Synthesis of Benzosiloles by Intramolecular anti-Hydroarylation via ortho-C-H Activation of Aryloxyethynyl Silanes

Article abstract of DOI:10.1021/jacs.7b08055

Straightforward synthesis of benzosiloles was achieved by the invention of Pd/acid-catalyzed intramolecular anti-hydroarylation of aryloxyethynyl(aryl)silanes via ortho-C-H bond activation. The aryloxy group bound to the ethynyl carbon is the key factor for this transformation.

Full text of DOI:10.1021/jacs.7b08055

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Communication
Synthesis of Benzosiloles by Intramolecular anti-Hydroarylation
via ortho-C–H Activation of Aryloxyethynyl silanes
Yasunori Minami, Yuta Noguchi, and Tamejiro Hiyama
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b08055 • Publication Date (Web): 21 Sep 2017
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Synthesis of Benzosiloles by Intramolecular anti-Hydroarylation via
ortho-C–H Activation of Aryloxyethynyl silanes
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Yasunori Minami,*,† Yuta Noguchi,‡ and Tamejiro Hiyama*,†
† Research and Development Initiative Chuo University, 1ꢀ13ꢀ27, Kasuga, Bunkyoꢀku, Tokyo 112ꢀ8551, Japan
‡ Department of Applied Chemistry, Chuo University, 1ꢀ13ꢀ27, Kasuga, Bunkyoꢀku, Tokyo 112ꢀ8551, Japan
Supporting Information Placeholder
Scheme 1. Metal-catalyzed annulation for the synthesis of
ABSTRACT: Straightforward synthesis of benzosiloles was
benzosiloles
achieved by the invention of Pd/acidꢀcatalyzed intramolecular
antiꢀhydroarylation of aryloxyethynyl(aryl)silanes via orthoꢀC–H
bond activation. The aryloxy group bound to the ethynyl carbon is
the key factor for this transformation.
(a) Cyclization of ortho-alkynylphenylsilanes (b) Annulation of aryl vinyl silanes
R2
E
X
E
Pd cat., base
(X = Br)
M cat.
R2
Si
Si
or
Y
2
base
Si
R1
R1
Si
Grubbs cat.
(X = vinyl)
2
R1
2
R1
2
(c) Intramolecular C-H Silylation
(d) Annulation of arylsilanes with alkynes
X
Rh, Fe, Pd cat.
(X = B(OH)2, Li,
Br, OTf, Y = Me)
Siloles and siloleꢀbased condensed aromatics have attracted
increasing attention due to their potential applications, e.g., buildꢀ
ing blocks for organic synthesis and πꢀconjugated functional maꢀ
terials as high electronꢀaffinity, holeꢀblocking and solidꢀstate
luminescence.1,2 For the synthesis of a benzosilole, metalꢀ
catalyzed annulation protocols are available. A typical example is
the annulation of oꢀalkynyl(aryl)silanes with additives such as
electrophiles (Scheme 1a).1g,2c,2d,3 Intramolecular MizorokiꢀHeck
reaction, olefin metathesis, and orthoꢀsilylation of (Z)ꢀ
phenylvinylsilane also lead to benzosilole formation (Scheme 1b,
c).4,5,6 Recently, annulations of alkynes with arylsilanes via Si–Me
or Si–H bond cleavage have been reported (Scheme 1d).7 Howevꢀ
er, as compared to dibenzosiloles, synthetic methods to access
benzosiloles are less developed. In light of the fact that alꢀ
kynyl(aryl)silanes can be easily prepared by the reaction of
chlorosilanes with terminal alkynes and used widely in organic
synthesis, e.g., to protect terminal alkynes, straightforward transꢀ
insertion using alkynyl(aryl)silanes via orthoꢀC–H bond activaꢀ
tion is an ideal synthetic method for benzosiloles (Scheme 1e).
