Silver-catalyzed asymmetric Sakurai-Hosomi allylation of ketones

Article abstract of DOI:10.1021/ja0553351

The complex of AgF and (R)-DIFLUORPHOS has been shown to be an effective catalyst for the asymmetric Sakurai-Hosomi allylation of simple ketones. A significaant improvement of the reactivity was observed by using THF as the solvent. The catalyst turnover

Full text of DOI:10.1021/ja0553351

Published on Web 09/30/2005
Silver-Catalyzed Asymmetric Sakurai-Hosomi Allylation of Ketones
Manabu Wadamoto and Hisashi Yamamoto*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
Received August 5, 2005; E-mail: yamamoto@uchicago.edu
Table 1. Optimal Reaction Conditionsa
Chiral homoallylic alcohols represent a class of compounds
that can be widely used in the synthesis of biologically active
compounds.1 Catalytic asymmetric Sakurai-Hosomi allylation is
one of the most reliable methods for providing these compounds
and has the advantage of having inexpensive, nontoxic, and stable
allyltrialkylsilanes2 and allyltrialkoxysilanes3 as nucleophiles. Al-
entry
ligand
catalyst, mol %
temp,
°
C
yield, %
ee, %b
though excellent asymmetric Sakurai-Hosomi allylations of alde-
hydes have been developed with these reagents, there are few
examples3b,4,5 of catalytic asymmetric allylation of simple ketones
despite the attractive structure of the chiral tertiary homoallylic
alcohols in organic synthesis.6
Many chiral silver-mediated asymmetric reactions have recently
been reported from our7 and other laboratories.8 We herein report
the development of a new catalyst system and its application to
the highly regio-, diastereo-, and enantioselective Sakurai-Hosomi
allylation of simple ketones.
1c
2
3
4
5
6
7
4
4
5
6
7
7
7
5
5
5
5
5
5
2
-78
-78
-78
-78
-78
-90
-78
35
90
90
95
96
40
86
63
63
46
74
82
86
81
a Reaction was conducted with 2.0 equiv of allyltrimethoxysilane at -78
°C for 12 h under Ar atmosphere. b The ee value was determined by HPLC.
c No addition of MeOH.
Initially we tested Sakurai-Hosomi allylation with acetophenone
and allyltrimethoxysilane using a catalytic amount of AgF and (R)-
BINAP (4) in MeOH.3a The desired product was not obtained at
all even under reflux conditions. Athough AgF is not soluble in an
aprotic solvent such as THF, the complex of AgF and 4 prepared
in MeOH is easily dissolved in THF. Further, the desired tertiary
alcohol with 63% ee was obtained by using this complex (Table 1,
entry 1). It is noteworthy that the product obtained was the tertiary
alcohol, not the silylated product. The alcoholic proton of this
product must originate from a small amount of MeOH used for
preparation of the catalyst. This observation indicated that the proton
transfer from MeOH should be much faster than silyl transfer
(Figure 1).3b,6a,9 Therefore, an additional equivalent of MeOH
improved the yield of the product significantly (entry 2).
Figure 1. Proposed catalytic cycle.
We recently documented that more than three complexes between
silver and diphosphine exist (Figure 2). According to these studies,
different reactivity and selectivity were given by different com-
plexes, and it is important to generate a single silver complex to
achieve a high stereoselective reaction.7b A survey of the 31P NMR
of 1:1 mixtures of AgF and ligands revealed that (R)-DIFLUOR-
PHOS (7) gave predominantly complex A, which is presumably
due to the poor electron donation ability of the phosphorus atoms.10
Significant improvement of enantioselectivity was observed by
using this solution.11 Finally, 86% ee was obtained by conducting
the reaction at -90 °C (entry 6). Using 2 mol % of catalyst provided
the same level of yield and enantioselectivity (entry 7).
As summarized in Table 2, the catalytic enantioselective ally-
lation of a variety of ketones was carried out using these conditions.
Although reactions proceed smoothly with either electron-
withdrawing or -donating groups attached to the aromatic ring, the
electron-withdrawing group gave higher enantioselectivity (entry
1). Remarkably, more than 90% ee was obtained from various
aromatic cyclic ketones (entries 6-9). Only the 1,2-addition took
place exclusively with both cyclic and acyclic conjugate enones
(entries 10-14). It is noteworthy that excellent enantiomeric
Figure 2. Ratio of three silver ligand complexes. A 1:1 mixture of AgF
ligand was used. The ratio of A/B was determined by 31P NMR at -78 °C.
See details in Supporting Information.
excesses (up to 96% ee) were observed in the reactions with a series
of R-halo unsaturated ketones.
Condensation of γ-substituted allyl metal reagents with simple
ketones is a challenging problem with regard to regio- (R/γ) and
stereoselectivity (E/Z or anti/syn).3b,12 The reactions between
acetophenone and various allyltrimethoxysilanes were investigated
using our new catalyst system (Table 3). Crotyltrimethoxysilane
gave branched syn products with high enantioselectivity from both
