Origin of the deactivation in styrene aziridination by aryl azides, catalyzed by ruthenium porphyrin complexes. Structural characterization of a Δ2-1,2,3-triazoline RuII(TPP)CO complex

Article abstract of DOI:10.1021/om050244y

The reaction of Ru(TPP)CO (TPP = dianion of tetraphenylporphyrin) with 1-(p-nitrophenyl)-5-methyl-5-phenyl-1,2,3-triazoline yielded a Δ²-1,2,3-triazoline ruthenium(II) porphyrin complex, which is responsible for the catalyst deactivation in the aziridination reaction of α-methylstyrene by p-nitrophenyl azide.

Full text of DOI:10.1021/om050244y

4710
Organometallics 2005, 24, 4710-4713
Origin of the Deactivation in Styrene Aziridination by
Aryl Azides, Catalyzed by Ruthenium Porphyrin
Complexes. Structural Characterization of a
∆2-1,2,3-Triazoline RuII(TPP)CO Complex
Simone Fantauzzi,† Emma Gallo,† Alessandro Caselli,† Fabio Ragaini,†
Piero Macchi,‡ Nicola Casati,‡ and Sergio Cenini*,†
Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Universita` di Milano, and
ISTM-CNR, and Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Universita`
di Milano, and ISTM-CNR, Via Venezian 21, 20133 Milano, Italy
Received April 1, 2005
Scheme 1
Summary: The reaction of Ru(TPP)CO (TPP ) dianion
of tetraphenylporphyrin) with 1-(p-nitrophenyl)-5-meth-
yl-5-phenyl-1,2,3-triazoline yielded a ∆2-1,2,3-triazoline
ruthenium(II) porphyrin complex, which is responsible
for the catalyst deactivation in the aziridination reaction
of R-methylstyrene by p-nitrophenyl azide.
The transition-metal-catalyzed aziridination of alk-
enes is an area of active investigation, owing to the
utility of the aziridine ring in organic synthesis.1 This
reaction is efficiently catalyzed by porphyrin metal
complexes using mainly PhIdNSO2Ar as the nitrogen
source.2 However, while the mechanism of related
epoxidation and cyclopropanation reactions has been
deeply investigated, relatively little is known on the
mechanism of aziridination,3 especially concerning the
role of possible organic reaction intermediates.
We have recently reported that the catalytic amina-
tion of benzylic C-H bonds forms amines and imines
using aryl azides as aminating agents in the presence
of cobalt(II) porphyrin complexes.4 Aryl azides represent
a versatile class of aminating reagents, also active in
the reaction with styrenes to form N-arylaziridines
using Ru(TPP)CO (TPP ) dianion of tetraphenylpor-
phyrin) as catalyst.5 To improve the yields and purity
of the aminated products, we have performed an opti-
mization study of the experimental conditions for the
reaction between p-nitrophenyl azide and several olefins
(Scheme 1). Some results obtained under the improved
conditions are reported in Table 1. The aziridination
reactions were followed by IR spectroscopy, monitoring
the intensity of the 2121 cm-1 absorption of the N3 group
Table 1. Ru(TPP)CO-Catalyzed Aziridination of
Selected Olefins by p-Nitrophenyl Azidea
c
run
substrate
yield (%)b
k/[Ru] (s-1 M-1
)
1
2
3
4
5
styrene
R-methylstyrene
2-allylphenol
trans-anethole
diphenylethylene
98
99
32
62
90
0.81
2.20
2.00
1.98
2.50
a Experimental conditions: Ru(TPP)CO (9 mg, 0.012 mmol) in
30 mL of refluxing benzene; Ru(TPP)CO/ArN3/olefin ratio 1/50/
250. b Determined by 1H NMR (2,4-dinitrotoluene as internal
standard) at complete conversion of the starting azide. c k was
determined using the equation ln A ) ln A0 - kt (A ) IR absor-
bance value at time t and A0 ) IR absorbance value at zero time).
of the azide. All reactions showed a first-order depen-
dence of the rate on azide concentration, and the
corresponding kinetics constants are also reported in
Table 1.
