Platinum-catalyzed allylation of carbon electrophiles with alkenylsilanes

Article abstract of DOI:10.1016/j.tetlet.2014.12.077

In the presence of catalytic amounts of PtCl₂ and AgSbF₆, (Z)-alkenylsilanes react with various carbon electrophiles at the γ-position to give allylation products. A plausible mechanism for the Pt-catalyzed allylation involves alkene migration of alkenylsilanes to allylsilanes and subsequent allylation of carbon electrophiles.

Full text of DOI:10.1016/j.tetlet.2014.12.077

Tetrahedron Letters 56 (2015) 713–716
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Platinum-catalyzed allylation of carbon electrophiles with
alkenylsilanes
Hidenori Kinoshita a, Ryosuke Kizu a, Gen Inoue b, Masayuki Fujimoto b, Masanori Saito a, Junji Ichikawa b,
Akira Hosomi b, Katsukiyo Miura a,
⇑
a Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University, Sakura-ku, Saitama 338-8570, Japan
b Division of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
In the presence of catalytic amounts of PtCl2 and AgSbF6, (Z)-alkenylsilanes react with various carbon
Received 1 November 2014
Revised 5 December 2014
Accepted 14 December 2014
Available online 19 December 2014
electrophiles at the c-position to give allylation products. A plausible mechanism for the Pt-catalyzed
allylation involves alkene migration of alkenylsilanes to allylsilanes and subsequent allylation of carbon
electrophiles.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Platinum(II) catalysis
Allylation
Alkenylsilane
Allylsilane
One-pot reaction
Allylsilanes have frequently been used for regiospecific allyla-
tion of various carbon electrophiles.1 Since they react with electro-
next tried the allylation of benzaldehyde dimethyl acetal (1a)
(Table 1).8 As expected 1a reacted with 2a (2 equiv) under catalysis
by PtCl2–2AgOTf (10 mol % Pt) at room temperature to give an ally-
lation product 3aa in 78% yield (syn/anti = 63:37) (entry 1). AgSbF6
as well as AgOTf served as an effective cocatalyst although AgBF4
and AgPF6 were not effective (entries 2–4). Without the cocatalyst,
the allylation was not observed (entry 5). The reaction using
AgSbF6 at 40 °C gave 3aa in high yields (entry 8). The use of AgSbF6
at 70 °C achieved an efficient allylation with reduced amounts of
2a (1.2 equiv) and PtCl2 (5 mol %) (entry 9).9 Under the same con-
ditions, the allylation with (E)-2a was rather slow and resulted in a
low yield of 3aa (entry 10).
philes only at the position
c to silicon, use of a- or c-substituted
allylsilanes gives allylation products as single regioisomers. Thus
far a large number of methods for regio-controlled synthesis of
a
- or c
-substituted allylsilanes have been developed.2,3 However,
convenient access to these allylsilanes from inexpensive commer-
cially available compounds is rather limited. There is much room
for development of regio-controlled allylation using more
accessible reagents. We herein report that (Z)-alkenylsilanes serve
as
c-substituted allylsilane equivalents for allylation of carbon
electrophiles under catalysis by a cationic Pt(II) species.4
We have previously reported the Pt(II)-catalyzed annulation of
hydroxyalkenylsilanes with aldehydes leading to 2,3-disubstituted
THPs and THFs.5 The reaction mechanism involves alkene migra-
tion of the alkenylsilane, the formation of an oxocarbenium ion
from the hydroxy group and an aldehyde, and intramolecular ally-
lation of the electrophilic carbon by the allylsilane part formed. On
the basis of the previous work, our interest was focused on the
Pt(II)-catalyzed intermolecular allylation of carbon electrophiles
with b-substituted alkenylsilanes.6 Initially the reaction of benzal-
dehyde with alkenylsilane 2a was carried out in the presence of
PtCl2–2AgOTf; however, no allylation product was obtained.7 We
The optimized conditions were applied to the allylation of var-
ious acetals (Table 2). Acetals 1b–d derived from 4-substituted
benzaldehydes efficiently underwent the Pt(II)-catalyzed allylation
with 2a to afford the corresponding homoallylic ethers 3ba–da
(entries 2–4). Under the same conditions, acetal 1e derived from
4-methoxybenzaldehyde was converted into a complex mixture
of products. This is probably due to the instability of the allylation
product 3ea under the Lewis acidic conditions. Shortening the
reaction time enabled the isolation of 3ea (entry 5). The allylation
of acetal 1f derived from cyclohexanecarbaldehyde proceeded in a
moderate yield (entry 6).
As shown in entries 7–11, the Pt(II)-catalyzed reaction was also
valuable for the allylation of aminals 4a–e derived from aromatic
aldehydes and methyl carbamate. With half amounts of PtCl2
⇑
Corresponding author. Tel.: +81 48 858 3514; fax: +81 48 858 3516.
E-mail address: kmiura@apc.saitama-u.ac.jp (K. Miura).
http://dx.doi.org/10.1016/j.tetlet.2014.12.077
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

