Selective reduction of formamides to O-silylated hemiaminals or methylamines with HSiMe2Ph catalyzed by iridium complexes

Article abstract of DOI:10.1039/c8dt05070h

The reaction of (4-methyl-pyridin-2-iloxy)ditertbutylsilane (NSi^tBu-H, 1) with [IrCl(coe)₂]₂ affords the iridium(iii) complex [Ir(H)(Cl)(κ²-NSi^tBu)(coe)] (2), which has been fully characterized including X-ray diffraction studies. The reaction of 2 with AgCF₃SO₃ leads to the formation of species [Ir(H)(CF₃SO₃)(κ²-NSi^tBu)(coe)] (3). The iridium complexes 2 and 3 are effective catalysts for the reduction of formamides with HSiMe₂Ph. The selectivity of the reduction process depends on the catalyst. Thus, by using complex 2, with a chloride ancillary ligand, it has been possible to selectively obtain the corresponding O-silylated hemiaminal by reaction of formamides with one equivalent of HSiMe₂Ph, while complex 3, with a triflate ligand instead of chloride, catalyzed the selective reduction of formamides to the corresponding methylamine.

Full text of DOI:10.1039/c8dt05070h

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Selective reduction of formamides to O-silylated
hemiaminals or methylamines with HSiMe2Ph
catalyzed by iridium complexes†
Cite this: DOI: 10.1039/c8dt05070h
Jefferson Guzmán, Ana M. Bernal, Pilar García-Orduña, Fernando J. Lahoz,
Luis A. Oro
and Francisco J. Fernández-Alvarez
*
The reaction of (4-methyl-pyridin-2-iloxy)ditertbutylsilane (NSitBu–H, 1) with [IrCl(coe)2]2 affords the
iridium(III) complex [Ir(H)(Cl)(κ2-NSitBu)(coe)] (2), which has been fully characterized including X-ray diffraction
studies. The reaction of 2 with AgCF3SO3 leads to the formation of species [Ir(H)(CF3SO3)(κ2-NSitBu)(coe)] (3).
The iridium complexes 2 and 3 are effective catalysts for the reduction of formamides with HSiMe2Ph.
The selectivity of the reduction process depends on the catalyst. Thus, by using complex 2, with a
chloride ancillary ligand, it has been possible to selectively obtain the corresponding O-silylated hemiam-
inal by reaction of formamides with one equivalent of HSiMe2Ph, while complex 3, with a triflate ligand
instead of chloride, catalyzed the selective reduction of formamides to the corresponding methylamine.
Received 22nd December 2018,
Accepted 25th February 2019
DOI: 10.1039/c8dt05070h
rsc.li/dalton
the reaction required to be heated at 418 K for 3 days to
Introduction
achieve a 60–87% yield.4 In 1998, Ito and collaborators
Amines are chemicals of great interest due to their numerous reported that the complex [RhCl(CO)(PPh3)3] (0.1 mol%) cata-
and different applications.1 They are commonly prepared by lyzed the reduction of tertiary amides with 2.1–2.5 equiv. of
stoichiometric reduction of amides with metal hydrides. H2SiEt2 to yield the corresponding tertiary amine.5 Since then,
However, the application of this type of reducing agent at the several examples of catalysts based on Ti,6 Mo,7 Fe,8 Ru,9 Co,10
industrial scale faces two main issues: (i) the generation of Rh,5,11 Ir,12 Pt,13 Cu,14 or Zn15 complexes effective for the
huge amounts of residues and (ii) the fact that metal hydrides reduction of amides to the corresponding amines using silicon
are moisture and air sensitive.2 That is why the development hydrides as reductants have been published.
of catalytic systems allowing the selective reduction of amides
In this context, as a continuation of our studies on the appli-
under mild reaction conditions constitutes an interesting cation of iridium systems as homogeneous catalysts for the
research field. During the last few years, some examples of reduction of unsaturated organic molecules with hydro-
efficient homogeneous catalysts for the reduction of amides to silanes,16 we have found that iridium(III) species with the
amines using hydrogen, hydroboranes or hydrosilanes as redu- monoanionic bidentate (4-methyl-pyridin-2-iloxy)ditertbutylsilyl
cing agents have been reported.3 In this context, the catalytic (NSitBu) ligand are effective catalysts for the reduction of forma-
hydrosilylation of amides has emerged as a chemical techno- mides to the corresponding methylamines under mild reaction
logy for the synthesis of amines with siloxanes as the only side conditions and low catalyst loading (0.5 mol%). Moreover, the
reaction products.3i,j
nature of the ancillary ligand influences the selectivity of these
To the best of our knowledge, the first examples of catalytic Ir–NSitBu based catalysts, thus by using a chloride derivative it
reduction of amides to amines, using hydrosilanes as reduc- has been possible to selectively reduce formamides to the
tants, were reported in 1962 by Calas et al.4 They used ZnCl2 as corresponding O-silylated hemiaminals, while Ir–NSitBu cata-
a catalyst for the reduction of RCONEt2 (R = Me, Et, Pr, Ph) to lysts bearing a triflate ligand catalyzed the selective reduction
the corresponding amine using HSiEt3 as a reducing agent; of formamides to the corresponding methylamines.
