Zinc-salen-catalyzed asymmetric alkynylation of alkyl acylsilanes

Article abstract of DOI:10.1002/adsc.200900177

Optically active tertiary propargylic alcohols are useful and versatile building blocks in organic synthesis, and their direct access by enantioselective addition of alkyne nucleophiles to ketones has achieved significant progress over the last ten years. In view of the potential applications of acylsilanes as useful synthetic equivalents of aldehydes, we described a general catalytic enantioselective addition of terminal alkynes to alkyl acylsilanes. After reaction optimization involving variation of solvent, tern-perature, catalyst ratio and various catalysts screen, the in situ generated Zn-salen complex was chosen as catalyst. With hexane as solvent, the silylated tertiary propargylic alcohols were obtained in satisfactory yields and ees for both aliphatic and aromatic alkynes.

Full text of DOI:10.1002/adsc.200900177

FULL PAPERS
DOI: 10.1002/adsc.200900177
Zinc-Salen-Catalyzed Asymmetric Alkynylation of Alkyl
Acylsilanes
Feng-Quan Li,a Shi Zhong,a Gui Lu,a,* and Albert S. C. Chana,b,*
a
Institute of Medicinal Chemistry, School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006, Peopleꢀs
Republic of China
Fax : (+86)-20-3994-3048; e-mail: lugui@mail.sysu.edu.cn
Open Laboratory of Chirotechnology of the Institute of Molecular Technology for Drug Discovery and Synthesis and
b
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon,
Hong Kong SAR, Peopleꢀs Republic of China
Received: March 20, 2009; Revised: June 6, 2009; Published online: July 16, 2009
Abstract: Optically active tertiary propargylic alco- perature, catalyst ratio and various catalysts screen,
hols are useful and versatile building blocks in organ- the in situ generated Zn-salen complex was chosen
ic synthesis, and their direct access by enantioselec- as catalyst. With hexane as solvent, the silylated ter-
tive addition of alkyne nucleophiles to ketones has tiary propargylic alcohols were obtained in satisfac-
achieved significant progress over the last ten years. tory yields and ees for both aliphatic and aromatic al-
In view of the potential applications of acylsilanes as kynes.
useful synthetic equivalents of aldehydes, we de-
scribed a general catalytic enantioselective addition Keywords: acylsilanes; alkynylation; asymmetric cat-
of terminal alkynes to alkyl acylsilanes. After reac- alysis; quaternary stereocenters; salen ligands; sily-
tion optimization involving variation of solvent, tem- lated propargylic alcohols
Introduction
ee).[9] Wang and co-workers have developed several
practical enantioselective additions of phenylacety-
Optically active tertiary propargyl alcohols are versa- lene to aromatic ketones with various chiral ligands
tile building blocks for the syntheses of pharmaceuti- such as BINOL,[10] Cinchona alkaloids,[11] oxazoli-
cals and natural products.[1] A direct access to these dines,[12] bis-sulfonamide diols[13] and Schiff bases.[14]
chiral compounds is the asymmetric alkynylation of In particular, for aliphatic ketones, the bis-sulfon-
ketones, either using preactivated metal acetylides
(such as zinc and silyl acetylides) as nucleophiles,[2] or with high enantioselectivities (91–94% ee).
