Highly efficient aluminum-catalyzed hydro-amination/-hydrazination of carbodiimides

Article abstract of DOI:10.1021/om100735q

The catalytic activity of commercially available [Al(NMe₂) ₃]₂ (1) and a dimethyl aluminum guanidinate complex toward the hydro-amination/-hydrazination of carbodiimides was studied. The guanidinate-supported complex 2 was

Full text of DOI:10.1021/om100735q

5946 Organometallics 2010, 29, 5946–5952
DOI: 10.1021/om100735q
Highly Efficient Aluminum-Catalyzed Hydro-amination/-hydrazination
of Carbodiimides
€
Jurgen Koller and Robert G. Bergman*
Department of Chemistry, University of California, Berkeley, California 94720, United States
Received July 26, 2010
The catalytic activity of commercially available [Al(NMe2)3]2 (1) and a dimethyl aluminum
guanidinate complex toward the hydro-amination/-hydrazination of carbodiimides was studied.
The guanidinate-supported complex 2 was prepared via salt metathesis reactions of AlMe2Cl and an
in situ generated lithium guanidinate reagent. X-ray crystallographic studies revealed the influence of
the guanidinate ligand on the Al metal center. Hydroamination reactions were successfully carried
out at room temperature with 2 as the catalyst, while 1 proved to be ineffective under these conditions.
On the contrary, both 1 and 2 were active toward the hydro-hydrazination of carbodiimides, which
were run at elevated temperatures (120 °C). Consequently, the reaction temperature had a significant
influence on the choice of the catalyst since the catalytically active species can be generated from
various precatalysts under different conditions. The formation of guanidines and aminoguanidines
showed a high functional group tolerance and typically proceeded with excellent yields at low catalyst
loadings. X-ray crystallographic studies of compound 4a revealed interesting structural features of
the previously unknown aminoguanidine products. The independently isolated Al aminoguanidinate
complex 5 showed catalytic activity toward hydro-hydrazination chemistry and provided valuable
evidence in support of the proposed reaction mechanism.
Introduction
biological properties.11-13 Aminoguanidines in particular
display dopamine β-oxidase inhibition and antihypertensive
properties.14
In recent years, the guanylation chemistry involving ar-
ylamines has enjoyed increasing attention sparked by the
discovery of highly active transition metal and lanthanide
catalysts.15-21 Additionally, group 1 and 2 metal complexes
have recently been shown to exhibit high activity toward this
organic transformation, thus expanding the list of active
guanylation catalysts to main group metals.22-24 Although
The catalytic addition of amine N-H bonds across car-
bodiimides, also known as guanylation, is a highly efficient
and atom economical route toward a series of substituted
guanidines. Besides their importance in biological and phar-
maceutical chemistry,1,2 guanidines enjoy extensive presence
in the literature as ancillary ligands for a wide variety of
transition, f-block, and main group metals due to their
electronic similarities to the widely used cyclopentadienyl
(Cp) ligand.3-10 Similarly, functionalized guanidines not
only are important building blocks for various natural
products but have also been shown to exhibit interesting
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*To whom correspondence should be addressed. Tel: þ1-510-642-
2156. Fax: þ1-510-642-7714. E-mail: rbergman@berkeley.edu.
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r
pubs.acs.org/Organometallics
Published on Web 10/20/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 22, 2010 5947
Scheme 1. Synthesis of Compound 2
˚
Table 1. Selected Bond Distances (A) and Angles (deg) for
Compound 2
the insertion of carbodiimides into Al-C/N bonds of Al
alkyl/amide species such as [Al(NMe2)3]2 (1) is known,25
examples of catalytic guanylation of amines using organo-
metallic catalysts involving such species have remained un-
documented until recently.26 Despite the similarities of pri-
mary arylamines and 1,1-disubstituted hydrazines, to the
best of our knowledge, metal-catalyzed hydroamination of
carbodiimides with hydrazines to form aminoguanidines
has not been reported. Herein we report the highly efficient
Al-catalyzed formation of substituted guanidines and
aminoguanidines via hydro-amination/-hydrazination of
various carbodiimides with arylamines and 1,1-disubstituted
hydrazines.
Al1-N1
Al1-N2
C1-N1
C1-N2
C1-N3
Al1-C32
Al1-C33
1.9287(15)
1.9239(16)
1.356(2)
1.354(2)
1.371(2)
N1-Al1-N2
N1-C1-N2
N1-C1-N3
N2-C1-N3
C32-Al1-C33
69.06(6)
107.39(15)
125.75(15)
126.87(16)
114.96(9)
1.9688(19)
1.9726(19)
Results and Discussion
Synthesis of 2. The guanidinate ligand precursor for
compound 2 was prepared by treating N,N0-bis(2,6-diiso-
propylphenyl)carbodiimide with LiNiPr2 at ambient tem-
perature to generate the corresponding lithium guanidinate
reagent as described in the literature.27-29 The intermediate
Li salt was not isolated but rather treated in situ with AlMe2Cl to
form 2 via salt metathesis, accompanied by elimination of LiCl
upon warming the solution to room temperature and stirring
overnight (Scheme 1). Analytically pure 2 was isolated as
colorless crystals via filtration after recrystallization of the crude
product from pentane at -35 °C. The 1H and 13C NMR spectra
show upfield shifted resonances at -0.16 and -6.89 ppm,
respectively, which correspond well to magnetically equivalent
Al-bound methyl moieties. A single resonance in the 13C NMR
spectrum at 165.91 ppm is diagnostic for the central C atom of
the guanidinate ligand and further corroborates the structural
assignment.16,30 The diastereotopic nature of the isopropyl
substituents on the flanking aryl groups is evidenced by well-
separated resonances (two doublets at 1.29 and 1.31 ppm) in the
1H NMR spectrum.
Figure 1. Thermal ellipsoids diagram of the molecular structure
of 2. Thermal ellipsoids are drawn at 50% probability. H atoms
are omitted for clarity.
