Visible Light-Induced Trifluoromethylation and Perfluoroalkylation of Cysteine Residues in Batch and Continuous Flow

Article abstract of DOI:10.1021/acs.joc.6b01031

We report a visible light-induced trifluoromethylation and perfluoroalkylation for cysteine conjugation using Ru(bpy)₃²⁺ as photocatalyst and inexpensive R_FI as coupling partner. The protocol allows the introduction of a variety of perfluoro alkyl groups (C1-C10) and a CF₂COOEt moiety. The reaction is high yielding (56-94% yield) and fast (2 h in batch, 12 examples). Process intensification in a photomicroreactor accelerated the reaction (5 min reaction time) and increased the yields (8 examples). Quantum yield investigations support a radical chain mechanism.

Full text of DOI:10.1021/acs.joc.6b01031

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Note
Visible light-induced trifluoromethylation and perfluoroalkylation
of cysteine residues in batch and continuous flow
Cecilia Bottecchia, Xiao-Jing Wei, Koen P. L. Kuijpers, Volker Hessel, and Timothy Noel
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01031 • Publication Date (Web): 03 Jun 2016
Downloaded from http://pubs.acs.org on June 3, 2016
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approaches were troubled with low yields and limited scope.10 A more convenient method was developed by Togni, Seebach
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et al. utilizing hypervalent iodine(III) trifluoromethylating reagents but still required cryogenic reaction conditions.6c,
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Furthermore, introduction of other perfluoroalkylated analogues requires the synthesis of unique hypervalent iodine(III)
perfluorinating reagents, which are either expensive, not commercially available, or difficult to synthesize. In order to develop
a general protocol for the perfluoroalkylation of cysteines, we turned our attention to RF–I reagents (RF = perfluoroalkyl)
which can generate electrophilic perfluoroalkyl radicals under visible light photocatalytic reactions conditions and are
commercially available and cost efficient (Scheme 1).12 We anticipated that such electrophilic perfluoroalkyl radicals can be
used to functionalize cysteine residues, thereby rendering a more general and practical approach to access
perfluoroalkylated cysteines. We also demonstrate that the use of continuousꢀflow photochemistry allows to accelerate this
transformation, which provides a convenient and scalable method able to handle gaseous reactants (e.g. CF3I) efficiently.
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Scheme 1: Trifluoromethylation strategies for cysteine modification
Based on our experience with the visibleꢀlight induced photocatalytic trifluoromethylation of aromatic thiols,13 we chose to
initiate our investigations by trifluoromethylating cysteine 1 with gaseous CF3I in the presence of Ru(bpy)3Cl2 as a
photocatalyst and tetramethylethaneꢀ1,2ꢀdiamine (TMEDA) in acetonitrile (Table 1). Irradiation of the reaction mixture was
achieved by a 24 W white CFL (compact fluorescent light). In the absence of any light or nitrogen base, no reaction product
(2a) could be obtained (Table 1, Entries 1 and 2). The formation of SCF3 product in the absence of any photocatalyst occurs
via homolytic cleavage of the CF3–I bond upon irradiation (Bond Dissociation Energy [CF3–I] = 52.6 ± 1.1 kcal/mol which
corresponds with 544 nm photons) (Table 1, Entry 3).14 However, a more efficient and faster reaction was observed in the
presence of Ru(bpy)3Cl2 (Table 1, Entry 4). When the reaction was conducted in MeOH, a lower product yield was obtained
(Table 1 Entry 5), while water proved to be an incompatible solvent which is mainly caused by solubility issues (Table1,
Entry 6). Further, the amount of CF3I could be lowered (from a large excess of 10 equivalents to 4 equivalents) without any
impact on the reaction yield (Table 1, Entries 4 and 7). The use of an inorganic base proved to be ineffective for this
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transformation (Table 1, Entries 8 and 9). Satisfyingly, in contrast to our observations with aromatic and other aliphatic thiols,
no disulfide byproduct formation could be observed in all cases (Table 1).
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Table 1 Optimization studies for the visible light-induced trifluoromethylation of cysteine 1 with CF3I
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Eq. of
CF3Ib
Yield
(%)c
Entry
Conditionsa
Solvent
Base
1
2
3
4
5
6
7
8
9
No light
CH3CN
CH3CN
CH3CN
CH3CN
MeOH
H2O
TMEDA
No base
TMEDA
TMEDA
TMEDA
TMEDA
TMEDA
KOAc
10
10
n.r.
n.r.
43 / 23
84
CFL
CFL, no catalyst
CFL
10 / 4
10
CFL
10
49
CFL
Sat.
4
n.r.
82
CFL
CH3CN
CH3CN
CH3CN
CFL
4
35
CFL
Na3PO4
4
23
aStandard reaction conditions: NꢀBocꢀLꢀCysꢀOMe (1) (0.5 mmol), Ru(bpy)3Cl2•6H2O
(3.75 mg, 1mol%) TMEDA (1 mmol), and CF3I in 5 ml CH3CN. bCF3I added directly to
the reaction mixture or via a stock solution in CH3CN, visible light irradiation, 2 hours.
cYield determined by 19FꢀNMR with addition of an internal standard (α,α,αꢀ
trifluorotoluene, 0.5 mmol).