To clarify the mechanism of the aziridination, we
performed a kinetic study of the model reaction between
R-methylstyrene and p-nitrophenyl azide catalyzed by
Ru(TPP)CO. The kinetics is always first order in azide
concentration and also shows a first-order dependence
on catalyst concentrations. A first-order dependence of
the rate on R-methylstyrene concentration is also ob-
served up to 30% (v/v) of olefin.6 At higher olefin
concentrations the reaction rate is lower than could be
expected on the basis of this kinetics (Figure 1).
To better understand these results, we studied the
uncatalyzed reaction between R-methylstyrene and p-
nitrophenyl azide at 75 °C. The kinetics is first order
in both aryl azide and R-methylstyrene, and 1-(p-
nitrophenyl)-5-methyl-5-phenyl-1,2,3-triazoline (1) was
isolated in 73% yield (Scheme 2).7
* To whom correspondence should be addressed. E-mail:
Sergio.Cenini@unimi.it. Fax: (+39)0250314405.
† Dipartimento di Chimica Inorganica, Metallorganica e Analitica.
‡ Dipartimento di Chimica Strutturale e Stereochimica Inorganica.
(1) (a) Katsuki, T. In Comprehensive Coordination Chemistry II;
McCleverty, J. A., Meyer, T. J., Eds.; Elsevier: Oxford, U.K., 2004;
Vol. 9, pp 207-264. (b) Mueller, P.; Fruit, C. Chem. Rev. 2003, 103,
2905-2919 and references therein.
(2) Liang, J.-L.; Huang, J.-S.; Yu, X.-Q.; Zhu, N.; Che, C.-M. Chem.
Eur. J. 2002, 8, 1563-1572.
(3) Au, S.-M.; Huang, J.-S.; Yu, W.-Y.; Fung, W.-H.; Che, C.-M. J.
Am. Chem. Soc. 1999, 121, 9120-9132.
(4) (a) Cenini, S.; Gallo, E.; Penoni, A.; Ragaini, F.; Tollari, S. Chem.
Commun. 2000, 2265-2266. (b) Ragaini, F.; Penoni, A.; Gallo, E.;
Tollari, S.; Li Gotti, C.; Lapadula, M.; Mangioni, E.; Cenini, S. Chem.
Eur. J. 2003, 9, 249-259.
(5) Cenini, S.; Tollari, S.; Penoni, A.; Cereda, C. J. Mol. Catal. A:
Chem. 1999, 137, 135-146.
1
The one- and two-dimensional H NMR spectra of 1
showed the presence in solution of only one of the two
(6) Since the boiling point of the solvent mixture changes as the
fraction of R-methylstyrene increases, all reactions were run at 75 °C.
Other experimental conditions are as given in the caption to Table 1.
10.1021/om050244y CCC: $30.25 © 2005 American Chemical Society
Publication on Web 08/26/2005

Communications
Organometallics, Vol. 24, No. 20, 2005 4711
of them have been structurally chacterized.10 Moreover,
most of the cycloadditions reported in the literature give
impure products and azido compounds react efficiently
only with electron-poor or strained alkenes, whereas
simple alkenes either do not react or react very slowly.
To investigate whether aziridine can be formed from
1 by N2 elimination, we analyzed the thermal and
photochemical stability of the triazoline molecule. Com-
pound 1 was heated at 180 °C in vacuo, and a decom-
position process was observed, but without the forma-
tion of aziridine; p-nitroaniline, propiophenone, and
1-phenylpropan-2-one (the last two in a 1/6 ratio) were
the only detected products. The presence of ketones in
the reaction mixture, formed by hydrolysis of the
corresponding imines obtained via dinitrogen elimina-
Figure 1. Dependence of the aziridination rate on R-me-
thylstyrene concentration. Experimental conditions are as
in the caption to Table 1, except for the R-methylstyrene
concentration, which varies.
