Stereoselective synthesis of tri- and tetrasubstituted α,β-unsaturated esters via copper-catalyzed coupling of enol triflates of β-ketoesters with Grignard-based zinc ate complexes

Article abstract of DOI:10.1055/s-2001-17479

Tri- and tetrasubstituted α,β-unsaturated esters were stereoselectively prepared via the copper-catalyzed couplings of the enol triflates of β-ketoesters with Grignard-based zinc ate complexes. Some insights into the mechanistic considerations are describ

Full text of DOI:10.1055/s-2001-17479

LETTER
1511
Stereoselective Synthesis of Tri- and Tetrasubstituted , -Unsaturated Esters
via Copper-catalyzed Coupling of Enol Triflates of -Ketoesters with
Grignard-based Zinc Ate Complexes
Stereoselective
S
ynthe
i
sis of
T
t
ri- and
s
T
etrasubs
u
ituted
,
-U
a
nsaturated
kEsters i Ide, Masaya Nakata*
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522,
Japan
Fax +81(45)5661551; E-mail: msynktxa@applc.keio.ac.jp
Received 30 July 2001
O
O
X
O
Abstract: Tri- and tetrasubstituted , -unsaturated esters were ste-
reoselectively prepared via the copper-catalyzed couplings of the
enol triflates of -ketoesters with Grignard-based zinc ate complex-
es. Some insights into the mechanistic considerations are described.
R1
R3
R1
R3
R2
R2
R4
O
H
O
Key words: stereoselective synthesis, enol triflates, copper cata-
lysts, Grignard reagents, zinc ate complexes
+
R1
R3
R1
R3
R2
R2
Scheme 1
The stereoselective synthesis of tri- and tetrasubstituted
, -unsaturated carbonyl compounds from -functional-
ized , -unsaturated carbonyl compounds is an important
objective in organic synthesis. Some synthetic methods
have already been developed that include:(1) the cou-
plings of -alkylthio- or -phenylthio- , -unsaturated
carbonyl compounds with dialkylcopper lithium1,2 or
Grignard-based copper reagents,3 (2) the couplings of the
enol acetates or benzoates of -ketocarbonyl compounds
with dialkylcopper lithium,4 (3) the couplings of -halo-
, -unsaturated ketones with dialkylcopper lithium,5 (4)
the couplings of the enol phosphates of -ketocarbonyl
compounds with dialkylcopper lithium6 or Grignard-
based copper reagents,7 (5) the couplings of the enol tri-
flates of -ketoesters with organostannanes in the pres-
We chose 113,14 the enol triflates of -ketoesters and ethyl
magnessium chloride as the Grignard reagent (Table 1).
First, the Gibbs conditions10b,c,12e were applied to these
substrates to obtain the required product 2. A 0.97 M THF
solution of EtMgCl (4 equiv) was added to a 1.02 M THF
solution of CuCN (2 equiv)-2LiCl at -78 °C. This was
warmed to 0 °C over a 1 h period. To this pale yellow so-
lution of Et2Cu(CN)(MgCl)2 was added at -30 °C a THF
solution of 1 (1 equiv) and the mixture was warmed to r.t.
over a 2 h period. After 1.5 h at r.t., the ethylated product
214 (R = Et) was almost quantitatively obtained (Table 1,
entry 1). CuI could also be used instead of CuCN
(Table 1, entry 2). The use of equal amounts of EtMgCl
and CuCN (i.e., EtCu(CN)MgCl) gave a much slower re-
action and a lower yield of 2 (Table 1, entry 3). When the
coupling reaction or the preparation of the copper reagent
was conducted at -78 °C, the yield of 2 decreased
(Table 1, entries 4 and 5). In these runs, neither the stere-
oisomer of 2 nor the reduction product 3 was produced.15
No reaction occurred with EtMgCl in the absence of the
copper salt (Table 1, entry 6).
ence of
a
catalytic Pd/Cu,8 with organocopper
reagents,9,10a or with Grignard-based copper re-
agents.8c,10b,c A drawback of these transformations is that
a reduction product (a hydrogen-incorporated product) is
sometimes obtained as the major product or by-product
(Scheme 1).4a,c,10a,11 There have been some discussion
about this phenomenon4a,10a,11d and about the same phe-
nomenon in related reactions;9,12 however, the actual spe-
cies responsible for donating a hydrogen atom is still
inconclusive. We report here a new methodology for the
stereoselective synthesis of tri- and tetrasubstituted , -
unsaturated esters via the copper-catalyzed couplings of
the enol triflates of -ketoesters with Grignard-based zinc
ate complexes and some insights into the mechanistic con-
siderations of this reaction.
