Room temperature tandem hydroamination and hydrosilation/protodesilation catalysis by a tricarbonylchromium-bound iridacycle

Article abstract of DOI:10.1039/c2cc35520e

A chromiumtricarbonyl-bound iridacycle displays novel catalytic virtues for the conversion of terminal aromatic alkynes into racemic N-phenyl, 1-arylethylamines by tandem hydro-amination and hydrosilation/protodesilation reactions under mild "one pot" con

Full text of DOI:10.1039/c2cc35520e

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COMMUNICATION
Room temperature tandem hydroamination and hydrosilation/
protodesilation catalysis by a tricarbonylchromium-bound iridacyclew
a
a
b
c
´ ´ ´ ´ ´
Wissam Iali, Frederic La Paglia, Xavier-Frederic Le Goff, Dusan Sredojevic,
Michel Pfeffera and Jean-Pierre Djukic*a
Received 30th July 2012, Accepted 30th August 2012
DOI: 10.1039/c2cc35520e
A chromiumtricarbonyl-bound iridacycle displays novel catalytic
virtues for the conversion of terminal aromatic alkynes into racemic
N-phenyl, 1-arylethylamines by tandem hydro-amination and hydro-
silation/protodesilation reactions under mild ‘‘one pot’’ conditions.
The principle of atom economy1 guides nowadays’ new develop-
Scheme 1 Reaction of mononuclear and binuclear iridacycles 1a and
1b with phenylacetylene at room temperature in MeOH for 7 h.
ments in the field of catalysis. This principle, which relates to the
effective management of reactants and reagents, takes all its
substance in the quest for procatalysts capable of performing
successive transformations,2 in a tandem one-pot fashion for
example. Our recent investigations of planar chiral3 iridacycles4,5
led us to investigate the reactivity of an array of iridacycles in
homogenous catalysis. Our attention was particularly attracted by
recent reports of Davies et al.6 and Jones et al.7 on the reactivity
of iridacycles towards unsaturated organic molecules. Mono-
nuclear iridacycles derived from aryl oxazolines6 and pyridines7
may insert alkenes and up to two alkynes to produce new
iridacycles. In this communication, we demonstrate that a
Cr(CO)3-containing iridacycle4 promotes readily the hydroamina-
tion of terminal alkynes into N-phenylimines8 as well as the
hydrosilation/protodesilation9 of the latter into the corresponding
benzylamines, two transformations that can be staged in a
one-pot tandem fashion under mild conditions.
A new class of p-complexes of iridacycles5 was investigated for
the possible influence of the electron-withdrawing p-bonded moiety,
i.e. Cr(CO)3, over possible reactions with terminal alkynes.
Preliminary explorations were carried out with iridacycles 1a and
1b5 under the conditions used by Jones et al. to promote the
dissociation of the Ir–Cl bond, i.e. in methanol.7 Whilst the reaction
of complex 1a with phenylacetylene reproduced the observations of
both Jones et al.7 and Davies et al.,6 that is the formation of a
product of double insertion, i.e. salt 2a (Scheme 1), the reaction of
complex 1b with the terminal alkyne produced a mixture containing
2a and a new iridium–acyl complex 2b (Scheme 1).
The structure of 2a was readily established by X-ray diffrac-
tion analysis (Fig. 1);w it shows obvious similarities with the
structure of the double insertion product reported by Davies
et al.6 wherein the insertion sequence for phenylacetylene
implies seemingly, the production of a vinyl-type intermediate
and the subsequent insertion of the electrophilic a-carbon of
the 2-phenylvinylidene ligand.
First principles-based DFT-D310 calculations performed on
a simplified Cp model of a cationic adduct of 1b with
phenylacetylene upon Ir–Cl bond cleavage indicated that the
Gibbs free enthalpy of conversion of a cationic p Ir–alkyne
coordinate into an Ir–vinylidene at 298.15 K (gas phase), i.e.
DGAB, amounts around ꢀ1.5 kcal molꢀ1 with a moderate
activation barrier DGzAB of ca. 17 kcal molꢀ1, which suggests that
a
´
Institut de Chimie de Strasbourg, UMR 7177, Universite de
Strasbourg, 4 rue Blaise Pascal, F-67000 Strasbourg, France.