Solid-State Structure of Compound 2. To confirm the
monomeric nature of 2, single crystals were grown from a
pentane solution at -35 °C and analyzed by X-ray crystal-
lography. Crystal data and structure refinement for 2 can be
found in Table S1 (Supporting Information). Selected bond
lengths and angles can be found in Table 1. Unsurprisingly,
the structural features are similar to other Al complexes
supported by guanidinate ligands.31 Compound 2 adopts a
distorted tetrahedral coordination environment as shown in the
ORTEP representation of 2 (Figure 1). The Al1-N1 and
˚
Al1-N2 bonds (1.9287(15) and 1.9239(16) A, respectively) of
the guanidinate ligand are almost identical, suggesting a high
degree of electron delocalization within the guanidinate back-
bone, an observation that is further substantiated by virtually
identical C1-N1 and C1-N2 bond distances of 1.356(2) and
(25) Chang, C. C.; Hsiung, C. S.; Su, H. L.; Srinivas, B.; Chiang,
M. Y.; Lee, G. H.; Wang, Y. Organometallics 1998, 17, 1595–1601.
(26) Rowley, C. N.; Ong, T.-G.; Priem, J.; Woo, T. K.; Richeson,
D. S. Inorg. Chem. 2008, 47, 9660–9668.
(27) Wilder, C. B.; Reitfort, L. L.; Abboud, K. A.; McElwee-White,
L. Inorg. Chem. 2006, 45, 263–268.
(28) Duncan, A. P.; Mullins, S. M.; Arnold, J.; Bergman, R. G.
Organometallics 2001, 20, 1808–1819.
(29) Wood; Yap, G. P. A.; Richeson, D. S. Inorg. Chem. 1999, 38,
5788–5794.
(30) Zhou, S.; Wang, S.; Yang, G.; Li, Q.; Zhang, L.; Yao, Z.; Zhou,
˚
1.354(2) A, respectively. Further electron delocalization involv-
ing the N3-based lone pair is restricted by a typical twist in the
guanidinate backbone (28.4°) due to the sterically demanding
isopropyl substituents on N3, as evidenced by the significantly
Z.; Song, H.-b. Organometallics 2007, 26, 3755–3761.
(31) Koller, J.; Bergman, R. G. Organometallics 2010, 29, 3350–3356.

5948 Organometallics, Vol. 29, No. 22, 2010
Koller and Bergman
Table 2. Catalytic Formation of Guanidines 3a-f
longer C1-N3 bond (1.371(2) A).32 The Al-C bond distances
˚
˚
of 1.9688(19) and 1.9726(19) A for Al1-C32 and Al1-C33,
respectively, reflect the symmetrical coordination of the guani-
dinate ligand to the Al metal center. The small bite angle of
69.06(6)° is characteristic for guanidinate ligands on relatively
small metals such as Al or Li and is significantly larger
compared to the corresponding angles in transition metal
guanidinate complexes.33,34 The notable deviation of the
C32-Al1-C33 bond angle (114.96(9)°) from the ideal 109.5°
for tetrahedral coordination has been documented for similar
compounds and is a further reflection of the ample space
generated by the small guanidinate bite angle. The flanking
aryl groups adopt a perpendicular orientation with respect to
the metal-ligand plane, which is most likely due to the sterically
demanding 2,6-substitution pattern.
entry
R
R0
product
catalyst
time (min)
yield (%)a
1
2
3
4
5
6
7
iPr
iPr
Cy
iPr
Cy
iPr
Cy
Me
Me
Me
H
H
F
3a
3a
3b
3c
3d
3e
3f
1
2
2
2
2
2
2
60
30
30
30
30
30
30
0
84
97
90
97
92
99
F
In recent reports of successful guanylation of carbodiimides
with aniline derivatives mitigated by mixed Al alkyl/halide
catalyst precursors, Al was established as a potent metal center
to catalyze these types of organic transformations.35 The cata-
lytic cycle was proposed to proceed via a highly active three-
coordinate Al amide species, a finding that tempted us to
investigate the catalytic activity of commercially available
[Al(NMe2)3]2 (1) toward the hydroamination of various carbo-
diimides. NMR-scale reactions (in C6D6) carried out at room
temperature revealed 1 to be inactive toward catalytic conver-
sion of N,N0-diisopropylcarbodiimide and p-toluidine to the
corresponding guanidine. Raising the temperature above 75 °C,
however, resulted in quantitative conversion to 1,3-diisopropyl-
2-p-tolylguanidine within a few minutes of heating time. This
finding is consistent with literature reports of the remarkable
thermal stability of Al amide/alkoxide dimers, preventing dis-
sociation of the dimer into reactive three-coordinate species
(eq 1).36,37
a Isolated yields.
Figure 2. Proposed catalytic cycle for the formation of guanidines.
exchange reaction with free aniline. The absence of diagnos-
tic methane signals in the 1H NMR spectra (at catalyst
loadings up to 20 mol %) suggests that the methyl moieties
on the metal center act as spectator ligands under the
reaction conditions and are not involved in the catalytic
activity. Although counterintuitive at first, this finding is
supported by reports of the remarkable stability of Al-Me
bonds toward protonolysis by anilines.38,39
The similarities of primary arylamines and 1,1-disubsti-
tuted hydrazines tempted us to investigate the corresponding
reactivity of hydrazines with carbodiimides to form amino-
guanidines. Initial NMR-scale reactions showed that the
reaction of 1,1-dimethylhydrazine and N,N0-diisopropyl-
carbodiimide in the presence of 2 (1 mol %) required
temperatures above 100 °C to proceed at a reasonable
rate. On the basis of the observations with the aniline deriva-
tives, we attempted the same reaction in the presence of 1
(1 mol %) as the catalyst since the dissociation of 1 into its
monomeric form [Al(NMe2)3] should be significant under
these conditions. Unsurprisingly, conversion of the substrates
to the corresponding aminoguanidine product was observed
within a few hours at 120 °C. Control experiments revealed
that catalysis is not influenced by the presence of a noncoor-
dinating base such as 1,2,2,6,6-pentamethylpiperidine, thus
ruling out the possibility of acid catalysis (Table 3, entry 1).
Additionally, no product formation was observed upon
The use of a monomeric Al source such as compound 2 is a
convenient way to avoid the need for elevated temperatures.
