Synthesis and kinetics of disassembly for silyl-containing ethoxycarbonyls using fluoride ions

Article abstract of DOI:10.1039/c7ra07876e

In this study, a series of silyl-containing ethoxycarbonates and ethoxycarbamates on electron poor anilines and phenols were synthesized and their kinetics of disassembly determined in real-time upon exposure to fluoride ion sources at room temperature. The results provide a greater understanding of stability and kinetics for silyl-containing protecting groups that eliminate volatile molecules upon removal, which will allow for simplification of orthogonal protection in complex organic molecules.

Full text of DOI:10.1039/c7ra07876e

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Synthesis and kinetics of disassembly for silyl-
containing ethoxycarbonyls using fluoride ions†
Cite this: RSC Adv., 2018, 8, 1884
b
b
Eugene Camerino,a Grant C. Daniels,b James H. Wynne
and Erick B. Iezzi
*
In this study, a series of silyl-containing ethoxycarbonates and ethoxycarbamates on electron poor anilines
and phenols were synthesized and their kinetics of disassembly determined in real-time upon exposure to
fluoride ion sources at room temperature. The results provide a greater understanding of stability and
kinetics for silyl-containing protecting groups that eliminate volatile molecules upon removal, which will
allow for simplification of orthogonal protection in complex organic molecules.
Received 17th July 2017
Accepted 22nd December 2017
DOI: 10.1039/c7ra07876e
rsc.li/rsc-advances
Silicon-based groups have demonstrated wide utility in organic kinetics compared to TEOC. As a result, we aimed to generate
syntheses, ranging from orthogonal protection of amine, quantitative kinetic data for the removal of TEOC and other
hydroxyl and acid functionalities in complex drug molecules1–3 silyl-containing ethoxycarbonyl groups from functionality by
to utilization as a trigger in self-immolative polymers and performing disassembly experiments with uoride ion sources
dendrimers.4–6 One of the commonly used silicon-based pro- at room temperature, as this information is relevant to those
tecting groups is 2-(trimethylsilyl)ethoxycarbonyl (TEOC),7 utilizing orthogonal protecting group chemistry in multi-step
which was originally developed and later optimized for N- synthesis and potentially in the processing of pharmaceuticals.
protection of amino acids to prevent formation of oligopep-
Herein, we report the kinetics of disassembly (i.e., rate
tides, but has since been utilized for protection of other func- constants (k) for removal) for a series of silyl-terminated
tionalities.8 The uniqueness of TEOC is that it can be ethoxycarbonyl protecting groups, such as TEOC, 2-(methyl-
preferentially removed with a mild de-blocking agent, such as diphenylsilyl)ethoxycarbonyl (MDPSEOC), and several extended
uoride ion, in the presence of other base-sensitive groups to chain versions of these molecules, from electron poor hydroxyl
produce low molecular weight and volatile by-products via and amine functionalities. To do this, we utilized non-
cascading bond cleavage, thereby easing purication of substituted and para-substituted phenols and anilines, as
complex mixtures and enabling greater product yields.
these molecules provided a UV-visible chromophore for real-
During our course of utilizing TEOC to protect amine and time detection during removal of the protecting groups. We
hydroxyl functionality on aliphatic oligomers we discovered also synthesized and determined the kinetics of disassembly for
there was a severe lack of kinetic data in the literature for several novel silyl-centered bis(ethoxycarbonyls), as we were
removal of TEOC using uoride ion, as most papers generally interested in studying the reaction of multiple uoride ions
stated the times for removal as occurring within hours with silicon and their inuence on the resulting kinetics.
at elevated temperatures7 to days at room temperature.9,10
Our study of these molecules commenced by attempting to
Furthermore, similar silyl-containing protecting groups, such determine the rate constants of disassembly for 4-nitrophenyl
as (2-phenyl-2-trimethylsilyl)ethoxycarbonyl (PSOC)11 and 2- (2-(trimethylsilyl)ethyl) carbonate (1a), a commercially available
(triphenylsilyl)ethoxycarbonyl (TPSEOC),12 were reported to be control,13 and synthesized 2-(trimethylsilyl)ethyl (4-nitrophenyl)
removed under milder uoride ion conditions and/or faster carbamate (1b), using solutions of different uoride salts in
times than TEOC due to a more readily formed uoro- organic solvents and water. It was initially found that tetrahy-
containing penta-coordinated intermediate as a result of drofuran (THF), dichloromethane, and acetonitrile provided
phenyl group incorporation. However, while these groups were good solubility of these organic molecules, with water and
reported to be removed within minutes to hours at room water/solvent combinations providing limited solubility.
