Silicon-Mediated Coupling of Carbon Monoxide, Ammonia, and Primary Amines to Form Acetamides

Article abstract of DOI:10.1002/anie.201904361

For the first time, a direct transformation of CO, NH₃, and primary amines into acetamides, mediated by a main-group element (silicon), is reported. Starting point is the selective deoxygenative reductive homocoupling of two CO molecules by the Fc-bis(silylene) 1 a (Fc=ferrocendiyl) as a reducing agent, which forms the ferrocendiyl-bridged disila(μ-O)(μ-CCO)ketene intermediate 2 a. Exposing 2 a to NH₃ (1 bar, 298 K) and benzylamine yields the Fc-disiloxanediamines [Fc(RHNSi-O-SiNHR)] 5 a (R=H) and 5 b (R=benzyl) under release of the respective acetamides H₃CC(O)NHR, as confirmed by ¹³C-isotope-labelling experiments. IR and NMR studies of the reaction reveal a four-step mechanism involving an N-silylated carboxamide that can be isolated and fully characterized. The striking reaction mechanism for this unprecedented transformation involves a facile Si?C bond cleavage and ammonolysis of a Si?O bond, and has been demonstrated experimentally and by quantum-chemical calculations.

Full text of DOI:10.1002/anie.201904361

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Accepted Article
Title: Silicon-Mediated Coupling of Carbon Monoxide, Ammonia and
Primary Amines to Form Acetamides
Authors: Matthias Driess, Marcel-Philip Luecke, Yuwen Wang, Arseni
Kostenko, and Shenglai Yao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201904361
Angew. Chem. 10.1002/ange.201904361
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10.1002/anie.201904361
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Silicon-Mediated Coupling of Carbon Monoxide, Ammonia and
Primary Amines to Form Acetamides
Marcel-Philip Luecke, Arseni Kostenko, Yuwen Wang, Shenglai Yao and Matthias Driess*
Dedicated to Professor Hansjörg Grützmacher
Abstract: For the first time, a direct transformation of CO, NH3 and
primary amines into acetamides, mediated by a main-group element
(silicon), is reported. Starting point is the selective deoxygenative
reductive homocoupling of two CO molecules by the Fc-bis(silylene)
1a (Fc = ferrocendiyl) as reducing agent to form the corresponding
ferrocendiyl-bridged disila(-O)(-CCO)ketene intermediate 2a.
Exposing 2a to NH3 (1 bar, 298 K) and benzylamine yield the
corresponding Fc-disiloxanediamines [Fc(RHSi-O-SiNHR)] 5a,
(R = H) and 5b (R = benzyl) under liberation of the acetamides
H3CC(O)NHR, respectively, as confirmed by 13C-isotopic labelling
experiments. IR and NMR studies of the reaction progress revealed
molecules with the strongest bonds in chemistry, could be
achieved in a hafnocene-mediated coupling reaction to yield
oxamide, H2N-CO-CO-NH2.[5] However, to the best of our
knowledge, a main-group element-mediated direct coupling of
CO and NH3 or primary amines affording an organic molecule is
currently unknown. Recently, our group reported the silicon-
mediated deoxygenative reductive homocoupling of two CO
molecules with the bis(silylene) 1a at room temperature to give
the respective disila(-O)(-CCO)ketene 2a (Scheme 1).[6]
a
three-step reaction mechanism including an N-silylated
carboxamide that could be isolated and fully characterized. The
striking reaction mechanism for this unprecedented transformation,
involving a facile Si-C bond cleavage and ammonolysis of a Si-O
bond, has been uncovered experimentally and by quantum chemical
calculations.
The activation and coupling of small molecules offers very
attractive pathways to a variety of functional organic compounds.
