Formation of Aromatic O-Silylcarbamates from Aminosilanes and Their Subsequent Thermal Decomposition with Formation of Isocyanates

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doi.org/10.1002/ejic.202100118

Formation of Aromatic O-Silylcarbamates from

Aminosilanes and Their Subsequent Thermal

Decomposition with Formation of Isocyanates

[a]

Franziska Gründler, Henrik Scholz, Marcus Herbig, Sandra Schwarzer, Jörg Wagler,

[a]

and Edwin Kroke*

A novel phosgene-free route to different isocyanates starts from

CO₂and aminosilanes (cf. silylamines) to form so-called

carbamoyloxysilanes (O-silylcarbamates), i.e., compounds with

to

partial

mono-insertion

(e. g.,

into

o-

C H (NHSiMe )(CH NHSiMe )) or intra-molecular condensation of

6

4

3

2

3

the intermediate insertion product in case of 1,8-

C H (NHSiMe ) to form 1H-perimidin-2(3H)-one and other side

1

2

3 4 5

the general motif R R NÀ COÀ OÀ SiR R R as potential precursors.

1

0

6

3 2

We focused on the insertion reaction of CO into SiÀ N bonds of

products. Thermal treatment of mono-silylated O-silylcarba-

2

substrates with cyclic (mostly aromatic) amine substituents, i.e.,

mates RHNÀ COÀ OÀ SiR’ resulted mainly in the formation of

3

PhNHSiMe₃,

(PhNH) SiMe ,

PhCH NHSiMe ,

p-(MeO)

substituted ureas (RHN) CO, whereas desired isocyanates could

2

3

2

C H NHSiMe ,

o-C H (NHSiMe ) ,

1,2-C H (NHSiMe ) , o-

not be detected in these cases. Therefore, we continued our

studies focussing on N,O-bissilylated precursors, which were

obtained by an additional N-silylation of the O-silylated

carbamates. This allowed the successful formation of isocya-

nates. As a sole byproduct hexamethyldisiloxane is formed. In

all cases, known as well as yet unknown substances were

6

4

3

6

4

3 2

6

10

3 2

C H (NHSiMe )(CH NHSiMe ) and 1,8-C H (NHSiMe ) . Com-

6

4

3

2

3

10

6

3 2

pared to previously investigated aminosilanes these reactions

are hindered due to the reduced nucleophilicity/basicity of the

N-atoms. Whereas slightly increased CO pressure (8 bar) and

prolonged reaction times (24 h) were sufficient to overcome

hindrance of the insertion into, e. g., PhNHSiMe , intermolecular

2

1

13

29

characterised by H, C and Si NMR spectroscopy, along with

X-ray diffraction analysis for crystallized solids.

3

effects in some two-fold NHSiMe functionalized substrates led

3

Introduction

In the context of the more and more evolving topic of carbon

dioxide utilization for the production of various crucial chem-

icals, we aim at employing CO as an oxygen source as well as a

2

carbon source in the synthesis of compounds such as (poly)

[

1–4,5,6,7]

siloxanes, urea derivatives and isocyanates.

One key step

Scheme 1. Generic reaction pathway of carbon dioxide insertion into amino-

silanes.

1

2

is the insertion reaction of CO into SiÀ NR R bonds of amino-

2

1

2

silanes with formation of the desired R R NC(O)OSi motif (cf.

carbamoyloxysilane, carbamatosilane, O-silylcarbamate; Si=tet-

ravalent silicon with three further substituents), as shown in

Scheme 1. These compounds may serve as precursors for

isocyanates, formation of which depends on cleavage of the O-

silylcarbamate motif with release of a suitable leaving group

sense as those compounds containing Si-bound amine groups,

i.e., SiÀ N bonds (cf. silylamines).

Mechanistic considerations of the insertion of CO into SiÀ N

2

1

[1,6]

R OSi. In this context, we refer to “aminosilanes” in a narrow

bonds have been published earlier.

In brief, at first a

nucleophilic approach of the N atom toward the CO carbon

2

atom occurs. Two alternative proceeding routes arise depend-

ing on the nitrogen-bonded substituents. (i) The CO oxygen

atom approaches the Si atom, thus forming a four-membered

2

[

a] F. Gründler, H. Scholz, Dr. M. Herbig, Dr. S. Schwarzer, Dr. J. Wagler,

Prof. Dr. E. Kroke

Inorganic Chemistry, TU Bergakademie Freiberg

Leipziger Strasse 29, 09596 Freiberg, Germany

E-mail: kroke@tu-freiberg.de

cyclic transition state, which was postulated by Kraushaar

[1]

et al., and represents a plausible pathway for any kind of

1,2

aminosilane (R may be combinations of H, alkyl, aryl, allyl). (ii)

Supporting information for this article is available on the WWW under

https://doi.org/10.1002/ejic.202100118

Alternatively, amine-bound hydrogen might attract and be

transferred to a CO oxygen atom and lead to the formation of

©

2021 The Authors. European Journal of Inorganic Chemistry published by

2

Wiley-VCH GmbH. This is an open access article under the terms of the

Creative Commons Attribution Non-Commercial NoDerivs License, which

permits use and distribution in any medium, provided the original work is

properly cited, the use is non-commercial and no modifications or adap-

tations are made.

a carbamic acid. In the latter case, the final silylcarbamate motif

is accomplished by migration of both the COOH proton and the

still N-bound silyl group. Strong bases might support the

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second pathway by deprotonation of the acid and thereby

stabilizing the intermediate form.

methoxy-substituted aniline derivatives) by the reaction of

aniline derivatives with hexamethyldisilazane and CO₂at

In our previous studies of O-silylcarbamate syntheses, the

CO insertion reaction included bubbling of CO into a solution

increased temperatures and using CoCl as a catalyst. Knausz

2

[20]

et al. also investigated N,O-bis(trimethylsilyl)-carbamates and

carbonyl compounds on the way to the formation of imines

and O-methyl-oximes. These results along with related research

on O-trimethylsilyl-N,N-dialkylcarbamates were summarized in a

2

of the aminosilane in THF at ambient pressure and low

[4]

temperature. For aliphatic aminosilanes the corresponding O-

silylcarbamates formed in high yields within 20 minutes or less.

The same procedure works equally fine for di-, tri- and tetra-

aminosilanes with sterically less demanding substituents. With

aromatic aminosilanes, however, similar reactions at ambient

[21]

review.

The CO insertion reaction into diaminosilanes of the type

2

(RR’N) SiMe has been thoroughly investigated in our group in

2

[1–3,7,8]

[1–3,22]

conditions failed.

the past.

The reports include a method to produce oligo-

Being the simplest representative of the class of aromatic

aminosilanes, a lot of experiments have been done using N-

trimethylsilylaniline (1), though a successful insertion reaction

and polysiloxanes along with N-substituted urea derivatives

starting from diaminosilanes (RR’N) SiMe in a reaction with CO

2

to yield insertion products (RR’NÀ COÀ O) SiMe and subsequent

2

[7]

[4]

by the mentioned procedure could not be achieved. The

formation of [O-(trimethylsilyl)carbamato]benzene (1a) has

been studied by different groups previous to the present work,

thermal decomposition affording the desired compounds. All

previous works focused on aliphatic side chains R and R’

bonded to the nitrogen atoms. In the present study, we

[9]

though. In doing so, Zoeckler and Laine found a way to

synthesize 1a starting from 1 and carbon dioxide including

transition metal catalysis and temperatures of 100°C and above

extended our investigations of CO insertion into SiÀ N bonds

2

onto the predominantly aromatic compounds shown in

Scheme 2.

[9]

in a Parr bomb reactor. Another synthesis pathway was

investigated by Kozyukov et al. By heating aniline and different

O- and N-silylcarbamates for several hours the desired product

As those compounds possess a high potential as precursors

due to the existing structure motif of isocyanates, we also

extended our research toward the synthesis of isocyanates. First

results are discussed subsequent to the carbon dioxide

insertion.

[

10]

[11]

could be obtained in yields of up to 40%. Knausz et al. and

the research group of Belova

[12]

were able to synthesize

compound 1a in yields of up to 45% by mixing aniline with

CO /hexamethyldisilazane, a so called N-siloxycarbonylating

2

reagent, either for a longer period or by heating to 60°C for a

few hours. Similar systems include CO /hydridosilane and CO /

2

[13]

N,N’-bis(trimethylsilyl)carbodiimide. Working with hexameth-

yldisilazane as well, Sheludyakov et al. were able to produce 1a

by mixing the silazane with aniline hydrochloride and carbon

[14]

dioxide.

Yamamoto et al. found a way to access 1a by

refluxing either tert-butyl- or isopropyl-trimethylsilyl-carbonate

[15]

along with aniline in diethyl ether for one hour.

The

formation of 1a in a procedure of heating a mixture of N-

phenyl-O-alkyl-carbamates and trimethylsilyl iodide up to 50°C

has been described by Jung and Lyster, affording the desired

[16]

[17]

product in 60% yield.

Quite recently, Fuchter et al.

published a way of synthesizing O-silylcarbamates by heating

N-silylamines in supercritical carbon dioxide as an intermediate

step to produce various ureas. In addition to aliphatic silyl-

amines, the aniline and p-anisidine derivatives have been part

of their investigations.

