Electrochemical and chemical syntheses of trifluoromethylating reagents and trifluoromethyl substituted compounds

DOI: 10.1002/zaac.200800206

Electrochemical and Chemical Syntheses of Trifluoromethylating Reagents and

Trifluoromethyl Substituted Compounds

Joseph Grobe* and Joachim Hegge

Münster/Germany, Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster

Received May 24th, 2008.

Abstract. The electroreduction of the halofluoromethanes CF₃Br,

CF₂Br₂and CF₂BrCl has been studied in high-pressure stainless

steel autoclaves at different cathodes [Pt, steel (V2A, V4A), glassy

carbon (GC)] and in various solvent-supporting electrolyte systems

(SSE), e.g. DMF/[Bu₄N]Br, NMP/[Bu₄N]BF₄etc.. The reduction

potentials for CF₃Br increase from Pt (Ϫ1.6 V) < V2A (Ϫ1.8 V) <

GC (Ϫ2.1 V) and are lower for CF₂Br₂and CF₂BrCl suggesting a

reductive cleavage of C-X bonds as the first step. CF₂Br₂and

CF₂BrCl show a two-step reduction in accord with the CϪX bond

energies (CϪF > CϪCl > CϪBr) and the “Perfluoro-effect“. The

electrolysis of CF₃Br in different SSE-systems with sacrificial zinc

or cadmium anodes has been reinvestigated with our experimental

set-up to elucidate the influence of the experimental conditions on

the type and ratio of the products. The observed products

CF₃MBrи2L and (CF₃)₂Mи2L (M ϭ Zn, Cd; L ϭ DMF or AN)

are the same as in the previous investigations, but are obtained in

different ratios, as a rule caused by a parallel chemical corrosion

of the respective anodes. By using aluminium as sacrificial anode

no CF₃Al compounds are formed. The CF₃species generated by

electroreduction of CF₃Br react with the solvents via hydrogen ab-

straction and formation of CF₃H. The current yield with respect

to the dissolution of the Al anode reaches 120 % indicating a con-

siderable chemical corrosion in addition to the anodic oxidation.

This result enabled a one-pot trifluoromethylation reaction of

NMP as organic carbonyl substrate and solvent with CF₃Br and

aluminium powder (ratio 3 : 2) at higher temperatures (> 70 °C).

The complete reaction of CF₃Br to give CF₃H and 1-methyl-2-

trifluoromethyl-4,5-dihydropyrrol allowed the isolation of the latter

by vacuum condensation and distillation in 45 % yield, rel. to the

CF₃Br used.

Gallium and indium were also applied as sacrificial anodes in com-

bination with CF₃Br as substrate. In both cases, anodic current

yields of about 280 % indicated an extreme chemical corrosion to-

gether with cathodic metal depositions corresponding to the ca-

thodic current yield. These deposits Ϫ in contrast to those of Zn

and Cd Ϫ do not react with CF₃Br in Grignard-type conversions

to CF₃Ga and CF₃In compounds. So, the observed products

(CF₃)_nMBr_3Ϫn·L (M ϭ Ga, In; n 1-3; L ϭ DMF, NMP) are obvi-

ously formed by chemical corrosion of the electro-activated anodes.

Finally, electrochemical and chemical trifluoromethylations were

successfully carried out, using R₃SiCl (R ϭ Me, Vi, Ph), Me₃MЈCl

(MЈ ϭ Ge, Sn) and aluminium anodes or Al-powder. The products

were characterized either after isolation or in the product solutions

by NMR-spectroscopic investigations.

Keywords: Trifluoromethylation (electrochemical/chemical); Tri-

fluoromethyl metal reagents (Zn, Cd, Ga, In); Trifluoromethyl

silicon, -germanium, -tin compounds

Introduction

Two possible pathways have been considered and tested:

(i) The electrochemical route to F₃CM or F₂XCM re-

agents or reactive intermediates (M ϭ Zn, Cd, Al etc.) by

cathodic reduction of halofluoro alkanes,

(ii) the thermally initiated reaction of halofluoro alkanes

with group 12 and 13 metals like Zn, Cd, Al, Ga, In with

or without suitable substrates.

