Guanylation of aromatic amines catalyzed by vanadium imido complexes

Article abstract of DOI:10.1016/j.jorganchem.2004.01.005

The reaction of formation of guanidines by coupling of carbodiimides and aromatic amines using imido vanadium complexes as catalyst have been investigated. Results demonstrate that the complex V(N-2,6-ⁱPr₂C₆H₃)

Full text of DOI:10.1016/j.jorganchem.2004.01.005

Journal of Organometallic Chemistry 689 (2004) 993–996
www.elsevier.com/locate/jorganchem
Communication
Guanylation of aromatic amines catalyzed by vanadium
imido complexes
*
ꢀ
Francisco Montilla , Antonio Pastor, Agustın Galindo
ꢀ
ꢀ
Departamento de Quımica Inorganica, Universidad de Sevilla, Aptdo 553, 41071 Sevilla, Spain
Received 13 November 2003; accepted 3 January 2004
Abstract
The reaction of formation of guanidines by coupling of carbodiimides and aromatic amines using imido vanadium complexes as
catalyst have been investigated. Results demonstrate that the complex V(N-2,6-iPr2C6H3)Cl3 is an effective catalyst for this process.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Imido; Vanadium; Guanylation
1. Introduction
derived from the [2 + 2] cycloaddition between the met-
al-nitrogen double bond and the double bond of the
unsaturated carbodiimide (Scheme 2).
Reactions involving imido complexes in which the
imido moiety undergoes chemical transformations have
experienced a remarkable growth in the last years due to
the role they play in many important reactions [1,2].
Richeson et al. have recently showed that the guanyla-
tion [3] of aromatic amines with carbodiimides is cata-
lyzed by imido titanium complexes [4]. The proposed
catalytic cycle begins with [2 + 2] addition of the car-
bodiimide to the titanium-imido moiety (Scheme 1). The
resulting diazametallacyclobutane intermediate, for
which exists experimental evidence [4], reacts with the
amine affording the corresponding guanidine.
NR
R
N
C
N
R
ArN
L2Ti NAr
[L= (Me2N)C(NiPr)2]
L2Ti NR
NAr
Ar NH2
R
R
N
H
N
H
Scheme 1.
On the other hand, recent works have described
carbodiimide metathesis reactions catalyzed by group 4
[5] and group 5 [6] imido complexes. Particularly, com-
pounds of the formula V(NAr)X3 (X ¼ Cl, OR) have
been successfully employed in this reaction [7]. The
proposed catalytic cycle for the process implies the
formation of a diazametallacyclobutane intermediate
Ar NH2
+
R
N
C
N
R
NAr
[1] (2% mol)
Toluene, ∆
R
R
N
H
N
H
ð1Þ
* Corresponding authors. Tel.: +34-954-557161; fax: +34-954-
R = iPr, Ar = 2,4,6-Me3C6H2, 2
R = iPr, Ar = 2-ClC6H4, 3
557153.
E-mail addresses: montilla@us.es (F. Montilla), galindo@us.es
R = Cy, Ar = 2,4,6-Me3C6H2, 4
(A. Galindo).
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.01.005