While a wide range of trimethylsilylated amine compounds

are known in the literature, the number of aromatic O-silylated

carbamates is limited. Quite some research has been done by

[18]

Xu et al. On the way of preparing various ureas from CO and

2

silylamines, they proposed the formation of silylcarbamates as

intermediate products. Their reaction procedure includes ex-

position of the aminosilane to 1–5 atm CO in a J-Young NMR

2

tube and refluxing the mixture at 75–150°C for several hours to

collect the desired urea afterwards. The described silylcarba-

mate, a derivative of p-anisidine, was only detected in NMR

[19]

spectra and could not be isolated, though. Knausz et al.

studied the formation of various trimethylsilylated N-aryl-

substituted carbamates (i.e., methyl-, chloro-, bromo- and

Scheme 2. Overview of aminosilanes investigated within this paper for the

insertion of carbon dioxide into SiÀ N bonds.

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Results and Discussion

proceeded toward a reaction scale of up to 50 mL, increasing

the amount of aminosilane used from approximately 1 g to

almost 10 g. Under the given parameters of 24 hours at room

Carbon dioxide insertion into aromatic aminosilanes

temperature and a CO pressure of 8 bar, the insertion reaction

2

Initial attempts at CO insertion into N-trimethylsilylaniline (1)

proceeds as smooth as on a small scale.

2

by gas bubbling under ambient conditions – analogous to the

very successful reactions of aminosilanes with aliphatic sub-

stituents on the N-atoms – failed, even upon increased reaction

time. Addition of the strong base 1,8-diazabicyclo(5.4.0)undec-

In order to gain insights into the reasons for the different

behaviour of aliphatic and aromatic aminosilanes at the

exposure to carbon dioxide and the need for a raised pressure,

calorimetric measurements of the CO insertion reaction were

2

7

-ene (DBU), which increases the total amount of dissolved

performed. Calorimetric analyses using aminosilane 1 revealed

an enthalpy of À 76 kJ/mol for the insertion reaction at a

carbon dioxide in the solvent due to the formation of a CO -

2

[23]

complex , afforded traces of the corresponding silylcarbamate

a. After an hour of gas bubbling at ice cooling, the amino-

pressure of 4 bar CO . Hence, the reaction is thermodynamically

2

1

favoured, but kinetically slightly hampered in this case. Steric

silane still represents the main compound in the product

mixture, though.

aspects might influence the insertion reaction of CO into the

2

SiÀ N bond of 1. Additionally, the lone pair of the nitrogen atom

is delocalized across the aromatic π-electron system, which

decreases the nucleophilicity of the amine and hampers the

insertion reaction on a kinetic level. This is most likely the main

reason for the observed differences between the two described

experimental methods.

A CH -linkage between the aromatic ring and the nitrogen

2

atom changes the behaviour completely, affording the silylcar-

bamate 2a easily by bubbling CO into N-trimeth-

2

[7]

ylsilylbenzylamine (2) in THF (Scheme 3).

Exposure of a THF solution of 1 to CO at 8 bar in an

2

autoclave (for picture see Supporting Information, Figure S1) for

A

similar insertion reaction can be observed using

2

4 hours at room temperature finally afforded pure 1a in good

dianilinodimethylsilane (PhNH) SiMe , which afforded the two-

2

yield, even without the use of a catalyst. The product, a white

solid, was recovered from the solution upon removal of the

solvent under reduced pressure. Subsequent recrystallization

from n-pentane finally afforded colorless needles of 1a in yields

of up to 80%. Spectroscopic analysis of the crystals confirmed

the identity and purity of the insertion product, and a single-

crystal X-ray diffraction analysis confirmed a structure which

had previously been reported by Sheludyakov et al. As our

structure determination delivered a model of somewhat better

bond precision, our data is included in the Supporting

Information (Figure S8). In the autoclave experiments, we

observed that addition of DBU supported both the insertion

reaction and crystallization as visible by spontaneous formation

of colorless needles immediately after opening the autoclave

container and transferring the pale-yellow liquid into a Schlenk

flask. Nonetheless, elevated CO₂pressure and prolonged

reaction times are still essential, as reducing the time to 2 hours

or the pressure to 2 bar lowered the product yield significantly.

While the first experiments were performed in the small

autoclave with an overall reaction volume of 5 mL, we

fold insertion product di(N-phenylcarbamoyloxy)dimethylsilane

by the common procedure (8 bar, 24 h) as a main product (for

experimental data see Supporting Information). The formation

of colorless needle-shaped crystals, which were suitable for X-

ray diffraction analysis, revealed the crystal structure of that yet

unknown compound (see Figure S57 in Supporting Informa-

tion). Side products included the proposed formation of a

mono-insertion product, where only one of two SiÀ N bonds

included carbon dioxide.

[14]

In the following, we studied the insertion reaction of CO₂

into some derivatives of 1. Starting from p-anisidine, which

carries a methoxy group in para-position, we synthesized N-

trimethylsilyl-p-anisidine (3) as a brownish liquid in a first step,

followed by the successful insertion reaction using the above-

mentioned autoclave method to obtain 4-[O-(trimethylsilyl)

carbamato]methoxybenzene (3a), which crystallized from the

mother liquor as colorless needles after partial removal of the

solvent. Single-crystal X-ray structure determination confirmed

the identity of the product and matches the results reported by

[24]

Belova et al. The CO insertion proceeds almost quantitatively,

2

leaving only small traces of the aminosilane in the reaction

solution. Complete removal of the solvent led to a brownish

solid, which was obtained in a yield of 91%.

1

,2-Bis(trimethylsilylamino)benzene (4), which bears another

trimethylsilylated amine function in ortho-position, also under-

goes CO₂insertion along the autoclave procedure with

formation of compound 4a. In this case, insertion of CO into

2

both SiÀ N bonds was observed in a basically quantitative

manner. Neither traces of the aminosilane 4 nor those of the

product of single CO insertion were detected. Under the same

2

conditions, a reaction starting from p-phenylendiamine via 1,4-

bis(trimethylsilylamino)benzene (5) proceeds in the same way

to yield the corresponding dicarbamate species, which has

Scheme 3. CO

2

insertion into the SiÀ N bonds of compounds 1 and 2

[

25]

following the common procedure of gas bubbling through a solution of the

aminosilane in THF.

been obtained using a different method by Mironov et al.

[

2]

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In addition to aromatic aminosilanes, which reveal some

kinetic hindrance of CO2 insertion, (+/À )- trans-1,2-bis

trimethylsilylamino)cyclohexane (6) is an alicyclic representa-

(

tive, which exhibits hindered CO₂insertion along the gas

bubbling protocol (affording a product mixture consisting of

larger amounts of the unconsumed aminosilane in addition to

the desired carbamato compound). For this case, the autoclave

procedure (8 bar, 24 h) proved useful once again, delivering the

desired product of two-fold CO -insertion in almost quantitative

2

yield. In addition to insufficient reaction time, steric aspects of

the two trimethylsilyl groups might influence the insertion of

CO into the SiÀ N bonds. With alkyl substituents of related

2

steric demand (e. g. cyclohexyl), simple aminosilanes were

reported to undergo CO insertion quantitatively under mild

2

conditions. The gas bubbling procedure afforded the desired

insertion product, O-(trimethylsilyl)carbamatocyclohexane, in an

isolated yield of 95% and gave crystals suitable for single-

crystal X-ray diffraction analysis, which revealed the crystal

structure of this so far unknown compound (see Supporting

Information, Figure S64).

Figure 1. Molecular structure of 7a in the crystal (displacement ellipsoids are

drawn at the 50% probability level, selected atoms are labelled, the lower

occupancy part of the disordered SiMe group at Si2 is omitted for clarity).

3

Only one of the three crystallographically independent (but conformationally

similar) molecules in the asymmetric unit is shown. Selected bond lengths

[

Å] and angles [°]: Si1À N1 1.725(2), Si2À O1 1.681(2), C11À O1 1.344(3),

An interesting observation was made with a combination of

aliphatic and aromatic amine environments, i.e., in N,N’-bis

C11À O2 1.229(3), C11À N2 1.325(3), Si1À N1À C4 131.0(2), Si2À O1À C11

1

25.4(2).

(trimethylsilyl)-2-aminobenzylamine (7). NMR spectroscopic

data acquired from the crude reaction mixture after the general

autoclave procedure suggest selective mono-insertion of CO₂

into the aliphatic silylamine group with formation of compound

separation O1···N1 of 3.000(3) Å (as well as between the

corresponding O and H atoms of the other two molecules in

the asymmetric unit with O···N separations of 3.012(3) and

3.061(3) Å). We assume it is due to these hydrogen bonds, and

7

a (Scheme 4).

2

9

In 7a, the Si NMR shift value of 2.8 ppm indicates a

nitrogen-bound SiMe group rather than a silyl carbamate. The

the hindered rotation of the aryl-NHÀ SiMe moiety resulting

3

latter tend to produce peaks in the range of 20–25 ppm as

therefrom, that even at CO pressure of 8 bar formation of the

2

29

confirmed by many of our own results (e. g., δ Si(1a)=

2.2 ppm). In addition to the signals of the main product 7a

another peak (at 23.8 ppm) with noticeably lower intensity was

dicarbamate is hindered.