1
tion,8,11 was confirmed by H NMR and GC-MS analy-
ses. The thermolysis of 1 probably did not afford the
corresponding aziridine because of the presence of the
methyl group on carbon 5 of the triazoline ring. It is
known8b that the nature of the decomposition products
of triazolines also depends on the electronic nature of
the substituents on the carbon atoms. If an alkyl group
is present on carbon 5, the rearrangement can also
proceed through a migration of the alkyl group from the
5- to the 4-carbon of the triazoline ring to afford an
imine, rather than by ring contraction to give an
aziridinine.
Scheme 2
The decomposition temperature (150 °C) was identi-
fied afterward by means of a TGA (thermal gravimetric
analysis) experiment.
Compound 1 was also irradiated with a low-pressure
mercury lamp at room temperature, but only decompo-
sition products were observed, probably due to compet-
ing processes associated with the excited aromatic nitro
group.12 The reaction of 1 equiv of 1 with Ru(TPP)CO
at room temperature was monitored by IR and 1H NMR
spectroscopy. The formation of aziridine was never
observed. Conversely, an immediate shift of the CO
stretching frequency and the 1H NMR triazoline signals
to higher field were respectively registered. These data
indicate that triazoline is not the precursor of aziridine
in the catalytic cycle. To confirm this hypothesis, we
investigated the reactivity of 1 with some RuII(Porph)-
CO complexes normally used as catalysts (eq 1).
Figure 2. Ortep drawing of 1. Selected bond lengths (Å):
N3-N2 ) 1.244(3); N2-N1 ) 1.373(2); N1-C5 ) 1.483-
(2); C4-C5 ) 1.546(3); C4-N3 ) 1.452(3). See the Sup-
porting Information for details.
RuII(Porph)CO + L f RuII(Porph)(L)CO
(1)
L ) 1; Porph ) TPP (2), p-ClTPP (3), p-MeOTPP (4)
possible regioisomers, that having the substituent-
bearing nitrogen of the aryl azide (N1) bonded to the
carbon bearing the phenyl and methyl groups (C5).
It should be emphasized that 1 is unusually stable,
and this allowed us to determine its molecular structure
by single-crystal X-ray diffraction analysis (Figure 2)
(see below).
Again, in all cases the formation of aziridine was not
observed, whereas triazoline ruthenium porphyrin com-
plexes were isolated in high yields (∼80%). Complexes
(9) (a) Wamhoff, H. In Comprehensive Heterocyclic Chemistry;
Katritzky, A. R., Rees, C. W., Eds.; Pergamon Press: Oxford, U.K.,
1984; Vol. 5, pp 669-732. (b) Fan, W. Q.; Katritzky, A. R. In
Comprehensive Heterocyclic Chemistry II; Katritzky, A. R., Rees, C.
W., Scriven, E. F. V., Eds.; Pergamon Press: Oxford, U.K., 1996; Vol.
4, pp 1-126.
In fact, despite the great number of 4,5-dihydro-1,2,3-
triazoles (∆2-1,2,3-triazoline) synthesized,8,9 only a few
(7) Synthesis of 1: p-nitrophenyl azide (100 mg, 0.61 mmol) was
dissolved in R-methylstyrene (30 mL). The resulting yellow solution
was stirred at 75 °C for 28 h, the solvent evaporated in vacuo, and the
residue purified by flash chromatography on silica gel (eluent dichlo-
romethane/n-hexane (9/1)) (73%). Recrystallization of the residue from
dichloromethane/methanol (1/1) gave crystals of 1 suitable for a
structural determination.
(8) (a) Kadaba, P. K.; Stanovnik, B.; Tisˇler, M. Adv. Heterocycl.
Chem. 1985, 37, 217-349. (b) Bourgois, J.; Bourgois, M.; Texier, F.
Bull. Soc. Chem. Fr. 1978, 9-10, 485-527.
