Merging organocatalysis with transition metal catalysis: Highly stereoselective α-alkylation of aldehydes

Article abstract of DOI:10.1021/ol3002859

The unprecedented cooperative systems involving a diarylprolinol silyl ether with various Lewis acids have been found to effect the highly enantioselective intermolecular α-alkylation of aldehydes. A wide variety of aldehydes and alcohols can be transformed into the desired highly functionalized aldehydes in high yields, excellent enantioselectivities, and good diastereoselectivities at room temperature under mild conditions.

Full text of DOI:10.1021/ol3002859

ORGANIC
LETTERS
2012
Vol. 14, No. 7
1716–1719
Merging Organocatalysis with Transition
Metal Catalysis: Highly Stereoselective
r-Alkylation of Aldehydes
Jian Xiao*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
P. R. China
xiaojian@dicp.ac.cn
Received February 5, 2012
ABSTRACT
The unprecedented cooperative systems involving a diarylprolinol silyl ether with various Lewis acids have been found to effect the highly
enantioselective intermolecular R-alkylation of aldehydes. A wide variety of aldehydes and alcohols can be transformed into the desired
highly functionalized aldehydes in high yields, excellent enantioselectivities, and good diastereoselectivities at room temperature under mild
conditions.
The past few decades have witnessed the revolutionary
advancement of organocatalysis, which introduced large
numbers of novel reactions to complement the existing
transition metal catalysis and enzyme catalysis.1 In this
context, the concept of merging organocatalysis with
transition metal catalysis has emerged as a promising
strategy to uncover new reactivities and new reactions that
are currently not viable.2 However, most asymmetric ex-
amples in this new area involve cooperative catalysis of
chiral BINOL-derived phosphoric acid with metal
catalysis,3 while the comparable combination of a Lewis
acid with a chiral secondary amine is not common.4,7À10
This is attributed to the ease of self-quenching of the Lewis
acid by the secondary amine or water released during the
catalytic cycle. However, the catalytic asymmetric inter-
molecular R-alkylation of aldehydes,5 a long sought-after
goal for transition metal catalysis,6 has been tackled by
several elegant cooperative systems comprising of chiral
secondary amines with transition metals quite recently.
The most impressive examples came from coupling of
MacMillan’s imidazolidinone catalysts with Ce(IV) salts
(1) For selected recents reviews, see: (a) Barbas, C. F., III Angew.
Chem., Int. Ed. 2008, 47, 42. (b) Melchiorre, P.; Marigo, M.; Carlone, A.;
Bartoli, G. Angew. Chem., Int. Ed. 2008, 47, 6138. (c) Mukherjee, S.;
Yang, J. W.; Hoffmann, S.; List, B. Chem. Rev. 2007, 107, 5471. (d)
Bertelsen, S.; Jørgensen, K. A. Chem. Soc. Rev. 2009, 38, 2178. (e)
MacMillan, D. W. C. Nature 2008, 455, 304. (f) Westermann, B.; Ayaz,
M.; Berkel, S. Angew. Chem., Int. Ed. 2010, 49, 846. (g) Grondal, C.;
Jeanty, M.; Enders, D. Nat. Chem. 2010, 2, 167.
(2) (a) Shao, Z.; Zhang, H. Chem. Soc. Rev. 2009, 38, 2745. (b)
Zhong, C.; Shi, X. Eur. J. Org. Chem. 2010, 2999.
(3) For a review, see: (a) Rueping, M.; Koenigs, R. M.; Atodiresei, J.
Chem.;Eur. J. 2010, 16, 9350. For selected examples, see: (b) Hu, W.;
Xu, X.; Zhou, J.; Liu, W.-J.; Huang, H.; Hu, J.; Gong, L.-Z. J. Am.
Chem. Soc. 2008, 130, 7782. (c) Han, Z.-Y.; Xiao, H.; Chen, X.-H.;
Gong, L.-Z. J. Am. Chem. Soc. 2009, 131, 9182. (d) Sorimachi, K.;
Terada, M. J. Am. Chem. Soc. 2008, 130, 14452. (e) Terada, M.; Toda, Y.