Departamento de Química Inorgánica – Instituto de Síntesis Química y Catálisis
Results and discussion
Synthesis and characterization of the catalyst precursors
Homogénea (ISQCH). Universidad de Zaragoza – CSIC, Facultad de Ciencias 50009,
Zaragoza, Spain. E-mail: paco@unizar.es
†Electronic supplementary information (ESI) available. CCDC 1857508. For ESI
In recent years, we have explored the potential of iridium17–20
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8dt05070h
and rhodium21 complexes with monoanionic tridentate NSiN
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ligands as hydrosilylation or silylation catalysts. The presence
of the silyl group labilizes the interaction of the metal with the
substrate trans located to the silicon atom, which positively
influences the catalytic activity of these species (Scheme 1).16
However, the preference of NSiN ligands for fac-coordination
hinders the interaction of the catalytically active site with
bulky substrates.
In this context, it should be mentioned that the chemistry
of iridium complexes with pyridine-2-iloxy-silyl based anions
as monoanionic bidentate ligands, which could afford less
sterically encumbered iridium catalysts, remains unexplored.22
In this work, the functionalized hydrosilane 4-methyl-
pyridin-2-iloxy-ditertbutyl silane (1) has been used as a ligand
precursor. Compound 1 has been obtained in 92% yield by
stirring for 5 hours a mixture of 4-methyl-2-hydroxy-pyridine,
ClSiHtBu2 and imidazole in THF at RT. The reaction of
[IrCl(coe)2]2 (coe = cis-cyclooctene) with two equivalents of
freshly prepared 1 in CH2Cl2 affords the Ir(III) complex
[Ir(H)(Cl)(κ2-NSitBu)(coe)] (2) (NSitBu = 4-methylpyridin-2-iloxy-
ditertbutylsilyl), which has been isolated as a yellow solid in
73.4% yield (Scheme 2).
Scheme 2 Synthesis of complexes [Ir(H)(X)(κ2-NSitBu)(coe)] (X = Cl, 2;
CF3SO3, 3).
The iridium complex 2 has been fully characterized by
elemental analysis, and 1H, 13C{1H} and 29Si{1H} NMR spec-
troscopy. In addition, the solid-state structure of 2 has been
determined by X-ray diffraction. Complex 2 exhibits a trigonal
bipyramidal iridium atom with the nitrogen atom and the cen-
troid of the olefinic bond of cyclooctene at apical positions
and equatorial sites occupied by silicon, chloro and hydrogen
atoms (Fig. 1).
The bidentate coordination of the NSitBu ligand exhibits
some similarities with geometrical parameters reported for tri-
dentate NSiN fragments. The nitrogen atom of the NSitBu frag-
ment is found to be trans located to the cyclooctene ligand
with Ir–N and Ir–O bond lengths in good agreement with
those of [Ir(H)(X)(κ3-NSiN)(coe)] compounds.17,20 In fact, the
main difference between Ir(κ2-NSi) bidentate and Ir(κ3-NSiN)
tridentate fragments concerns the Ir–Si bond length, which is
found to be longer in complex 2 (2.2853(6) compared to values
Fig. 1 Molecular structure of complex 2: selection of bond distances
(Å) and angles (°): Ir–Cl, 2.3950(6); Ir–Si, 2.2853(6); Ir–N, 2.0947(18); Ir–
Ct 2.0612(15); Ir–H, 1.61; Cl–Ir–Si, 131.76(2); Cl–Ir–N, 88.47(5); Cl–Ir–
Ctª, 97.44(4); Cl–Ir–H, 123.8; Si–Ir–N, 81.72(6); Si–Ir–Ct, 98.42(5); Si–
Ir–H, 100.0; N–Ir–Ct, 171.74(5); N–Ir–H, 77.8; Ct–Ir–H, 94.1. Ct is the
centroid of the olefinic bond of the coe ligand.
