Asymmetric nozaki-hiyama propargylation of aldehydes: Enhancement of enantioselectivity by cobalt co-catalysis

Article abstract of DOI:10.1002/anie.201002751

It takes two to tango! A combination of partially reduced chiral H ₈-TBOx chromium catalyst 1 and achiral cobalt porphine co-catalyst 2 (Ar=p-anisyl) led to an enhancement in enantioselectivity by suppression of the background process that presumably proceeds through an organomanganese species.

Full text of DOI:10.1002/anie.201002751

Angewandte
Chemie
DOI: 10.1002/anie.201002751
Asymmetric Catalysis
Asymmetric Nozaki–Hiyama Propargylation of Aldehydes:
Enhancement of Enantioselectivity by Cobalt Co-Catalysis**
Dmitry L. Usanov and Hisashi Yamamoto*
The addition of organochromium reagents to carbonyl com-
pounds, pioneered by Nozaki and Hiyama in late 1970s, has
been an important tool in contemporary organic synthesis
because of a number of unique features, such as mild reaction
conditions, high chemoselectivity, and compatibility with a
wide range of functional groups.[1,2] However, the necessity to
use superstoichiometric amounts of toxic chromium reagents
was among the major drawbacks. In 1996 Fꢀrstner and Shi
reported the first example of this reaction being catalytic in
chromium, which turned Nozaki–Hiyama chemistry into a
more environmentally benign methodology.[3] This seminal
report gave rise to a number of asymmetric applications of
catalytic Nozaki–Hiyama (NH) processes.[4]
However, the existing methods involve relatively high
catalyst loadings (10 mol%); as such, the development of a
protocol employing lesser amounts of chromium would be
highly desirable. In addition, room for improvement with
respect to enantioselectivity still remains for a range of
substrates. For instance, for aromatic aldehydes, enantiose-
lectivities exceeding 80% ee have not been achieved using
NH methodology.
Recently, our group developed a tethered bis(8-quinoli-
nato) (TBOx) chromium complex 1,[11] which was successfully
used as a highly stereoselective catalyst for asymmetric
pinacol coupling,[12] asymmetric NH allylation[13] and allenyl-
ation[14] of aldehydes, as well as for the asymmetric synthesis
of 1,3-butadien-2-ylcarbinols (Scheme 1).[15] In light of these
applications, we envisioned that 1 could potentially be used as
a catalyst in asymmetric NH propargylation reactions.
Chiral homopropargyl alcohols are among the products
which are potentially accessible using NH methodology. Most
of the asymmetric methods that provide access to these
compounds involve the use of chiral allenyl reagents.[5]
A
number of enantioselective procedures involving achiral
allenyl species have also been reported.[6] A catalytic enan-
tioselective NH propargylation reaction is thus an advanta-
geous alternative to these approaches owing to the ready
availability of propargyl halides as the propargyl moiety
sources.[7]
The first examples of enantioselective NH propargylation
were reported by Cozzi, Umani-Ronchi, and co-workers and
involved the use of chromium complexes derived from chiral
salen ligands (10 mol%, up to 56% ee).[8] In 2004, Inoue and
Nakada described a superior procedure involving a tridentate
carbazole ligand (10 mol%), which furnished most of the
products with 51–82% ee (the catalyst showed exceptional
enantiocontrol for pivalaldehyde, 98% ee).[9] Recently Kishi
and co-workers reported the implementation of NH prop-
Scheme 1. TBOx derivatives used in Nozaki–Hiyama chemistry.
We started our studies using the conditions previously
used for other NH processes:[12,13] 0.5 mmol of benzaldehyde,
3 equivalents of manganese, 1 equivalent of TESCl, and
1.5 equivalents of propargyl bromide were mixed in 2 mL of
solvent in the presence of 3 mol% of 1 for 40 hours at room
temperature. A solvent screen demonstrated that only a
limited number of solvents were suitable for the propargyla-
tion reaction; tetrahydrofuran appeared to be superior to the
majority of other common solvents (60% conversion after
40 h, 67% ee). Interestingly, 2-methyltetrahydrofuran showed
a slight increase of enantioselectivity compared to tetrahy-
drofuran (72% ee). However, the latter was used as a solvent
for the further preliminary screening experiments for reasons
of convenience.
argylation into the total synthesis of halichondrin.[10]
A
number of homopropargyl alcohols were obtained in 55–
94% yield and 46–93% ee using chiral sulfonamide ligands;
the scope of the study was mostly focused on aliphatic
substrates.