J. Am. Chem. Soc. 2009, 131, 6354. (f) Yang, T.; Ferrali, A.; Sladojevich,
F.; Campbell, L.; Dixon, D. J. J. Am. Chem. Soc. 2009, 131, 9140.
(4) (a) Zhao, G.-L.; Ullah, F.; Deiana, L.; Lin, S.; Zhang, Q.; Sun, J.;
(5) For reviews, see: (a) Melchiorre, P. Angew. Chem., Int. Ed. 2009,
48, 1360. (b) Alba, A.-N.; Viciano, M.; Rios, R. ChemCatChem 2009, 1,
437. (c) Tak-Tak, L.; Dhimane, H.; Dalko, P. I. Angew. Chem., Int. Ed.
2011, 50, 12146. For selected examples without a metal, see: (d) Shaikh,
R. R.; Mazzanti, A.; Petrini, M.; Bartoli, G.; Melchiorre, P. Angew.
Chem., Int. Ed. 2008, 47, 8707. (e) Cozzi, P. G.; Benfatti, F.; Zoli, L.
Angew. Chem., Int. Ed. 2009, 48, 1313. (f) Benfatti, F.; Benedetto, E.;
ꢀ
Cozzi, P. G. Chem.;Asian J. 2010, 5, 2047. (g) Gomez-Bengoa, E.;
Landa, A.; Lizarraga, A.; Mielgo, A.; Oiarbide, M.; Palomo, C. Chem.
Sci. 2011, 2, 353. (h) Xiang, S.; Zhang, B.; Zhang, L.; Cui, Y.; Jiao, N.
Chem. Commun. 2011, 47, 5007. (i) Brown, A. R.; Kuo, W. H.; Jacobsen,
E. N. J. Am. Chem. Soc. 2010, 132, 9286. For asymmetric R-alkylation of
ketones, see: (j) Zhang, L.; Cui, L.; Li, X.; Li, J.; Luo, S.; Cheng, J.-P.
Chem.;Eur. J. 2010, 16, 2045.
ꢀ
Ibrahem, I.; Dziedzic, P.; Cοrdova, A. Chem.;Eur. J. 2010, 16, 1585. (b)
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Jensen, K. L.; Franke, P. T.; Arrοniz, C.; Kobbelgaard, S.; Jørgensen,
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r
10.1021/ol3002859
Published on Web 03/22/2012
2012 American Chemical Society

or Ru(II) salts, namely, SOMO catalysis7 or photoredox
catalysis,8 which constitutes the landmark work for the
combination of aminocatalysis with metal catalysis to
address currently underdeveloped reactions. Thereafter,
Nishibayashi developed a highly enantioselective pro-
pargylation of aldehydes with propargylic alcohols cata-
lyzed by diarylprolinol silyl ether with a ruthenium
complex.9 A stereoselective R-alkylation of aldehydes with
allylic alcohols or propargylic alcohols by use of the
MacMillan catalyst with InBr3 or In(OTf)3 was achieved
by Cozzi.10 However, these examples suffered from the use
of a single substrate, need for cryogenic conditions as well
as high catalyst loading. Up to until now, no systematic
study of the chiral secondary amine-transition metal cat-
alytic system is presented. Thus, the development of a new
highly efficient cooperative system involving a chiral sec-
ondary amine with a new transition metal for the highly
enantioselective intermolecular R-alkylation of aldehydes
is of great importance and poses a formidable challenge in
organic synthesis. As part of our ongoing research pro-
gram addressing new catalytic systems for asymmetric
catalysis,11 herein we report the unprecedented coopera-
tive systems of diarylprolinol silyl ether12 with CuCl, IrCl3,
or InBr3 to effect the highly enantioselective intermolecu-
lar R-alkylation of aldehydes with alcohols at room tem-
perature under very mild conditions with a wide substrate
scope.