With optimal conditions in hand, we investigated the scope of this photoꢀinduced trifluoromethylation protocol (Scheme 2).
Two Lꢀcysteine derivatives with a different amine protecting group were efficiently trifluoromethylated in excellent isolated
yield (compounds 2a and 2b). Notably, good to excellent yields were also obtained for dipeptides BocꢀLeuꢀCysꢀOMe (4,
56%) and BocꢀPheꢀCysꢀOMe (5, 94%), thus showing the selectivity of our methodology in the presence of other amino acid
residues.
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Scheme 2: Direct photo-induced trifluoromethylation of cysteine residues in batch. Reaction conditions: Cysteine
derivative (0.5 mmol), Ru(bpy)3Cl2•6H2O (1mol%), TMEDA (1 mmol), and CF3I (2 mmol) in 5 ml CH3CN. 24 W white
CFL, 2 hours.
Several studies have shown that the introduction of multiple highly fluorinated amino acids can significantly alter the
properties of proteins.15 For example, due to the less polarizable nature of C–F bonds compared to C–H bonds, a
perfluoroalkylated cysteine residue could change the overall acidity of the protein or could easily participate in hydrophobic
interactions in a biological environment. Specifically, the modified residue could be harbored within hydrophobic pockets of
proteins and enzymes, therefore providing an enabling tool for the investigation of hydrophobic proteinꢀprotein or even
proteinꢀmembrane interactions.5b Moreover, the synthetic access to various perfluoroalkylated amino acids would be of high
interest to generate a small library of compounds, which can be used for rapid screening of such interactions. In addition,
long fluorous tags can be used to recover peptides and proteins by enabling extraction techniques with fluorinated
solvents.16 To showcase the utility of our methodology for the preparation of highly fluorinated cysteine residues, the scope
of the perfluoroalkyl coupling partner was further expanded by using a wide variety of commercially available
perfluoroalkylated iodides (Scheme 3). A complete range of perfluoroalkylꢀsubstituted cysteines, bearing perfluoroalkylated
chains of variable length (C3 to C10), was obtained in good to excellent yields (60ꢀ90% isolated yield). Moreover, derivative
3h, bearing an ethyl difluoroacetyl moiety, could be obtained in good yield (75% isolated yield). This compound constitutes
an intermediate of interest for the preparation of difluoromethylꢀsubstituted compounds or for the introduction of 18F via Agꢀ
catalyzed decarboxylative fluorination.3a
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Scheme 3: Direct photo-induced perfluoroalkylation of cysteine in batch. Reaction conditions: N-Boc-L-Cys-OMe
(0.5 mmol), Ru(bpy)3Cl2•6H2O (1mol%) TMEDA (1 mmol), and RF-I (1 mmol) in 5 ml CH3CN. 24 W white CFL, 2 hours.
Next, we focused our research efforts to transfer our trifluoromethylation and perfluoroalkylation protocol to a continuousꢀ
flow microreactor. In general, such devices provide a more homogeneous irradiation/energy distribution and an increased
gasꢀliquid mass transfer.17 The observed process intensification of photochemical transformations in microreactors often
results in increased yields, reduced reaction times and easy scaleꢀup.18 The microflow setup consists of
perfluoroalkoxyalkane microcapillary tubing (PFA, 760 ꢁm ID, 2.0 m, 883 ꢁL) wrapped around a plastic holder, which is
placed into a 3Dꢀprinted beaker (See Figure 1C and Supporting Information).19 The reactor is subjected to irradiation
generated by a blue LED stripe (1 m length, 78 Lumen, 3.12 Watt). The use of such miniaturized light sources, instead of
CFL light sources, allows to increase the overall photonic efficiency and to minimize unproductive heat generation.20 For the
trifluoromethylation protocol, CF3I gas was dosed into the liquid stream via a mass flow controller (Figure 1A). For the
perfluoroalkylation protocol, the two liquid phases were introduced in the reactor by means of syringe pumps (Figure 1B).
For both, the two streams were combined in a Tefzel Tꢀmicromixer (500 ꢁm ID) upon entering the photomicroreactor.
Notably, because of the high solubility of CF3I gas in acetonitrile, the formation of a slug flow regime was not observed. A
significant acceleration of the reaction rate was observed for both the trifluoromethylation and perfluoroalkylation chemistry,
thus affording the formation of the desired products in higher yields than in batch and within only 5 minutes residence time
(Scheme 4, 62ꢀ92%).
