Reactivity of the nickel(0)-CO2-imine system: New pathway to vicinal diamines

Article abstract of DOI:10.1021/om900765m

Nickela-2-oxazolidinones, formed by oxidative coupling of imines and CO₂ with Ni⁰, react with LiCl under mild conditions (4 °C, 1 atm) to afford vicinal diamines in up to 89% yield. The reaction is the first organometallic example of

Full text of DOI:10.1021/om900765m

Organometallics 2010, 29, 1997–2000 1997
DOI: 10.1021/om900765m
Reactivity of the Nickel(0)-CO2-Imine System: New Pathway to
Vicinal Diamines
€
Alexandre Graet, Laura Sinault, Maxime B. Fusaro, Anne-Laure Vallet, Candace Seu,
James L. Kilgore, and Marc M. Baum*
Department of Chemistry, Oak Crest Institute of Science, 2275 E. Foothill Boulevard, Pasadena,
California 91107
Received September 2, 2009
Summary: Nickela-2-oxazolidinones, formed by oxidative
coupling of imines and CO2 with Ni0, react with LiCl under
mild conditions (4 °C, 1 atm) to afford vicinal diamines in up to
89% yield. The reaction is the first organometallic example of
reductive imine coupling requiring CO2. In this system, CO2 is
participatory and is not incorporated in the reaction products.
These results represent an important addition to our under-
standing of the reactivity of metallacycles derived from CO2
and unsaturated compounds.
compounds,12 including alkenes,13-16 dienes,17-19 al-
kynes,20-24 aldehydes (Y = O),25-27 and imines (Y =
NR3).28 Recent efforts primarily have been dedicated to further
developing the chemistry of oxanickelacyclopentenes, formed
by the oxidative coupling of Ni0 with CO2 and alkynes (1b). For
example, Mori et al. reported the regio- and stereoselective
synthesis of tri- and tetrasubstituted carboxylated alkenes,4
while Louie and co-workers used CO2, diynes, and a catalytic
amount of nickel complex in an efficient, intramolecular [2 þ
2 þ 2] cycloaddition system.2,29,30
Carbon dioxide (CO2) is a highly appealing C1 building
block for industrial organic synthesis,1-5 since this potent
greenhouse gas is inexpensive, abundant, nontoxic, noncor-
rosive, nonexplosive, and nonflammable. The thermody-
namic stability and kinetic inertness of CO2, however,
present significant challenges to converting CO2 into useful
materials. Nickel(0) has a particular affinity for CO2, as
evidenced by the large number of characterized monomeric
and oligomeric Ni-CO2 complexes.6-11 The regioselective
co-oligomerization of CO2, Ni0, and unsaturated com-
pounds via an oxidative coupling reaction (Scheme 1) takes
advantage of this affinity to afford the five-membered
chelate ring 1. The reaction has been shown to proceed at
the electron-rich Ni0 center with a variety of unsaturated
Scheme 1. Nickel-Mediated, Regioselective Oxidative Coupling
of Double-Bonded Compounds (Y = CR3R4, O, NR3) and
Alkynes with CO2
Although nickela-2-oxazolidinones 2 are formed in high
yield (ca. 90%) from aliphatic and aromatic imines,28 further
investigations on their reactivity have not been reported.
Herein, we describe the first results of attempted Ni-C
insertion reactions in nickela-2-oxazolidinones and the un-
expected, high-yielding production of vicinal diamines by a
Ni-CO2-imine intermediate.
*To whom correspondence should be addressed. Tel: þ1-626-817-
0883. Fax: þ1-626-817-0884. E-mail: m.baum@oak-crest.org.
(1) Walther, D.; Ruben, M.; Rau, S. Coord. Chem. Rev. 1999, 182, 67.
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r
2010 American Chemical Society
Published on Web 04/15/2010
pubs.acs.org/Organometallics

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1998 Organometallics, Vol. 29, No. 9, 2010
Graet et al.
