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\DOI{10.5802/crchim.341}
\datereceived{2024-07-12}
\daterevised{2024-09-06}
\dateaccepted{2024-09-17}
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\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{review}

\title{A journey into trifluoromethylchalcogenation: some reagents from
Lyon}

\alttitle{Voyage au sein de la trifluorom\'{e}thylchalcog\'{e}nation :
quelques r\'{e}actifs lyonnais}

\author{\firstname{Thierry} \lastname{Billard}\CDRorcid{0000-0002-2937-9523}}
\address{Institute of Chemistry and Biochemistry (ICBMS, UMR CNRS 5246), 
CNRS, Universit\'{e} Lyon 1, CPE Lyon, 1 rue Victor Grignard, 69622 Lyon, France}
\email{Thierry.billard@univ-lyon1.fr}

\keywords{\kwd{Trifluoromethoxylation}
\kwd{Trifluoromethylthiolation}
\kwd{Trifluoromethylselenolation}
\kwd{Trifluoromethanesulfenamide}
\kwd{Trifluoromethylselenosulfonate}
\kwd{DNTFB}
\kwd{Fluorine}}

\altkeywords{\kwd{Trifluorom\'{e}thoxylation}
\kwd{Trifluorom\'{e}thylthiolation}
\kwd{Trifluorom\'{e}thyls\'{e}l\'{e}nolation}
\kwd{Trifluorom\'{e}thanesulf\'{e}namide}
\kwd{Trifluorom\'{e}thyls\'{e}l\'{e}nosulfonate}
\kwd{DNTFB}
\kwd{Fluor}}

\dedicatory{Dedicated to my former mentor, B. Langlois}

\thanks{CNRS, French Ministry of Education and Research, CPE Lyon.}

\begin{abstract}
The development of emerging fluorinated groups is an active field of
research in recent years. In particular, the fusion of fluorinated
moieties with chalcogens has attracted considerable interest. This has
led to a strong demand for the development of new efficient reagents to
perform various direct trifluoromethylchalcogenations. Herein, we
present an overview of the results obtained in our laboratory in Lyon,
which has actively contributed to this quest during the 
last\unskip\break 
15 years. \vspace*{2pt}
\end{abstract}

\begin{altabstract}
Le d\'{e}veloppement de groupes fluor\'{e}s \'{e}mergents est un
domaine de recherche tr\`{e}s actif ces derni\`{e}res ann\'{e}es. En
particulier, l'association de groupements fluor\'{e}s avec des
chalcog\`{e}nes a suscit\'{e} un int\'{e}r\^{e}t tout particulier. Cela
a induit une forte demande pour d\'{e}velopper de nouveaux r\'{e}actifs
efficaces permettant d'effectuer diverses
trifluorom\'{e}thylchalcog\'{e}nations directes. Dans cet article, nous
allons pr\'{e}senter un aper\c{c}u des principaux r\'{e}sultats de
notre laboratoire lyonnais, qui a activement contribu\'{e} \`{a} cette
qu\^{e}te au cours des 15 derni\`{e}res ann\'{e}es.
\end{altabstract}

\maketitle

\vspace*{14pt}

\twocolumngrid

\end{noXML}

\let\rmmu\upmu

\section{Introduction}\label{sec1}

Due to the intrinsic properties of the fluorine atom, fluorinated
compounds possess unique  properties~\cite{1,2,3}. Consequently,
fluorinated molecules play an important role in a wide range of
applications, such as  materials~\cite{4,5,6,7,8}, battery
technology~\cite{9,10,11},  agrochemistry~\cite{12,13,14}, medicinal
chemistry~\cite{15,16,17,18,19,20,21}, medical 
imaging~\cite{21,22,23,24,25,26,27}\,{\ldots}. In order to develop new
compounds with increasingly specific properties for targeted
applications, the research of new fluorinated groups for the
functionalization of various molecules has seen a growing interest in
the last  decade~\cite{28}. In this respect, the association of
fluorinated moieties with heteroatoms has been particularly studied,
with an important focus on  chalcogens~\cite{29}. 