between 2.2196(14) and 2.2356(12) Å) found for Ir(κ3-
NSiN).17,20 This feature may be related to a lower constraint
exerted by only one Ir–Si–O–C–N iridacycle in 2. However, not
only the C–O–Si bond angles (close to 120°), but also the irida-
cycle characteristics are comparable in bidentate and triden-
tate ligands, as the twisted 2T1 five-membered Ir–Si–O–C–N
iridacycle in 2 shows puckering amplitude and phase angle
values23 (q = 0.188(1) Å, φ = −158.4(1)°) similar to those
previously described for [Ir(H)(CF3CO2)(κ3-NSiN)(coe)]17b and
[Ir(H)(CF3(CF2)2CO2)(κ3-NSiN)(coe)]17a complexes. Moreover, it
should be kept in mind that Ir–Si bond length values in triden-
tate NSiN complexes are the shortest iridium-silicon bond so
far reported in the literature.24 Electronic effects due to the
different metal environment in 2 should not be neglected,
although they are not expected to also influence Si–O and Si–C
Scheme 1 Examples of synthesis of previously reported Ir-NSiN and
Rh-NSiN catalyst precursors.16
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bond lengths, which are found to be longer in 2. In contrast, transformation of 4a into 5a is faster than in the case of the 2-
the metal center geometry affects the Ir–Cl bond length catalyzed reactions. In agreement with that, the full conversion
(2.3950(6) Å, Cl–Ir–Si: 131.76(2)°), which is notably shorter of N-formylpyrrolidine into 5a using 3 was achieved in 1 h,
1
than the one found trans to the silicon atom in [Ir(H)Cl(κ3- while using 2 as a catalyst precursor 4 h were required. H and
NSiN)(coe)] (2.5229(10) Å, Cl–Ir–Si: 170.51(4)°).20
29Si{1H} NMR spectra of these reactions confirm that the start-
The reaction of complex 2 with one equivalent of the silver ing HSiMe2Ph is transformed into the corresponding siloxane
salt AgCF3SO3 in CH2Cl2 quantitatively affords the expected O(SiMe2Ph)2.
1
transmetalation product [Ir(H)(CF3SO3)(κ2-NSitBu)(coe)] (3),
Moreover, the H NMR spectra of the above-described cata-
which has been isolated as a yellow solid in 75% yield, and lytic reactions evidence the presence of a single resonance at
1
characterized by elemental analysis and NMR spectroscopy δ 1.50 ppm, which correlates in the H–13C HSQC spectra with
(Scheme 2).
a resonance that appears in the 13C{1H} NMR spectra at
1H NMR spectra of complexes 2 and 3 show the signal δ 27.0 ppm, due to cyclooctane resulting from the hydrogen-
assigned to the Ir–H proton as a singlet at δ −20.68 and ation of the coe ligand, and the formation of unidentified
−27.46 ppm, respectively. The coordination of the olefin to the iridium-hydride intermediate species, which agrees with the
iridium atom is corroborated by two multiplet resonances need for a catalyst activation step.
corresponding to the CH protons of the coe ligand, which
To explore the effect of the catalyst loading, we have per-
1
appear at δ 4.82 and 3.39 ppm (2), and δ 4.93 and 3.58 ppm formed experiments using 0.5 mol% of complexes 2 and 3. H
(3). In agreement with the solid structure determined for NMR studies of the 2-catalyzed (0.5 mol%) reaction of
complex 2 (Fig. 1), the two tBu groups of complexes 2 and 3 are N-formylpyrrolidine with two equivalents of HSiMe2Ph at
not equivalent in solution. Thus, two singlet resonances for 298 K show that, analogously to the above-described experi-
the tBu protons at δ 1.11 and 1.00 ppm and δ 1.20 and ments carried out in the presence of 5.0 mol% of 2, the fast
0.96 ppm were observed in the 1H NMR spectra of complexes 2 transformation of N-formylpyrrolidine into 4a takes place
and 3, respectively. These findings support that the structure during the first minutes of the reaction. Afterwards, the slow
of 3 could be similar to that found for 2 (Fig. 1). The remain- transformation of 4a into 5a is observed (Fig. 2). Under the
1
ing resonances of the H and 13C{1H} NMR spectra agree with same reaction conditions, 298 K, C6D6 and two equivalents of
the structure proposed for these complexes. The 29Si{1H} NMR HSiMe2Ph, complex 3 (0.5 mol%) promote the reduction of
spectra of 2 and 3 show a singlet resonance at δ 41.9 (2) and N-formylpyrrolidine to afford 5a (Fig. 3). In this case, similar
45.8 ppm (3), which is low field shifted with respect to the to that observed for the 3-catalyzed (5.0 mol%) reaction, 5a
resonance observed for 1. Moreover, the presence of CF3 in was the only reaction product observed along the reaction.
complex 3 has been confirmed by 19F{1H} NMR spectroscopy, Therefore, these findings allow us to conclude that using a
which shows a singlet resonance at δ −78.55 ppm.
catalyst loading of 0.5 mol% of complexes 2 and 3, it is poss-
ible to achieve a reasonable catalytic activity.
Ir-NSitBu catalyzed reduction of formamides with hydrosilanes.
