Allylic substitution reactions with Grignard reagents catalyzed by imidazolium and 4,5-dihydroimidazolium carbene-CuCl complexes

Article abstract of DOI:10.1016/j.jorganchem.2005.07.096

Imidazolium and 4,5-dihydroimidazolium carbene-CuCl complexes effectively catalyzed the substitution reaction of allylic compounds with Grignard reagents in an S_N2′-selective fashion. It was noteworthy that the amount of the imidazolium carbene

Full text of DOI:10.1016/j.jorganchem.2005.07.096

Journal of Organometallic Chemistry 690 (2005) 6001–6007
www.elsevier.com/locate/jorganchem
Allylic substitution reactions with Grignard reagents catalyzed by
imidazolium and 4,5-dihydroimidazolium carbene–CuCl complexes
*
Sentaro Okamoto , Satoshi Tominaga, Naoko Saino, Kouki Kase, Kentaro Shimoda
Department of Applied Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
Received 13 May 2005; received in revised form 26 July 2005; accepted 26 July 2005
Available online 31 August 2005
Abstract
Imidazolium and 4,5-dihydroimidazolium carbene–CuCl complexes effectively catalyzed the substitution reaction of allylic com-
pounds with Grignard reagents in an SN20-selective fashion. It was noteworthy that the amount of the imidazolium carbene-CuCl
complex could be reduced to 0.001 mol% and the catalysis recorded a high TON (105). Based on the experimental results, the ate-
type complex(es) such as [(imidazolium carbene)-CuR2]ꢀ(MgX)+ was postulated as an active species.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: N-Heterocyclic carbene; Copper; SN20; Allylic substitution; Enatioselective reaction; Grignard reagent; Catalysis
1. Introduction
mental results regarding catalytic activities, stoichiome-
tric reactions, stereospecificity and enantioselectivity
(see Fig. 1).
Copper-catalyzed allylic substitution reactions with
Grignard reagents have received widespread acceptance
as valuable methods for preparation of a-substituted
alkenes [1,2]. The regioselectivity, stereoselectivity and
stereospecificity of the substitution are dependent upon
the structure of the substrates and the Grignard reagents
and are also influenced by the reaction conditions
including copper salt, solvent(s), temperature and the
addition order of reagents [1,2]. Investigation has also
been focused on enantioselective reactions using copper
catalysts with chiral ligands [3]. Recently, we have devel-
oped a novel procedure for SN20-selective reaction with
a Grignard reagent which was catalyzed by copper
N-heterocyclic carbene (NHC)[4] complexes and it has
been applied to enantioselective substitution by using
chiral modified NHC ligands [5,6]. Herein, we discuss
more detailed features of this catalysis based on experi-
2. Results and discussion
2.1. Catalytic activity of imidazolium and 4,5-dihydro-
imidazolium carbene–CuCl complexes
In a previous paper [7], we described the characteris-
tic features, regarding the structure of allylic substrates
and Grignard reagents, and solvent effects, of the
NHC–CuCl-catalyzed allylic substitution reaction and
found that the reaction was reasonably general: (i) The
reaction in ethyl ether proceeded in a highly SN20-selec-
tive fashion (>94:6). (ii) The reaction in THF was rela-
tively slow and an allylic substrate having an ester as a
leaving group gave an SN2 product mainly but reaction
of allylic chloride proceeded SN20-selectively. (ii) A vari-
ety of allylic substrates and Grignard reagents except for
aryl magnesium halides could be used and afforded
SN20-product predominantly.
*
Corresponding author.
E-mail address: okamos10@kanagawa-u.ac.jp (S. Okamoto).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.096

6002
S. Okamoto et al. / Journal of Organometallic Chemistry 690 (2005) 6001–6007
carbonate gave mainly SN2-product in THF (entries 1
and 3–5). It was noteworthy that the amount of the
NHC–CuCl complex could be reduced to 0.001 mol%
and the catalysis recorded a high TON (105) (entry 7).
