Synthesis of 7H-benzo[e]naphtho[1,8-bc]silines by rhodium-catalyzed [2 + 2 + 2] cycloaddition

Article abstract of DOI:10.1246/cl.200025

An efficient synthesis of 7H-benzo[e]naphtho[1,8-bc]silines has been developed by a rhodium-catalyzed [2 + 2 + 2] cycloaddition of alkynyl(8-alkynyl-1-naphthyl)silanes with internal alkynes. High chemoselectivity can be realized by employing P(2-MeOC₆H₄)₃ as the ligand for rhodium. Preliminary investigation of its asymmetric variant has also been described to create a silicon stereogenic center with relatively high enantioselectivity.

Full text of DOI:10.1246/cl.200025

Advance Publication Cover Page
Synthesis of 7H-Benzo[e]naphtho[1,8-bc]silines by Rhodium-Catalyzed [2 + 2 + 2]
Cycloaddition
Takumi Maesato and Ryo Shintani*
Advance Publication on the web January 31, 2020
doi:10.1246/cl.200025
© 2020 The Chemical Society of Japan
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1
Synthesis of 7H-Benzo[e]naphtho[1,8-bc]silines by Rhodium-Catalyzed [2 + 2 + 2]
Cycloaddition
Takumi Maesato and Ryo Shintani*
Division of Chemistry, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University,
Toyonaka, Osaka 560-8531, Japan
E-mail: shintani@chem.es.osaka-u.ac.jp
1
2
3
4
5
6
7
8
An efficient synthesis of 7H-benzo[e]naphtho[1,8-
45 with diphenylacetylene (2a) as
a
model substrate
bc]silines has been developed by a rhodium-catalyzed [2 + 2
+ 2] cycloaddition of alkynyl(8-alkynyl-1-naphthyl)silanes
with internal alkynes. High chemoselectivity can be realized
by employing P(2-MeOC6H4)3 as the ligand for rhodium.
Preliminary investigation of its asymmetric variant has also
been described to create a silicon stereogenic center with
relatively high enantioselectivity.
46 combination toward the synthesis of benzonaphthosiline 3aa
47 (Table 1). The use of PPh3 as the ligand in toluene at 100 °C
48 did provide product 3aa, but only in 13% yield even after full
49 consumption of substrate 1a (entry 1). In stark contrast, we
50 subsequently found that 96% yield of 3aa could be achieved
51 by changing the ligand to P(2-MeOC6H4)3 (entry 2).12 It is
52 worth mentioning that neither P(2-MeC6H4)3 nor P(4-
53 MeOC6H4)3 could provide 3aa with that high efficiency
54 (entries 3 and 4). In addition, the use of binap, which is often
55 used as an effective ligand for various rhodium-catalyzed [2
56 + 2 + 2] cycloaddition reactions,8 gave no desired compound
57 3aa in this case (entry 5).