However, this sort of transformation has remained unexplored, in
contrast to indene synthesis which starts with propargylarenes.8
Thus, development of an annulation strategy using alꢀ
kynyl(aryl)silanes is critical for achieving synthetic diversity toꢀ
ward the enhancement of the chemistry of siloles. Herein we reꢀ
port a straightforward synthesis of benzosiloles by palladiꢀ
um/acidꢀcatalyzed intramolecular antiꢀhydroarylation of alꢀ
kynyl(aryl)silanes via C–H bond activation.
R3
Y
Ir cat.
Si
R4
Si
R1
or
SiPh2
H
2
Si
R1
Ph2
+
oxidant (X = Y = H)
R3
R4
2
(e) This work: trans-hydroarylation using alkynyl aryl silanes
R'
H
R'
m
R'
H
Cl
Si
Si
R2
Si
R2
R2
well developed
yet to be solved
transꢀaddition to the alkynyl moiety to form benzosiloles.
We initiated our investigations using 2,6ꢀdiisopropylꢀ
phenoxy(tertꢀbutyldiphenylsilyl)ethyne (1a) based on our previꢀ
ously reported hydroalkylation conditions10f and gratifyingly
found that an intramolecular antiꢀhydroarylation proceeded
smoothly under Pd(dba)2/PEt3/PivOH catalytic conditions to form
product 2a in 95% yield (Table 1, Entry 1).12 A preparative gramꢀ
scale reaction using 1a (1.20 g) was carried out, and 2a was sucꢀ
cessfully obtained in 93% yield (1.11 g, Entry 2). The other carꢀ
bonaceous groups such as 3,5ꢀxylyl (1b) and neopentyl (1c) on
oxygen gave products in lower yields (Entries 3 and 4). These
results show that the diisopropylphenyl group on oxygen is the
most suitable for this annulation due likely to its bulkiness and
lack of hydrogen atoms at the orthoꢀpositions. The reaction using
various phenylsilylalkynes 1dꢀ1h was carried out. Substrates 1d
and 1e having less bulky methyl groups on silicon gave the correꢀ
sponding products (2d and 2e) under suitable conditions, however,
with diminished stability of the substrates and decreased reaction
rates (Entries 5 and 6). On the other hand, carbonaceous substituꢀ
ents bulkier than methyl groups led to an increase in product yield.
Triphenylsilylalkyne 1f also afforded product 2f at a higher temꢀ
perature due to its lower reactivity (Entry 7). In the case of two nꢀ
butyl groups on silicon, annulation proceeded to give 2g in a high
yield although high catalyst loading was required (Entry 8). More
bulky isopropylꢀsubstituted substrate 1h resulted in the best yield
of 2h (98%, Entry 9). Such bulky effects may be the result of the
ThorpeꢀIngold effect, which pushes the phenyl group closer to the
To achieve the metalꢀcatalyzed annulation using alꢀ
kynyl(aryl)silanes, orthoꢀC–H bond activation directed by an
alkynyl group is absolutely required. However, it is difficult to
use an alkynyl group as a directing group for C–H bond activaꢀ
tion.9 We previously disclosed that aryl silylethynyl ethers underꢀ
went Pdꢀcatalyzed annulation with alkynes and alkenes via orthoꢀ
C–H bond activation to construct various complex oxacycles.10,11
Based on these findings, we envisaged that an aryl C–H bond
ortho to an oxyethynylsilyl group can be activated and undergo
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Table 2. Synthesis of condensed siloles a
alkynyl group and stabilizes the silyl group. Of note, the aryloxy
Entry
1
Time (h) 2 (%)b
group is crucial for this annulation: tBuPh2Si(C≡CPh) (3) did not
undergo the annulation at all, indicating that an electronꢀdeficient
alkynyl group derived from oxygen promotes the hydroarylation
due to an effective interaction with the palladium(0) complex.