Optimization of the catalytic conditions
Catalytic reaction of formamides with HSiMe2Ph in the pres-
ence of 2 (0.5 mol%)
1H NMR studies of the reaction of N-formylpyrrolidine with
two equivalents of HSiMe2Ph at 298 K in the presence of cata-
lytic amounts of 2 or 3 (5.0 mol%) evidenced that both cata-
lysts promote the selective reduction of N-formylpyrrolidine to
N-methylpyrrolidine. In the case of the 2-catalyzed (5.0 mol%)
reactions, the immediate formation of (CH2)4NCH2OSiMe2Ph
(4a), which was slowly transformed into (CH2)4NMe (5a), was
observed (Scheme 3). However, using 3 (5.0 mol%) as a catalyst
precursor, the reaction of N-formylpyrrolidine with two equiva-
lents of HSiMe2Ph did not evidence the formation of 4a along
the reaction. This could suggest that in the presence of 3, the
O-Silylated hemiaminals have been proposed as highly reactive
intermediates in the reduction of amides to amines with sila-
nes.3j,12c,25 Therefore, the above-described results found for 2
inspired us to explore its potential application as a catalyst for
Fig. 2 Kinetic profile of the 2-catalyzed (0.5 mol%) reaction of
N-formylpyrrolidine with two equivalents of HSiMe2Ph in C6D6 at 298 K.
The discontinuous lines only represent a connection between the data
points.
Scheme 3 Catalyzed reduction of N-formylpyrrolidine with HSiMe2Ph
in the presence of catalytic amounts (5.0 mol%) of complex 2. Using
complex 3, the formation of 4a could not be observed.
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observed. A different behavior was observed when complex 3
was used as a catalyst precursor. Thus, the 3-catalyzed reac-
tions of formamides with one equivalent of HSiMe2Ph afford
mixtures of the starting formamide and the corresponding
methylamine. These results agree with the above-described
higher activity of 3 in comparison with 2 as hydrosilylation
catalysts.
It should be mentioned that the reaction temperature influ-
ences the selectivity of the 2-catalyzed (0.5 mol%) reaction of
N-formylpyrrolidine with one equivalent of HSiMe2Ph. Indeed,
on heating at 323 K, under the above-described conditions, the
formation of 5a as a side reaction product together with 4a
could not be avoided. Thus, the best selectivities to the corres-
ponding O-silylated hemiaminal were obtained at 298 K.
In the case of the 2-catalyzed (0.5 mol%) reactions of C6D6
solutions of N-formylpyrrolidine, N,N-dimethylformamide or
N,N-diisopropylformamide with 2.1 equivalents of HSiMe2Ph,
the full conversion of the starting formamide into the corres-
ponding O-silylated hemiaminal takes place in a few minutes
(Fig. 2). However, the subsequent reduction of the hemiaminal
was found to be very slow. Indeed, after 14 h only an 80%, 15%
or 23% of the corresponding methylamine was observed.
Fig. 3 Kinetic profile of the 3-catalyzed (0.5 mol%) reaction of
N-formylpyrrolidine with two equivalents of HSiMe2Ph in C6D6 at 298 K.
The discontinuous lines only represent a connection between the data
points.
the synthesis of O-silylated hemiaminals from the reaction of
formamides with one equivalent of HSiMe2Ph (Table 1). 1H
NMR studies of the 2-catalyzed (0.5 mol%) reaction of C6D6
solutions of N-formylpyrrolidine, dimethylformamide, and
N-methyl-N-phenyl-formamide with one equivalent of
HSiMe2Ph at 298 K show the quantitative formation of the
corresponding O-silylated hemiaminal, (CH2)4NCH2OSiMe2Ph
(4a), Me2NCH2OSiMe2Ph (4b), and MePhNCH2OSiMe2Ph (4c),
which was selectively obtained (>99%) after 0.8 h, 6 h and
24 h, respectively. In the case of N,N-diphenylformamide, the
reaction was also found to be selective to the formation of the
corresponding O-silylated hemiaminal, however it is much
more slower than that carried out using the abovementioned
formamides. Indeed, after 24 h of the reaction, only 50% of
conversion was observed. Under the same reaction conditions,
the selective reduction of N,N-diisopropylformamide could not
be achieved. Indeed, after 16 hours of the reaction, 82% of
conversion of the formamide to give a mixture of the corres-
ponding O-silylated hemiaminal Me2PhSiOCH2NiPr2 (4e)
(80%) and N,N-diisopropylmethylamine (5e) (20%) was
3-Catalyzed (0.5 mol%) reduction of amides to the
corresponding amine
The 3-catalyzed (0.5 mol%) reaction of N-formylpyrrolidine
with one equivalent of HSiMe2Ph affords an equimolar
mixture of the corresponding methylamine and the starting
N-formylpyrrolidine. The above-described outcomes prompted
us to explore the potential of complex 3 as a catalyst precursor
for the reduction of formamides with HSiMe2Ph. Thus, the
reactions of N-formylpyrrolidine, N,N-dimethylformamide,
N-methyl-N-phenylformamide, N,N-diphenylformamide and
N,N-diisopropylformamide with 2.1 equivalents of HSiMe2Ph
in the presence of 3 (0.5 mol%) were comparatively studied by
1H NMR spectroscopy in C6D6 at 323 K (Table 2).