2

Extension of the aromatic system to a naphthalene core

decreased the ability to include carbon dioxide at the given

parameters. The use of 1,8-bis(trimethylsilylamino)naphthalene

2

9

observed in the Si NMR spectrum. We suspect that the second

insertion reaction of CO into the aromatic aminosilane group

(8) did not lead to the desired CO insertion product at all. The

2

with formation of the dicarbamate species does occur to some

extent, but is kinetically much more hampered.

steric effects of the two NHSiMe₃groups in peri-position

presumably pose a greater steric hindrance. Even prolonged

The identity of the main product 7a was confirmed by

crystallographic structure determination (Figure 1). After apply-

reaction time (up to 72 h) and a pressure of 8 bar CO only

resulted in a partial conversion of the aminosilane (see

Scheme 5).

2

ing a CO pressure of 8 bar to the aminosilane for a period of

2

3 hours and removing the solvent afterwards, a yellow, highly

The aminosilane 8 apparently forms two rotation isomers

viscous liquid was received. Upon storage in the refrigerator,

needle shaped crystals formed, which were suitable for X-ray

crystallographic measurements. The crystal structure revealed

intramolecular hydrogen bonds between O2 and H1n with a

due to the orientation of the two NHSiMe groups. Hence, the

3

2

9

Si-inept NMR spectrum of 8 shows two peaks, at 1.4 and

3.4 ppm. After application of 8 bar CO for 72 h a bunch of

2

9

signals is found in the Si-inept NMR spectrum of the crude

product mixture (for spectra see Supporting Information, Fig-

ure S48). The peaks of the starting material 8 (1.2 ppm,

3

.3 ppm) remain the peaks with the highest intensity in the

same intensity pattern, indicating an equilibrium state in

solution. A group of four signals is found in the area of 23 ppm,

with a peak at 22.8 ppm showing the second highest intensity

of the full spectrum. We associate those signals with oxygen-

bound SiMe groups of the di- and mono-insertion product 8a

3

and 8b, each compound appearing in two rotation isomers

themselves. The signals for the corresponding nitrogen-bound

Scheme 4. Proposed CO

2

insertion into one SiÀ N bond of compound 7 with

formation of 7a based on the results of NMR spectroscopic measurements.

SiMe group of the mono-insertion product appear at 13.0 and

3

2

9

Associated Si NMR shifts are listed in blue.

1

3.8 ppm: As expected, two signals for the rotation isomers. In

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current focus of our research. The approach of obtaining

isocyanates by thermal cracking of silyl carbamates has been

[26]

reported in the literature. Greber and Kricheldorf found a way

of reducing the decomposition temperature of carbamates by

silylation of carbamic acid chlorides and urethanes producing

mono-silylated N-silylcarbamates. Finally, thermal cracking of

the N-silylcarbamates yielded the corresponding isocyanates.

[27]

Mironov

summarized routes starting from silylamines and

related silicon compounds to obtain isocyanates, such as direct

phosgenation of N-silylamines, thermolysis of mono-silylated N-

silylurethanes and thermolysis of silylhydroxamic acids. Mironov

also reported the formation of isocyanates in high yields upon

heating mono-silylated O-silylcarbamates or carbamates along

with PhSiCl . As trimethylsilanol is liberated during decomposi-

3

tion of the O-silylcarbamate, large amounts of the isocyanate

undergo hydrolysis yielding the symmetric dialkylurea. Similar

reactions have been mentioned by Lebedev et al. to transfer this

synthesis onto isocyanates with other aliphatic and allylic side

Scheme 5. Rotation isomers of 8 and proposed insertion products 8a and

b.

8

[28]

[29]

[30]

chains.

In contrast, Oertel et al.

and Breederveld et al.

described the O-silylcarbamates as mostly stable to thermal

treatment, reporting distillation rather than decomposition

upon heating.

addition, the appearance of hexamethyldisiloxane (7.2 ppm)

can be detected. The latter would be the side product during

condensation of a product of single CO₂insertion with

formation of a urea, which formed clusters of beige crystalline

plates (which appeared red because of the adherent mother

liquor) after several days and was identified as 1H-perimidin-

In previous studies, we already investigated the thermal

decomposition of O-silylcarbamates, thereby setting the focus

on aliphatic diaminosilanes as mentioned in the introduction. A

postulated mechanism for thermal decomposition of such

compounds includes the intermediate formation of isocyanates,

which react quickly with free amine to yield the observed urea

2

(3H)-one by X-ray crystal structure determination (Figure 2).

[2,22]

derivatives.

The isolation of isocyanates remained problem-

atic, though, as the side reaction with the amine (affording the

corresponding urea derivative) was always favoured. A similar

decomposition pathway has been postulated by Knausz et al.

Preparation of isocyanates

[

31]

The investigated class of compounds might be suitable as

precursors for crucial chemicals such as urea derivatives,

siloxanes and isocyanates. Especially the latter became the

in 1983 for aliphatic and allylic trimethyl-substituted O-

silylcarbamates refuting the thesis of Mironov et al., which has

been mentioned above. By autosilylation the corresponding

N,O-bissilylated carbamate species is formed in the condensed

phase along with an equivalent of both carbon dioxide and

amine (see Scheme 6). At temperatures around 100–120°C the

intermediate bissilylated species decomposes with formation of

the desired isocyanate and release of hexamethyldisiloxane,

Figure 2. Molecular structure of the cyclic urea derivative 1H-perimidin-

2

(3H)-one formed by the insertion reaction of CO into 8 (displacement

ellipsoids drawn at the 50% probability level). Selected bond lengths [Å] and

angles [°]: O1À C11 1.245(2), N1À C1 1.403(2), N1À C11 1.359(2), N2À C9

1

.404(2), N2À C11 1.360(2), O1À C11À N1 121.72(13), O1À C11À N2 121.53(13),

Scheme 6. Postulated pathway for the formation of urea derivatives by

thermal treatment of O-silylcarbamates.

[

31]

N1À C11À N2 116.74(12).

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though a quick reaction with the amine occurs, eventually

forming the corresponding 1,3-dialkylurea. This postulated

pathway has been adopted by Fuchter et al. in the synthesis of

[17]

O-silylcarbamates and ureas in supercritical CO₂. Neither of

them reports the analytical proof not to mention the isolation

of the isocyanates, though.

Taking the above mentioned information into account, we

extended our own research onto aromatic O-silylcarbamates. As

the insertion reaction of carbon dioxide afforded the corre-

sponding O-silylcarbamate in high yields, thermolysis experi-

ments followed. So far, we have been investigating the thermal

decomposition of 1a in a distillation apparatus for micro

quantities with cold finger (short-path distillation) by heating

the white solid in a vacuum. Already 10 minutes after starting

the experiment the deposition of a white solid in upper parts of

the still was visible. After brief interruption of the experiment

Figure 4. Molecular structure of 1b in the crystal (displacement ellipsoids

drawn at the 50% probability level). Selected bond lengths [Å] and angles

(for taking a sample of the white solid for NMR spectroscopic

[

°]: C2À N1 1.4421(18), N1À Si2 1.7936(13), N1À C1 1.3654(19), C1À O1

analysis) heating was resumed, which finally led to the

condensation of a clear colorless liquid at the cold finger and

more solid in higher parts of the still. NMR studies revealed the

presence of 1a in every sample reaching from the bottom

vessel of the still through to the safety flask after the still, which

was cooled by liquid nitrogen. In the latter, the accumulation of

the more volatile hexamethyldisiloxane (HMDSO) along with

aniline was detected. Both compounds next to unconsumed 1a

were found on the cold finger as well. The white solid turned

out to contain traces of 1,3-diphenyl urea in addition to the O-

silylcarbamate. Comparison with commercially available phenyl

isocyanate confirmed the absence of noticeable amounts of

that compound in any sample after thermal treatment (cf. grey

boxes in Figure 3). These results agree with the postulated

mechanism for the thermal decomposition of O-silylcarbamates

1

.3555(17), C1À O2 1.2083(19), O1À Si1 1.6890(11), N1À C1À O1 110.99(13),

Si2À À N1À C1 122.30(10), C1À O1À Si1 123.62(10).

tained by using trimethylsilyl triflate. The silylating agent is

added dropwise to a mixture of the O-silylcarbamate and Et N

3

in n-pentane while stirring and cooling in an ice bath. The

formation of a slushy precipitate at the bottom of the flask was

observed, which is easily removed from the mixture by freezing

and decanting the liquid supernatant. Removal of solvent (from

the supernatant) under reduced pressure yielded the desired

derivative 1b as a white solid, which was recrystallized from

THF to afford crystals suitable for X-ray crystal structure

determination (Figure 4). This method revealed the structure of

[31]

by Knausz et al.

the

so-far

unknown

molecule

N,O-bis(trimethylsilyl)

To inhibit the side reaction of the isocyanate group with

any amine formed, we replaced the nitrogen-bound hydrogen

atom in another silylation procedure, which afforded a N,O-

bissilylated carbamate. Only partial conversion was achieved

using trimethylchlorosilane, though better results were ob-

carbamatobenzene (1b). It represents a rather novel compound

as no aromatic N,O-bissilylated carbamates are described in the

literature to the current date, while the unsubstituted ‘parent’

N,O-bissilylated carbamate Me SiHNÀ COÀ OÀ SiMe may be used

3

1

3

29

13

Figure 3. C-APT and Si NMR spectra of various samples after thermal treatment of 1a. For better visualisation only part of the C NMR spectra are shown.

