Copper(I)-catalyzed allylic substitution of silyl nucleophiles through Si-Si bond activation

Article abstract of DOI:10.1002/adsc.201100819

We report the first copper(I)-catalyzed reaction between a disilane and allylic carbonates to produce allylsilanes. The silyl nucleophilic substitution proceeded primarily in a S_N2 fashion whereas S_N2' products were also obtained through σ-π-σ isomerization of the copper(III) intermediate depending on the substrate's steric bulk. Copyright

Full text of DOI:10.1002/adsc.201100819

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DOI: 10.1002/adsc.201100819
Copper(I)-Catalyzed Allylic Substitution of Silyl Nucleophiles
À
through Si Si Bond Activation
Hajime Ito,a,* Yuko Horita,a,b and Masaya Sawamurab
a
Division of Chemical Process Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan
Fax : (+81)-(0)11-706-6561; e-mail: hajito@eng.hokudai.ac.jp
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
b
Received: October 20, 2011; Published online: March 13, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100819.
Abstract: We report the first copper(I)-catalyzed
reaction between a disilane and allylic carbonates
to produce allylsilanes. The silyl nucleophilic substi-
tution proceeded primarily in a SN2 fashion whereas
SN2’ products were also obtained through s-p-s iso-
merization of the copperACTHNUTRGNE(UNG III) intermediate depend-
ing on the substrateꢀs steric bulk.
Keywords: copper; homogeneous catalysis; metala-
tion; silanes; synthetic methods
Scheme 1. Copper(I)-catalyzed silylation of a,b-unsaturated
carbonyl compounds (a) and allylic esters (b) with disilanes.
Reactions between silyl nucleophiles and carbon elec-
trophiles provide indispensable synthetic procedures catalysts (Hosomi–Sakurai allylation).[7,8] The devel-
for organosilicon compounds. Trialkylsilylmetal com- opment of an efficient preparation method for allylsi-
pounds that are prepared from stoichiometric silyl lanes has attracted significant interest for a long
halides and low-valent metals, typically lithium, have time.[1–10] Many studies have been devoted to the reac-
been extensively studied especially in combination tion between stoichiometric silylmetal/copper(I) re-
with stoichiometric copper(I) salts.[1] Recent studies agents and allylic electrophiles.[9]
have focused on more convenient procedures such as
Oestreich and co-workers recently reported a cop-
the catalytic generation of silyl nucleophiles through per(I)-catalyzed silylation using silylborane as well as
s-bond metathesis reactions of transition metal com- silylzinc reagents.[5c,9f] However, there have been no
[3]
[4]
[5]
plexes[2] (Si Si/Pd, Si B/Rh, Si B/Cu or Si Si/ reports on the use of disilanes as the silyl group
Cu[6]). The practical advantage of these procedures source in copper(I)-catalyzed allylsilane synthesis. We
is that they enable the use of easily storable and are interested in investigating disilane/copper(I) catal-
À
À
À
À
commercially
available
precursors
such
as ysis and thus report here the regioselective silylation
À
À
PhMe2Si B
N
ophile sources. We have previously reported the first presence of CuCl catalyst and K
(O-t-Bu). The regio-
example of a copper(I)-catalyzed reaction between selectivity was primarily SN2 but it also depended
disilanes and a,b-unsaturated carbonyl compounds heavily on the steric nature of the substrates; the SN2’
À
where the Si Si bond is cleaved by s-bond metathesis product was found to be the major compound when
to form a silylcopper(I) intermediate, which under- the SN2 product had more steric congestion [Scheme 1
goes a formal nucleophilic addition to a,b-unsaturated (b)].
carbonyl compounds [Scheme 1 (a)].[6a]
We initially examined the silyl substitution of allylic
Allylsilanes are particularly useful organosilane re- carbonate (E)-1a using our previous catalyst system,
agents because of their high stereo- and regioselective CuOTf (5 mol%) and PBu3 (5 mol%) in the presence
addition reactions with aldehydes using Lewis acid of disilane 2 (Table 1, entry 1), which is the most ef-
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Table 1. Copper(I)-catalyzed silyl substitution of allylic carbonate 1a.[a]
Entry
Ligand
K
–
(O-tBu) [mol%]
CuX
T [8C]
Yield[b] [%]
3a/3a’
1
2
3
Bu3P
none
none
none
none
none
none
PPh3
Xphos
dppe
dppp
Xantphos
–
CuOTf
CuCl
CuCl
CuCl
CuCl
CuCl
none
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl/NHC
CuCl
100
0
0
-30
-30
0
0
0
0
0
0
6
60
37
44
0
–
–
10
100
100
200
0
100
100
100
100
100
100
100
100
90:10
94:6
95:5
–
4
5[c]
6
7
8
9
10
11
12
13[d]
14
0
–
57
47
53
61
43
61
59
90:10
83:17
86:14
86:14
86:14
90:10
86:14
0
0
0
0
2,2’-bipy
[a]
Reaction conditions: 1a (0.25 mmol), 2 (0.25 mmol), CuCl (0.0125 mmol), ligand (0–0.125 mmol), KACTHNUTRGNE(NUG O-t-Bu) (1.0M,
0.25 mL, 0.25 mmol), THF (0.5 mL).