R
N
N
R
N
N
CuCl
CuCl
1c
1a: R = Me
1b: R = H
O
O
2.2. Comparison with stoichiometric reactions
Ph
Ph
Ar
Ar
N
N
N
N
N
N
To discuss the active species in the catalysis, we
carried out the corresponding stoichiometric reactions
and compared the results with those in the catalytic
reactions (Table 2). Thus, the reaction of allylic carbon-
ate 2a and i-PrMgCl with a stoichiometric amount of 1
(entry 2) was found to be much slower than that with a
catalytic amount of 2 (entry 1) or resulted in no reaction
(entry 5). On the other hand, a mixture of 1 equiv. of 1
and 2 equiv. of i-PrMgCl could convert 2 to 3 and 4 very
quickly with a high regioselectivity (entries 3 and 6).
Based on these results, the major active catalyst in the
present reaction can be postulated to be an ate-complex
such as the type [(NHC)CuR2]ꢀ(MgX)+ (ii) but not a
complex of the type (NHC)CuR (i) [8].
CuCl
CuCl
CuCl
1g
1d: Ar = Ph
1e: Ar = 2-naphtyl
1f
Fig. 1. Utilized NHC–CuCl complexes.
R'MgX
cat. 1
R
R
L
R
R'
+
0 oC, 1 h
R'
4
3
2
SN2-product
(L = leaving group)
SN2'-product
ð1Þ
As revealed from the additional results shown in
Table 1, which were obtained by the reactions of
cinnamyl carbonate 2a or chloride 2b with i-PrMgCl
catalyzed by imidazolium carbene complex 1a and 4,5-
dihydroimidazolium carbene complex 1c, 1a as well as
1c could efficiently catalyze the reaction in a highly
SN20-selective way (entries 1 and 2). It was observed
again that the reaction of allyl chloride in THF pro-
ceeded SN20-selectively as well as that in ether but allyl
RMgX
RMgX
N
N
R'
N
N
R'
N
N
R'
R'
R'
R'
R Cu- +MgX
Cu
CuCl
R
R
i
1
ii
ð2Þ
Table 1
1-Catalyzed substitution of 2 (R = Ph) with i-PrMgCla
Entry
2 (R = Ph)
1 (mol%)
Solvent
3:4b
Total yield
1
2
3
4
5
6
7d
a
2a: L = OCO2 Et
2a
1a (1)
1c (1)
Et2O
Et2O
THF
Et2O
THF
Et2O
Et2O
98:2
98:2
8:92
96:4
89:11
96:4
92:8
Quant.
Quant.
21%c
Quant.
Quant.
Quant.
Quant. (98%)e
2a
2b: L = Cl
2b
2b
2b
1a (1)
1a (1)
1a (1)
1a (1)
1a (0.001)
A mixture of 1 (0.001–1 mol%), i-PrMgCl (1.5 equiv., 0.7–1.3 M in ether) and 2a (1.0 equiv.) was stirred for 1 h at 0 ꢁC, unless otherwise
indicated.
b
Determined by 500 or 600 MHz 1H NMR analysis of the crude mixture.
78% of 2a was recovered.
The reaction was performed for 10 h at 0 ꢁC.
Isolated by column chromatography.
c
d
e
Table 2
Reaction of 2a with i-PrMgCl under various conditions
Entry
1 (equiv.)
i-PrMgCl (equiv.)
Conditions
Product (%)
Recovered 2a
3
4
1
2
3
4
5
6
1a (0.01)
1a (1.5)
1a (1.3)
1c (0.01)
1c (1.5)
1c (1.3)
(1.5)
(1.3)
(2.6)
(1.3)
(1.3)
(2.6)
0 ꢁC, 30 min
0 ꢁC, 12 h
0 ꢁC, 5 min
0 ꢁC, 15 min
0 ꢁC, 24 h
98
13
93
98
0
2
<1
5
2
0
0
86
2
0
ꢁ100
0 ꢁC, 5 min
86
4
0

S. Okamoto et al. / Journal of Organometallic Chemistry 690 (2005) 6001–6007
6003
2.3. Stereospecificity
R
M
O
syn-SN2'
O
The results shown in Table 3, which were obtained by
copper-catalyzed substitution reactions of cyclic allyl
acetate 2d, indicate their reaction course and stereospec-
ificity. Thus, deuterated allylic acetate 2d [9], which was
prepared from carvone by reduction with LiAlD4 and
the following acetylation, was treated with 1.5 equiv.
of i-BuMgBr in the presence of 1 mol% of copper salt.
