Rhodium-catalyzed enantioselective conjugate silyl transfer: 1,4-Addition of silyl boronic esters to cyclic enones and lactones

Article abstract of DOI:10.1002/anie.200601747

(Chemical Equation Presented) Missing linkage: A chiral cationic rhodium complex catalyzes the enantioselective conjugate addition of silyl anion equivalents to cyclic α,β-unsaturated ketones (X = CH₂) and esters (X = O; see scheme). This unpre

Full text of DOI:10.1002/anie.200601747

Angewandte
Chemie
Asymmetric Catalysis
DOI: 10.1002/anie.200601747
Rhodium-Catalyzed Enantioselective Conjugate
Silyl Transfer: 1,4-Addition of Silyl Boronic Esters
to Cyclic Enones and Lactones**
Christian Walter, Gertrud Auer, and Martin Oestreich*
Silicon connected to a stereogenic carbon atom is an
important functional group in asymmetric synthesis. The
synthetic equivalence of silicon and oxygen, which is achieved
[1]
ꢀ
by the stereospecific oxidative degradation of a C Si bond,
Scheme 1. Strategies for the transition-metal-catalyzed enantioselective
construction of a-chiral b-silylated carbonyl compounds.
ꢀ
as well as several stereoselective C C bond-forming reactions
involving a-chiral silanes[2] underscore the synthetic potential
of silicon. Conversely, the number of effective methods for
ꢀ
the direct enantioselective formation of C Si bonds is
rhodium-catalyzed (asymmetric) conjugate silyl transfer
through the use of silyl boronic esters as the silyl anion
limited[3] and, therefore, novel asymmetric transition-metal-
catalyzed reactions will certainly plug a “synthetic hole”.
The 1,4-addition of stoichiometric amounts of silicon-
based cuprate reagents to prochiral a,b-unsaturated carbonyl
source.[16] Herein we report an unprecedented conjugate C Si
ꢀ
bond-forming reaction, which, for the first time, enables the
catalytic asymmetric 1,4-addition of nucleophilic silicon to
cyclic a,b-unsaturated carbonyl compounds with excellent
levels of stereoinduction.
ꢀ
compounds is one of the standard C Si bond-forming
reactions.[4] Although copper-catalyzed variants are available
today,[5,6] their extension to chirally modified catalysts has so
far failed.[7] Copper-catalyzed conjugate silyl transfer of
disilanes has also been reported, but again only for a racemic
series.[8] To date, the palladium-catalyzed enantioselective
1,4-disilylation[9] of acyclic enones using disilanes has
remained the sole example (B!A, Scheme 1).[10] The impor-
tance of b-silyl carbonyl compound A (either as a masked
aldol itself or as a functionalized precursor for the preparation
of a-chiral silanes)[2b] has resulted in the development of
alternative catalyst-controlled routes towards A. These
approaches, however, rely on the more established rhodium-
Our search for catalysts that would facilitate silyl transfer
from pinacol-derived silyl boronic ester 11[17] to 2-cyclo-
hexenone (2) commenced with a systematic screening of
rhodium (pre)catalysts (Scheme 2 and Table 1). For this,
typical reaction conditions for the rhodium-catalyzed 1,4-
addition of aryl boronic acids served as a reasonable reference
point (1,4-dioxane/H2O solvent mixtures at elevated reaction
temperatures).[14] Two crucial observations emerged from
these initial investigations: Both the presence of a base and
the absence of strongly coordinating counterions were critical
for product formation. Replacement of chloride by weakly or
noncoordinating perchlorate led to the achiral cationic
catalyst [(dppp)Rh(cod)]ClO4.[18]
We were pleased to find that a combination of this catalyst
(5.0 mol%) and an equimolar amount of the free bidentate
ligand dppp (5.0 mol%) in the presence of triethylamine as
the base (1.0 equiv) promoted the desired 1,4-addition—
KOH (1.0 equiv) performed equally well[19] (Scheme 2 and
Table 1). The rhodium-catalyzed silyl transfer from 11
(2.5 equiv) to cyclic enones 1–3 in aqueous 1,4-dioxane at
508C afforded racemic products 6–8 in good yields (Table 1,
entries 1–3). Acyclic enone 4 was less reactive under identical
reaction conditions and required double the amount of
catalyst and ligand, which then gave rac-9 in good yield
catalyzed enantioselective formation of C C bonds[11] (C!A,
ꢀ
Scheme 1) and copper-catalyzed enantioselective formation
[12]
of C H bonds (D!A, Scheme 1).[13]
ꢀ
Inspired by the rhodium-catalyzed 1,4-addition of aryl
boronic acids,[14] and aware of the diverse transition-metal-
[15]
ꢀ
catalyzed chemistry of Si B compounds, we envisioned a
[*] Dipl.-Chem. C. Walter,[+] Dr. G. Auer, Prof. Dr. M. Oestreich[+]
Institut für Organische Chemie und Biochemie
Albert-Ludwigs-Universität Freiburg
Albertstrasse 21, 79104 Freiburg im Breisgau (Germany)
Fax: (+49)761-203-6100
E-mail: martin.oestreich@orgmail.chemie.uni-freiburg.de
[+] New address:
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstrasse 40, 48149 Münster (Germany)
Fax: (+49)251-83-36501
E-mail: martin.oestreich@uni-muenster.de
[**] Financial support for this work was provided by the Deutsche
Forschungsgemeinschaft (Emmy Noether-Programm, 2001-2006),
the Fonds der Chemischen Industrie, and the Aventis Foundation
(Karl-Winnacker-Stipendium, 2006-2008). We thank Gerd Fehren-
bach for performing the HPLC analyses.