A plausible mechanism of this reaction is as follows
(Scheme 2).4a,10a,12e,f The ate complex [Et2Cu(CN)-
(MgCl)2], derived from a 2:1 mixture of EtMgCl and
CuCN, is oxidatively added by the enol triflate A, giving
the Cu(III) species B. The reductive elimination from B
gives the desired alkylated product C.
2EtMgCl + CuCN
Et2Cu(CN)(MgCl)2
Et
OTf
O
R
OMe
Et
O
Et Cu
O
A
Me
R
OMe
R
OMe
C
Me
B
Me
Synlett 2001, No. 10, 28 09 2001. Article Identifier:
1437-2096,E;20010,0,10,1511,1515,ftx,en;Y14201ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214
EtCu
2Mg • 2Cl • CN • OTf
Scheme 2

1512
M. Ide, M. Nakata
LETTER
Table 1 Copper-promoted Ethylation of 1
H
OMe
O
TfO
OMe
O
R
OMe
O
TrO(CH2)3
TrO(CH2)3
TrO(CH2)3
+
in THF
Me
Me
1
Me
2 (R = Et)
3
Entry
Reagents (equiv for 1)
Conditions for copper reagents Conditions for coupling
Yield (%) of 2
1
2
3
4
5
6
EtMgCl (4), CuCN (2)-2LiCl
EtMgCl (4), CuI (2)
-78 °C to 0 °C, 1 h
-78 °C to 0 °C, 1 h
-78 °C to 0 °C, 1 h
-78 °C to 0 °C, 1 h
-78 °C, 15 min
-78 °C
-30 °C to r.t., 2 h; r.t., 1.5 h
-30 °C to r.t., 2 h; r.t., 1.5 h
-30 °C to r.t., 2 h; r.t., 4 h
-78 °C, 8.5 h
98
98
EtMgCl (2), CuCN (2)-2LiCl
EtMgCl (4), CuCN (2)-2LiCl
EtMgCl (4), CuCN (2)-2LiCl
EtMgCl (4)
40a
74b
93c
NRd
-78 °C, 2.5 h
-78 °C, 4.5 h
a The starting material 1 was recovered in ~60% yield.
b The starting material 1 was recovered in ~25% yield.
c The starting material 1 was recovered in ~5% yield.
d No reaction.
We next investigated the use of catalytic amounts of cop-
per in this reaction (Table 2). To a solution of 1 (1 equiv)
in THF were added at -78 °C a 0.1 M THF solution of
CuCN (20 mol%)-2LiCl and a 0.97 M THF solution of Et-
MgCl (4 equiv). After 4.5 h at -78 °C, the ethylated prod-
uct 2 (R = Et) was obtained in only 16% yield together
with the reduction product 3 in 80% yield (R = H, Table 2,
entry 1). The use of CuCl2 (10 mol%) instead of CuCN
gave the same result (Table 2, entry 2). The reduction
product 3 was also the major product when BuMgCl, Ph-
MgBr and vinyl MgBr were used (Table 2, entries 3, 4,
and 5). The yield of 2 was very low with t-BuMgCl
(Table 2, entry 6). Interestingly, the methylated product 2
(R = Me) was obtained in 86% yield when MeMgCl was
used (Table 2, entry 7).
Table 2 Copper-catalyzed Alkylation of 1
TfO
OMe
O
R
OMe
O
TrO(CH2)3
TrO(CH2)3
in THF, -78 °C
Me
Me
2 or 3 (R = H)
1
Entry Reagents (equiv for 1) Time/h Yield (%) of 2 Yield(%)
of 3
1
2
3
4
5
6
7
EtMgCl (4), CuCN (20 4.5
mol%)-2LiCl
16 (R = Et)
17 (R = Et)
21 (R = Bu)
32 (R = Ph)
80
76
72
66
EtMgCl (4), CuCl2 (10 0.5
mol%)-2LiCl
BuMgCl (4), CuCl2 (10 0.5
mol%)-2LiCl
The mechanism in Scheme 3 is what we assume occurs to
give the reduction product 3. R1Cu, obtained from a 2:1
mixture of R1MgX and Cu(II), couples with R1MgX to af-
PhMgBr (4), CuCl2 (20 5
mol%)-2LiCl
ford R1 CuMgX. This ate complex is oxidatively added
2
vinyl MgBr (4), CuCl2
(20 mol%)-2LiCl
5
32 (R = vinyl) 68
by the enol triflate A, giving the Cu(III) species B. The re-
ductive elimination from B furnishes the desired alkylated
product C, and R1Cu is regenerated. However, when this
step is slow, transmetallation would occur between the ex-
cess R1MgX and B to give the vinyl Grignard species D
and R1 Cu. R1Cu is regenerated from R1 Cu. The final
t-BuMgCl (4), CuCl2 (10 5
mol%)-2LiCl
4a (R = t-Bu)
0
0
MeMgCl (4), CuCl2 (10 0.5
mol%)-2LiCl
86 (R = Me)
3
3
work-up produces the reduction product E from D. For t-
BuMgCl, the ate complex (t-Bu2CuMgX) or Cu(III) spe-
cies B is not effectively generated at -78 °C. In the case of
MeMgCl, the reductive elimination step (B to C) is fast
even at -78 °C, giving the methylated product 2 (R = Me).
