Palladium-catalyzed silylene-1,3-diene [4 + 1] cycloaddition with use of (aminosilyl)boronic esters as synthetic equivalents of silylene

Article abstract of DOI:10.1021/ja907170p

(Chemical Equation Presented) Silylboronic esters bearing a dialkylamino group on the silicon atoms reacted with 1,3-dienes in the presence of a palladium catalyst to give silacyclopent-3-enes (i.e., 2,5-dihydrosiloles) in high yields via efficient silyle

Full text of DOI:10.1021/ja907170p

Published on Web 11/02/2009
Palladium-Catalyzed Silylene-1,3-Diene [4 + 1] Cycloaddition with Use of
(Aminosilyl)boronic Esters as Synthetic Equivalents of Silylene
Toshimichi Ohmura, Kohei Masuda, Ichiro Takase, and Michinori Suginome*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Katsura, Kyoto 615-8510, Japan
Received August 25, 2009; E-mail: suginome@sbchem.kyoto-u.ac.jp
Table 1. Screening of Reaction Conditionsa
Carbon-silicon bond formations involving silylene and its
equivalents have received increasing attention in the synthesis of
organosilicon compounds.1 Silylene species can be generated by
dehalogenation of dihalosilanes as well as thermolysis or photolysis
of oligosilanes, disilenes, and strained cyclic organosilanes and
undergo cycloaddition with unsaturated hydrocarbons to give cyclic
entry
silylborane
ligand (Pd/P)
% yield of 3ab % yield of 4c
organosilanes.2 Among the formation of silacarbocycles, cycload-
dition with 1,3-dienes is attractive because the reaction provides
silacyclopent-3-enes that serve as useful intermediates in organic
synthesis.3,4 However, control of the reaction is rather difficult,
because the reaction proceeds via the transient formation of
vinylsilacyclopropanes,5 which then rearrange via biradical inter-
mediates to lead to the formation of undesired side products6 along
with loss of stereospecificity.7 As a result, synthesis of silacyclopent-
3-enes via silylene transfer to 1,3-dienes has been limited to isoprene
and 2,3-dimethyl-1,3-butadiene.3,8
1
2
3
4
5
6
7
8
9
1a (R2N ) Et2N)
PPh3 (1/1.2)
17
36 (4a)
99 (4a)
81 (4a)
19 (4a)
81 (4a)
5 (4a)
4 (4a)
92 (4b)
67 (4c)
1a
PMePh2 (1/1.2)
PMePh2 (1/2.4)
PMePh2 (1/4.8)
PMe2Ph (1/1.2)
PMe3 (1/1.2)
none
62 (63)d
1a
1a
1a
1a
61
11
59
3
0
15
28
1a
1b (R2N ) Me2N)
1c (R2N ) pyrrolidino) PMePh2 (1/1.2)
PMePh2 (1/1.2)
a 1a-c (0.30 mmol), 2a (0.33 mmol), Pd(dba)2 (3.0 µmol), and
ligand (0-14.4 µmol) were stirred in C6D6 (0.15 mL) at room
temperature unless otherwise noted. b GC yield based on silylborane.
Transition-metal-catalyzed silylene transfer would be an attractive
method for overcoming the limitations of the thermal or photo-
chemical reactions. However, no efficient silylene transfer to 1,3-
dienes has been reported,9 despite recent successes in catalytic
silylene transfer from silacyclopropanes or other silylene precursors
to alkynes,10 alkenes,11 and R,ꢀ-unsaturated carbonyl compounds.12
We recently found that silylboronic esters bearing a dialkylamino
group on the silicon atoms reacted as silylene equivalents in the
presence of terminal alkynes and a palladium catalyst, giving 2,4-
disubstituted siloles.13 Our effort was then focused on application
of the new silylene transfer system to 1,3-dienes. Herein, we
describe palladium-catalyzed reaction of silylboronic esters with
1,3-dienes, leading to efficient access to silacyclopent-3-enes.14
Reactions of 1,3-decadiene (2a) with silylboronic esters 1a-c15
bearing dialkylamino groups on the silicon atoms were examined
(Table 1). In the presence of Pd(dba)2 (1.0 mol %) and PPh3 (1.2
mol %), Et2N-substituted 1a reacted slowly with 2a in C6D6 at room
temperature to give silacyclopent-3-ene 3a and aminoboronic ester
4a in 17 and 36% yields, respectively (entry 1). It should be
remarked that silylborane 1a did not afford silaboration products16
but served as a silylene equivalent under the reaction conditions.
We found that the yields of 3a and 4a were greatly improved when
the reaction was carried out with Pd/PMePh2 catalyst (entry 2). A
palladium complex bearing PMe2Ph also showed high catalyst
ability (entry 5), whereas almost no reaction took place with PMe3
(entry 6). The reaction proceeded smoothly with Pd/P ratios of 1:1.2
to 1:2.4 (entries 2 and 3), while a slow reaction was observed with
higher ligand/Pd ratios (entries 4 and 7). Reaction of Me2N-