ODipp
H
MeO
H
MeO
ODipp
ODipp
ODipp
1
20
Si
Si
iPr
iPr iPr
iPr
1i
2i, 98
Table 1. Reaction of oxyethynylsilylarenesa
ODipp
H
Ph2N
H
Ph2N
Pd(dba)2 (5 mol%)
R2
9
O
PEt3 (5 mol%)
H
2
20
Si
Si
O
R2
PivOH (10 mol%)
iPr
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
iPr iPr
iPr
H
toluene (1.0 M)
Si
1j
2j, 99
Si
R1
R1 R1
R1
ODipp
H
F3C
H
1
2
F3C
Entry
1
1
R1
R2
Temp (°C) Time
2 (%)b
2a, 95
2a, 93
2b, 30
2c, 10e
2d, 80
2e, 54
2f, 89
2g, 86
2h, 98
3c
60
Si
Si
iPr
iPr iPr
iPr
1a
1a
1b
1c
1d
1e
1f
tBu, Ph
tBu, Ph
tBu, Ph
tBu, Ph
Me, Ph
Me2
Dipp
100
100
120
140
90
10 h
46 h
24 h
13 h
7 d
1k
1l
2k, 85
2c
Dipp
ODipp
H
ODipp
3d
3,5ꢀXyl
CH2tBu
Dipp
3
H
ODipp
+
H
4d
4
24
Si
Si
Si
1
iPr
iPr
5d
iPr iPr
H
iPr
iPr
2l + 2’l, 73 (71:29)
6d,f
Dipp
90
9 d
OMe
OMe
ODipp
ODipp
H
2
H
7
Ph2
Dipp
140
100
120
22 h
40 h
22 h
ODipp
8 d
9
1g
1h
Bu2
Dipp
5d
26
H +
Si
Si
Si
6
MeO
iPr
iPr
iPr2
Dipp
iPr iPr
H
iPr
iPr
1m
F
2m + 2’m, 86 (33:67)
a 1, Pd(dba)2 (5 mol%), PEt3 (5 mol%), PivOH (10 mol%), in toluene (1.0
M). b Isolated yield. c 1a (1.20 g, 2.72 mmol) was used to give 2a (1.11 g,
2.52 mmol). d Pd(dba)2/PEt3 (10 mol%). e NMR yield. f PnBu3 as a ligand.
PivOH = tBuCO2H, Dipp = 2,6ꢀiPr2ꢀC6H3, 3,5ꢀXyl = 3,5ꢀMe2ꢀC6H3.
2
H
ODipp
6
7
24
24
Si
iPr iPr
1n
2’n, 82
ODipp
H
The scope of the present annulation was examined (Table 2).
Electronꢀdonating groups such as methoxy and diphenylamino at
the paraꢀposition did not affect the reactivity, giving 2i and 2j
quantitatively, whereas the electronꢀwithdrawing trifluoromethyl
group had moderate reactivity and required a high catalyst loading
for a good yield of 2k (Entries 1ꢀ3). The reaction of 2ꢀ
naphthyl(ethynyl)silane 1l underwent the hydroarylation to give
linear naphthosilole 2l as the main product via lessꢀhindered C3–
H bond activation in 1l together with the generation of bent naphꢀ
thosilole 2’l via C1–H bond activation (Entry 4). When 1m bearꢀ
ing a methoxy group at C3 was used in the reaction at 100 °C, the
C2–H bond ortho to the methoxy group was more reactive, giving
rise to 2’m as the major product (Entry 5). In the case of 3ꢀ
fluorinated 1n, the C2–H bond was selectively activated and 2’n
was obtained in good yield (Entry 6).13 The orthoꢀmethoxy group
in 1o did not interfere with the hydroarylation to form 2o (Entry
7). Heteroarylated alkynylsilanes could be applied to this annulaꢀ
tion reaction. Substrates 1pꢀs having 2ꢀ and 3ꢀthienyl and 2ꢀ
benzothienyl groups on silicon were transformed to the correꢀ
sponding thienosiloles 2pꢀs (Entries 8ꢀ11). In the case of 3ꢀthienyl
isomer 1r, the C2–H bond was solely activated without cleavage
of the C4–H bond. The TMS group on the thieno group was tolerꢀ
ated (Entry 9). NꢀMethylindole 1t could be used in the annulation
reaction and the corresponding tricycle 2t was obtained in a high
yield (Entry 12).