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1

3

as a silylating reagent itself, forming only CO and NH as side

products.

of HMDSO at 1.7 ppm arises with the highest intensity in the C

NMR spectra of the outer fraction. Additionally, the presence of

benzyl isocyanate was confirmed (46.4, 126.6, 127.8, 128.7,

137.0 ppm). The peak location and intensity pattern agrees well

with the spectrum of a commercially available sample of benzyl

isocyanate. Next to that, in very low intensity a peak at

21.2 ppm along with a few peaks in the shift range of phenyl C

atoms can be observed. Performing that procedure afforded

1.82 g colorless liquid in the outer bulb starting from 3.13 g of

2b. Thermolysis of a larger sample of 2b (8.38 g) with a raised

heating rate (direct heating to 300°C without steps at 200°C

and 250°C) gave 1.74 g colorless liquid in the outer bulb with a

2

3

[32]

By replacing the N-bound hydrogen atom by a trimethylsilyl

group the thermal decomposition toward the isocyanate and

HMDSO should be favoured. Our experiments of the thermal

treatment of 1b revealed the Kugelrohr (ball tube) distilling

apparatus to be a more suitable device for the decomposition

reaction. We performed the thermal treatment of 1b in a three-

parted Kugelrohr still yielding the residue, the intermediate

fraction inside the oven as well and the outer fraction outside

the heating chamber (and cooled by water). While the residue

and the intermediate fraction contained a mixture of various

aromatic species, which could not be identified clearly, the third

fraction in the outer bulb consisted exclusively of the desired

phenyl isocyanate and the side product HMDSO, as detected by

NMR spectroscopy. After an overall decomposition time of

significant increase of the intensity of peaks mentioned last

(21.5, 125.5, 128.4, 129.2, 137.9 ppm). In addition, the corre-

sponding signals of benzyl isocyanate vanish completely.

Interestingly, the peaks with increased intensity can be assigned

[33]

3

hours no traces of the starting N,O-bissilylcarbamate were

to toluene (cf. 21.3, 125.6, 128.5, 129.3, 137.7 ppm ). Next to

that, another peak with significant intensity was observed at

0.9 ppm, which we assigned to trimethylsilyl isocyanate. The

corresponding signal for the isocyanate group can be found at

124.0 ppm as a broad peak of very low intensity. In the

found in any fraction. Comparison with commercially available

phenyl isocyanate clearly proved the presence of that com-

pound in the outer fraction. Additionally, this result was

confirmed by ATR-IR spectroscopy (Figure 5), where the corre-

À 1

29

sponding isocyanate band was clearly visible at 2258 cm (cf.

associated Si-inept NMR spectrum the peak for trimethylsilyl

À 1

Ph-NCO: 2247 cm ).

isocyanate was found at 4.2 ppm. Our own measurements of a

commercially available sample of trimethylsilyl isocyanate along

with an HMBC NMR analysis of the outer fraction confirmed the

assignments (see Scheme 7). We suspect the toluene formation

being enabled by carbonization of the residue, which turns dark

brown during thermal treatment, or by traces of impurities in

the starting material such as triethylamine for example.

In the following, we extended our research towards the

synthesis of different isocyanates, e. g. benzyl isocyanate, by the

described route. The silylation of benzyl amine to gain the N-

benzyl substituted aminosilane 2, followed by insertion of CO₂

to yield 2a and silylation of the nitrogen atom with formation

of ω-[N,O-bis(trimethylsilyl)carbamato]toluene (2b) proceeded

in high yields and afforded the desired compounds in high

purity as confirmed by NMR spectroscopy.

Thermal treatment of 2b using a Kugelrohr created the Conclusions

desired benzyl isocyanate in the outer fraction. Whereas the

residue and the intermediate fraction contain various silicon-

In our present study we investigated the insertion of CO into

silicon nitrogen bonds of aromatic aminosilanes with formation

2

9

containing species according to the Si-inept NMR spectra,

HMDSO can be observed as the sole silicon-containing com-

of À NÀ COÀ OÀ SiÀ units and successfully establishing an auto-

1

3

pound in the outer fraction. Those results are confirmed by

C

clave method to provide the desired products in good yields.

NMR spectroscopy, where several peaks are visible in the

spectra of the residue and the intermediate fraction. The peak

Instead of gas bubbling of CO through a THF solution of

2

aminosilane, the starting material has to be exposed to the gas

at a pressure of up to 8 bar. Reactions in a scale of up to 10 g

aminosilane were performed successfully. Compounds with

limited steric hindrance, i.e. those containing only a single

NÀ SiMe group as found in PhNHSiMe₃and p-(MeO)

3

C H NHSiMe , easily include carbon dioxide at the given

6

4

3

parameters. With the newly established method the insertion

reaction even proceeded effectively with compounds posing

more demanding steric requirements, whereas gas bubbling

Figure 5. ATR-IR spectra of decomposition fractions in comparison to

commercially available phenyl isocyanate.

Scheme 7. Observed products upon thermal treatment of 2b.

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led to a product mixture. Only the extension of the aromatic

system toward a naphthalene derivative still causes problems,

as we were not able to clearly identify the desired carbamato-

compound so far, and intermediates may already decompose

with formation of side products, among them the cyclic urea

derivative 1H-perimidin-2(3H)-one.

Embarking in the topic of replacing phosgene in the

industrial process of synthesizing isocyanates we managed to

successfully decompose aliphatic and aromatic N,O-bissilylated

carbamate species to gain the desired isocyanate along with

hexamethyldisiloxane as a side product. Herein, we show the

general proof-of-principle to form isocyanates from amino-

silanes and CO₂without using phosgene. The mentioned

procedure still holds potential for optimization, e. g. in-situ

silylation to mask the nitrogen bonded proton or variation of

thermolysis parameter. Combination of the aminosilane syn-

possibility to work under fully inert conditions, the miniclave could

be loaded and unloaded in an argon atmosphere. Both autoclave

systems were charged with CO from Linde with a purity of 5.3.

2

Solids and liquids have been analysed by nuclear magnetic

1

13

29

resonance spectroscopy. H, C and Si NMR spectra of liquid

samples were recorded on BRUKER Avance III 500 MHz or BRUKER

Nanobay 400 MHz spectrometers. Tetramethylsilane was used as

1

13 29

internal standard [δ( H, C, Si)=0 ppm]. Additional pictures of NMR

spectra are included in the Supporting Information. Crystals

obtained have been examined by single-crystal X-ray diffraction

analysis, either for determination of the unit cell (for compounds of

known structure) or for full structure refinement. Single-crystal X-

ray diffraction data were collected on a STOE IPDS-2/-2T diffrac-

tometer (equipped with a low-temperature device) with MoÀ Kα

radiation (λ=0.71073 Å) using ω scans. Preliminary structure

[34]

models were derived by solution with ShelXS or ShelXT and the

structures were refined by full-matrix least-squares cycles based on

2

[35]

F

for all reflections using ShelXL . The positions of N-bound

hydrogen atoms have been refined without constraints. All other

hydrogen atoms were included in the refinement in idealized

positions (riding model).

thesis and CO insertion might be interesting as well. Further

2

investigations on that part include an upscaling to produce the

pure isocyanate as well as variation of the carbamate species to

increase the variety of products accessible along this route.

Especially phosgene-sensitive, low-scale isocyanates produced

by smaller companies come into focus of our future research to

avoid high costs for safety equipment necessary for the use of

phosgene. In addition to that, the introduced synthesis pathway

can be adapted to produce industrially relevant diisocyanates.

While the production of common diisocyanate compounds

such as TDI and MDI is well established, we aim for the

synthesis of special chemicals with a small-scale need.

Elemental analysis was performed on a Vario MICRO cube in CHNS

mode. Sample preparation occurred under inert conditions inside

an argon-flooded glovebox.

Synthesis of Aminosilanes 1to-8

Compounds 1–3 were produced by dropwise addition of trimeth-

ylchlorosilane to a solution of the associated amine in the solvent

with intense stirring and cooling in an ice bath. (The solution

contained triethylamine as a sacrificial base for formation of

triethylamine hydrochloride as a white precipitate.) Upon addition

of the silane, the reaction mixture was allowed to attain room

temperature and was continuously stirred overnight. Afterwards,

the precipitate was filtered off, and removal of the solvent from the

filtrate (condensation into a cold trap under reduced pressure)

afforded the aminosilane as a liquid residue. All relevant data are

summarized in Table 1 and Table 2.

Experimental Section

The syntheses of aminosilanes as well as the insertion reactions

were carried out in a dry argon atmosphere using standard Schlenk

techniques. The chemicals were purchased from commercial

suppliers and purified by distillation. Solvents were dried using a

solvent purification system by MBRAUN (for THF) or by storage over

molecular sieves for deuterated solvents and n-pentane. Solids

were dried in a dynamic vacuum at room temperature for at least

one hour. Insertion reactions of carbon dioxide were either

performed in a smaller, self-made autoclave with a reaction volume

of up to 5 mL or in a miniclave of Büchi AG manufactured by C3

Prozess- und Analysentechnik GmbH. The latter could hold up to

For the preparation of aminosilanes 4–8, the starting materials were

refluxed for 30 minutes after dropwise addition of trimeth-

ylchlorosilane to a mixture of the corresponding amine and

triethylamine in the listed solvent. Afterwards, the described

procedure of compound 1 was applied.

