Grignard allylic substitution reaction catalyzed by 1,2,3-triazol-5-ylidene magnesium complexes

Article abstract of DOI:10.1016/j.tetlet.2012.12.124

Allylic chlorides and phosphates reacted with alkyl-Grignard reagent in an S_N2′-selective manner in the presence of a catalytic amount of 1,2,3-triazol-5-ylidenes to provide α-branched alkenes. Copyright

Full text of DOI:10.1016/j.tetlet.2012.12.124

Tetrahedron Letters 54 (2013) 1360–1363
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Tetrahedron Letters
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Grignard allylic substitution reaction catalyzed by 1,2,3-triazol-5-ylidene
magnesium complexes
⇑
Ryosuke Nomura, Yuji Tsuchiya, Hiroyuki Ishikawa, Sentaro Okamoto
Department of Material and Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
a r t i c l e i n f o
a b s t r a c t
Allylic chlorides and phosphates reacted with alkyl-Grignard reagent in an SN20-selective manner in the
Article history:
Received 17 November 2012
Revised 14 December 2012
Accepted 20 December 2012
Available online 10 January 2013
presence of a catalytic amount of 1,2,3-triazol-5-ylidenes to provide a-branched alkenes.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Allylic substitution
Grignard reagent
SN20-selective
12,3-Triazol-5-ylidene
Controlling the reactivities and selectivities of nucleophilic
organometallic compounds, RmMXn, is key to the success of selec-
tive bond formation. The electronic and/or steric nature of these
compounds can be altered by the coordination of Lewis-basic sol-
vents or chelating agents and/or by complexation with other inor-
ganic or organometallic materials such as LiCl and Li(acac).1 Such
reactivity control is accepted as a versatile means of developing
and tuning these reactions. However, most of these schemes re-
quire stoichiometric or solvent quantities of the controlling re-
agent.2 Catalytic use of Lewis-basic reagents is most efficient for
controlling the regiochemical and stereochemical course of a reac-
tion, as exemplified by asymmetric catalyses developed for the Le-
wis-base-activated aldol3 and allylation4 reactions of carbonyl
compounds.
Recently, Lewis-base-catalyzed, Cu-free allylic substitution
reactions with organomagnesium and organozinc reagents have
been reported. Lee and Hoveyda reported that in asymmetric
allylic alkylation reactions with Grignard reagents using chiral N-
heterocyclic carbenes5 (NHCs) as Lewis-base catalysts,6 magne-
sium 4,5-dihydroimidazol-2-ylidene alkoxide was postulated as
an active catalyst. This was based on the observation that the lack
of a phenolic OH in the side-chain of the precursor imidazolium
salt caused low reactivity and selectivity. Kondo et al. reported that
the allylic substitution of 4-chloro-2-alkylbutenoates with dialkyl-
zinc reagents in the presence of catalytic phosphazene base (t-Bu-
P4 base) and LiCl selectively yields SN20 products.7 However, these
investigations have been restricted to the reaction of 4-chloro-2-
alkylbutenoates (tiglic acid esters). Alexakis and co-workers have
reported an improved, efficient asymmetric allylic substitution
procedure based on chiral imidazol-2-ylidenes with a side-chain
bearing a phenolic OH as an N-substituent.8 These findings clearly
demonstrate the effects of Lewis-base coordination in altering the
reactivity of RMgX and R2Zn reagents. Recently, we have developed
c-selective Grignard allylic substitution reactions of c-alkyl- or -
aryl-substituted allylic chlorides and phosphates, 1 catalyzed by
imidazol-2-ylidenes (imd-2 carbenes) as well as imidazole-5-yli-
denes (imd-5 carbenes) (Eq. 1), which can be possibly generated
in situ from the corresponding imidazolium salts such as 4a–d
and 5a–e (Scheme 1).9
It has been observed that the reactions with imidazol-2-ylidene
(imd-2) were generally slower and more selective while those
with imidazol-5-ylidene (imd-5) were faster and less selective.
Herein we report the
c-selective Grignard substitution reac-
tions of allylic chlorides and phosphates 1 catalyzed by 1,2,3-tria-
zol-5-ylidenes (taz-5 carbenes),10 which may be generated in situ
from the corresponding triazolium salts 6a–d (Scheme 1). It was
found that, in comparison to catalysis with imd-2 or imd-5, reac-
tions with taz-5 were reasonably rapid with a higher SN20-(
c-
)selectivity, which enables the construction of a quaternary carbon
center from 3,3-disubstituted allylic chlorides. It has been reported
that 1,2,3-triazol-5-ylidenes were coordinated to various metals
and their donor properties were estimated to be compatible with
the most basic 2-imidazolylidenes, albeit being slightly less basic
than other abnormal carbenes.11 This report presents the first
example of catalytic use of 1,2,3-triazol-5-ylidene magnesium
⇑
Corresponding author. Tel: +81 45 481 5661; fax: +81 45 413 9770.
E-mail address: okamos10@kanagawa-u.ac.jp (S. Okamoto).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.12.124