Scheme 2. Rhodium-catalyzed conjugate silyl transfer (see Table 1).
dppp=1,3-bis(diphenylphosphanyl)propane, cod=1,5-cyclooctadiene.
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Table 1: Rhodium-catalyzed conjugate silyl transfer (see Scheme 2).
Table 2: Catalytic asymmetric conjugate silyl transfer (see Scheme 3).
Yield [%][a]
Entry Substrate
n
X
Base Product Yield [%][a] ee [%][b] [a]D
[c]
Entry
Substrate
Product
1
2
3
4
1
2
3
5
0
1
2
1
CH2 Et3N (S)-6
CH2 Et3N (S)-7
CH2 Et3N (S)-8
70
45
22
58
97
96
92
96
ꢀ131
ꢀ85.8
ꢀ66.9
ꢀ38.1
1
82
O
Et3N (S)-10
[a] Yield of analytically pure product isolated by flash chromatography on
silica gel. [b] HPLC analysis on Daicel columns provided baseline
separation of the enantiomers: Chiralpak AD-H column (n-heptane/
iPrOH 300:1 at 208C) for 6: tr =21.8 min for (S)-6 and 23.7 min for (R)-6;
Chiralpak AD-H column (n-heptane/iPrOH 100:1 at 208C) for 7: tr =
9.48 min for (S)-7 and 10.7 min for (R)-7; Chiralcel OD-H column (n-
heptane/iPrOH 100:1 at 208C) for 8: tr =13.5 min for (S)-8 and 14.9 min
for (R)-8; Chiralpak AD-H column (n-heptane/iPrOH 100:1 at 208C) for
10: tr =19.5 min for (S)-10 and 20.5 min for (R)-10. [c] c=2.21 for (S)-6,
2.86 for (S)-7, 1.07 for (S)-8, and 1.77 for (S)-10 in CHCl3 at 208C.
2
76
81
77
76
3
4[b]
thus far best reaction 1!(S)-6 (70%, 97% ee), which resulted
in a substantial decrease in the yield and, importantly, the
enantioselectivity (47%, 85% ee). An excess of the free
ligand, (S)-binap, was indispensable to ensure high enantio-
selectivities. The catalyst alone induced distinctly diminished
enantioselection (55%, 74% ee).
The influence of the base on the yield and stereoinduction
of related rhodium-catalyzed processes is known.[14,19] A brief
survey of different organic and inorganic bases confirmed
these findings for 1!(S)-6 (Table 3). Sterically hindered
5
[a] Yield of analytically pure product isolated by flash chromatography on
silica gel. [b] [(dppp)Rh(cod)]ꢀClO4 (10 mol%) and dppp (10 mol%).
+
(Table 1, entry 4). Besides cyclic a,b-unsaturated ketones 1–3,
lactone 5 displayed similar reactivity and provided rac-10
cleanly (Table 1, entry 5).
Table 3: Influence of the base on enantioselectivity (see Scheme 3).
