Tetrahedron Letters 48 (2007) 3367–3370
Selective N-monoalkylation of anilines catalyzed by a
cationic ruthenium(II) compound
Sipra Naskar and Manish Bhattacharjee*
Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India
Received 1 December 2006; revised 6 March 2007; accepted 14 March 2007
Available online 19 March 2007
Abstract—[(PPh3)2Ru(CH3CN)3Cl][BPh4] has been found to catalyze the selective monoalkylation of anilines by alcohols.
Ó 2007 Elsevier Ltd. All rights reserved.
Selective monoalkylation of amines is of fundamental
importance.1–6 For example, monoalkylation of aniline
is important in industry.6 The most common method
of N-alkylation is reaction of an alkyl halide and an
amine in the presence of base.7 However, this method
is undesirable from an environmental point of view.7
Since the monoalkylated products have a tendency to
react further to produce di- and tri-alkylated amines,1,8
N-alkylation using alkyl halides suffers from non-selec-
tivity. There are a large number of reports on the alkyl-
ation of aniline by reaction with alcohols in the presence
of solid acid catalysts.9 However, in most cases, mono-
and dialkylated products are obtained. A recent report
describes the monoalkylation of aniline with alcohols
in the presence of an iridium catalyst in toluene.7 Ruthe-
nium complexes have also been used as catalysts for
N-alkylation with alcohols.10,11 In most cases, dialkyl-
ated products were obtained as the major products.10,11
Recently, we reported the synthesis and structure of
the cationic ruthenium complex, [RuCl(PPh3)2-
(CH3CN)3]+[BPh4]À, which was found to be an effective
catalyst for transfer hydrogenation of aldehydes and
ketones.12 Also, [RuCl(PPh3)2(CH3CN)3]+[BPh4]À was
found to catalyze selective reduction of C@C double
bonds in a,b-unsaturated ketones by formic acid.13 We
were interested in exploring the catalytic activity of this
ruthenium complex for alkylation of amines. We report
herein [RuCl(PPh3)2(CH3CN)3]+[BPh4]À (1) catalyzed
selective monoalkylation of aniline by alcohols under
solvent-free conditions.
A mixture of aniline (1 mmol) and 1 (0.01 mmol) was
treated with an excess of alcohol (10 cm3) and anhy-
drous K2CO3 (1 mmol) under argon in a Schlenk flask.
The reaction mixture was then heated at 80–110 °C for
10 h. (Scheme 1). The reaction mixture was cooled to
room temperature, diluted with water (50 cm3), ex-
tracted with ether and after work-up, the products were
isolated and purified by column chromatography. The
reactions were monitored by TLC and the products were
isolated in low to high yield (Table 1). The products
were characterized by elemental analyses and 1H
NMR spectroscopy.
The conversion is dependent on the chain length of the
alkyl group of the alcohol. The highest conversion was
observed when methanol was used as the alkylating
agent and the conversion gradually decreases as the
chain length increases (Table 1). The most noteworthy
fact is that only monoalkylation was observed,
although, an excess of alcohol was used in all the reac-
tions. When secondary alcohols, such as 2-propanol
and 2-butanol, were used as alkylating agents, we did
*
Corresponding author. Tel.: +91 3222 283302; fax: +91 3222
Scheme 1.
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.03.075
Naskar, Sipra