R. Nomura et al. / Tetrahedron Letters 54 (2013) 1360–1363
1361
Scheme 3. Reaction of 1a with n-BuMgCl.
Table 1
Reaction of 1a with n-BuMgCl in the presence of 5 mol % of an imidazolium salt, 4a–d
or 5a–e or triazolium salt 6a–da
Run
Catalyst (5 mol %)
Conversionb
2a:3ab
(a:c)
1c
2c
3c
4c
5
6
7
8
4a
4b
5a
5b
6a
6b
6c
6d
43
36
99
96
96
99
93
98
1:99
2:98
4:96
6:94
1:99
2:98
1:99
5:95
a
Compound 1a (1.0 mmol), n-BuMgCl (1.5 mmol, 0.90 M in THF), 4, 5, or 6
Scheme 1. N-heterocyclic carbine-catalyzed allylic Grignard substitution.
(0.05 mmol), THF (5 mL).
b
Determined by 1H NMR analysis of the crude mixture using an internal
standard.
c
complexes in organic transformation and comparison of their reac-
tivity with that of imidazol-2-ylidene and imidazole-5-ylidene
complexes.
Previous results (see Ref. 9).
mainly attribute to an electronic reason (runs 1, 2, and 6). The
TLC analyses of the reaction mixtures indicated that the reaction
with 5a was faster than that with 6b. Overall, 1,2,3-triazol-5-yli-
denes (taz-5 carbenes) was able to combine both high reactivity
and SN20-selectivity.
With these results in hand, we further studied the reaction of
various allylic substrates 1 illustrated in Figure 1 and several Grig-
nard reagents in the presence of 5 mol % of the representative salts
of 4a (or 4b), 5a, or 6b (Table 2).14 For comparison, 4a, 4b, and 6b
having the same substituents at the carbene–carbon b were
chosen.
For the reaction of a secondary Grignard reagent (i-PrMgCl)
(runs 1–3), only taz-5 carbene from 6b gave satisfactory yield
and selectivity (run 3). The 4b- and 5b-catalyzed reactions showed
much lower selectivity. Meanwhile, the reaction with the
aryl-Grignard reagent (PhMgCl) resulted in a low selectivity to pre-
Trisubstituted 1,2,3-triazolium salts 6a–d were synthesized by
the copper-catalyzed azide-alkyne [3+2] cycloaddition (Hüisgen
reaction)12 followed by inter- or intra-molecular N-alkylation reac-
tion (Scheme 2).13
Initially, we investigated the reaction of trans-1-chloro-3-non-
ene (1a) with n-BuMgCl in the presence of various imidazolium
salts, 4a,b or 5a,b or triazolium salts 6a–d to compare their cata-
lytic activity and selectivity (Scheme 3). The reactions were
quenched after 6 h and the results are shown in Table 1. The reac-
tion in the absence of an imidazolium or triazolium salt was very
slow and yielded only a trace amount of the SN2 product 2a (data
not provided). As shown in the table, all imidazolium and triazoli-
um salts cleanly catalyzed the substitution reaction to yield a mix-
ture of 2a and 3a. In all cases, the SN20 product 3a was obtained
with high selectivity. The 1,2,3-trisubstituted imidazolium salts
5a,b and 1,3,4-trisubstituted 1,2,3-triazolium salts 6a–d, which
may be deprotonated by the reaction with a Grignard reagent to
respectively generate imd-5 or taz-5 carbenes, effectively cata-
lyzed the reaction to complete in <6 h at room temperature (runs
3–8). Meanwhile, the reactions with 1,3-disubstituted imidazoli-
um salts 4a and 4b —precursors of the corresponding imd-2 carb-
enes—were relatively slow (runs 1 and 2), when the reaction for 6 h
did not complete and over 55% of the substrate 1a was recovered.
Since 4a (or 4b) and 6b have the same substituents (mesityl) at the
carbene–carbon b, the observed difference of reactivity may
dominantly give the
a-substituted product (runs 4 and 5). The
6b-catalyzed reactions with silyloxymethyl- and secondary alkyl-
substituted substrates, 1b and 1c, also proceeded very selectively
(runs 6 and 7). The imd-2 or taz-5 carbene from 4a or 6b, respec-
tively, selectively catalyzed the reaction of cinnamyl chloride (1d)
and phosphate 1e to afford the corresponding SN20 product 3d in
excellent yield (runs 8, 10, 11, and 13) but the reactions with
imd-5 carbene from 5a showed relatively low selectivity (runs 9
and 12). All 4b-, 5a-, 6b- and IMes-CuCl-catalyzed reactions of
the conjugated diene substrate 1f produced a mixture of SN20
(a
-) and SN20
(c-) products 2f and 3f, but the e-substituted
Scheme 2. Preparation of 1,2,3-triazolium salts 6a–d.
Figure 1. Allylic substrates.