in the presence of chiral Brønsted base catalysts.[3]
Among the carbonyl derivatives, acylsilanes are
ACHTUNGTRENaNGNU mide diol 4 effectively provided the desired products
During the past five years, significant improvements very useful substrates,[15,16] which are known to be the
have been made in the asymmetric alkynylation of ke- synthetic equivalents of aldehydes due to the possibil-
tones, especially with mild organozinc reagents.[4] Tan ity of a stereospecific desilylation with fluoride ion,
and co-workers reported the stoichiometric addition especially in the case of chemically and configuration-
of an in situ prepared alkynylzinc to some highly acti- ally unstable substrates such as a-amino aldehydes
vated ketones with up to 98% ee for the synthesis of (Scheme 2).[17,18] From a synthetic standpoint, it is
efavirenz.[5,6] Cozzi realized the first general catalytic highly desirable to develop an efficient approach to
enantioselective addition of terminal alkynes to unac- enantiomerically pure silylated tertiary propargylic al-
tivated ketones with the zinc-salen 1 complex as cata- cohols.[19] Although several catalytic asymmetric reac-
lyst (Scheme 1). This protocol was proven to be more tions using acylsilanes have been developed,[20,21] to
effective with aliphatic ketones.[7] Nearly at the same the best of our knowledge, the enantioselective alk-
time, Chan et al. also reported that a combination of
ACHTUNGTRENyNUNG nylation of acylsilanes has not been reported so
camphorsulfonamide 2 and Cu
AHCTUNGTRENNUNG
enantioselective addition of an alkynylzinc to aromat- catalytic alkynylation of alkyl acylsilane with both ali-
ic ketones in high yield and up to 97% ee.[8] Subse- phatic and aromatic alkynes. Using the in situ gener-
quently Katsuki et al. demonstrated that salen ligand ated Zn-salen complex as catalyst, optically active si-
3 could serve as a quite efficient catalyst for the asym- lylated propargylic alcohols were obtained in good
metric alkynylation of aliphatic ketones (up to 93% yields and high enantioselectivities.
Adv. Synth. Catal. 2009, 351, 1955 – 1960
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Feng-Quan Li et al.
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Scheme 1. Various efficient ligands for the asymmetric alkynylation of aliphatic ketones.
pared with the camphorsulfonamide catalyst, the
salen system showed significant improvements
Table 1. Asymmetric addition of phenylacetylene to acetyl-
trimethylsilane catalyzed by camphorsulfonamides.[a]
Scheme 2. Acylsilane as stable synthetic equivalent of a-
amino aldehyde.
Results and Discussion
Preliminary studies were carried out with the model
reaction of acetyltrimethylsilane and phenylacetylene.
As camphorsulfonamides have been proved to be an
efficient catalyst for the production of chiral tertiary
propargylic alcohols,[8] firstly we chose a series of
camphorsulfonamides to catalyze the asymmetric al-
kynylation of alkyl acylsilane (Table 1) under the op-
timal conditions described in our previous work.[8]
To our surprise, in the presence of Me2Zn,
CuACHTUNGTRENNUNG(OTf)2 and camphorsulfonamide, the reaction gave
moderate to good yields but poor enantioselectivity
(less than 30%), which was quite different from the
excellent behavior of other aliphatic ketones, for ex-
ample, 88% ee for 3-methyl-2-butanone.[8] We attrib-
uted this to the influence of the SiR3 moiety, which
compared with the CR3 moiety, is more electron-re-
leasing, and thereby partially neutralizes and stabiliz-
es more the positive charge at the carbonyl carbon
atom, making the reaction less favorable.
Entry
Ligand
Yield [%][b]
ee [%][c]
1
2
3
4
2
5
6
7
82
45
42
59
18
23
2
27
Since the bifunctional catalyst Zn
proved more effective for the enantioselective alkyn-
ACHTUNGTREN(NGNU salen) has been
[a]
Acylsilane:alkyne:Me2Zn:CuACTHUNGRTNEUNG(OTf)2:ligand=
0.4:1.2:1.2:0.04:0.04. All reactions were carried out in
CH2Cl2, 08C for 2 days.
ACHTUNGTRENNUNGylation of aliphatic ketones than for aromatic ke-
tones,[7] using 20 mol% salen (R,R)-1 and 3 equiv. of
[b]
[c]
Isolated yield.
zinc phenylacetylide, we studied the asymmetric alk-
ACHTUNGTRENNUNGynylation of acylsilane at room temperature. Com-
The enantiomeric excess was determined by HPLC anal-
ysis using a Chiralcel OD-H column.
1956
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Zinc-Salen-Catalyzed Asymmetric Alkynylation of Alkyl Acylsilanes
FULL PAPERS
Table 2. Enantioselective alkynylation of acetyltrimethyl-
ACHTUNGTRENNUNG
silane catalyzed by Me2Zn and salen ligand.[a]
addition of 1-ethynylcyclohexene to acylsilane, and
observed high yield and moderate enantioselectivity
(entry 6).
