Organic Letters
Letter
a
Table 1. Reaction Conditions
b
b
b
entry
catalyst
ligand
B−Si
3a
4
5
1
2
3
4
5
6
7
8
[Rh(cod)2]BF4
[RhCl(cod)]2
[RhCl(cod)]2
[RhCl(cod)]2
[RhCl(cod)]2
[RhCl(cod)]2
[RhCl(cod)]2
[RhCl(cod)]2
[RhCl(cod)]2
[RhCl(cod)]2
Pd(dba)2
DPPM
DPPM
DPPE
DPPP
DPPB
DPPPent
PMePh2
−
DPPM
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2a
2b
8
37
0
0
0
68
0
0
0
0
51
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2
c
0
e
d
9
10
11 ,
87 (80)
d
DPPM
PMePh2
PMePh2
72
4
0
fg
h
23
0
29
78 (70)
f
h
i
12
Pd(dba)2
a
1a (0.10 mmol), 2a (0.10 mmol), catalyst (5 mol % metal), ligand (5.5 mol %), and ClCH2CH2Cl (0.1 mL) were stirred at 80 °C for 6 h.
b
c
d
e
f
Determined by 1H NMR. 10 mol %. ClCH2CH2Cl (0.5 mL) was used. Isolated yield in a 3 mmol scale reaction. Reaction was carried out in
g
h
i
toluene (0.2 mL) for 12 h. At 110 °C. 12 mol %. Isolated yield in a 0.1 mmol scale reaction. DPPM: Ph2PCH2PPh2. DPPE: Ph2P(CH2)2PPh2.
DPPP: Ph2P(CH2)3PPh2. DPPB: Ph2P(CH2)4PPh2. DPPPent: Ph2P(CH2)5PPh2.
the dienyl moiety of A into the Rh−Si and Rh−C bond affords
E and subsequently F, leading to C−Si bond-forming
cyclization with the elimination of X to afford B. Herein we
describe the first catalytic [4 + 2 + 1] cycloaddition that allows
the participation of one atom component other than carbon.
The efficient construction of silicon-containing seven-mem-
bered rings was achieved by the rhodium-catalyzed reaction of
dienynes with borylsilane based on the catalytic cycle involving
the rhodium silylenoid, which efficiently prohibits undesired [4
+ 2], [2 + 2 + 1], and [4 + 1] cycloadditions (A to G, H, and I
in Scheme 1).14
Reactions of diethyl malonate-derived dienyne 1a with
(pin)B−Si(Oi-Pr)Me2 (2a,15 1 equiv) were examined in 1,2-
dichloroethane at 80 °C for 6 h in the presence of a rhodium
catalyst (5 mol % Rh) and a phosphorus ligand (5.5 mol %)
(Table 1). A combination of [Rh(cod)2]BF4 and bis-
(diphenylphosphino)methane (DPPM), which has been
reported as an effective catalyst for the intramolecular [4 +
2] cycloaddition of dienynes (A to G in Scheme 1),16 was
tested first (entry 1). Although the [4 + 2] cycloaddition of 1a
proceeded preferentially to afford 4, even in the presence of 2a,
we found that the reaction also afforded a small amount of
seven-membered silacarbocycle 3a, indicating that the [4 + 2 +
1] cycloaddition took place by the rhodium catalyst (entry 1).
The formation of 4 was completely inhibited by changing the
catalyst precursor from cationic [Rh(cod)2]BF4 to neutral
[RhCl(cod)]2, by which 3a was formed selectively in 37% yield
with the formation of alkoxyboronate 6a (entry 2). To improve
the yield of 3a, other phosphorus ligands were examined
(entries 3−7). However, bidentate phosphine ligands having a
larger bite angle were less effective, with no formation of 3a
(entries 3−6). Either a rhodium with PMePh2 (P/Rh = 2) or a
phosphine-free rhodium showed poor catalytic activity (entries
7 and 8). These results indicate that the small bite angle of
DPPM is more preferable for efficient [4 + 2 + 1]
cycloaddition. Finally, we achieved the high-yield formation
of 3a (87%) by performing the reaction under a lower
concentration (ca. 0.2 M), by which side reactions including
the oligomerization of 1a were effectively suppressed (entry 9).
The reaction was applicable to a larger-scale synthesis of 3a
(0.80 g, 80%, entry 9). The reaction with (pin)B−Si(NEt2)-
Me2 (2b)15 also proceeded efficiently to afford 3a in good yield
with the formation of 6b (entry 10). On the contrary, the
reaction with Me3Si−SiMe3, which has been used as a silylene
equivalent in the rhodium-catalyzed alkyne−alkyne−silylene [2
+ 2 + 1] cycloaddition,17 did not afford 3a under the
conditions identical to entries 9 and 10. It is noted that
Pd(dba)2 with PMePh2 promoted [4 + 1] cycloaddition (A to
I in Scheme 1) rather than [4 + 2 + 1] and [4 + 2]
cycloadditions (entries 11 and 12). The palladium-catalyzed
reaction of 1a with 2b (1.2 equiv) in toluene at 80 °C
selectively afforded 5 in 70% isolated yield (entry 12).
The rhodium-catalyzed [4 + 2 + 1] cycloaddition of various
dienynes with 2a was conducted in 1,2-dichloroethane at 80
°C using [RhCl(cod)]2 with DPPM (Scheme 2). In the
reaction of dialkyl malonate-derived 1a−c, the yields of the
cyclized products increased in the order of dimethyl (3b, 71%),
diethyl (3a, 80%), and diisopropyl esters (3c, 84%), indicating
that the cycloaddition is accelerated by the Thorpe−Ingold
effect. Silyl ether 1d and methyl ether 1e also reacted smoothly
to afford 3d and 3e in 79 and 69% yield. The penta-2,4-dione-
derived 1f afforded 3f in good yield, where the relatively acidic
carbonyl α-hydrogen and the electrophilic carbonyl group did
not affect the reaction. The methyl-substituted diene moiety in
B
Org. Lett. XXXX, XXX, XXX−XXX