1362
R. Nomura et al. / Tetrahedron Letters 54 (2013) 1360–1363
Table 2
Reaction of various 1 with RMgCl in the presence of 5 mol % of an imidazolium salt or triazolium salta
Run
1
R
Catalyst (5 mol %)
Products
Yield (%), time (h)
98 (13)
2:3b
(a:c)
1
1a
i-Pr
4b
21:79
2
3
1a
1a
i-Pr
i-Pr
5a
6b
2a0 + 3a0
2a0 + 3a0
36 (11)
99 (6)
58:42
2:98
4
5
1a
1a
Ph
Ph
5a
6b
98 (24)
98 (10)
85:15
65:35
2a00 + 3a00
6
1b
n-Bu
6b
96 (3)
6:94
7
8
1c
n-Bu
n-Bu
6b
4a
99 (12)
92 (4)
10:90
8:92
1d
9c
10c
11
1d
1d
1e
1e
1e
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
5a
6b
4a
5a
6b
2d + 3d
2d + 3d
2d + 3d
2d + 3d
2d + 3d
92 (2)
15:85
3:97
4:96
32:64
11:89
94 (1.5)
99 (0.5)
99 (0.5)
98 (0.5)
12c
13c
14
1f
n-Bu
4b
96 (4)
35:65
15
16
17
1f
1f
1f
n-Bu
n-Bu
n-Bu
5a
6b
2f + 3f
2f + 3f
2f + 3f
96 (4)
99 (4)
99 (3)
11:89
9:91
11:89
IMes-CuClc,d
18
1g
n-Bu
6b
99 (6)
3:97
19c
20
1h
1h
n-Bu
n-Bu
6b
6b
99 (9)
91 (4)
37:63
4:96
2h + 3h
a
b
c
1 (1.0 mmol), RMgCl (1.5 mmol, 0.90–1.2 M in THF), 4, 5 or 6 (0.05 mmol), THF (5 mL).
Determined by 1H NMR analysis of the crude mixture using an internal standard.
Previous results (see Ref. 9).
d
IMes = 1,4-di(2,4,6-trimethylphenyl)imidazol-2-ylidene.
compound(s) were not detected (runs 14–17). Among them, 5a-
and 6b-catalyzed reactions were highly SN20-selective and comple-
mentary to the copper-catalyzed reaction. It was found that catal-
ysis with taz-5 carbene from salt 6b can be applied to the
construction of a quaternary carbon center. Thus, the c,c-disubsti-
tuted substrates, 1g and 1h, could be readily converted into the
corresponding SN20 product with a high selectivity by employing
1,2,3-triazolium salt catalyst 6b (runs 18 and 20), while using
imd-5 carbene from 5a again resulted in low selectivity (run 19).
As shown in Scheme 4, taz-5 from 6b also catalyzed c-selective
propargylic substitution reactions. Thus, the reaction of propargy-
lic phosphate 7 with n-BuMgCl in the presence of 5 mol % of 6b in
THF at room temperature proceeded smoothly in an SN20 manner
to predominantly produce 1,1-dibutyl-1,2-propadiene (8) (68% iso-
lated yield).
We postulated the organometallic compounds involved in the
reaction mixture using triazolium salts 6, as illustrated in Scheme 5,
where 1,2,3-triazolium salts 6 were deprotonated by the reaction
with one equivalent of Grignard reagent to generate the corre-
Scheme 5. Postulated mechanism.
sponding carbene, 1,2,3-triazol-5-ylidene, and determined yields
of the corresponding NHC–MgX2 (I) complexes. Under catalytic
conditions, subsequent substitution and complexation of the
resulting NHC–MgX2 complex with the Grignard reagents might
yield NHC–MgRX (II), NHC–MgR2 (III), an ate complex (NHC–
MgR3)ꢀ(MgX)+ (IV), and their solvated derivatives.
To confirm the active intermediate(s) among this scheme, stoi-
chiometric reactions were investigated (Table 3). Thus, to a mix-
Scheme 4. Propargylic substitution catalyzed by 6b.