temperature the studies provide minimal details on the actual Furthermore, uoride ion sources, such as tetrabutylammo-
nium uoride trihydrate (TBAF$3H2O) and a 1 : 1 molar ratio of
TBAF/CsF, provided good solubility in the aforementioned
solvents and several solvent/water combinations, whereas
aAmerican Society for Engineering Education, Chemistry Division, U.S. Naval Research
Laboratory, Washington, DC 20375, USA
bChemistry Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA. solubility of cesium uoride (CsF), sodium uoride (NaF),
E-mail: erick.iezzi@nrl.navy.mil
potassium uoride (KF), and stannous uoride (SnF2) were
† Electronic supplementary information (ESI) available: Experimental details,
limited. It should be noted that a 1 : 1 TBAF/CsF mixture in THF
synthetic procedures, characterization data, NMR spectra, UV-visible spectra,
provided nearly identical rate constants of disassembly during
rate plots and GC-MS chromatograms. See DOI: 10.1039/c7ra07876e
1884 | RSC Adv., 2018, 8, 1884–1888
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Table
1
Rate constants of disassembly and percentage of silyl-
initial studies as TBAF in THF. However, to maintain uoride
ion solution consistency it was determined that all kinetic
experiments would be conducted in THF using commercially
available TBAF in THF.
terminated carbonates and carbamates remaining once treated with
TBAF under non-stirred conditions
Approx. %
remaining
k (MÀ1 minÀ1
2196 Æ 1380
)
Kinetic experiments for 1a and 1b were conducted as 0.1 mM
solutions in THF using 1 ml cuvettes and an Agilent 8453 UV-
Visible Spectrophotometer. To initiate disassembly, 4 molar
equivalents of 1.0 M TBAF in THF were added separately to 1a
and 1b at room temperature (Scheme 1), then the absorbance of
the solutions were recorded up to 120 minutes. The cuvettes
were not stirred upon addition of the uoride salt solution in
effort to prevent an increase in the rate of disassembly due to
turbidity. Silyl-terminated carbonate 1a, which has a maximum
absorbance (lmax) at 270 nm, decreased within minutes as a new
absorbance at lmax of 423 nm appeared, corresponding to
4-nitrophenolate (2a).14 Ethylene and carbon dioxide were
detected via gas chromatography/mass spectrometry (GC-MS),
thereby supporting the proposed mechanism of disassembly
as shown in Scheme 1.7 The rate constant of disassembly for 1a
was found to be 2196 Æ 1380 MÀ1 minÀ1 for the second order
reaction (Table 1). For silyl-terminated carbamate 1b, addition
Entry
Structure
1a
0
1b
1c
38.5 Æ 0.3
355 Æ 28
68
41
1d
77.1 Æ 0.2
27
of 1.0 M TBAF in THF resulted in rapid formation of an provided a basis for larger aliphatic groups, which are discussed
absorbance at lmax of 444 nm, which corresponded to 4-nitro- in subsequent paragraphs.