a The starting material 1 was recovered in ~95% yield.
Synlett 2001, No. 10, 1511–1515 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Stereoselective Synthesis of Tri- and Tetrasubsituted , -Unsaturated Esters
1513
3). In addition, when the reaction mixture was quenched
with I2, the ethylated product 2 (R = Et) and the vinyl io-
dide 2 (R = I) were produced in 28% and 72% yields, re-
spectively (Table 3, entry 4).
2R1MgX + CuCl2
0.5 R1-R1 + 2MgXCl
R1 R1
R1
R1 Cu
R1
R1Cu
These results can be explained by the mechanism shown
in Scheme 4. The vinyl Grignard species D instead of
R1MgX couples with R1Cu to give the ate complex, to
which oxidatively added R2Y (EtI, MeI, or I2), affording
the new Cu(III) species B’. Two kinds of products, C and
C’, were produced from B’. Therefore, we expected that,
to prevent the transmetallation step (B to D in Scheme 3),
less nucleophilic organometallic species than Grignard re-
agents would be preferable for this catalytic cycle. There-
fore, we selected organozincates.17
R1MgX
MgX
O
R1
O
R12CuMgX
R
OMe
R
OMe
D
Me
C
Me
OTf
O
work-
up
R
OMe
R1
A
Me
H(D)
O
R1 Cu
O
R1MgX
MgX(OTf)
OMe
R
OMe
R
R1Cu
MgX
O
E
Me
B
Me
R2Cu
R
OMe
Scheme 3
D
Me
R1
O
R2
O
R1 Cu
R
O
In order to support this assumption, the following reac-
tions were examined (Table 3). To a THF-DMPU (10:1)
solution of 1 (1 equiv) were added at -78 °C a 0.11 M THF
solution of CuCl2 (20 mol%)-2LiCl and a 0.97 M THF so-
lution of EtMgCl (4 equiv). After 4.5 h at -78 °C, excess
CD3OD was added to the reaction mixture. After 1 h at -
78 °C, the ethylated product 2 (R = Et) and the reduction
product 3 (R = D, 95% deuterium incorporation) were ob-
tained in 47% and 48% yields, respectively (Table 3, en-
try 1). This result indicates that no hydrogen source exists
in the reaction medium at -78 °C and the hydrogen incor-
poration occurs during the quenching process. Moreover,
quenching the reaction mixture with EtI increased the
yield of 2 (R = Et) to 82% and decreased the yield of 3
(R = H) to 16% (Table 3, entry 2).16 When the reaction
mixture was quenched with MeI, the ethylated product 2
(R = Et) was obtained in 32% yield together with the 53%
yield of the methylated product 2 (R = Me, Table 3, entry
MgX
R
OMe
R
OMe
R2
OMe
C
Me
C'
Me
Me
R2Y
R1 Cu
O
MgXY
R
OMe
B' Me
Scheme 4
To a 0.5 M THF solution of ZnCl2 (1.5 equiv) was added
at 0 °C a 1.02 M THF solution of EtMgCl (4.5 equiv); af-
ter 0.5 h at 0 °C, a 0.1 M THF solution of CuCl2 (15
mol%)-2LiCl and 1 (1 equiv) in THF-DMPU (10:1) were
added at -30 °C. The reaction mixture was warmed to r.t.
over a 2 h period. After 4 h at r.t., the ethylated product 2
(R = Et) was obtained in 98% yield (Table 4, entry 1).18
Neither the stereoisomer of 2 nor the reduction product 3
was produced. The use of a 2:1 or 1:1 mixture of EtMgCl
and ZnCl2 was less satisfactory (Table 4, entries 2 and 3).
DMPU was essential for this coupling (Table 4, entry 4).