994
F. Montilla et al. / Journal of Organometallic Chemistry 689 (2004) 993–996
NR'
R' N
C N R'
RN
X3V NR
X3V NR'
R N
C N R'
R N
C
N
R'
NR
R N
C
N
R
R'N
X3V NR'
X3V NR
Scheme 2.
Taking into consideration the similarity of both in-
Consequently, a diazavanadacyclobutane, formed from
the [2 + 2] addition of the carbodiimide to the V ¼ NAr
group should be the intermediate implied in this process.
However, the resulting diazavanadacyclobutane is a
coordinatively unsaturated species and the efforts to
isolate it by reaction of 1 with one equivalent of car-
bodiimide have been unproductive because the reaction
yields a mixture of products. Attempts to isolate a re-
lated species by trapping it with a donor co-ligand are
currently in progress.
termediates and following our interest in the area of
imido complexes of group 5 [8] and 6 metals [9], we have
extended our results on imido complexes of vanadium to
the study of the assembling reaction between carbodii-
mides and aromatic amines. To the best of our knowl-
edge, this communication represents the first report of
group 5 metal catalyzed guanylation of aromatic amines
with carbodiimides.
We have also evaluated the activity of other imido and
oxo vanadium compounds, such as V(NAr)Cl3(dme) [8a],
V(NAr)(iPr-dtc)3 [8b], V(NAr)[(OCH2CH2)3N] [12]
(Ar ¼ 2,6-iPr2C6H3), V2O5 and VO(acac)2. Table 2 col-
lects the results obtained in the preparation of guanidine
2. Complex V(NAr)Cl3(dme) was found to be an effective
catalyst. This result was not surprising in view of the ca-
pacity of this complex to lose the coordinated dimeth-
oxyethane ligand at high temperature to afford the
unsaturated complex 1. By contrast, other imido vana-
dium complexes, such as V(NAr)(iPr-dtc)3 and V(NAr)
[(OCH2CH2)3N], were found to be inactive for the pro-
cess. The lack of activity observed for the seven-coordi-
nate complex V(NAr)(iPr-dtc)3 may be associated with
the difficulty in reaching the diazametallacyclobutane
intermediate due to the overcrowded vanadium environ-
ment [8b]. The lack of activity observed for complex
2. Results and discussion
Compound V(N-2,6-iPr2C6H3)Cl3 (1) [10] has re-
vealed to be an efficient catalyst for the guanylation of
aryl amines (Eq. (1)). The reactions were carried out in
toluene at 105 °C with the addition of 2% of catalyst
precursor 1, giving good yields of the corresponding new
guanidines 2–4 (Table 1). The guanidine yield was found
to be strongly dependent on the temperature. At room
temperature, no reaction of 2,4,6-trimethylaniline with
diisopropylcarbodiimide was observed. Conversely, at
105 °C, the conversion to guanidine 2 was complete after
approximately 24 h (see Section 3).
We assumed that the proposed mechanism for the
guanylation reaction catalyzed by titanium imido com-
plexes (Scheme 1) is also operative in our process [11].
Table 2
Effect of the catalyst in the guanylation of arylaminesa
Table 1
Guanylation of aromatic amines using V(N-2,6-iPr2C6H3)Cl3 as
catalysta
Entry Catalyst
Product Conversion
(%)b
1
2
3
4
5
6
7
None
–
2
2
–
–
–
2
No reaction
84
75
Entry
R group of
carbodiimide
Amine aryl
group
Product Conversion
(%)b
V(N-2,6-iPr2C6H3)Cl3
V(N-2,6-iPr2C6H3)Cl3(dme)
V(N-2,6-iPr2C6H3)(iPr-dtc)3
V(N-2,6-iPr2C6H3)[(OCH2CH2)3N]
V2O5
1
2
3
iPr
iPr
Cy
2,4,6-Me3C6H2
2-ClC6H4
2,4,6-Me3C6H2
2
3
4
84
88
80
No reaction
No reaction
No reaction
44
a Conditions: ratio carbodiimide:aniline:catalyst ¼ 1:1:0.02, t ¼ 24 h,
T ¼ 105 °C, solvent ¼ toluene.
VO(acac)2
a See conditions in Section 3.
b Isolated yields.
b Isolated yields.

