Enantioselective alkynylation of aldehydes with 1-haloalkynes catalyzed by tethered bis(8-quinolinato) chromium complex

Article abstract of DOI:10.1021/ja1102822

The first example of Cr-catalyzed asymmetric alkynylation of aldehydes with 1-iodo- and 1-bromoalkynes was developed. The use of tethered bis(8-quinolinato) chromium catalyst (3 mol %) allowed preparation of enantioenriched propargyl alcohols with good yields and enantioselectivities up to 92% ee. 1-Bromoalkynes can be activated by the introduction of a cobalt porphine cocatalyst, which enables shorter reaction times without any loss of enantiocontrol.

Full text of DOI:10.1021/ja1102822

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pubs.acs.org/JACS
Enantioselective Alkynylation of Aldehydes with 1-Haloalkynes
Catalyzed by Tethered Bis(8-quinolinato) Chromium Complex
Dmitry L. Usanov and Hisashi Yamamoto*
Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, United States
S Supporting Information
b
Chart 1. TBOx Chromium Catalyst and Cobalt Porphine
Co-catalyst for Asymmetric Alkynylation of Aldehydes
ABSTRACT: The first example of Cr-catalyzed asymmetric
alkynylation of aldehydes with 1-iodo- and 1-bromoalkynes
was developed. The use of tethered bis(8-quinolinato)
chromium catalyst (3 mol %) allowed preparation of
enantioenriched propargyl alcohols with good yields and
enantioselectivities up to 92% ee. 1-Bromoalkynes can be
activated by the introduction of a cobalt porphine co-
catalyst, which enables shorter reaction times without any
loss of enantiocontrol.
successfully applied to asymmetric Nozaki-Hiyama allylation,20a
allenylation,20b and propargylation20c of aldehydes as well as
to asymmetric pinacol coupling20d and synthesis of 1,3-butadien-2-
ylcarbinols.20e Herein, we report the first example of Cr-catalyzed
enantioselective alkynylation of aldehydes with 1-haloalkynes
mediated by complex 1.
hiral propargyl alcohols are useful and versatile building
C
blocks in organic synthesis.1 The general synthetic approaches
to these compounds involve asymmetric reduction of ynones2
and metal-mediated asymmetric alkynylation of the carbonyl
group.3-11 The methods of the latter group are based on the use
of a chiral inductor alongside with either stoichiometric amounts of
organometallic reagent4-7 or a combination of an alkyne, an
organic base, and catalytic amounts of a metal source;8-11 systems
based on zinc,5,8 titanium,6 copper,7,9 indium,10 and ruthenium11
reagents have been reported. Generation of a metal acetylidefrom
an alkyne is the unifying concept of these methods. However,
alternative approaches to access propargyl alcohols in the
enantioenriched form remain rather underrepresented;12 in this
context, we became interested in the possible expansion of redox
chemistry into this field of asymmetric catalysis.
1-Haloalkynes can be easily prepared from the corresponding
terminal alkynes in good to excellent yields.21 We started our
studies on Cr-catalyzed asymmetric alkynylation on benzaldehyde
as a model substrate using the conditions previously reported for
other TBOxCrCl-mediated processes.20a,20c,20d To our delight,
the use of phenyliodoacetylene smoothly furnished the desired
propargyl alcohol with good yield and promising enantioselec-
tivity (Table 1, entry 1). The bromo and chloro analogues
demonstrated somewhat higher stereoselectivities at the expense
of considerably longer reaction times (entries 2 and 3). The effect
of the silyl chloride on the reaction outcome was not as profound
as in the case of TBOxCrCl-mediated asymmetric propargylation;20c
the use of bulkier silyl chlorides resulted in slightly higher ee values,
which were accompanied with decreased reactivities (entries 4-9).
Lowering the reaction temperature to 0 °C also resulted in
enhanced enantiocontrol; however, the reaction rate de-
creased dramatically (entry 10). Solvent screening identified
a rather narrow range of suitable solvents (entries 11-13);22
the reaction carried out in 2-methyltetrahydrofuran yielded
the product with the highest enantioselectivity of 88% ee.
Interestingly, the same increase in stereoselectivity was ob-
served when N-methylimidazole (20 mol %) was used as an
additive to the reaction run in tetrahydrofuran (Table 1, entry
14). For convenience and the optimum reaction outcome,
these conditions were chosen for the substrate/iodoalkyne
screening experiments.23
The reaction of 1-iodoalkynes with aldehydes and ketones
mediated by stoichiometric amounts of CrCl2 was reported by
Takai and Oshima in 1985.13 This method provided access to
propargyl alcohols in mild conditions and allowed selective
alkynylation of an aldehyde moiety in the presence of a keto
group. In 1987, Kishi implemented a modified protocol into the
synthesis of Halichondrin B fragment; as low as 0.01% of nickel
chloride was used as a catalyst alongside with stoichiometric amounts
of chromium.14 Since then, Cr-mediated alkynylation has been
extensively used in synthesis due to such advantages as compat-
ibility with labile aldehydes and functional group tolerance.15,16
In 1996, F€urstner reported17a this reaction using catalytic
amounts of chromium(II) chloride, and subsequent total synthe-
sis implementations were published later on.17b Few other
protocols catalytic in chromium have also been described.18
However, despite the high synthetic potential of this methodol-
ogy, no enantioselective variants have ever been reported.
In 2004, our group developed a tethered bis(8-quinolinato)
(TBOx) chromium complex 1 (Chart 1),19 which has been
Received: November 16, 2010
Published: January 7, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja1102822 J. Am. Chem. Soc. 2011, 133, 1286–1289
|

