Helical chiral 2,2′-bipyridine N-monoxides as catalysts in the enantioselective propargylation of aldehydes with allenyltrichlorosilane

Article abstract of DOI:10.1021/ol200102c

A highly enantioselective synthesis of homopropargylic alcohols is achieved by using the new helical chiral 2,2′-bipyridine N-monoxide catalyst and allenyltrichlorosilane. This method can be further extended to the enantio- and regioselective propargylati

Full text of DOI:10.1021/ol200102c

ORGANIC
LETTERS
Helical Chiral 2,20-Bipyridine
N- Monoxides as Catalysts in the
Enantioselective Propargylation
of Aldehydes with Allenyltrichlorosilane
2011
Vol. 13, No. 7
1654–1657
Jinshui Chen, Burjor Captain, and Norito Takenaka*
Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables,
Florida 33146-0431, United States
n.takenaka@miami.edu
Received January 16, 2011
ABSTRACT
A highly enantioselective synthesis of homopropargylic alcohols is achieved by using the new helical chiral 2,20-bipyridine N-monoxide catalyst
and allenyltrichlorosilane. This method can be further extended to the enantio- and regioselective propargylation of N-acylhydrazones.
Optically active homopropargylic alcohols are highly
useful chiral building blocks in organic synthesis due to the
synthetic versatility of an acetylene unit.1 The asymmetric
propargylation of aldehydes provides direct access to this
class of compounds.2 Significant progress has been made
in the development of chiral propargylation reagents and
diastereoselective additions of propargylic anion equiva-
lents to chiral aldehydes.2,3 Enantioselective catalytic ap-
proaches include Barbier type additions,1a,b,4 Lewis acid or
base catalysis with metalloallene reagents,5 and the Cu-
catalyzed propargylation with a propargyl borolane.6
Despite these creative efforts, the catalytic asymmetric
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J. Org. Chem. 2003, 68, 4008–4013. (e) Marino, J. P.; Mcdure, M. S.;
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r
10.1021/ol200102c
Published on Web 03/02/2011
2011 American Chemical Society

propargylation of aldehydes remains a challenge. Some of
these methods suffer from difficulties associated with low
regioselectivity (competing allenylation) and/or reactivity.
In some instances, the protocol requires the use of toxic
reagents, such as tin compounds.
effectively to the putative hypervalent silicate complex.12,13
In light of this observation, we were interested in exploring
the reactivity of allenyltrichlorosilane in Lewis base-
catalyzed addition to aldehydes since it represents a more
challenging substrate as mentioned above.
Chiral Lewis base catalysis of polychlorosilane-mediated
transformations was pioneered by Denmark and co-workers,
and such Lewis base catalysts have been shown to promote
several synthetically useful reactions, providing valuable
implication for further study.7 Allenyltrichlorosilane is an
attractive candidate as a nucleophilic partner in CdO and
CdN propargylation reactions because of its mildness, re-
giospecificity, and low toxicity, although it is considerably
less reactive than analogous allyltrichlorosilane.8,9 However,
to our knowledge, the report by Nakajima and co-workers is
the only example of the use of this reagent in asymmetric
catalysis (20 mol % of axial chiral 3,30-dimethyl-2,20-
biquinoline N,N0-dioxide,10 35-65% yields, 23-52%
ees).11a Herein, we report the discovery of a new bidentate
Lewis base that efficiently catalyzes the addition of alle-
nyltrichlorosilane toaromatic aldehydes with highlevels of
enantioselectivity and yields. The proposed stereochemical
models that account for the observed levels and trends in
enantioselectivity are provided.
Table 1. Evaluation of Catalystsa
entry
cat.
temp (°C)
time (h)
yield (%)b
eec
1
2
3
4
5
6
1
-78
-40
-20
-20
-20
-78
24
24
24
24
24
3
7
48
80
85
90
99
68
54
48
48
34
84
1
1
2
3
4d
Recently, we reported that helical chiral pyridine
N-oxides 1-3 (Table 1) efficiently activate SiCl4, and the
chirality of an helicene can indeed be communicated
a Allenyltrichlorosilane (1.5 equiv) was used. b Yield of the isolated
product. c Ee value was determined by HPLC analysis on a chiral phase.
d See Scheme 1.