The ultimate aim of this strategy is to synthesize more organic
commodities (hydrocarbons, alcohols, ketones, carbonic acids
etc.) and fine chemicals with far less or even without
consumption of fossil resources. With respect to heteroatom-
containing organic molecules, the bottom-up synthesis of basic
C-N compounds such as amines, amides and cyanides from
COx and ammonia as simple building blocks is of particular
interest. The importance of CO serving as a versatile C1-building
block and NH3 as a source of N atoms for C-N bond formation
cannot be overestimated. Among the most important reactions
in (bio)organic chemistry is the sequential CO-NH (amide) bond
formation because of its widespread occurrence in nature and
role for pharmaceuticals, agrochemicals and dyes.[1,2] The
conventional approach to amides includes the reaction of
carboxylic acids and halides (RCOX, X = OH, halogen) with NH-
amines. A transition-metal(TM)-mediated formation of amides
can be achieved as shown in the case of the palladium-
catalyzed aminocarbonylation of aryl halides with NH3 in the
presence of CO.[3] Carbonyl TM complexes and ammonium salts
as non-gaseous sources of CO and NH3, respectively, have also
been applied successfully in aminocarbonylations.[4] The
energetically more demanding reductive cleavage of N2 as a
source of nitrogen and homologation of CO, two simple diatomic
Scheme 1. Formation of acylamines 6a, b via disilaketene 2a. The latter
results from deoxygenative reductive homocoupling of two CO molecules
with the corresponding bis(silylene) 1a.
With the disilaketene 2a in hand, we now learned that its ketene
moiety possesses an unprecedented reactivity towards
ammonia and primary amines affording acetamides. This type of
[7,8]
reactivity is currently unknown for diorganoketenes
and
disilaketenes. [9] Herein, we report on the unexpected formation
and striking mechanism of the acetamides 6a, b through
ammonolysis of the disilaketenes 2a.
Treatment of 2a with NH3 under ambient conditions (1 bar,
298 K) furnished acetamide 6a along with the corresponding Fc-
disiloxanediamine 5a, which could be crystallized from THF at
– 30 °C. Similarly, the acetamide 6b and disilyldiamine 5b were
obtained upon reaction of 2a with N-benzylamine in toluene at
room temperature (Scheme 2); 5b could be isolated as yellow
crystals in 64 % yields. Disiloxanediamine 5a crystallizes in the
monoclinic space group P21/c with two THF molecules in the unit
cell, while 5b in the triclinic space group P1. Both compounds
feature a central structure motif of an disiloxanediamine
[O(SiNHR)2] with typical Si-N bond distances (5a: 1.712(2) Å,
5b: 1.703(2) Å) for silylamines (1.70-1.76 Å).[10b]
MSc. M.-P. Luecke, Dr. A. Kostenko, MSc. Y. Wang, Dr. S. Yao, Prof.
Dr. M. Driess
Department of Chemistry: Metalorganics and Inorganic Materials
Technische Universität Berlin
Strasse des 17. Juni 115, Sekr. C2, D-10623 Berlin Germany;
E-mail: matthias.driess@tu-berlin.de
Supporting information for this article is given via a link at the end of the
document.
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10.1002/anie.201904361
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mixture revealed a clean reaction progress to the corresponding
acylation products 6a,b and 5a,b (see Supporting Information).
Monitoring the reaction progress of 13C-labeled disilaketene
2a-13C with NH3 in C6D6 suggested the formation of cyclic N-
silylated carboxamide 3a-H (Scheme 3) as exemplified by the
observation of two doublets at  = 175.8 (13C=O) and 31.5 ppm
(Si13CH2) in the 13C NMR spectrum with a characteristic value of
the 13C-13C coupling constant of 47.0 Hz.[12] The 29Si,1H-HMQC
NMR spectrum shows two cross peaks at  = - 79.5 ppm (1H:  =
4.50 ppm) and - 24.5 ppm (1H:  = 2.98 ppm), suggesting the
formation of the cyclic N-silylated carboxamide 3a-H as the first
observable intermediate. 3a-H is not isolable and decomposes
in solution within a few minutes in the absence of NH3. To gain
further mechanistic insights, a bulky tert-butyl group was
Scheme 2. Reaction of 2a with NH3 and benzylamine to
disiloxanediamines 5a, 5b and acetamides 6a, 6b, respectively.