Upon stirring N,N0-dialkylcarbodiimides with various ani-
line derivatives in the presence of 2 (1 mol %), formation of
the corresponding guanidines was observed within minutes
at room temperature (Table 2, entries 2-7). Substitution of
the aniline derivative in the para position with either elec-
tron-donating or -withdrawing groups was tolerated by
the catalyst, and the reactions proceeded under equally
mild conditions. Analogous to the AlMexCl3-x-catalyzed
systems,35 the reaction is proposed to proceed via amine
exchange to form a catalytically active three-coordinate Al
species (structure I, Figure 2), which can subsequently insert
carbodiimides to form Al guanidinate species similar to the
precatalyst itself (structure II, Figure 2). The catalytically
active species can then be regenerated by another amine
(32) Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.;
Procopiou, P. A. Dalton Trans. 2008, 4474–4481.
(33) Aeilts, S. L.; Coles, M. P.; Swenson, D. G.; Jordan, R. F.; Young,
V. G. Organometallics 1998, 17, 3265–3270.
(34) Rische, D.; Baunemann, A.; Winter, M.; Fischer, R. A. Inorg.
Chem. 2006, 45, 269–277.
(35) Zhang, W.-X.; Li, D.; Wang, Z.; Xi, Z. Organometallics 2009, 28,
882–887.
(36) Chisholm, M. H.; DiStasi, V. F.; Streib, W. E. Polyhedron 1990,
9, 253–255.
(37) Brazeau, A. L.; DiLabio, G. A.; Kreisel, K. A.; Monillas, W.;
(38) Waggoner, K. M.; Power, P. P. J. Am. Chem. Soc. 1991, 113,
3385–3393.
Yap, G. P. A.; Barry, S. T. Dalton Trans. 2007, 3297–3304.
(39) Timoshkin, A. Y. Coord. Chem. Rev. 2005, 249, 2094–2131.

Article
Table 3. Catalytic Formation of Aminoguanidines 4a-g
Organometallics, Vol. 29, No. 22, 2010 5949
entry
R
NR02
product
yield (%)a
1
2
3
4
5
6
7
iPr
iPr
iPr
Cy
Cy
NMe2
pip
NPh2
NMe2
pip
4a
4b
4c
4d
4e
4f
88 (86)b (89)c
93
90
99
98
84
95
2,6-iPr-C6H3
2,6-iPr-C6H3
NMe2
pip
4g
a Isolated yields. b Reaction run in the presence of 2 mol % of
1,2,2,6,6-pentamethylpiperidine. c Reaction run with 1 mol % of 5 as
the catalyst.
Figure 3. Thermal ellipsoids diagram of the molecular structure
of 4a. Thermal ellipsoids are drawn at 50% probability. H atoms
(except H1 and H2) are omitted for clarity.
heating the substrates at 135 °C for 4 h in the absence of any
catalyst.
˚
Table 4. Selected Bond Distances (A) and Angles (deg) for
Compound 4a
Encouraged by the NMR-scale experiments, we attempted
the synthesis of various aminoguanidines on a preparative
scale. The reactions proceeded with generally excellent yields
(Table 3, entries 1-7), and the products were isolated on a
gram scale as colorless crystalline solids. Both alkyl- and
aryl-substituted carbodiimides were tolerated by the cata-
lyst, and even the sterically demanding 2,6-diisopropylphe-
nyl substituents did not have a significant impact on product
formation. Furthermore, various substituents (alkyl, cyclic
alkyl, aryl) on the hydrazine moiety were incorporated
successfully into the final products without any significant
change in the overall excellent reaction yields.
C1-N1
C1-N2
C1-N3
N3-N4
N1-H1
N2-H2
1.3807(14)
1.3616(14)
1.3098(14)
1.4640(12)
0.877(15)
0.826(14)
N1-C1-N2
N1-C1-N3
N2-C1-N3
C1-N3-N4
N3-N4-C8
N3-N4-C9
116.80(10)
123.47(10)
119.62(10)
110.22(8)
108.32(9)
107.69(9)
The aforementioned hydrogen-bonding interaction between
H1 and N4 of compound 4a seems to be sustained in solution
(on the NMR time scale at room temperature), as evidenced by
chemically inequivalent isopropyl groups on N1 and N2 as well
as a significantly downfield shifted resonance corresponding
to H1 (5.84 ppm) compared to the resonance assigned to H2
To establish the structural conformation of the amino-
guanidine products, single crystals of compound 4a were
grown from a concentrated pentane solution at -20 °C and
analyzed by X-ray crystallography. Crystal data and struc-
ture refinement for 4a can be found in Table S6 (Supporting
Information). Compound 4a shows interesting structural
features, as shown in the ORTEP representation of 4a
(Figure 3). Selected bond lengths and angles can be found
in Table 4. The trigonal coordination around C1 (sum of
bond angles 359.9°) is consistent with sp2 hybridization. The
1
(2.78 ppm). Remarkably similar H NMR patterns can be
observed for all other aminoguanidine products, indicating that
this bonding scheme is common for this class of molecules. This
H-bonding interaction is also manifested in the infrared spectra
of compounds 4a-g, where relatively sharp peaks correspond-
ing to the H-bonded N-H functionalities (shifted to lower
wavenumbers compared to typical secondary amine stretches)
are apparent. These experimental findings are in accordance
with a recent computational study by Bharatam et al., where
this structural arrangement has been predicted to be the most
stable conformation for aminoguanidines.42
In analogy with the guanidine chemistry, the catalytic
cycle is proposed to proceed via a formal amine exchange
reaction resulting in the generation of the catalytically active
Al species (structure I, Figure 4). The κ2-N,N0 (“side-on”)
coordination ability of the hydrazide ligand likely stabilizes
the proposed intermediate and thus lowers its reactivity
toward carbodiimide insertion. Similar coordination com-
plexes involving hydrazide ligands coordinated to Al have
been reported in the literature and may explain the need for
substantially higher temperatures to achieve catalytic turn-
over (compared to the guanylation reactions).43 Subsequent
activation of the carbodiimide by the Al hydrazide inter-
mediate (structure II, Figure 3) followed by protonation with
˚
short C1-N3 bond distance of 1.3098(14) A is indicative of a
C-N double bond and is remarkably similar to structures
obtained from the guanidine counterparts.30 Similar C1-N1
˚
and C1-N2 bond lengths (1.3807(14) and 1.3616(14) A,
respectively) further corroborate this finding. The unusually
˚
long N3-N4 bond (1.4640(12) A) is indicative of a N-N
single bond with very little electron delocalization between
the N atoms, a phenomenon that is typically observed in
hydrazines.40,41 Average bond angles of 109.0° around N4
are in good agreement with a tetrahedral arrangement
around N4, further limiting the ability of electron delocaliza-
tion through population of a formally sp3-hybridized orbital.