714
H. Kinoshita et al. / Tetrahedron Letters 56 (2015) 713–716
Table 1
Table 3
Optimization of reaction conditionsa
Allylation with other alkenylsilanes 2a
Entry
AgX
Temp (°C)
Time (h)
Yieldb (%)
syn/antic
Entry
1a or 4a
2: R
Product
Yieldb (%)
syn/antic
1
2
3
4
5
6
7
8
AgOTf
AgSbF6
AgPF6
AgBF4
none
AgOTf
AgOTf
AgSbF6
AgSbF6
AgSbF6
rt
rt
rt
rt
24
24
24
24
24
12
1.5
10
4
78
60
28
8
63:37
73:27
72:28
70:30
—
64:36
61:39
74:26
63:37
61:39
1d
2
3
4
5
1a
1a
1a
4a
4a
4a
2b: Ph
3ab
3ac
3ad
5ab
5ae
5af
76
65
61
60
62
86
70:30
68:32
70:30
50:50
60:40
67:33
2c: PhCO2(CH2)3
2d: MeO(CH2)3
2b: Ph
2e: TIPSO(CH2)3
2f: Me
rt
0
40
70
40
70
70
71
72
89
84
26f
6
a–c
See footnotes a–c in Table 2.
The reaction time was 4 h.
9d
10d,e
d
24
a
Reaction conditions: 1a (0.50 mmol), 2a (Z/E = >98:2, 1.00 mmol), PtCl2
alkenylsilanes 2b, 2e, and 2f to afford the corresponding homoal-
lylic amines 5ab, 5ae, and 5af, respectively (entries 4–6).
The Lewis or Brønsted acid-catalyzed three-component
coupling of aldehydes, oxygen or nitrogen donors, and allylsilanes
(0.05 mmol), AgX (0.10 mmol), and CH2Cl2 (1.5 mL, entries 1–5) or (CH2Cl)2 (1.5 mL,
entries 6–9).
b
Isolated yield.
c
Determined by 1H NMR analysis.
d
With 2a (0.60 mmol), PtCl2 (0.025 mmol), and AgSbF6 (0.05 mmol).
The E-isomer of 2a ((E)-2a, Z/E = 3:97) was used.
(E)-2a was recovered in 60% yield.
e
is valuable as
a direct approach to homoallylic ethers or
amines.10,11 The Pt(II)-catalyzed allylation using alkenylsilanes is
applicable to the three-component coupling. As shown in Scheme 1,
when a mixture of benzaldehyde, trimethyl orthoformate, and 2a
was treated with catalytic amounts of PtCl2 and AgSbF6, homoal-
lylic ether 3aa was formed in good yields. The three-component
coupling reaction using methyl carbamate, a nitrogen donor, also
proceeded successfully.
f
Table 2
Allylation of acetals 1 and aminals 4a
In the Pt(II)-catalyzed allylation of 1a with 2a, the formation of
allylsilane 6a was observed. This implies that the allylation pro-
ceeds by alkene migration of 2a and subsequent allylation of 1a
with 6a. To ascertain the postulated mechanism, we first examined
alkene migration of 2a. When only 2a was treated with a catalytic
amount of PtCl2 or AgSbF6, the isomerization of 2a to 6a did not
occur in each case. The combined use of PtCl2 and AgSbF6 effec-
tively promoted the isomerization to give 6a (E/Z = ca. 7:1) mainly
with (E)-2a and other isomers (Scheme 2).12,13 We next examined