ODipp
ODipp
H
Si
Si
iPr
iPr
MeO
iPr iPr
MeO
1o
2o, 80
ODipp
H
H
8
9c
S
20
24
24
24
24
S
Si
Si
iPr
iPr iPr
iPr
1p
2p, 89
ODipp
H
H
ODipp
Me3Si
Me3Si
S
S
Si
Si
iPr
iPr iPr
iPr
1q
2q, 45
ODipp
H
2
S
S
ODipp
H
10
11
12
Si
Si
4
iPr
iPr iPr
iPr
1r
2r, 98
ODipp
H
ODipp
H
S
S
Si
Si
iPr
iPr iPr
iPr
1s
2s, 96
ODipp
H
ODipp
N
N
Si
Si
iPr
Me
Me
iPr iPr
iPr
1t
2t, 73
a 1, Pd(dba)2 (5 mol%), PEt3 (5 mol%), PivOH (10 mol%), in toluene (1.0
When 1ꢀnaphthyl(ethynyl)silane 1u was used in this hyꢀ
droarylation under optimized conditions, C2–H bond activation
occurred to give naphthosilole 2u in conjunction with C8–H bond
activation to form 1ꢀsilaphenalene 4 (eq 1).14 The observed selecꢀ
tivity may be attributed to the steric influence.
M), at 120 °C. b Isolated yield. c Pd(dba)2/PEt3 (10 mol%). d 100 °C.
Pd(dba)2 (5 mol%)
ODipp
PEt3 (5 mol%)
iPr
Si
2
ODipp
PivOH (10 mol%)
H
iPr
(1)
+
Si
Si
toluene (1.0 M)
120 °C, 24 h
The successful reaction was applied to the double annulation
of 4,4’ꢀbis(oxyethynylsilyl)biphenyl 5. The reaction underwent
double hydroarylation using the same molar ratios of the catalysts
as in the single annulation to give 5,5’ꢀbibenzosilole 6 (eq 2).
iPr
H
iPr iPr
iPr
8
ODipp
4, 20%
1u
2u, 43%
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iPr iPr
Si
Pd(dba)2 (10 mol%)
PEt3 (10 mol%)
PivOH (20 mol%)
Finally, the synthetic transformation of the products was exꢀ
amined (Scheme 3). Reduction of 2a took place with Et3SiH unꢀ
der KOtBu conditions19 to provide silaindanes 7. Subsequent oxiꢀ
dation by DDQ gave 8.20 Furthermore, trifluoroacetic acid atꢀ
tacked the C–Si bond in 2 at ꢀ78 °C to form a silyl esters 9 due to
the high electronꢀdonating ability of C2. Subsequent treatment
with TsOH catalyst gave a 2ꢀacetylphenylsilanol 10 with eliminaꢀ
tion of the Dipp group.
DippO
H
toluene (1.0 M)
120 °C, 34 h
H
ODipp
Si
iPr
iPr
5
iPr iPr
Si
ODipp
H
H
(2)
DippO
Si
iPr
6, 69%
iPr
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
To gain an insight into the mechanism, pentadeuteratedꢀ
phenyl substrate 1hꢀd5 was used in the annulation in the presence
of stoichiometric amounts of Pd(0)/PEt3/PivOH. The 2ꢀ
hydrogenated product 2hꢀd5 was mainly obtained in 90% yield
(15% D at C2; eq 3). Moreover, 1h underwent this reaction with 1
equiv of Pd(0)/PEt3 and AcOHꢀd to form the 2ꢀdeuterated product
2hꢀd with 55% deuterium incorporation at C2 (eq 4). The fact that
the deuteration ratio by use of AcOHꢀd was higher than that using
1hꢀd5 clearly indicated that the 2ꢀhydrogen in the product was
derived from the acid. Of note, no H/D scrambling at C7 suggestꢀ
ed that C–H cleavage was irreversible. The kinetic isotope effect
(KIE) observed was 2.83, indicating that the rateꢀlimiting step was
the C–H cleavage step which proceeded via a concertedꢀ
metalationꢀdeprotonation pathway.