The replacement of dimethylzinc by diethylzinc af-
forded a higher reactive alkynylzinc, and hence short-
ed the reaction time to one day for both aliphatic and
aromatic alkynes. Here 1-ethynylhexene and phenyl-
AHCTUNGTREGaNNUN cetylene were chosen as the representatives of ali-
phatic alkynes and aromatic alkynes, respectively, all
the conditions influencing the reaction are summar-
ized in Table 3.
Table 3. Enantioselective alkynylation of acetyltrimethyl-
AHCTUNGTRENNUNG
silane catalyzed by Et2Zn and salen ligands.[a]
En- Ligand
try
Alkyne
T
Yield ee
[8C] [%][b] [%][c]
1
2
N
r.t.
0
79
66
58
63
3
U
r.t.
0
89
82
66
90
59
55
1
Entry Alkyne
Solvent T [8C] Yield [%][b] ee [%][c]
4
1
hexane r.t.
hexane
78
82
62
89
76
92
90
72
76
69
72
38
65
47
5[d]
r.t.
r.t.
2
0
6
47
3
hexane À20
[a]
4
CH2Cl2 r.t.
[b]
[c]
[d]
5
CH2Cl2 0
6
toluene r.t.
toluene r.t.
7[d]
(Table 2), the reaction was completed in two days to
afford the silylated tertiary propargylic alcohol in
79% yield, but with moderate enantiomeric excess
8
toluene 0
toluene 0
82
66
81
79
78
63
86
81
9[d]
10
hexane
hexane
0
0
11[e]
(58% ee for acetACHTUNGTRENNUNGyltrimethylsilane, entry 1). The use of
low temperature (08C) slowed down the reaction con-
siderably, but only a slight increase in ee (63% ee,
entry 2) was observed.
Electron-rich aromatic terminal alkynes such as p-
methoxyphenylacetylene displayed higher nucleophil-
ic reactivity than unsubstituted phenylacetylene,
hence afforded higher conversion but with compara-
ble or a bit lower ee (entries 3 and 4). We also tried
the asymmetric alkynylation of acylsilane with the
[a]
Acylsilane:alkyne:Et2Zn:ligand=0.4:1.2:1.2:0.08. All re-
actions were carried out for 1 day.
Isolated yield.
Determined by HPLC analysis.
Me2Zn was used and the reaction time was 2 days.
10 mol% (R,R)-1 was used.
[b]
[c]
[d]
[e]
When varying reaction parameters such as solvent,
chiral Schiff base amino alcohol (1R,2S)-8, but race- temperature, and the quantity of salen ligand, we
mic product was obtained (entry 5). Again the com- found that the solvent strongly influenced the result
mercially available compound (R,R)-1 appeared more of the reaction. Hexane was found to be the best sol-
suitable for this reaction as in the case of many other vent for both aliphatic alkynes and aromatic alkynes.
metal-salen promoted reactions.[7]
The reaction in CH2Cl2 was quite sensitive to temper-
For possible further transformations of the alkene ature, low temperature significantly decreased the ee
group, such as epoxidation, hydroxylation and ozonol- values (entry 4 versus 5). Reducing the amount of
ysis, the asymmetric addition of 1-ethynylcyclohex- ligand from 20 mol% to 10 mol% showed a slight
ACHTUNGTRENNUNG
ene[14c] to acylsilane should have more potential appli- drop in the enantiomeric excess (entry 10 versus 11).
cations for the synthesis of complex bioactive natural
Under the optimal reaction conditions (Table 3
products. We also carried out the catalytic asymmetric entry 10), ligand (R,R)-1 was employed to catalyze
Adv. Synth. Catal. 2009, 351, 1955 – 1960
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Feng-Quan Li et al.
FULL PAPERS
Table 4. Asymmetric addition of various alkynes to acetyltri-
methylsilane and acetyldimethylphenylsilane.[a]
Entry R1
Alkynes
Yield [%][b] ee [%][c]
Scheme 3. Disilylation
butyn-2-ol 9.
of
2-(trimethylsilyl)-4-phenyl-3-
1
2
Me
Ph
81
72
86
88
3
4
5
6
Me
Me
Me
88
88
82
86
83
76
solely desilylated product, although product racemiza-
tion was observed.