E- and Z-isomers. Surprisingly, starting from racemic allylsilanes
such as 2d-e, we observed the optically pure product almost
exclusively. Thus, in sharp contrast with the previous study of the
SE′ allylation using a combination of chiral allylsilanes and Lewis
acid,1 the present catalyst provides high diastereo- and enantiose-
lectivity regardless of chirality of the starting allylsilane. We may
9
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J. AM. CHEM. SOC. 2005, 127, 14556-14557
10.1021/ja0553351 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Substrate Scopea
Catalyst turnover was increased by the addition of MeOH, and the
system was further improved based on the structure of the complex
of silver and ligand. The allylation uniformly provided optically
active syn isomers. Detailed mechanisms of this reaction including
allylation from racemic allylsilane are now under investigation.
Acknowledgment. Support of this research was provided by
SORST project of Japan Science and Technology Agency (JST).
We thank Takasago International Corporation for its generous gift
of (R)-SEGPHOS.
entry
ketone
product
yield, %
ee, %b
1
2
3
4
5
6
7
8
9
10
11
12
13
14c
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
3ba
3ca
3da
3ea
92
63
62
98
42
89
63
78
74
97
92
92
96
56
90
78
92
95
65
91
93
92
92
96
96
94
84
95
Supporting Information Available: Experimental procedures,
spectral data for all new compounds, and characterization data. This
material is available free of charge via the Internet at http://pubs.acs.org.
3fa
References
3ga
3ha
3ia
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3ja
3kad
3lad
3mad
3nad
3oad
a Reaction was conducted with 5 mol % of catalyst, 2.0 equiv of
allyltrimethoxysilane at -78 °C for 12 h under Ar atmosphere. b The ee
value was determined by HPLC or GC. c A quantity of 4.0 equiv of
allyltrimethoxysilane was used. d Only 1,2-adduct was observed.
(4) For catalytic enantioselective addition to simple ketones using allylborane,
see: Wada, R.; Oisaki, K.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc.
2004, 126, 8910.
Table 3. Catalytic Diastereo- and Enantioselective Allylationa
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ketones using allyltin, see: (a) Casolari, S.; D’Addario, D.; Tagliavine,
E. Org. Lett. 1999, 1, 1061. (b) Cunningham, A.; Woodward, S. Synthesis
2002, 43. (c) Kim, J. G.; Waltz, K. M.; Garcia, I. F.; Kwiatkowski, G.
D.; Walsh, P. J. J. Am. Chem. Soc. 2004, 126, 12580. (d) Kii, S.; Maruoka,
K. Chirality 2003, 15, 67. (e) Teo, Y.-C.; Goh, J.-D.; Loh, T.-P. Org.
Lett. 2005, 7, 2743.
(6) Selected examples of catalytic enantioselective addition of other nucleo-
philes to simple ketones: (a) Dosa, P. I.; Fu, G. C. J. Am. Chem. Soc.
1998, 120, 445. (b) Yabu, K.; Masumoto, S.; Yamasaki, S.; Hamashima,
Y.; Kanai, M.; Du, W.; Curran, D. P.; Shibasaki, M. J. Am. Chem. Soc.
2001, 123, 9908. (c) Denmark, S. E.; Fan, Y. J. Am. Chem. Soc. 2002,
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9544. (e) Moreau, X.; Bazan-Tejeda, B.; Campagne, J.-M. J. Am. Chem.
Soc. 2005, 127, 7288.
entry
allylsilane
yield %
syn/antib
ee, %c
(7) (a) Yanagisawa, A.; Nakashima, H.; Ishiba, A.; Yamamoto, H. J. Am.
Chem. Soc. 1996, 118, 4723. (b) Momiyama, N.; Yamamoto, H. J. Am.
Chem. Soc. 2004, 126, 5360.
(8) (a) Longmire, J. M.; Wang, B.; Zhang, X. J. Am. Chem. Soc. 2002, 124,
13400. (b) Josephsohn, N. S.; Snapper, M. L.; Hoveyda, A. H. J. Am.
Chem. Soc. 2004, 126, 3734. (c) Yanagisawa, A.; Touge, T.; Arai, T.
Angew. Chem., Int. Ed. 2005, 44, 1546.
1
2
2b R1 ) Me, R2 ) H
2c R1 ) H, R2 ) Me
2d n ) 1
60
95
72
74e
97
90/10
90/10
96/4
96/4
>99/1
95
93
98
99
98
3
4d
5
2d n ) 1
2e n ) 3
(9) A similar mechanism was proposed by: Kobayashi, S.; Kiyohara, H.;
Nakamura, Y.; Matsubara, R. J. Am. Chem. Soc. 2004, 126, 6558.
(10) Jeulin, S.; Duprat de Paule, S.; Ratovelomanana-Vidal, V.; Genet, J.-P.;
Champion, N.; Dellis, P. Angew. Chem., Int. Ed. 2004, 43, 320.
(11) The active species is not clear in this reaction, because minor complexes
are able to dominate a reaction.
a Reaction was conducted with 5 mol % of catalyst, 3.0 equiv of
allylsilane at -40 °C for 36 h under Ar atmosphere. c Diastereomer ratio
was determined by 1H NMR. b The ee value was determined by HPLC.
c Major isomer. d A quantity of 0.5 equiv of allylsilane was used. e Deter-
mined from allylsilane.
(12) Yasuda, M.; Hirata, K.; Nishino, M.; Yamamoto, A.; Baba, A. J. Am.
Chem. Soc. 2002, 124, 13442.
therefore conclude that allylsilvers rapidly interconvert between two
diastereomeric complexes (eq 1).13
In summary, we have developed a new AgF catalyst and
demonstrated high enantioselective allylation of simple ketones.
(13) (a) Trost, B. M.; Crawley, M. L. Chem. ReV. 2003, 103, 2921 (b) Zanoni,
G.; Gladiali, S.; Marchetti, A.; Piccinini, P.; Tredici, I.; Vidari, G. Angew.
Chem., Int. Ed. 2004, 43, 846.
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