Scheme 3. Synthetic transformation
Et3SiH (3 eq)
KOtBu (3 eq)
DDQ
toluene
140 °C, 19 h
CH2Cl2
100 °C
Si
Si
Ph
Ph
ODipp
tBu
tBu
7, 75%
8, 70%
Si
R'
R
2a (R = tBu, R' = Ph)
ODipp
O
CF3CO2H
(1-3 eq)
TsOH
(10 mol%)
2h (R = R' = iPr)
O
CF3
OH
ClCH2CH2Cl
-78 °C, 14 h
H2O (0.25 M)
90 °C, 7 h
Si
R
Si
R'
iPr iPr
10, 60% (NMR)
32% (isolated)
O
9a, 78% (R = tBu, R' = Ph)
9b, 99% (R = R' = iPr)
Pd(dba)2 (1 eq)
PEt3 (1 eq)
PivOH (1 eq)
In conclusion, the present study presents an unprecedented
straightforward synthesis of benzosiloles by the intramolecular
transꢀinsertion of alkynylsilyl groups into an orthoꢀC–H bond.
This method is widely applicable toward a broad range of prodꢀ
ucts from readily available alkynyl(aryl)silanes. The Pd(0)/pivalic
acid catalytic conditions are effective for the hydroarylation, by
initially activating the alkynyl group, followed by C–H activation.
Current efforts are directed toward the development of similar
hydroarylation reactions for straightforward formation of various
condensed (hetero)aromatics.
ODipp
15% D
D
ODipp
2
D4
(3)
H/D
D3
Si
toluene (0.1 mL)
120 °C, 12 h
Si
7
D
iPr
iPr iPr
iPr
>99% D
1h-d5, 0.01 mmol
2h-d5, 90%
Pd(dba)2 (1 eq)
PEt3 (1 eq)
AcOD (1 eq)
ODipp
2
55% D
H
ODipp
(4)
H/D
Si
toluene (0.1 mL)
120 °C, 18 h
Si
iPr
7
iPr
iPr iPr
H
<1% D
1h, 0.01 mmol
2h-d, 91%
A probable reaction mechanism is shown in Scheme 2 using
ASSOCIATED CONTENT
Supporting Information
1h as a representative substrate. First, a pivalic acidꢀcoordinated
palladium complex I with an η2ꢀalkyne is generated, which then
undergoes synꢀhydropalladation15 to provide vinyl palladium
pivalate II. An alternative pathway leading to II could be asꢀ
sumed: oxidative addition of pivalic acid to palladium(0) can give
hydridopivaloxypalladium(II), which reacts with 1h to generate II.
Dipp and two isopropyl groups on silicon accelerate the stereoiꢀ
somerization16 due to steric repulsion, forming Zꢀcomplex III. It
is also assumed that the oxygen atom assists the isomerization by
the donation of its lone pair electron to the C–C double bond.
Subsequent C–H activation via the CMD pathway17 gives palꢀ
ladacycle IV followed by reductive elimination to furnish the
product 2h and regenerate palladium(0) and pivalic acid.18
Detailed experimental procedures and characterization data of
new compounds are available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* yminami@kc.chuoꢀu.ac.jp
* thiyama@kc.chuoꢀu.ac.jp
Notes
The authors declare no competing financial interests.
Scheme 2. Proposed mechanism
ACKNOWLEDGMENT
tBu
O
tBu
PEt3
This research was supported by JST, ACTꢀC (JPMJCR12Z1), and
a GrantꢀinꢀAid for Young Scientists (B) (25870747 to Y.M.) from
the JSPS. Y.M. also acknowledges the Asahi Glass Foundation
and the Sumitomo Foundation.
OH
Pd
O
iPr
O
PEt3
H
Pd
1h
O
iPr
iPr
iPr
Si
iPr
Si
O
iPr
iPr
iPr
I
II
Pd(0)
PEt3
REFERENCES
PivOH
tBu
tBu
O
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OH
PEt3
O
PEt3
O
Pd
2h
Pd
iPr
iPr
Si
iPr
iPr
Si
O
iPr
iPr
O
iPr
iPr
H
H
IV
III
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ODipp
Pd cat.
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R
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Si
iPr
Straightforward
trans-Hydroarylation
iPr iPr
iPr
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