These studies allow us to conclude that by using a low
loading of 3 (0.5 mol%), it is possible to achieve the selective
reduction of N-formylpyrrolidine, N,N-dimethylformamide,
N-methyl-N-phenylformamide, N,N-diphenylformamide and
Table 1 Results from the reaction of formamides with HSiMe2Ph in
C6D6 at 298 K using 2 (0.5 mol%) as a catalyst precursor
Table 2 Results from the reaction of amides with HSiMe2Ph in C6D6 at
323 K using 3 (0.5 mol%) as a catalyst precursor
Time
(h)
Hemiaminala
(%)
TOF1/2
Formamide
(h−1
)
N-Formylpyrrolidine
0.8
6.0
24
24
16.0
>99
>99
98
750
240
15
4.2
600
Formamide
Time (h) Aminea (%) TOF1/2 (h−1
)
N,N-Dimethylformamide
N-Methyl-N-phenylformamide
N,N-Diphenylformamide
N,N-Diisopropylformamide
N-Formylpyrrolidine
N,N-Dimethylformamide
N-Methyl-N-phenylformamide 15
N,N-Diphenylformamide
N,N-Diisopropylformamide
0.25
8.5
>99
>99
90
4000
50
120
15
50
>80b
15
70
a % was calculated by 1H NMR integration. b 82% of conversion based
on the starting formamide to afford a mixture 4 : 1 of the corres-
ponding O-silylated hemiaminal and methylamine, respectively.
2.5
>99
343
a % was calculated by 1H NMR integration.
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N,N-diisopropylformamide to quantitatively afford the corres- (iii) reduction of the O-silylated hemiaminal to the corres-
ponding methylamine, which was characterized by compari- ponding methylamine.
son of their 1H, 13C{1H} NMR spectra with those reported in
the literature (Table 2).
The performance of the catalytic reactions depends very
much on the nature of the formamide. Indeed, a few minutes
are required to transform N-formylpyrrolidine into
N-methylpyrrolidine while hours were needed to complete the
Experimental
General considerations
reduction of the other formamides (Table 2). This behavior All experiments and manipulations were carried out under rig-
could be due to a complex balance of electronic and steric orous exclusion of air using Schlenk-type techniques and an
factors. For instance, it draws attention that the reduction of MBraun glovebox when necessary. Solvents were dried and dis-
N,N-dimethylformamide requires more time than that of N,N- tilled under argon by standard procedures prior to use or puri-
diisopropylformamide. This could not be attributable to steric fied by using
factors since the N-formyl group is more hindered in the latter Technologies). NMR spectra were obtained on a Bruker
than in N,N-dimethylformamide. ARX-300, Bruker AV-300 MHz or Varian Gemini 2000.
a Solvent Purification System (Innovative
The higher activity found for N,N-diisopropylformamide in Chemical shifts (δ), reported in ppm, were referenced to the
comparison with N,N-dimethylformamide and the poor reac- residual peaks of deuterated solvents. The amides and the
tivity observed for N,N-diphenylformamide in comparison with silicon-hydrides were purchased from commercial sources.
N-methyl-N-phenylformamide (Table 2) shows that the nucleo- The amides were dried over A4 molecular-sieves.
philic character of the oxygen atom of the formyl group, inde-
4-Methylpyridine-2-yloxyditertbutylsilane (1). A solution of
pendent of the steric hindrance around the nitrogen atom, HSitBu2Cl (1.50 mL, 7.42 mmol) in THF (10 mL) was slowly
exerts a positive effect on the catalytic activity of the catalytic added to a THF solution (10 mL) of 2-hydroxy-4-methyl-
systems based on 2 and 3. This suggests that the iridium– pyridine (0.80 g, 7.33 mmol) and imidazole (0.58 g,
NSitBu promoted reduction of formamides with HSiMe2Ph 8.52 mmol) at RT. After the addition, the resulting mixture was
could take place via no-classic mechanisms,26 similar to those stirred for 5 h. Then the solvent was removed in vacuo. The
proposed for other iridium based catalysts.12
product was extracted with pentane (3 × 5 mL) and dried
1H NMR studies show that in the absence of formamide, under reduced pressure to afford a pale-yellow oil. Yield: 1.69 g
complexes 2 and 3 react with an excess of HSiMe2Ph (4 equiva- (92%). High resolution mass spectrometry (ESI+): calc. m/z =
lents) to afford mixtures of unidentified hydride species, 252.1778; found m/z = 252.1782 (M+). 1H NMR (300 MHz,
cyclooctane and traces of free cyclooctene. Under the same CD2Cl2, 298 K): δ 7.95 (m, 1H, C6–H), 6.72 (m, 1H, C5–H), 6.57
conditions, the reaction of complex 2 with HSiMe2Ph is slower (m, 1H, C3–H), 4.50 (s, 1H, Si–H), 2.28 (s, 3H, py–CH3), 1.06 (s,
than with 3. These results agree with the outcomes from the 18H, Si–tBu). 13C{1H} NMR (75 MHz, CD2Cl2, 298 K): δ 163.3 (s,
catalytic studies and show that the ancillary ligand shows a C2), 151.0 (s, C4), 147.3 (s, C6), 119.0 (s, C5), 113.2 (s, C3), 27.5
relevant role in the catalyst activation step, which in the case (s, CH3–tBu), 21.0 (s, py–CH3), 20.2 (s, Si–C). 29Si{1H} NMR
of the triflate derivative 3 is faster than for 2.