9
Keywords: Benzonaphthosilines, Rhodium catalyst, [2
10 + 2 + 2] Cycloaddition
11
5H-Dibenzo[b,d]siloles (dibenzosiloles) and related
12 compounds belong to a class of silicon-bridged π-conjugated
13 compounds that can be applied to various optoelectronic
14 materials.1 In addition to the conventional synthetic process
15 based on the reaction of 2,2’-dimetalated biaryls with
16 dichlorosilanes,2 dibenzosiloles have been synthesized using
17 a variety of methods including those under transition-metal
18 catalysis.3 In contrast, structurally related 7H-
[RhCl(C2H4)2]2 (8 mol% Rh)
R
R
R
PPh3 (8 mol%)
R
NaBArF4 (16 mol%)
R1
R1
Si
Si
(2)
MeCN (1.0 equiv)
CH2Cl2, 40 °C, 16 h
R2
1 (0.10 M)
R2
(ArF = 3,5-(CF3)2C6H3)
4: up to 93% yield
58
19 benzo[e]naphtho[1,8-bc]silines
(benzonaphthosilines)
59 Table 1. Rhodium-catalyzed [2 + 2 + 2] cycloaddition of 1a
60 with 2a: Ligand effect.a
20 possessing a 6-membered silacycle have been significantly
21 less explored to date,4 which is likely due to the lack of their
22 efficient synthetic methods. In fact, only three isolated
23 approaches were reported to date as far as we are aware: (1)
[RhCl(C2H4)2]2
Me
(8 mol% Rh)
ligand (16 mol%)
NaBArF4 (16 mol%)
Me
Me
Me
Ph
Si
Ph
Ph
Si
Ph
24 dehydrogenative
C–C
coupling
of
methyl(1-
+
toluene, 100 °C, 18 h
Ph
25 naphthyl)(phenyl)silane by pyrolysis,5 (2) oxidative Si–C
26 coupling of 1-naphthyltriphenylsilane by a radical process,6
27 (3) ruthenium-catalyzed cyclization of 5,5-dimethyl-10-(2-
28 propyn-1-ylidene)-5,10-dihydrodibenzo[b,e]siline,7 and all
29 of them suffer from either low chemical yields or low chemo-
30 selectivity. To provide a solution to this methodological
31 deficiency, herein we describe the first efficient and selective
32 synthesis of benzonaphthosilines 3 by a rhodium-catalyzed [2
Ph
Ph
Ph
1a (0.10 M)
2a (1.0 equiv)
3aa
61
entry
ligand
yield (%)b
1
2
3
4
5
PPh3
13
96c
17
30
0
P(2-MeOC6H4)3
P(2-MeC6H4)3
P(4-MeOC6H4)3
binapd
33
+
2
+
2] cycloaddition of alkynyl(8-alkynyl-1-
34 naphthyl)silanes 1 with internal alkynes 2 (eq 1),8 including
35 its enantioselective variant for the construction of a silicon
36 stereogenic center.9,10
62 aCompound 4a was obtained in 3–11% yield as side product in
1
63 all entries. bDetermined by H NMR against internal standard
c
d
64 (dimethyl terephthalate). Isolated yield. 8 mol% of ligand was
65 used.
R
R
R
R
R3
R1
Si
cat. Rh+/P(2-MeOC6H4)3
toluene, 60–120 °C
R1
Si
(1)
+
R3
66
Under the conditions using P(2-MeOC6H4)3 as the
R2
R4
67 ligand, various benzonaphthosilines 3 can be synthesized in
68 reasonably high yields (Scheme 1).13 For example, in addition
69 to compound 3aa having a dimethylsilylene bridging unit,
70 diphenylsilylene-bridged benzonaphthosiline 3ba can be
71 obtained in a similarly high yield (91% yield). Introduction
72 of methoxy groups on naphthalene is also tolerated to give
73 3ca (64% yield). With regard to the substituents on the
74 alkynes of 1, both R1 and R2 can be either aryl or alkyl groups
75 as shown in the formation of 3da–3ja (64–95% yield). The
R2
R4
1
2
3: up to 96% yield
37
38
In 2015, we reported that compounds 1 undergo
39 intramolecular alkynylsilylation of alkynes to give
40 compounds 4 with a cationic Rh/PPh3 catalyst in the presence
41 of MeCN (eq 2), and replacement of MeCN with internal
42 alkynes led to the formation of benzonaphthosilines 3 via [2
43 + 2 + 2] cycloaddition, albeit with low efficiency.11 Based on
44 this previous result, we initially examined a reaction of 1a

2
Me Me
Si
1
2
3
4
5
6
7
8
9
reaction partner
2
can also be changed from
Br
Me
diphenylacetylene to various other diarylacetylenes as well as
dialkylacetylenes to give corresponding products 3ab–3ah
with similar efficiency (65–91% yield). Unsymmetric alkyne
2i (R3 = Ph, R4 = Me) gave product 3ai as a regioisomeric
mixture in the ratio of 67/33, but relatively high
regioselectivity (84/16–91/9) could be achieved by using
more biased unsymmetric alkynes such as 2j–2l to give 3aj–
3al.14 For the reaction of 1i with 2l, product 3il was obtained
+
Me
1k (0.10 M)
2m (1.0 equiv)
[RhCl(C2H4)2]2
(8 mol% Rh)
P(2-MeOC6H4)3
(16 mol%)
Me
Me
Pd(OAc)2 (16 mol%)
PCy3•HBF4 (20 mol%)
tBuCO2H (24 mol%)
Si
Me
NaBArF4 (16 mol%)
(3)
toluene
100 °C, 18 h
K2CO3 (2.4 equiv)
toluene, 100 °C, 8 h
Me
10 in 62% with almost perfect regioselectivity.14 It is worth
11 noting that the reaction of 1k with 1-bromo-2-
12 (phenylethynyl)benzene (2m) followed by palladium-
13 catalyzed intramolecular C–H arylation15 leads to
14 naphthotriphenylenosiline 5, a new silicon-bridged π-
15 conjugated compound, in a facile manner (eq 3).