E-mail: djukic@unistra.fr; Fax: +33 (0)3 68 85 00 01;
Tel: +33 (0)3 68 85 15 23
b
´ ´ ´ ´
Laboratoire ‘‘Heteroelements et Coordination’’, Ecole Polytechnique,
CNRS, F-91128 Palaiseau Cedex, France
c Faculty of Chemistry, University of Belgrade, Studentski trg,
SRB-11000 Belgrade, Serbia
Fig. 1 CSD mercury ellipsoid drawing of the molecular structure of 2a
(30% probability).w Distorted molecule of solvent and atoms of hydrogen
have been omitted for the sake of clarity. Selected interatomic distances (A)
and angles (1):Ir1–C14 2.082(2), Ir1–N1 2.103(2), Ir1–C12 2.160(2),
Ir1–C13 2.221(2), C11–C12 1.476(3), C12–C13 1.441(3), C13–C14
1.417(3), C14–C15 1.328(3), N1–Ir1–C12 88.91(8).
w Electronic supplementary information (ESI) available: Full experi-
mental parts and characterisations; computational details, i.e. coordi-
nates, vibrational modes and references. CCDC 893285 (2a). For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2cc35520e
c
10310 Chem. Commun., 2012, 48, 10310–10312
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Table 1 Performance of catalyst 1b in the hydroamination of 3a into
4a (R1, R2 = H, eqn (1)) under various experimental conditions
Entry Catalyst Solvent Salt
T (1C) t (h) Conversiond
1
2
3
4
5
6
7
1aa
1ba
1ba
1bb
1bb
1bb
2aa
CH3OH
CH2Cl2 NaBArF4c 40
CH3Ph NaBArF4 100
CH3OH
CH3OH
CH3OH
CH3OH
—
25
10
12
24
2
1.5
2
0
0
c
30
20
100
95
0
—
—
—
—
65
40
25
25
10
Scheme 2 Equilibration of the alkyne and vinylidene Ir intermediates
A and B in the model ground state and transition state TSAB (n(gas)
530.1i cmꢀ1, n(COSMO MeOH) 543.2i cmꢀ1) with selected distances
for gas phase geometries: Gibbs free enthalpies (298.15 K) of reaction
DGAB and activation Gibbs free enthalpies DGzAB were computed in the gas
and in the solvated (COSMO MeOH) state at the (ZORA) PBE-D3(0)/all
a
b
5 mol%, 1 eq. aniline. 1 mol%, 1 eq. aniline. c 10 mol%. d Determined
1
by H NMR spectroscopy.
either CH2Cl2 or in toluene (Table 1, entries 2 and 3).15 Only
low conversion without decomposition of the catalyst was
achieved when toluene and NaBArF4 were used as the solvent
and co-catalyst, respectively, at 100 1C. It was found that the best
conditions for a quantitative formation of 4a were by running the
reaction in the absence of any ionic co-catalyst, at room tem-
perature in methanol (entries 5 and 6, Table 1). The order of
introduction of the reagents was found essential for optimal
production of N-phenylimines: the best results were achieved
when the catalyst was added to a solution of arylacetylene and
aniline in freshly distilled methanol. Quantitative transforma-
tions could be achieved within 2 h with a molar 1% loading of 1b
at room temperature. Table 2 lists a range of results obtained
with various aromatic mono- (3a–e) and di-ynes (3f–g) as well as
with one aliphatic alkyne, i.e. 1-hexyne 3h (eqn (2)). For all
aromatic monoalkynes, the yields in imines 4a–e were quantita-
tive or nearly so. For para disubstituted diyne 3f, the yield in
di-imine 5f was limited to 33% as a probable consequence of the
electron-withdrawing deactivating effect of the para-imino group
at 4f. In turn, double hydroamination of 3g into 5g (entry 7,
Table 2) could be achieved in 50% yield, the monohydroamina-
tion product 4g being produced in equal yields.
electron TZP level; natural charges at atoms 1 and 2, i.e. qNPA1 and qNPA2
,
were obtained from natural population analysis (NPA).