Other desilylation approaches such as protecting
the hydroxy group prior to desilylation to avoid the
possible rearrangement, and changing the SiMe3
group to SiMe2Ph or SiMe2ACTHNUGRTNEUNG(t-Bu), which is more
Me
Me
74
78
72
76
stable towards the desilylation and rearrangement re-
action for its large steric hindrance and electronic in-
fluences, may help to retain the configuration during
the desilylation. Related work is still being carried
out in our laboratory.
7
[a]
Acylsilane:alkyne:Et2Zn:ligand=0.4:1.2:1.2:0.08. All re-
actions were carried out in hexane, 08C for 1 day.
Isolated yield.
[b]
[c]
Determined by HPLC analysis.
Conclusions
the asymmetric addition of various alkynes to acylsi-
lanes. For both aromatic and aliphatic alkynes, chiral In summary, a highly efficient catalytic enantioselec-
silylated tertiary propargylic alcohols were generated tive addition of terminal alkynes to alkyl acylsilanes is
in 72–88% ee (Table 4 entries 1–7). Aromatic alkynes described. In the presence of Zn-salen complex, the
exhibited better yields and ee values than aliphatic al- expected silylated tertiary propargylic alcohols were
kynes (entries 1–4 versus 5–7). Electron-rich alkynes obtained in satisfactory yields and ees for both ali-
such as para-methoxyphenylacetylene showed compa- phatic and aromatic alkynes, thus proving the general-
rable ee with phenylacetylene (entry 1 versus 3), but ity of the method. Further studies focusing on the
higher ee than electron-deficient alkynes such as para- uses of homochiral a-alkoxy and a-amino acylsilanes
bromophenylacetylene (entry 1 versus 4). Better to synthesize polyfunctionalized propargylic alcohols
enantioselectivity was achieved with acetyldimethyl- as versatile intermediates for the stereoselective syn-
phenylsilane as substrate (entry 2), this could be ex- thesis of biologically active compounds are underway.
plained by the large steric hindrance of the Me2PhSi
moiety, which favored the stereoselective addition to
afford high selectivity but a slightly lower yield. Good
Experimental Section
enantioselectivity could also be obtained for silyl-sub-
stituted aliphatic alkynes such as trimethylsilylacetyle-
ne (entry 7).
General Information
The replacement of the silyl group with a proton
was also investigated. Treatment of 2-(trimethylsilyl)-
All reactions were carried out under an atmosphere of nitro-
gen in flame-dried glassware with magnetic stirring. Unless
otherwise stated, commercial reagents purchased from Alfa
4-phenyl-3-butyn-2-ol
9
with TBAF in THF
(Scheme 3) led to 4-phenyl-2-buten-2-one 11 in com- Aesar, Acros and Aldrich chemical companies were used
without further purification. Purification of reaction prod-
ucts was carried out by flash chromatography using Qing
Dao Sea Chemical Reagent silica gel (200–300 mesh).
1H NMR spectra were recorded on a Varian (300 MHz) or a
Bruker (400 MHz) spectrometer and the spectra were refer-
enced internally to the residual proton resonance in CDCl3
(d=7.26 ppm), or with tetramethylsilane (TMS, d=
0.00 ppm) as the internal standard. Chemical shifts were re-
ported as parts per million (ppm) in the d scale downfield
from TMS. 13C NMR spectra were recorded on a Varian
plete conversion rather than the expected desilylACHTUNTRGNEUNGated
propargylic alcohol 10, which can be rationalized in
terms of a facial Brook rearrangement, occurring in
the initial intermediate formed by attack of the fluo-
ride ion on the silicon atom, followed by isomeriza-
tion. We have also tested other reagents such as CsF,
K2CO3 and got similar results. To our delight, in the
presence of NaOMe, 16% desilylated product was ob-
tained albeit 82% product was rearranged. Further
study revealed that KF can catalyze the reaction with (300 MHz) or a Bruker (400 MHz) spectrometer with com-
1958
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Adv. Synth. Catal. 2009, 351, 1955 – 1960

Zinc-Salen-Catalyzed Asymmetric Alkynylation of Alkyl Acylsilanes
FULL PAPERS
plete proton decoupling, and chemical shifts are reported in
ppm from TMS with the solvent as the internal reference
(CDCl3, d=77.0 ppm). HPLC analyses were conducted on a
Shimadzu 10 A or Agilent 1200 instrument using Daicel col-
umns (0.46 cm diameterꢂ25 cm length). The reactions were
carried out in solvents distilled from standard drying agents.