Table 1. Ni0-CO2 Mediated Dimerization of Imines
R2 compd meso:dla yieldb (%) imine (%)
Scheme 2. Attempted Synthesis of 3,4-Dihydro-2H-1,3-oxazin-
2-ones from Imines, Alkynes, CO2, and Ni0
R1
Ph
p-OMePh p-OMePh
p-CF3Ph p-OMePh
p-OMePh p-CF3Ph
p-CF3Ph p-CF3Ph
Ph
4
5
6
7
8
1.02
1.45
0.83
0.99
1.05
80.7
11.5
58.5
35.0
17.6
7.7
63.6
ND
36.0
32.0
a Determined by 1H NMR analysis66 (see the Supporting In-
formation). b Determined by GC and 1H NMR analysis.
Scheme 4. Formation of Imidazolidines 9 and 10 (R1 = R2 =
R3 = Ph)
Scheme 3. Ni0-CO2 Mediated Dimerization of Imines
Formation of imidazolidines 9 from the corresponding
diamines and either a carbonyl or imino compound is
straightforward (Scheme 4), but the origin of the CdX
component was unclear. Compound 10 was consistently
present in samples of 4 that had been chromatographed on
untreated silica gel, and this imidazolidine also appeared
when purified diamine samples (chromatographed on deac-
tivated silica;see the Supporting Information) were stored
in CDCl3. It therefore seemed likely that C2 of 10 must come
from the diamine product itself. A possible explanation was
offered by Shimizu and Makino,31 who found that vicinal
diamines readily undergo oxidative cleavage to the corre-
sponding imines in the presence of Lewis acids. The conver-
sion of dl-bisamine 4a into pentaphenyl imidazolidine 10 was
followed by 1H NMR spectroscopy of a crude, aerobic
reaction mixture in CDCl3 over a 4 week period (Figure 1).
The highly stereoselective, if not stereospecific, nature of this
reaction was surprising. Imidazolidine 10 steadily formed at
the expense of 4a, but the concentration of meso-diamine 4b
remained constant in the same sample. The formation of 10 in
purified diamine samples was also promoted by exposure to
untreated silica gel. It was subsequently found that treatment
of silica with triethylamine largely suppressed imidazolidine
formation (data not shown), and similarly, the reaction in
CDCl3 was suppressed by filtering the solvent through basic
alumina powder, both results being consistent with acid
catalysis of the cleavage reaction. There is no literature pre-
cedent for stereoselective decomposition of diaryl diamines,
and we can offer no satisfactory explanation for this result.
Periasamy et al. reported the formation of an imidazoli-
dine (R1 = R3 = Ph, R2 = CH3) in 48% yield as a byproduct
in the corresponding imine coupling reaction using a TiCl4/
Et3N system.32 Given the oxidative lability of many 1,2-
diamines, it is surprising that reports of imidazolidine by-
product are not more common in the literature.
Our initial plan was to build nitrogen-containing hetero-
cyclic products via the insertion of alkynes into the Ni-C
bond of nickelacycles 2, with subsequent reductive elimina-
tion from the seven-membered chelate ring to afford 3,4-
dihydro-2H-1,3-oxazin-2-ones 3 according to Scheme 2.
Numerous reactions were attempted by combining Ni-
(COD)2 with a wide range of substituted imines and metal
ligands in the presence of CO2. Under all conditions tried,
none of the cyclic products were detected and only starting
imines, as well as aldehyde and amine from imine hydrolysis,
were isolated (see Table S1 in the Supporting Information for
a complete list of reaction components). The addition of
lithium chloride, a Lewis acid intended to activate the
nickelacycle, led to the formation of compounds with peaks
1
in the 4.5-6.0 ppm region in the H NMR spectrum of
the crude reaction mixture’s organic fraction (L2 = bpy;
imine = N-benzylideneaniline; alkyne = 1-hexyne). The
reaction products were found by IR spectroscopy not to contain
any carboxylate moiety, and olefinic signals expected from
alkyne adducts were also absent. NMR and IR spectroscopic
analysis in combination with high-resolution mass spectro-
metry showed these reaction products to be vicinal diamines.
To tetrahydrofuran solutions of Ni(COD)2 under argon
were added equimolar amounts of bpy and imine followed by
sparging with CO2 (1 atm) at 4 °C and the addition of LiCl
(2 equiv) to afford vicinal diamines (Scheme 3) in low to
excellent unoptimized yields (Table 1). The reaction afforded
the highest yields with electron-neutral systems (R1 = R2 =
Ph). The vicinal diamines were obtained as diastereomeric
(meso:dl) mixtures with modest diastereoselectivities. The
highest stereoselectivity was observed when R2 was p-OMePh
(Table 1).