the allylation of 1a with 6a. A catalytic amount of PtCl2 or AgSbF6
did not serve for the allylation of 1a. In contrast, the combination
of PtCl2 with AgSbF6 achieved an efficient allylation of 1a
(Scheme 3).8 Judging from these results, the allylation mechanism
involves a consecutive isomerization-allylation process, in which a
Lewis acidic, cationic Pt(II) species generated from PtCl2 and
AgSbF6 promotes both steps by activation of the C–C double bond
and the acetal C–O bond.14
Entry
1 or 4: R1
Time (h)
Product
Yieldb (%)
syn/antic
1
2
1a: Ph
4
10
24
4
0.5
2
24
24
24
24
24
24
3aa
3ba
3ca
3da
3ea
3fa
5aa
5ba
5ca
5da
5ea
5fa
84
93
76
74
26
65
91
93
84
86
74
0
63:37
70:30
68:32
40:60
47:53
75:25
62:38
62:38
68:32
64:36
55:45
—
1b: 4-Br-C6H4
1c: 4-O2N-C6H4
1d: 4-Me-C6H4
1e: 4-MeO-C6H4
1f: Cy
3
4d
5
6
7
8
9
10
11
12
4a: Ph
4b: 4-Br-C6H4
4c: 4-O2N-C6H4
4d: 4-Me-C6H4
4e: 4-MeO-C6H4
4f: Cy
a
Reaction conditions in entries 1–6: an acetal 1 (0.50 mmol), 2a (0.60 mmol),
PtCl2 (0.025 mmol), AgSbF6 (0.05 mmol), and (CH2Cl)2 (1.5 mL) at 70 °C. Reaction
conditions in entries 7–12: an aminal (1.00 mmol), 2a (1.20 mmol), PtCl2
The present Pt(II)-catalyzed one-step reaction is not useful for
stereoselective synthesis of homoallylic ethers and amines. The
low stereoselectivity observed is probably due to relatively high
reaction temperature at the allylation step. Therefore, to improve
the stereoselectivity, we attempted a one-pot allylation by a
4
(0.025 mmol), AgSbF6 (0.05 mmol), and (CH2Cl)2 (3.0 mL) at 70 °C.
b
Isolated yield.
Determined by 1H NMR analysis.
c
d
With PtCl2 (0.05 mmol) and AgSbF6 (0.10 mmol).
(2.5 mol %) and AgSbF6 (5 mol %), the aminal allylation gave
homoallylic amines 5aa–ea in good to high yields. Unlike 1e,
aminal 4e underwent a successful allylation without a rapid
decomposition of 5ea (entry 11). Aminal 4f derived from cyclohex-
anecarbaldehyde did not react with 2a at all (entry 12).
Other alkenylsilanes also served as allylating agents under the
Pt(II) catalysis (Table 3). The reaction of 1a with alkenylsilanes
2b (R = Ph) gave the corresponding homoallylic ethers 3ab in good
yields (entry 1). Alkenylsilanes 2c and 2d bearing an oxygen
functionality showed lower reactivity than 2a (entries 2 and 3).
It may be due to deactivation of the Pt(II) species by coordination
of the functional group. Aminal 4a as well as 1a reacted with
Scheme 1. Three-component coupling reaction.