[*] D. L. Usanov, Prof. Dr. H. Yamamoto
Department of Chemistry, The University of Chicago
5735 South Ellis Avenue, Chicago, IL 60637 (USA)
Fax: (+1)773-702-0805
Tuning the size of the silyl group of the product scavenger
led to a bell-shaped dependence: increasing the size first
resulted in an enhancement of the enantioselectivity (Table 1,
entries 1–3); however, bulkier silyl chlorides (Table 1,
entries 4 and 5) furnished the product with inferior ee
values. An inverse dependence of the reactivity on the size
of the scavenger was observed over the tested range. These
E-mail: yamamoto@uchicago.edu
Homepage: http://yamamotogroup.uchicago.edu/
[**] This work was made possible by the generous support of the NSF
(CHE-0717618). We thank Dr. Guoyao Xia and Dr. Marina Naodovic
for their involvement in the preliminary stage of this project.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002751.
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Table 1: The effect of silyl chloride on the catalytic asymmetric Nozaki–
Hiyama propargylation of benzaldehyde.
which is in compliance with the observed stereoselectivity
profile. Background experiments confirmed this assumption
(Table 2, entries 7 and 8).
It should be noted that the previously reported reactivities
and enantioselectivities appeared to be different from those
obtained subsequently;[15] numerous experiments with differ-
ent batches of TBOx CrCl were consistent with the result
shown in Table 1, entry 2. The reasons for this phenomenon
are currently under investigation.
We considered an idea to prepare a partially reduced H8-
TBOx ligand, which gives rise to complex 2, in order to test
whether the introduction of aliphatic moieties can exert any
impact on the reactivity and stereocontrol of the catalysis.
This type of modification of the TBOx backbone has not been
tested thus far.[11–15] To our delight, complex 2 furnished the
homopropargyl alcohol with appreciably enhanced enantio-
selectivity of 80% ee (Table 2, entry 9). In order to further
enhance the enantiopurity of the product, we focused our
studies on the suppression of the background process.
In the conventional NH catalytic cycle, the electrons are
supposedly transferred from the chromium(II) species to the
propargyl halide (Scheme 2, A). In 1989 Takai and Utimoto
Entry
Silyl chloride
Conversion [%][a]
ee [%][b]
1
2
3
4
5
TMSCl
TESCl
n-Pr3SiCl
TBDMSCl
TTMSSCl
79
68
45
26
18
33
67
70
62
61
[a] Determined by 1H NMR analysis of the crude material after the
workup. [b] Determined by HPLC on a chiral OD-H column. TMSCl=
trimethylsilyl chloride, TES=triethylsilyl chloride, TBDMSCl=tert-butyl-
dimethylsilyl chloride, TTMSS=tris(trimethylsilyl)silyl chloride.
observations led to the conclusion that the silyl chloride is
directly involved in the stereo-determining step, possibly via
Lewis acid activation of the substrate.
The reaction temperature appeared to have a dramatic
impact on the catalysis efficiency. Thus, at 08C, almost no
reactivity was observed along with significant loss of enantio-
selectivity (Table 2, entry 1). In contrast, carrying out the
reaction at 408C (Table 2, entry 3) resulted in enhanced
enantiocontrol and higher conversion.
Table 2: Screening of temperature and propargyl source in catalytic
asymmetric Nozaki–Hiyama propargylation of benzaldehyde.
Entry
Catalyst
X
T [8C]
Conversion [%][a]
ee [%][b]
Scheme 2. Putative mechanism for the Nozaki–Hiyama propargylation:
standard (A) and co-catalyzed (B) routes.
1
2
3
1
1
1
1
1
1
–
–
2
Br
Br
Br
Cl
Cl
I
Cl
Br
Br
0
RT
40
RT
40
RT
RT
RT
RT
5
68
>95
10
27
>95
–
14
67
73
74
79
28
–
4[c]
5[c]
6
reported the chromium-mediated preparation of secondary
alcohols from aldehydes and alkyl halides catalyzed by
vitamin B12 or cobalt phthalocyanine 3.[18] The key point of
the procedure was the formation of alkyl radicals from the
CoI/CoII/CoIII cycle, which were then captured by chromium
species able to attack the carbonyl group. The formation of
stabilized radicals from the corresponding halides by means
of cobalt(I) complexes is well-known.[19] Addition of cobalt
phthalocyanine 3 (Scheme 3) was also reported to be
7
8
9
12
94
–
80
[a] Determined by 1H NMR analysis of the crude material after the work-
up. [b] Determined by HPLC on a chiral OD-H column. [c] The pinacol
coupling product was formed in appreciable extent.