In the course of our diverse “Electrochemical metal atom

syntheses” of transition metal complexes with low-valent

metal atoms [1] one of the recent projects was aimed at the

development of methods for the disposal of aerosol propel-

lants on the basis of halofluoro alkanes (Frigen, Freon,

Halon) as an alternative to the high temperature oxidative

combustion. The idea was to transform these compounds

into CF₃or CF₂X containing derivatives as suitable syn-

thons for fluoro-organic syntheses.

While the more reactive CF₃I was used as trifluoromethylating

agent already in the 1980^thby Haszeldine [2], only few attempts

are reported on the application of the very stable fire-extinguishing

propellant CF₃Br. Thus Naumann et al. [3] investigated the Grig-

nard analogous reaction of CF₃Br with zinc powder yielding F₃CZn

complexes of fairly low reactivity, which, however, are very well

´ ´

suited for the preparation of CF₃I. The French group of Nedelec

´

and Perichon [4] has thoroughly studied the electrochemical acti-

vation of CF₃Br and applied this method to the trifluoromethyl-

ation of carbonyl compounds, sulfur dioxide and haloarenes using

a sacrificial anode. A real break-through was the reaction devel-

oped by Ruppert in 1984 [5], using P(NEt₂)₃in combination with

* Prof. Dr. Dr. h.c. Joseph Grobe

Institut für Anorganische und Analytische Chemie

Westfälische Wilhelms-Universität Münster

Corrensstr. 36

Ϫ

CF₃Br for the in situ generation of the nucleophile CF₃resulting

D-48149 Münster/Germany

E-mail: grobe@uni-muenster.de

in the efficient preparation of Me₃SiCF₃and other main group

Z. Anorg. Allg. Chem. 2008, 634, 1975Ϫ1990

J. Grobe, J. Hegge

element derivatives. In the early 1990^thOlah and Prakash used the

socalled “Ruppert’s Reagent” Me₃SiCF₃in combination with tetra-

butyl ammonium fluoride as catalyst for reactions with carbonyl

compounds (aldehydes, ketones, enones) and prepared a variety of

F₃C-substituted organic derivatives [6Ϫ8]. Ruppert’s Reagent is not

limited to reactions with simple carbonyl compounds, but can also

be used as a valuable synthon for the introduction of CF₃-groups

into natural products like steroids [9], amino acids [10, 11] and

carbohydrates [12, 13]. In addition, activated and cyclic carbonic

acid esters are useful precursors for the trifluoromethylation with

Me₃SiCF₃. With tris-(dimethylamino)-sulfonium difluoro-trimethyl

silicate [(Me₂N)₃SO₃]SiF₂Me₃(TAS-Fluoride) as catalyst also nu-

merous chloro-substituted compounds can be trifluoromethylated,

e. g. arylsulfenyl-, -sulfinyl- and sulfonyl chlorides [14, 15].

• Direct reduction at inert cathodes,

• indirect reduction with the aid of appropriate redox

mediators,

• cathodic reduction with the aid of sacrificing anodes.

According to studies of Rhozkov et al. [29] perfluoro alkyl

halides undergo direct reduction at platinum cathodes in

acetonitrile at more negative potentials than the corre-

sponding alkyl halides. The potentials are also dependent

on the molecular structure, the number and length of the

R_Fgroups and the halide function X, as shown by

the following row of decreasing reduction potentials:

(R_F)₃CI

> (R_F)₃CBr > (R_F)₂CFI > (R_F)CF₂I ഠ

´

(CF₃)₂CFBr ഠ (R_F)₃CCl. By cyclic voltammetry Saveant

et al. [30, 31] observed a strong dependence of the reduction

potential [V vs. SCE] on the material of the cathode.