5: 61% yield
19
20
Although the origin of regioselectivity for unsymmetric
21 alkynes is not entirely clear, it could be explained by
22 considering the reaction pathways of the present [2 + 2 + 2]
23 cycloaddition reaction of 1a with alkyne 2 (Scheme 2). If the
[RhCl(C2H4)2]2 (8 mol% Rh)
P(2-MeOC6H4)3 (16 mol%)
NaBArF4 (16 mol%)
R
R
24 reaction
proceeds
through
the
formation
of
R
R
R3
R1
Ar1
Ar2
Si
25 rhodacyclopentadiene A from 1a and the rhodium catalyst via
26 pathway I, differentiation of the two carbon–rhodium bonds
27 for the subsequent insertion of alkyne 2 in a position- and
28 regio-selective manner would be highly difficult. In contrast,
29 the selective formation of rhodacyclopentadiene B over C
30 from 1a, 2, and the rhodium catalyst through pathway II could
31 account for the observed regioselectivity, and this selectivity
32 would be reasonable based on the stereo- or electronic bias of
33 the alkyne 2.
Ar1
Ar2
R1
Si
+
R3
toluene, 100 °C, 18 h
R2
R4
R2
R4
1 (0.10 M)
2 (1.0 equiv)
3
R
Me
Si
Me
R
Me
Me
Si
Ph
MeO
Ph
Ph
Si
R
Ph MeO
Ph
Ph
Ph
Ph
Ph
Ph
Ph
pathway I
3aa (R = Me): 96% yield
3ba (R = Ph): 91% yield
3ca: 64% yield
3da (R = 4-MeOC6H4):
Me
Me
Me
Si
(70 °C)
94% yield (60 °C)
3ea (R = 3-BrC6H4):
95% yield (60 °C)
Me
RhI-catalyst
Si
Ph
Me
R (2)
Ph
3fa (R = nPr): 79% yield
Ph
RhIII
Me
Me
difficult to differentiate
X
1a
Ph
Me
Me
Si
Ph
A
Me
Me
pathway II
Me
Si
Ph
Ph
X
Si
Me
Si
Me
Me
Me
Me
Ph
Ph
Ph
RhI-catalyst
Ph
Si
Ph
R
Si
Ph
Me
R
RhIII
Ph
R
RhIII
Me
Ph
X
1a
+
3ga (X = MeO): 64% yield 3ia (R = Me): 70% yield 3ab (X = N-carbazolyl):
3ha (X = Cl): 73% yield
3ja (R = nPr): 92% yield
65% yield
3ac (X = MeO): 83% yield
3ad (X = Cl): 84% yield
Ph
Me
Ph
R
Me
B
C
2
34
3ae (X = CO2Me): 80% yield
35 Scheme 2. Possible reaction pathways I and II for the present
36 rhodium-catalyzed [2 + 2 + 2] cycloaddition of 1a with 2.