We then tested the influence of the type of propargyl
halide on the reaction outcome: the reactivity of the halide
increased according to the series Cl < Br< I, accompanied by
a decrease of the enantioselectivity (Table 2, entries 2, 4, and
6). Based on this observation, together with temperature
studies, we hypothesized that propargyl halides can react with
manganese to form organomanganese species, which can
furnish the homopropargyl alcohol with compromised enan-
tiocontrol. Even though oxidative insertion of manganese to
organic halides is considered to be largely unknown,[3b]
a
number of reports indicate that this pathway is feasible for the
most-reactive halides.[16] The rate of formation of organo-
manganese species would increase in the order Cl < Br< I,
Scheme 3. Cobalt additives used in asymmetric Nozaki–Hiyama prop-
argylation.
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Table 4: Asymmetric Nozaki–Hiyama propargylation catalyzed by 2.
beneficial for the reaction rate for some of the NH
processes.[20] Inspired by those reports, we assumed that
cobalt co-catalysis was able to accelerate the initial radical
formation step and thus could lead to enhancement of the
enantioselectivity of the TBOx CrCl-mediated asymmetric
NH propargylation (Scheme 2, B).
Entry
R
t [h]
Yield [%][a]
ee [%][b]
1[c]
2
Ph
40
48
90
75
83
63
40
73
54
78
81
79
64
40
91
86
69
51
74
70
42
37
91
89
78
88
88
92
84
86
75
89
92
84
84
93
92
80
87
34
29[e]
The use of [(Ph3P)3CoICl] or [(Ph3P)2CoIICl2] as additives
did result in an appreciable acceleration of the process;
p-MeO-C6H4
o-F-C6H4
3
however,
a substantial drop in enantioselectivity was
4[c]
5
48
observed; the effect was even more profound for cobalt
phthalocyanine 3 (Table 3). These experiments indicated that
p-Cl-C6H4
1-naphthyl
2-naphthyl
150
100
100
60
120
78
6[c]
7
8[c]
9
Table 3: Screening of cobalt additives in Nozaki–Hiyama propargylation
of benzaldehyde.
10[c]
11
12
13[c]
14
15[c]
16
17[c]
18
19
=
(E)-PhCH CH
(E)-PhCH C(Me)
90
60[d]
=
60[d]
25
2-furanyl
2-thienyl
Entry
Cat.
X
Additive
Conversion [%][a]
ee [%][b]
35
1
2
3
4
5
6
1
1
1
1
2
2
2
2
2
2
Br
Br
Br
Br
Br
Cl
Br
Cl
Br
Br
[(PPh3)3CoICl]
>95
>95
>95
81
>95
39
>95
36
>95
>95
45
59
10
78
85
91
91
90
92
90
60[d]
60[d]
90
[(PPh3)2CoIICl2]
3
4
4
4
4
4
5
5
PhCH2CH2
cyclohexyl
90
[a] Yield of isolated product after purification on silica gel. [b] Deter-
mined by HPLC on a chiral column. [c] Reactions were carried out in 2-
methyltetrahydrofuran in the presence of 20 mol% of CaCO3.[17] [d] The
reaction time was not optimized. [e] Determined by 1H NMR analysis
after esterification with (R)-MTPACl.
7[d,e]
8[c,d]
9
10[d,e]
[a] Determined by 1H NMR analysis of the crude material after the
workup. [b] Determined by HPLC on a chiral OD-H type column.
[c] 3 mol% of the additive was used. [d] 2-Methyltetrahydrofuran was
used as a solvent. [e] CaCO3 (20 mol%) was used as an additive.[17]
furan demonstrated higher enantioselectivities and the prod-
ucts were isolated in somewhat lower yields compared to the
reactions in tetrahydrofuran.[17] Aromatic aldehydes showed
superior reactivity and stereoselectivity profiles with respect
to aliphatic substrates. This procedure could also be applied to
some of a,b-unsaturated and heteroaromatic aldehydes to
give homopropargyl alcohols with enantioselectivities of 84–
93% ee.
