60 ◦C with no catalyst, and under these conditions no formation
of acetophenone phenylhydrazone (1) was observed after 45 hours.
The ability of [Ir(bpm)(CO)2]BArF (2) to catalyse the hydroami-
nation of phenylethyne with phenylhydrazine was also tested under
conditions that were not inert. The sample was prepared in air
using CDCl3 which had not been pre-dried or degassed and the
conversion obtained was comparable to that performed under an
inert atmosphere, although slightly slower in the beginning of the
reaction (Table 2). This result demonstrates the robust nature of
the BArF- complex 2, and its potential application in industry
compared to many more air-sensitive catalysts.
With optimum reaction conditions for the intermolecular
hydroamination of phenylethyne with phenylhydrazine using 2
established as CDCl3 as solvent at 60 ◦C and toleune-d8 as solvent
at 100 ◦C the reaction scope was extended to include other terminal
alkynes (4-ethynyltoluene, 1-hexyne and 1-octyne) and substi-
tuted hydrazines (N-aminopiperidine, 1,1-dimethylhydrazine and
methylhydrazine) (Table 3).
The overall efficiency of conversion of the two aromatic alkynes
was higher than the efficiency of conversion of the two aliphatic
alkynes, with conversions of approximately 70% for phenylethyne
and 4-ethynyltoluene after 21 hours, but only 48% for 1-octyne
at the same temperature. The para-substituent on the phenyl
ring of 4-ethynyltoluene had minimal affect on the efficiency of
the catalysed hydroamination reaction when compared to the
efficiency of conversion of the unsubstituted phenylethyne.
The efficiency of the catalysed conversions of 1-octyne and
1-hexyne to 2-octanone phenylhydrazone (7) and 2-hexanone
phenylhydrazone, respectively, were almost identical, when per-
formed under the same reaction conditions (CDCl3 at 60 ◦C). The
catalysed conversion of 1-octyne to the hydrazone 7 over 22 hours
was increased by a factor of 2 when the temperature of the reaction
was raised from 60 ◦C to 100 ◦C. This is opposite to the trend
observed for the aromatic alkynes, which showed comparable, if
not lower, conversions when temperatures were raised.
The low efficiency of conversion of 1-hexyne and 1-octyne to
the expected hydrazones was most likely due to the decomposition
of the starting materials or conversion of the starting material
into other products. Possible side-products being formed during
the catalysed reaction could be those due to anti-Markovnikov
addition of the hydrazine to the terminal alkyne. Previous work
performed by Odom and co-workers10 on the metal catalysed
hydroamination of terminal alkynes with 1,1-dimethylhydrazine
indicated that the regioselectivity of the reaction was influenced
by the electronic structure of the alkyne substrate. However, in the
reactions described here there was no evidence of the formation of
either of the E- or Z-isomers of the product of anti-Markovnikov
addition.
methylhydrazone. The efficiency of the catalysed addition of
the two di-substituted hydrazines, 1,1-dimethylhydrazine and N-
aminopiperidine, to phenylethyne was similar in CDCl3 at 60 ◦C,
with conversions of around 25% after 42 hours for each. Increasing
the temperature to 100 ◦C only improved the conversion of
N-aminopiperidine to N-(1-phenylethylidene)-1-piperidinamine
from 28% to 35% after 21 hours, with no significant change after
that time.
The catalysed intermolecular hydroamination of phenylethyne
was also performed using aniline in place of a hydrazine substrate.
The efficiency of conversion of phenylethyne with aniline to N-
(1-phenylethylidene)-benzenamine in CDCl3 at 60 ◦C was far less
than the efficiency of the hydroamination reaction using phenyl-
hydrazine. This indicates that substituted hydrazines undergo
catalysed intermolecular hydroamination of terminal alkynes
much more readily than primary amines when the catalytic system
described in this work is used.
Microwave heating, as opposed to conventional thermal heat-
ing, has been shown to dramatically improve the efficiency
of both standard synthetic, as well as catalysed, reactions.19,20
The efficiency of catalysed hydroamination of phenylethyne with
phenylhydrazine using [Ir(bpm)(CO)2]BArF (2) (5 mol%) was
tested using microwave irradiation as the heating mechanism in
order to improve the yield of the hydrazone product, however
the rate of conversion to acetophenone phenylhydrazone (1) was
not improved by employing microwave irradiation, even when the
maximum power (300 W) was maintained over the whole heating
time. Comparable conversions of substrates to 1 were obtained
when the catalysed reaction was performed in both CDCl3 at 60 ◦C
and toluene-d8 at 100 ◦C for the two heating methods.
The best previously reported catalysed addition of phenylhy-
drazine to phenylethyne to yield 1 is with a system incorporating
(Ph3P)Au(CH3) (0.2 mol%) and NH4PF6 (1 mol%) which pro-
moted a conversion of 99% after 4 hours.12 The efficiency of this
catalyst for other substrates was not demonstrated, however. Other
previously reported intermolecular hydroamination reactions with
1,1-disubstituted hydrazines were catalysed by titanium complexes
and achieved conversions of alkynes to hydrazone products of up
to 88% in 2 hours.10 These reactions, however, produced a mixture
of Markovnikov and anti-Markovnikov products, depending on
the substituent on the alkyne group.
In conclusion, we have demonstrated the efficient and selective
intermolecular hydroamination of alkynes with hydrazines using
Rh(I) and Ir(I) metal complexes. [Ir(bpm)(CO)2]BArF (2) was
found to be the best catalyst for the conversion of phenylethyne
and phenylhydrazine to a single product, acetophenone phenyl-
hydrazone (1), promoting a conversion of 70% after 22 hours.
The complexes reported here containing the BArF- counterion
increased the rate of conversion for the hydroamination reaction
by at least a factor of 2, when compared to tetraphenylborate com-
plexes of similar structures. Overall, the catalysed hydroamination
reactions performed with both substrates containing an aromatic
group were the most efficient at yielding the hydrazone products.
In most of the catalysed hydroamination reactions performed the
substrates were consumed before full conversion of substrates to
products could be achieved, indicating side-reactions were taking
place.
The hydroamination of phenylethyne with phenylhydrazine was
much more successful than the hydroamination reactions using
other substituted hydrazines, with a conversion of alkyne to ace-
tophenone phenylhydrazone (1) more than twice that of the other
substituted hydrazines (79% compared to N-aminopiperidine
which gave the next highest conversion of 35%). Methylhydrazine
gave a very poor level of hydroamination with only 12% conversion
after 42 hours. Monitoring the reaction by 1H NMR spectroscopy
showed that over 80% of the methylhydrazine had been consumed
after 5 hours leading to the formation of by-products, which
would account for the low conversion to the desired acetophenone
Financial support from the Australian Research Council
and The University of New South Wales is gratefully
6370 | Dalton Trans., 2008, 6368–6371
This journal is
The Royal Society of Chemistry 2008
©