Catalytic amination reactions mediated by Co(II) Schiff base complexes

Article abstract of DOI:10.1016/j.jorganchem.2005.01.021

Co(acacen), 1, (acacen = 2,11-dihydroxy-4,9-dimethyl-5,8-diaza-2,4,8,10- dodecatetraene dianion) was found to be a highly efficient catalyst for the allylic amination of non activated alkenes, using N-(p-toluensulfonyl) iminophenyliodinane (PhINTs) as nitrene precursor. This reactivity has been extended to the less reactive C-H bond of toluene. The effect of reaction times and of added cosolvent on yields and selectivities was investigated. Under the best conditions, allylic amines were obtained in a 40-70% isolated yield. A complex derived from the stoichiometric reaction of Co(acacen), 1, with PhINTs has been isolated and spectroscopically characterized. Such a complex, although not able to transfer its NTs moiety to alkenes, is still active in catalyzing allylic amination of cyclohexene.

Full text of DOI:10.1016/j.jorganchem.2005.01.021

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Journal of Organometallic Chemistry 690 (2005) 2142–2148
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Catalytic amination reactions mediated by Co(II) Schiff
base complexes
*
Alessandro Caselli, Emma Gallo, Fabio Ragaini, Alessandro Oppezzo, Sergio Cenini
`
Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Universita di Milano, and ISTM-CNR, Via Venezian 21, 20133 Milano, Italy
Received 20 December 2004; accepted 18 January 2005
Available online 19 February 2005
Abstract
Co(acacen), 1, (acacen = 2,11-dihydroxy-4,9-dimethyl-5,8-diaza-2,4,8,10-dodecatetraene dianion) was found to be a highly effi-
cient catalyst for the allylic amination of non activated alkenes, using N-(p-toluensulfonyl)iminophenyliodinane (PhI@NTs) as nit-
rene precursor. This reactivity has been extended to the less reactive C–H bond of toluene. The effect of reaction times and of added
cosolvent on yields and selectivities was investigated. Under the best conditions, allylic amines were obtained in a 40–70% isolated
yield. A complex derived from the stoichiometric reaction of Co(acacen), 1, with PhI@NTs has been isolated and spectroscopically
characterized. Such a complex, although not able to transfer its NTs moiety to alkenes, is still active in catalyzing allylic amination
of cyclohexene.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Cobalt; Schiff bases; N-(p-toluensulfonyl)iminophenyliodinane; Amination reactions; Allylic amines
1. Introduction
enes have received increasing attention. Stoichiometric
allylic amination of olefins has been performed with sul-
fur [11,12] and selenium [13] imido compounds and pro-
ceeds with retention of the double bond position. On the
other hand several indirect routes mostly based on ene
reactions of organonitrogen compounds, including
azo- [14–16], nitroso- [17–20], and N-sulfinylcarbamate
derivatives [21], exhibit high regioselectivities with dou-
ble bond transposition, but require additional N–N or
N–O reduction steps to produce allyl amines. The me-
tal-catalyzed direct electrophilic amination of alkenes
represents an attractive way for the synthesis of allylic
amines. A stoichiometric allylic amination employing
molybdenum oxaziridines has been introduced by
Sharpless [22] and in 1992 Nicholas reported the first
catalytic version of this reaction using phenylhydroxyl-
amine as nitrogen-containing reagent and a dioxomo-
lybdenum(VI) complex as catalyst [23]. Since then
some molybdenum- [24], iron- [25–29] or more recently
copper-catalyzed [30,31] processes employing arylhydr-
The development of new methods for the direct and
selective synthesis of organonitrogen compounds from
hydrocarbons represents one of the great challenges in
both academy and industry. In contrast to hydrocarbon
oxidation, which has been developed into several impor-
tant industrial and laboratory processes for the produc-
tion of oxygenated compounds [1–6], the metal
catalyzed intermolecular nitrogen-atom transfer reac-
tion is an attractive goal that remains largely elusive
[7,8]. Commercially important examples are few, i.e.,
the Mo–Bi catalyzed ammoxidation of propylene to
acrylonitrile [9] and the Ni catalyzed hydrocyanation
of butadiene to adiponitrile [10]. During the last decade,
laboratory methods for the N-functionalization of alk-
*
Corresponding author. Tel.: +39 02 50314392; fax: +39 02
50314405.
E-mail address: sergio.cenini@unimi.it (S. Cenini).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.01.021