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A) Gas-liquid trifluoromethylation of cysteine in flow
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6
CF3I
PFA microreactor
760 ꢀm ID, 883 ꢀL
MFC
1.72 mL—minꢁ1
7
8
9
NꢀBocꢀCysꢀOMe,
Ru(bpy)3Cl2
TMEDA, ACN
CF3
O
C
S
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176 ꢀL—minꢁ1
MeO
HN
R'
B) Perfluoroalkylation of cysteine in flow
NꢀBocꢀCysꢀOMe,
Ru(bpy)3Cl2, ACN
88.3 ꢀL—minꢁ1
PFA microreactor
760 ꢀm ID, 883 ꢀL
RF
O
S
R
Iꢀ F,TMEDA, ACN
C
MeO
88.3 ꢀL—minꢁ1
HN
OtBu
O
(C)
Figure 1. Schematic representation of the microflow setups for (A) gas-liquid trifluoromethylation and (B)
perfluoroalkylation of cysteine. (C) Picture of the photomicroreactor. (More details about the setup can be found in
the Supporting Information.
Ru(bpy)3Cl2
RF-I, TMEDA
RF
O
C
SH
O
C
S
MeO
MeO
HN
HN
R'
R'
PFA microreactor
760 ꢀm ID, 883 ꢀL
Trifluoromethylations
CF3
CF3
O
C
S
O
C
S
S
O
CF3
H
N
MeO
MeO
O
OMe
tBu
N
OtBu
Me
HN
HN
H
O
Bn
O
O
O
BocꢀPheꢀCysꢀOMe
2a
5
2b
91%
79%
85%
Perfluoroalkylations
F F
F
F F
F
F
F
F
F
CF3
CF3
O
C
S
O
S
O
C
S
CF3
F
F
F
F F
F
F
F
C
MeO
MeO
MeO
HN OtBu
HN OtBu
HN OtBu
O
3a
O
O
3b
3c
70%
70%
92%
F
F F F F
F
F2
OEt
C
CF3
O
C
S
O
C
S
F
F F F
O
MeO
MeO
HN OtBu
HN
OtBu
O
O
3d
62%
3h
81%
Scheme 4: Direct photo-induced trifluoromethylation and perfluoroalkylation of cysteine in batch and continuous
flow. Reaction conditions: N-Boc-L-Cys-OMe (0.5 mmol), Ru(bpy)3Cl2•6H2O (1mol%) TMEDA (1 mmol), and RF-I (4
equiv. for CF3I, 2 equiv. for RF-I) in CH3CN are mixed with a T-mixer and irradiated with an array of. 3.12 W blue
LEDs, 5 minutes residence time.
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A plausible mechanism for this process is outlined in Scheme 5. Upon absorption of blue light, [Ru(bpy)3]2+ undergoes a
metalꢀtoꢀligand charge transfer which is subsequently reductively quenched by TMEDA. SternꢀVolmer quenching
experiments indeed demonstrated that this step occurs under our reaction conditions. Next, [Ru(bpy)3]+ is oxidized to its
ground state generating an electrophilic RF radical. This radical can subsequently react with cysteine to establish the S–RF
linkage. In order to generate a neutral species, the radical anion needs to undergo another single electron transfer step
(SET). This can be either done with [TMEDA]●+ (chainꢀterminating SET) or with RFI (chain propagating SET) to generate
another RF radical. In order to elucidate this step and to update our previously proposed mechanism on the photocatalytic
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trifluoromethylation of aromatic thiols, we calibrated the quantum yield of this transformation against the oxidation of 1,9ꢀ
21
diphenylanthracene with singlet oxygen.13,
The obtained quantum yield value was Φ = 126 which demonstrates that
indeed a chain propagating SET step is present in the lightꢀinduced perfluoroalkylation of cysteine (see Supporting
Information).22
Scheme 5 Proposed mechanism of the Ru(bpy)32+-catalyzed radical perfluoroalkylation of cysteine residues.
In summary, we have developed a visible lightꢀinduced photocatalytic route to prepare a wide variety of trifluoromethylated
and perfluoroalkylated cysteine residues. The mild reaction conditions and the broad scope renders our methodology
amenable to the synthesis of perfluoroalkylated cysteines, which can be subsequently introduced in standard peptide
synthesis protocols. Moreover, the implementation of a continuousꢀflow photomicroreactor afforded increased product yields
(on average 10% more product formation compared to batch) and reduced reaction times (5 minutes vs 2 hours in batch).
Experimental Section
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All components as well as reagents and solvents were used as received without further purification, unless stated otherwise. The product
isolation was performed using silica, and TLC analysis was performed using silica on aluminum foils TLC plates with visualization under
ultraviolet light (254 nm and 365 nm) or appropriate TLC staining. 1HꢀNMR, 13CꢀNMR and 19FꢀNMR spectra were recorded on ambient
temperature using a 400 MHz spectrometer. 1HꢀNMR spectra are reported in parts per million (ppm) downfield relative to TMS (0.00 ppm)
and all 13C NMR spectra are reported in ppm relative to CDCl3 (77.23 ppm). Known products were characterized by comparing to the
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1
corresponding HꢀNMR and 13CꢀNMR from literature. All reactions were monitored by TLC, and 19FꢀNMR. The IR spectra were recorded on
an FTꢀIR spectrometer. HRMS (ESI/APCI multimode ionization source, TOFꢀMSD analyzer), were measured with direct infusion in a 50:50
flow of 5mM NH4OAc in water / MeOH.