phenylamide (2b),15 although this peak quickly decreased as
The rate constants of 1c and 1d were found to be 355 Æ
a new absorbance at lmax of 366 nm was formed. The latter 28 MÀ1 minÀ1 and 77.1 Æ 0.2 MÀ1 minÀ1 (Table 1), respectively,
corresponded to 4-nitroaniline (2c),16 the conjugate acid of 2b, with 1d rst forming 4-nitrophenylamide (2b) at 444 nm before
and is proposed to have resulted from beta-elimination of protonating to form 4-nitroaniline (2c) at 366 nm. These results
a proton on the tetrabutylammonium cation, thereby forming are consistent with the greater electron withdrawing nature of
tributylamine and 1-butene as side products via a Hofmann substituted phenols compared to anilines, in addition to the
elimination.17 1H-NMR and GC-MS analysis of the solution aer reported increase in electrophilicity of the silicon atom, and
disassembly supports this hypothesis. The rate constant of hence faster removal, due to phenyl group attachment.12
disassembly for 1b was determined to be 38.5 Æ 0.3 MÀ1 minÀ1
,
However, the rate constant of 1c was slower than 1a, which
and the slower rate compared to 1a is consistent with contradicted this rationale. To address the issue, we synthe-
substituted anilines possessing a larger pKa value than sized 1a and compared it to the rate constant of disassembly for
substituted phenols. 2-(methyldiphenylsilyl)ethyl (4-nitro- the commercially available version, yet we found no change in
phenyl) carbonate 1c and 2-(methyldiphenylsilyl)ethyl (4-nitro- the results. Aer viewing the rst few seconds of the absorbance
phenyl)carbamate 1d were subsequently synthesized and their spectra we noticed the absorbance of 1a was decreasing slightly,
rate constants of disassembly determined. Although similar to even before addition of TBAF, which signied molecule insta-
the triphenylsilyl and 2-phenyl-2-trimethylsilyl ethoxycarbonyl bility at room temperature. This resulted in a faster rate
protecting groups reported in the literature,11,12 we chose the constant compared to 1b when calculated via Excel plot. We
methyldiphenylsilyl group because it was a hybrid of the currently do not have the ability to study the kinetics at reduced
aforementioned groups, the kinetics of phenyl-containing silyl temperatures, but the instability supports literature examples
groups had not been reported, and the lone methyl group for the use of 1a at 0 ꢀC to protect organic functionality. On the
contrary, 1c was stable at room temperature for weeks, and thus
may be useful as an alternative protecting group for orthogonal
chemistry.
Once changes in the UV-visible spectra no longer appeared to
occur, thus signifying that disassembly had slowed or ceased,
the lowest absorbance of the silyl-terminated carbonates and
carbamates was used to calculate the concentration of non-
disassembled starting material. This, in-turn, could be repre-
sented as an approximate percentage of remaining starting
material as shown in Table 1. For instance, 1a completely dis-
assembled within 5 minutes of treatment with 1.0 M TBAF in
THF, whereas about 68% of 1b remained in solution aer
120 minutes. The percentage of 1c remaining aer 16 minutes
was about 41%, whereas about 73% of 1d remained aer
Scheme 1 Products and proposed mechanism of disassembly for
reaction of silyl-terminated carbonates and carbamates with TBAF.
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120 minutes. As discussed in the aforementioned paragraph, with rate constants of 118 711 Æ 14 000 MÀ1 minÀ1 and
the complete disassembly of 1a resulted from an instability at 123 825 Æ 4000 MÀ1 minÀ1, respectively (Table 2). All reactions
room temperature. However, all other molecules likely ceased to generated ethylene, carbon dioxide and a single new absor-
disassemble within the measured time period due to reduced bance at lmax of 423 nm, corresponding to 4-nitrophenolate
collisions with uoride ion as a result of not stirring the solu- (2a), and in all cases the central linker was ejected via intra-
tions. For these examples, the percentage of arene chromo- molecular cyclization to form oxizolidin-2-one (4a) or 3-methyl-
phores (2a–2c) generated during disassembly was equivalent to oxizolidin-2-one (4b), thus supporting the proposed mechanism
the percentage of disassembled starting materials.
shown in Scheme 2 and demonstrating that the rates were
Based on the success with 4-nitrophenol and 4-nitroaniline signicantly enhanced by the entropic increase.18,19 Detection of
as leaving groups we decided to synthesize and investigate the the oxizolidinones were conrmed by GC-MS analysis of the
change in rate constants for silyl-terminated ethoxycarbonates reaction solutions aer disassembly. The rate constants for the
and ethoxycarbamates using alterative substituents, such as molecules containing N-methyl linkages (3b and 3d) were found
4-methoxy, 3-triuoromethyl, 4-methyl, and 4-hydrogen, on to be greater than those with non-N-alkylated linkages (3a and
phenol and aniline. However, in all cases, the absorbance of the 3c), which is consistent with the greater nucleophilicity of the
disassembled phenolate or amide signicantly overlapped with resulting secondary amide ion (i.e., aer loss of CO2) compared
those of the starting materials, thus the rate constants could not to those of primary amine ions. Furthermore, molecules with
be accurately determined via UV-visible spectroscopy.