(10) (a) Gieren, A. Chem. Ber. 1973, 106, 288-311. (b) Ferguson,
G.; Lough, A. J.; Mackay, D.; Weeratunga, G. J. Chem. Soc., Perkin
Trans. 1 1991, 3361-3369. (c) Kaas, K. Acta Crystallogr. 1973, B29,
1458-1463. (d) Mloston, G.; Urbaniak, K.; Linden, A.; Heimgartner,
H. Helv. Chim. Acta 2002, 85, 2644-2656. (e) Murata, S.; Mori, Y.;
Satoh, Y.; Yoshidome, R.; Tomioka, H. Chem. Lett. 1999, 597-598. (f)
Furin, G. G.; Gatilov, Y. V.; Bagryanskay, I. Y.; Zhuzhgov, E. L. J.
Fluorine Chem. 2001, 110, 21-24.
(11) Hirakawa, K.; Tanabiki, Y. J. Org. Chem. 1982, 47, 280-284.
(12) Scheiner, P. Tetrahedron 1968, 24, 2757-2765.

4712 Organometallics, Vol. 24, No. 20, 2005
Communications
Table 2. Dependence of the Ratio kcatalytic reaction
kuncatalyzed reaction on r-Methylstyrene
Concentrationa
/
R-methylstyrene
concn (mol/L)
kcatalytic reaction/
kuncatalyzed reaction
1.92
3.84
5.77
7.69
370
186
37
9
a Experimental conditions: 100 mg of p-nitrophenyl azide (0.610
mmol). In the catalyzed reaction, Ru(TPP)CO (9 mg, 0.012 mmol)
was also present. The reactions were run in 30 mL of benzene/R-
methylstyrene at 75 °C.
2,13 3, and 4 were characterized by elemental analysis
and IR and NMR spectroscopy (see the Supporting
Information). Complex 2 was also characterized by
X-ray crystal analysis (see below). The most notable
Figure 3. Ortep drawing of 2. Selected bond lengths (Å)
and angles (deg): Ru-N3 ) 2.143(5); N3-N2 ) 1.280(98);
N2-N1 ) 1.365(8); N1-C5 ) 1.48(1); C4-C5 ) 1.55(1);
C4-N3 ) 1.468(9); Ru-C1 ) 1.829(7); C1-O1 ) 1.157(8);
<Ru-Nporph> ) 2.053; Ru-N3-N2 ) 121.3(4); Ru-N3-
C4 ) 128.1(4); Ru-C1-O1 ) 179.0(6). See the Supporting
Information for details.
1
feature in the H NMR spectra is a strong high-field
shift of the signals relative to the two C4 protons and
to the methyl group of the triazoline moiety due to the
porphyrin ring current effect.
To better understand the role of triazoline in the
catalytic cycle, a reaction was carried out between
R-methylstyrene (1.9 M in benzene) and p-nitrophenyl
azide using complex 2 as catalyst or with the addition
of a slight excess of 1 to the reaction mixture containing
a catalytic amount of Ru(TPP)CO. In both cases the
reaction rate decreased drastically, strongly indicating
that 1, rather than being an intermediate, is an inhibi-
tor of the catalytic cycle (k(Ru(TPP)CO)/k(2)/k(Ru(TPP)-
CO/1)1:1.11) ) 36/3.5/1). These data suggest a compe-
tition between 1 and p-nitrophenyl azide for the
coordination to the catalytic center. To support this
hypothesis, the 1H NMR of a mixture of 2 and aryl azide
at 75 °C was analyzed. The formation of 14% of free
triazoline strongly points to a ligand substitution reac-
tion. The presence of free triazoline was also confirmed
by a GC-MS analysis of the reaction mixture.
line. The first one, a triazoline complex of palladium-
(II), was obtained by one of us some years ago by
reacting cis-cyclooctene with phenyl azide in the pres-
ence of palladium chloride.14 It must be noted that
examples of ∆2-1,2,3-triazolines employed as ligands for
transition metals are extremely rare, and in almost all
of them the metal center is σ-bonded to an anionic
nitrogen of the heterocycle. These triazoline complexes
were synthesized directly in the coordination sphere of
the metal by the reaction of azido complexes with
olefins.15
The Ru(TPP)(L)CO (L ) 1) complex has a remarkable
thermal stability, compared with that of free 1. The TGA
and DSC (differential scanning calorimetry) analyses
showed that 2 decomposes, in a multistep process, only
above 250 °C: i.e. 100 °C over the free triazoline (vide
supra).
