Angewandte
Communications
Chemie
[
a]
[a]
Table 1: Reaction development.
Table 2: Substrate scope of silacyclization.
[
b]
Entry
Variation from “standard conditions”
Yield of 2a [%]
[
c]
1
2
3
4
5
6
7
8
none
86 (77)
Use of Ag CO instead of DMBQ
Use of CuCl instead of DMBQ
Use of BQ instead of DMBQ
Use of Pd(TFA) instead of Pd(OAc)2
At 1008C
Without DMBQ
Without Pd(OAc)2
5
2
3
19
28
47
76
14
0
2
2
[a] Reaction conditions: 1a (0.20 mmol), ODCS (0.5 mmol), catalyst
(
10 mol%), oxidant (0.5 mmol) in toluene (0.5 mL), 48 h, 1208C, under
Ar; [b] GC yield; [c] isolated yield; 2,5-dimethyl-1,4-benzoquinone
DMBQ), 1,4-benzoquinone (BQ).
(
bonds at the indole C6 and C7 positions was unambiguously
confirmed by X-ray crystallographic analysis. The use of
DMBQ in the system appeared to be crucial for suppressing
t
oxidation of the N-P Bu motif and maintaining the catalytic
cycle,
2
[
15]
whereas other conventional oxidants, such as
Ag CO , CuCl , and BQ, reacted to produce lower yields
2
3
2
(
entries 2–4). Reactions with palladium catalysts lacking OAc
produced considerably lower yields than those catalyzed by
Pd(OAc) (entry 5). Good reactivity (in terms of the reaction
2
outcome) was maintained upon lowering the reaction temper-
ature to 1008C (entry 6). Finally, control experiments showed
poor conversion without the use of the DMBQ oxidant
(
entry 7), and silylation did not proceed in the absence of the
palladium catalyst (entry 8).
We next evaluated the scope and generality of this method
under optimized reaction conditions (Table 2). Indoles bear-
ing electron-neutral and electron-donating substituents, such
as methyl (1b–1d), phenyl (1e), thioether (1 f), ether (1g) and
alkenyl (1g) groups at C3-C5 positions, were well tolerated,
affording the corresponding silacycles 2b–2g in 41–75%
yields. These reactions proceed without any interference from
halides, such as the F (2i and 2j) and Cl (2k) units. Indoles
containing electron-withdrawing groups, such as ester (1l)
[
a] Reaction conditions: 1 (0.20 mmol), ODCS (0.5 mmol), Pd(OAc)2
(10 mol%), DMBQ (0.5 mmol) in toluene (0.5 mL), 48 h, 1208C, under
Ar; isolated yield; [b] at 1408C.
III
and acetyl (1m), were also suitable for P -directed silacyc-
lization with ODCS, providing products 2l and 2m in 74%
and 55% yields, respectively. The reaction of indole 1n with
a methyl group at the C2 position did not inhibit the process,
including 5H-benzo[b]carbazole 1w, 7H-dibenzo[c,g]-
carbazole 1x and 5,11-dihydroindeno[1,2-b]carbazole 1y,
were also examined, and the silacyclization products (2w–
2y) were obtained in 46–64% yields.
but the smaller N-PCy2 (Cy = cyclohexyl) directing group
t
exhibited higher reactivity than the N-P Bu motif, affording
2
the desired product 2n in a 63% yield. The 2,3-disubstituted
indole skeletons 1o–1q also underwent cyclization to form
Further investigations were conducted to demonstrate the
practicability of the ODCS reagent (Scheme 2). First, the
ODCS reagent could also be used for the late-stage CÀH
2
o, 2p, and 2q in 46–71% yields. In addition to indoles, the
silacyclization of carbazoles 1r–1s occurred with excellent
site selectivity for ortho and meta positions to the N-PCy2
group. The regioselectivity of the silylation of unsymmetrical
carbazoles with methyl (1t), methoxy (1u) and Cl (1v) groups
at the less hindered benzene core led to a high level of steric
control by substituents ortho to the reacting CÀH bond. In
modification of complex molecules (Scheme 2a). When
complex indole molecules 3a and 3b were subjected to the
developed system, high levels of selectivity were observed for
CÀH silylation in products 4a and 4b, despite multiple
potentially reactive positions. Second, silacyclization with the
ODCS reagent could be used for rapidly modification of
addition, several highly p-extended heterocyclic skeletons,
Angew. Chem. Int. Ed. 2021, 60, 7066 –7071
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