Moreover, this lone pair on N4 is further immobilized by a
hydrogen-bonding interaction with H1 (bond length of
˚
2.071(14) A). Additionally, a weak hydrogen bond between
˚
H2 and N3 of a neighboring molecule (bond length 2.522(15) A)
is present in the solid-state structure.
(40) Schlegel, H. B.; Skancke, A. J. Am. Chem. Soc. 1993, 115, 7465–
7471.
(41) Koller, J.; Ajmera, H. M.; Abboud, K. A.; Anderson, T. J.;
(42) Bharatam, P. V.; Iqbal, P.; Malde, A.; Tiwari, R. J. Phys. Chem.
A 2004, 108, 10509–10517.
(43) Uhl, W.; Molter, J.; Koch, R. Eur. J. Inorg. Chem. 2000, 2255–
McElwee-White, L. Inorg. Chem. 2008, 47, 4457–4462.
2262.

5950 Organometallics, Vol. 29, No. 22, 2010
Koller and Bergman
The commercially available [Al(NMe2)3]2 (1) dimer is active in
transformations that occur at temperatures above 75 °C,
whereas monomeric species such as the guanidinate-supported
dimethyl Al complex 2 show increased catalytic activity at
low reaction temperatures. The presented synthetic protocols
allow convenient access to various guanidine and previously
unknown aminoguanidine species at low catalyst loadings with
the added benefit of facile product isolation procedures. X-ray
crystallographic analysis of [(iPr)NHC(NNMe2)NH(iPr)] (4a)
provides valuable insight into the interesting bonding behavior
of aminoguanidines and confirms computational predictions
involving such species. The independently prepared Al amino-
guanidinate complex 5 proved to be catalytically active and
thus corroborates the proposed reaction mechanism.
Experimental Section
General Procedures. Unless otherwise noted, all reactions and
manipulations were performed in an inert atmosphere (N2)
glovebox or using standard Schlenk and high-vacuum line
techniques. Glassware was dried overnight at 150 °C before use.
1H and 13C NMR spectra were recorded at room tempera-
ture (except where noted) on Bruker AVB 400 MHz, AVQ 400 MHz,
or DRX 500 MHz spectrometers using residual protons of
deuterated solvents for reference. Elemental analyses were
performed at the University of California, Berkeley, Microana-
lytical Facility, on a Perkin-Elmer 2400 Series II CHNO/S
analyzer. IR spectra were measured neat on a Nicolet iS10
FT-IR spectrometer with a diamond attenuated total reflective
(ATR) accessory. Peak intensities are reported as broad (b),
weak (w), medium (m), or strong (s). Only peaks in the func-
tional group region (4000-1300 cm-1) are reported. High-
resolution mass spectral data were obtained at the QB3/Chemistry
Mass Spectrometry Facility operated by the College of Chemistry,
University of California, Berkeley. Electrospray injection (ESI)
mass spectra were recorded on a LTQ Orbitrap (Thermo) instru-
ment. Low-resolution mass spectral data were obtained on a
6890N gas chromatograph system coupled to a 5973N mass
selective detector (Agilent Technologies). Sealed NMR tubes were
prepared by attaching the NMR tube directly to a Kontes high-
vacuum stopcock via a Cajon Ultra-Torr reducing union followed
by flame-sealing on a vacuum line.
Figure 4. Proposed catalytic cycle for the formation of amino-
guanidines.
Scheme 2. Synthesis of Compound 5
free hydrazine results in formation of the aminoguanidine
product while simultaneously regenerating the active species.
To provide further evidence for the proposed reaction
mechanism, we attempted the independent synthesis of an Al
aminoguanidinate complex akin to structure II (Figure 4). In
fact, a simple amine exchange reaction of 1 with two equiva-
lents of aminoguanidine 4a at 100 °C provided access to
compound 5 (Scheme 2), which was isolated as a pale yellow
oil. The 1H NMR spectrum of 5 shows resonances that are
remarkably similar to those of 4a in addition to resonances
corresponding to magnetically equivalent dimethylamide
functionalities on the Al metal center. As expected, the
hydrogen-bonded N-H moiety of 4a is activated preferen-
tially during the amine exchange reaction, as evidenced by
the disappearance of the N-H resonance of the free ligand at
5.84 ppm. The emergence of the remaining N-H proton as a
doublet, in combination with the apparent doublet of septets
assigned to the methine proton of an isopropyl substituent,
corroborates the structural assignment. Subjecting com-
pound 5 to catalytic conditions revealed its activity toward
hydro-hydrazination chemistry with isolated reaction yields
similar to those of reactions catalyzed by 1 (Table 3, entry 1).
These findings are consistent with the proposed reaction
mechanism and indicate that Al aminoguanidinate com-
plexes such as 5 may in fact be intermediates during the
conversion of 1,1-dialkylhydrazines and carbodiimides to
aminoguanidines.
Materials. Unless otherwise noted, reagents were purchased
from commercial suppliers and used without further purification.
Pentane, hexane, diethyl ether, and toluene were purified by
passing the degassed solvents through a column of activated
alumina (type A2, size 12-32, Purifry Co.) under nitrogen pressure
followed by additional sparging with N2 prior to use. Deuterated
solvents (Cambridge Isotope Laboratories) and 1,1-diphenyl-
hydrazine were degassed by three freeze-pump-thaw cycles and
˚
stored over activated 3 A molecular sieves. [Al(NMe2)3]2 was
purchased from Aldrich and sublimed prior to use. Aniline,
4-fluoroaniline, 1,1-dimethylhydrazine, N-aminopiperidine, and
N,N0-diisopropylcarbodiimide were distilled from CaH2 and
˚
stored over activated 3 A molecular sieves.