Complex 2 is not stable in the presence of strong
donor ligands, and a ligand substitution occurs. The
reaction of 2 with tert-butyl isocyanide or dimethyl
sulfoxide afforded 1 and RuII(TPP)(tBuNC)2 (5)16 and
RuII(TPP)(DMSO)CO (6), respectively, which were iso-
lated and characterized (see the Supporting Informa-
tion).
An interesting comparison between the molecular
geometries of the isolated and complexed triazoline is
possible, having determined the structures of both 1 and
2 (Figures 2 and 3, respectively) by single-crystal X-ray
diffraction experiments.
The kinetics of the triazoline formation indicates
that, at high olefin concentration, 1 is formed at a rate
competitive with that of the catalytic reaction and
can react with Ru(TPP)CO to generate 2. As reported
in Table 2, the decrease of the ratio kcatalytic reaction
/
kuncatalyzed reaction is proportional to the increase of R-me-
thylstyrene concentration. The GC-MS analysis of the
reaction mixture after a catalytic reaction run with
R-methylstyrene (7.69 M) showed the presence of 1,
whereas the 1H NMR analysis of the residue after
evaporation of all volatiles revealed the presence of the
typical signals of the aliphatic protons of 2 at negative
field.
The formation of the catalytically inactive complex 2
reduces the active catalyst amount and consequently the
reaction rate, thus explaining the effect observed when
the solution contains more than 30% (v/v) of R-methyl-
styrene (see Figure 1).
While 1 spontaneously resolves during the crystal-
lization process (in the orthorhombic space group
P212121), giving rise to a racemate, the reaction in eq 1
(14) Porta, F.; Pizzotti, M.; La Monica, G.; Finessi, L. A.; Cenini,
S.; Bellon, P. L.; Demartin, F. J. Chem. Soc., Dalton Trans. 1984,
2409-2414.
To the best of our knowledge, complex 2 is the second
reported transition-metal complex of a neutral triazo-
(15) (a) Kemmerich, T.; Nelson, J. H.; Takach, N. E.; Boehme, H.;
Jablonski, B.; Beck, W. Inorg. Chem. 1982, 21, 1226-1232. (b) Paul,
P.; Nag, K. Inorg. Chem. 1987, 26, 2969-2974. (c) Guilard, R.; Perrot,
I.; Tabard, A.; Richard, P.; Lecomte, C.; Liu, Y. H.; Kadish, K. M. Inorg.
Chem. 1991, 30, 27-37. (d) Guilard, R.; Jagerovic, N.; Tabard, A.;
Richard, P.; Courthaudon, L.; Louati, A.; Lecomte, C.; Kadish, K. M.
Inorg. Chem. 1991, 30, 16-27. (e) Fru¨hauf, H.-W. Chem. Rev. 1997,
97, 523-596.
(13) Synthesis of 2: 1 (125 mg, 0.44 mmol) was added to a benzene
(50 mL) suspension of Ru(TPP)CO (311 mg, 0.42 mmol). The resulting
red solution was stirred at 75 °C for 15 min and concentrated to 5 mL,
and n-hexane (30 mL) was added. The resulting violet solid was
collected by filtration, washed with n-hexane, and dried in vacuo (85%).
Recrystallization of 2 from ethyl ether gave crystals suitable for a
structural determination.