All alkynes and ketones were freshly distilled under normal
or reduced pressure before use. Ligand (R,R)-1 was synthe-
sized according to the procedure of Jacobsen and co-work-
ers.[23] Acetyldimethylphenylsilane was synthesized accord-
ing to the procedure of Scheidt and co-workers.[24]
(+)-4-(4-Methoxyphenyl)-2-(trimethylsilyl)-3-butyn-2-
ol
88% yield isolated after 24 h reaction. 86% ee determined
by HPLC analysis (Daicel Chiralcel OD-H column,
IPA:hexane=10:90, 254 nm, 0.5 mLminÀ1). Retention time:
(minor)=12.2 min; [a]2D5: +45.0 (c 1.00 g/
tACTHNUTRGENN(UG major)=8.8 and tACHUTNGTRENNUNG
1
100 mL, CH2Cl2); H NMR (300 MHz, CDCl3): d=7.21–7.17
(m, 2H), 6.70–6.65 (m, 3H), 3.63 (s, 3H), 1.42 (s, 3H), 0.08
(s, 9H); 13C NMR (75 MHz, CDCl3): d=159.0, 132.7, 115.3,
113.6, 91.7, 85.9, 61.1, 55.1, 25.4, À4.4.
General Procedure for Asymmetric Addition of
Alkynes to Ketones
Phenylacetylene (132 mL, 1.2 mmol) and a 1.0M solution of
diethylzinc in hexane (1.2 mL, 1.2 mmol) were added to a
dry flask with a hexane solution (1.0 mL) of (R,R)-1 ligand
(44 mg, 0.08 mmol). The mixture was stirred at room tem-
perature for 1 hour. Then the flask was put into a 08C cold
bath with continuous stirring for 1 hour. Acetyltrimethylsi-
lane (58 mL, 0.4 mmol) was sequentially added to the flask
via syringe. The resulting solution was allowed to stir at 08C
for 24 h. The mixture was then quenched with 1.0 mL satu-
rated NH4Cl solution, extracted with ether (3ꢂ3 mL), dried
over anhydrous Na2SO4 and concentrated under reduced
pressure. The corresponding propargylic alcohol was puri-
fied via flash chromatography (silica gel) using 2% ethyl
acetate in petroleum ether as eluent. The enantiomeric
excess of the product was determined by HPLC analysis on
a Daicel Chiralcel OD-H column.
(+)-4-(4-Bromophenyl)-2-(trimethylsilyl)-3-butyn-2-ol
88% yield isolated after 24 h reaction, 83% ee determined
by HPLC analysis (Daicel Chiralcel OD-H column,
IPA:hexane=5:95, 254 nm, 0.5 mLminÀ1). Retention time:
(minor)=9.1 min; [a]2D5: +40.0 (c 1.00 g/
tACTHNUTRGENN(UG major)=8.4 and tACHUTNGTRENNUNG
1
100 mL, CH2Cl2); H NMR (400 MHz, CDCl3): d=7.34–7.32
(m, 2H), 7.16–7.14 (m, 2H), 1.45 (s, 3H), 0.10 (s, 9H);
13C NMR (100 MHz, CDCl3): d=134.3, 132.9, 132.0, 122.2,
122.0, 94.4, 85.3, 61.5, 25.3, À4.5.
(+)-2-(Trimethylsilyl)-4-phenyl-3-butyn-2-ol
(+)-4-Cyclohexenyl-2-(trimethylsilyl)-3-butyn-2-ol
81% yield isolated after 24 h reaction. 86% ee determined
by HPLC analysis (Daicel Chiralcel OD-H column,
IPA:hexane=10:90, 254 nm, 0.5 mLminÀ1). Retention time:
82% yield isolated after 24 h reaction. 76% ee determined
by HPLC analysis (Daicel Chiralcel OD-H column,
IPA:hexane=2:98, 254 nm, 0.5 mLminÀ1). Retention time:
tACHTUNGTRENNUNG(major)=8.0 and tACHTUNGTRENNUNG
(minor)=9.7 min; [a]2D5: + 48.0 (c 1.00 g/
1
100 mL, CH2Cl2); H NMR (300 MHz, CDCl3): d=7.30–7.27
(m, 2H), 7.18–7.16 (m, 3H), 1.46 (s, 3H), 0.11 (s, 9H);
13C NMR (75 MHz, CDCl3): d=131.3, 128.1, 127.8, 123.2,
93.2, 86.3, 61.3, 25.5, À4.3.