This interest in CF$_{3}$O, CF$_{3}$S and CF$_{3}$Se moieties is due to
their specific properties. Indeed, these substituents possess
electronic properties that can modify the electron density of the
molecules and thus some of their behaviors 
(Table~\ref{tab1})~\cite{30}. In addition, these groups possess a high
lipophilicity, which will contribute to the improvement of the
physicochemical properties of compounds, such as their membrane
permeation to increase their bioavailability 
(Table~\ref{tab1})~\cite{31,32,33,34}. Finally, some unusual
conformational modifications can also be attributed to these 
moieties~\cite{35}.

The chemistry of CF$_{3}$O, CF$_{3}$S and CF$_{3}$Se groups is an old
topic that has been studied since the 1960s. In 2010, the field
received a new impetus, in particular with the development of new
reagents that allow the direct introduction of CF$_{3}$E (E ${=}$ O, S,
Se) groups onto  molecules~\cite{29,36,37,38,39}. Based on our
experience in this chemistry, our laboratory was one of the pioneering
groups that, fifteen years ago, initiated a re-examination of the
chemistry of CF$_{3}$E moieties by developing new reagents to perform
direct trifluoromethyl chalcogenations. In this account, we will give
an overview of the reagents we have developed and summarize our
contributions to the field.

\section{Back to the beginning}\label{sec2}
Our research interest in the 
trifluoromethylchalco{\hbreak}genyl 
groups began in
the mid-1990s with the trifluoromethylation of sulfur and selenium
derivatives. We initially developed
radical trifluoromethylations of
disulfides and diselenides using UV photolysis of 
trifluoromethyl
thioesters
(Scheme~\ref{sch1}a) and
trifluoromethanethio(seleno)sulfonates
(Scheme~\ref{sch1}b)~\cite{40}. 
In the case of trifluoromethaneselenosulfonates, similar results have
been obtained by thermal activation  (Scheme~\ref{sch1}c)~\cite{41}.
Since trifluoromethaneselenosulfonates were synthesized from Langlois'
reagent (CF$_{3}$SO$_{2}$Na) and diselenides, a one-pot process was
also successfully carried  out (Scheme~\ref{sch1}d)~\cite{41}.

\begin{scheme}
\includegraphics{sc01}
\vspace*{.7pc}
\caption{\label{sch1}Radical trifluoromethylation of disulfides and
diselenides.}
\vspace*{-.5pc}
\end{scheme}

\begin{table}
\caption{\label{tab1}Hammett $(\rmsigma)$, Swain--Lupton $(F, R)$
electronic parameters and Hansch--Leo  $(\rmpi_R)$ lipophilicity
parameters}
\begin{tabular}{cccccc}
\thead
Group & $\rmsigma_m$ & $\rmsigma_p$ & $F$ & $R$ & $\rmpi_R$ \\
\endthead
CF$_{3}$O & 0.38 & 0.35 & 0.39 & ${-}$0.04 & 1.04 \\ 
CF$_{3}$S & 0.40 & 0.50 & 0.36 & \mn0.14 & 1.44 \\ 
CF$_{3}$Se & 0.44 & 0.45 & 0.43 & \mn0.02 & 1.61
\botline
\end{tabular}
\end{table}

More interestingly, nucleophilic trifluoromethylation of disulfides and
diselenides was subsequently developed by employing the commercially
available Ruppert--Prakash reagent (CF$_{3}$SiMe$_{3}$) 
(Scheme~\ref{sch2}a)~\cite{42}. However, disulfides and diselenides are
often difficult to obtain as starting\unskip\break 
materials and only one part of
the dichalcogenide is utilized. To circumvent these drawbacks, the more
accessible thiocyanates and selenocyanates were considered as
surrogates and provided to be highly valuable starting materials for
the efficient synthesis of trifluoromethylsulfides and 
trifluoromethylselenides (Scheme~\ref{sch2}b)~\cite{43}.

\begin{scheme}
\includegraphics{sc02}
\vspace*{.8pc}
\caption{\label{sch2}Nucleophilic trifluoromethylation of
dichalcogenides and chalcogenocyanates.}
\vspace*{-1pc}
\end{scheme}

This last strategy remains a valuable tool in the synthesis of a wide
range of compounds, as evidenced by its continued use in the 
literature~\cite{44,45,46,47,48}. It is also worth noting that the
reaction has been extended to other fluorinated groups, starting from
the corresponding silylated  reagents
(R$_{\mathrm{F}}$SiMe$_{3}$)~\cite{45,49,50}.