such an interconversion is likely to occur spontaneously in
solution, as already suggested by Leong et al.11 In rationalizing
the formation of 2a, this calculation also provides an explanation
for the formation of 2b. Indeed, the ‘‘umpolung’’12 occurring at
the ethynyl moiety upon vinylidene’s formation (Scheme 2) lays
the ground for the nucleophilic addition of adventitious water
preferably at position 1. Deliberate introduction of large amounts
of water in the reaction medium resulted in a drastic reversal of the
2a : 2b ratio to ca. 1 : 3; no products of catalysed hydration of
phenylacetylene were observed though.13 1H and 13C NMR and
IR spectra of complex 2b provided sound proofs of the structure
of the Ir–acyl 2b (cf. ESIw), the X-ray structure of which is not
disclosed in this article for the sake of conciseness. To some
extent 2b presents spectroscopic features similar to Fukuzumi’s
acyliridium complex (cf. ESIw).13 The peculiar reactivity of 1b
with alkynes in the presence of water led us to consider other
potential nucleophiles such as primary amines in order to
attempt the formation of Ir–aminovinyl intermediates.
Primary amines such as tBuNH2, PhCH2NH2 and PhNH2 were
probed in reactions with phenylacetylene and 1b in stoichiometric
proportions under an inert atmosphere. In the former two cases,
no transformation of either the organic substrates or the complex
ensued. However in CDCl3, 1H NMR analyses indicated that the
alkylamines would form unstable adducts with 1b, the isolation of
which was not sought further.
Table 2 Performance of catalyst 1b in the hydroamination of 3a–h by
aniline at 25 1C in MeOH (eqn (1))
ð1Þ
Time
(h)
Substrate/
product(s)
Conversion
(%)
In contrast, the reaction of phenylacetylene 3a with aniline led
to the quantitative production of the Markovnikov product of
hydroamination,14 i.e. N-phenylimine 4a.
Entry
R1
R2
1
2
3
4
5
6
7
8
H
H
H
Me
H
H
H
2a
2a
2a
2a
3a
3b
3b
3b
3a/4a
3b/4b
3c/4c
3d/4d
3e/4e
3f/4f, 5f
3g/4g, 5g
3h/4h
95
95
100
100
90
67, 33
50, 50
50
This unexpected result led us to attempt a similar experiment
with complex 1a; the latter happened to convert exclusively into
2a without any single trace of imine being formed aside (entry 1,
Table 1). Complexes 2a (entry 7, Table 1) and 2b were found
catalytically inactive. This peculiar catalytic property of 1b
was investigated by varying the conditions of the catalysis by
first checking the possible promoting action of NaBArF4 in
OMe
NMe2
CF3
CRCH
H
H
CRCH
—
—
a
b
1 mol% 1b, 1 eq. aniline. 2.5 mol% 1b, 2.3 eq. aniline.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 10310–10312 10311

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The ability of 1b to promote sequential catalytic transforma-
tions was probed under ‘‘one pot’’ conditions with a series
of aromatic terminal alkynes, targeting their conversion to
N-phenyl,1-arylethylamines. A 1 : 1 : 1.3 mixture of alkyne
(3a–e), aniline and Et3SiH was dissolved in methanol in the
presence of 2.5 mol% of 1b at 25 1C. Scheme 3 shows that all
conversions in amines 6a–e are higher than 50% and in three
cases they are equal to or higher than 80%.
In conclusion, our study shows that a Cr(CO)3-bound
iridacycle such as 1b can readily promote the tandem transfor-
mation of terminal alkynes into N-phenylamines. To our knowl-
edge 1b outperforms other catalysts15,19 by the mild conditions
required particularly for the ‘‘one pot’’ intermolecular hydro-
amination–hydrosilation/protodesilation of terminal alkynes.20
These results bear a particular meaning here because the irida-
cycles in question are planar-chiral by essence. Our ongoing
efforts are now focussed on the synthesis of enantio-enriched
chiral iridacycles as it appears that iridacycles similar to 1b
display identical catalytic activity (results not shown here) and
that other iridacycles are active hydrosilation catalysts.
Fig. 2 Quantitative hydrosilation/protodesilation of aromatic imines
promoted by 1b: reactions were carried out with a 1 : 1.2 imine/Et3SiH
ratio.