2

00 mL reaction volume in a PTFE inlet or up to 50 mL in an

Erlenmeyer flask. While the small autoclave did not offer the

Table 1. Synthesis and analysis data of aminosilanes 1 to 3.

N-trimethylsilylaniline (1)

N-trimethylsilylbenzylamine (2)

N-trimethylsilyl-p-anisidine (3)

amine

Me SiCl

Et

solvent

product

yield

4.28 g (46.00 mmol)

5.12 g (47.16 mmol)

5.11 g (50.50 mmol)

100 ml n-pentane

white solid

5.03 g (46.94 mmol)

5.17 g (47.59 mmol)

4.91 g (48.52 mmol)

160 ml Et O

2

colorless liquid

5.61 g (45.52 mmol)

5.12 g (47.16 mmol)

4.89 g (48.32 mmol)

100 ml n-pentane

orange liquid

3

N

7.13 g

7.45 g

7.64 g

(

43.13 mmol, 94%)

(41.54 mmol, 89%)

(39.11 mmol, 86%)

C D

6 6

6.70 (m, 2 H, ArH), 6.36 (m, 2 H, ArH), 3.45 (s, 3 H,

OMe), 3.07 (s, 1 H, HN), 0.15 (s, 9 H, SiMe

152.5 (ArC), 141.1 (ArC), 117.4 (ArCH), 116.2,

115.0 (ArCH), 55.1 (OMe), 0.0 (SiMe

1.8

NMR solv. CDCl

6.66–6.59 (m, 5 H, ArH), 3.29 (s, 1

400 MHz) H, HN), 0.21 (s, 9 H, SiMe

3

CDCl

7.16 (m, 2 H, ArH), 7.05 (m, 2 H, ArH), 3.76 (d, 2 H,

3

1

H NMR

3

(

3

)

J=8.0 Hz, CH

2

), 0.55 (s, 1 H, HN), 0.00 (s, 9 H, SiMe

3

)

3

)

1

3

C NMR

147.4 (ArC), 129.2 (ArCH), 117.5

ArCH), 116.1 (ArCH), 0.0 (SiMe

2.7

144.2 (ArC), 128.1 (ArCH), 126.8 (ArCH), 126.3 (ArCH),

(

3

)

45.9 (CH

3.9

2

), 0.0 (SiMe

3

)

3

)

29

Si NMR

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Table 2. Synthesis and analysis data of aminosilanes 4 to 8.

1

,2-bis

1,4-bis

(trimethylsilylamino)

benzene (5)

(+/À )-trans-1,2- bis

(trimethylsilylamino)

cyclohexane (6)

N,N’-bis(trimethylsilyl)-2-

amino-benzylamine (7)

1,8-bis

(trimethylsilylamino)

naphthalene (8)

(

trimethylsilylamino)

benzene (4)

amine

Me

Et

solvent

5.01 g (46 mmol)

SiCl 10.00 g (92 mmol)

5.00 g (46.25 mmol)

10.03 g (92.30 mmol)

9.38 g (92.68 mmol)

140 ml THF

9.51 g (83 mmol)

18.10 g (167 mmol)

16.95 g (167 mmol)

100 ml THF

5.00 g (41 mmol)

9.00 g (83 mmol)

8.40 g (83 mmol)

100 ml THF

7.30 g (46 mmol)

10.50 g (97 mmol)

9.33 g (92 mmol)

100 ml THF

3

N

9.42 g (93 mmol)

100 ml THF

product orange liquid

yellow solid

colorless liquid

yellow liquid

red solid

yield

NMR

10.35 g (40.99 mmol, 88%) 9.68 g (38.33 mmol, 83%)

14.34 g (55.46 mmol, 67%) 6.48 g (23.77 mmol, 58%)

12.52 g (41.38 mmol, 90%)

CDCl

3

CDCl CDCl

3

CDCl CDCl

3

solv.

1

3

H NMR 6.80 (m, 2 H, ArH), 6.71 (m, 6.50 (m, 4 H, ArH), 3.05 (s,

2.15 (d, 2 H, J=7.0 Hz),

1.81 (m, 2 H), 1.57 (m, 2

H), 1.19 (m, 2 H), 1.03 (m,

7.08 (m, 1 H, ArH), 7.02 (m, 7.21 (m, 2 H, ArH), 7.14 (m,

2

0

H, ArH), 3.06 (s, 2 H, HN), 2 H, HN), 0.22 (s, 18 H,

.21 (s, 18 H, SiMe SiMe

1 H, ArH), 6.77 (m, 1 H,

ArH), 6.65 (m, 1 H, ArH),

2 H, ArH), 6.65 (m, 2 H,

ArH), 5.49 (s, 2 H, HN), 0.23

3

)

3

)

2

H), 0.67 (s, 2 H, HN), 0.00 5.20 (s, 1 H, HN), 3.84 (d, 2 (s, 18 H, SiMe

3

)

3

(

s, 18 H, SiMe

3

)

H, J=6.0 Hz, CH

2

), 0.30 (s,

9

SiMe

H, SiMe

3

), 0.12 (s, 9 H,

3

)

13

C NMR 137.6 (ArC), 120.2 (ArCH),

19.7 (ArCH), 0.4 (SiMe

138.3 (ArC), 117.1 (ArCH),

58.8 (ring C), 36.7 (ring

CH ), 25.5 (ring CH ), 0.9

SiMe

147.3 (ArC), 129.5 (ArCH),

128.1 (ArCH), 128.1 (ArC),

117.1 (ArCH), 115.9 (ArCH), 120.4 (ArCH), 115.4 (ArCH),

144.4 (ArC), 137.4 (ArC),

125.7 (ArCH), 121.3 (ArC),

1

3

)

0.0 (SiMe

3

)

2

(

3

)

4

5.5 (CH

2

), 0.4 (SiMe

3

)

3

),

0.1 (SiMe

3

)

À 0.4 (SiMe

29

Si

NMR

3.8

2.0

1.6

1.6, 5.5

3.6

the experimental data gave an average enthalpy value of

O-(trimethylsilyl)carbamatobenzene (1a)

À 76.7+/À 5.2 kJ/mol for the insertion reaction at a pressure of

Using the small autoclave, aminosilane 1 (0.80 g, 4.84 mmol) was

4 bar CO using 54 mg (0.33 mmol) and 99 mg (0.60 mmol) amino-

2

dissolved in dry THF (2 mL) and exposed to 4 bar of CO₂for

silane, respectively.

2

4 hours without stirring. After transferring the pale yellow liquid to

a Schlenk flask and removing the solvent under reduced pressure, a

white solid was obtained. Yield: 0.82 g (3.92 mmol, 81%). Recrystal-

lization from n-pentane afforded colorless needles suitable for

single-crystal X-ray analysis. Data of crystal structure determination

ω-[O-(trimethylsilyl)carbamato]toluene (2a)

The aminosilane 2 (2.69 g, 15.00 mmol) was dissolved in dry THF

1

(50 mL). CO insertion reaction was performed by gas bubbling

2

are contained in the Supporting Information. H NMR (400 MHz,

twice for 15 minutes at room temperature. The gas was dried prior

to use by gas bubbling through pure sulfuric acid. After separating

the solvent, the product was obtained as a white solid. Yield: 2.50 g

THF-d8): δ 7.46–6.92 (m, 5H, ArH), 6.58 (s, 1H, NH), 0.30 (s, 9H,

13

SiMe₃). C NMR (101 MHz, THF-d8): δ 153.0 (CO), 140.8 (ArC), 129.4

29

(

ArCH), 123.1 (ArCH), 118.9 (ArCH), 0.1 (SiMe₃). Si NMR (79 MHz,

[7]

1

(

11.19 mmol, 75%). H NMR (400 MHz, CDCl ): δ 7.37–7.31 (m, 5H,

3

13

THF-d8): δ 22.2.

ArH), 5.62 (s, 1H, NH), 3.89–3.78 (m, 2H, CH ), 0.37 (s, 9H, SiMe ).

C

2

3

For a large-scale reaction in the bigger autoclave, a solution of 1

NMR (101 MHz, CDCl ): δ 155.8 (CO), 155.3 (CO), 143.1 (ArC), 138.8

3

(

7.56 g, 45.73 mmol) in dry THF (20 mL) was exposed to 8 bar of

(ArCH), 128.6–128.5 (ArCH), 127.5–126.8 (ArCH), 45.1 (CH ), À 0.03

2

9

CO₂for 24 h. Following the described procedure led to a pale-

yellow solid, which was dried under reduced pressure for 3 h. Yield:

8

(SiMe ). Si NMR (79 MHz, CDCl ): δ 23.6, 22.6.

3 3

Insertion reaction using the big autoclave afforded the pure

product in higher yields. A solution of the aminosilane 2 (6.78 g,

.62 g (41.18 mmol, 90%).

EA: calculated N 6.69%, C 57.38%, H 7.22%; found N 6.79%, C

37.83 mmol) in THF (20 mL) was charged with 8 bar CO for 1 hour.

2

5

7.14%, H 7.053%.

By removal of the solvent 2a (7.90 g, 35.35 mmol, 93%) was gained

as a white solid. NMR data confirmed the identity and purity of the

product.