methyldiphenylsilyl groups (3c and 3d) proved to have faster
We next aimed to determine the rate constants of disas- rates of disassembly than those with trimethylsilyl groups
sembly for silyl-terminated molecules with extended chains (3a– (3a and 3b), which, as previously mentioned, is consistent with
3h), as we were interested to learn if the molecules would fully literature reports for enhanced reactivity at the silicon atom
disassembly and eliminate cyclic oxizolidinones (Scheme 2), upon phenyl group attachment.
and if so, how their rates differed from the non-extended
As shown in Table 2, approximately 13–15% of extended
molecules (1a–1d). We also aimed to conrm whether or not carbonates 3a–3d remained in solution aer monitoring the
these extended molecules would provide an increased rate reaction with TBAF for 10 minutes. Because the UV-vis and GC-
constant of disassembly when phenyl groups were attached to MS data supports the proposed mechanism shown in Scheme 2
the silicon atom. To accomplish this, we developed a synthetic the percentage of 4-nitrophenolate (2a) produced during
route to insert ethanolamine or N-methyl ethanolamine disassembly was likely equivalent to the percentage of starting
between the ethoxycarbonyl and aromatic chromophore material disassembled.
(Scheme S1 of ESI†). Upon addition of 4 molar equivalents of
Following the results of the extended carbonates we decided
1.0 M TBAF in THF, silyl-terminated molecules 3a and 3b to synthesize and determined the rate constants of disassembly
resulted in complete disassembly with rate constants of 42 638 for several silyl-terminated extended chain carbamates (3e–3h,
Æ 1720 MÀ1 minÀ1 and 46 049 Æ 1860 MÀ1 minÀ1, respectively, Schemes 2 and S1 of ESI†). However, when each of these
whereas molecules 3c and 3d resulted in complete disassembly carbamates (lmax at 321 nm) were treated with 4 molar equiva-
lents of 1.0 M TBAF in THF we observed the minor formation of
a species with a similar absorption to 4-nitrophenylamide (2b,
lmax of 445 nm), but we did not observe a peak for 4-nitroaniline
(2c, lmax of 366 nm) or the appearance of any other peaks.
Furthermore, the absorbance peak for the starting materials
never decreased beyond 70%, even aer monitoring the reac-
tion for 24 hours, and treatment with 100 equivalents of TBAF
did not inuence the decrease in the starting materials.
Although extend chain carbamates 3e–3h did not fully disas-
semble, we report the rates for loss of the starting materials
during the rst minutes of exposure to TBAF (Table 2).
To understand why the complete disassembly of 3e–3h
ceased to proceed we performed additional experiments using
3e as the representative carbamate. For instance, aer addition
of TBAF to 3e in THF, GC-MS analysis showed peaks for
oxazolidin-2-one (4a) and 4-nitroaniline (2c) (see ESI†). This
suggested that complete disassembly was possible, although
the elevated temperature at the inlet likely facilitated disas-
sembly. Furthermore, while the addition of TBAF at room
temperature showed a new spot via TLC, we were unable to
purify and analyse the by-product. Thus, we assume that uo-
ride ion attack at room temperature resulted in cleavage of the
Scheme 2 Products and proposed mechanisms of disassembly for
silyl group and elimination of ethylene and carbon dioxide, but
that formation of the oxazolidinone ring did not occur due to
reaction of silyl-terminated extended chain carbonates and carba-
mates with TBAF.
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Table 2 Rate constants of disassembly and percentage of silyl-
terminated extended chain carbonates and carbamates remaining
once treated with TBAF under non-stirred conditions
Approx. %
remaining
Entry Structure
k (MÀ1 minÀ1
)
3a
42 638 Æ 1720
46 049 Æ 1860
16
3b
3c
15
118 711 Æ 14 000 16
Scheme 3 Products and proposed mechanism of disassembly for
reaction of silyl-centered bis(ethoxycarbonyls) with TBAF.