Yield and regioisomeric ratio were determined by GC analysis. The starting material was consumed within 1 h in almost
[b]
all cases and the main by-product was 4a.
10 mol% of CuCl was used.
ChloroACHTUNGTRENNUNG[1,3-bis(2,6-diisopropylphenyl)-imidazole-2-ylidene]copper(I) was used.
[c]
[d]
fective catalyst for the silylation of a,b-unsaturated likely responsible for the silyl substitution reaction
carbonyl compounds.[6a] However, no reaction oc- (entries 6 and 7). The desired product was scarcely
curred, even at 1008C. We then examined another obtained when other disilanes such as hexamethyldisi-
copper(I) catalyst,
(10 mol%) and CuCl (5 mol%) but only found that gave lower yields than those for allylic carbonates 1a.
the corresponding allylsilane product 3a had formed We next examined the impact of the ligand and ex-
in low yield (entry 2, 6%). A reaction with a larger pected the reaction to be attenuated by the nature of
amount of K(O-t-Bu) (100 mol%) improved the yield the ligand. However, a notable improvement in the
a combination of KACHTNUTGNRUE(GN O-t-Bu) lane were used instead of 2. Allylic acetates or ethers
ACHTUNGTRENNUNG
(60%) in the presence of a catalytic amount of CuCl yield and regioselectivity was not found for the reac-
(5 mol%) (entry 3). The reaction was complete within tions with PPh3, Xphos, dppe, dppp, Xantphos, an N-
1 h at 08C with a full conversion of the substrate. Silyl heterocyclic carbene ligand and 2,2’-bipyridine ligands
ether 4a was the only side product and no other prod- (entries 7–14). In all these reactions the (E)-configu-
ucts were detected. The SN2 product (E)-3a was the ration stereoisomer was found to be the major prod-
major product with high a/g regioselectivity (3a/3a’= uct with high selectivity (>20:1).
90:10). The regioselectivity was improved in this reac-
tion at À308C (3a/3a’=94:6) but the yield decreased substitution of the disilane using various allylic sub-
to 37% (entry 4). Additional K
(O-t-Bu) did not sig- strates (Table 2).[11] (E)-1b, which is a regioisomeric
nificantly improve the yield (entry 5, 44%). Both substrate of (E)-1a in terms of the allylic system, af-
(O-t-Bu) and CuCl were necessary to promote this forded the a-product (E)-3b in a moderate yield and
reaction indicating that the copper(I) species that was regioselectivity (entry 1, 52%, a/g=79:21). The reac-
generated from disilane/CuCl/K(O-t-Bu) is most tion with excess carbonate resulted in a slight im-
We then examined the copper(I)-catalyzed silyl
AHCTUNGTRENNUNG
KACHTUNGTRENNUNG
AHCTUNGTRENNUNG
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Copper(I)-Catalyzed Allylic Substitution of Silyl Nucleophiles through Si Si Bond Activation
Table 2. Copper(I)-catalyzed silyl substitution of allylic carbonates 1.[a]
Entry
Substrate
Product
Yield [%][b]
1[c]
52 (a/g, 79:21)
75 (a/g, 88:12)
72 (a/g, 85:15)
2[c,d]
3[e]
(E)-1b
(E)-3b
4[f]
66 (a/g, 95:5)
68 (a/g, 0:100)
5[e]
6[e]
48% (a/g, 58:42)
7[e]
8[e]
55 (a/g, 99:1)
39 (a/g, 1:>99)
9[e]
48
72
10[f]
11[f]
66
12[e]
66 (a/g, 91:9)
[a]
Reaction conditions: 1a (0.5 mmol), 2 (0.5 mmol), CuCl (0.025 mmol), KACTHNUTRGNEU(GN O-t-Bu) (1.0M, 0.5 mL, 0.5 mmol), additional
THF (0.5 mL).
[b]
Yield and regioisomeric ratio were determined by NMR analysis after chromatographic purification. In almost all the
cases, 1a was fully consumed at 1 h and the main by-product was the silyl ether 4.
0.25 mmol scale.
1a (0.25 mmol), 2 (0.168 mmol) were used. Yield was based on the disilane.
1.2 mL of THF were added.
[c]
[d]
[e]
[f]
1.0 mL of THF was added.
provement in the regioselectivity (75%, a/g=88:12) mediate (entries 4 and 5).[9] The reaction of (Z)-1d
(entry 2). A substrate with methyl and butyl groups gave both the a-product [(Z)-3d] and the g-product
around an internal allylic system [(E)-1c] gave the 3e with a very low regioselectivity (a/g=58:42). It is
corresponding a-product (E)-3c with a similar regio- noteworthy that the a-products retained the double
selectivity (a/g=85:15) compared to that of (E)-1b bond configuration of the starting material [entries 4
(entry 3). The reactions with the substrates containing and 6, (E)-3d], whereas the g-product resulted in
a terminal allylic system [(E)-1d and (E)-1e] both re- a mixture of E/Z isomers (entry 5). The more sterical-
sulted in the selective production of (E)-3d where the ly demanding substrate 1f with dimethyl substitution
À
new Si C bond formed at the terminal position of the at the g-position of the leaving group resulted in very
product. This gave the typical regioselectivity often high regioselectivity (entry 7, a/g=99:1). The sub-
found in the reaction with a p-allyl copperACTHNUGRTNEUNG(III) inter- strate derived from the tertiary allylic alcohol [(E)-1g]
Adv. Synth. Catal. 2012, 354, 813 – 817
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Hajime Ito et al.
