reactions through the formation of monoacyl boronate
intermediates.5,6 However, these arylboronic acids are not
effective for the catalytic dehydrative self-condensation of
carboxylic acids because of the much lower nucleophilicity
of carboxylic acids than amines and alcohols. In 2006,
Whiting and co-workers reported arylboronic acid 1a
bearing an (N,N-diisopropylamino)methyl group at the
2-position as an efficient catalyst for amide condensation
reaction.7 In some cases, 1a is more advantageous than
electron-deficient arylboronic acids, although the role of
the (N,N-diisopropylamino)methyl group is unclear for
the amide condensation reaction.8 On the basis of these
previous results, we envisioned that an arylboronic acid of
general type 1 bearing an (N,N-dialkylamino)methyl
group7,9-14 would promote the dehydrative self-condensa-
tion of carboxylic acids under mild conditions through
activation of the other carboxyl group using the (N,N-
dialkylamino)methyl group as a Brønsted base in a mono-
acyl boronate intermediate 2 (Figure 1). We report here
Brønsted base-assisted boronic acid catalysis for the in-
tramolecular dehydrative condensation of di- and tetra-
carboxylic acids.
Figure 1. Working hypothesis: Brønsted base-assisted boronic
acid catalysis.
(10 mol %) that promoted the intramolecular dehydrative
condensation of phthalic acid (Table 1). The reaction was
conducted in nonane (bp 151 °C) under azeotropic reflux
conditions with the removal of water. Arylboronic acids
bearing electron-withdrawing substituents, some of which
Table 1. Catalytic Dehydrative Condensation of Phthalic Acida
According to our working hypothesis, we first investi-
gated the catalytic activities of various arylboronic acids
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€
a The reaction of phthalic acid (2.5 mmol) was conducted in the
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€
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1
b Determined by H NMR analysis. c Boiling point. d Data in parenth-
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eses refer to conversion yield in the presence of 1,2,2,6,6-pentamethylpi-
peridine (20 mol %).
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