(HMBC 29Si-1H, 298 K): δ 12.0.
It has been observed that using 2, the reduction of forma-
Synthesis of [Ir(Cl)(H)(κ2-NSitBu)(coe)] (2). A solution of com-
mides to the corresponding O-silylated hemiaminal is faster pound 1 (0.500 g, 1.988 mmol) in CH2Cl2 (5 mL) was slowly
than the subsequent reduction of the O-silylated hemiaminals added to a suspension of [IrCl(coe)2]2 (0.890 g, 0.993 mmol)
to the corresponding methylamines. Conversely, when 3 was in CH2Cl2 (10 mL) at 273 K. The mixture was then stirred at
used as a catalyst, the corresponding O-silyl hemiaminal was room temperature for 16 h. The solvent was removed in vacuo
never observed, which shows that in this case the first and the residue was precipitated with pentane (3 × 5 mL) at
reduction is slower than the second one. This different behav- 195 K. The washing solutions were combined and dried
iour could be the reason behind the high selectivity to O-silyl in vacuo to afford a bright-yellow solid. Yield: 0.860 g (73.4%).
hemiaminals found for 2 (Scheme 4).
Therefore, the overall catalytic process could be understood Found: C 44.85; H 6.70; N 2.62. 1H NMR (300 MHz, CD2Cl2,
three-step process involving (i) catalyst activation, 298 K): δ 8.43 (m, 1H, C6–H), 6.79 (s, 1H), 6.75 (m, 1H, C3–H),
Anal. calcd for C22H39ClIrNOSi: C, 44.84; H 6.67; N 2.38.
as
a
(ii) reduction of formamide to O-silylated hemiaminal and 4.82 (m, 1H, CH-coe), 3.39 (m, 1H, CH-coe), 2.47 (m, 2H,
CH2-coe), 2.32 (m, 3H, py–CH3), 2.19–1.97 (m, 2H, CH2-coe),
1.89–1.20 (m, 8H, CH2-coe), 1.11 (s, 9H, Si-tBu), 1.00 (s, 9H,
Si-tBu), −20.68 (s, 1H, Ir–H). 13C{1H} NMR (75 MHz, CD2Cl2,
298 K): δ 154.3 (s, C4), 147.6 (s, C6), 117.5 (s, C5), 111.0 (s, C3)
71.2 (s, CH-coe), 62.0 (s, CH-coe), 31.2 (s, CH2-coe), 30.5 (s,
CH2-coe), 29.4 (s, CH3-tBu), 28.4 (s, CH3-tBu), 27.1 (s, CH2-
coe), 26.3 (s, CH2-coe), 26.1 (s, CH2-coe), 24.8 (s, Si–C), 24.1
(s, Si–C), 21.0 (s, py–CH3). 29Si{1H} NMR (HMBC 29Si-1H,
Scheme 4 Representation of the rate controlling step of the 2- and 3-
catalyzed reduction of formamides with HSiMe2Ph.
298 K): δ 41.9.
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Synthesis of [Ir(CF3SO3)(H)(κ2-NSitBu)(coe)] (3). CH2Cl2 128.2, 118.8, 114.7 (s, pH), 77.8 (s, CH2), 37.6 (s, N–CH3), −1.3
(10 mL) was added to a light-protected Schlenk flask contain- (s, Si–CH3).29Si{1H} NMR (HMBC 29Si-1H, 298 K): δ 5.3.
1
ing 2 (0.300 g, 0.510 mmol) and silver triflate (0.150 g,
Ph2NCH2OSiMe2Ph (4d): H NMR (300 MHz, C6D6, 298 K):
0.583 mmol). The resulting mixture was stirred for 5 h at room δ 5.16 (s, 2H, CH2), 0.17 (s, 6H, Si–CH3). 13C{1H} NMR
temperature. The solution was filtered through Celite and (75 MHz, C6D6, 298 K): δ 77.1 (s, CH2), −1.4 (s, Si–CH3).
dried in vacuo; the residue was then washed with pentane (3 × 29Si{1H} NMR (HMBC 29Si-1H, 298 K): δ 6.2.