Crystallographic Analysis. X-ray structural determinations
were performed at CHEXRAY, University of California,
Berkeley. Single crystals of 2 and 4a were coated in Paratone-N
oil, mounted on a Kaptan loop, transferred to a Bruker APEX-I
CCD area detector instrument, centered in the beam, and cooled
by a nitrogen flow low-temperature apparatus that was pre-
viously calibrated by a thermocouple placed at the same position
as the crystal. Preliminary orientation matrices andcell constants
were determined by collection of 60 10 s frames, followed by spot
integration and least-squares refinement. An arbitrary hemi-
sphere of data was collected, and the raw data were integrated
using SAINT. Cell dimensions reported were calculated from all
reflections with I > 10σ. The data were corrected for Lorentz
and polarization effects, but no correction for crystal decay was
Conclusions
We have established aluminum as a highly active metal
toward the hydro-amination/-hydrazination of carbodiimides.

Article
Organometallics, Vol. 29, No. 22, 2010 5951
applied. Data were analyzed for agreement and possible absorp-
tion using XPREP. An empirical absorption correction based on
S3 comparison of redundant and equivalent reflections was
applied using SADABS.44 The structures were solved using
SHELXS and refined on all data by full-matrix least-squares
with SHELXL-97.45 Thermal parameters for all non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were cal-
culated in ideal positions and were riding on their respective
carbon atoms. Nitrogen-bound hydrogen atoms were located on
the electron density map and fully refined. For compound 2, a
total of 358 parameters were refined in the final cycle of refine-
ment using 4564 reflections with I > 2σ(I) to yield R1 and wR2 of
4.65% and 11.29%, respectively. For compound 4a, a total of
136 parameters were refined in the final cycle of refinement using
2042 reflections with I > 2σ(I) to yield R1 and wR2 of 3.99% and
10.02%, respectively. Refinement was done using F2. ORTEP
diagrams werecreatedusing the ORTEP-3 software package and
rendered using Pov-Ray 3.6.
(s, Ph), 130.68 (s, Ph), 130.53 (s, Ph), 124.06 (s, Ph), 50.77 (s,
NCH), 34.35 (s, Cy), 26.30 (s, Cy), 25.63 (s, Cy), 21.24 (s, CH3).
MS (EI, þve): m/z (%) 313 (29) [M]þ.
1,3-Diisopropyl-2-phenylguanidine (3c): 46 colorless solid. Yield:
1
197 mg (90%, 0.898 mmol). H NMR (benzene-d6, 400 MHz):
δ 7.29-7.25 (m, 2H, Ph), 7.14 (m, 2H, Ph), 6.94-6.91 (m, 1H, Ph),
3.64 (bs, 2H, NCH(CH3)2), 3.41 (bs, 2H, NH), 0.89 (d, 12H, J =
6.0 Hz, NCH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ 152.04
(s, N3C), 149.88 (s, Ph), 130.04 (s, Ph), 124.10 (s, Ph), 121.76 (s, Ph),
43.59 (s, NCH(CH3)2), 23.57 (s, NCH(CH3)2). MS (EI, þve): m/z
(%) 219 (47) [M]þ.
1,3-Dicyclohexyl-2-phenylguanidine (3d): 46 colorless solid. Yield:
290 mg (97%, 0.968 mmol). 1H NMR (benzene-d6, 400 MHz): δ
7.30-7.27 (m, 2H, Ph), 7.19 (m, 2H, Ph), 6.96-6.92 (m, 1H, Ph),
3.61 (bd, 2H, J = 6.0 Hz, NH), 3.47 (bs, 2H, NCH(CH3)2),
1.93-1.91 (m, 4H, Cy), 1.51-1.47 (m, 4H, Cy), 1.39-1.37 (m,
2H, Cy), 1.17-1.08 (m, 4H, Cy), 0.97-0.80 (m, 6H, Cy). 13CNMR
(benzene-d6, 101 MHz): δ 152.17 (s, N3C), 149.79 (s, Ph), 130.04
(s, Ph), 124.22 (s, Ph), 121.75 (s, Ph), 50.77 (s, NCH), 34.29
(s, Cy), 26.26 (s, Cy), 25.61 (s, Cy). MS (EI, þve): m/z (%) 299
(35) [M]þ.
[(2,6-iPr2C6H3)NC(N(iPr)2)N(2,6-iPr2C6H3)]AlMe2 (2). To a
stirred solution of LiN(iPr)2 (241 mg, 2.25 mmol) in Et2O (10 mL)
was added a solution of N,N0-bis(2,6-diisopropylphenyl)carbo-
diimide (816 mg, 2.25 mmol) in Et2O (10 mL). The resulting
mixture was stirred at ambient temperature for 60 min. The pale
yellow solution was subsequently cooled to 0 °C, and AlClMe2
(2.50 mL, 0.9 M solution in heptane, 2.25 mmol) was added
dropwise. The resulting mixture was allowed to warm to room
temperature and stirred for another 18 h. All volatiles were
removed in vacuo, and the product was extracted with pentane
(2ꢀ 5 mL). The combined extracts were filtered and cooled to -35 °C
to yield pure 2 as colorless crystals, which were isolated by
16
1,3-Diisopropyl-2-(4-fluorophenyl)guanidine (3e): colorless
solid. Yield: 219 mg (92%, 0.923 mmol). 1H NMR (benzene-d6,
400 MHz): δ 6.89 (d, 4H, J = 6.8 Hz, Ph), 3.60 (bs, 2H,
NCH(CH3)2), 3.33 (bd, 2H, J = 6.8 Hz, NH), 0.88 (d, 12H,
J = 5.9 Hz, NCH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ
160.10 (s, N3C), 157.73 (s, Ph), 150.23 (s, Ph), 147.94 (d, J = 2.2
Hz, Ph), 124.88 (d, J = 7.6 Hz, Ph), 116.49 (d, J = 21.8 Hz, Ph),
43.55 (s, NCH(CH3)2), 23.55 (s, NCH(CH3)2). MS (EI, þve):
m/z (%) = 237 (100) [M]þ.