The reaction catalyzed by CuCl or CuCN afforded
mainly an SN2-product and it was found that minor
SN20 reaction proceeded in an anti fashion (entries 3
and 4). The 1b-catalyzed reaction ‘‘in THF’’ gave similar
results and the reaction was found to be relatively slow
(entry 2). Meanwhile, it was noteworthy that the 1b-cat-
alyzed reaction ‘‘in ether’’ gave SN20-products predomi-
nantly and the reaction proceeded in a syn pathway
mainly (entry 1). These results suggest that coordination
between the acetoxy group and a metal atom present in
an active species might affect the 1b-catalyzed reaction
in ether (Fig. 2) [10].
D
anti-SN2'
Fig. 2. anti-SN20 vs. syn-SN20.
2.4. Enantioselectivity of the reactions catalyzed by chiral
NHC–CuCl complexes [11]
Table 4 shows the results of the substitution reaction
of 4-(silyloxy)-but-2-en-1-ol derivatives with n-C6H13-
MgBr using C2-chiral NHC–CuCl 1d–1g as a catalyst
precursor. Because the complexes 1d–1g in their solid
state were somewhat unstable toward moisture and/or
air, the THF solution containing 1d–f was, respectively,
prepared from the corresponding imidazolium salt,
t-BuONa and CuCl [6a] and the resulting solution was
stored under Ar. The solution was charged into the
Table 3
i-BuMgBr
(1.5 eq.)
catalyst
(1 mol%)
D
D
AcO D
D
+
+
2d
SN2'-syn
SN2
SN2'-anti
Entry
Catalyst
Conditions
SN2
SN20-anti
SN20-syn
Total yield (%)
1
2
3
4
1b
1b
Et2O, 0 ꢁC, 2 h
THF, 0 ꢁC, 12 h
Et2O, 0 ꢁC, 3 h
Et2O, 0 ꢁC, 3 h
35
90
93
92
7
10
7
58
0
0
97
71
76
74
CuCl
CuCN
8
0
Table 4
n-C6H13MgBr
catalyst
ether, -20 oC
R1
R1
OR
n-C6H13
R2
R2
20 h
Entry
R1
R2
OR
Catalyst (mol%)
% e.e.a (config.)a
SN20:SN2
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
TBSO
TBSO
TBSO
TBSO
TBSO
TBSO
TBSO
TBSO
TBSO
TBSO
TBSO
TBSO
H
OAc
OCO2Et
OP(O)(OEt)2
Cl
O-(2-pyridyl)
OAc
1d (5)
1d (5)
1d (5)
1d (5)
1d (5)
1f (1)
1e (5)
1e (5)
1e (5)
1g (1)
1e (5)
1e (5)
40 (R)
5 (R)
8 (R)
87:13
96:4
99:1
77:23
91:9
49:51
95:5
99:1
98:2
98:2
97:3
86:14
8 (S)
36 (R)
52 (R)
60 (R)
55 (R)
70 (R)
50 (R)
38 (S)
60 (S)
OAc
OC(O)NMe2
O-(2-pyridyl)
O-(2-pyridyl)
OAc
10
11
12
H
O-(2-pyridyl)
All reactions proceeded quantitatively.
a
For determination, see [7].

6004
S. Okamoto et al. / Journal of Organometallic Chemistry 690 (2005) 6001–6007
reaction vessel and THF was removed under reduced
pressure prior to use. As revealed from the results shown
in entries 1–5, enantiomeric excess (e.e.) of the SN20
product was highly dependent on the nature of the lea-
ving group and the coordinative moiety such as acetoxy
and 2-pyridyloxy groups gave better e.e. The complex
with sterically demanding N-substituents (1e) or 4,5-
dihydroimidazolium carbene-CuCl (1f) gave better e.e.