General procedure for the trifluoromethylation of cysteine residues in batch (GP1, for compounds 2a-b, 4, 5): In an ovenꢀdried vial
equipped with a magnetic stirrer and a PTFE septum, 3.75 mg (1 mol %) of Ru(bpy)3Cl2•6H2O was added to a mixture of NꢀBocꢀLꢀCysꢀOMe
(117.65 mg, 0.5 mmol), N,N,N’,N’ꢀtetramethylethylenediamine (TMEDA) (116.24 mg, 1.0 mmol) and α,α,αꢀTrifluorotoluene (73.1 mg, 0.5
mmol, internal standard) in CH3CN. The fluorinating agent (2 mmol, 4 equiv. for CF3I) was added dropwise to the reaction mixture. For the
insertion of CF3I, stock solutions of known concentrations in CH3CN were prepared and immediately used for the reaction. The vial was
subjected to visible light irradiation with a 24W white CFL. The reaction was stirred at 1000 rpm for 2 hours. The reaction mixture was preꢀ
adsorbed onto silica, dried in vacuo and purified by flash chromatography to yield the fluorinated product.
General procedure for the perfluoroalkylation of cysteine residues in batch (GP2, for compounds 3a-h): In an ovenꢀdried vial
equipped with a magnetic stirrer and a PTFE septum, 3.75 mg (1 mol %) of Ru(bpy)3Cl2•6H2O was added to a mixture of NꢀBocꢀLꢀCysꢀOMe
(117.65 mg, 0.5 mmol), N,N,N’,N’ꢀtetramethylethylenediamine (TMEDA) (116.24 mg, 1.0 mmol) and α,α,αꢀTrifluorotoluene (73.1 mg, 0.5
mmol, internal standard) in CH3CN. The fluorinating agent (IꢀCF2R, 1 mmol, 2 equiv) was added dropwise to the reaction mixture. The vial
was subjected to visible light irradiation with a 24W white CFL. The reaction was stirred at 1000 rpm for 2 hours. The reaction mixture was
preꢀadsorbed onto silica, dried in vacuo and purified by flash chromatography to yield the fluorinated product.
General procedure for the trifluoromethylation of cysteine residues in a continuous-flow microreactor (GP3, for compounds 2a-b,
5): A 10 mL syringe containing 7.5 mg (1 mol%) of Ru(bpy)3Cl2•6H2O, NꢀBocꢀLꢀCysꢀOMe (235.3 mg, 1 mmol, 0.1 M), α,α,αꢀTrifluorotoluene
(146.1 mg, 1 mmol, internal standard) and TMEDA (232.4 mg, 2.0 mmol) in 10 mL CH3CN was mounted on a syringe pump. The liquid
flowrate was fixed at 176.6 ꢁL/min. The liquid stream was merged with gaseous CF3I in a TꢀMixer before entering the reactor. CF3I was
added to the reaction mixture at a flowrate of 1.72 mL/min by means of a mass flow controller. After reaching steady state, a reaction sample
was collected until 0.5 mmol of product was collected in a vial kept in the dark. The reaction mixture was preꢀadsorbed onto silica, dried in
vacuo and purified by flash chromatography to yield the trifluoromethylated product
General procedure for the perfluoroalkylation of cysteine residues in a continuous-flow microreactor (GP4, for compounds 3a-d,
3h): A 5 mL syringe containing 7.5 mg (1 mol%) of Ru(bpy)3Cl2•6H2O, NꢀBocꢀLꢀCysꢀOMe (235.3 mg, 1.0 mmol, 0.2 M), α,α,αꢀ
Trifluorotoluene (146.1 mg, 1 mmol, internal standard) in 5 mL CH3CN and a 5 mL syringe containing TMEDA (232.4 mg, 2.0 mmol) and the
fluorinating agent (IꢀCF2R, 2 mmol,2 eq.) in 5 mL CH3CN were mounted on a single syringe pump. The liquid flowrate (per syringe) was fixed
at 88.3 ꢁL/min. The two liquid streams were merged in a TꢀMixer. After reaching steady state, a reaction sample was collected until 0.5 mmol
of product was collected in a vial kept in the dark. The reaction mixture was preꢀadsorbed onto silica, dried in vacuo and purified by flash
chromatography to yield the fluorinated product.