(minor)=9.3 min; [a]2D5: +33.0 (c 1.00 g/
tACTHNUTRGENN(UG major)=8.8 and tACHUTNGTRENNUNG
1
100 mL, CH2Cl2); H NMR (300 MHz, CDCl3): d=5.96–5.93
(m, 1H), 2.02–1.99 (m, 4H), 1.57–1.49 (m, 4H), 1.38 (s, 3H),
0.06 (s, 9H) ; 13C NMR (75 MHz, CDCl3): d=133.9, 120.5,
90.4, 88.0, 61.2, 29.5, 25.6, 22.4, 21.6, À4.4.
(+)-2-(DimethylACHTUNGTRENNUNG(phenyl)silyl)-4-phenyl-3-butyn-2-ol
(+)-2-(Trimethylsilyl)-3-nonyn-2-ol
72% yield isolated after 24 h reaction. 88% ee determined
by HPLC analysis (Daicel Chiralcel OD-H column,
IPA:hexane=5:95, 254 nm, 0.5 mLminÀ1). Retention time:
74% yield isolated after 24 h reaction. 72% ee determined
by HPLC analysis (Daicel Chiralcel OD-H column,
IPA:hexane=1:99, 208 nm, 0.5 mLminÀ1). Retention time:
(minor)=13.9 min; [a]2D5: +17.0 (c
tACHTUNGTRENNUNG(major)=11.0 and tACTHUNGTRENNNUG
1.00 g/100 mL, CH2Cl2); 1H NMR (300 MHz, CDCl3,): d=
7.61–7.58 (m, 2H), 7.30–7.27 (m, 4H), 7.20–7.18 (m, 4H),
1.44 (s, 3H), 0.42 (s, 6H); 13C NMR (75 MHz, CDCl3): d=
134.6, 131.3, 129.6, 128.1, 127.9, 127.7, 123.2, 93.2, 86.8, 61.3,
25.6, À5.8, À6.0.
(minor)=10.4 min; [a]2D5: +15.0 (c 1.00 g/
tACTHNUTRGENN(UG major)=9.7 and tACHUTNGTRENNUGN
1
100 mL, CH2Cl2); H NMR (300 MHz, CDCl3): d=2.17–2.12
(t, J=6.88 Hz, 2H), 1.47–1.40 (m, 2H), 1.35 (s, 3H), 1.32–
1.27 (m, 2H), 1.24–1.18 (m, 2H), 0.85–0.80 (t, J=6.84 Hz,
Adv. Synth. Catal. 2009, 351, 1955 – 1960
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1959

Feng-Quan Li et al.
FULL PAPERS
3H), 0.05 (s, 9H); 13C NMR (75 MHz, CDCl3): d=86.8,
84.0, 61.2, 31.1, 28.8, 25.9, 22.3, 19.0, 14.1, À4.4.
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78% yield isolated after 24 h reaction, 76% ee determined
by HPLC analysis (Daicel Chiralcel OD-H column,
IPA:hexane=2: 98, 254 nm, 0.5 mLminÀ1). Retention time:
(minor)=7.9 min; [a]2D5: +18.0 (c 1.00 g/
tACHTUNGTRENNUNG(major)=7.5 and tACHUTNGTRENNUNG
1
100 mL, CH2Cl2); H NMR (400 MHz, CDCl3): d=1.38 (s,
3H), 0.10 (s, 3H), 0.07 (s, 3H); 13C NMR (100 MHz,
CDCl3): d=110.3, 90.4, 61.6, 25.2, 0.0, À4.7.
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1
Light yellow oil. H NMR (400 MHz, CDCl3): d=7.44–7.41
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Acknowledgements
We thank the National Natural Science Foundation of China
(Contract grant number: 20772161), the Scientific Research
Foundation for the Returned Overseas Chinese Scholars and
the Program for New Century Excellent Talents in University
(both from the State Education Ministry of China) for finan-
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