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Scheme 2. A plausible reaction mechanism for the copper(I)-catalyzed silylation of allylic carbonate with disilane.
bearing two methyl groups at the a-position of the Experimental Section
leaving group then afforded only the g-product 3g
(entry 8, a/g=1:99) avoiding steric congestion. The si- General Remarks
lylation of carbonates with cyclic structures gave the
Materials were obtained from commercial suppliers and pu-
corresponding allylic silanes (entries 9–11). This catal-
rified by the standard procedure unless otherwise noted.
ysis has moderate functional group compatibility; the
Dry THF was purchased and used as received. NMR spectra
acetal group in (E)-1k can tolerate the silylation con-
ditions (entry 12).
The reaction profile given in Table 2 indicates that
the reaction involves allylic isomerization through the
were recorded on a JEOL JNM-ECX400P spectrometer
(1H: 400 MHz and 13C: 99.5 MHz) or a Varian Gemini 2000
(1H: 300 MHz, 13C: 75.6 MHz). Tetramethylsilane (1H) and
CDCl3 (13C) were employed as external standards, respec-
tively. CuCl (ReagentPlusꢂ grade, 224332–25G, ꢁ99%) and
p-allyl copperACHTUNGTRENNUNG(III) intermediate (Scheme 2). The silyl-
KACTHNUGRTENUNG(O-t-Bu)/THF (1.0M, 328650–50ML) were purchased
copper(I) species 5 was first formed through s-bond
from Sigma–Aldrich Co. and used as received. Mesitylene
was used as the internal standard for determining NMR
yields. Recycle preparative gel permeation chromatography
was conducted with a JAI LC-9101 using CHCl3 as eluent.
Low- and high-resolution mass spectra were recorded at the
Center for Instrumental Analysis, Hokkaido University.
metathesis between the disilane, CuCl and KOR.[12]
À
The oxidative addition of the C O bond on the cop-
per(I) center (A) gave a silylcopper
N
When the steric bulk between two substituents
around the allylic system is similar (R1 ꢀR2), a subse-
quent reductive elimination (B) occurs at the a-posi-
tion to give the SN2 product 3a at a much faster rate
than that of s-p-s isomerization. On the other hand,
when the difference in the steric demands of the a-
position is significantly larger than that of the g-posi-
Representative Procedure for Copper(I)-Catalyzed
Allylic Substitution with Disilane
Copper chloride (2.5 mg, 0.025 mmol) was placed in an
oven-dried reaction vial. The vial was sealed with a screw
cap containing a silicon-coated rubber septum. After the
vial had been evacuated and backfilled with nitrogen, THF
À
tion, the Si C bond forms at the sterically less con-
gested g-position (C) by the s-p-s isomerization of
the allylic copperACHTUNGTRENNUNG
(III) species (R1 >R2), producing the
(1.0 mmol), disilane
2 (0.5 mmol), and KACHTNUTGNRUE(GN O-t-Bu)/THF
SN2’ product 3a’. The erosion of E/Z isomeric purity
that was observed in the SN2’ reaction product
(Table 2, entry 5) is consistent with this explanation.
In summary, we report the first example of allylsi-
lane synthesis by the copper(I)-catalyzed reaction of
a disilane. Similar to other silylmetal/copper(I) sys-
(1.0M, 0.5 mL, 0.5 mmol) were added through the rubber
septum at 08C. Then the allylic carbonate 1 (0.5 mmol) was
added dropwise. After the reaction was complete, the reac-
tion mixture was passed through a short silica column elut-
ing with 1% Et2O/hexane mixed solvent. The crude mixture
was further purified by column chromatography (SiO2,
hexane). When the pure product was not obtained by sever-
al column chromatography purifications, the yield of the re-
action was determined by 1H NMR analysis of the crude
material using an internal standard. The further purification
of the product could be conducted with a recycle gel perme-
ation chromatography.
À
tems the regioselectivity for the newly formed Si C
bond depends on the structure of the substrate. Al-
though there is room for improvement in product
yield as a synthetic method of allylsilanes, these re-
sults provide interesting insights into novel silyl
copper species that are generated by the disilane/
Cu(I)/excess KOR system as well as provide a new
convenient procedure for allylsilane synthesis with
a copper(I) catalyst.
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Copper(I)-Catalyzed Allylic Substitution of Silyl Nucleophiles through Si Si Bond Activation
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Acknowledgements
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This work was financially supported by Funding Program
for Next Generation World-Leading Researchers (NEXT
Program, No. G002) from Japan Society for the Promotion
of Science (JSPS).
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