Me
Me
Me
Me
Me
Me
Si
Ph
Si
Ph
Si
Ph
R
and
37
We have also begun to develop an asymmetric variant
R
R
Me
38 of this process. On the basis of the ligand effect for the
39 nonasymmetric reactions (Table 1) as well as our previous
40 report on the asymmetric synthesis of related silicon-
41 stereogenic dibenzosiloles,10d,e we conducted a reaction of
42 prochiral 1l with 3-hexyne (2g) by employing 3’-methylated
43 (R)-MeO-mop ((R)-L1).16 As shown in eq 4, the reaction
44 proceeded at 30 °C to give silicon-stereogenic
45 benzonaphthosiline 3lg in 62% yield with 80% ee, and a
46 slight improvement in the yield and ee was achieved (87%
47 yield, 81% ee) by modification of the aryl groups on the
48 phosphorus atom of the chiral ligand from phenyl to 3,5-
49 dimethyl-4-methoxyphenyl ((R)-L2). Similarly, the reaction
50 of 1l with 1,4-dimethoxy-2-butyne (2h) under Rh/(R)-L2
51 catalysis gave 3lh in 77% yield with 82% ee.
Ph
R
Ph
Me
Ph
3af (R = 3,5-Me2C6H3):
3ai/3ai’ (R = Ph): 89% yield (rs = 67/33)
91% yield 3aj/3aj’ (R = 2,6-Me2C6H3):
3ag (R = Et): 79% yield
58% yield (rs = 84/16)
3ah (R = CH2OMe): 68% yield 3ak/3ak’ (R = SiMe3):
69% yield (rs = 91/9) (120 °C)
3al/3al’ (R = CO2Me): 82% yield (rs = 90/10)
Me
Si
Me
Ph
CO2Me
Me
Me
3il: 62% yield (rs > 99/1)
16
17 Scheme 1. Rhodium-catalyzed [2 + 2 + 2] cycloaddition of 1
18 with 2: Scope.

3
Ar
50
51
52
53
54
55
56
57
58
59
60
61
62
63
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[RhCl(C2H4)2]2
(8 mol% Rh)
ligand (8 mol%)
NaBArF4 (16 mol%)
Ar
Ar
Cy
Si
Cy
R
R
Si
Ar
*
(4)
+
7
8
toluene, 30 °C, 18 h
R
Me
(Ar = 4-MeOC6H4)
(1.0 equiv)
Me
R
2g (R = Et)
1l (0.10 M)
2h (R = CH2OMe)
3’
Me
ligand
product
3lg
yield (%) ee (%)
9
OMe
PAr2
(R)-L1
(R)-L2
(R)-L2
62
87
77
80
81
82
ligand =
3lg
3lh
(R)-L1: Ar = Ph
(R)-L2: Ar = 3,5-Me2-4-MeOC6H2
64 10
65
1
66
2
3
4
5
6
7
8
9
In summary, we have developed an efficient synthesis
67
of benzonaphthosilines by a rhodium-catalyzed [2 + 2 + 2]
cycloaddition of alkynyl(8-alkynyl-1-naphthyl)silanes with
internal alkynes through the use of P(2-MeOC6H4)3 as ligand
for rhodium. We have also described our preliminary
investigation of its asymmetric variant, creating a silicon
stereogenic center with relatively high enantioselectivity.
Future studies will be directed toward the development of
68
69
70
71
72
73
74
75
10 functional organic molecules based on the present structural
11 motifs.
76
77 11
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supplementary crystallographic data for this paper. These data can
12
79 12
80
13 Support has been provided in part by a Grant-in-Aid for
14 Scientific Research (B) (16H04145), the Ministry of
15 Education, Culture, Sports, Science and Technology, Japan.
16 We thank Prof. Takeshi Naota and Dr. Soichiro Kawamorita
17 at Osaka University for the measurement of fluorescence
18 spectra. We thank Mr. Tomohiro Tsuda for the help of X-ray
19 crystallographic analysis.
81
82 13
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84 14
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by
via
emailing
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or
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23 References and Notes
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4
5