candidate. Among the various Lewis acid catalysts
screened, most of them do not function in the cooperative
system (see the Supporting Information), and only the
successful examples are listed in Table 1. It was found that
1 equiv of AuCl3 can afford good yield, while almost no
Table 1. Screening of the Lewis Acidsa
entry
Lewis acid
AuCl3
x (mol %)
yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
9
100
50
50
20
20
20
50
20
10
79
44
51
89
53
38
66
77
43
77
86
93
97
53
73
96
94
93
CdCl2 2H2O
3
CuCl2 2H2O
3
CuCl
CuBr
CuI
IrCl3
IrCl3
IrCl3
At the outset, the model reaction between butylaldehyde
and xanthydrol was carried out with a range of Lewis acid
catalysts (20À100 mol %) to uncover the best Lewis acid
a Reactions were conducted with butyraldehyde (0.4 mmol), xanthy-
drol (0.1 mmol), catalyst I (0.01 mmol), and Lewis acids in 0.5 mL of
CH2Cl2 at room temperature. b Isolated yield. c The ee was determined
by HPLC analysis on a chiral stationary phase.
(6) For representative examples, see: (a) Evans, D. A.; Ennis, M. D.;
Mathre, D. J. J. Am. Chem. Soc. 1982, 104, 1737. (b) Schmierer, R.;
Grotemeier, G.; Helmchen, G.; Selim, A. Angew. Chem., Int. Ed. 1981,
20, 207. (c) Meyers, A. I.; Williams, D. R.; Erickson, G. W.; White, S.;
Druelinger, M. J. Am. Chem. Soc. 1981, 103, 3081. (d) Helmchen, G.;
Wierzchowski, R. Angew. Chem., Int. Ed. 1984, 23, 60. (e) Myers, A. G.;
Yang, B. H.; Chen, H.; McKinstry, L.; Kopecky, D. J.; Gleason, J. L. J.
Am. Chem. Soc. 1997, 119, 6496.
product was observed with catalytic AuCl3 (Table 1, entry 1).
Interestingly, CdCl2 2H2O and CuCl2 2H2O can provide
3
3
good results, implying that the carbocation intermediate
was not quenched by the hydrate (Table 1, entries 2À3).
Gratifyingly, we found that 20 mol % CuCl can afford
89% yield and 97% ee, but the analogous CuBr and CuI
gave far inferior results (Table 1, entries 4À6). Eventually,
it was found that 20 mol % IrCl3 gave 77% yield and 94%
ee, with both higher and lower catalyst loadings being less
effective (Table 1, entries 7À9). This promising result
represents the rare examples of IrCl3 as a Lewis acid
catalyst for organic transformations.13 It is noteworthy
that this new finding is consistent with the classification of
Lewis acids on the basis of aldehyde and aldimine selectiv-
ity by Kobayashi, wherein the effective Lewis acids for this
reactionsuchasCuCl2 and CuCl are inGroup B (weakand
aldimine selective), while CdCl2 and IrCl3 are classified
under Group C (inactive) in that domain.14
(7) (a) Beeson, T. D.; Mastracchio, A.; Hong, J.-B.; Ashton, K.;
MacMillan, D. W. C. Science 2007, 316, 582. (b) Jang, H.-Y.; Hong,
J.-B.; MacMillan, D. W. C. J. Am. Chem. Soc. 2007, 129, 7004. (c) Kim,
H.; MacMillan, D. W. C. J. Am. Chem. Soc. 2008, 130, 398. (d) Sibi, M.;
Hasegawa, M. J. Am. Chem. Soc. 2007, 129, 4124.
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Chem. Soc. 2008, 130, 16494. (c) Nagib, D. A.; Scott, M. E.; MacMillan,
D. W. C. J. Am. Chem. Soc. 2009, 131, 10875. (d) Wilson, J. E.; Casarez,
A. D.; MacMillan, D. W. C. J. Am. Chem. Soc. 2009, 131, 11332. (e)
Allen, A. E.; MacMillan, D. W. C. J. Am. Chem. Soc. 2010, 132, 4986.