In summary, we have developed a highly enantioselective
catalytic system for the Nozaki–Hiyama propargylation of
aldehydes employing lowered catalyst loading (3 mol%);
enantioselectivities of 84–93% ee, hitherto unapproachable
for aromatic, heteroaromatic, and a,b-unsaturated aldehydes
using NH chemistry, were obtained for a range of substrates.
A beneficial influence of the partially reduced binaphthyl
moiety on enantiocontrol was observed, thus resulting in the
development of a new type of H8-TBOx ligand. The
important role of the background reaction presumably
caused by organomanganese compounds formed in situ was
demonstrated; the issue was efficiently overcome by intro-
duction of an additional cobalt-mediated catalytic cycle,
which resulted in enhancement of enantioselectivity. Com-
mercially available porphine complexes, hitherto never used
in NH processes, were found to serve as unique auxiliaries,
whereas conventional cobalt additives were shown to be
ineffective.
cobalt complexes were able to transfer the propargyl moiety
directly onto the substrate; this suggestion was confirmed by
blank experiments carried out in the absence of the TBOx
chromium complex.[21] In an attempt to eliminate this path-
way, we tested commercially available tetraarylporphine
complex 4. Anisyl groups, orthogonal to the porphine ring
plane, were expected to provide steric hindrance for the direct
attack on the substrate. To our delight, in addition to
increased reactivity, we observed enhancement of enantiose-
lectivity with respect to the process catalyzed solely by 1 and 2
(Table 3, entries 4 and 5). 2-Methyltetrahydrofuran also
appeared to be superior to tetrahydrofuran under the
conditions of cobalt co-catalysis (Table 3, entry 7). When
propargyl chloride was employed under the modified con-
ditions, the same level of enantiocontrol was observed
(Table 3, entries 6 and 8 versus 7).
Interestingly, when the less sterically hindered complex 5
was used as an additive, enhancement of the enantioselectiv-
ity was also observed to a comparable extent with that
provided by complex 4 (Table 3, entries 9 and 10).[22] Thus, the
background reaction catalyzed by 3 was faster than the one
mediated by porphine complexes, presumably owing to
electronic effects rather than steric.[21]
We believe that the use of cobalt porphine co-catalysts
can be beneficial for other asymmetric NH-type methods
hitherto poorly explored, presumably owing to low stereo-
selectivity levels caused by background reactions. Studies in
these directions are currently undergoing in our group.
A range of various aldehydes were tested in order to study
the scope and limitations of the developed procedure
(Table 4). Generally, reactions run in 2-methyltetrahydro-
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Communications
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Experimental Section
Representative procedure for asymmetric propargylation: a flame-
dried argon-filled test tube sealed with a sleeve stopper was charged
with 4 (4 mg, 0.005 mmol), (H8-TBOx)CrCl 2 (12 mg, 0.015 mmol),
manganese powder (81 mg, 1.47 mmol), and CaCO3 (10 mg,
0.01 mmol). 2-Methyltetrahydrofuran (2 mL) was then added, and
the mixture was left stirring at room temperature for 10 minutes.
Then propargyl bromide (55 mL, 0.73 mmol) was added dropwise
followed by benzaldehyde (50 mL, 0.49 mmol) and chlorotriethylsi-
lane (83 mL, 0.49 mmol). The mixture was stirred vigorously at room
temperature (228C) for 40 hours. The reaction mixture was then
quenched with aqueous saturated NaHCO3 solution and extracted
with dichloromethane. The extracts were dried over anhydrous
Na2SO4, and concentrated under vacuum. 1m TBAF solution in
tetrahydrofuran was then added (1 mL, 1 mmol), and the mixture was
left at room temperature for 1 hour. It was then diluted with
dichloromethane and washed with brine. The aqueous layer was
back extracted with dichloromethane. The combined organic phases
were dried over anhydrous Na2SO4, and the solvent was evaporated.
Column chromatography of the resulting residue on silica gel in 1:10
ethyl acetate/hexanes solvent system afforded the homopropargyl
alcohol (53 mg, 75% yield, 91% ee).
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Received: May 6, 2010
Revised: June 26, 2010
Published online: September 21, 2010
Keywords: asymmetric catalysis · chromium · cobalt ·
.
homopropargyl alcohols · propargylation
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[22] The idea to test complex
5 in order to gain additional
information about the phenomenon of cobalt co-catalysis was
kindly suggested by a reviewer of this manuscript.
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