substituted 1b gave 3a in much lower yield than did 1a, while
retaining the high yield of aminoborane 4b (entry 8). This result
may indicate that silylene extrusion, which forms 4b, and trapping
of the silylene by 1,3-dienes take place sequentially on the
1H NMR yield. d Isolated yield in the reaction of 1a (0.43 mmol) with
c
2a (0.38 mmol).
Table 2. Palladium-Catalyzed Reaction of Silylboronic Esters with
Dienesa
entry
Si-B
R1
R2
R3
yield (%)b
1c
2
1a
5
Ph
Ph
H
H
H
H
H
H
Ph
n-Bu
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
Ph
2b
2b
2c
2d
2e
2f
93 (3b)
77 (6b)
82 (6c)
84 (3d)
77 (3e)
71 (3f)
71 (3g)
82 (3h)
94 (3i)
92 (3j)
79 (6k)
84 (3l)
3
5
4
5
6
7
1a
1a
1a
1a
1a
1a
1a
5
(CH2)2CO2Me
(CH2)2CN
(CH2)2CHdCMe2 2g
OSiMe2Ph
Ph
OSiMe2Ph
Me
8c
9c
10c
11
12
13
14c
2h
2i
2j
2k
2l
Me
1a
1a
1a
n-C6H13 n-C6H13
-(CH2)4-
Ph
2m 84 (3m)
2n 86 (3n)
Me
Me
a 1a or 5 (0.40 mmol), 2 (0.44-0.68 mmol), Pd(dba)2 (4.0 µmol),
and PMePh2 (4.8 µmol) were stirred in toluene (0.2 mL) at room
temperature (for 1a) or at 50 °C (for 5) unless otherwise noted.
b Isolated yield. c 1a (0.44 mmol) and 2 (0.40 mmol) were employed.
palladium. On the other hand, slower formation of both 3a and 4c
was observed in the reaction with pyrrolidino-substituted 1c
(entry 9).
Mono- and disubstituted 1,3-butadienes were then subjected to
the reaction with Et2N-substituted 1a or diphenylsilyl analogue 5
9
16624 J. AM. CHEM. SOC. 2009, 131, 16624–16625
10.1021/ja907170p CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
in the presence of the Pd/PMePh2 catalyst (Table 2). Silylboronic
esters 1a and 5 reacted with 1-phenyl-1,3-butadiene (2b) at room
temperature to give 2-phenyl-silacyclopent-3-enes 3b and 6b,
respectively (entries 1 and 2). The reaction of diphenyl derivative
required higher reaction temperature than the dimethylsilylene
transfer. Reactions of 1a or 5 with 1,3-butadienes 2c-h bearing a
substituent at C2 gave the corresponding silacyclopent-3-enes 3c-h
in 71-85% yields (entries 3-8). It should be noted that functional
groups such as ester, nitrile, and isolated C-C double bonds did
not affect the formation of silacyclopent-3-enes (entries 5-7). The
reaction was applicable to dienol silyl ether 2h (entry 8). Reactions
of 1,3- and 2,3-disubstituted 1,3-butadienes 2i-m and 1,2,3-
trisubstituted 2n also gave the corresponding products 3i-n in good
yields (entries 9-14).17
The catalytic silylene transfer using silylboronic esters was
applicable to 1,3-dienes that had been reported to give undesired
products under the thermal or photochemical conditions.6,7 1,3-
Butadiene (2o) reacted with 5 to give 6o in 84% yield without
incorporation of a second 1,3-butadiene (eq 1).6c Stereospecific ring
formation took place in the reaction of 1a with either stereoisomer
of 5,7-dodecadiene (2p): (E,E)-2p gave cis-3p (eq 2), whereas
selective formation of trans-3p was observed in the reaction of
(E,Z)-2p (eq 3).7 Selective formation of 7-silanorbornene 6q (88%)
was achieved by reaction of 1,3-cyclohexadiene (2q) with 5, in
which no ring-opening products were formed (eq 4).6a
that 2,4- and 2,5-diaryl-1-silacyclopent-3-enes were easily converted
into the corresponding siloles by treatment with p-chloranil or
DDQ.19 Readily available dienes 2 (R1 ) H, R2 ) Ph, R4 ) Ar or
R1 ) Ph, R2 ) H, R4 ) Ar) were converted into 3 and then oxidized
with p-chloranil or DDQ to give 7-11 in high total yields.
In conclusion, we have established an efficient method to
synthesize silacyclopent-3-enes via palladium-catalyzed silylene
transfer to 1,3-dienes from silylboronic esters. The reaction was
applicable to a wide variety of 1,3-dienes with high functional group
tolerance and stereospecificity.
Acknowledgment. This work is supported by Grant-in-Aid for
Scientific Research on Priority Areas (Nos. 19027031 and 20036029,
“Synergy of Elements”) from Ministry of Education, Culture,
Sports, Science and Technology, Japan. K.M. acknowledges JSPS
for fellowship support.
Supporting Information Available: Experimental details and
characterization data of the products. This material is available free of
charge via the Internet at http://pubs.acs.org.
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a Total yields based on dienes 2 are shown.
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