Toward the continuous-flow synthesis of chiral tertiary alcohols by enantioselective addition of organozinc reagents to ketones using nanosize isoborneol ligands

Article abstract of DOI:10.1016/j.tetasy.2008.02.002

The catalytic enantioselective addition of different organozinc reagents, such as alkyl or in situ generated phenylzinc derivatives to simple aryl ketones, was accomplished using titanium tetraisopropoxide and chiral ligands derived from trans-1-arenesulf

Full text of DOI:10.1016/j.tetasy.2008.02.002

Available online at www.sciencedirect.com
Tetrahedron: Asymmetry 19 (2008) 537–541
Toward the continuous-flow synthesis of chiral tertiary alcohols
by enantioselective addition of organozinc reagents to ketones
using nanosize isoborneol ligands
*
*
´
Vicente J. Forrat, Diego J. Ramon and Miguel Yus
Instituto de Sı´ntesis Orga´nica (ISO) and Departamento de Quı´mica Orga´nica, Facultad de Ciencias, Universidad de Alicante,
Apdo. 99, E-03080-Alicante, Spain
Received 21 January 2008; accepted 1 February 2008
Available online 4 March 2008
Dedicated to Professor Yoshinori Yamamoto on the ocassion of his 65th birthday
Abstract—The catalytic enantioselective addition of different organozinc reagents, such as alkyl or in situ generated phenylzinc deriva-
tives to simple aryl ketones, was accomplished using titanium tetraisopropoxide and chiral ligands derived from trans-1-arenesulfonyl-
´
amino-2-isoborneolsulfonylamidocyclohexane and Frechet-type dendrons with up to 2.5 nm of diameter, giving the corresponding
tertiary alcohols with enantioselectivities up to >99%. A simple and efficient procedure for the synthesis of the ligands used is also
described, involving the radical addition of the corresponding thiol dendron to the chiral styryl-isoborneol derivative.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
to aldehydes11 in the presence of titanium tetraisopropox-
ide,12 with very broad ranging results.
The high economical and environmental cost of a single use
chiral ligand has forced the development of new alterna-
tives, such as the anchoring of a chiral ligand onto inert
polymers.1 However, this strategy usually renders lower
results, in comparison to the homogeneous version.
Another alternative is the anchoring of a chiral ligand to
a dendrimeric skeleton (dendron).2 In this way, the reaction
is performed under homogeneous conditions and the den-
drimer can be usually recovered by precipitation. More
interesting is the use of nanosize ligands,3 since they allow
the reaction to be carried out under continuous-flow condi-
tions,4 by the use of membranes thus avoiding the leaching
of the nanosize catalyst from the reaction vessel.5 One of
the most used dendrons is a polyether having a repeated
As an integrated part of our program on the design and
synthesis of chiral sulfonamides,13 as well as their use as
chiral promoters14 in the addition of organozinc reagents
to ketones,15 obtaining molecules with a challenging qua-
ternary stereocenter,16 we envisaged that the incorporation
´
of the chiral isoborneolsulfonamide sub-unit into a Frechet
dendron will not only lead to a new dendrimeric ligand for
application in catalysis, but will also provide a unique
opportunity to study the influence of the shape and archi-
tecture of the dendrimer on the enantioselectivity.
Herein, we report the first synthesis of nanosize ligands
´
derived from isoborneolsulfonamide and Frechet dendritic
´
3,5-dioxybenzyl structure (Frechet-type dendron), which
wedges, which were joined using a new radical strategy, as
well as their use in the enantioselective alkylation and
arylation17 of ketones.
has been attached to different chiral sub-units. Among
them, dendrimers containing hydroxymethylpyridine,6
TADDOL,7 BINOL,8 and 2-aminoalcohols9,10 have been
used in the enantioselective addition of dialkylzinc reagents
2. Results and discussion
Among the three general strategies to place a chiral sub-
unit into a dendrimer, (a) peripheral, (b) building blocks
or (c) core, we choose the last one, with the hope that the
*
Corresponding authors. Tel.: +35 965 90 35 48; fax: +35 965 90 35 49
(M.Y.); e-mail addresses: djramon@ua.es; yus@ua.es
0957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.02.002