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General procedure for the synthesis of dipeptides Boc-Leu-Cys-OMe and Boc-Phe-Cys-OMe: The dipeptides used as starting
materials for the synthesis of derivatives 4 and 5 and were prepared through a twoꢀstep procedure adapted from literature.23 Step 1:
Formation of the thioester derivative: LꢀBocꢀLeuꢀOH or LꢀBocꢀLeuꢀOH (1.0 equiv.) and DCM or ethyl acetate (0.5 mmol/mL) were added
to an ovenꢀdried flask and placed in an ice bath (0°C). Then, DCC (1 equiv.) and HOBt•H2O (1 equiv.) were added together with thiophenol
(1 equiv.). The flask was closed with a PTFE septum and the reaction mixture was placed under argon atmosphere. The reaction was
checked for completion by TLC (4 ꢀ24 hours). The reaction mixture was washed with HCl (1M), NaHCO3 (sat) and brine. The organic layer
was dried with MgSO4. The crude was absorbed on silica gel and purified with PE:EtOAC 7:1. Step 2: Native chemical ligation: LꢀCysteine
methyl ester HCl (1.0 equiv.) and the thioester derivative (1.0 equiv.) were added to MeOH (0.25 mmol/mL) in an ovenꢀdried flask kept under
argon atmosphere and closed with a PTFE septum. Next tributylphopshine (0.6 equiv.) was added by the use of a disposable syringe and the
reaction mixture was stirred at r.t. until completion (24 hours). The crude was evaporated under vacuo, and redissolved in EtOAc. The
organic layer was extracted with H2O (3 times) and with brine (3 times). The organic layer was then dried with MgSO4 and concentrated on
silica gel under vacuo. The purification by column chromatography afforded the desired dipeptides BocꢀLeuꢀCysꢀOMe (45%) and BocꢀPheꢀ
CysꢀOMe (24%) (DCM:MeOH 9:1 + 1% acetic acid).
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Methyl N-(tert-butoxycarbonyl)-S-(trifluoromethyl)-L-cysteinate (2a)11 was made according to GP1 on a 0.5 mmol scale. The crude
product was purified by flash chromatography (petroleum ether: ethyl acetate 100:0 to 90:10) yielding 124.4 mg (0.41 mmol, 82%) of
derivative 2a as a white solid (Mp: 67.6ꢀ67.9 °C). The reaction according to GP3 on a 0.5 mmol scale afforded. 120 mg (0.39 mmol, 79 %) of
1
product 2a after 5 minutes residence time. H NMR (399 MHz, Chloroformꢀd) δ 5.35 (s, 1H), 4.62 (s, 1H), 3.79 (s, 3H), 3.58 – 3.23 (m, 2H),
1.45 (s, 9H). 13C NMR (101 MHz, Chloroformꢀd) δ 170.1, 154.9, 130.5 (q, J = 306.4 Hz), 80.6, 53.0, 52.9, 32.1, 28.2. 19F NMR (376 MHz,
Chloroformꢀd) δ ꢀ41.0 HRMS (ESI) calculated for C5H9F3NO2S [MꢀBoc+H]+: 204.0306; found: 204.0308. IR (ATR, cmꢀ1): 3358, 3000, 1724,
1674, 1519, 1369, 1342, 1328, 1292, 1251, 1161, 1145.
Methyl N-acetyl-S-(trifluoromethyl)-L-cysteinate (2b)10 was made according to GP1 on a 0.5 mmol scale. The crude product was purified
by flash chromatography (petroleum ether: ethyl ether 1:1) yielding 110.3 mg (0.45 mmol, 90%) of derivative 2b as a white solid (Mp: 68.6ꢀ
69.4 °C). The reaction according to GP3 on a 0.5 mmol scale afforded 111.5 mg (0.46 mmol, 91%) of product 2b. 1H NMR (400 MHz,
Chloroformꢀd) δ 6.72 (s, 1H), 5.01 – 4.60 (m, 1H), 3.74 (s, 3H), 3.59 – 3.10 (m, 2H), 2.00 (s, 3H13C NMR (101 MHz, Chloroformꢀd) δ 170.3,
170.1, 130.5 (q, J = 306.4 Hz), 53.0, 51.9, 31.6, 22.8, 15.2.19F NMR (376 MHz, Chloroformꢀd) δ ꢀ41.1. HRMS (ESI) calculated for
C7H11F3NO3S [M+H]+: 246.0412; found: 246.0401. IR (ATR, cmꢀ1): 3317, 2958, 1741, 1734, 1641, 1537, 1340, 1255, 1105, 1039.
Methyl N-(tert-butoxycarbonyl)-S-(perfluoropropyl)-L-cysteinate (3a) was made according to GP2 on a 0.5 mmol scale. The crude
product was purified by flash chromatography (petroleum ether: ethyl acetate 100:0 to 90:10) yielding 121 mg (0.3 mmol, 60 %) of derivative
3a as yellow oil. The reaction according to GP4 on a 0.5 mmol scale afforded 141.2 mg (0.35 mmol, 70 %) of compound 3a.1H NMR (399
MHz, Chloroformꢀd) δ 5.37 (s, 1H), 4.61 (s, 1H), 3.78 (s, 3H), 3.60 – 3.28 (m, 2H), 1.44 (s, 9H). 13C NMR (100 MHz, Chloroformꢀd) δ 170.2,
155.1, 128.0 – 120.8 (m), 117.4 (dt, J = 288.1, 33.3 Hz), 114.0 – 106.9 (m), 80.7, 53.2, 52.9, 30.9, 28.2. 19F NMR (376 MHz, Chloroformꢀd) δ
ꢀ76.30 – ꢀ82.31 (m), ꢀ84.76 – ꢀ89.66 (m), ꢀ124.08. HRMS (ESI) calculated for C7H9F7NO2S+ [MꢀBoc+H]+: 304.0237; found: 304.0238: 254. IR
(ATR, cmꢀ1): 3367, 2982, 1724, 1518, 1336, 1180, 1161, 1112.