H. Kinoshita et al. / Tetrahedron Letters 56 (2015) 713–716
715
Table 4
Allylation of other carbon electrophilesa
Entry
7: E-Y
Product
Isolated yield (%)
Method Ab Method Bb
1
7a: Ph2CH-OMe
8a
8a
8a
8d
8d
8f
8g
8h
8i
60, 81c,d
84
43
2
3
4
5
6
7
8
9
7b: Ph2CH-OAc
7c: Ph2CH-Cl
7d: p-AnCH2-OAc
7e: p-AnCH2-Cl
7f: MeOCH2-Cl
7g: Ac-OAc
94
0
77, 98c
0e
Tracee, 79f
71g
Scheme 2. Pt-catalyzed alkene migration.
0
0
74
56
65, 74h
41
7h: Bz-OBz
7i: Piv-OPiv
Trace, 85i
51, 93j
60
a
Reaction conditions: 7 (1.00 mmol), 2a (1.40 mmol), PtCl2 (0.025 mmol), AgSbF6
(0.050 mmol) and (CH2Cl)2 (3 mL).
b
Method A: a mixture of all the listed compounds was stirred at 70 °C for 24 h.
Method B: first, a mixture of 2a, PtCl2, and AgSbF6 in (CH2Cl)2 was stirred at 70 °C
for 2 h. Then, 7 was added. The resultant mixture was stirred at 70 °C for 24 h.
c
With 2a (1.60 mmol).
With PtCl2 (0.10 mmol) and AgSbF6 (0.20 mmol).
With recovery of 7d.
d
Scheme 3. Pt-catalyzed allylation.
e
f
Allylation with BF3ÁOEt2 (1.00 mmol) at 30 °C for 2 h.
g
two-step method using an additional Lewis acid (Scheme 4). The
former step is the Pt(II)-catalyzed alkene migration of an alkenylsi-
lane at 70 °C and the latter step is the Lewis acid-promoted allyla-
tion of a carbon electrophile with the resultant allylsilane at lower
temperatures. When the latter step was conducted with BF3ÁOEt2
in MeCN at À30 °C, the allylation of acetal 1a with 2a gave 3aa
in 96% yield with 87% syn selectivity. Without solvent displace-
ment from CH2Cl2 to MeCN, the syn selectivity dropped to 81%.
The stepwise allylation of 1f also showed much better stereoselec-
tivity than the one-step reaction. A similar stereochemical out-
come was obtained in the allylation of aminal 4a. Although 4f
did not undergo the one-step allylation with 2a (Table 2, entry
12), the stepwise method using BF3ÁOEt2 efficiently converted 4f
into the allylation product 5fa with high syn selectivity.
The Pt(II)-catalyzed allylation with alkenylsilanes is applicable
to other carbon electrophiles (Table 4). Ether 7a and ester 7b
underwent the one-step allylation (method A) with 2a to afford
the product 8a (entries 1 and 2). As expected from the leaving
groups, 7b showed higher reactivity than 7a. Adding 7b after the
isomerization of 2a, that is, a stepwise method (method B)
promoted the allylation more efficiently. Interestingly, method A
Allylation at 30 °C for 18 h.
h
Allylation for 4 h.
i
Allylation with BF3ÁOEt2 (0.50 mmol).
j
Allylation with BF3ÁOEt2 (0.40 mmol) for 20 h.
completely suppressed the allylation of chloride 7c, while the reac-
tion by method B formed 8a in good yields (entry 3). Similar results
were observed in the reaction of chlorides 7e and 7f (entries 5 and
6). In these cases using method A, the chlorides 7 were decom-
posed and the isomerization of 2a to 6a hardly proceeded. This is
probably due to fast chloride abstraction from 7 by AgSbF6. Allyla-
tion of ester 7d resulted in failure even by method B. However, it
was successfully achieved by a modified method using BF3ÁOEt2
(entry 4). Carboxylic anhydrides were also allylated under the Pt(II)
catalysis (entries 7–9). The use of BF3ÁOEt2 was effective in the ally-
lation of carboxylic anhydrides 7h and 7i.
In conclusion, we have developed novel allylation reactions of
carbon electrophiles with (Z)-alkenylsilanes, which serve as pre-
cursors of
c-substituted allylsilanes under catalysis by PtCl2 and
AgSbF6. It is noteworthy that the Pt(II) catalysis promotes both
alkene migration of alkenylsilanes and subsequent allylation.14
Although the one-step allylation of acetals and aminals does not
show good stereoselectivities, the stepwise allylation using an
additional Lewis acid is useful for stereoselective synthesis of
homoallylic ethers and amines. The reasonable compatibility with
functional groups, a wide range of carbon electrophiles and easy
access to (Z)-alkenylsilanes make these allylation reactions syn-
thetically useful.4
Acknowledgment
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Area ‘Synergistic Effects for Creation of Func-
tional Molecules’ (No. 20036010) from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan.
Supplementary data
Supplementary data (experimental details and characterization
data 1H NMR, 13C NMR, IR, elemental analysis) associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2014.12.077.
Scheme 4. Diastereoselective allylation.

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H. Kinoshita et al. / Tetrahedron Letters 56 (2015) 713–716
6. A related allylation of aldehydes with alkenylboronates has been reported by
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