Calorimetric measurements

EA: calculated N 6.27%, C 59.16%, H 7.67%; found N 6.81%, C

59.02%, H 7.408%.

Calorimetric measurements using the aminosilane 1 were per-

formed on a Setaram C80 instrument with a 9.5 cm stainless steel

3

vessel. Measurements were conducted isothermal at 30°C in a

pressure range from vacuum to 5 bar. Prior to measurements the

vessel was evacuated for several hours. A sample of 50 to 100 mg

pure aminosilane was inserted into the vessel in an argon

atmosphere under ambient conditions. Carbon dioxide (from Linde,

purity of 5.3) was provided by a pressurised tank. Regulation of the

gas exposition was ensured by a stopcock separating the vessel

and the gas tank. After inserting the deflated vessel into the

instrument, equilibration of the setup was obtained before starting

4-[O-(trimethylsilyl)carbamato]methoxybenzene (3a)

A mixture of 3 (6.50 g, 33.28 mmol) and DBU (0.1 mL) in dry THF

(

20 mL) was exposed to 8 bar of CO for 24 hours without stirring.

2

After transferring the orange liquid to a Schlenk flask and removing

the solvent under reduced pressure, a brownish solid was obtained

(

7.23 g, 30.21 mmol, 91%). Recrystallization from mother liquor

afforded pale brown, needle-shaped crystals, which were used for

unit cell determination by X-ray diffraction. H NMR (400 MHz,

CDCl ): δ 8.15 (s, 1H, 3a, NH), 7.28 (d, J=8.9 Hz, 2H, 3a, ArH), 6.71

1

the measurement and loading the vessel with CO . The time period

2

3

of the measurement varied between 18 to 21 hours. Evaluation of

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3

13

(

d, J=9.0 Hz, 2H, 3a, ArH), 6.59 (d, J=8.9 Hz, 2H, 3, ArH), 6.48 (d,

(s, 9H, SiMe ), 0.26 (s, 9H, SiMe ). C NMR (101 MHz, CDCl ): δ 155.8

3

13

J=8.9 Hz, 2H, 3, ArH), 3.59 (s, 1H, 3, NH), 0.23 (s, 9H, SiMe₃).

C

(CO), 155.7 (CO), 146.4 (ArC), 130.7 (ArCH), 129.0 (ArCH), 124.3

2

9

NMR (101 MHz, CDCl ): δ 155.8 (CO), 153.0 (ArCÀ O), 141.3 (3,

(ArCH), 116.8 (ArCH), 115.4 (ArCH), 43.0 (CH ), À 0.0 (SiMe ). Si NMR

3

2

3

ArCÀ N), 132.6 (ArCÀ N), 120.3 (ArCH), 117.2 (ArCH), 116.2 (ArCH),

(79 MHz, CDCl ): δ 23.3 (OÀ SiMe ), 2.8 (NÀ SiMe ).

3

2

9

1

14.9 (ArCH), 114.2 (ArCH), 55.3 (OMe), 0.0 (SiMe ). Si NMR

3

(

79 MHz, CDCl ): δ 22.6 (OÀ SiMe ), 7.3 (Me SiOSiMe ), 1.7 (3, NH-

3

SiMe ).

1,8-bis(trimethylsilylamino)naphthalene (8)

3

No reaction observed during exposure to 8 bar CO for 24 hours.

2

1

,2-bis[O-(trimethylsilyl)carbamato]benzene (4a)

A mixture of 8 (2 mL) in dry THF (10 mL) was exposed to 8 bar CO₂

pressure for 72 h. After transferring the deep red liquid to a Schlenk

The aminosilane 4 (0.51 g, 2.02 mmol) was dissolved in dry THF

1

flask an NMR sample of the crude solution was analyzed. H NMR

(

2 mL) and exposed to 6 bar of CO for 24 hours at room temper-

2

13

(

400 MHz, CDCl ): numerous peaks. C NMR (101 MHz, CDCl₃):

3

ature without stirring. Eliminating the solvent under reduced

2

9

numerous peaks. Si NMR (79 MHz, CDCl ): δ 24.9 (8b, OÀ Si), 23.8

3

pressure afforded a highly viscous liquid with a yellow-brownish

1

(8a, OÀ Si), 23.6 (8b, OÀ Si), 22.8 (8a, OÀ Si), 13.8 (8b, NÀ Si), 13.0 (8b,

color. Yield: 0.60 g (1.76 mmol, 87%). H NMR (400 MHz, CDCl ): δ

3

1

3

NÀ Si), 7.2 (HMDSO), 3.3 (8), 1.2 (8).

7

.38–6.58 (m, 4H, ArH), 3.32 (m, 2H, NH), 0.25 (s, 18H, SiMe₃).

C

NMR (101 MHz, CDCl ): δ 153.3 (CO), 126.1 (ArC), 124.6 (ArCH), 120.0

3

Removal of solvent under reduced pressure for reducing the

volume of the solution to ca. 50% and additional storage in the

refrigerator led to the formation of white crystal blocks, which

appeared red due to adherent mother liquor (*). Single-crystal X-ray

structure determination revealed the structure of 1H-perimidin-

29

(

^A^r^C^H⁾^,⁰^.²⁽^Sⁱ^M^e3⁾^. Si NMR (79 MHz, CDCl ): δ 24.5.

3

1

,4-bis[O-(trimethylsilyl)carbamato]benzene (5a)

2

(3H)-one.

A solution of 5 (9.68 g, 38.33 mmol) in dry THF (20 mL) was exposed

to 8 bar of CO for 24 hours at room temperature without stirring.

1

2

H NMR (400 MHz, CDCl ): δ 9.45 (s, 2H, NH), 7.16–7.00 (m, 4H, ArH),

3

The formation of a yellow precipitate inside a brownish liquid was

observed during opening of the autoclave. The reaction mixture

along with the precipitate was transferred into a Schlenk flask with

the aid of 3×10 mL of dry THF. Removal of the solvent under

reduced pressure afforded a yellow solid, which was dried in a

3

6

.45 (dd, 2H, J=7.3 Hz, ArH), 5.05 (s, 1H, NH, 8), 3.59 (THF), 1.73

13

(THF), 0.12 (s, 8), 0.01 (TMS). C NMR (101 MHz, CDCl ): δ 151.5

3

(

1

CO), 147.0 (*), 138.9 (ArCÀ N), 135.8 (ArC), 128.6 (ArCH), 126.6 (*),

18.9 (ArCH), 115.2 (ArC), 111.2 (*), 104.8 (ArCH), 67.4 (THF), 25.2

(THF).

1

dynamic vacuum for 2 hours. Yield: 12.26 g (36.00 mmol, 94%). H

EA of 1H-perimidin-2(3H)-one: calculated N 15.21%, C 71.73%, H

4

NMR (400 MHz, CDCl ): δ 6.57 (m, 4H, ArH), 3.35 (m, 2H, NH), 0.34 (s,

3

13

.38%; found N 15.39%, C 72.19%, H 4.538%.

1

2

8H, SiMe₃). C NMR (101 MHz, CDCl ): δ 152.2 (CO), 138.7 (ArC),

3

29

19.4 (ArCH), 116.8 (ArCH), 0.0 (SiMe₃). Si NMR (79 MHz, CDCl ): δ

3

4.5, 23.9.

Thermal decomposition of 1a

a (1.12 g, 5.35 mmol, white solid) was placed in a small Schlenk

EA: calculated N 8.23%, C 49.38%, H 7.10%; found N 8.96%, C

0.30%, H 7.09%.

1

5

flask and attached to a short-path distillation apparatus under inert

conditions. The thermolysis experiment was performed under

reduced pressure with constant stirring and heating in a silicone oil

bath to a temperature of approximately 190°C. A safety flask cooled

(

₆+_a/₎À )-trans-1,2-bis[O-(trimethylsilyl)carbamato]cyclohexane

with liquid nitrogen was installed between the still and the Schlenk

line. The formation of a white solid in the neck of the flask, joint of

the still and along the water-cooled cold finger was observed after

Exposing a solution of 6 (0.98 g, 3.79 mmol) in dry THF (2 mL) to

^b^a^r^o^f^C^O2 to for 24 h at room temperature without stirring

6

afforded a white solid (after removal of the solvent under reduced

1

0 minutes. The temperature at the thermometer in the head of

1

pressure). Yield: 0.90 g (2.60 mmol, 67%). H NMR (400 MHz, CDCl ):

numerous peaks. C NMR (101 MHz, CDCl ): δ 155.7 (CO), 55.5 (ring

CH), 32.6 (ring CH ), 24.8 (ring CH ), 0.1 (SiMe ). Si NMR (79 MHz,

CDCl ): δ 22.7.

°

3

the still remained at 25 C. The heating was stopped, parts of the

white solid were removed for NMR analysis and the heating was

resumed afterwards. Shortly after, the collection of a colorless liquid

at the cold finger was observed. 20 minutes after beginning of

heating the solid starting material was fully molten and white

crystals formed in the joint between Schlenk flask and still. After

13

3

29

2

3

2

(

-trimethylsilylamino-ω-[O-(trimethylsilyl)carbamato]toluene

7a)

30 minutes the heating was stopped and the device was allowed to

attain room temperature. Afterwards, NMR samples were taken

from the cold finger, the safety flask and the residue. Analysis

results are summarized in Table 3.