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A. Caselli et al. / Journal of Organometallic Chemistry 690 (2005) 2142–2148
2143
oxylamines have been reported. These reactions proceed
regioselectively with N-functionalization at the less
substituted olefinic carbon. Another approach employs
an amine in the presence of an oxidant as the aminating
agent, thus avoiding the preliminary synthesis of the
hydroxylamine [32]. We recently introduced a new syn-
thetic way to produce allyl amines, using the more read-
ily available nitroarenes as aminating agents, under
reducing conditions (CO pressure) and in the presence
of Ru3(CO)12/diimine as catalyst [33–36]. Subsequently,
cyclopropanation of olefins [55–58], to the best of our
knowledge their use has never been extended to the ami-
nation reactions. Tailored tetradentate Schiff base com-
plexes with two axial sites open to ancillary ligands, are
very much like porphyrins, but more easily prepared and
their use is gaining an increasing attention [59]. Nowa-
days active and well designed Schiff base ligands are
considered ‘‘privileged ligands’’ [60] because they are
easily prepared by the condensation between aldehydes
and amines and are able to stabilize different metals in
various oxidation states. We report here the results ob-
tained using simple Schiff base complexes of Co(II) as
catalysts in the amination reactions of alkenes employ-
ing PhI@NTs as nitrene precursor.
the use of [Cp( )Fe(CO) ] as catalyst [37], the photoas-
*
2 2
sisted version of this reaction [38] and the use of nitros-
oarenes as aminating agents [39] have been reported. On
the other hand it has long been established that nitrenes
or nitrene precursors, such as N-(p-toluensulfonyl)imin-
ophenyliodinane (PhI@NTs) can add to an alkene,
forming an aziridine, or insert into the allylic C–H bond,
forming an allylamine [40]. Despite the fact that the azir-
idination reaction has been recently developed and quite
efficient systems are now available for the enantioselec-
tive addition of the tosyl nitrenoid species to alkenes
[8,41–48], the selective insertion of nitrenoids into the
allylic C–H bond has been explored only to a low extent
[49,50].
In our ongoing study on amination reactions cata-
lyzed by transition metals, we have recently reported
that porphyrin complexes of cobalt(II) are able to acti-
vate aromatic azides for the amination under mild con-
dition of allylic C–H bonds [51] and for the even more
difficult activation of the C–H bonds of saturated organ-
ic compounds, to give secondary amines and imines
[52,53] (Scheme 1).
2. Results and discussion
2.1. Catalytic reactions with cyclohexene
To evaluate the efficiency of Co(acacen), 1, (acacen =
2,11-dihydroxy-4,9-dimethyl-5,8-diaza-2,4,8,10-dodeca-
tetraene dianion) as catalyst for the intermolecular
nitrogen-atom insertion into unactivated C–H bonds,
the reaction of cyclohexene with N-(p-toluensulfo-
nyl)iminophenyliodinane (PhI@NTs) was studied
(Scheme 2). The same reaction was repeated in the pres-
ence of Co(TPP), 2, (TPP = tetraphenylporphyrin dian-
ion) as catalysts and the results obtained with the two
systems were compared.
The results obtained employing different experimen-
tal conditions are reported in Table 1. The best results
were obtained with a catalyst/PhI@NTs molar ratio
1:15, employing a mixture of cyclohexene/C6H6 4:1 as
solvent. Under these conditions the reaction was com-
plete in 8 h (the reaction can be followed by the dissolu-
tion of PhI@NTs and monitored by TLC), yielding the
allylic amine 3 as the sole cyclohexene derived aminated
product (67% isolated yield based on the starting N-(p-
toluensulfonyl)iminophenyliodinane; Entry 4, Table 1).
When cyclohexene was employed as the substrate, the
Co(II) porphyrin complexes tested showed a remarkable
selectivity for the allylic amine (53–61%) production
[51]. Diazo compounds are isoelectronic with aromatic
azides and recently we have shown for the first time that
Co(II) porphyrin complexes may also act as catalysts for
the cyclopropanation reactions [54]. Although cobalt(II)
Schiff bases complexes are excellent catalysts for the
cat.
ArCH2R + Ar'N3
+ Ar'N3
ArC(R) NAr'+ArC(R)(H) NHAr'+ Ar'NH2 + N2
cat.
+ N2
NHAr'
R2
R1
R2
[Co(TPP)]: R1 = Ph, R2 = H
R2
R2
R1
R2
[Co(p-MeOTTP)]: R1 = p-MeOC6H4, R2 = H
[Co(p-ClTPP)]: R1 = p-ClC6H4, R2 = H
[Co(OEP)]: R1 = H, R2 = Et
N
N
R1
R2
cat =
CoII
R1
N
N
R2
R2
Scheme 1.