\vspace*{-3.8pt}

\section{Direct trifluoromethylthiolation}\label{sec3}
Although the strategy developed previously using thiocyanates was
effective, it required two steps, with the preliminary synthesis of the
corresponding thiocyanate. In light of these considerations, we sought
to explore a more elegant and intuitive approach from a retrosynthetic
point of view: the direct trifluoromethylthiolation. At the beginning
of this investigation, the subject was relatively understudied and
essentially limited to nucleophilic reactions, with the particularly
sensitive CF$_{3}$S$^-$  anion~\cite{36,51}. With regard to an
electrophilic approach, the few results obtained were mainly  with
CF$_{3}$SCl~\cite{36,51}, a highly toxic gas~\cite{52}.\looseness=-1

\begin{scheme*}
\includegraphics{sc03}
\vspace*{.5pc}
\caption{\label{sch3}Direct electrophilic trifluoromethylthiolations
with trifluoromethanesulfenamides.}
\vspace*{-1.1pc}
\end{scheme*}

In order to propose a safer reagent, we focused our interest on
trifluoromethanesulfenamides, which could be either liquid or solid and
could behave as an electrophilic donor of the CF$_{3}$S moiety due to
the polarization of the S--N bond. This approach was inspired by our
previous work on the synthesis of trifluoromethylsulfinamidines, where
we  had discovered their rearrangement
under acidic conditions to 
trifluoromethanesulfenamides~\cite{53}. With the development of an
efficient synthetic method, we have proceeded to the synthesis of
several reagents for the electrophilic trifluoromethylthiolation
reaction  (Figure~\ref{fig1})~\cite{54}.

\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}Trifluoromethanesulfenamide\newline 
reagents for electrophilic trifluorometylthiolation.}
\end{figure}

Reagents \textbf{BB13} and \textbf{BB23} are the most commonly used to
perform various electrophilic trifluoromethyl thiolations. 
\textbf{BB23} exhibits superior reactivity compared  to
\textbf{BB13}~\cite{55}, due to its higher  electrophilicity~\cite{56}.
They have been used in a variety of reactions to form C--SCF$_{3}$
bonds (e.g., cross-coupling reactions, S$_{\mathrm{E}}$Ar,
electrophilic additions, reactions with organometallics). They have
also been used with heteronucleophiles to form heteroatom--SCF$_{3}$
bonds. These reagents have been used not only by our research group but
also by other laboratories worldwide 
(Scheme~\ref{sch3})~\cite{29,51,57,58,59}.

Reagent \textbf{BB33H} has been specifically designed for use in
superacidic conditions to generate a superelectrophile. In
collaboration with S. Thibaudeau, the trifluoromethylthiolation of some
substrates that failed under standard conditions was successfully
carried out under these conditions. This often resulted in unusual
regioselectivity  (Scheme~\ref{sch4})~\cite{60}.

\begin{scheme}
\includegraphics{sc04}
\vspace*{.5pc}
\caption{\label{sch4}Trifluoromethylthiolation in\newline
superacid
conditions.}
\vspace*{-1pc}
\end{scheme}

Interestingly, \textbf{BB23} can also be used in nucleophilic
trifluoromethylthiolations through an Umpolung reactivity. In fact, 
\textbf{BB23} reacts with Bu$_{4}$NI to form the transient CF$_{3}$SI
species, which represents an inverse polarization of the S--I 
bond~\cite{61}. A second equivalent of Bu$_{4}$NI can then attack the
iodine atom to release the CF$_{3}$S$^-$ anion, which can be involved
in various nucleophilic reactions  (Scheme~\ref{sch5})~\cite{62,63,64}.

\begin{scheme}
\includegraphics{sc05}
\vspace*{.5pc}
\caption{\label{sch5}Umpolung reactivity of \textbf{BB23}.}
\vspace*{-1.5pc}
\end{scheme}

Finally, this family of reagents has been extended to include other
fluorinated moieties.\unskip\break
Consequently, a variety of reagents with similar
reactivity
have been developed to introduce different fluoroalkylthio
groups onto organic substrates. The products obtained have undergone
further post-functionalizations, using the SCF$_{2}$CO$_{2}$Me and
SCF$_{2}$SO$_{2}$Ph moieties, to obtain molecules with other
fluorinated  moieties (Scheme~\ref{sch6})~\cite{65,66}.