Methyl N-(tert-butoxycarbonyl)-S-(perfluorobutyl)-L-cysteinate (3b) was made according to GP2 on a 0.5 mmol scale. The crude product
was purified by flash chromatography (petroleum ether: ethyl acetate 100:0 to 90:10) yielding 190.2 mg (0.42 mmol, 84%) of derivative 3b as
yellow oil. The reaction according to GP4 on a 0.5 mmol scale afforded 209.3.mg (0.46 mmol, 92 %) of compound 3b.1H NMR (399 MHz,
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Chloroformꢀd) δ 5.49 (s, 1H), 4.58 (s, 1H), 3.74 (s, 3H), 3.56 – 3.25 (m, 2H), 1.40 (s, 9H). 13C NMR (100 MHz, Chloroformꢀd) δ 170.2, 155.1,
127.8 – 123.4 (m), 122.7 – 120.5 (m), 117.4 (dt, J = 288.1, 33.3 Hz), 114.6 – 105.3 (m), 80.7, 53.2, 52.9, 30.9, 28.2. 19F NMR (376 MHz,
Chloroformꢀd) δ ꢀ81.39 (t, J = 9.8 Hz), ꢀ85.67 – ꢀ88.16 (m), ꢀ119.94 – ꢀ121.26 (m), ꢀ125.32 – ꢀ126.23 (m). HRMS (ESI) calculated for
C8H9F9NO2S+ [MꢀBoc+H]+: 354.0205; found: 354.0206. IR (ATR, cmꢀ1): 3370, 2997, 1724, 1681, 1518, 1348, 1225, 1198, 1163, 1136.
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Methyl N-(tert-butoxycarbonyl)-S-(perfluoropentyl)-L-cysteinate (3c) was made according to GP2 on a 0.5 mmol scale. The crude
product was purified by flash chromatography (petroleum ether: ethyl acetate 100:0 to 90:10) yielding 178.7 mg (0.36 mmol, 71%) of
derivative 3c as yellow oil. The reaction according to GP4 on a 0.5 mmol scale afforded 176.1.mg (0.35 mmol, 70%) of compound 3c. 1H
NMR (399 MHz, Chloroformꢀd) δ 5.37 (s, 1H), 4.63 (s, 1H), 3.79 (s, 3H), 3.61 – 3.27 (m, 2H), 1.44 (s, 9H). 13C NMR (100 MHz, Chloroformꢀd)
δ 170.0, 154.8, 127.8 – 126.3 (m), 124.6 – 123.5 (m), 122.3 – 120.3 (m), 119.3 – 117.9 (m), 115.9 (d, J = 33.1 Hz), 80.6, 53.0, 52.9, 30.9,
28.1. 19F NMR (376 MHz, Chloroformꢀd) δ ꢀ80.47 – ꢀ81.14 (m), ꢀ85.48 – ꢀ87.66 (m), ꢀ119.18 – ꢀ120.38 (m), ꢀ121.60 – ꢀ122.76 (m), ꢀ125.63 – ꢀ
127.13 (m). HRMS (ESI) calculated for C9H9F11NO2S+ [MꢀBoc+H]+: 404,0173; found: 404,0171. IR (ATR, cmꢀ1): 3371, 2997, 2949, 1726,
1680, 1518, 1288, 1223, 1099.
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Methyl N-(tert-butoxycarbonyl)-S-(perfluorohexyl)-L-cysteinate (3d) was made according to GP2 on a 0.5 mmol scale. The crude
product was purified by flash chromatography (petroleum ether: ethyl acetate 100:0 to 90:10) yielding 171.4 mg (0.31 mmol, 62%) of
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derivative 3d as yellow oil. The reaction according to GP4 on a 0.5 mmol scale afforded 171.6 mg (0.31 mmol, 62%) of compound 3d. H
NMR (399 MHz, Chloroformꢀd) δ 5.40 (s, 1H), 4.62 (s, 1H), 3.78 (s, 3H), 3.55 – 3.25 (m, 2H), 1.43 (s, 9H 13C NMR (100 MHz, Chloroformꢀd)
δ 170.0, 154.8, 125.5 (dt, J = 292.5, 34.1 Hz), 121.6 – 120.8 (m), 117.1 (dt, J = 288.5, 33.2 Hz), 114.4 – 112.2 (m), 111.6 – 109.6 (m), 109.2
– 107.1 (m), 80.5, 53.0, 52.8, 30.8, 28.1. 19F NMR (376 MHz, Chloroformꢀd) δ ꢀ80.11 – ꢀ81.65 (m), ꢀ85.12 – ꢀ88.04 (m), ꢀ119.78, ꢀ121.51, ꢀ
122.94, ꢀ125.67 – ꢀ127.15 (m). HRMS (ESI) calculated for C10H9F13NO2S+ [MꢀBoc+H]+: 454,0141; found: 454.0153. IR (ATR, cmꢀ1): 3379,
2980, 2951, 1728, 1695, 1682, 1518, 1317, 1199, 1188, 1163, 1147.