The 29Si NMR chemical shift of  = - 57.7 (5a) and - 45.6 ppm
(5b) in C6D6 are in accordance with the different coordination
modes of the ligand L (L = [PhC(NtBu)2], Scheme 2) at the Si1
atom in 5a (chelating) and 5b (monodentate), in line with the
molecular structures in the solid state (Fig. 1).
t
introduced by treatment of disilaketene 2a with BuNH2, which
could increase the stability of the proposed intermediate through
an equilibrium of a cyclic O-silylated carboximidate and its
tautomer (N-silylated carboxamide, Scheme 4).[13] Reaction of
2a with tBuNH2 in deuterated benzene at room temperature led
to the selective formation of O-silylated carboxamide 3a`-tBu in
accordance with the better shielding of the carbon nucleus of the
CO moiety at = 154.2 ppm compared to that of 3a-H
( = 175.8 ppm) in the 13C NMR spectrum, and a slight red shift
of the C=N absorption band at v = 1656 cm-1 in the IR
spectrum.[13] The protons of the methylene moiety gave rise to a
set of two doublets (1H:  = 2.95 and 3.42 ppm, 2JH,H = 14.0 Hz)
as evidenced by the H,C-HSQC NMR spectrum (see Supporting
Information). The proposed equilibrium shown in Scheme 3 is
supported by Density Functional Theory (DFT) calculations
revealing a rather small free energy difference of ∆G = 6.5 kcal
mol-1 between the favored O-silylated carboxamide 3a`-tBu
[14]
tautomer and 3a-tBu at room temperature.
However, the
chemical shift of the 13C NMR resonance of the CO moiety in
3a`-tBu remains unchanged over a wide temperature range from
- 80 °C to + 40 °C in d8-THF. Reaction of the 13CO-labeled
t
disilaketene 2a-13C with BuNH2 furnishes the expected 13C
labeled complex 3a`-tBu-13C in solutions, containing
a
[13C-13C(=NtBu)-O)] moiety in accordance with multinuclear
NMR spectroscopic data and a bathochromic shifted C=N
absorption band (ν = 1621 cm-1, ∆ν = 35 cm-1) in the FT-IR
spectrum. Further evidence is given by the 29Si NMR spectrum
Figure 1. Molecular structures of 5a (top) and 5b (bottom) with thermal
ellipsoids at 50 % probability level. Hydrogen atoms, except of those at
N1 and N2, and solvent molecules are omitted for clarity. Selected
distances [Å] and angles [°]: Compound 5a Si1-O1 1.6214(16), Si1-N1
1.712(2); Si1-O1-Si2 135.53(10), O1-Si1-N1 109.49(10), O1-Si2-N2
94.80(10), ∑Si1 = 323.45°. Compound 5b Si1-O1 1.6388(17), Si1-N1
1.703(2), Si2-N2 1.713(2); Si1-O1-Si2 142.08(12), O1-Si1-N1
102.56(10), O1-Si2-N2 106.14(10), ∑Si1 = 314.4°.