R. Nomura et al. / Tetrahedron Letters 54 (2013) 1360–1363
1363
Table 3
Results of the stoichiometric reactions of 1d with 6a (1 equiv.) and n-BuMgCl (1.95–3.77 equiv)a
c
e
Run
1
Equivalent of Grignardb (equiv as an anion source)
1.95 (0.95)
Postulated reagent typesd (equiv)
Conversion, % (2d:3d)
>99 (49:51)
I (0.05)
+II (0.95)
II (0.14)
+III (0.86)
III (0.23)
+IV (0.77)
2
3
4
2.86 (1.86)
>99 (17:83)
>99 (10:90)
>99 (12:88)
3.77 (2.77)
1.5 with 5 mol % of 6a (catalytic)
a
Compound 1d (1.0 mmol), n-BuMgCl (1.95–2.77 mmol, 0.90 M in THF), 6a (1.0 mmol), THF (5 mL), 0 °C, 38, 12, 12, and 9 h for runs 1, 2, 3, and
4, respectively.
b
The Grignard reagent was titrated prior to use.
Equivalent for deprotonation of 6a was deducted.
c
d
e
Postulated considering the reagent stoichiometry and the mechanism depicted in Scheme 5.
Determined by 1H NMR analysis of the crude mixture using an internal standard.
(e) Nakajima, M.; Tomioka, K.; Koga, K. Tetrahedron 1993, 49, 9751; (f) Noyori,
R.; Kitamura, M. Angew. Chem., Int. Ed. Engl. 1991, 30, 49.
3. Denmark, S. E.; Beutner, G. L.; Wynn, T.; Eastgate, M. D. J. Am. Chem. Soc. 2005,
127, 3774.
4. (a) Chelucci, G.; Thummel, R. P. Chem. Rev. 2002, 102, 3129; (b) Denmark, S. E.;
Fu, J. Chem. Rev. 2003, 103, 2763.
ture of triazolium salt 6a (1.0 equiv) in THF was added dropwise a
THF solution of n-BuMgCl (equiv indicated in the table) at 0 °C.
After being stirred for 1 h at room temperature, a solution of 1d
(1.0 equiv) in THF was added.
An analysis of the hypothetical mechanisms illustrated in
Scheme 5 and the reagent stoichiometry suggests that the use of
1.95, 2.86, and 3.77 equiv of the Grignard reagent, respectively,
would generate the following combinations of the reagents:
NHC–MgX2/NHC–MgXR [I (0.05 equiv) + II (0.95 equiv)] (run 1),
NHC–MgXR/NHC–MgR2 [II (0.14 equiv) + III (0.86 equiv)] (run 2),
and NHC–MgR2/[(NHC-MgR3)ꢀ (MgX)+] [III (0.23 equiv) + IV
(0.77 equiv)] (run 3), because one equivalent of the Grignard re-
agent (1.0 equiv to 6a) was consumed in the deprotonation of 6a.
The reaction mixture comprising I and II was relatively slow and
afforded a ꢁ1:1 mixture of 2d and 3d (run 1). The reaction with re-
agent III (a mixture with II) afforded 2d and 3d in a ratio of 17:83
(run 2), with selectivity slightly lower than that of the catalytic
reaction (run 4). The reaction involving reagent IV showed the
SN20-selectivity, compatible with that observed under catalytic
conditions (run 3). These results suggest that the reaction under
catalytic conditions may involve the complexes [NHC–MgR2] (III)
and/or [(NHC–MgR3)ꢀ(MgX)+] (IV) as an active species.
In summary, we have demonstrated that 1,2,3-triazol-5-
ylidenes, generated in situ from the corresponding 1,3,4-trisubsti-
tuted 1,2,3-triazolium salts, effectively catalyzed the SN20-selective
substitution reaction of allylic and propargylic compounds with al-
kyl-Grignard reagents, in which 1,2,3-triazol-5-ylidene magnesium
complexes are proposed as the active species based on the results
of stoichiometric reactions. Since the method is operationally sim-
ple and requires only readily synthesizable 1,3,4-trisubstituted
1,2,3-triazolium salts formed by Click reaction,12,15 this system will
be useful in many applications including asymmetric reactions
using chiral 1,2,3-triazol-5-ylidenes. Comparing with imidazol-
2-ylidenes and imidazole-5-ylidenes, it was found that 1,2,3-tria-
zol-5-ylidenes were a better choice because they were able to
combine both high reactivity and SN20-selectivity. Investigation in
this direction is underway in our laboratories.
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14. Typical procedure for 6-catalyzed reaction of 1: To a mixture of 3-methyl-1,4-
diphenyl-1H-1,2,3-triazol-3-ium iodide (6a, 18.2 mg, 0.05 mmol, 5 mol %) and
THF (5 mL) was added
a THF solution of n-BuMgCl (1.65 mL, 0.91 M,
References and notes
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(2a) in total 92% yield (isolated).
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