\begin{scheme}
\includegraphics{sc06}
\vspace*{.5pc}
\caption{\label{sch6}Other reagents and post-function{\hbreak}alizations to
obtain compounds with various fluorinated moieties.}
\vspace*{-1.2pc}
\end{scheme}

The CF$_{3}$S$^-$ salts have been described in the literature as
difluorothiophosgene  generators~\cite{67,68,69,70,71}. Since
\textbf{BB23} can release this anion under iodide activation, we
considered using the  \textbf{BB23} reagent to synthesize thiocarbamoyl
fluorides. The reaction \mbox{requires} the presence of calcium salts, which
facilitate the degradation of the trifluorothiomethanolate anion to
difluorothiophosgene. This intermediate can then be reacted with amines
to yield the expected thiocarbamoyl fluorides 
(Scheme~\ref{sch7})~\cite{72}.

\begin{scheme}
\includegraphics{sc07}
\vspace*{.6pc}
\caption{\label{sch7}Synthesis of thiocarbamoyl fluorides using
\textbf{BB23}.}
\vspace*{-1pc}
\end{scheme}

\section{Direct trifluoromethylselenolation}\label{sec4}
In our ongoing investigation of the direct
trifluoromethylchalcogenation reaction, we continued our chalcogen
exploration by moving down the column to focus our attention on
selenium.

Although selenium is toxic in high doses, it is also an essential trace
element in human  physiology~\cite{73,74}. In addition, some
trifluoromethylselenolated compounds have recently shown interesting
properties as potential anticancer  agents~\cite{47,48}. 

At the beginning of this project, the chemistry of CF$_{3}$Se was
essentially limited to the use of the sensitive CF$_{3}$Se$^-$ 
anion~\cite{29,38,75,76}. However, the synthesis of this anion requires
cumbersome conditions, starting with the use of toxic red selenium. The
electrophilic CF$_{3}$SeCl species has also been poorly described, but
its inefficient and tedious  synthesis~\cite{77} and its toxicity and
volatility have severely limited its  use~\cite{29,38,76}.

We decided to reinvestigate the chemistry of CF$_{3}$SeCl, but under
safe conditions, by generating it {in situ} with a quantitative
reaction  (Scheme~\ref{sch8})~\cite{49}, inspired by the work of E.
Magnier~\cite{78}. 

\begin{scheme}
\includegraphics{sc08}
\vspace*{.6pc}
\caption{\label{sch8}{In situ} generation of CF$_{3}$SeCl from benzyl
trifluoromethyl selenide.}
\vspace*{-1pc}
\end{scheme}

Thus, by using benzyl trifluoromethyl selenide, a readily accessible
prereagent, we were able to perform a series of direct 
electrophilic
trifluoromethylselenolations~\cite{29,38,76}, by the {in situ}
preparation of CF$_{3}$SeCl  (Scheme~\ref{sch9})~\cite{49}. 

\begin{scheme}
\includegraphics{sc09}
\vspace*{.5pc}
\caption{\label{sch9}Electrophilic trifluoromethylselenolations with
CF$_{3}$SeCl.}
\vspace*{-1.2pc}
\end{scheme}

Notably, this method has been successfully extended to other
fluorinated groups. This has been achieved by the synthesis of various
benzyl fluoroalkyl selenides as prereagents, which generate
R$_{\mathrm{F}}$SeCl  species~\cite{49}. This has opened the way to
various fluoroalkylselenolations  (Scheme~\ref{sch10})~\cite{29,38,76}.

\begin{scheme}
\includegraphics{sc10}
\vspace*{.5pc}
\caption{\label{sch10}Various prereagents for electrophilic
fluoroalkylselenolations.}
\vspace*{-.5pc}
\end{scheme}

Despite the efficiency of this method, the main problem is the
simultaneous production of an equivalent of benzyl chloride during the
synthesis of CF$_{3}$SeCl  (Scheme~\ref{sch8}). Consequently, this
reactive species can interfere in some reactions with side reactions.
This is well illustrated by the reactions with Grignard or lithium
reagents, where two equivalents of organometallic species were required
to achieve satisfactory  yields~\cite{79}.