1
filtration. Yield: 518 mg (40%, 1.00 mmol). H NMR (benzene-
1,3-Dicyclohexyl-2-(4-fluorophenyl)guanidine (3f): 16 colorless
solid. Yield: 315 mg (99%, 0.992 mmol). 1H NMR (benzene-d6,
400 MHz): δ 6.96-6.89 (m, 4H, Ph), 3.52 (bd, 2H, J = 6.7 Hz,
NH), 3.45 (bs, 2H, NCH(CH3)2), 1.90 (m, 4H, Cy), 1.51-1.47
(m, 4H, Cy), 1.39-1.36 (m, 2H, Cy), 1.16-1.07 (m, 4H, Cy),
0.97-0.82 (m, 6H, Cy). 13C NMR (benzene-d6, 101 MHz):
δ 160.12 (s, N3C), 150.11 (s, Ph), 148.06 (d, J = 3.0 Hz, Ph),
124.98 (d, J = 7.4 Hz, Ph), 116.50 (d, J = 21.8 Hz, Ph), 50.72 (s,
NCH), 34.28 (s, Cy), 26.24 (s, Cy), 25.57 (s, Cy). MS (EI, þve):
m/z (%) 317 (43) [M]þ.
General Procedure for Formation of Aminoguanidines. A Schlenk
bomb was charged with the respective carbodiimide (5 mmol), 1,
1-disubstituted hydrazine (5 mmol), 1 (0.05 mmol), and 5 mL of
toluene. The clear solution was heated at 120 °C for 4 h followed by
solvent removal in vacuo after cooling to ambient temperature. The
resulting white solid was extracted with 2 ꢀ 10 mL of pentane, and
the solution was subsequently filtered to remove all insolubles. The
pure product was obtained as a colorless solid via crystallization
from pentane at -20 °C.
d6, 500 MHz): δ 7.09 (bs, 6H, Ph), 3.98 (sept, 2H, NCH(CH3)2),
3.73 (sept, 4H, CH(CH3)2), 1.31 (d, 12H, J = 6.9 Hz, CH(CH3)2),
1.29 (d, 12H, J = 6.7 Hz, CH(CH3)2), 0.80 (d, 12H, J = 7.0 Hz,
NCH(CH3)2), -0.16 (s, 6H, AlCH3). 13C NMR (benzene-d6,
125 MHz): δ 165.91 (s, N3C), 145.65 (s, Ph), 139.73 (s, Ph),
126.14 (s, Ph), 124.55 (s, Ph), 50.19 (s, NCH(CH3)2), 28.70 (s,
CH(CH3)2), 27.62 (s, CH(CH3)2), 24.13 (s, CH(CH3)2), 23.97 (s,
NCH(CH3)2), -6.89 (s, AlCH3). Anal. Calcd for C33H54N3Al: C,
76.25; H, 10.47; N, 8.08. Found: C, 75.88; H, 10.52; N, 8.15.
General Procedure for Formation of Guanidines. A 20 mL vial
equipped with a Teflon stir bar was charged with the respective
carbodiimide (1 mmol), aniline (1 mmol), and 4 mL of hexane. A
solution of 2 (0.01 mmol) in hexane (1 mL) was added via pipet,
and the solution was stirred at ambient temperature for 30 min,
during which time the product precipitated as a white solid. The
vial was removed from the glovebox, and the product was
collected via filtration, washed with 5 mL of cold (-20 °C)
pentane, and subsequently dried in vacuo. Spectroscopic data
are in good agreement with literature data for known com-
pounds 3a-f.16,19,46
[(iPr)NHC(NNMe2)NH(iPr)] (4a): colorless solid. Yield: 0.82 g
(88%, 4.39 mmol). 1H NMR (benzene-d6, 400 MHz): δ 5.84 (d,
1H, J = 9.2 Hz, NH), 4.18 (m, 1H, NCH(CH3)2), 2.99 (dsept,
1H, J = 10.1, 6.3 Hz, NCH(CH3)2), 2.78 (d, 1H, J = 5.7 Hz,
NH), 2.51 (s, 6H, NN(CH3)2), 1.08 (d, 6H, J = 6.4 Hz, NCH-
(CH3)2), 0.90 (d, 6H, J = 6.3 Hz, NCH(CH3)2). 13C NMR
(benzene-d6, 101 MHz): δ 157.01 (s, N3C), 48.84 (s, NN(CH3)2),
44.01 (s, NCH(CH3)2), 42.50 (s, NCH(CH3)2), 24.16 (s, NCH-
(CH3)2), 23.72 (s, NCH(CH3)2). IR (neat): ν~ 3310 (b), 3255 (w),
2970 (m), 2939 (w), 2853 (b), 1598, (s), 1530 (s), 1466 (w), 1443
(w), 1378 (m), 1362 (s), 1332 (w). HRMS (ESI): calcd for
[C9H23N4]þ 187.1917, found 187.1915.
1,3-Diisopropyl-2-p-tolylguanidine (3a): 19 colorless solid. Yield:
196 mg (84%, 0.840 mmol). 1H NMR (benzene-d6, 400 MHz): δ
7.08 (s, 4H, Ph), 3.67 (bs, 2H, NCH(CH3)2), 3.45 (bs, 2H, NH),
2.18 (s, 3H, CH3), 0.91 (d, 12H, J = 5.8 Hz, NCH(CH3)2). 13C
NMR (benzene-d6, 101 MHz): δ 150.09 (s, N3C), 149.38 (s, Ph),
130.68 (s, Ph), 130.53 (s, Ph), 123.95 (s, Ph), 43.59 (s, NCH(CH3)2),
23.62 (s, NCH(CH3)2), 21.23 (s, CH3). MS (EI, þve): m/z (%) 233
(64) [M]þ.