of 3a`-tBu-13C with a singlet at  = - 86.8 ppm and a doublet at
13
 = - 29.6 ppm (1J29 C = 66 Hz), confirming the presence of a
Si,
Si-O and Si-13C bond, respectively. 3a`-tBu-13C is stable in
solutions over a longer period of time and does not further react
with tBuNH2 upon prolonged heating (60 °C for 12 h). However,
exposure of 3a`-tBu to NH3 (1 bar, 298 K) in C6D6 led to the
formation of 5a and N-tert-butylacetamide 6c as confirmed by
ESI mass spectrometry, NMR- and FT-IR spectroscopy
(Supporting Information).[11b] Monitoring the reaction progress of
3a-tBu with NH3 (1 bar, 298 K) in C6D6 indicated the formation of
Silylamines are well known and generally prepared by
ammonolysis or aminolysis of the Si-Cl bond in chlorosilanes.[10]
The formation of the known acetamides 6a and 6b was
confirmed by ESI mass spectrometry, NMR- and IR-
spectroscopy with characteristically strong carbonyl stretching
vibration modes.[11a,b] Isotopic labeling experiments using 13C
labeled 2a-13C confirmed the assignment. The FT-IR absorption
spectra obtained from the reaction mixtures (2a + amine) after
24 h exhibit a strong absorption at 1644 and 1646 cm-1 assigned
to the C=O stretching vibration of 6a and 6b, respectively. The
13C NMR signals of the corresponding carbon nuclei were
observed at  = 172 ppm (6a) and  = 168 ppm (6b).[11a,b] In-situ
heteronuclear NMR spectroscopic analysis of the reaction
the
proposed
silylamine
[SiNH2]
(15N: = 30.9 ppm)
intermediate 4a-tBu featuring a new singlet signal in the 1H-NMR
spectrum at  = 2.46 ppm (Scheme 3). The second cross peak
in the 1H,15N-HMQC NMR spectrum at lower field
(15N: = 133.4 ppm) corresponds to the CO-NHtBu amide
proton, which exhibits a singlet at  = 7.43 ppm in the 1H NMR
spectrum (see Supporting Information).
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Scheme 3. Proposed mechanism for the amination of 2a with tBuNH2 and NH3.
2
The methylene protons (1H:  = 2.84 ppm, JH,H = 11.8 Hz) still
remained in the 1H NMR spectrum and gave rise to a cross peak
in the 1H,29Si-HMQC NMR spectrum at  = – 25.6 ppm [SiC2H2].
The second cross peak at  = - 78.6 ppm (29Si), coupling to the
protons at  = 2.46 ppm, attributed to the O-Si-NH2 silylamine
moiety. Further evidence stems from the low-field shift for the
carbonyl 13C nucleus in the 13C-NMR spectrum at = 172.5 ppm
(3a`-tBu, C=N: 154.2 ppm) and a red shifted strong IR
absorption band at ν = 1644 cm-1 (3a`-tBu: ν = 1656 cm-1, C=N)
assigned to the C=O stretching vibration mode of the alkylamide
4a-tBu. Additional absorption bands at ν = 3331 and 3345 cm-1
stem from the N-H bonds. Based on the higher stability of the Si-
O vs. Si-N bond we propose that the N-silylated carboxamide
3a-tBu (Scheme 3) is more reactive towards ammonolysis of the
Si-N bond to give 5a. This is supported by the large σ*(Si-N)
bond character of the [LUMO+3] frontier orbitals of 3a-tBu
(Supporting Information).
Scheme 4. Conversion of disilaketene 2b with NH3 yielding the cyclic
N-silylated carboxamide 3b.
Notably,
a derivative of the proposed cyclic N-silylated
carboxamide 3a-H, compound 3b, could be isolated in 72 %
yields as colorless crystals through reaction of the less flexible
xanthene-linked disilaketene 2b with NH3 (Scheme 4,
Supporting Information). 3b crystallizes in the triclinic space
group P1 with a non-planar six membered Si2C2NO δ-lactam
core as central structural motif (Fig. 2). The amination of
disilaketene 2b takes place at the carbonyl C1atom via
nucleophilic attack of NH3 followed by a 1,3-H migration to the
where a strained four-membered Si2CO-ring is formed (Scheme
4). Upon formation of the Si-N bond a second 1,3-H migration
occurs, leading to the final product 3b with a six-membered
δ-lactam structure (Fig. 3). C2 atom upon 2b` A possible two-
step reaction involving an enol of an amide as intermediate was
not observed, indicating a concerted mechanism involving
addition across the C=C bond of the ketene.[15]
Fig. 3. Molecular structure of 3b at 50 % probability level. Hydrogen
and solvent atoms are omitted for clarity. Bond lengths [Å]: Si1-C2
1.797(1), C1-C2 1.450(9), Si2-N5 1.809(3), C1-N5 1.414(9). Bond angle
[°]: Si1-C2-C1 123.01(3), C2-C1-O3 119.67(1), C2-C1-N5 122.30(5).
