Amination of benzylic C–H bonds by aryl azides catalysed by CoII(porphyrin)
complexes. A new reaction leading to secondary amines and imines†
Sergio Cenini,*a Emma Gallo,a Andrea Penoni,a Fabio Ragainia and Stefano Tollarib
a Dipartimento di Chimica Inorganica, Metallorganica e Analitica and CNR Center, Via Venezian 21, 20133 Milano,
Italy. E-mail: Sergio.Cenini@unimi.it
b Dipartimento di Scienze Chimiche, Fisiche e Matematiche, Via Valleggio 11, 22100 Como, Italy
Received (in Cambridge, UK) 2nd August 2000, Accepted 5th October 2000
First published as an Advance Article on the web
CoII(porphyrin) complexes catalyse the reaction of aromatic
azides, ArN3, with hydrocarbons containing a benzylic
group, R1R2R3CH, to give the corresponding amines,
R1R2R3C-NHAr. When at least one of the R1–R3 sub-
stituents is hydrogen, the catalytic reaction proceeds further
to give the imines R1R2CNNAr in good yields.
The reaction quantitatively gives one equivalent of imine and
one of p-YC6H4NH2 per equivalent of amine while no reaction
is observed in the absence of the catalyst.
Main by-products of reaction (1) are the azide-derived aniline
and the diarylazo compound.
Besides Co(TPP) (TPP = dianion of meso-tetraphenylpor-
phyrin) even Co(p-MeOTPP) [p-MeOTPP = dianion of meso-
tetrakis(4-methoxyphenyl)porphyrin], Co(p-ClTPP) [p-ClTPP
The development of metal-mediated intermolecular nitrogen
atom transfer reactions, such as the amination of saturated C–H
bonds, is a topic of high current interest.1–8 Reasonably good
results have only been obtained by using [N-(p-tolylsulfonyl)-
imino]phenyliodinane (PhINNTs) and related derivatives as the
nitrogen source. However these compounds can only be
obtained from the corresponding sulfonamides.9 Recently, we
have developed a methodology for the intermolecular allylic
amination of unactivated olefins by nitroarenes in the presence
of CO and ruthenium complexes as catalysts.10 However, in
these reactions only the allylic C–H bond of an olefin could be
functionalised.
Aromatic azides, ArN3, represent a wide class of easily
synthesised compounds. We have recently reported that por-
phyrin complexes of cobalt(II) are able to activate aromatic
azides, for the amination under mild conditions of allylic C–H
bonds11 and we report here that the same catalytic system also
results in the more difficult activation of the C–H bonds of
saturated organic compounds, to give secondary amines and
imines. The use of organic azides as aminating agents extends
considerably the number of nitrogen containing organic com-
pounds that can be potentially obtained in catalytic reactions.
Such a reaction has never been reported before. It should also be
pointed out that although the condensation of ketones with
primary amines is a textbook route to ketimines, the procedure
is sometimes not practical for the synthesis of aromatic
ketimines.12,13
=
dianion of meso-tetrakis(4-chlorophenyl)porphyrin] and
Co(OEP) (OEP = dianion of b-octaethylporphyrin) were tested
as catalysts. Representative results are reported in Table 1. Note
that the stoichiometry in eqns. (1) and (2) implies that a
maximum yield of 50% can be obtained with respect to the aryl
azide when the imine is the product.
Steric hindrance lowers the selectivity, but does not prevent
the reaction and with sec-butylbenzene, the amine was formed
in 10–15% yield (Table 1).
The reaction is sensitive to the nature of the azide employed.
With toluene as substrate and Co(TPP) as catalyst, electron-
withdrawing substituents in the para position of the aromatic
azide not only increase the yields of the imine, but also notably
accelerate the reaction (Table 2).
To clarify the mechanism of this reaction we performed some
stoichiometric reactions and a kinetic study. When Co(TPP)
was reacted with excess p-O2NC6H4N3 at 75 °C in benzene the
diazoarene p-O2NC6H4NNNC6H4NO2-p was catalytically ob-
tained. When the same reaction was repeated at a lower
1
temperature in C6D6, a new complex was observed whose H
NMR signals are identical to the ones of the product obtained by
reaction of Co(TPP) with p-O2NC6H4NNNC6H4NO2-p. Ob-
servation of only one set of resonances for the aryl groups of the
ArNNNAr moiety of this complex indicates that the aromatic
groups of the ‘ArN’ moieties are equivalent. According to these
preliminary results and to some similar data reported in the
literature,14 we propose that the reaction product of Co(TPP)
with ArN3 is the complex [Co(TPP)]2(ArNNNAr), having an
aryldiazene ligand bridging two porphyrin complexes.
By reaction of hydrocarbons containing a benzylic group
with p-nitrophenyl azide catalysed by Co(II)(porphyrin) deriva-
tives, the corresponding amines have been obtained [eqn.
(1)].
A
kinetic study was also performed employing p-
O2NC6H4N3 and toluene as substrates and Co(TPP) as catalyst.
The kinetics are first order in Co(TPP) and in azide, but show a
saturation behaviour with respect to toluene concentration, the
other solvent being benzene (Fig. 1, ESI†). When the selectivity
was examined as a function of time for different toluene–
benzene mixtures the following observations were made. (a)
The amount of imine and p-nitroaniline grow at the same rate,
in accord with the stoichiometry reported in eqns. (1) and (2).
(b) The rate of formation of p-O2NC6H4NNNC6H4NO2-p is
proportional to the one of the imine for each reaction, indicating
the same dependency from the azide concentration. (c) The rate
of formation (and the final selectivity) of imine increases with
the toluene concentration (up to 100% toluene in the solvent
mixture) while the one of the diarylazo compound decreases in
the same series, indicating a common intermediate and
competition for the formation of the two products.
(1)
When at least one of the R1–R3 substituents is hydrogen, the
catalytic reaction proceeds further to give the corresponding
imines [eqn. (2)]:
(2)
Reaction (2) has been independently observed by preforming
the amines PhCH2-NHC6H4Y-p and reacting them with p-
YC6H4N3 (Y = NO2, Cl, OMe) in the presence of the catalyst.
† Formulae of the catalysts and of the organic products and Fig. 1, the
influence of the toluene concentration on the value of kapp are available as
suppdata/cc/b0/b006136k
All of the aforementioned data can be accounted for by the
mechanism shown in Scheme 1.
DOI: 10.1039/b006136k
Chem. Commun., 2000, 2265–2266
This journal is © The Royal Society of Chemistry 2000
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Cenini