Methyl N-(tert-butoxycarbonyl)-S-(perfluoroheptyl)-L-cysteinate (3e) was made according to GP2 on a 0.5 mmol scale. The crude
product was purified by flash chromatography (petroleum ether : diethylether 16:1 to 8:1 )yielding 269.5 mg (0.44 mmol, 90 %) of derivative
3e as yellow oil. 1H NMR (399 MHz, Chloroformꢀd) δ 5.42 (s, 1H), 4.62 (s, 1H), 3.77 (s, 3H), 3.61 – 3.27 (m, 2H), 1.43 (s, 9H). 13C NMR (100
MHz, Chloroformꢀd) δ 170.2, 155.1, 127.8 – 123.6 (m), 122.2 – 120.3 (m), 117.3 (dt, J = 288.5, 33.0 Hz), 114.5 – 112.4 (m), 112.2 – 109.9
(m), 109.2 – 107.2 (m), 106.7 – 104.5 (m), 80.7, 53.2, 53.0, 31.0, 28.3. 19F NMR (376 MHz, Chloroformꢀd) δ ꢀ81.07 (t, J = 10.0 Hz), ꢀ84.97 –
ꢀ88.54 (m), ꢀ119.78, ꢀ121.39, ꢀ122.18, ꢀ122.93, ꢀ126.00 – ꢀ126.69 (m). HRMS (ESI) calculated for C11H9F15NO2S+ [MꢀBoc+H]+: 504,0109;
found: 504,0114. IR (ATR, cmꢀ1): 3377, 2991, 1728, 1693, 1681, 1518, 1321, 1230, 1193, 1149.
Methyl N-(tert-butoxycarbonyl)-S-(perfluorooctyl)-L-cysteinate (3f) was made according to GP2 on a 0.5 mmol scale. The crude product
was purified by flash chromatography (petroleum ether :ether 10:1 to 6:1) yielding 218.9 mg (0.34 mmol, 67%) of derivative 3f as yellow oil.
1H NMR (399 MHz, Chloroformꢀd) δ 5.40 (s, 1H), 4.63 (s, 1H), 3.78 (s, 3H), 3.56 – 3.30 (m, 2H), 1.43 (s, 9H). 13C NMR (100 MHz,
Chloroformꢀd) δ 170.2, 155.0, 127.4 (d, J = 33.4 Hz), 124.3 (t, J = 33.9 Hz), 119.5 – 118.2 (m), 118.1 – 117.2 (m), 115.8 (t, J = 33.4 Hz),
112.0 – 109.8 (m), 109.5 – 107.0 (m), 80.8, 53.2, 53.1, 31.1, 28.3.19F NMR (376 MHz, Chloroformꢀd) δ ꢀ81.02 (t, J = 10.0 Hz), ꢀ84.55 – ꢀ
89.24 (m), ꢀ119.59 – ꢀ119.88 (m), ꢀ121.20 – ꢀ121.46 (m), ꢀ121.80 – ꢀ122.27 (m), ꢀ122.74 – ꢀ123.14 (m), ꢀ126.25 – ꢀ126.46 (m). HRMS (ESI)
calculated for C12H9F17NO2S+ [MꢀBoc+H]+: 554,0077; found: 554.0076. IR (ATR, cmꢀ1): 3383, 2991, 1695, 1681, 1516, 1369, 1195, 1118,
1082.
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Methyl N-(tert-butoxycarbonyl)-S-(perfluorodecyl)-L-cysteinate (3g) was made according to GP2 on a 0.5 mmol scale. The crude
product was purified by flash chromatography (petroleum ether : ether 12:1 to 8:1) yielding 225.9 mg (0.3 mmol, 60 %) of derivative 3g as a
white solid (Mp: 65.5ꢀ67.1 °C). 1H NMR (399 MHz, Chloroformꢀd) δ 5.38 (s, 1H), 4.64 (s, 1H), 3.79 (s, 3H), 3.65 – 3.22 (m, 2H), 1.44 (s, 9H).
13C NMR (100 MHz, Chloroformꢀd) δ 170.2, 155.0, 127.7 – 127.0 (m), 126.3 – 125.5 (m), 124.9 – 123.9 (m), 122.0 – 121.1 (m), 119.4 –
118.0 (m), 116.5 – 115.1 (m), 114.4 – 113.0 (m), 111.9 – 109.7 (m), 109.3 – 106.8 (m), 80.8, 53.2, 53.1, 31.1, 28.3. 19F NMR (376 MHz,
Chloroformꢀd) δ ꢀ80.48 – ꢀ81.23 (m), ꢀ85.58 – ꢀ87.69 (m), ꢀ119.21 – ꢀ119.96 (m), ꢀ120.84 – ꢀ121.55 (m), ꢀ121.55 – ꢀ122.26 (m), ꢀ122.62 – ꢀ
123.05 (m), ꢀ125.67 – ꢀ126.94 (m). HRMS (ESI) calculated for C14H9F21NO2S+ [MꢀBoc+H]+: 654,0013; found: 654,0019. IR (ATR, cmꢀ1): 3383,
2982, 1728, 1695, 1684, 1516, 1198, 1141.