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Fig. 4. DFT-Derived potential energy surface (PES) of the proposed mechanism. L = chelating PhC(NtBu)2 ligand in all intermediates.
Acknowledgements
The calculated potential energy surface for the proposed
This
work
was
supported
by
the
Deutsche
mechanism of transformation of carboxamide
3
to
Forschungsgemeinschaft [Germany´s Excellence Strategy –
EXC 2008/1– 390540038 (UniSysCat) and with a PhD fellowship
by the Einstein Foundation Berlin (M.-P.L.). Y.W. gratefully
acknowledges financial support by the China Scholarship
Council. A.K. is grateful to the Alexander von Humboldt
Foundation for a postdoctoral fellowship. We thank Paula
Nixdorf (TU Berlin) for her assistance in the XRD measurements
and Dr. Terrance J. Hadlington (TU Berlin) for helpful
discussions.
disiloxanediamine 5 is presented in Figure 4.[14] Coordination of
ammonia to A results in formation of intermediate B at ∆G =
8.7 kcal mol-1. B undergoes an ammonolysis of the Si-N bond
via the four-membered TS(B-C) (∆G = 26.4 kcal mol-1) forming
the α-silylated acetamide (C) at ∆G = 26.4 kcal mol-1, that
further rearranges to the silyl enolether D (at ∆G = 2.3 kcal mol-
1) via TS(C-D) at ∆G = 23.5 kcal mol-1. This type of
rearrangement was previously reported for a α-boryl(silyl)
substituted N,N-dimethylacetamide leading to the isolation of
silylethers.[16] Coordination of NH3 to D forms intermediate E (at
∆G = 5.3 kcal mol-1) allowing the transfer of an H atom from NH3
to the vinyl moiety via the six-membered TS(E-F) at ∆G = 18.5
kcal mol-1 yielding the final disilyldiamine F and acetamide. The
barrier for the xanthene-linked carboxamide for Si-N bond
ammonolysis (3b + NH3) is 4 kcal mol-1 higher in energy
explaining the spacer depending reactivity thereof. Additional
mechanisms for this transformation were also considered but
could be excluded energetically (see Supporting Information).
Keywords: disilaketene • carbonylamination • ammonolysis •
carbonylation • silylenes
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In summary, the direct coupling of CO, NH3 and primary amines
into acetamides could be accomplished in a silicon-mediated
reaction sequence. Exposure of disilaketene 2a to NH3 (1 bar,
298 K)
and
benzylamine
yields
the corresponding
disiloxanediamines 5a and 5b under liberation of the acetamides
H3CC(O)NHR (R = H, benzyl), respectively, as confirmed by
13C-isotopic labelling experiments. Further studies revealed a
three-step reaction mechanism including an N-silylated
carboxamide 3b which could be isolated and fully characterized.
As a primary example, the strikingly facile Si-C bond cleavage is
accomplished by an intramolecular rearrangement to give the
silylated enolether D, resulting in an unprecedented Si-O bond
ammonolysis under ambient conditions.
[4]
[5]
[6]
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[8]
[9]
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[14] The geometries were optimized at B3LYP-D3 level with s6-31g* basis
for iron and def2-SVP basis set for all other atoms. The tBu and Ph
groups of the amidinato substituents were replaced by methyl groups.
For details see Supporting Information.
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Entry for the Table of Contents
M.-P. Luecke, A. Kostenko, Y. Wang, S.
Yao, M. Driess*
Page No. – Page No.
Silicon-Mediated Coupling of Carbon
Monoxide, Ammonia and Primary
Amines to Form Acetamides
What silicon can do: In the presence of a bis(silylene), CO, NH3 and primary amines
can be transformed into corresponding acetamides. Starting point is a silicon-based
reaction sequence via the ferrocendiyl-bridged disila(μ-O)(μ-CCO)ketene
intermediate, derived from a deoxygenative reductive homocoupling of two CO
molecules. Spectroscopic results and theoretical calculations revealed a three-step
reaction mechanism, including an N-silylated carboxamide that could even be
isolated and characterized.
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