3d
123 825 Æ 4000
70.7 Æ 38.4a
13
53
equivalents of 1.0 M TBAF in THF we detected 4-nitrophenolate
(2b), followed by the formation of 2c, and calculated a rate
constant of disassembly of 967 Æ 14 MÀ1 minÀ1 (Table 3). This
rate was about a 25-fold increase when compared to that of
38.5 MÀ1 minÀ1 for trimethylsilyl-terminated ethoxycarbamate
1b. The signicant increase in rate implied that once the rst
uoride ion bonded with silicon the resulting uorinated silyl
group increased in electrophilicity, thereby increasing the
rate of attack by a second uoride ion as shown for the
proposed mechanism in Scheme 3. Silyl-centered molecule
3e
3f
122 608 Æ 15 654a 34
3g
1134 Æ 787a
38
(diphenylsilanediyl)bis(ethane-2,1-diyl)
bis((4-
nitrophenyl)carbamate) (5b), when treated with 8 molar
equivalents of TBAF in THF, demonstrated a rate constant of
1733 Æ 30 MÀ1 minÀ1, which followed a similar trend for silyl-
terminated molecules where the electrophilicity of silicon was
increased upon phenyl group attachment. It is important to
note that 8 molar equivalents of TBAF were used in order to
maintain a ratio of 4 molar equivalents per UV-detectable end
group. However, when 4 molar equivalents of TBAF were
utilized the rates for 5a and 5b were 50% slower, thus con-
rming the second order reaction. Nevertheless, these rates
were still faster when compared to silyl-terminated molecules
1b and 1d.
3h
244 851 Æ 39 000a 26
a
Rates based on disappearance of starting material within rst
minutes.
the greater thermal barrier to cyclization at the carbamate
linkage (i.e., compared to similar carbonates). As a result,
protonation of the amide anion may have occurred, possibly by
reaction with tetrabutylammonium ion via a Hofmann elimi-
nation. Protonation of the amide anion would also explain why
the starting material never fully decreased, as the by-product
would have a similar absorbance as the starting material.
Akin to the extended chain carbonates, the absorbance of
extended chain carbamates 3e–3h aer 10 minutes could be
used to calculate an approximate percentage of non-
disassembled starting material. However, unlike the carbon-
ates, the carbamates did not completely disassemble, thus the
percentage of generated arene chromophores could not be
determined.
Table 3 Rate constants of disassembly and percentage of silyl-
centered bis(ethoxycarbonates) and bis(ethoxycarbamates) remaining
once treated with TBAF under non-stirred conditions
Approx. %
remaining
Entry Structure
k (MÀ1 minÀ1
967 Æ 14
)
5a
5b
5c
35
34
31
1733 Æ 30
Our focus then turned to the synthesis of novel silyl-centered
molecules, such as bis(ethoxycarbamates) (5a and 5b) and
bis(ethoxycarbonates) (5c and 5d) (Scheme S2 of ESI†).
Upon treatment of (dimethylsilanediyl)bis(ethane-2,1-diyl)
bis((4-nitrophenyl)carbamate) (5a, Scheme 3) with 8 molar
38 358 Æ 2200
5d
404 699 Æ 64 100 16
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In contrast to the bis(ethoxycarbamates), silyl-centered silicon aer addition of the rst uoride ion. These molecules
bis(ethoxycarbonates) (5c and 5d) demonstrated a signicant may have applications in self-immolative materials for detect-
increase in their rates of disassembly compared to silyl- ing uoride ion concentrations in water.
terminated carbonates. For instance, the rate constant for 5c
was 38 358 Æ 2200 MÀ1 minÀ1, about a 39-fold increase
Conflicts of interest
compared to 5a. However, the rate for 5d was 404 699 Æ 64 100
MÀ1 minÀ1, about a 233-fold increase compared to 5b and about
a 10-fold increase compared to 5c (Table 3). The large increase
There are no conicts to declare.
in rate for 5d is most likely a cumulative effect due to the
increased electrophilicity at the silyl group as a result of phenyl Acknowledgements
group inclusion and the rst reaction with uoride ion, the
This work was supported by the Naval Research Laboratory Base
increased amount of TBAF present, and the greater electron
Program. E. C. would like to thank nancial support from the
American Society for Engineering Education (ASEE).
withdrawing nature of the leaving group.
The rates of disassembly for the silyl-centered molecules
were signicantly faster than those for all silyl-terminated
molecules presented in this paper. However, disassembly
signicantly slowed around 15–20 minutes for the bis(ethox-
References
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minutes of reaction (Table 3). Even aer 120 minutes of reaction
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