.
Angewandte
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nones[8] from aromatic imidates and isocyanates under
rhenium catalysis and its application to the synthesis of
polyimide derivatives.
Table 1: Reactions between several benzimidates 1 and p-tolylisocyanate
(2a).[a]
Through the optimization of the reaction conditions
(catalysts, solvents, temperatures, and reaction times), we
found that the reaction of (Z)-methyl N-phenylbenzimidate
(1a) with p-tolyl isocyanate (2a) in the presence of the
rhenium catalyst [Re2(CO)10] in toluene at 1508C for 24 hours
proceeded smoothly and gave (E)-3-(phenylimino)-2-(p-tol-
yl)isoindolin-1-one (3a) in 99% yield [Eq. (1); for details of
Entry
1
3
Yield [%][b]
1
2
3
4
5
6
R=4-MeO: 1b
R=4-CF3: 1c
R=4-Br: 1d
R=4-CN: 1e
R=4-CO2Me: 1 f 3 f: 80
R=3-Me: 1g
3b: 80
3c: 68
3d: 90
3e: 60
3g+3g’: 81, 3g/3g’=63:37
the optimization of the reaction conditions, see Table S1 in
the Supporting Information].[9,10] After the reaction, meth-
1
anol was observed in the H NMR spectra as a side product.
The structure of 3a was determined by single-crystal X-ray
structure analysis (see the Supporting Information).[11] In this
reaction, the imino group (the directing group) remains after
the reaction, and this result is quite different from previous
reports in which directing groups did not remain after the
transformations.[6,7]
7
8
R=2-Me: 1h
3h: 36
1i
3i+3i’: 98, 3i/3i’=48:52
The scope of the imidates 1 was investigated (Table 1).
The desired 3-imino-1-isoindolinones 3b–f were obtained in
good to excellent yield using the imidates 1b–f having either
an electron-donating or electron-withdrawing group
(entries 1–5). In these cases, the functional group remained
unchanged during the reaction. An imidate with a substituent
at the meta-position, 1g, provided a mixture of two regioisom-
ers, 3g and 3g’ (entry 6). The reaction was affected by steric
hindrance from a substituent at the ortho-position (entry 7).
Although an imidate with a naphthyl group, 1i, had high
reactivity, a mixture of two regioisomers, 3i and 3i’, was
generated (entry 8). The reaction of the N-alkyl imidate 1j
gave the desired product 3j in 84% yield (entry 9). The
9
1j
3j: 84
10
1k
3k: 72
À
reaction also proceeded at the olefinic C H bond and the
corresponding cyclic product 3k was produced in 72% yield
(entry 10).
[a] Used 1.0 equiv 2a. [b] Yield of the isolated product.
Next, we investigated the scope of isocyanates (Table 2).
The annulation reaction proceeded in excellent yield using
aryl isocyanates (2b–e) having either an electron-donating or
electron-withdrawing group (entries 1–4). In entry 4, the
desired product 3o was obtained quantitatively without loss
of the bromine atom, and the reaction was not inhibited by
steric hindrance (entry 5). 1-Naphthyl isocyanate (2g) also
provided the corresponding 3-imino-1-isoindolinone 3q
quantitatively (entry 6). The 3-imino-1-isoindolinones 3r
and 3s were obtained in 88% and 87% yields, respectively,
from the corresponding primary and secondary alkyl isocya-
nates 2h and 2i.[12]
The proposed mechanism for the rhenium-catalyzed
formation of the 3-imino-1-isoindolinones 3 is as follows
(Scheme 1): 1) oxidative addition of methyl benzimidate 1 to
the rhenium catalyst (C H bond activation),[6] 2) insertion of
À
2 into the formed rhenium–carbon bond,[6b] 3) intramolecular
nucleophilic cyclization,[6b] and 4) reductive elimination and
elimination of methanol to give 3 and regenerate the rhenium
catalyst. In fact, methanol was formed during the reaction as
mentioned above. In this reaction, the elimination of meth-
anol is important for retaining the imino group in the
products.[13]
Notably, the yield of 3a was also high even on gram scale.
When 845 mg of 1a was used as the substrate, the reaction
2
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
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