(5) For selected references, see: (a) Hanawa, H.; Uraguchi, D.;
Konishi, S.; Hashimoto, T.; Maruoka, K. Chem.;Eur. J. 2003, 9,
4405–4413. (b) Konishi, S.; Hanawa, H.; Maruoka, K. Tetrahedron:
Asymmetry 2003, 14, 1603–1605. (c) Yu, C.-M.; Kim, J.-M.; Shin, M.- S.;
Cho, D. Tetrahedron Lett. 2003, 44, 5487–5490. (d) Evans, D. A.;
Sweeney, Z. K.; Rovis, V; Tedrow, J. S. J. Am. Chem. Soc. 2001, 123,
12095–12096. (e) Denmark, S. E.; Wynn, T. J. Am. Chem. Soc. 2001, 123,
6199–6200. (f) Yu, C.-M.; Yoon, S.-K.; Choi, H.-S.; Baek, K. Chem.
Commun. 1997, 763–764. (g) Keck, G. E.; Krishnamurthy, D.; Chen, X.
Tetrahedron Lett. 1994, 55, 8323–8324.
Scheme 1. Synthesis of a New Bidentate Catalyst
(6) (a) Fandrick, D. R.; Fandrick, K. R.; Reeves, J. T.; Tan, Z.; Tang,
W.; Capacci, A. G.; Rodriguez, S.; Song, J. J.; Lee, H.; Yee, N. K.;
Senanayake, C. H. J. Am. Chem. Soc. 2010, 132, 7600–7601. For a
related copper catalyzed asymmetric propargylation of ketones, see:
(b) Shi, S.-L.; Xu, L.-W.; Oisaki, K.; Kanai, M.; Shibasaki, M. J. Am.
Chem. Soc. 2010, 132, 6638–6639.
(7) For selected reviews, see: (a) Denmark, S. E.; Beutner, G. L.
Angew. Chem., Int. Ed. 2008, 47, 1560–1638. (b) Benaglia, M.; Guizzetti,
S.; Pignataro, L. Coord. Chem. Rev. 2008, 252, 492–512. (c) Malkov,
A. V.; Kocovsky, P. Eur. J. Org. Chem. 2007, 29–36. (d) Orito, Y.;
Nakajima, M Synthesis 2006, 9, 1391–1401.
(8) (a) Schneider, U.; Sugiura, M.; Kobayashi, S. Adv. Synth. Catal.
2006, 348, 323–329. (b) Schneider, U.; Sugiura, M.; Kobayashi, S.
Tetrahedron 2006, 62, 496–502. (c) Kobayashi, S.; Nishio, K. J. Am.
Chem. Soc. 1995, 117, 6392–6393.
(9) For discussions, see: (a) Curtis-Long, M. J.; Aye, Y. Chem.;Eur.
J. 2009, 15, 5402–5416. (b) Denmark, S. E.; Fu, J. Chem. Rev. 2003, 103,
2763–2793. Allenylstannanes are considerably less reactive than allyl-
stannanes. see:(c) Lequan, M.; Guillerm, et G. J. Organomet. Chem.
1973, 54, 153–164.
We initiated our investigation by evaluating the reaction
ofallenyltrichlorosilane withbenzaldehydeusing catalyst1
(Table 1). To our delight, 1 did catalyze the reaction and
provided the product with good enantioselectivity
although the yield was poor (entry 1). Higher reaction
temperature was necessary to increase the yield, but it
adversely affected the enantioselectivity (entries 2 and 3).