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Table 1. Optimization of Cr-Catalyzed Asymmetric
Alkynylationa
Table 2. TBOxCrCl-Catalyzed Asymmetric Alkynylationa
entry
R
R0
Ph
time (h)
yield (%)b
ee (%)c
entry
X
R3Si
TES
solvent
THF
time (h) yield (%)b ee (%)c
1
Ph
20
18
84
84
45
36
45
36
42
36
32
48
36
48
24
36
82
88
75
72
45
79
85
58
82
52
66
70
87
87
76
82
88
84
79
86
69
82
89
83
92
87
87
75
79
85
72
84
1
I
15
60
60
15
15
36
36
36
36
160
15
15
15
20
32
20
81
83
89
88
82
75
86
80
87
87
91
87
88
82
88
89
81
2
2-furanyl
Ph
3
PhCHdC(Me)d
o-MeC6H4
o-ClC6H4
o-FC6H4
Ph
2
Br TES
Cl TES
THF
THF
THF
74
3
30 (35)
88
4
Ph
4
I
I
I
I
I
I
I
I
I
I
I
TMS
5
Ph
5
TMSSiMe2 THF
79
6
Ph
6
n-Pr3Si
n-Bu3Si
TTMSS
TBDMS
TES
THF
83
7
p-MeC6H4
p-ClC6H4
1-naphthyl
2-naphthyl
p-MeO2CC6H4
Ph
Ph
7
THF
68
8
Ph
8
THF
46 (50)
44 (82)
91
9
Ph
9
THF
10
12e
13
14
15
16
17
Ph
10d
11e
12
13
14f
15g
16g
THF
Ph
TES
DME
2-MeTHF
MeCN
THF
25 (67)
63 (75)
68 (93)
82
t-Bu
t-Bu
n-Bu
n-Bu
n-Bu
PhCHdC(Me)d
TES
TES
Ph
TES
2-furanyl
PhCHdC(Me)d
Br TES
TES
THF
74
a Abbreviations: TES, triethylsilyl; NMI, N-methylimidazole. b Isolated
yields after column chromatography. c Determined by HPLC analysis.
d (E)-isomer. e As the product is unstable toward TBAF, desilylation was
accomplished using 0.15 M HCl/aqueous THF.
I
THF
75
a Abbreviations: TES, triethylsilyl; TTMSS, tris(trimethylsilyl)silyl;
TBDMS, tert-butyldimethylsilyl; 2-MeTHF, 2-methyltetrahydrofuran.
b Isolated yields after column chromatography. For the reactions
stopped before completion, conversions of benzaldehyde are given in
parentheses. c Determined by HPLC analysis. d The reaction was carried
out at 0 °C. e A 2:1 ratio of pinacol coupling:alkynylation products was
detected. f 20 mol % of N-methylimidazole was used as an additive. g1
mol % of complex 2 was added.
the use of (R,R)-N,N0-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclo-
hexanediaminochromium(III) chloride24 under conditions cor-
responding to Table 1, entry 1, resulted in isolation of the
racemic product in 58% yield after 60 h. This observation
emphasizes the well-matched design of the TBOx backbone for
both reactivity and stereoselectivity of asymmetric alkynylation.
In summary, we have developed the first example of Cr-
catalyzed asymmetric alkynylation of aldehydes with 1-iodo- and
1-bromoalkynes. A range of propargyl alcohols was obtained with
generally good yields and high enantioselectivities (up to 92%
ee). Introduction of cobalt porphine co-catalyst 2 was found to
enhance the reactivity of 1-bromoalkynes without any loss of the
enantiocontrol. Further investigations on the potential of the
developed methodology and mechanistic studies of the catalytic
cycle25 are currently underway in our group.
A range of substrates were tested in the reaction with phenyl-
and alkyl-substituted 1-iodoalkynes under catalysis by complex 1
(Table 2). Aromatic, heteroaromatic, and R,β-unsaturated alde-
hydes were tolerated; aliphatic substrates demonstrated unsatis-
factory results. Reactions involving ortho-substituted aromatic
aldehydes required somewhat longer times for completion. The
presence of the ester moiety was well tolerated (entry 12). The
use of 1-iodo-3,3-dimethylbut-1-yne resulted in lowered enan-
tioselectivities (entries 13 and 14) compared to the n-butyl-
substituted analogue (entries 15-17).
We have recently demonstrated that the implementation of
Co porphine co-catalysis into TBOxCrCl-mediated asymmetric
Nozaki-Hiyama propargylation could increase the rate of the
catalytic process.20c Likewise, this approach appeared to be
beneficial for the methodology reported herein. The rate of the
reaction involving phenylbromoacetylene could be increased by
the addition of 1 mol % of commercially available complex 2; the
enantioselectivity level remained unchanged, whereas the reac-
tion time decreased 2-fold (Table 1, entry 15 vs 2). In the case of
the analogous iodoalkyne, the product was obtained with some-
what lower enantiomeric purity, which is presumably due to the
background process (entry 16).
’ ASSOCIATED CONTENT
S
Supporting Information. Complete experimental proce-
b
dures and characterization data for the prepared compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
yamamoto@uchicago.edu
We were curious about comparison of the catalytic perfor-
mance of TBOxCrCl (cis-β-configuration of the metal) in
asymmetric alkynylaton with a structurally different catalyst,
namely Cr(salen) system (trans-configuration). Interestingly,
’ ACKNOWLEDGMENT
This work was made possible by the generous support of the
NSF (CHE-0717618).
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dx.doi.org/10.1021/ja1102822 |J. Am. Chem. Soc. 2011, 133, 1286–1289