(16) Lee, F.-W.; Choi, M.-Y.; Cheung, K.-K.; Che, C.-M. J. Orga-
nomet. Chem. 2000, 595, 114-125.

Communications
Organometallics, Vol. 24, No. 20, 2005 4713
produces a solid-state phase of 2, probably tetragonal
and acentric, but of very low crystallinity. Any attempt
to solve that structure (and even to unambiguously
identify the crystalline system) was unsuccessful, but,
after recrystallization in Et2O, a racemic phase (mono-
clinic C2/c) including the clathrated solvent was ob-
tained and could be solved, despite poor diffraction by
the crystals. The coordination of the triazoline to Ru
occurs via N3, which is more basic and less sterically
hindered than N2. The N2-N3 bond is elongated upon
complexation (1.244(3) Å in 1, 1.279(8) Å in 2), suggest-
ing that some π-bonding redistribution occurs after
coordination. The Ru-CO bond distance (1.828(7) Å) lies
in the range of Ru porphyrins with axial coordination
to a monoaza heterocyclic ligand (such as pyridine).17
The conformation of the triazoline about the Ru-N
bond is very likely imposed by the crystal packing more
than by intramolecular effects. Molecules of 2 pack in
columns elongated along the monoclinic axis b (although
the molecular principal axis and b are not aligned but
form an angle of 28°) and leave channels, again parallel
to b, where the solvent is hosted, though disordered. For
further information about the X-ray characterization see
the Supporting Information.
identification of 2, the first example of a ruthenium
porphyrin complex of a neutral triazoline, allowed us
to study for the first time the influence of triazoline on
the catalytic aziridination of olefins. We have shown
that in the present system, quite unexpectedly, the
triazoline is not an intermediate in the catalytic cycle
and may even act as an inhibitor. It is worth pointing
out that in the present case the aziridine cannot be
obtained without the assistance of the metal, and a more
in-depth mechanistic study of the aziridination reaction
is in progress. Preliminary kinetic data suggest that an
azide complex of the type Ru(TPP)CO(ArN3) is the
active species involved in the amination step, rather
than an imido complex such as Ru(TPP)(CO)(NAr). It
1
is important to note that the H NMR and IR spectra
recorded at the end of the catalytic reaction revealed
the presence of the unmodified catalyst Ru(TPP)CO.
Acknowledgment. We thank the MIUR (Program-
mi di Ricerca Scientifica di Rilevante Interesse Nazio-
nale, PRIN 2003033857) for financial support. We are
grateful to Dr. Lucia Carlucci (Dipartimento di Chimica
Strutturale e Stereochimica Inorganica, Universita` di
Milano, and ISTM-CNR, Via Venezian 21, 20133 Mil-
ano, Italy) for performing thermal gravimetric analyses
and differential scanning calorimetry.
In conclusion, we have reported a new aspect in the
chemistry of ∆2-1,2,3-triazolines. Thermal and photo-
chemical reactions of triazolines that afford aziridines,
by ring contraction and evolution of dinitrogen, have
been widely reported in the literature.8b,12,18 On the
other hand, nothing is known about the role of triazoline
when the reactions between azides and olefins are
performed in the presence of a metal catalyst.19 The
Supporting Information Available: Text, tables, and
figures giving synthetic procedures, characterization data, and
X-ray crystal structure data; X-ray data are also given as CIF
files. This material is available free of charge via the Internet
at http://pubs.acs.org.
(17) Little, R. G.; Ibers, J. A. J. Am. Chem. Soc. 1973, 95, 8583-
8590.
OM050244Y
(18) (a) Scheiner, P. J. Org. Chem. 1965, 30, 7-10. (b) Scheiner, P.
J. Am. Chem. Soc. 1966, 88, 4759-4760. (c) Texier, F.; Carrie´, R. Bull.
Soc. Chem. Fr. 1971, 10, 3642-3648.
(19) Omura, K.; Uchida, T.; Irie, R.; Katsuki, T. Chem. Commun.
2004, 2060-2061 and references therein.