The influence of substituents para to the ethynyl group in
arylacetylenes was investigated by competitive experiments,
staging the substituted phenylacetylenes 3c–e, 3a and a default
amount of aniline in a 1 : 1 : 1 ratio in the presence of 1 mol%
of 1b at 25 1C in methanol. Within 3 h each experiment
reached quantitative conversion of aniline, leading, respectively,
to mixtures containing the product pairs 4c–4a, 4d–4a and
4e–4a in 9 : 1, 4 : 6 and 2 : 8 ratios respectively. This result is
consistent with the assumption that the rate determining step
could be the addition of aniline to a polarized cationic p
Ir–alkyne intermediate akin to A (Scheme 2). Further outcome
in catalysis for 1b arose by submitting an array of imines
(Fig. 2) to the conditions of hydrosilation/protodesilation, that
is to their 1b-promoted reaction with Et3SiH in methanol
followed by hydrolysis. All attempted reactions led to quanti-
tative conversion of the imines into the corresponding racemic
chiral monoamines and diamines, such as 7f, or the pincer
ligand precursor 7g that was produced as a 1 : 1 mixture of
the (S,R) and (S*,S*) diastereomers, and the cyclic alkaloid
salsolidine 6i. The peculiar efficiency of 1b in promoting
hydrosilation was assigned to the possible intervention of an
Ir–hydrido intermediate,4 a key intermediate for the transfer of
the hydritic H atom to the electrophilic imine substrate.
Notes and references
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Soc. Rev., 2008, 37, 406–425.
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S. D. Zaric, X.-F. LeGoff and L. Ricard, Organometallics, 2007,
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5 J.-P. Djukic, W. Iali, M. Pfeffer and X.-F. LeGoff, Chem. Commun.,
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6 D. L. Davies, O. Al-Duaij, J. Fawcett and K. Singh, Organo-
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7 L. Li, Y. Jiao, W. W. Brennessel and W. D. Jones, Organometallics,
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Clues for the possible formation of a hydrido–Ir intermedi-
ate were obtained in an experiment carried out in dry d8-THF
wherein addition of N,N-dimethylaminopyridine to a mixture
of 1b with Et3SiH initiated the release of H2 and the appearance
of a new signal at ꢀ15.1 ppm in the 1H NMR spectrum of the
resulting solution, which was assigned to the typical resonance
of an Ir-bound hydrido ligand.4,16 Complex 1b was also found
to readily promote the dehydrogenative methoxylation of
Et3SiH in methanol:17 the dependence of the rate of release
of H2 gas on the catalyst’s concentration was qualitatively
evidenced indirectly by following the variation of the voltage
raise rate in a modified H2/air fuel cell18 (cf. ESIw).
11 E. Kumaran and W. K. Leong, Organometallics, 2012, 31, 4849–4853.
12 D. Seebach, Angew. Chem., Int. Ed. Engl., 1979, 18, 239–258.
13 S. Ogo, K. Uehara, T. Abura, Y. Watanabe and S. Fukuzumi,
J. Am. Chem. Soc., 2004, 126, 16520–16527.
14 M. Nobis and B. Driessen-Holscher, Angew. Chem., Int. Ed., 2001,
40, 3983–3985.
15 R.-Y. Lai, K. Surekha, A. Hayashi, F. Ozawa, Y.-H. Liu,
S.-M. Peng and S.-T. Liu, Organometallics, 2007, 26, 1062–1068.
16 Y. Hu, L. Li, A. P. Shaw, J. R. Norton, W. Sattler and Y. Rong,
Organometallics, 2012, 31, 5058–5064.
17 L. D. Field, B. A. Messerle, M. Rehr, L. P. Soler and
T. W. Hambley, Organometallics, 2003, 22, 2387–2395.
18 C. Boulho and J.-P. Djukic, Dalton Trans., 2010, 39, 8893–8905.
19 E. Kumaran and W. K. Leong, Organometallics, 2012, 31,
1068–1072.
Scheme 3 ‘‘One pot’’ transformation of terminal arylethynes into
N-phenyl,1-arylethylamines promoted by 1b.
20 L. D. Field, B. A. Messerle and S. L. Wren, Organometallics, 2003,
22, 4393–4395.
c
10312 Chem. Commun., 2012, 48, 10310–10312
This journal is The Royal Society of Chemistry 2012