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2144
A. Caselli et al. / Journal of Organometallic Chemistry 690 (2005) 2142–2148
O
S
O
O
S
O
O
S
O
O
S
O
H
75 ˚C
cat.
I
N
N
N
H2N
+
+
+
3
5
4
O
N
O
Co(II)
or
Co(TPP)
cat. =
N
2
1
Scheme 2.
Table 1
Amination of cyclohexene by PhI@NTs in the absence or in the
for the catalytic reactions. No reaction occurred. After
8 h no trace of the allylic amine could be detected and only
the unreacted aziridine 4 was recovered. Second, we
added a known amount of the isolated aziridine 4 (2 moles
per mol of the catalyst) to the reaction mixture (Entry 5).
After 8 h we obtained the expected amount of allylic
amine 3 (61% with respect to the starting N-(p-tolu-
ensulfonyl)iminophenyliodinane only) and an amount
of the aziridine 4 corresponding exactly to the added
one (97.8%). At this stage, the reason for the different
selectivities observed after 3 and 8 h is unclear, but the
experimental data collected allows us to exclude that azi-
ridine is an intermediate in the formation of the allylic
amine. More probably the formation of the allylic amine
can occur through two independent mechanisms and in
one of this two mechanisms one sufficiently stable inter-
mediate is formed which can evolve into the aziridine
and the sulfonamide during the work up when the reac-
tion is quenched before completeness. In the presence of
the catalyst, this intermediate is instead slowly trans-
formed into the allylic amine. It should be noted that,
when the reaction is stopped before completion
(undissolved PhI@NTs still present), the starting N-(p-
toluensulfonyl)iminophenyliodinane is recovered as sul-
fonamide after column chromatography. This explains
the high yield of sulfonamide at short reaction times.
The added benzene plays a relevant role: under these
conditions both the catalyst and the aminating agent are
soluble in the reaction medium. Iminoiodinanes are rel-
atively strong oxidants, with very limited solubility in
most solvents [61]. The use of CH2Cl2 as a cosolvent
causes a significant drop in the selectivity of the reaction
(Entry 6). To investigate the possible role of adventi-
tious traces of water on the reaction mixture in the for-
mation of the p-toluensulfonamide we run the reaction
in the presence of molecular sieves. The beneficial effect
of molecular sieves in decreasing the amount of the
amide formed in similar reactions has already been re-
ported in the literature [42]. Unfortunately in our case
we did not observe any positive effect upon the addition
of molecular sieves (Entry 7). Anyway when water was
added as a cosolvent (Co(acacen), 1, displays a remark-
able solubility in neutral aqueous solutions) p-toluen-
sulfonamide was recovered as the only product and no
trace of the allylic amine was detected (Entry 8).
presence of Co(II) catalystsa
Entry Catalyst
t
Cosolvent Allylic
amine
Aziridine Sulfonamide
(5) (%)b
(h)
(4) (%)b
(3) (%)b
1
None
8
3
3
8
8
3
3
8
3
8
C6H6
C6H6
C6H6
C6H6
C6H6
CH2Cl2
C6H6
H2O
4.4
14.3
24.1
66.9
61.9
8.8
3.9
8.7
26.5
–
90.5
77.7
47.2
29.8
28.9
79.0
45.5
85.0
79.7
30.1
2
1
1
1
1
1
1
1
2
2
3c
4c
5d
6
d
–
8.8
8.8
–
7e
14.5
–
8
9e
CH2Cl2
C6H6
9.0
8.7
–
10
64.5
a
Typical experimental conditions: catalyst = 8.7 mg, mol ratio
PhI@NTs/catalyst = 20, in 8 ml of cyclohexene plus 2 ml of cosolvent,
at 75 ꢁC.
b
Isolated yields with respect to starting PhI@NTs.
Mol ratio PhI@NTs/catalyst = 15.
In the presence of added 4 (12.9 mol% with respect to PhI@NTs),
c
d
mol ratio PhI@NTs/catalyst = 15. The added aziridine, 4, (97.8%) was
recovered unchanged at the end of the reaction.
e
˚
In the presence of 4 A molecular sieves.
The only observed byproduct of the reaction is the cor-
responding p-toluensulfonamide, 5, (30%), which can be
formed either during the reaction or during the work-up
(see later).
Interestingly when the reaction is stopped after 3 h
(Entry 3), not only the yield of the isolated allylic amine
3 decreases, as expected, but an equal amount of the cor-
responding aziridine 4 is obtained as well. The lack of
selectivity on a shorter time scale could apparently seem
odd, but can be rationalized taking into account the fol-
lowing considerations. The uncatalyzed reaction pro-
ceeds slowly to give equimolar amounts of aziridine 4
and allylic amine 3 (Entry 1). Two different mechanisms
should be operating with similar rates. Apparently the
catalyst can accelerate both processes (Entries 2 and 3).
It may be supposed that at longer reaction times the
formed aziridine should be converted into the allylic
amine. To check whether or not the aziridine is an inter-
mediate in the formation of the allylic amine we run two
different experiments. First, we treated the isolated aziri-
dine 4 with 1 under the same reaction conditions used