Spectral data for imidazolidine 10 initially seemed to be
inconsistent with the assigned structure. Mass spectrometric
analysis gave an m/z value of 451.3 ([M-H]+ calcd for
C33H28N2, 451.2), but the 1H NMR spectrum showed a
pattern of ring proton signals more complex than originally
Following the assignment of 1H NMR signals from
diamine mixture 4, it was clear that other imine-derived
substances were present in the product mixtures. One com-
pound that appeared in variable amounts in different pre-
parations ultimately proved to be the imidazolidine 10
(Scheme 4).
(31) Shimizu, M.; Makino, H. Tetrahedron Lett. 2001, 42, 8865.
(32) Periasamy, M.; Srinivas, G.; Karunakar, G. V.; Bharathi, P.
Tetrahedron Lett. 1999, 40, 7577.

Communication
Organometallics, Vol. 29, No. 9, 2010 1999
Figure 2. Side products in the Ni0-CO2 mediated dimerization
of imines;. 12a (R1 = p-OMePh): HRMS-FAB (m/z) [M þ H]þ
calcd for C23H21F3O2N 400.1524, found 400.1507. 12b (R1 = p-
CF3Ph): HRMS-FAB (m/z) [M þ H]þ calcd for C23H15F9N
476.1060, found 476.1046.
obtained. When the solvent was not presaturated with CO2,
the isolated nickelacycle decomposed to quantitatively lib-
erate the starting imine. Nickelaoxazolidinone 2 could also
be prepared from Ni(acac)2 and DIBAH in excellent yields (see
the Supporting Information) or generated in situ affording,
following the addition of LiCl, diamines 4. The LiCl can be
added with the bpy, at the beginning of the reaction, or after the
CO2, at the very end, with equivalent results. No diamine
products were observed when LiCl was substituted in the above
reactions with a range of Lewis acids (ZnI2, SbI3, BF3) that have
been used previously as additives to activate nickelalactones
toward reaction with alkyl iodides.16,38 When MnI2 was used,
trace amounts (4%) of diamines 4 were obtained along with
unreacted imine. These results suggest a unique role for the LiCl
in the dimerization reaction, possibly both as a weak Lewis acid
and as a source of nucleophilic chloride anion.
Figure 1. Evolution of organic product mole fractions from Ni-
1
mediated imine coupling analyzed by H NMR spectroscopy
(CDCl3): (white circles) dl-1,2-diamine 4a; (black circles) meso-
1,2-diamine 4b; (gray circles) imidazolidine 10; (white squares)
N-benzylideneaniline; (white triangles) benzaldehyde.
expected, including a pair of doublets at δH 4.95 and 5.13
ppm (JH4-H5 = 7.2 Hz). It is known that for imidazolidines 9
with small C2 substituents (e.g., R3 = CH3) rapid ring inver-
sion leads to apparent magnetic equivalence for H4 and H5
to produce a singlet for these methine protons at δH
3.50 ppm,32-35 but none of the compounds in these studies
included larger C2 substituents. The low-field resonances in our
product could be explained by assuming that the C2 phenyl
group is rotated out of the plane of the imidazolidine ring so that
both C4 and C5 methine protons display significant downfield
shifts. Subsequently, we learned that 10 had been reported as a
minor product of the silyl radical-promoted coupling of
Reacting nickelaoxazolidinone 2 (R1 = R2 = Ph) with 1
equiv of imine (R1 = R2 = Ph) in the presence of CO2 and
LiCl afforded the diamines 4 in 89% yield (based on 2) with
98% recovery of imine. Similar yields of the diamines 4 were
obtained in the absence of additional imine, starting with the
corresponding nickelacycle and LiCl. These results suggest
that the diamines are not formed through insertion of free
imine, or iminium ion formed by the reaction of trace
carbonic acid with the imine, into the nickelacycle Ni-C
bond. The involvement of an iminium ion is also unlikely,
because the addition of organic bases (DABCO, benzyl-
amine, aniline, triethylamine) to the reaction mixture did
not reduce the yield of diamine 4, which even formed in the
presence of 3 equiv of triethylamine.