than that obtained with 1d (entry 1 vs. entries 6 and
7). Inversion of the product configuration was observed
when (E)-allylic substrates were used instead of (Z)-iso-
mers (entries 11 and 12). The highest enantiomeric ratio
of 85:15 was attained by the reaction of (Z)-2-pyridyl
ether with catalyst 1e (entry 9). The fact that the reac-
tion with conformationally rigid NHC-CuCl 1g with
(S,S)-configuration gave the SN20 product with (R)-con-
figuration (entry 10) suggests the conformation of
N-substituents in the active catalysts derived from 1d,
1e and 1f, the (R,R)-isomers of which produced (R)-
product predominantly (vide infra).
group and +MgX counter cation [10]. Use of THF
instead of diethyl ether as a solvent may inhibit the coor-
dination to retard the reaction and cause change of the
regioselectivity.
To discuss the enantioselection with 1d–f, conforma-
tion of their 1-arylethyl side-chain in the active species is
important and comparison of the results of the 1d–f
catalyzed reactions with those with 1g suggest that their
active structure would be as (b) shown in Fig. 3. Thus,
the aryl moieties present in complexes (b) derived from
1d–f with the R,R-configuration should occupy the same
quadrants around an imidazolium carbene-Cu axis as
those which two i-Pr groups do in the rigid complex
(c) derived from 1g with S,S-configuration. In such con-
formation shown as (b) two hydrogen atoms Ha and
Ha0 locate around sterically hindered Cu atom similarly
to the conformation of the side chain in the solid state
structure reported for 1,3-bis(cyclohexyl)imidazolin-2-
ylidene copper chloride (d) [6d].
These findings and the discussion described here will
support further study for development of NHC–CuCl
complexes giving better enantioselectivity of the present
allylic substitution reaction and development of other
reactions catalyzed by NHC–CuCl. Investigation along
this line is under way in our laboratories.
2.5. Proposing transition structure
Although elucidation of the origin of the regioselec-
tivity of the reaction remains difficult at this time, the
results of the reactions with a stoichiometric amount
of 1, the solvent effect and the effect of the coordinative
nature of the leaving groups on regioselectivity, stereo-
specificity and enantioselectivity observed here suggest
reaction mechanism as follows: The reaction might
involve an ate-complex such as the type [(NHC)CuR2]ꢀ
(MgX)+ as an active species which could be generated
via the (NHC)CuR complex (Eq. 2). The generated ate
complex would react with the allylic substrate through
the seven-membered cyclic structure (a) shown in
Fig. 3 which consists of p-complexation between the
Cu atom and the carbon–carbon double bond of the
substrate and the coordination between the leaving
3. Experimental
3.1. General
NMR spectra were recorded in CDCl3 at 600, 500
1
and 270 MHz for H and 150, 125 and 67.5 MHz for
13C, respectively, on JEOL JNM-ECA600, 500 and
–EX270 spectrometers. Chemical shifts are reported in
parts per million (ppm, d) relative to Me4Si (d 0.00) or
1
residual CHCl3 (d 7.26 for H and d 77.0 for 13C). IR
spectra were recorded on an FT-IR spectrometer
N
R
R
N
R'
R'
X
Cu- +Mg
O
O
(a)
O
O
N
H
N
N
N
N
N
Hα'
H
Hα
Hα'
Hα
Cu-R2
Cu-R2
Cu
X
(d)
(c)
(b)
Fig. 3. Postulated transition structure and conformations of side-chains in NHC-copper complexes.

S. Okamoto et al. / Journal of Organometallic Chemistry 690 (2005) 6001–6007
6005
(HITACHI 270-30) and are reported in wave numbers
(cmꢀ1). All reactions sensitive to oxygen and/or mois-
ture were performed under an argon atmosphere. Dry
solvents (THF and ethyl ether) were purchased from
Kanto Chemicals. Grignard reagents were prepared
from magnesium turnings and the corresponding organ-
ic halide in ethyl ether or THF, titrated by an acid–base
titration using aqueous 1.0 M HCl and 0.5 M NaOH,
and stored under argon. Imidazolium salts, i.e., 1,3-
bis(2,4,6-trimethylphenyl)-3H-imidazol-1-ium chloride
3.3. Allylic substitution reactions with Grignard reagents
catalyzed by imidazolium carbene–CuX complexes
3.3.1. Reaction with 1 mol% of NHC–CuCl complexes 1
To a suspension of an imidazolium or 4,5-dihydroim-
idazolium carbene–CuCl complex
1
(0.01 mmol,
1 mol%) in ether (1 mL) was added dropwise a solution
of Grignard reagent (1.5 mmol) at 0 ꢁC and the resulting
mixture was stirred for 15 min. To this was added drop-
wise a solution of allylic substrate 2 (1.0 mmol) and the
mixture was stirred for 1–2 h. After confirming the
completion of the reaction by TLC analysis, saturated
aqueous NH4Cl was added. The mixture was extracted
with hexanes (2 · 10 mL) and the combined organic
layers were washed with brine, dried over MgSO4, con-
centrated. The 500 or 600 MHz 1H NMR analysis of the
crude residue determined the regioselectivity. Chroma-
tography on silica gel gave the substitution products.