For a nonphotolytic approach, see: (f) Allen, A. E.; MacMillan,
D. W. C. J. Am. Chem. Soc. 2010, 132, 4986.
(9) Ikeda, M.; Miyake, Y.; Nishibayashi, Y. Angew. Chem., Int. Ed.
2009, 49, 7289.
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P. G. Chem.;Eur. J. 2010, 16, 11237. (b) Sinisi, R.; Vita, M. V.;
Gualandi, A.; Emer, E.; Cozzi, P. G. Chem.;Eur. J. 2011, 17, 7404.
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10.1021/jo2025114. (b) Xiao, J.; Zhao, K.; Loh, T. P. Chem. Comm. 2012,
48, 3548. (c) Xiao, J.; Zhao, K.; Loh, T. P. Chem. Asian J. 2011, 6, 2890.
(d) Xiao, J.; Lu, Y. P.; Liu, Y. L.; Wong, P. S.; Loh, T. P. Org. Lett. 2011,
13, 876. (e) Xiao, J.; Xu, F. X.; Lu, Y. P.; Liu, Y. L.; Loh, T. P. Synthesis
2011, 1292. (f) Xiao, J.; Xu, F. X.; Lu, Y. P.; Loh, T. P. Org. Lett. 2010,
12, 1220. (g) Xiao, J.; Wong, Z. Z.; Lu, Y. P.; Loh, T. P. Adv. Synth.
Catal. 2010, 352, 1107. (h) Xiao, J.; Liu, Y. L.; Loh, T. P. Synlett 2010,
2029. (i) Xiao, J.; Loh, T. P. Org. Lett. 2009, 11, 2876.
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(13) (a) Karame, I.; Tommasino, M. L.; Lemaire, M. Tetrahedron
Lett. 2003, 44, 7687. (b) Ding, F.; William, R.; Gorityala, B. K.; Ma, J.;
Wang, S.; Liu, X. W. Tetrahedron Lett. 2010, 51, 3146.
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7876. (b) Mielgo, A.; Palomo, C. Chem.;Asian J. 2008, 3, 922. (c)
Jensen, K. L.; Dickmeiss, G.; Jiang, H.; Albrecht, Ł.; Jørgensen, K. A.
Acc. Chem. Res. 2012, 45, 248.
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2000, 6, 3491. For an example on the different behavior of various
metallic chlorides, see: (b) Alcaide, B.; Almendros, P.; Campo, T. M.
Chem.;Eur. J. 2008, 14, 7756.
Org. Lett., Vol. 14, No. 7, 2012
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With the optimized conditions in hand, the two coopera-
tive catalytic systems, I/CuCl and I/IrCl3, were employed
to investigate the generality of aldehydes and activated
alcohols (Table 2). Various aldehydes can be tolerated in
this reaction. Both linear and bulky aldehdyes, as well as
aldehydes bearing an oxygen atom, reacted with xanthydrol
to produce chiral xanthene derivatives in high yields
(63À96%) and excellent enantioselectivities (90À99% ee)
(Table 1, enties 1À9), rendering this methodology highly
valuable, considering the biochemical and pharmaceutical
significance of xanthene derivatives.15 Other activated alco-
hols, such as 9H-thioxanthen-9-ol (Table 1, entry 10) and
bis(4-(dimethylamino)-phenyl)methanol (Table 1, entry 11)
also work very well to afford the corresponding products in
high yields and high levels of stereochemical control. For all
these substrates, IrCl3 and CuCl exhibited similar proper-
ties, affording similar yields and similar enantioselectivities.
The vital importance of ferrocene-based chiral ligands
and catalysts in asymmetric catalysis16 encouraged us to
investigate the reaction of ferrocenyl(phenyl)methanol
(2d) with aldehydes. It was found that I/CuCl and I/IrCl3
fail to work for ferrocene-based activated alcohols. Even-
tually, we found that I/InBr3 is an effective cooperative
system for this new important class of compounds (see the
Supporing Information). For instance, reaction of pro-
pionaldehyde and butyraldehyde with 2d furnished 3l
and 3m in higher than 90% ee for both diastereoisomers
(Table 3, entries 1À2). Comparable results were also
obtained with 3l by use of I/Zn(OTf)2 system. Catalyst I
(10mol %) with 10 mol %Zn(OTf)2 can give 3l in 88% yield
with excellent enantioselectivities (94% ee for anti, 92% ee
for syn) and similar diastereoselectivity (anti:syn = 47:53).