Sterically less demanding catalysts 2 and 3 did not improve
the reactivity much (entries 4 and 5). Mechanistically, this
reaction is thought to involve coordination of one or two
Lewis bases to allenyltrichlorosilane to generate a more
Lewis acidic species capable of activating aldehyde toward
propargylation, analogous to other reactions involving
Lewis bases and polychlorosilanes.14 Thus, the reaction
(10) 3,30-Dimethyl-2,20-biquinoline NN0-dioxide developed by
Nakajima and co-workers is among the most successful catalysts
reported to date for allylation of aldehydes with allyltrichlorosilane.
(a) Nakajima, M.; Saito, M.; Shiro, M.; Hashimoto, S. J. Am. Chem.
Soc. 1998, 120, 6419–6420. Also see(b) Malkov, A. V.; Bell, M.;
Castelluzzo, F.; Kocovsky, P. Org. Lett. 2005, 7, 3219–3222 and ref 7c.
(11) (a) Nakajima, M.; Saito, M.; Hashimoto, S. Tetrahedron: Asym-
metry 2002, 13, 2449–2452. For a related allenylation of aldehydes, see:
(b) Iseki, K.; Kuroki, Y.; Kobayashi, Y. Tetrahedron: Asymmetry 1998,
9, 2889–2894.
(13) For azahelicenes as asymmetric catalysts, see: (a) Takenaka, N.;
Chen, J.; Captain, B.; Sarangthem, R. S.; Chandrakumar, A. J. Am.
Chem. Soc. 2010, 132, 4536–4537. (b) Samai, M.; MiSek, J.; Stara, I. G.;
Stary, I. Collect. Czech. Chem. Commun. 2009, 74, 1151–1159.
(12) (a) Chen, J.; Takenaka, N. Chem.;Eur. J. 2009, 15, 7268–7276.
(b) Takenaka, N.; Sarangthem, R. S.; Captain, B. Angew. Chem., Int. Ed.
2008, 47, 9708–9710.
Org. Lett., Vol. 13, No. 7, 2011
1655

can be either first or second order in a monodentate Lewis
base catalyst. Since the beneficial effects of bidentate
catalysts were described for some analogous reactions,14,15
we decided to pursue the possibility that 2-pyridine sub-
stituted N-oxide 4 (Scheme 1) might serve as a bidentate
Lewis base catalyst16 (i.e., first order in the catalyst).
Catalyst 4 was readily derived from 1 in high yield.17 To
our surprise, 4 displayed significantly increased reactivity
and selectivity, completing the reaction at ;78 °C in only
3 h (entry 6).18
always provided higher enantioselectivities than 4-substi-
tuted counterparts did regardless of the kind of functional
groups. An aliphatic aldehyde was also found to be a good
substrate for the present catalyst although somewhat long-
er reaction time was required (entry 18). At the end of these
reactions, the catalysts were recovered (ca. 80%) and
reused without loss in activity and selectivity.
Table 2. Asymmetric Propargylation of Aldehydesa
entry
R
yield (%)b
eec
1d
2d
3d
4
Ph
87
86
84
92
92
88
92
90
74
82
96
96
92
96
94
94
86
96
59
2-naphthyl
4-Br-Ph
86
95
Figure 1. Solid-state structure of (P)-4.
4-Cl-Ph
90
5
6f
4-F-Ph
93
4-NO2-Ph
4-CF3-Ph
4-MeO-Ph
4-Me-Ph
2-Br-Ph
55e
80
Collectively considering all the data we have obtained so
far, including the crystal structure of 4 (Figure 1), we were
able to propose the stereochemical models19 that are con-
sistent with the observed sense of enantioselection and the
aforementioned trends (Scheme 2). The Si face addition is
favored due to the expected π-stacking between the bound
aldehyde and the helicene framework. In contrast, the 11,
12- benzo unit is very close to the aldehyde bound in the Re
face addition mode (steric repulsion). This expected steric
repulsion is also consistent with the large enantioselectivity
difference between 4- and 2-MeO substituted aldehydes
whose π-stacking interactions with the helicene framework
should not be efficient (Table 2, entries 8 and 15). For
comparison, we synthesized catalyst 5 and tested it
(Scheme 2, eq 1). It was equally reactive but significantly
less selective than 4 as predicted by the stereochemical
models. While a definitive answer regarding the mode of
2-pyridine substituted catalyst 4 is still to be firmly
7
8
80
9
85
10d
11
12
13
14f
15
16
17f
18g
93
2-Cl-Ph
97
2-F-Ph
98
2-NO2-Ph
2-CF3-Ph
2-MeO-Ph
2-Me-Ph
2-Br-4-Me-Ph
Cy
87
95
78
90
92
61 h(80)i
a Allenyltrichlorosilane (1.5 equiv)was used. b Yield of the isolated
product. c Ee value was determined by HPLC analysis on a chiral phase.