1,3-Dicyclohexyl-2-p-tolylguanidine (3b): 19 colorless solid. Yield:
303 mg (97%, 0.967 mmol). 1H NMR (benzene-d6, 500 MHz): δ
7.14-7.09 (m, 4H, Ph), 3.63 (bs, 2H, NH), 3.50 (bs, 2H, NCH-
(CH3)2), 2.19 (s, 3H, CH3), 1.94 (m, 4H, Cy), 1.50 (m, 4H, Cy),
1.38 (m, 2H, Cy), 1.14-1.10 (m, 4H, Cy), 0.97-0.85 (m, 6H, Cy).
13C NMR (benzene-d6, 125 MHz): δ 149.95 (s, N3C), 149.50
[(iPr)NHC(Npip)NH(iPr)] (4b): colorless solid. Yield: 1.06 g
(93%, 4.67 mmol). 1H NMR (benzene-d6, 400 MHz): δ 5.92 (d,
1H, J = 9.4 Hz, NH), 4.20 (m, 1H, NCH(CH3)2), 3.01 (m, 1H,
NCH(CH3)2), 2.98 (m, 2H, NCH2), 2.82 (d, 1H, J = 6.1 Hz,
NH), 2.67 (m, 2H, NCH2), 1.61-1.58 (m, 4H, CH2), 1.51 (m,
1H, CH2), 1.16 (m, 1H, CH2), 1.09 (d, 6H, J = 6.4 Hz, NCH-
(CH3)2), 0.94 (d, 6H, J = 6.3 Hz, NCH(CH3)2). 13C NMR
(benzene-d6, 101 MHz): δ 156.88 (s, N3C), 58.04 (s, NCH2),
44.13 (s, NCH(CH3)2), 42.55 (s, NCH(CH3)2), 26.93 (s, CH2),
(44) SADABS; Bruker-AXS: Madison, WI, 2001.
(45) SHELXTL6; Bruker-AXS: Madison, WI, 2000.
(46) Shen, H.; Chan, H.-S.; Xie, Z. Organometallics 2006, 25, 5515–
5517.

5952 Organometallics, Vol. 29, No. 22, 2010
Koller and Bergman
24.81 (s, CH2), 24.28 (s, NCH(CH3)2), 23.72 (s, NCH(CH3)2).
IR (neat): ν~ 3310 (b), 2961 (m), 2933 (m), 2825 (w), 1596 (s), 1540
(s), 1469 (w), 1441 (w), 1378 (m), 1360 (m). HRMS (ESI): calcd
for [C12H27N4]þ 227.2230, found 227.2231.
(s, Ph), 122.92 (s, Ph), 47.76 (s, NN(CH3)2), 29.04 (s, CH(CH3)2),
28.13 (s, CH(CH3)2), 24.47 (s, CH(CH3)2), 24.20 (s, CH(CH3)2).
IR (neat): ν~ 3367 (b), 3308 (w), 2957 (m), 2929 (w), 2866 (w), 1649
(s), 1586 (w), 1491 (s), 1435 (m), 1381 (w), 1360 (w), 1329 (w).
HRMS (ESI): calcd for [C27H43N4]þ 423.3482, found 423.3474.
[(2,6-iPr2C6H3)NC(Npip)N(2,6-iPr2C6H3)] (4g). Due to solu-
bility issues, compound 4g was extracted with diethyl ether (2 ꢀ
10 mL). The pure product was crystallized from diethyl ether as
a colorless solid. Yield: 2.20 g (95%, 4.76 mmol). 1H NMR
(benzene-d6, 400 MHz): δ 7.29-7.21 (m, 5H, Ph), 7.14-7.12 (m,
1H, Ph), 7.09 (m, 1H, NH), 4.47 (s, 1H, NH), 3.70 (sept, 2H,
CH(CH3)2), 3.44 (sept, 2H, CH(CH3)2), 2.85 (m, 2H, NCH2),
1.48 (m, 2H, CH2), 1.39 (d, 12H, J = 7.0 Hz, CH(CH3)2), 1.36
(m, 6H, CH2), 1.18 (d, 12H, J = 6.9 Hz, CH(CH3)2). 13C NMR
(benzene-d6, 101 MHz): δ 147.49 (s, N3C), 146.82 (s, Ph), 144.11
(s, Ph), 141.15 (s, Ph), 134.64 (s, Ph), 123.55 (s, Ph), 123.38 (s,
Ph), 122.91 (s, Ph), 57.64 (s, NCH2), 29.14 (s, CH(CH3)2), 28.19
(s, CH(CH3)2), 26.28 (s, CH2), 24.54 (s, CH(CH3)2), 24.22 (s,
CH(CH3)2), 23.02 (s, CH2). IR (neat): ν~ 3390 (b), 2955 (b), 2863
(w), 1644 (s), 1587 (m), 1490 (s), 1464 (m), 1437 (m), 1380 (w),
1358 (w), 1330 (w). HRMS (ESI): calcd for [C30H47N4]þ
463.3795, found 463.3782.
[(iPr)NHC(NNPh2)NH(iPr)] (4c): colorless solid. Yield: 1.38 g
1
(90%, 4.45 mmol). H NMR (benzene-d6, 400 MHz): δ 7.44
(d, 4H, J = 8.2 Hz, Ph), 7.19 (d, 4H, J = 8.3 Hz, Ph), 6.88 (t, 2H,
J = 7.2 Hz, Ph), 5.19 (d, 1H, J = 9.2 Hz, NH), 4.27 (m, 1H,
NCH(CH3)2), 2.95 (d, 1H, J = 7.1 Hz, NH), 2.80 (m, 1H, NCH-
(CH3)2), 1.10 (d, 6H, J = 6.4 Hz, NCH(CH3)2), 0.68 (d, 6H, J =
6.3 Hz, NCH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ 159.63
(s, N3C), 150.63 (s, Ph), 149.93 (s, Ph), 144.01 (s, Ph), 129.87 (s, Ph),
129.50 (s, Ph), 129.39 (s, Ph), 122.41 (s, Ph), 122.17 (s, Ph), 121.35 (s,
Ph), 120.09 (s, Ph), 118.52 (s, Ph), 44.14 (s, NCH(CH3)2), 43.05 (s,
NCH(CH3)2), 23.92 (s, NCH(CH3)2), 23.84 (s, NCH(CH3)2). IR
(neat): ν~ 3401 (w), 3368 (w), 2978 (w), 2965 (m), 2978 (b), 1587 (s),
1516 (m), 1485 (s), 1381 (w), 1303 (w). HRMS (ESI): calcd for
[C19H27N4]þ 311.2230, found 311.2229.