To overcome this drawback, a new reagent was designed that was
isolatable, stable, non-volatile and easy to handle. Building upon our
previous experience in the chemistry of  selenosulfonates~\cite{80}, we
decided to capture the CF$_{3}$SeCl with a sulfinate salt to form a
trifluoromethyl  selenosulfonate~\cite{81}. Consequently, the
trifluoromethyl toluene selenosulfonate  \textbf{BG13} was successfully
obtained in good yield using the sodium toluene sulfinate salt 
(Scheme~\ref{sch11}). This liquid compound showed stability,
purifiability on silica gel, and  non-volatility~\cite{81}.

\begin{scheme}
\includegraphics{sc11}
\vspace*{.5pc}
\caption{\label{sch11}Synthesis of the trifluoromethyl toluene
selenosulfonate \textbf{BG13}.}
\vspace*{-1.2pc}
\end{scheme}

This new reagent has shown remarkable efficacy in facilitating
trifluoromethylselenolations  (Scheme~\ref{sch12}A)~\cite{39},
including cross-coupling reactions~\cite{81} and aromatic and vinylic
C--H  functionalizations~\cite{82,83}.

\begin{scheme*}
\includegraphics{sc12}
\vspace*{.5pc}
\caption{\label{sch12}Reactivity of the trifluoromethyl toluene
selenosulfonate \textbf{BG13}.}
\vspace*{-.5pc}
\end{scheme*}

The homolytically cleavable SO$_{2}$--Se bond allows the  \textbf{BG13}
reagent to participate in radical reactions under photocatalytic or
photolytic conditions  (Scheme~\ref{sch12}B)~\cite{32,39,84,85}. In
collaboration with the group of E. Magnier, we were able to design new
fluorinated groups, namely (trifluoromethylselenyl)methylchalcogenyl
(CF$_{3}$SeCH$_{2}$E, E ${=}$ O, S, Se), which have a high lipophilicity
parameter (Hansch--Leo lipophilicity parameter up to  2.24)~\cite{32}.

Building on the Umpolung reactivity of the trifluoromethyl thiolating
reagent  \textbf{BB23}  (Scheme~\ref{sch5}), we also succeeded in
performing nucleophilic trifluoromethyl selenolation in the presence of
Bu$_{4}$NI  (Scheme~\ref{sch12}C)~\cite{86}. Interestingly, similar
nucleophilic reactivity has also been observed under reductive
conditions  (Scheme~\ref{sch12}C) by using organic reducer 
(TDAE)~\cite{87}, metallic reducer (Fe)~\cite{88} or 
electrochemistry~\cite{89}.

In conclusion, this trifluoromethyl toluene selenosulfonate reagent
(\textbf{BG13}) proved to be a versatile reagent capable of performing
electrophilic, nucleophilic or radical reactions, depending on the
conditions.

In addition, more highly fluorinated homologues of \textbf{BG13} have
been developed with similar reactivity to give fluoroalkylselenolated 
compounds~\cite{39}.

\section{Direct trifluoromethoxylation}\label{sec5}
After our previous successful experiences with CF$_{3}$S and CF$_{3}$Se
chemistry, we decided to move up the chalcogen column to tackle the
very challenging CF$_{3}$O chemistry. Although trifluoromethoxylated
compounds are in high demand, particularly in medicinal 
chemistry~\cite{90}, direct trifluoromethoxylation reactions are still 
underdeveloped~\cite{29,37,90,91,92,93,94,95}. In addition, the
majority of the reagents are often sophisticated and require the use of
expensive and multistep synthesis.

We decided to focus our efforts on the nucleophilic
trifluoromethoxylation. The main challenge associated with this
approach is the instability of the CF$_{3}$O$^-$ anion, which quickly
collapses to difluorophosgene by expelling a fluoride anion. To
overcome this drawback, we initially considered using the commercially
available trifluoromethyl triflate  (\textbf{TFMT}) to generate the
CF$_{3}$O$^-$ anion {in situ}~\cite{96}. In the presence of fluoride
anion, \textbf{TFMT} releases the CF$_{3}$O$^-$ anion, which can then
be captured by an electrophile to perform nucleophilic substitutions 
(Scheme~\ref{sch13})~\cite{97}.

\begin{scheme}
\includegraphics{sc13}
\vspace*{.8pc}
\caption{\label{sch13}Nucleophilic trifluoromethoxylation with
\textbf{TFMT}.}
\vspace*{-1.2pc}
\end{scheme}

Although this strategy has yielded promising results, \textbf{TFMT} is
an expensive reagent and is highly volatile, making it difficult to
handle. In addition, the better yields are generally obtained with a
stoichiometric amount of silver fluoride (AgF).