1
5 mL) to afford a yellow solid. Yield: 270 mg (75%). Anal. calcd
iPr2NCH2OSiMe2Ph (4e): H NMR (300 MHz, C6D6, 298 K):
i
for C23H39F3IrNO4SSi: C, 39.3; H, 5.59; N, 1.99; S, 4.56. Found: δ 4.61 (s, 2H, CH2), 3.11 (sp, J = 6.6 Hz, 2H, Pr), 1.11 (d, J =
C, 39.01; H, 5.76; N, 1.88; S, 4.49. H NMR (300 MHz, CD2Cl2, 6.6 Hz, 12 H, Pr), 0.34 (s, 6H, Si–CH3). 13C{1H} NMR (75 MHz,
1
i
i
298 K): δ 8.17 (m, 1H, C6–H), 6.87(s, 1H, C3–H), 6.82 (m, 1H, C6D6, 298 K): δ 73.3 (s, CH2), 48.8 (s, N–CH), 22.9 (s, Pr), −1.2
C3–H), 4.93 (m, 1H, CH-coe), 3.58 (m, 1H, CH-coe), 2.37 (s, 3H, (s, Si–CH3). 29Si{1H} NMR (HMBC 29Si-1H, 298 K): δ 1.7.
py–CH3), 2.30–1.25 (m, 12H, CH2-coe), 1.20 (s, 9H, Si-tBu), 0.96
(CH2)4NCH3 (5a): H NMR (300 MHz, C6D6, 298 K): δ 2.34
1
(s, 9H, Si-tBu), −27.46 (s, 1H, Ir–H). 13C{1H} NMR (75 MHz, (m, 4H, N–CH2), 2.25 (s, 3H, CH3), 1.63 (m, 4H, CH2). 13C{1H}
CD2Cl2, 298 K): δ 156.4 (s, C4), 147.9 (s, C6), 118.5 (s, C5), 112.1 NMR (75 MHz, C6D6, 298 K): δ 56.5 (s, N–CH2), 42.3 (s, N–
(s, C3), 76.3(s, CH-coe), 68.5 (s, CH-coe), 31.6 (s, CH2-coe), 30.4 CH3), 24.6 (s, CH2).
(s, CH2-coe), 29.8 (s, CH3-tBu + CH2-coe), 28.4 (s, CH3-tBu),
N(CH3)3 (5b): 1H NMR (300 MHz, C6D6, 298 K): δ 2.07 (s,
26.8 (s, CH2-coe), 26.5 (s, CH2-coe), 26.2 (s, CH2-coe), 24.8 (s, 9H, CH3). 13C{1H} NMR (75 MHz, C6D6, 298 K): δ 47.7 (s,
Si–C), 21.88 (s, py-CH3). 29Si{1H} NMR (HMBC 29Si-1H, 298 K): N–CH3).
δ 45.8. 19F{1H} NMR (282 MHz, CD2Cl2): δ −78.55.
PhN(CH3)2 (5c): 1H NMR (300 MHz, C6D6, 298 K):
δ 7.27–7.19 (m, 2H, Ph), 6.77 (t, J = 7.3 Hz, 1H, Ph), 6.64 (d,
Catalytic reaction of N-formylpyrrolidine with HSiMe2Ph in the 2H, Ph), 2.55 (s, 6H, N–CH3). 13C{1H} NMR (75 MHz, C6D6,
presence of catalytic amounts of species 2, and 3 (5 mol%)
298 K): δ 129.4, 117.1, 113.1 (s, Ph), 40.3 (s, N–CH3).
1
Ph2NCH3 (5d) H NMR (300 MHz, C6D6, 298 K): δ 2.93 (s,
In a glovebox, the catalyst precursor 2 was added to an NMR
tube (5.0 mg, 8.48 μmol) and dissolved in C6D6 (0.5 mL).
N-Formylpyrrolidine (16.2 μL, 16.9 mmol) and HSiMe2Ph
(54.5 μL, 35.5 mmol) were added to this solution and the
resulting mixture was studied by NMR spectroscopy. The same
process was followed with complex 3 (5.0 mg, 7.11 μmol).
N–CH3). 13C{1H} NMR (75 MHz, C6D6, 298 K): δ 40.1 (s,
N–CH3).
1
iPr2NCH3 (5e): H NMR (300 MHz, C6D6, 298 K): δ 2.84 (sp,
J = 6.5 Hz, 2H, N–CH), 2.09 (s, 3H, N–CH3), 0.97 (d, J = 6.5 Hz,
12H, CH3). 13C{1H} NMR (75 MHz, C6D6, 298 K): δ 50.7 (s,
N–CH), 30.9 (s, N–CH3), 20.2 (s, CH3).
