Silylboranes bearing dialkylamino groups on silicon as silylene equivalents: Palladium-catalyzed regioselective synthesis of 2,4-disubstituted siloles

Article abstract of DOI:10.1021/ja073896h

Silylpinacolboranes bearing dialkylamino groups on the silicon atom served as synthetic equivalents of silylene in palladium-catalyzed reactions with terminal alkynes, leading to the formation of 2,4-disubstituted siloles in high yield. It was found that the amino group on the silicon atom was critically important for the reaction; no silole products were found in reactions using silylpinacolboranes carrying aryl, chloro, or alkoxy groups on the silicon atoms. Site-selective bromination of 1,1-dimethyl-2,4-diphenylsilole followed by Migita-Kosugi-Stille coupling with (arylalkynyl)tributylstannanes gave novel π-conjugated siloles with good total yields. Copyright

Full text of DOI:10.1021/ja073896h

Published on Web 01/16/2008
Silylboranes Bearing Dialkylamino Groups on Silicon as Silylene
Equivalents: Palladium-Catalyzed Regioselective Synthesis of
2,4-Disubstituted Siloles
Toshimichi Ohmura, Kohei Masuda, and Michinori Suginome*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Katsura, Kyoto 615-8510, Japan
Received May 30, 2007; E-mail: suginome@sbchem.kyoto-u.ac.jp
Table 1. Palladium-Catalyzed Reaction of Silylboranes 1-4 with
Siloles have received much attention in materials sciences
because of their unique electronic properties, arising mainly from
their low-lying LUMO.1 Particular interest has been focused on
the application of π-conjugated siloles to light-emitting materials.2
As the properties of siloles largely depend upon the substituents
on the silole ring, much effort has been devoted to the development
of new methods for their efficient and selective synthesis.2,3
Transition-metal-catalyzed [2 + 2 + 1] cyclization of an alkyne
(2 equiv) with a silylene equivalent is one of the most attractive
routes to substituted siloles. Nickel- and palladium-catalyzed
reactions have been developed with exploration of organosilicon
reagents such as hydrodisilane,4 alkynyldisilane,5 trisilacyclopro-
pane,6 (hydrosilyl)stannane,7 silacyclopropene,8 and silacyclopro-
pane.9 These reaction systems allowed synthesis of 2,3,4,5-
tetrasubstituted, 2,3,4-trisubstituted, and 3,4-disubstituted siloles,
which had been difficult to synthesize.
As part of our study of silylborane,10 we recently established
synthetic access to silylboranes that are functionalized on the silicon
atom.11 By using this method, silylboranes bearing chloro, alkoxy,
and dialkylamino groups on silicon are easily prepared on a practical
scale. Our interest was then focused on the reactivity of these novel
silylboranes in transition-metal-catalyzed silaboration of unsaturated
organic molecules. In this paper, we disclose our unexpected finding
on the efficient use of amino-substituted silylboronic esters as a
silylene equivalent and selective formation of 2,4-disubstituted
siloles, for which no efficient synthetic access has so far been
established.12
Reactions of 1-octyne (5a) with silylboranes 1-4,11 bearing
phenyl, chloro, methoxy, and dialkylamino groups on silicon, were
carried out in the presence of 1.0 mol % of CpPd(η3-C3H5) and
1.2 mol % of PPh3 (Table 1).13 Addition of Ph-substituted
silylborane 1 to 5a took place slowly at room temperature (70 h
for full conversion), giving 1-boryl-2-silyl-1-alkene 6 with a
quantitative yield (entry 1). Large rate acceleration was observed
when the reaction was carried out with Cl-substituted silylborane
2, resulting in efficient formation of alkene 7 within 15 min (entry
2). Moderate rate acceleration was also observed in the addition of
MeO-substituted silylborane 3 (entry 3). These results suggest that
the reaction rate of the silaboration critically depends upon the
electronic nature of the substituents on the silicon.
Diethylamino-substituted silylborane 4a11 was then reacted with
5a under the same reaction conditions (entry 4). The starting 4a
was completely consumed for 80 min at room temperature, but no
silaboration products, such as 9a, were found in the reaction
mixture. We found that 2,4-disubstituted silole 10a and 3,4-silole
10a′ were formed in good total yield (79%, 10a:10a′ ) 77:23).
The formation of silole was accompanied by the formation of
(diethylamino)pinacolborane (11a), which suggests the involvement
of Pd-silylene species in the catalytic system.14 This silole
formation was found to be general for amino-substituted silylboranes
5aa
yield (%)b
c
entry
silylborane
time (h)
6
−9
10 (10a:10a
′
)
11
1
2
3
1 (X ) Ph)
70
0.2
4
1.3
1.5
1.5
99d (6)
92 (7)
80 (8)
0 (9a)
0 (9b)
0 (9c)
0
0
0
0
0
0
2 (X ) Cl)
3 (X ) OMe)
4
4a (X ) NEt2)
4b (X ) NMe2)
4c (X ) pyrrolidino)
79 (77:23) 56 (11a)
55f (69:31) 77f (11b)
69f (75:25) 82f (11c)
5e
6e
a 1-4 (0.40 mmol), 5a (0.96 mmol), CpPd(η3-C3H5) (4.0 µmol), and
PPh3 (4.8 µmol) were stirred in toluene (0.2 mL) at room temperature unless
otherwise noted. b Isolated yield based on silylborane. c Determined by 1H
NMR analysis. d GC yield. e Carried out in C6D6. f 1H NMR yield.
