctyl iodide at high temperature to yield compound 3.
8
The resolution of rac-3 was accomplished by column chromatography of the corresponding
9
diastereomeric bis-(S)-10-camphorsulfonates 4a and 4b, followed by alkaline hydrolysis to yield
10
the optically pure (S)-3 and (R)-3 (Scheme 1).
*
Corresponding author. E-mail: ksc@cuhk.edu.hk
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01519-7

8814
Scheme 1.
i
11
The diethylzinc addition to benzaldehyde catalyzed by perfluoroalkyl-BINOL/Ti(O Pr)4 was
demonstrated in a fluorous biphasic system (Eq. (1)). To 20 mol% of (S)-perfluoroalkyl-BINOL
i
3
dissolved in perfluoro(methyldecalin), 1.4 equiv. of Ti(O Pr) in hexane solution was added to
4
give a two-layer system. After heating to 45°C, a homogenous reaction system was obtained.
Then 3 equiv. of Et Zn in a hexane solution and PhCHO were added sequentially. After stirring
2
for 1 hour at 45°C, the reaction was cooled to 0°C and two layers were obtained. The upper
i
organic layer was withdrawn for analysis. Repeated addition of Ti(O Pr) and reactants
4
completed another reaction cycle. Nine cycles of the reaction were performed and no significant
decrease in the chemical yield or the enantiomeric excess of the product was observed (Table 1).
i
Poor results were obtained when Ti(O Pr) was absent in the tenth cycle. This observation is
4
consistent with previously reported work and it is suggested that this is due to the requirement
i
12
for free Ti(O Pr) as a co-catalyst in the reaction.
4
(1)

8815
Table 1
FBC result of Et Zn addition to benzaldehyde catalyzed by S-3
2
a
b
Reaction cycle
Yield (%)
e.e.
1
2
3
4
5
6
7
8
69
80
79
79
80
79
80
79
79
51
54
57
58
58
55
58
57
56
55
5
9
0
c
1
a
b
c
Determined by HPLC integration against 4-methoxyphenethyl alcohol as the internal standard.
Determined by HPLC using Daicel Chiralcel OD-H column, the major isomer had the (S) configuration.
i
Ti(O Pr) was absent.
4
Other aryl aldehydes also underwent Et Zn addition in this fluorous biphasic system catalyzed
2
by (S)-perfluorooctyl-BINOL 3 (Eq. (2)). For the more electron deficient 4-chlorobenzaldehyde,
the reaction was a little faster than that with benzaldehyde and was completed in 45 minutes
according to TLC analysis with a quantitative yield of 6A being obtained. On the contrary, the
reaction of the more electron rich 4-methoxybenzaldehyde was slower. The yield of 6B was
around 90% after 2 hours. The FBC phenomena has been observed in both cases. There were
no significant changes in the chemical yield, as well as in the enantioselectivities of products after
three reaction cycles (Table 2).
(2)
Table 2
FBC of Et Zn addition to arylaldehydes catalyzed by S-3
2
a
d
Reaction cycles
6A
6B
b
c
e
c
Yield (%)
e.e.
Yield (%)
e.e.
1
2
3
99
98
99
54
51
51
91
92
91
37
41
40
a
Reaction time was 45 minutes.
Determined by HPLC integration against 4-methoxyphenethyl alcohol as the internal standard.
Determined by HPLC using Daicel Chiralcel OD-H column, the major isomer had the (S) configuration.
Reaction time was 2 hours.
Determined by HPLC integration against 3-nitrobenzyl alcohol as the internal standard.
b
c
d
e

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In summary, we have synthesized a new type of tetraperfluorooctyl-BINOL and demonstrated
asymmetric catalytic carbonꢀcarbon bond formation in a fluorous biphasic system. Further
studies are being carried out.
Acknowledgements
We thank the Hong Kong Research Grants Council for financial support (CUHK4023/98P).
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. For rac-3: H NMR (300 MHz, CDCl ) l 5.73 (brs, 2H), 7.27 (d, 2H, J=7.6 Hz), 7.56 (d, 2H, J=8.9 Hz), 7.87
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2
0
20
D D
1
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1
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f
9
5
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