A solution of aminosilane 7 (0.55 g, 2.02 mmol) in dry THF (2 mL)

was exposed to 6 bar of CO₂for 24 hours at room temperature

without stirring. Afterwards, removal of the solvent under reduced

pressure afforded a highly viscous, yellow liquid. Yield: 0.57 g

N,O-bis(trimethylsilyl)carbamatobenzene (1b)

(1.80 mmol, 89%).

The O-silylated carbamate 1a (6.87 g, 32.82 mmol) was dissolved in

Storage in the refrigerator gave needle-shaped crystals after several

days. Analysis of the crystals by single-crystal X-ray diffraction

revealed the crystal structure of the target compound. Data is

collected in Table 7.

dry n-pentane (100 mL) and mixed with Et N (3.40 g, 33.64 mmol)

3

under constant stirring. While cooling with an ice bath, trimeth-

ylsilyltriflate (7.28 g, 32.73 mmol) was added dropwise, forming a

cloudy precipitate, which settles on the bottom of the flask when

stirring is interrupted. After 45 minutes of stirring under ice cooling

the stirring was continued for another 2 hours at room temper-

ature. Afterwards, the precipitate was frozen in a mixture of dry ice

1

3

H NMR (400 MHz, CDCl ): δ 7.07 (dt, 1H, J=7.7 Hz, ArH), 6.96 (dd,

3

1

H, J=7.4 Hz, ArH), 6.76 (dd, 1H, J=8.0 Hz, ArH), 6.58 (dt, 1H, J=

7

.3 Hz, ArH), 5.48 (s, 1H, NH), 4.65 (s, 1H, NH), 4.18 (m, 2H, CH ), 0.33

2

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Table 3. NMR analysis data of the thermal decomposition of 1a.

10 min

CDCl

1

a

residue

CDCl

cold finger

CDCl

safety flask

CDCl

NMR

CDCl

3

solv.

1

H NMR 7.38–7.05 (m, 5H, ArH),

numerous peaks

6

9

.65 (s, 1H, NH), 0.34 (s,

H, SiMe ).

3

13

C NMR 153.0 (CO),

146.4 (1a),

138.1 (1a),

129.3 (1a),

129.0 (1a),

123.4 (1a),

118.6 (1a),

157.2, 152.2, 141.5, 138.2,

129.1, 128.7, 126.5, 123.4,

118.6, 116.0, 115.2, 2.0, 0.0 116.0, 115.1, 1.9, 0.0

146.4, 138.1, 129.3, 129.0,

123.4, 120.7, 118.5, 117.4,

146.4, 138.2, 129.3, 129.0,

123.4, 118.5, 115.1, 68.0

(THF), 25.6 (THF), 1.9

(HMDSO), 0.0 (TMS).

1

0

40.8 (ArC),

29.4 (ArCH),

23.1 (ArCH),

18.9 (ArCH),

3

.1 (SiMe ).

1

0

15.1 (1a),

.9 (HMDSO),

.0 (1a, TMS).

29

Si

NMR

22.2

Inept: 24.7 (1a), 7.3

(HMDSO), 0.0 (TMS)

24.5 (1a), 23.5, 10.7, 7.4

(HMDSO), 2.8, 0.0 (TMS)

24.6 (1a), 7.4 (HMDSO), 0.0 24.6 (1a), 7.4 (HMDSO), 0.0

(TMS) (TMS)

and isopropanol, and the liquid supernatant was separated by

decantation. The precipitate fraction was washed with 20 ml of n-

pentane. From the combined supernatant and washing, the solvent

was removed under reduced pressure to yield a white solid, which

was dried in a dynamic vacuum for a period of 2 hours to afford

intensive yellow. After cool down to room temperature the

fractions were transferred into Schlenk flasks and samples for NMR

spectroscopic measurements were taken. Due to the numerous

1

3

peaks in the C NMR spectra of the residue and the intermediate

2

9

fraction, only the associated Si NMR data are listed in Table 4.

7

.69 g (27.32 mmol, 83%) of 1b. Recrystallization from dry THF

gave crystals suitable for single-crystal X-ray structure determina-

tion, which revealed the crystal structure of 1b. Data is collected in

Table 7.

ω-[N,O-bis(trimethylsilyl)carbamato]toluene (2b)

A solution of the O-silylated carbamate 2a (30.33 g, 135.80 mmol)

1

H NMR (400 MHz, CDCl ): δ 7.11–6.86 (m, 5H, ArH), 0.17 (s, 9H,

in dry diethylether (100 mL) was mixed with Et N (14.48 g,

3

13

SiMe ), 0.00 (s, 9H, SiMe ). C NMR (101 MHz, CDCl ): δ 157.0 (CO),

143.07 mmol) under constant stirring and cooling with an ice bath.

Trimethylsilyltriflate (32.62 g, 146.76 mmol) was added dropwise,

leading to the formation of a cloudy precipitate, which settles on

the bottom of the flask when the stirring is interrupted. The stirring

was continued at room temperature until the next day. Afterwards,

the precipitate was frozen in a mixture of dry ice and isopropanol,

and the liquid supernatant was separated by decantation. The

solvent was removed from the supernatant under reduced pressure

to yield 2b (37.58 g, 127.17 mmol, 94%) as a yellow, viscous liquid.

3

1

41.4 (ArC), 128.7 (ArCH), 128.5 (ArCH), 126.3 (ArCH), 0.5 (SiMe ),

3

29

À 0.2 (SiMe ). Si NMR (79 MHz, CDCl ): δ 23.4 (OÀ SiMe ), 10.7

3

(NÀ SiMe ).

3

EA: calculated N 4.98%, C 55.47%, H 8.24%; found N 5.07%, C

5.01%, H 8.215%.

5

Thermal decomposition of 1b

1

H NMR (400 MHz, CDCl ): δ 7.18 (m, 5H, ArH), 4.41 (s, 2H, CH ), 0.33

3

2

A sample of 1b (approx. 1 g) was transferred into the bottom ball

of the three-parted Kugelrohr (ball tube) and connected to the

Kugelrohr distillation device under inert conditions. The bottom

ball (residue) and the intermediate fraction were placed inside the

glass oven, while the outer bulb was cooled by water. The

thermolysis experiment was performed under ambient pressure

and with constant rotation of 20 rpm. The oven was heated to

13

(s, 9H, SiMe ), 0.19 (s, 9H, SiMe ), 0.09 (s, 18H, HMDSO). C NMR

3

(

101 MHz, CDCl ): δ 158.2 (CO), 140.4 (ArC), 128.4 (ArCH), 126.6

3

(ArCH), 126.4 (ArCH), 48.1 (CH ), 2.0 (HMDSO), 0.8 (SiMe ), 0.0

2

3

29

(

7

SiMe₃). Si NMR (79 MHz, CDCl ): δ 22.6 (OÀ SiMe ), 10.9 (NÀ SiMe ),

3

.1 (HMDSO).

1

70°C, to 200°C and to 230°C for 30 minutes, respectively. At a

Thermal decomposition of 2b

temperature of approx. 100°C melting of the white solid com-

menced and was completed at around 140°C, thus forming a pale

yellow liquid. At 200°C, condensation of a colorless liquid in the

middle bulb was observed. Up to a temperature of 230°C colorless

liquid reached the outer bulb as well, while the residue turned an

A sample of 2b (3.13 g, 10.60 mmol) was placed in the bottom ball

of the three-parted Kugelrohr (ball tube) and connected to the

Kugelrohr distillation device under inert conditions. The bottom

ball (residue) and the intermediate fraction were stored inside the

Table 4. NMR analysis data of the thermal decomposition of 1b.

1

b

residue

intermediate fraction

CDCl

outer fraction

CDCl

134.0 (ArC), 129.7 (ArCH), 125.8

(ArCH), 124.9 (ArCH), 120.7

(NCO), 2.0 (HMDSO)

NMR solv. CDCl

3

CDCl

3

1

3

C NMR

157.0 (CO),

numerous peaks

1

(

0

41.4 (ArC), 128.7 (ArCH), 128.5

ArCH), 126.3 (ArCH),

.5 (SiMe ),

À 0.2 (SiMe

23.4 (OÀ SiMe

3

).

29

Si NMR

3

), 10.7 (NÀ SiMe

3

)

10.8, 8.6, 7.4 (HMDSO), 4.5

23.4 (1b), 14.0, 12.5, 10.7 (1b),

7.1 (HMDSO)

10.1, 7.3 (HMDSO), 4.5

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Table 5. NMR analysis data of the thermal decomposition of 2b in a three-parted Kugelrohr with stepwise heating.

2

b

residue

intermediate fraction

outer fraction

CDCl

NMR solv. CDCl

3

CDCl

3

CDCl

3

13

C NMR

158.2 (CO),

numerous peaks

144.1,

1

4

0

40.4 (ArC),

137.0 (ArC),

128.7 (ArCH),

127.8 (ArCH),

126.6 (ArCH),

120.7 (NCO),

46.4 (CH ),

2

1.7 (HMDSO)

28.4 (ArCH),

26.6 (ArCH),

26.4 (ArCH),

2

8.1 (CH ),

.8 (SiMe

.0 (SiMe

3

),

).