In the absence of the copper salt, the reaction did not pro-
ceed (Table 4, entry 5).
Table 3 Trapping Experiments of Vinyl Grignard Species
1) EtMgCl (4),
CuCl2 (20 mol%)-
TfO
OMe
O
R
OMe
O
2LiCl, THF-DMPU,
-78 °C, 4.5 h
TrO(CH2)3
TrO(CH2)3
2) excess additive,
-78 °C, 1 h
Me
1
Me
2 or 3 (R = H)
We propose the reaction mechanism depicted in
Scheme 5. Et3ZnMgCl, obtained from a 3:1 mixture of Et-
MgCl and ZnCl2, reduces Cu(II) to Cu(I). The resulting
EtCu couples with Et3ZnMgCl to produce Et2CuMgCl.
The insertion of this ate complex to the enol triflate A af-
fords the Cu(III) species B. The reductive elimination
from B furnishes the desired alkylated product C, and
EtCu is regenerated. It is noteworthy that the zinc ate
complex (Et3ZnMgCl) is less nucleophilic than the Grig-
nard reagents; therefore, the transmetallation step is pre-
vented and the reductive elimination step is preferred.
Entry
Additive (equiv for 1) Yield (%) of 2 Yield (%) of 3
1
CD3OD (100)
EtI (100)
47 (R = Et)
82 (R = Et)
48a
16
5
2
3b
MeI (100)
32 (R = Et)
53 (R = Me)
4b
I2 (10)
28 (R = Et)
72 (R = I)
0
a 95% Deuterium incorporation.
b DMPU was not used.
Synlett 2001, No. 10, 1511–1515 ISSN 0936-5214 © Thieme Stuttgart · New York

1514
M. Ide, M. Nakata
LETTER
Et
OMe
O
Me
OMe
O
Table 4 Copper-catalyzed Ethylation of 1 with Ethylzincate
TrO(CH2)3
TrO(CH2)3
TrO(CH2)3
TrO(CH2)3
TfO
OMe
O
R
OMe
O
1) EtMgCl, ZnCl2,
0 °C, 0.5 h
TrO(CH2)3
TrO(CH2)3
95%
Bu
98% Me
Me
2) 1, -30 °C to r.t.,
2 h; r.t., 4 h
Me
1
Me
2 (R = Et)
i-Pr
OMe
O
OMe
O
Entry Reagents (equiv for 1)
Solvent
Yield (%) of 2
96% Me
88% Me
1
2
3
4
5
EtMgCl (4.5), ZnCl2 (1.5), THF-DMPU 98
CuCl2 (15 mol%)-2LiCl
t-Bu
OMe
O
TrO(CH2)3
EtMgCl (3.0), ZnCl2 (1.5), THF-DMPU 26a
CuCl2 (15 mol%)-2LiCl
94% Me
EtMgCl (1.5), ZnCl2 (1.5), THF-DMPU NRb
CuCl2 (15 mol%)-2LiCl
Figure 1
EtMgCl (4.5), ZnCl2 (1.5), THF
CuCl2 (15 mol%)-2LiCl
49c
Et
OMe
O
Et
EtMgCl (4.5), ZnCl2 (1.5) THF-DMPU NRb
TrO(CH2)3
98%
TrO(CH2)3
98%
Me
a The starting material 1 was recovered in ~70% yield.
b No reaction.
Me
O
OMe
c The starting material 1 was recovered in ~50% yield.
Et
OMe
O
Et
TrO(CH2)3
98%
TrO(CH2)3
98%
6EtMgCl + 2ZnCl2
2Et3ZnMgCl
O
OMe
4MgCl2
CuCl2
TrO
Et
OMe
O
TrO
Et
0.5 Et-Et + 2Et2Zn + 2MgCl2
Me
EtCu
Me
Me
Me
Me
O
83%
TrO
94%
TrO
OMe
Et3ZnMgCl
Et2Zn
Et
O
Et
OMe
O
Et
Et2CuMgCl
R
OMe
C
Me
Me
O
OTf
O
98%
97%
OMe
Et
R
OMe
Figure 2
Et Cu
O
A
Me
R
OMe
In summary, we have demonstrated that tri- and tetrasub-
stituted , -unsaturated esters were stereoselectively pre-
pared via the copper-catalyzed couplings of the enol
triflates of -ketoesters with Grignard-based zinc ate com-
plexes in high yields. The zinc ate complexes (R3ZnMgX)
are milder nucleophiles than Grignard reagents. This was
critical for preventing hydrogen incorporation in the cop-
per-catalyzed conditions. Moreover, from a synthetic
point of view, the experimental procedures are easier than
those of the stoichiometric copper-mediated reactions in
both preparation of reagents and work-up.20
MgCl(OTf)
Et3ZnMgCl
B
Me
Scheme 5
Under the above optimized conditions, other alkylated
products were secured (Figure 1). In addition to the ethyl
group, the Me (MeMgCl in ether), Bu (BuMgCl in THF),
i-Pr (i-PrMgCl in THF) and t-Bu (t-BuMgCl in THF)
groups were easily incorporated into 1 in high yields with-
out the formation of the stereoisomer or the reduction
product.19
TrO
Finally, the incorporation of the ethyl group into other
CHO
TrO(CH2)3
CHO
TrO
I
enol triflates of
-ketoesters were examined
(Figure 2).13,14 In all the cases examined, satisfactory re-
sults were obtained. Neither the stereoisomer nor the re-
duction product, was produced.