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2145
The reaction of cyclohexene with PhI@NTs was also
run in the presence of Co(TPP), 2, that, as already men-
tioned in the introduction, has been successfully applied
by us as a catalyst for the allylic amination of non-acti-
vated double bonds [51] and the amination of benzylic
hydrocarbons [52,53] by arylazides. The results are sim-
ilar to those observed for the reactions catalyzed by 1
(Entries 9 and 10). It should be mentioned that
PhI@NTs failed to give benzilic amination when the
reaction was conducted in presence of 2 as catalyst
[52,53].
a clean reaction and only p-toluensulfonamide, 5, was
recovered along with traces of the aziridine 11 (Entries
6 and 7). In this case, products recovery from the crude
was complicated by the formation of polystyrenic
byproducts in large amounts.
We have reported thus far the efficiency of 1 in acti-
vating PhI@NTs for the amination under mild condi-
tions of allylic C–H bonds. The same catalytic system
can also perform the more difficult activation of the
C–H bond of the methyl group of toluene. The reaction
does not stop to the formation of the secondary amine
12 (Eq. (1)) but proceeds further, at least in part, to give
the imine 13 according to Eq. (2):
2.2. Catalytic reactions with other alkenes and with
toluene
O
H
Co(acacen)
N
S
O
+ PhI
+
PhI=NTs
We next examined the allylic amination of different
alkenes under the optimized conditions. The results are
summarized in Table 2 (see Chart 1).
12
ð1Þ
As expected the reaction of cyclopentene gave the
allylic amine 6 in high isolated yield (63%, Entry 2).
Only a 4% of the aziridination product 7 could be de-
tected after 8 h. The reaction failed to give the allylic
amination when a linear alkene like 1-octene was used
(Entry 3). Moreover, only traces of the aziridine 8 could
be detected. It should be noted that a drop in yields of
the aziridination of linear alkenes by aryliminophenylio-
dinanes catalyzed by rhodium complexes has already
been reported and that no allylic amination was ob-
served [50]. We obtained fairly good results with a-
methylstyrene. The allylic amine 9 was obtained in
40% yield after 8 h (Entry 5), while stopping the reaction
after 3 h yielded equal amounts of the aziridine 10
(14.5%) and the allylic amine 9 (14.7%) (Entry 4). Sty-
rene, which cannot give the allylic amine, failed to give
O
H
Co(acacen)
PhI=NTs
+
N S
O
O
O
N S
+ PhI
+
H2N S
O
13
O
5
ð2Þ
When a toluene (10 ml) solution of Co(acacen), 1,
was treated with PhI@NTs (catalyst/substrate molar ra-
tio 1:14) the reaction proceeded in 3 h to give 4-methyl-
N-[1-phenyl-meth-(E)-ylidene]-benzenesulfonamide, 13,
in a 45.4% isolated yield, along with N-benzyl-4-
methyl-benzenesulfonamide, 12, (4.2%) and p-toluen-
Table 2
Amination of different alkenes by PhI@NTs catalyzed by 1a
Entry
1c
Substrate
Allylic amine (%)b
Aziridine (%)b
Sulfonamide (5) (%)b
t (h)
3
10.6 (6)
11.7 (7)
74.6
2
8
3
63.6 (6)
4.3(7)
29.0
69.3
3c
-
Traces (8)
4c
3
8
14.5 (9)
40.0 (9)
14.7 (10)
65.2
44.8
5
-
6c
3
8
-
-
Traces (11)
Traces (11)
45.7
35.5
7
a
Catalyst = 8.7 mg, mol ratio PhI@NTs/catalyst 1:15, in 8 ml of alkene plus 2 ml of C6H6, at 75 ꢁC.
Isolated yields with respect to starting PhI@NTs.
5 ml of alkene plus 5 ml of C6H6.
b
c