3

29

Si NMR

22.6 (OÀ SiMe

3

), 10.9 (NÀ SiMe

3

)

22.3, 15.9, 7.4 (HMDSO), À 21.8

29.6, 27.5, 22.7 (2b), 19.2, 11.0

7.3 (HMDSO)

(

2b), 9.8, 7.1 (HMDSO), 3.3, 2.2

glass oven and the outer bulb was cooled by water. The thermolysis

experiment was performed under ambient pressure and with

constant rotation of 20 rpm. For a period of 45 minutes, the

apparatus was tilted at a 45-degree angle to allow some reflux. The

oven was heated to 200°C, to 250°C and to 300°C for 30 minutes,

each. Beginning at a temperature of approx. 250°C the collection of

a dark brown color and increased in viscosity. After cool down to

room temperature the fractions were transferred into Schlenk flasks

and samples for NMR spectroscopic measurements were taken. Due

1

3

to the numerous peaks in the C NMR spectra of the residue and

2

9

the intermediate fraction, only the associated Si NMR data are

2

9

listed in Table 5. The overall intensity of the signals in the Si NMR

spectrum of the residue is very low resulting in a bad signal-to-

noise ratio.

a clear, colorless liquid in the outer bulb was observed. With the

increased temperature of 300°C the residue progressively adopted

Table 6. NMR analysis data of the thermal decomposition of 2b in a three-parted Kugelrohr with direct heating to 300°C.

2

b

residue

intermediate fraction

outer fraction

CDCl

137.9 (toluene, ArC),

129.2 (toluene, ArCH),

128.4 (toluene, ArCH),

125.5 (toluene, ArCH),

21.5 (toluene, CH3),

2.0 (HMDSO),

NMR solv. CDCl

3

CDCl

numerous peaks

3

CDCl

numerous peaks

3

13

C NMR

158.2 (CO),

1

4

0

40.4 (ArC),

28.4 (ArCH),

26.6 (ArCH),

26.4 (ArCH),

2

8.1 (CH ),

.8 (SiMe

.0 (SiMe

3

),

).

0.8 (Me

3

SiNCO)

3

29

Si NMR

22.6 (OÀ SiMe

3

), 10.9 (NÀ SiMe

3

)

16.9, 7.4 (HMDSO), 4.4,

À 21.8

7.4 (HMDSO), À 21.9

7.3 (HMDSO), 4.2 (Me SiNCO)

3

Table 7. Crystallographic and structure refinement data for compounds 1b and 7a.

N,O-bis(trimethylsilyl) carbamatobenzene (1b)

2-trimethylsilylamino-ω-[O (trimethylsilyl)carbamato]

toluene (7a)

empirical formula

formula weight

temperature [K]

crystal system

space group

a [Å]

C

13

H

23NO

2

Si

2

14 26 2 2 2

C H N O Si

310.55

180(2)

Triclinic

P-1

10.9652(4)

14.3342(5)

18.7251(6)

92.844(3)

101.678(3)

101.501(3)

281.50

160(2)

Monoclinic

P2 /c

1

13.1210(7)

7.3166(2)

17.6436(10)

90

90.266(5)

90

1693.79(14)

4

1.104

0.205

b [Å]

c [Å]

α [°]

β [°]

γ [°]

3

volume [Å ]

2811.93(17)

6

1.100

0.192

Z

3

1

calc [mg/cm ]

À 1

absorption coefficient [mm

F(000)

]

608

1008

reflections collected/unique

29493/3694

R(int)=0.0313]

27102/9887

[R(int)=0.0414]

[

data/restraints/parameters

goodness-of-fit on F

3694/0/169

1.057

9887/7/600

1.046

2

Final R indices [I>2sigma(I)]

R indices (all data)

extinction coefficient

R1=0.0364, wR2=0.0916

R1=0.0491, wR2=0.1000

n/a

0.277 and À 0.305

R1=0.0483, wR2=0.1125

R1=0.0759, wR2=0.1238

0.0038(6)

0.534 and À 0.345

.

À 3

largest diff. peak and hole [eÅ

]

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Table 8. Data of unit cell determination and structure refinement for compounds 1a, perimidone, two modifications of N-(trimethylsilyl)

carbamatocyclohexane and di(N-phenylcarbamoyloxy)dimethylsilane.

N-(trimethylsilyl) carbamato- perimidone

benzene (1a)

N-(trimethylsilyl)

carbamato-

cyclohexane

N-(trimethylsilyl)

carbamato-

cyclohexane

di(N-phenyl carbamoyloxy)

dimethylsilane

empirical formula

formula weight

temperature [K]

crystal system

space group

a [Å]

C

10

H

15NO

2

Si

C

11

H

8

N

2

O

C

10

H

21NO

2

Si

C

10

H

21NO

2

Si

16 18 2 4

C H N O Si

330.41

150(2)

209.32

200(2)

184.19

180(2)

Monoclinic

P2 /c

14.0569(16)

4.5923(3)

13.1708(16)

99.505(9)

838.55(15)

4

215.37

145(2)

Monoclinic

P2 /c

11.1378(6)

12.3898(5)

9.4183(5)

97.513(4)

1288.52(11)

4

215.37

180(2)

Monoclinic

P2 /c

11.0607(6)

12.4051(9)

9.7141(5)

100.490(4)

1310.59(14)

4

Orthorhombic

Pbca

9.5592(4)

19.3737(11)

12.2437(6)

90

2267.50(19)

8

1.226

Orthorhombic

Pca2

1

27.9900(19)

5.1720(3)

11.0443(11)

90

1598.8(2)

4

b [Å]

c [Å]

β [°]

3

volume [Å ]

Z

3

1

calc [mg/cm ]

1.459

0.097

1.110

0.162

1.091

0.160

1.373

0.169

À 1

absorption coefficient [mm

]

0.183

F(000)

896

15031/2219

384

472

696

reflections collected/unique

data/restraints/parameters

12857/1824

[R(int)=0.0466]

1824/0/135

1.042

20134/3113

[R(int)=0.0480]

3113/0/134

1.084

13105/3159

[R(int)=0.0279]

3159/7/201

1.044

16379/3711

[R(int)=0.0470]

3711/1/218

1.054

[

R(int)=0.0405]

2219/0/134

1.064

2

goodness-of-fit on F

final R indices [I>2sigma(I)]

R1=0.0347,

wR2=0.0810

R1=0.0516,

wR2=0.0908

0.210 and À 0.274

R1=0.0370,

wR2=0.0918

R1=0.0558,

wR2=0.1006

R1=0.0375,

wR2=0.0884

R1=0.0490,

wR2=0.0953

R1=0.0400,

wR2=0.1041

R1=0.0503,

wR2=0.1114

R1=0.0398,

wR2=0.0960

R1=0.0580,

wR2=0.1052

R indices (all data)

.

À 3

largest diff. peak and hole [eÅ

]

0.226 and À 0.184 0.304 and À 0.248 0.263 and À 0.262 0.375 and À 0.311

Direct heating of 2b (8.38 g, 28.36 mmol) to 300°C in a three-

parted Kugelrohr for 2.5 hours resulted in the formation of toluene

instead of the desired benzyl isocyanate. The experiment was

carried out under reflux for 1 hour by tilting the apparatus to a 45-

degree angle before moving back to a horizontal orientation for

another 1.5 hours. While the residue turned brown and highly

viscous, a colorless liquid with a white precipitate accumulated

inside the outer bulb. After cool down to room temperature the

fractions were transferred into Schlenk flasks using CDCl₃for

washing. NMR spectroscopic measurements were performed after-

Acknowledgements

The authors are grateful to B. Kutzner for NMR measurements.

Additional thanks go to G. Walter (Institut für Physikalische

Chemie) for calorimetric measurements as well as J. Görbing for

collection of experimental data within the framework of her B.Sc.

thesis. This work was executed within the research group

‘

‘Chemical utilization of carbon dioxide with aminosilanes

13

wards. Due to the numerous peaks in the C NMR spectra of the

(CO2Sil)” with financial support of the European Union (European

regional development fund), the Ministry of Science and Art of

Saxony (SMWK) and the Sächsische Aufbaubank (SAB). Open

access funding enabled and organized by Projekt DEAL.

2

9

residue and the intermediate fraction, only the associated Si NMR

data are listed in Table 6. The overall intensity of the signals in the

29

Si NMR spectrum of the residue is very low resulting in a bad

signal-to-noise ratio.

Crystal structure data

Conflict of Interest

Table 7 and Table 8 contain crystal structure data for all com-

pounds, which are mentioned in this manuscript and were analyzed

by X-ray diffraction analysis.

The authors declare no conflict of interest.

Deposition Numbers 2060544 (for 1a ), 2060545 (for di(N -phenyl-

carbamoyloxy)dimethylsilane), 2060546 (for perimidone), 2060547

Keywords: Carbon dioxide · Insertion · Isocyanates · Silanes ·

Silyl amines

(for 1b ), 2060548 (for 7a ), 2060549 (for N-(trimethylsilyl)

carbamatocyclohexane at 180 K), and 2060550 (for N-(trimethylsilyl)

carbamatocyclohexane at 145 K) contain the supplementary crys-

tallographic data for this paper. These data are provided free of

charge by the joint Cambridge Crystallographic Data Centre and

Fachinformationszentrum Karlsruhe Access Structures service

www.ccdc.cam.ac.uk/structures.

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