Reaction of different amides with HSiMe2Ph in the presence of
complexes 2 or 3 (0.5 mol%)
Crystal structure determination of complex 2
Single crystal X-ray diffraction data were collected at 100(2) K
with graphite-monochromated Mo Kα radiation (λ = 0.71072 Å)
using narrow frame rotation (Δω = 0.3°) on a Bruker Smart
APEX diffractometer. Measured intensities were integrated and
corrected for absorption effects with SAINT+27 and SABABS28
programs, included in the APEX3 package. The structure was
solved by direct methods with SHELXS-201329 and refined
by full-matrix least-squares refinement on F2 with the
SHELXL-201730 program, included in the WingX package.31
Several refinement strategies have been tested for hydride
The desired amount of amide (0.254 mmol) and the corres-
ponding equivalents of HSiMePh2 (1 eq., 0.254 mmol; 2 eq.,
0.534 mmol) were added to an NMR tube. To this, a C6D6 solu-
tion (0.5 mL) of complex 2 (0.75 mg, 1.27 μmol) was added
and the resulting mixture was studied by NMR spectroscopy.
The same process was followed with complex 3 (0.75 mg,
1.07 μmol).
Selected NMR data of the products
1
(CH2)4NCH2OSiMe2Ph (4a): H NMR (300 MHz, C6D6, 298 K): ligand refinement. Its free refinement from the observed posi-
δ 7.61, 7.24 (m, 5H, Si–Ph), 4.52 (s, 2H, CH2–O), 2.75 (m, 4H, tion leads to unrealistic geometrical parameters (Si–Ir–H: 64.1
N–CH2), 1.60 (m, 4H, N–CH2), 0.35 (s, 6H, SiMe2). 13C{1H} degree angle). Moreover, no evidence of agostic interactions
NMR (75 MHz, C6D6, 298 K): δ 77.5 (s, N–CH2O), 48.8 (s, has been observed in the NMR spectra. Therefore, the hydride
N–CH2), 24.9 (s, CH2) −1.3 (s, SiMe). 29Si{1H} NMR (HMBC ligand position has been calculated from the consideration of
29Si-1H, 298 K): δ 4.7.
the molecular geometry and a potential energy minimization
Me2NCH2OSiMe2Ph (4b): H NMR (300 MHz, C6D6, 298 K): calculation with the HYDEX program.32 The calculated posi-
δ 7.59. 7.24 (m, 5H, Si–Ph), 4.31 (s, CH2, 2H), 2.29 (s, N–CH3, tion has been fixed and the isotropic atomic displacement
3H), 0.33 (s, Si–CH3, 6H). 13C{1H} NMR (75 MHz, C6D6, 298 K): parameter has been constrained to be 1.5 times the Ueq of the
δ 82.6 (s, CH2), 41.2 (s, N–CH3), −1.36 (s, Si–CH3). 29Si{1H} metal atom. CCDC 1857508† contains the supplementary crys-
1
NMR (HMBC 29Si-1H, 298 K): δ 4.5.
tallographic data for this paper.
PhMeNCH2OSiMe2Ph (4c): 1H NMR (300 MHz, C6D6,
298 K): δ 7.52 (m, Ph), 7.19 (m, Ph), 6.83 (m, Ph), 4.78 (s, 2H,
Crystal data compound 2
CH2), 2.62 (s, 3H, N–CH3), 0.26 (s, 6H, SiMe2). 13C{1H} NMR C22H39ClIrNOSi, M = 589.28; yellow prism 0.080 × 0.090 ×
3
ˉ
(75 MHz, C6D6, 298 K): δ 148.8, 138.2, 133.9, 129.9, 129.3, 0.240 mm ; triclinic P1, a = 8.8806(4), b = 10.5584(4), c =
Dalton Trans.
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12.7712(5) Å, α = 94.4790(10), β = 91.3980(10), γ = 94.6700(10)°, Fund and Ministerio de Economía y Competitividad of Spain
V = 1189.30(8) Å3; Z = 2; Dc = 1.646 g cm−3; μ = 5.788 mm−1
for a PTA contract. The authors would like to acknowledge the
17 208/5722 reflections measured/unique (Rint 0.0172), use of Servicio General de Apoyo a la Investigación-SAI,
number of data/restraint/parameters 5722/0/258, R1(F2)
;
=
=
Universidad de Zaragoza.
0.0172 (5534 reflections, I > 2σ(I)) and wR(F2) = 0.0404 (all
data), final GoF = 1.101, largest difference peak: 1.156 e Å−3
.
References
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Acknowledgements
The financial support from MINECO/FEDER project CTQ2015-
67366-P and DGA/FSE group E07 is gratefully acknowledged.
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