538
V. J. Forrat et al. / Tetrahedron: Asymmetry 19 (2008) 537–541
asymmetric core could force the achiral branch into a spe-
cific arrangement, thus enhancing the anisotropy around of
isoborneol–titanium center, as enzymes do, and therefore
increasing the enantioselectivity of the reaction.
Measurements of the specific rotations of dendrimers 2
showed that the [a]D values decreased when increasing
the generation number. This is in agreement with earlier
findings7,25 which showed that an attachment of achiral
branches to a chiral core leads to a kind of dilution effect
of the specific rotation. The molar rotation [U]D values
showed a higher value for the first generation in compari-
son to zero and second generations (Fig. 1).
While G0-SH (benzylmercaptan) is comercially available,
the dendritic wedges G1-SH18 and G2-SH19 were prepared
in 50% and 34% yields, respectively, following a convergent
approach starting for both from 3,5-dihydroxybenzyl
alcohol.20 The benzylation of the above catechol under
standard conditions followed by the mesylation of the
bencylic alcohol,21 substitution with potassium thioacetate,
and the reduction of the thioester22 gave the expected G1-
SH. However, for the preparation of related G2-SH, after
mesylation, the obtained benzylic sulfonate derivative was
reacted again with another equivalent of 3,5-dihydroxybenz-
yl alcohol, rendering the corresponding benzylic alcohol of
the second generation,23 which was transformed into the
corresponding sulfanyl derivative as above. The final radi-
cal addition of sulfanyl derivatives Gn-SH to the chiral
styryl-isoborneol compound 117f initiated by AIBN24 gave
the expected chiral dendrimers 2 in good yields (Scheme 1),
practically independent of the generation prepared, with
this approach being used for the first time in the prepara-
150
100
50
16
14
12
10
8
2b
2a
2b
2a
2c
2c
6
4
2
0
0
1 (0)
3 (1)
7 (2)
1 (0)
3 (1)
7 (2)
number of benzylic groups
(generation)
number of benzylic groups
(generation)
Figure 1. Comparison between the specific and molar rotation for
dendrimers 2.
The CD spectra of 10À4 M solutions of compound 1 and
dendrimers 2 in acetonitrile were recorded in order to study
the influence of dendritic wedges on the spectroscopic
properties. To avoid distortion of the dichotic absorption
caused by the presence of an increasing number of benzene
chromophores in the dendrimer, the obtained De values
were divided by the number of phenyl rings present in each
compound. All compounds showed a small positive Cotton
effect. While dendrimers 2 present a maximum value at
230 nm, decreasing the intensity with increasing genera-
tion, the initial core sub-unit 1 showed the maximum at
higher values, ca. 260 (Fig. 2).
´
tion of Frechet dendrimers.
SH
OH
SO2
NH
+
O
O
O
O
NH
Ph
Ph
SO2
Ph
O
O
Ph
n
n
G0-SH: benzylmercaptan
G1-SH: n = 0
G2-SH: n = 1
1
AIBN
(0.3 equiv.)
THF, 70 ºC, 48 h
Ph
Figure 2. CD spectra of compounds 1 (dark blue), 2a (green), 2b (red),
and 2c (pale blue).
Ph
O
O
OH
SO2
NH
Unfortunately, all the aforementioned chiroptical proper-
ties seem to show that the anisotropy does not increase
around the core due to the specific spatial arrangement of
O
O
NH
n
SO2
´
the Frechet dendron. Although under some reaction condi-
S
tions, this fact could be changed.
Next, we performed calculations to visualize the molecular
shape of second generation dendrimer 2c. First, in order to
search for a set of low-energy conformers, the conforma-
tional analysis was carried out by molecular mechanics.
Then, the resulting structures were further optimized by a
PM3 method showing that the maximum differences
between the heats of formation of conformers were lower
than 5 kcal/mol, and the globular structures have an
O
Ph
O
2a (70 %)
Ph
n
2b: n = 0 (68 %)
2c: n = 1 (62 %)
´
Scheme 1. Radical preparation of Frechet dendrimeric isoborneolsulfon-
amide ligands 2.