having dimethylamino and pyrrolidino groups on the silicon atom
(entries 5 and 6).
To improve regioselectivity (10a:10a′), we tested various tertiary
phosphine ligands in the reactions of 4a with 5a.15 We found that
the highest regioselectivity was attained with sterically hindered
P(t-Bu)2(2-biphenyl) (12), which afforded 10a and 10a′ with the
ratio of 90:10, although the reaction was slower than the original
reaction conditions using PPh3 (entry 1 in Table 2).
Various terminal alkynes were subjected to reaction with 4a in
the presence of palladium catalysts (Table 2). It was found that
easily available Pd(dba)2 could be used for this reaction.16 In the
presence of 1.0 mol % of Pd(dba)2 with 1.2 mol % of either 12 or
the analogous bulky phosphine P(t-Bu)2[2-(2′-methylbiphenyl)] (13),
reactions of functionalized or unfunctionalized aliphatic alkynes
5a-d gave the corresponding siloles 10a-d in good yields with
high regioselectivities (10:10′ ) 90:10-96:4, entries 1-4). Reac-
tions of aromatic alkynes were carried out with the Pd/PPh3 catalyst
(entries 5-12). Phenylacetylene (5e) and alkynes 5f-h bearing
electron-donating groups yielded 10e-h in yields of 80-96% with
high regioisomeric ratios (entries 5-8), whereas the reaction of
CF3-substituted 5i gave a lower yield with good regioselectivity
(94:6, entry 9). Sterically demanding arylacetylenes 5j-l reacted
with 4a with no drop in reaction rate to give 10j-l with higher
regioselectivities (95:5-99:1, entries 10-12).
We then carried out the reaction of 5e with (Et2N)Ph2Si-B(pin)
(14)11 in the presence of the Pd/PPh3 catalyst (eq 1). Although the
9
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J. AM. CHEM. SOC. 2008, 130, 1526-1527
10.1021/ja073896h CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Regioselective Synthesis of 2,4-Disubstituted Siloles via
Palladium-Catalyzed Reaction of 4a with Terminal Alkynesa
Acknowledgment. This work is supported in part by Research
for Promoting Technological Seeds from JST, and Grant-in-Aid
for Scientific Research on Priority Areas (No. 19027031, “Synergy
of Elements”) from Ministry of Education, Culture, Sports, Science
and Technology, Japan.
Supporting Information Available: Experimental details and
characterization data of the products. This material is available free of
charge via Internet at http://pubs.acs.org.
yield
(%)c
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5a (R ) n-C6H13)
5b (R ) n-C8H17)
5c [R ) (CH2)2OTBDMS]
5d [R ) (CH2)3Cl]
5e (R ) Ph)
5f (R ) 4-MeC6H4)
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5h (R ) 4-Me2NC6H4)
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5l (R ) 1-naphthyl)
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10j
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10l
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(14) In the absence of alkyne, silylborane 4a slowly reacted at room temperature
in the presence of the Pd/PPh3 complex (10 mol %), resulting in the
formation of 11a (84% yield after 48 h) with organosilicon compounds,
which exhibited 1H NMR signals in the region of -1.0 to 0.6 ppm.
Although they were hardly identifiable, dodecamethylcyclohexasilane was
detected by 1H NMR and GCMS analysis as a very minor (<1%)
component.
Conversion of the 2,4-disubstituted silole 10e to novel π-con-
jugated 2,3,5-trisubstituted siloles was demonstrated (Scheme 1).
Site-selective bromination of 10e was achieved by treatment with
N-bromosuccinimide (NBS) at room temperature, giving 2-bromo-
1,1-dimethyl-3,5-diphenylsilole (16). Migita-Kosugi-Stille cou-
pling of 16 with alkynyltributylstannanes under Fu’s conditions17
afforded 17a and 17b in 78 and 52% yields from 10e, respectively.18
In conclusion, we have established new synthetic access to 2,4-
disubstituted siloles via Pd-catalyzed reaction of terminal alkynes
with (dialkylaminosilyl)pinacolboranes, which serve as new silylene
equivalents. Mechanism of the reaction is currently under investiga-
tion. The elimination of (dialkylamino)borane seems to be the key
driving force for the reaction as recently demonstrated in the Ru-
catalyzed reaction system.19
(15) 10a:10a′ (PR3): 80:20 (PCyPh2); 82:18 (PCy2Ph); 77:23 (PCy3); 68:32
[P(t-Bu)3]; 86:14 [PPh2(2-biphenyl)]; 88:12 [PCy2(2-biphenyl)]; 90:10 [P(t-
Bu)2(2-biphenyl)].
Scheme 1. Site-Selective Functionalization of 10e
(16) Pd(OAc)2 and PdCl2(CH3CN)2 were also effective as catalyst precursors
of the reaction, although prolonged reaction time was needed because of
longer induction period than Pd(dba)2.
(17) Littke, A. F.; Schwarz, L.; Fu, G. C. J. Am. Chem. Soc. 2002, 124, 6343.
(18) 10e: UV/vis λmax 338 nm (ꢀ 2.0 × 103); FL λmax 452 nm, Φf 0.13. 17a:
UV/vis λmax 403 nm (ꢀ 2.5 × 103); FL λmax 456 nm, Φf 0.069. 17b: UV/
vis λmax 420 nm (ꢀ 2.2 × 104); FL λmax 516 nm, Φf 0.015. The
photophysical data were measured in CHCl3. Quantum yields (Φf) were
determined with reference to quinine sulfate in 0.1 M H2SO4 (excited at
366 nm).
(19) Ueno, S.; Chatani, N.; Kakiuchi, F. J. Am. Chem. Soc. 2007, 129, 6098.
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