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Methyl N-(tert-butoxycarbonyl)-S-(2-ethoxy-1,1-difluoro-2-oxoethyl)-L-cysteinate (3h) was made according to GP2 on a 0.5 mmol
scale. The crude product was purified by flash chromatography (petroleum ether: ethyl acetate 100:0 to 90:10) yielding 134.2 mg (0.38 mmol,
75%) of derivative 3h as yellow oil. The reaction according to GP4 on a 0.5 mmol scale afforded 144.7.mg (0.40 mmol, 81%) of compound
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3h. H NMR (400 MHz, Chloroformꢀd) δ 5.36 (s, 1H), 4.55 (s, 1H), 4.31 (q, J = 7.1 Hz, 2H), 3.73 (s, 3H), 3.48 – 3.18 (m, 2H), 1.40 (s, 9H),
1.32 (t, J = 7.1 Hz, 3H).13C NMR (101 MHz, Chloroformꢀd) δ = 170.4, 161.4 (t, J=32.6), 154.9, 120.0 (t, J=287.4), 80.4, 63.8, 53.0, 52.8, 30.9,
28.2, 13.8. 19F NMR (376 MHz, Chloroformꢀd) δ = ꢀ80.9 – ꢀ82.4 (m). HRMS (ESI) calculated for C8H14F2NO4S+ [MꢀBoc+H]+: 258,0612; found:
258,0616. IR (ATR, cmꢀ1): 3387, 2980, 1755, 1714, 1504, 1247, 1161, 1010.
Methyl N-((tert-butoxycarbonyl)-L-leucyl)-S-(trifluoromethyl)-L-cysteinate (4) was made according to GP1 on a 0.50 mmol scale. The
crude product was purified by flash chromatography (petroleum ether: ether: 6:1 to 3:1) yielding 117 mg (0.28 mmol, 56%) of derivative 4 as
a white solid (Mp: 78.8ꢀ79.3 °C). 1H NMR (400 MHz, Chloroformꢀd) δ 7.22 (s, 1H), 5.00 (s, 1H), 4.80 (q, J = 5.3 Hz, 1H), 4.14 (s, 1H), 3.75 (s,
3H), 3.52 – 3.24 (m, 2H), 1.74 – 1.58 (m, 2H), 1.51 – 1.44 (m, 1H), 1.42 (s, 9H), 0.91 (t, J = 6.8 Hz, 6H). 13C NMR (101 MHz, Chloroformꢀd) δ
172.8, 169.7, 155.6, 130.5 (q, J = 306.4 Hz), 80.2, 53.1, 52.9, 51.8, 40.8, 31.4, 28.2, 24.7, 22.8, 21.9. 19F NMR (376 MHz, Chloroformꢀd) δ ꢀ
41.03. HRMS (ESI) calculated for C16H27F3N2NaO5S+ [M+Na]+: 439,1485; found: 439,1487. IR (ATR, cmꢀ1): 3334, 2958, 1755, 1686, 1654,
1514, 1367, 1153, 1103.
Methyl N-((tert-butoxycarbonyl)-L-phenylalanyl)-S-(trifluoromethyl)-L-cysteinate (5)6c was made according to GP1 on a 0.50 mmol
scale. The crude product was purified by flash chromatography (petroleum ether: ethyl ether 50:50 to 0:100) yielding 211.5 mg (0.47 mmol,
94%) of derivative 5 as a white solid (Mp: 112.2ꢀ112.9 °C). The reaction according to GP3 on a 0.5 mmol scale afforded 191.3.mg (0.43
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mmol, 85%) of compound 5. H NMR (400 MHz, Chloroformꢀd) δ 7.41 – 7.16 (m, 5H), 6.83 (s, 1H), 4.94 (s, 1H), 4.82 (s, 1H), 4.42 (s, 1H),
3.79 (s, 3H), 3.58 – 3.27 (m, 2H), 3.20 – 3.04 (m, 2H), 1.45 (s, 9H). 13C NMR (101 MHz, Chloroformꢀd) δ 171.5, 169.5, 155.5, 141.8 – 129.7
(m), 129.4, 129.0, 128.9, 127.2, 80.7, 55.8, 53.1, 52.0, 38.0, 31.6 (d, J = 2.2 Hz), 28.4. 19F NMR (376 MHz, Chloroformꢀd) δ ꢀ40.9. HRMS
(ESI) calculated for C19H25F3N2NaO5S+ [M+Na]+: 473,1329; found: 473,1323. IR (ATR, cmꢀ1): 3325, 2972, 1741, 1681, 1666, 1516, 1439,
1298, 1220, 1153
Acknowledgements:
T.N. and C.B and X.J.W. would like to acknowledge the European Union for a Marie Curie ITN Grant (Photo4Future, Grant
No. 641861). Further financial support for this work was provided by a VIDI grant (T.N., SensPhotoFlow, No. 14150) and a
Marie Curie CIG grant (T.N., Grant No. 333659).
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Description of reaction setꢀups, light sources and their emission spectra, quantum yield measurements and spectral data of
all products. This material is available free of charge via the Internet at http://pubs.acs.org
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