Indole skeleton is a unique structure motif ubiquitously
present in a large number of biologically active natural
products and pharmaceuticals.17 Therefore, to develop
new efficient methods for the synthesis of optically pure
indole derivatives is of great importance for asymmetric
synthesis.18,19 Reaction of indole-based activated alcohols
with aldehydes provides a facile access to a large variety of
optically active carbonyl-functionalized indole derivatives,
although such type of reaction is extremely rare.5e,11bÀc,19
Hence, we attempted to extend the scope to indole-based
substrates. It was found that the cooperative system
I/InBr3 is an effective catalytic system for indolyl alcohols.
For instance, phenyl(2-phenyl-1H-indol-3-yl)methanol
Table 2. Substrate Scopea
(15) (a) Poupelin, J. P.; Saint-Rut, G.; Foussard-Blanpin, O.; Nar-
cisse, G.; Uchida-Ernouf, G.; Lacroix, R. Eur. J. Med. Chem. 1978, 13,
67. (b) Naya, A.; Sagara, Y.; Ohwaki, K.; Saeki, T.; Ichikawa, D.;
Iwasawa, Y.; Noguchi, K.; Ohtake, N. J. Med. Chem. 2001, 44, 1429. (c)
Ornstein, P. L.; Arnold, M. B.; Bleisch, T. J.; Wright, R. A.; Wheeler,
W. J.; Schoepp, D. D. Bioorg. Med. Chem. Lett. 1998, 8, 1919. (d)
Goodell, J. R.; Puig-Basagoiti, F.; Forshey, B. M.; Shi, P.-Y.; Ferguson,
D. M. J. Med. Chem. 2006, 49, 2127.
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(16) (a) Togni, A. Angew. Chem., Int. Ed. 1996, 35, 1475. (b) Arrayas,
R. G.; Adrio, J.; Carretero, J. C. Angew. Chem., Int. Ed. 2006, 45, 7674.
(c) Dai, L.-X.; Tu, T.; You, S.-L.; Deng, W.-P.; Hou, X.-L. Acc. Chem.
Res. 2003, 36, 659. (d) Colacot, T. J. Chem. Rev. 2003, 103, 3101. (e)
Miyake, Y.; Nishibayashi, Y.; Uemura, S. Synlett 2008, 1747.
(17) For reviews, see: (a) Humphrey, G. R.; Kuethe, J. T. Chem. Rev.
2006, 106, 2875. (b) Bandini, M.; Eichholzer, A. Angew. Chem., Int. Ed.
2009, 48, 9608. (c) Kochanowska-Karamyan, A. J.; Hamann, M. T.
Chem. Rev. 2010, 110, 4489.
a Unless noted, reactions were conducted with aldehyde (0.4 mmol),
alcohol (0.1 mmol), catalyst I (0.01 mmol), IrCl3 or CuCl (0.02 mmol) in
0.5 mL of CH2Cl2 at room temperature. b Isolated yields. c The ee was
determined by HPLC analysis on a chiral stationary phase.
1718
Org. Lett., Vol. 14, No. 7, 2012

which afforded low yields and low enantioselectivities (11%
ee) in previous report,5d provided excellent enantioselectiv-
ities (98% ee for 3p, 99% ee for 3q) and good to excellent
diastereoselectivities (75/25À98/2), albeit in moderate yields.
Acetaldehyde, an active and unique aldehyde candidate
in enamine catalysis,20 can smoothly react with 2e to furnish
the product 3r in good yields and good enantioselectivities in
the presence of 20 mol % I/InX3 (X = F, Cl, Br), of which
InF3 gave the best results (88% yield, 78% ee) (Scheme 1).