i
ii
Me
iii
Figure 3
Synlett 2001, No. 10, 1511–1515 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Stereoselective Synthesis of Tri- and Tetrasubsituted , -Unsaturated Esters
1515
(13) The -ketoester corresponding to 1 was prepared from i
(Figure 3) and methyl propionate [i)LDA, THF, -78 °C, 1 h,
then i, -78 °C, 1 h. ii)PCC, CH2Cl2, r.t., 1 d]. The -
ketoesters corresponding to the ethylated products depicted
in Figure 2 were prepared from ii (Figure3) + methyl
acetoacetate (NaH, BuLi, THF, 0 °C, 0.5 h, then ii, 0 °C, 1
h), iii (Figure 3) + methyl propionate [i) LDA, THF, -78 °C,
1 h, then iii, -78 °C, 2 h. ii) PCC, CH2Cl2, r.t., 1 d], and iii +
methyl acetate [i) LDA, THF, -78 °C, 1 h, then iii, -78 °C, 2
h. ii) PCC, CH2Cl2, r.t., 1 d]. The (Z)- and (E)-enol triflates
were prepared under the Gibbs conditions8b from the -
ketoesters. All the enol triflates used in these experiments
are stereochemically pure. The compound ii was prepared
from 1,3-propanediol by tritylation (TrCl, Et3N, CH2Cl2, r.t.,
12 h) followed by iodination (I2, Ph3P, imidazole, CH2Cl2,
r.t., 1.5 h). The compounds i and iii were prepared by the
reported procedures, see: (a) Jones, G. B.; Hynd, G.; Wright,
J. M.; Sharma, A. J. Org. Chem. 2000, 65, 263. (b) Ide, M.;
Nakata, M. Bull. Chem. Soc. Jpn. 1999, 72, 2491.
(14) Satisfactory analytical data (NMR and IR spectra, elemental
analyses and/or HRMS) were obtained for all new
compounds. The double-bond geometry was confirmed by
NMR nOe experiments.
(15) In contrast, when the coupling reaction and the preparation
of the copper reagent were conducted at -78 °C in the
absence of LiCl, the reduction product 3 was obtained as the
major product.
(16) The effectiveness of the alkylhalide addition to the reaction
mixture has already been described by Casey4c and Weiler.6a
See also: Corey, E. J.; Katzenellenbogen, J. A.; Gilman, N.
W.; Roman, S. A.; Erickson, B. W. J. Am. Chem. Soc. 1968,
90, 5618.
(17) It is well known that organozincates are less nucleophilic
than Grignard reagents. For example, see: (a) Uchiyama,
M.; Kameda, M.; Mishima, O.; Yokoyama, N.; Koike, M.;
Kondo, Y.; Sakamoto, T. J. Am. Chem. Soc. 1998, 120,
4934. (b) Uchiyama, M.; Kondo, Y.; Sakamoto, T. J. Synth.
Org. Chem. Jpn. 1999, 57, 1051.
(18) In the case of Et3ZnMgCl prepared from a 1:1 mixture of
Et2Zn and EtMgCl, the ratio of 2 (R = Et) to 3 was 6:1.
(19) Unfortunately, the phenyl and vinyl groups could not
effectively be incorporated under the optimized conditions.
(20) For 1,4-addition reactions and substitution reactions using
organozincates in the presence of the catalytic amounts of
copper, see: (a) Tückmantel, W.; Oshima, K.; Nozaki, H.
Chem. Ber. 1986, 119, 1581. (b) Tsunashima, K.; Ide, M.;
Kadoi, H.; Hirayama, A.; Nakata, M. Tetrahedron Lett.
2001, 42, 3607.
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