d Absolute configurations were determined. See Supporting Informa-
tion. e Low yield is presumably due to the poor solubility of the aldehyde.
f (M)-4 catalyst was used. g (R)-Isomer is major. h Twelve hour reaction.
i Thirty-six hour reaction.
Next, we systematically examined a series of aromatic
aldehydes using catalyst 4. The results are summarized in
Table 2, from which certain trends were gleaned. (1) In the
case of 4-substituted aldehydes, electron-deficient alde-
hydes (entries 3-7) are better substrates than electron-rich
aldehydes (entries 8 and 9) in terms of enantioselectivity.
(2) For 2-substituted counterparts, both electron-deficient
and electron-rich aldehydes provided excellent enantios-
electivities (entries 10-17). (3) 2-substituted aldehydes
(15) For selected references, see: (a) Pu, X.; Qi, X.; Ready, J. M.
J. Am. Chem. Soc. 2009, 131, 10364–10365. (b) Chelucci, G.; Baldino, S.;
Pinna, G. A.; Benaglia, M.; Buffa, L.; Guizzetti, S. Tetrahedron 2008, 64,
7574–7582. (c) Nakajima, M.; Saito, M.; Uemura, M.; Hashimoto, S.
Tetrahedron Lett. 2002, 43, 8827–8829.
(16) For selected references for 2,20-bipyridine N-monoxides as Lewis
base catalysts, see: (a) Malkov, A. V.; Orsini, M.; Pernazza, D.; Muir,
K. W.; Langer, V.; Meghani, P.; Kocovsky, P. Org. Lett. 2002, 4, 1047–
1049. (b) Malkov, A. V.; Bell, M.; Orsini, M.; Pernazza, D.; Massa, A.;
Herrmann, P.; Meghani, P.; KoCovsky, P. J. Org. Chem. 2003, 68, 9659–
9668.
(17) Tagawa, Y.; Nomura, M.; Yamashita, H.; Goto, Y.; Hamana,
M. Heterocycles 1999, 51, 2385–2397.
(18) For comparison, we tested the corresponding p-tol analogue of 4
(Py = p-tol). It hardly produced the product even after 24 h at ambient
temperature.
(19) The proposed stereochemical models in Scheme 2 respect the
stereoelectronic effect, which dictates that the Lewis-basic N-oxide
oxygen be positioned trans to the allenyl group to enhance its nucleo-
philicity. For discussions, see: refs 14f, 16 and Traverse, J. F.; Zhao, Y.;
Hoveyda, A. H.; Snapper, M. L. Org. Lett. 2005, 7, 3151–3154.
(14) For selected references, see: (a) Denmark, S. E.; Eklov, B. M.;
Yao, P. J.; Eastgate, M. D. J. Am. Chem. Soc. 2009, 131, 11770–11787.
(b) Malkov, A. V.; Ramirez-Lopez, P.; Biedermannova, L.; Rulfiek, L.;
Dufkova, L.; Kotora, M.; Zhu, F.; Kocovsky, P. J. Am. Chem. Soc.
2008, 130, 5341–5348. (c) Denmark, S. E.; Fu, J.; Coe, D. M.; Su, X.;
Pratt, N. E.; Griedel, B. D. J. Org. Chem. 2006, 71, 1513–1522.