In contrast to the direct reduction at inert electrodes, very

Therefore, our first goal was the development of a new electro-

chemical synthesis for “Ruppert’s Reagent” Me₃SiCF₃using sacri-

ficial anodes. This technique was used already in the 1920^thfor the

preparation of lead and bismuth alkyls from the corresponding me-

tal anodes in diethyl zinc or triethyl aluminium solutions with

ethylsodium guaranteeing the essential conductivity [16]. In the

1950^thZiegler et al. [17] studied the anodic dissolution of alu-

minium in the presence of organic halides and/or various or-

ganometallic species. Later, numerous syntheses for organometallic

compounds on the basis of sacrificial anodes have been developed

[18] and in some cases used for commercial preparations, e. g. for

tetraethyllead [19].

Ϫ

probably giving CF₃besides CF₃radicals, the indirect re-

•

duction with redox mediators selectively yields CF₃, as

demonstrated by the indirect reduction of CF₃Br in the

presence of styrene affording the dimer of the CF₃adduct

[31].

•

The cathodic reduction of perfluoroalkyl halides with the

aid of a sacrificial anode corresponds to that of alkyl hal-

Ϫ

ides with the transfer of 2 electrons giving R_Fand X^Ϫ

Ϫ

followed by the nucleophilic attack of R_Fat suitable elec-

trophilic centres E^ϩof various substrates. This method, of

course, has some advantages. Due to a controlled anodic

reaction the reduction can be carried out under simple ex-

perimental conditions in a one-compartment electrolytic

cell. For example, Sibille et al. [32] have shown that the

electrochemical reaction of CF₃Br with aldehydes using a

sacrificial Zn anode proceeds almost quantitatively with re-

duction of anodically produced Zn^2ϩsalts followed by the

chemical reduction of CF₃Br with the deposited Zn metal

yielding the CF₃-substituted alcohols.

Our extensive electrochemical studies, following metal carbonyl

preparations of an Italian group [20], first were based on acetylace-

tonates of transition metals [21], but later sacrificial anodes were

used in combination with CO and phosphane ligands, and a new

method of “Metal Atom Synthesis” was developed [1, 22, 23].

Lehmkuhl et al. for the first time simultaneously employed the po-

tentials of both anode and cathode for the preparation of dicyclo-

octatriene nickel and iron(II) ethanolate [24, 25]. The latter was

used for an efficient ferrocene synthesis.

On the basis of this experience and in view of the poten-

tial of halofluoroalkanes as “Ozone Killers“, we planned

the use of the fire extinguishing propellant CF₃Br in the

electrochemical preparation of Me₃SiCF₃with the ad-

ditional goal to develope a disposal technique for such com-

pounds as an alternative to the high temperature plasma

combustion with oxygen. Large amounts of Me₃SiCF₃

could also help to satisfy the considerable demand for

bioactive compounds with CF₃-substituents [26] in pharma-

ceutical and agricultural chemistry.

1.1 Electrochemical reduction of CF₃Br, CF₂Br₂and

CF₂BrCl at different cathodes

´

The above mentioned study of Saveant et al. [30, 31] was

carried out to answer the question whether the reduction of

perfluoroalkyl bromides and iodides at inert electrodes

occurs via radical or carbanionic intermediates. Except for

CF₃Br, which allows both radical and carbanionic mecha-

nisms, as a rule, radicals are involved.

Our interest was focussed on the reduction potentials of

CF₃Br, CF₂Br₂and CF₂BrCl at Pt, Glassy Carbon (GC)

and V2A or V4A steel cathodes. Information was collected

by cyclic voltammetry and one-sweep current/voltage meas-

urements, as shown in Figures 1 and 2, respectively, for

CF₃Br in DMF with Bu₄NBF₄as supporting electrolyte.

Table 1 presents the redox-potentials (V vs. SCE) of

CF₃Br at different electrodes determined by cyclic voltam-

metry measurements in comparison with the results in ref.

[30]. In addition, re-oxidation peaks following the CF₃Br

reduction are given.