The following spectral data were recorded using a mix-
ture of 3 and 4.
[4c,12],
1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-3H-
imidazol-1-ium chloride [13], (R,R)-1,3-bis(1-phenyl-
ethyl)-3H-imidazol-1-ium chloride [5c,14], (R,R)-1,
3-bis[1-(1-naphtyl)ethyl]-3H-imidazol-1-ium chloride [5c,
14], and (S,S)-1,6-diisopropyl-1,2,5,6-tetrahydro-3,4-
dioxa-7a-aza-6a-azonia-cyclopenta[a]-pentalene triflate
[15] were prepared from the corresponding amines
according to the reported procedures. Other chemicals
are commercially available, unless otherwise indicated,
and were used as received.
3.2. Preparation of NHC–CuCl complexes 1a–c
3.3.2. Reaction with 0.001 mol% of NHC–CuCl complex
1a
3.2.1. 1,3-Bis(2,6-dimethyl-phenyl)imidazolin-2-ylidene
copper chloride (1b)
In a flask a THF solution of 1a (100 lL, 0.001 M,
10ꢀ4 mmol) was charged and THF was removed in
vacuo. Under Ar atmosphere to this were added ether
(1.5 mL), i-PrMgCl (11.5 mL, 1.3 M in ether, 15 mmol)
and cinnamyl chloride (1.39 mL, 10 mmol) at 0 ꢁC.
The mixture was stirred for 10 h at 0 ꢁC. The resulting
white suspension was poured into aqueous 1 M HCl
(30 mL) and the mixture was extracted with ether
(2 · 20 mL), washed with saturated aqueous NaHCO3
(10 mL), dried over MgSO4, and concentrated. The res-
idue was passed through a short silica gel column with
hexanes and concentrated to give a mixture of 3 and 4
(R = Ph, R0 = i-Pr, total 1.57 g, 98% yield) in a ratio
of 92:8.
To a mixture of 1,3-bis(2,6-dimethylphenyl)-3H-imi-
dazol-1-ium chloride (313 mg, 1.0 mmol), CuCl (89 mg,
0.90 mmol) and t-BuONa (96 mg, 1.0 mmol) was added
THF (7 mL). The resulting suspension was stirred for
6 h at room temperature and filtered through a pad of
Celite. The filtrate was concentrated in vacuo to give
1b (264 mg, 78% yield) as a red-brown powder. M.p.
1
268 ꢁC; H NMR (CDCl3, 500 MHz) d 7.31 (t, J = 7.5,
2H), 7.21 (d, J = 8.0, 4H), 7.10 (s, 2H), 2.17 (s, 12H);
13C NMR (CDCl3, 125 MHz) d 178.5, 137.4, 134.9,
129.6, 128.8, 122.1, 17.8; IR (neat) 3166, 1695, 1599,
1557, 1479, 1443, 1404, 1335, 1290, 1230, 1161, 1110,
1038, 948, 786, 735, 693, 609. Anal. Calc. for C19H2-
N2OClCu: C, 60.79; H, 5.37; N, 7.46. Found: C,
60.95; H, 5.54; N, 7.47%.
Spectral data of 4-methyl-3-phenyl-1-pentene obtai-
ned as the SN20 product by the reaction shown in Table
1 and 2 were identical with those reported [3e].