Table 3. Substrate Scopea
Scheme 1. Reaction of Acetaldehyde with 2e
In summary, the unprecedented cooperative chiral sec-
ondary amineÀLewis acid systems of I/CuCl, I/IrCl3, and
I/InBr3 have been discovered to catalyze the highly
enantioselective intermolecular R-alkylation of aldehydes.
A large variety of aldehydes and alcohols can be trans-
formed to the desired highly functionalized aldehydes in
high yields, excellent enantioselectivities and good diastereo-
selectivities. In particular, even the problematic and intri-
guing (1H-indol-3-yl)(phenyl)methanol and acetaldehyde
can also be well-tolerated. Furthermore, the mild condi-
tions and simple operation make this methodology very
practical. Lastly, the systematic study of the synergistic
catalysis21 of a chiral secondary amine with Lewis acids
paved the way for future new applications. Given the
versatility of Lewis acids in electrophilic activation, we
believe that these cooperative systems will trigger many
new asymmetric reactions.22
a Unless noted, reactions were conducted with aldehyde (0.4 mmol),
activated alcohol (0.1 mmol), catalyst I (0.01 mmol), InBr3 (0.01 mmol)
in 0.5 mL of CH2Cl2 at room temperature. b Isolated yield. c The dr (anti:
syn) was determined by chiral HPLC on a chiral stationary phase. d The
ee (%) was determined by chiral HPLC on a chiral stationary phase, and
absolute configuration was determined by comparison of chiral HPLC
retention time with those reported in the literature. See the Supporting
Information
Acknowledgment. Financial support from the National
Natural Science Foundation of China (No. 21102142) is
gratefully acknowledged. This paper is dedicated to Prof
Teck-Peng Loh on the occasion of his 50th birthday. We
thank Prof Teck-Peng Loh (Nanyang Technological Uni-
versity) for the generous help and Mr. Kai Zhao (Nanyang
Technological University) for some experiments.
(2e) reacted with both propionaldehyde and butyralde-
hyde to afford good yields and high enantioselectivities
(90À97% ee) with good diastereoselectivities (Table 3,
entries 3À4). Intriguingly, the problematic substrate (1H-
indol-3-yl)(phenyl)methanol (2f), without 2-substitution,
Supporting Information Available. Additional experi-
mental procedures and spectrascopic data of new pro-
ducts. This material is available free of charge via the
Internet at http://pubs.acs.org.
(19) (a) Han, B.; Xiao, Y.-C.; Yao, Y.; Chen, Y.-C. Angew. Chem.,
Int. Ed. 2010, 49, 10189. (b) Xu, B.; Guo, Z. L.; Jin, W. Y.; Wang, Z. P.;
Peng, Y. G.; Guo, Q. X. Angew. Chem., Int. Ed. 2012, 51, 1059.
(20) For a review on organocatalytic reactions with acetaldehyde,
see:Alcaide, B.; Almendros, P. Angew. Chem., Int. Ed. 2008, 47, 4632.
(21) Allen, A. E.; MacMillan, D. W. C. Chem. Sci. 2012, 3, 633.
(22) These catalytic systems cannot work well for allylic and pro-
pargylic alcohols.
(18) For reviews, see: (a) Palmieri, A.; Petrini, M.; Shaikh, R. R. Org.
Biomol. Chem. 2010, 8, 1259. (b) Loh, C. C. J.; Enders, D. Angew. Chem.,
Int. Ed. 2012, 51, 46. (c) Bartoli, G.; Bencivennia, G.; Dalpozzob, R.
Chem. Soc. Rev. 2010, 39, 4449. (d) Zeng, M.; You, S. L. Synlett 2010, 9,
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J.-W.; Song, L.; Feng, Z.; Gong, L.-Z. Org. Lett. 2009, 11, 4620. (f)
Liang, T.; Zhang, Z.; Antilla, J. C. Angew. Chem., Int. Ed. 2010, 49, 9734.
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Y.; Jiang, G. F. Chem. Sci. 2011, 2, 803. (h) Guo, C.; Song, J.; Luo,
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The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 7, 2012
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