F. Montilla et al. / Journal of Organometallic Chemistry 689 (2004) 993–996
995
V(NAr)[(OCH2CH2)3N] is clearly related with the high
inertness of this species [12].
for 24 h. The volatiles were removed and the residue was
extracted with diethyl ether. Cooling to )20 °C gave a
white solid of N-2,4,6-trimethylphenyl,N0; N00-diisopro-
pylguanidine (2), which was isolated by filtration and
dried in vacuum. Yield: 84%. MS (m/z): 262 (M+H)þ.
Finally, we have also investigated the activity of some
oxo vanadium compounds. V2O5 displayed not catalytic
activity, while VO(acac)2 shows some activity in the
guanylation. The inactivity of the former can be attrib-
uted to the low solubility of the vanadium compound in
the reaction media. The moderate activity of VO(acac)2
was not unexpected considering this complex was an
efficient catalyst for carbodiimide metathesis [7].
In conclusion, we have shown that vanadium imido
complexes are efficient catalysts for the guanylation of
carbodiimides using aromatic amines. Further studies
are in progress in order to confirm the mechanism
proposed for the process.
i
1H-NMR (CDCl3): d 0.89 – 1.21 (br, CH3, Pr), 2.27 (s,
6H, o-CH3), 2.30 (s, 3H, p-CH3), 3.39 (br, 2H, NH),
i
4.10 (br, CH, Pr), 6.92 (s, 2H, C6H2). 13C{1H}-NMR
(CDCl3): d 18.1 (s, o-CH3), 20.7 (s, p-CH3), 23.5 (CH3,
i
iPr), 43.1 (br, CH, Pr), 128.7, 130.7, 143.3, 148.0 (s,
C6H2), quaternary carbon ˜C@N– not observed. Anal.
Calc. for C16H27N3(261.22): C, 73.51; H, 10.41; N,
16.07. Found: C, 72.79; H, 10.26; N, 15.62%.
Following a similar procedure were obtained gua-
nidines 3 and 4. N-2-chlorophenyl,N0; N00-diisopro-
pylguanidine (3). Yield: 88%. MS (m/z): 254 (M+H)þ.
i
1H-NMR (CDCl3): d 1.15 (d, 12H, 6.4 Hz, CH3, Pr),
i
3. Experimental
3.46 (br, 2H, NH), 3.76 (br, 2H, CH, Pr), 6.87 (m, 2H,
C6H4), 7.11 (t, 1H, 6.6 Hz, C6H4), 7.32 (t, 1H, 7.8 Hz,
i
All preparations and other operations were carried out
under dry oxygen-free nitrogen atmosphere following
conventional Schlenk techniques. Solvents were dried and
degassed before use. Carbodiimides and anilines were
purchased from Aldrich and they were used as supplied.
Compounds V(N-2,6-iPr2C6H3)Cl3 (1) [10], V(N-2,6-
iPr2C6H3)Cl3(dme) [8a], V(N-2,6-iPr2C6H3)(iPr-dtc)3
[8b] were prepared as previously reported. V(N-2,6-
iPr2C6H3)[(OCH2CH2)3N] was prepared by reaction of
V(N-2,6-iPr2C6H3)Cl3(dme) and (HOCH2CH2)3N in the
presence of Et3N [12]. Infrared spectra were recorded on
C6H4). 13C{1H}-NMR (CDCl3): d 23.4 (s, CH3, Pr),
43.3 (s, CH, Pr), 122.4, 125.2, 127.5, 129.9 (s, C6H4),
i
146.9, 149.9 (s, C6H4), quaternary carbon ˜C@N– not
observed. Anal. Calc. for C13H20N3Cl(253.13): C, 61.53;
H, 7.94, N, 16.56. Found: C, 61.49; H, 7.98; N, 16.01%.
N-2,4,6-trimethylphenyl,N0; N00-dicyclohexylguanidine
(4). Yield: 80%. MS (m/z): 342 (M+H)þ. 1H-NMR
(CDCl3): d 1.06 – 1.61 (br, 22H, CH2 and CH on cy-
clohexyl), 2.02 (s, 6H, o-CH3), 2.16 (s, 3H, p-CH3), 3.41
(br, 2H, NH), 6.74 (s, 2H, C6H2). 13C{1H}-NMR
(CDCl3): d 18.0 (s, o-CH3), 20.6 (s, p-CH3), 24.9, 25.4,
34.1 (s, CH2 on cyclohexyl), 49.8 (br, CH on cyclo-
hexyl), 128.1, 130.5, 143.4, 147.7 (s, C6H2), quaternary
carbon ˜C@N– not observed. Anal. Calc. for
C22H35N3(341.28): C, 77.37; H, 10.33, N, 12.30. Found:
C, 76.64; H, 11.31; N, 12.06%.
Perkin–Elmer Model 883 spectrophotometer. 1H and 13
C
NMR spectra were run on Bruker AMX-300 spectrom-
eter. Microanalyses (C, H, N) were carried out by the
Microanalytical Service of the University of Sevilla. Mass
spectra were recorded on Kratos MS80-RFA FAB tech-
nique using thioglycerol as a matrix.
The same procedure was employed in the catalyst
assays (Table 2) with different vanadium complexes
i
3.1. Influence of the temperature
using the same reaction conditions (ratio PrN@C@
NiPr:2,4,6-Me3C6H2NH2:catalyst ¼ 1:1:0.02, T ¼ 105
°C, t ¼ 24 h, solvent ¼ toluene).
Reactions at different temperatures were monitored
by NMR spectroscopy. 1 equivalent of vanadium com-
plex 1 and 50 equivalent for both diisopropylcarbodii-
mide and 2,4,6-trimethylaniline were mixed in
deuterated toluene. (Me3Si)2O was added as an internal
standard and the NMR tube was heated at the required
temperature for 17 h. Conversions to guanidine 2 were
determined by 1H-NMR integration relative to standard
O(SiMe3)2. The reaction does not proceed at a reason-
able rate below 105 °C.
Acknowledgements
This work was supported by European Commission
(Contract HPMF-CT-2002-01609), MCYT (BQU2001-
ꢀ
3715) and Junta de Andalucıa. We thank the Servicio de
Espectrometrıa de Masas and the Servicio General de
ꢀ
ꢀ
Microanalisis (University of Sevilla) for the corre-
sponding analyses.
3.2. Guanylation reaction
References
A solution of 1 (0.064 mmol), 2,4,6-trimethylaniline
(0.42 ml, 3.2 mmol) and diisopropylcarbodiimide
(0.50 ml, 3.2 mmol) in 10 ml of toluene was heated in a
thick-walled glass vessel with Teflon stopcock at 105 °C
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of amine as both a reagent and base. In this case, an alternative
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