V. J. Forrat et al. / Tetrahedron: Asymmetry 19 (2008) 537–541
Table 1. Enantioselective alkylation of ketones using dendrimeric ligands 2
539
HO R2
R1 R3
O
Ti(OPri)4 (110 mol %)
+
R32Zn
R1
R2
2 (5 mol %), PhMe, 25 ºC
4a: R3 = Me
5
4b: R3 = Et
3
Entry
Ligand
R1
Ph
4-MeC6H4
4-FC6H4
R2
R3
Time (d)
Alcohol
Yielda (%)
eeb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
2a
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2b
2b
2c
2c
2c
Me
Me
Me
Me
Me
Me
Et
Me
Me
Me
Me
Me
Me
Et
Et
Et
Et
Et
Et
Et
Me
Et
Et
Et
Et
Et
Et
Me
Et
Et
Et
4
4
3
2
4
2
8
4
4
2
2
3
2
7
4
5
3
5a
5b
66
30
92
97
52
98
25
53
29
92
98
98
97
21
59
34
96
95 (S)
86 (À)
81 (À)
93 (À)
75 (À)
88 (+)
>99c (R)
97 (S)
96 (À)
85 (À)
90 (À)
57 (À)
88 (+)
>99c (R)
99 (S)
5c
5d
4-BrC6H4
2-Naphthyl
PhC„C
Ph
5e
5f
ent-5a
5a
5b
Ph
4-MeC6H4
4-FC6H4
4-BrC6H4
2-Naphthyl
PhC„C
Ph
Ph
4-MeC6H4
PhC„C
5c
5d
5e
5f
ent-5a
5a
5b
Me
Me
Me
98 (À)
5f
86 (+)
a Isolated yield after column chromatography.
b Determinated by HPLC using Chiracel columns; the absolute configuration or the sign of the predominant enantiomer is indicated in parentheses.
c Only one enantiomer detected.
approximate diameter of 2.5 nm (Fig. 3), with these num-
bers being similar when a titanium atom was added to form
the expected complex.
benzylsulfanyl derivative 2a, first generation 2b (n = 0) or
second generation 2c (n = 1), practically rendered the same
Table 2. Enantioselective phenylation of ketones using dendrimeric
ligands 2
HO R2
i, PhMe, 70 ºC, 16 h
+
Et2Zn
BPh3
6
R1
Ph
ii, 2 (5 mol %), Ti(OPri)4 (110 mol %)
R1COR2 3, PhMe, 25 ºC, 1-4 d
4b
5
Entry
L
R1
R2
No
Yielda (%)
eeb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
2a
2a
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2b
2b
2b
2c
2c
2c
Bun
Me
Me
Me
Me
Me
Me
Me
Et
Me
Me
Me
Me
Me
Me
Me
Et
5g
5h
5i
97
51
83
98
98
97
91
55
76
65
67
88
90
91
89
51
95
97
84
34 (R)
80 (À)
80 (À)
85 (+)
86 (+)
83 (+)
>99c (+)
72 (+)
35 (R)
80 (À)
84 (À)
83 (+)
84 (+)
64 (+)
78 (+)
78 (+)
84 (À)
81 (+)
75 (+)
4-MeC6H4
3-MeC6H4
4-CF3C6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
4-BrC6H4
Bun
4-MeC6H4
3-MeC6H4
4-CF3C6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
4-BrC6H4
3-MeC6H4
4-CF3C6H4
4-BrC6H4
Figure 3. Calculated globular structure of ligand 2c.
5j
Finally, the activity of chiral dendrimers 2 was tested
through the enantioselective alkylation of ketones 3 to give
the expected tertiary alcohols in good results (Table 1). The
nature of the ketone has any important effect on the enantio-
selectivity, with a,b-unsaturated compounds giving lower
ee (compare entries 1, 8, and 15 with 6, 13, and 17, respec-
tively). Also, the chemical yield was clearly influenced by
the ketone nature, with alkyl aryl ketones possessing an
electron-donating group at the para-position giving worse
chemical yields than those possessing electron-withdrawing
groups (compare entries 2 and 3, 4 in Table 1). The
bulkiness of the aryl substituent also has a detrimental
effect, not only on the chemical yield but on the enantio-
selectivity (compare entries 1 and 5 in Table 1). The nature
of the dialkylzinc reagent, methyl or ethyl, has an impor-
tant effect on the reaction rate, as well as on the chemical
yield (compare entry 1 with 7). Finally, comparing all
ligands, it is worthy of note that the results obtained were
homogeneous independent of the dendrimeric wedge used;
5k
5l
5m
5n
5g
5h
5i
5j
5k
5l
5m
5n
5i
Me
Me
Me
5j
5m
a Isolated yield after column chromatography.
b Determinated by HPLC using Chiracel columns; the absolute configu-
ration or the sign of the predominant enantiomer is indicated in
parentheses.
c Only one enantiomer detected.

540
V. J. Forrat et al. / Tetrahedron: Asymmetry 19 (2008) 537–541
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After the success obtained with the alkylation process, we
focused our efforts on the arylation process. Due to the
instability of pure diphenylzinc, we chose to prepare the
corresponding phenylzinc reagent by the transmetallation
of triphenylborane 6 with diethylzinc 4b at 70 °C, and the
in situ obtained reagent was submitted to further reaction
with ketones 3 in the presence of nearly stoichiometric
amounts of titanium tetraisopropoxide and substoichio-
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First, it should be pointed out that the results using 2-hexa-
none are quite different with regard to the enantioselectiv-
ities from those obtained with alkyl aryl ketones, since the
differentiation between the two acyclic alkyl groups is far
more difficult than between aryl and alkyl groups (compare
entries 1 and 2 in Table 2). As in the previous alkylation
process, the results are quite homogeneous, independent
of the generation ligand used (compare entries 2–6 and
10–14 in Table 2). The electronic character of the substitu-
ent on the aryl group has a minimal effect on the chemical
yield (Table 2, entries 2 and 4). However, in the phenyla-
tion process, the bulkiness of the alkyl group of the ketone
has a dramatic effect on the chemical yield and enantio-
selectivity (entries 7 and 8 in Table 2).
3. Conclusion
In conclusion, we have developed a new strategy to attach
´
chiral styryl derivatives to an achiral Frechet dendron by a
radical approach. The dendrimers obtained have been suc-
cessfully used in the catalytic enantioselective nucleophilic
alkylation and arylation of simple ketones. These com-
plexes have diameters in the range of nanosize, that would
permit their use in a continuous-flow membrane reactor,
although so far the reaction rates make this probability
very difficult. Work is currently in progress in order to ful-
fill all these requirements.
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