The above results suggest that the C-C bond in the diamine
coupling reaction is formed by the reaction of two metalla-
cycles, possibly via a bimetallic intermediate, with a key
involvement of LiCl. Decarboxylation affords diamines 4-8,
which are freed from the metal upon mild oxidative workup.
Vicinal diamines are recurring structural motifs in biolo-
gically active compounds,39-44 are useful chiral auxiliaries
and ligands in asymmetric synthesis,45-49 are intermediates
1
PhCHdNPh36 and that this compound’s H NMR spectrum
matched that of our material. Compound 10 was synthesized
independently by heating an authentic sample of 4a under reflux
with benzaldehyde dimethyl acetal and CF3CO2H in methanol,
again providing material with matching 1H NMR spectra.
When R2 = p-CF3Ph, additional reaction products were
observed in the Ni0-CO2 mediated dimerization of imines
(Figure 2), presumably through elimination of 4-(trifluoro-
methyl)aniline under mildly acidic/hydrolytic conditions to
afford the corresponding tautomeric enamine (11)-imine
(12) pair. An equilibrium between these two forms typically
is reached within 2 days.37
The reductive coupling of imines to 1,2-diamines described
here is unusual because it requires a low-valent bpyNi(COD)
system, CO2, and LiCl. The omission of any of these
components results in quantitative recovery of starting
materials. The requirement of CO2 for reductive imine
coupling suggested that the reaction proceeded via nicke-
laoxazolidinone 2. When this complex (R1 = R2 = Ph),
synthesized and isolated independently, was suspended in
THF saturated with CO2 and allowed to react with LiCl
(1 equiv) under a CO2 atmosphere, vicinal diamines 4 were
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Graet et al.
2000 Organometallics, Vol. 29, No. 9, 2010
in the synthesis of N-heterocyclic carbene ligands,50 and find
application in radiopharmaceuticals.51,52 The reductive cou-
pling of imines to vicinal diamines, a less studied variant on
the well-known pinacol coupling of carbonyl compounds,
has been reported using numerous reagents.32,50,53-72 De-
spite the wide range of metal-containing reagents used in
these transformations, nickel-based reductants have received
little attention. Catalytic nickel(II) iodide has been shown to
significantly improve the samarium(II) iodide mediated
coupling of imines,64 and Rieke Ni, generated in situ, was
shown to promote the pinacol coupling of various carbonyl
compounds.73 The advantages of the variant reported here
include good to excellent yields of vicinal diamines where R1
and R2 are aryl groups. Our route also affords negligible
amounts (<5%) of monoamines resulting from reduction
of the corresponding imine. Imine reduction to the mono-
amine is a significant, competitive process in many literature
methods.54,55,58,61,62,74-76
In summary, nickelaoxazolidinones, formed by oxidative
coupling of CO2 with aryl imines, afford vicinal diamines in
the presence of LiCl. The reactions proceed under mild
conditions (4 °C, 1 atm) starting with Ni(COD)2 or Ni
(acac)2 and represent the first organometallic example of
reductive imine coupling requiring CO2. In this system, CO2
is participatory and is not incorporated in the reaction
products. These results represent an important addition to
the growing repertoire of metallacycle chemistry derived
from CO2 and unsaturated substrates.
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Acknowledgment. We gratefully acknowledge the
National Science Foundation (Award Number 0723265),
the Ralph M. Parsons Foundation, and the Oak Crest
Institute for financial support and the California Institute
of Technology for a Summer Undergraduate Research
Fellowships (SURF) award (to C.S.). We thank the Ecole
ꢀ
ꢀ
Superieure de Chimie Organique et Minerale (ESCOM,
France) for placing students (A.G., L.S., M.B.F., A.-L.
V.) in the Oak Crest program and the California Institute
of Technology Division of Chemistry and Chemical
Engineering for providing access to NMR and MS facili-
ties. We also thank Nathan Dalleska, John A. Moss,
Mona Shahgholi, and David Vander Velde for valuable
input.
Supporting Information Available: Text, figures, and tables
giving information on experimental procedures and spectral
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