[(Cy)NHC(NNMe2)NH(Cy)] (4d): colorless solid. Yield: 1.32
g (99%, 4.94 mmol). 1H NMR (benzene-d6, 400 MHz): δ 6.05 (d,
1H, J = 9.7 Hz, NH), 3.90 (m, 1H, NCH(CH3)2), 2.99 (d, 1H,
J = 7.1 Hz, NH), 2.86 (m, 1H, NCH(CH3)2), 2.53 (s, 6H,
NN(CH3)2), 2.15-2.11 (m, 2H, Cy), 1.84-1.81 (m, 2H, Cy),
1.60-1.53 (m, 4H, Cy), 1.45-1.19 (m, 6H, Cy), 1.12-0.94 (m,
8H, Cy). 13C NMR (benzene-d6, 101 MHz): δ 156.93 (s, N3C),
51.31 (s, NCH), 49.50 (s, NCH), 48.90 (s, NN(CH3)2), 34.87 (s,
Cy), 34.35 (s, Cy), 26.74 (s, Cy), 26.30 (s, Cy), 25.83 (s, Cy), 25.49
(s, Cy). IR (neat): ν~ 3329 (w), 3277 (w), 2928 (s), 2850 (m), 1596
(s), 1526 (s), 1447 (w), 1387 (w), 1346 (w). HRMS (ESI): calcd
for [C15H31N4]þ 267.2543, found 267.2544.
[(Cy)NHC(Npip)NH(Cy)] (4e): colorless solid. Yield: 1.50 g
(98%, 4.91 mmol). 1H NMR (benzene-d6, 400 MHz): δ 6.18 (d,
1H, J = 9.7 Hz, NH), 3.93 (m, 1H, NCH), 3.01 (d, 1H, J =
7.2 Hz, NH), 2.90 (m, 3H, NCH and CH2), 2.63 (m, 2H, CH2),
2.14 (m, 2H, CH2), 1.82 (m, 2H, CH2), 1.62-1.36 (m, 11H,
CH2), 1.30-0.98 (m, 11H, CH2). 13C NMR (benzene-d6, 101
MHz): δ 156.87 (s, N3C), 58.08 (s, NCH2), 51.19 (s, NCH), 49.52
(s, NCH), 34.83 (s, CH2), 34.42 (s, CH2), 27.06 (s, CH2), 26.76 (s,
CH2), 26.34 (s, CH2), 25.82 (s, CH2), 25.30 (s, CH2), 24.84 (s,
CH2). IR (neat): ν~ 3334 (b), 3276 (w), 2931 (s), 2851 (m), 2802
(w), 1595 (s), 1526 (s), 1446 (w), 1415 (w), 1388 (w), 1360 (w),
1346 (w). HRMS (ESI): calcd for [C18H35N4]þ 307.2856, found
307.2856.
[(iPr)NC(NNMe2)NH(iPr)]Al(NMe2)2 (5). A Schlenk bomb
was charged with a solution of 1 (500 mg, 1.57 mmol) and 4a
(585 mg, 3.14 mmol) in 5 mL of toluene. The clear solution was
heated at 100 °C for 60 min. After cooling to ambient tempera-
ture, 2 mL of pentane was added, and all insolubles were
removed by filtration. Removal of all volatiles yielded analyti-
cally pure 5 as a pale yellow oil. Yield: 820 mg (87%, 2.73 mmol).
1H NMR (benzene-d6, 400 MHz): δ 4.09 (dsept, 1H, NCH-
(CH3)2), 3.10 (bd, 1H, J = 7.4 Hz, NH), 3.04 (sept, 1H, NCH-
(CH3)2), 2.78 (s, 12H, AlN(CH3)2), 2.32 (s, 6H, NN(CH3)2), 1.17
(d, 6H, J = 6.3 Hz, NCH(CH3)2), 1.04 (d, 6H, J = 6.4 Hz,
NCH(CH3)2). 13C NMR (benzene-d6, 101 MHz): δ 161.56 (s,
N3C), 49.22 (s, NN(CH3)2), 44.17 (s, NCH(CH3)2), 42.52 (s,
NCH(CH3)2), 41.69 (s, AlN(CH3)2), 23.94 (s, NCH(CH3)2),
23.36 (s, NCH(CH3)2). Anal. Calcd for C13H33N6Al: C, 51.97;
H, 11.07; N, 27.97. Found: C, 51.67; H, 11.44; N, 27.64.
Acknowledgment. This work was supported by the
Director, Officeof Science, Officeof Basic EnergySciences,
and the Division of Chemical Sciences, Geosciences, and
Biosciences of the U.S. Department of Energy at LBNL
under Contract DE-AC02-05CH11231.
[(2,6-iPr2C6H3)NHC(NNMe2)NH(2,6-iPr2C6H3)] (4f): color-
less solid. Yield: 1.78 g (84%, 4.21 mmol). 1H NMR (benzene-d6,
400 MHz): δ 7.29-7.20 (m, 5H, Ph), 7.13-7.10 (m, 1H, Ph), 6.94
(s, 1H, NH), 4.30 (s, 1H, NH), 3.66 (sept, 2H, CH(CH3)2), 3.40
(sept, 2H, CH(CH3)2), 1.90 (s, 6H, NN(CH3)2), 1.36 (bd, 18H, J =
6.9 Hz, CH(CH3)2), 1.17 (d, 6H, J = 6.9 Hz, CH(CH3)2). 13C
NMR (benzene-d6, 101 MHz): δ 147.61 (s, N3C), 146.76 (s, Ph),
144.03 (s, Ph), 141.06 (s, Ph), 134.40 (s, Ph), 123.53 (s, Ph), 123.37
Supporting Information Available: X-ray data are available as
crystallographic information files (CIF) for compounds 2 and
4a. 1H and 13C NMR data of all compounds as pdf files. This
material is available free of charge via the Internet at http://
pubs.acs.org.