(d) Denmark, S. E.; Fu, J.; Lawler, M. J. J. Org. Chem. 2006, 71,
1523–1536. (e) Malkov, A. V.; Dufkova, L.; Farrugia, L.; Kocovsky, P.
Angew. Chem., Int. Ed. 2003, 42, 3674–3677. (f) Denmark, S. E.; Fu, J.
J. Am. Chem. Soc. 2000, 122, 12021–12022.
1656
Org. Lett., Vol. 13, No. 7, 2011

Scheme 2. Proposed Stereochemical Models
Scheme 3. Propargylation of an Acylhydrazonea
a Allenyltrichlorosilane (1.5 equiv) was used. The reaction was not
optimized.
reported, to our knowledge, for the Lewis base-promoted
additions of not only allenyltrichlorosilane but also more
reactive allyltrichlorosilane to N-acylhydrazones.23
In summary, we have developed a highly enantioselec-
tive propargylation of aromatic aldehydes with allenyltri-
chlorosilane. In the course of this study, we have identified new
helical chiral 2,20-bipyridine N-monoxides that exhibited ex-
cellent reactivity for the relatively unreactive allenyltrichloro-
silane. Ongoing studies include mechanistic study of new
catalysts, and expansion of the scope and utility of the reaction.
established, and while we would like to refrain from spec-
ulating on the reason for its remarkably high reactivity at the
current stage, we can not completely rule out a possibility of
the stabilizing cation-π type interaction20 between the sili-
cate and the helicene framework due to its unique architec-
ture. This would increase the concentration of a reactive
silicate species in the reaction.
Encouraged by the excellent reactivity observed with
catalyst 4, we wondered whether it could add allenyltri-
chlorosilane to an acylhydrazone.8a Interestingly, it did
catalyze the reaction and provided the product in good
enantioselectivity (Scheme 3).21,22 It is worthy of mention
that substoichiometric enantioinduction has not been
Acknowledgment. We thank the Florida Department of
Health(07KN-12), the AmericanCancerSociety(IRG-98-
277-07), the Sylvester Comprehensive Cancer Center, and
the University of Miami for support. We also thank Prof.
Stanislaw Wnuk (Florida International University) for the
use of their polarimeter.
Supporting Information Available. Experimental pro-
cedures, characterization data, and X-ray crystallo-
graphic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
(20) For selected review, see: Ma, J. C; Dougherty, D. A. Chem. Rev.
1997, 97, 1303–1324.
(21) For selected references for the asymmetric synthesis of homo-
propargylic amines, see: (a) Fandrick, D. R.; Johnson, C. S.; Fandrick,
K. R.; Reeves, J. T.; Tan, Z.; Lee, H.; Song, J. J.; Yee, N. K.;
Senanayake, C. H. Org. Lett. 2010, 12, 748–751. (b) Brawn, R. A.;
Panek, J. S. Org. Lett. 2009, 11, 4362–4365. (c) Gonzalex, A. Z.;
Soderquist, J. A. Org. Lett. 2007, 9, 1081–1084.
(22) For catalytic enantioselective propargylation of CdN electro-
philes, see: (a) Wisniewska, H. M.; Jarvo, E. R. Chem. Sci. 2011,
DOI: 10.1039/c0sc00613k. (b) Kagoshima, H.; Uzawa, T.; Akiyama, T.
Chem. Lett. 2002, 298–299.
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Garcia, M. V.; Khiar, N. Org. Biomol. Chem. 2010, 8, 4388–4393.
(b) Fulton, J. R.; Kamara, L. M.; Morton, S. C; Rowlands, G. J. Tetra-
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Khiar, N. Org. Lett. 2007, 9, 2215–2218. (d) Garcia-Flores, F.; Flores-
Michel, L. S.; Juaristi, E. Tetrahedron Lett. 2006, 47, 8235–8238.
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N. Org. Lett. 2005, 7, 1307–1310. (f) Ogawa, C; Sugiura, M.; Kabayashi, S.
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Org. Lett., Vol. 13, No. 7, 2011
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