To propose an alternative reagent that is inexpensive, non-volatile and
reactive in metal-free conditions, we investigated the use of
2,4-dinitro-trifluoromethoxybenzene  (\textbf{DNTFB}). A
S$_{\mathrm{N}}$Ar process allows \textbf{DNTFB} to generate a
trifluoromethoxide anion upon reaction with  \textbf{DMAP} 
(Scheme~\ref{sch14})~\cite{98}. Surprisingly, this salt
(\textbf{DDPyOCF}$_{\boldsymbol{3}}$) was found to be stable in
solution (up to 6 weeks).

\begin{scheme}
\includegraphics{sc14}
\vspace*{.7pc}
\caption{\label{sch14}Nucleophilic trifluoromethoxylation with
\textbf{DDPyOCF}$_{\mathbf{3}}$ arising from \textbf{DNTFB}.}
\vspace*{-.4pc}
\end{scheme}

With this stable salt, in collaboration with the group of F. Leroux,
nucleophilic substitutive trifluoromethoxylations could be carried out
under mild and metal-free conditions with good yields and over a wide
range of substrates  (Scheme~\ref{sch14}). Furthermore, a rational
guideline for the reaction conditions depending on the starting
compounds was\unskip\break  
proposed~\cite{99}.

Although the \textbf{DDPyOCF}$_{\boldsymbol{3}}$ salt is stable, it
remains sensitive in the presence of a hydrogen bond donor to then
generate difluorophosgene. We took advantage of this collapse to
propose an efficient synthesis of carbamoyl fluorides by simply mixing
amines with  \textbf{DDPyOCF}$_{\boldsymbol{3}}$ 
(Scheme~\ref{sch15})~\cite{100}. Thanks to this work, an in-depth study
of carbamoyl fluorides has also been realized. We have demonstrated the
remarkable stability of these compounds in aqueous solutions up to pH
10 and good stability in conditions that mimic the physiological
environment. Finally, an isotopic exchange with fluorine-18 opens the
way to the potential use of [$\mbox{}^{18}$F]carbamoyl fluorides as a
label in PET  imaging~\cite{100}.

\begin{scheme}
\includegraphics{sc15}
\vspace*{.5pc}
\caption{\label{sch15}Synthesis of carbamoyl fluorides with
\textbf{DDPyOCF}$_{\boldsymbol{3}}$.}
\vspace*{-1pc}
\end{scheme}

This ability to generate the difluorophosgene {in~situ} under safe
conditions has also been applied to the synthesis of acid fluorides 
(Scheme~\ref{sch16}). The strategy has been extended to a one-pot
process to directly obtain amides or esters by converting 
\textbf{DDPyOCF}$_{\boldsymbol{3}}$ as a potential coupling 
reagent~\cite{101}.

\begin{scheme}
\includegraphics{sc16}
\vspace*{.5pc}
\caption{\label{sch16}Synthesis of acyl fluorides, amides or esters
with \textbf{DDPyOCF}$_{\boldsymbol{3}}$.}
\vspace*{-1.2pc}
\end{scheme}

\section{Conclusion}\label{sec6}
In summary, over the past 15 years, we have developed several efficient
reagents to perform trifluoromethoxylations, trifluoromethylthiolations
and trifluoromethylselenolations of a wide range of substrates under a
variety of conditions. The adventure continues as we continue to
explore the versatility of these reagents to propose more efficient
tools for the organic chemist's toolbox. 

\section*{Declaration of interests}
The author does not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and has
declared no affiliations other than his research institution.

\section*{Funding}
The CNRS, the French Ministry of Education and Research and CPE Lyon
are thanked for their financial support.

\section*{Acknowledgments}
The author gratefully acknowledges all the students and collaborators
who have contributed to these works. A special warm thanks to F. Leroux
and E.~Magnier for the strong and friendly collaboration we pursue from
year to year. The French Fluorine Network (GIS-FLUOR) is also warmly
thanked for its unconditional support.

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