3.2.2. 1,3-Bis(2,4,6-trimethyl-phenyl)imidazolin-2-
ylidene copper chloride (1a)
3.4. Chiral modified NHC–CuCl complexes-catalyzed
reactions
1H NMR (500 MHz, CDCl3)d 7.06 (s, 2H), 7.00 (s,
4H), 2.34 (s, 6H), 2.10 (2, 12H); 13C NMR (125 MHz,
CDCl3) d 178.7, 139.2, 134.9, 134.4, 129.3, 122.2, 21.1,
17.6; IR (KBr) 2914, 1485, 1400, 1234, 1076, 932, 862,
702 cmꢀ1. Anal. Calc. for C21H24ClCuN2: C, 62.52; H,
6.00; N, 6.94. Found: C, 62.33; H, 6.16; N, 6.86%.
THF solutions of compounds 1d–f were prepared
from the corresponding imidazolium salts, t-BuONa
and CuCl in THF [6a]. The resulting mixture was fil-
tered through a pad of Celite and the filtrate was stored
under Ar because the corresponding complexes in their
solid state were unstable toward moisture and/or air.
The THF-solution containing 1d–1f, respectively, thus
obtained was charged into the reaction vessel and
THF was removed under reduced pressure prior to use.
In a flask a THF solution (0.05 mmol) thus prepared
was charged and THF was removed in vacuo. Under Ar
atmosphere to this were added ether (1.0 mL), i-PrMgCl
3.2.3. 1,3-Bis(2,4,6-trimethyl-phenyl)imidazolidin-2-
ylidene copper chloride (1c)
The spectral data (1H and 13C NMR) were in good
agreement with those reported [6d]. Anal. Calc. for
C21H26ClCuN2: C, 62.21; H, 6.46; N, 6.91. Found: C,
62.34; H, 6.16; N, 6.98%.

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S. Okamoto et al. / Journal of Organometallic Chemistry 690 (2005) 6001–6007
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¨
(1.15 mL, 1.3 M in ether, 1.5 mmol) and 4-(silyloxy)-
but-2-en-1-ol derivative (1.0 mmol) at ꢀ20 ꢁC. The mix-
ture was stirred for 20 h at ꢀ20 ꢁC. To this was added
saturated aqueous NH4Cl (10 mL) and the mixture
was extracted with ether (2 · 20 mL), washed with satu-
rated aqueous NaHCO3 (10 mL), dried over MgSO4,
and concentrated. The residue was passed through a
short silica gel column with hexanes and concentrated
to give tert-butyldimethyl(2-hexylbut-3-enyloxy)silane.
Enantiomeric excess of the product was determined by
conversion to the corresponding MTPA-esters [16] after
protodesilylation and their 1H NMR analysis. The abso-
lute configuration was confirmed by conversion to the
known 2-(tert-butyldimethyl-silyloxymethyl)octan-1-ol
[17] by ozonolysis and the following reduction with
NaBH4 and comparison of the optical rotation with that
reported.
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(1968) 141;
3.4.1. tert-Butyldimethyl(2-hexylbut-3-enyloxy)silane
1H NMR (CDCl3, 270 MHz) d 5.62 (ddd, J = 8.4,
9.7, 17.8 Hz, 1H), 4.97-5.07 (m, 2H), 3.52 (dd, J = 6.3,
11.2 Hz, 1H), 3.49 (dd, J = 6.3, 11.2 Hz, 1H), 2.07–
2.23 (m, 1H), 1.10–1.60 (m, 10H), 0.89 (s, 9H), 0.88 (t,
J = 7.1 Hz, 3H), 0.07 and 0.04 (2s, each 3H); 13C
NMR (CDCl3, 67.5 MHz) d 140.6, 115.2, 66.7, 31.8,
30.8, 29.4, 27.0, 26.0, 22.7, 18.4, 14.1, ꢀ5.3, ꢀ5.4; IR
(neat) 3076, 2926, 2854, 1644, 1470, 1383, 1365, 1254,
1101, 1008, 942, 915, 837, 777 cmꢀ1. Anal. Calc. for
C16H34OSi; C, 71.04; H, 12.67. Found; C, 70.64; H,
13.00.
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