We then addressed the elusive enantioselective 1,4-
Entry
Substrate
n
Base
Product Yield [%]
ee [%]
addition of nucleophilic silicon to cyclic substrates by using
1
2
3
4
5
1
1
1
1
2
0
0
0
0
1
TMP
(S)-6
(S)-6
(S)-6
(S)-6
(S)-7
75
0
71
68
45
96
–
98
59
78
[18]
the chiral catalyst [((S)-binap)Rh(cod)]ClO4
(5.0 mol%)
morpholine
K3PO4
KOH
with additives (S)-binap (5.0 mol%) and triethylamine
(1 equiv)[19] (Scheme 3 and Table 2). To our delight, conjugate
silylation of cyclic enones 1–3 proceeded with remarkably
high enantioselectivities (Table 2, entries 1–3). Unfortunately,
the yields of the isolated products (S)-6–(S)-8 decreased with
increasing ring size.[20] Thus, a,b-unsaturated lactone 5 under-
went the silyl transfer with excellent enantioselectivity and in
reasonable yield (Table 2, entry 4) whereas acyclic enone 4
was completely inert under these reaction conditions. The
absolute configuration of (S)-6[21] and (S)-7[22] was assigned
unambiguously by comparison with reported data, while
those of (S)-8 and (S)-10 were assigned on the basis of similar
HPLC characteristics on a chiral stationary phase and the sign
of the optical rotation.[22]
KOH
2,2,6,6-tetramethylpiperidine (TMP) was comparable with
triethylamine (Table 3, entry 1). In contrast, (S)-6 was not
produced when morpholine was used as the base (Table 3,
entry 2). Inorganic bases such as K3PO4 and KOH also gave
different results: stereoinduction as high as 98% ee was
obtained with the former (Table 3, entry 3), whereas a
pronounced negative effect was seen with the latter for 1 as
well as 2 (Table 3, entries 4 and 5).
In summary, we have presented a novel transition-metal-
ꢀ
catalyzed C Si bond-forming reaction. In the presence of
cationic rhodium complexes, the Si B fragment functions as
ꢀ
We generally used catalyst and ligand loadings of
5.0 mol% in these reactions. We reduced the amount of
[((S)-binap)Rh(cod)]ClO4 and (S)-binap to 1.0 mol% for the
an equivalent of nucleophilic silicon, which is efficiently
transferred onto acyclic as well as cyclic a,b-unsaturated
carbonyl compounds. Chiral [((S)-binap)Rh(cod)]ClO4 has
effected an unprecedented asymmetric 1,4-addition to cyclic
acceptors in high enantiomeric excesses. This transformation
might also pave the way for synthetically useful tandem
processes. Extension of this line of research including
mechanistic investigations[23] is our current focus.
Experimental Section
General procedure: A Schlenk tube equipped with a magnetic stir bar
in an argon atmosphere was charged with the rhodium catalyst
Scheme 3. Catalytic asymmetric conjugate silyl transfer (see Tables 2
and 3). binap=2,2’-bis(diphenylphosphanyl)-1,1’-binaphthyl.
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Angewandte
Chemie
(5.0 mol%) and the ligand (5.0 mol%). The compounds were then
dissolved in deoxygenated 1,4-dioxane/H2O 10:1 (ca. 0.5m based on
substrate). The a,b-unsaturated acceptor (1.0 equiv), the silyl boronic
ester (2.5 equiv), and the base (1.0 equiv) were then added and the
reaction mixture maintained at 508C for 16 h. After cooling the
mixture to room temperature, silica gel was added and the solvents
were evaporated under reduced pressure. The residue was subjected
to flash column chromatography on silica gel using cyclohexane/ethyl
acetate solvent mixtures as the eluent.
[19] R. Itooka, Y. Iguchi, N. Miyaura, J. Org. Chem. 2003, 68, 6000 –
6004.
[20] The diminished reactivity of [((S)-binap)Rh(cod)]ClO4 com-
pared to [(dppp)Rh(cod)]ClO4 might indicate that electron-rich
phosphines will enhance the reaction rate. A survey of chiral
ligands will be the subject of future investigations.
[21] [a]D = + 105 (c = 3.08 in CHCl3 at 208C) for (R)-7 (personal
communication from Professor Pierre Deslongchamps): a) S.
Trudeau, P. Deslongchamps, J. Org. Chem. 2004, 69, 832 – 838;
b) G. Sarakinos, E. J. Corey, Org. Lett. 1999, 1, 811 – 814.
[22] R. W. Barnhart, X. Wang, P. Noheda, S. H. Bergens, J. Whelan,
B. Bosnich, J. Am. Chem. Soc. 1994, 116, 1821 – 1830.
[23] T. Hayashi, M. Takahashi, Y. Takaya, M. Ogasawara, J. Am.
Chem. Soc. 2002, 124, 5052 – 5058.
Received: May 3, 2006
Published online: July 21, 2006
Keywords: asymmetric catalysis · boron ·
.
homogeneous catalysis · rhodium · silicon
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