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(22) No reaction was observed in diethyl ether, 1,4-dioxane, methyl
tert-butyl ether, cyclopentyl methyl ether, dimethylformamide, toluene,
hexanes, or dichloromethane.
(23) The effects of the factors leading to enantioselectivity enhance-
ment (lowered temperature, use of 2-methyltetrahydrofuran as a
solvent, use of tri-n-propylsilyl chloride, addition of N-methylimidazole)
appeared to be nonadditive; various combination thereof did not result
in any further increase of the ee of the product.
(24) Martínez, L. E.; Leighton, J. L.; Carsten, D. H; Jacobsen, E. N.
J. Am. Chem. Soc. 1995, 117, 5897–5898.
(25) A question whether organomanganese species play an impor-
tant role in the catalysis was raised during the review process. The
organomanganese compound synthesized by transmetalation of
PhCtC;Li with MnI2 was reacted with benzaldehyde in the presence
of TBOxCrCl and furnished racemic products both in the presence and
in the absence of TESCl (however, TBOxCrCl-mediated alkynylation
with phenyliodoacetylene in the presence of 1.2 equiv of LiI proceeded
with 88% ee). We have also found that if the reaction is run under the
conditions corresponding to Table 1, entry 1, without the chromium
complex (carefully worked up after 15 h), the crude NMR contained
benzaldehyde and phenyliodoacetylene in the unchanged ratio; no
phenylacetylene was observed. On the basis of these observations, the
formation of organomanganese compounds in the TBOxCrCl-mediated
alkynylation can be excluded. The role of manganese is apparently
limited to conversion of CrIII into CrII, with the latter being able to
transfer electrons to the alkynyl halide.
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