Results and Discussion

1 Electrochemical properties of halofluoroalkanes

Whereas numerous publications on the mechanism of elec-

tron transfer processes on halons in gaseous plasma phases

exist [27], reports on the electrochemical reduction were

rare, when we started our investigations. Fuchigami [28] has

published a review summarizing the published information

up to 1994. The results observed are based on three differ-

ent techniques:

1976

www.zaac.wiley-vch.de

Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

Syntheses of Trifluoromethylating Reagents and Trifluoromethyl Substituted Compounds

ure gave a molar solution. The concentration of the sup-

porting salt Bu₄NBF₄was adjusted to 0.1 mole/l.

Figure 1 Cyclic polarogram of CF₃Br in a 0.1 molar Bu₄NBF₄

solution in DMF at a V2A steel cathode

Figure 2 Current density/voltage curves (j/E) of CF₃Br at diffe-

rent cathodes

Table 1 Reduction Potentials (V vs. SCE) of Halofluoro methanes

at different Cathodes in 0.1 m Bu₄NBF₄/DMF, deduced from CV

measurements^a)

Cathode

Substrate

Red. Peak 1

V^b)

Red. Peak 2

V

Oxid. Peak

V

GC

Pt

V2A steel

GC

Pt

V2A steel

GC

Pt

CF₃Br

CF₂Br₂

CF₂BrCl

Ϫ2.1 (Ϫ2.07)

Ϫ1.6 (Ϫ1.55)

Ϫ1.8.(Ϫ1.90)

Ϫ1.2

Ϫ0.8

Ϫ1.7

Ϫ1.0

Ϫ0.8

Ϫ1.7

Ϫ

Ϫ2.0

Ϫ1.7

Ϫ2.3

Ϫ2.1

Ϫ2.0

Ϫ2.6

ϩ1.45

ϩ1.15

ϩ1.70

ϩ1.1/ϩ1.45

ϩ0.9/ϩ1.3

ϩ1.6/Ϫ

ϩ1.3

ϩ1.25

Ϫ

V2A steel

^a)Conditions: Concentration of the substrates in solution 0.01 m

^b)Literature values for CF₃Br [31] in ().

The characteristic forms of the cyclic polarograms differ

only slightly, but occur at different peak potentials for the

different electrodes. The easiest reduction is observed at

Ϫ1.6 V for platinum. For V2A steel and glassy carbon ca-

thodic shifts of 0.2 and 0.5 V, respectively, are observed. A

different behaviour shows up for the oxidation peaks:

1.15 V (Pt), 1.45 V (GC) and 1.7 V (V2A). Variations of

aprotic solvents (DMF, NMP, DME, AN)¹⁾or supporting

electrolytes (Bu₄NBF₄, Bu₄NPF₆, Bu₄NClO₄) have only

little effect on the observed potentials.

Figure 3 High pressure cell for electrolytic preparations

Since our interest was focussed on preparative electro-

lyses, more useful information was obtained via evaluation

of one-sweep current density/voltage curves which can be

recorded by a slow stepwise increase of the electrode poten-

tial, e.g. 1 mV/s.

Figure 2 shows the graph obtained for the reduction of

CF₃Br in DMF solution at the different cathodes. To make

sure that the concentration of CF₃Br is sufficiently high

during the registration period, the investigation was carried

out in special electrolysis autoclaves, e.g. in Figure 3 and in

reference [1]. The amount of CF₃Br introduced under press-

The measurement was started at 0.0 V, decreased by

1 mV/s and the resulting current density recorded. The

measurement was finished at Ϫ2.8 V. At Pt and GC elec-

trodes a noticeable current was observed at negative poten-

tials below Ϫ1.6 V whereas at V2A the current flow started

at about Ϫ1.9 V. With this method the reduction of CF₃Br

at GC begins at slightly more positive, at V2A at more

negative potentials than anticipated on the basis of the cyc-

lic measurements. At current densities up to ca. 4 mA cm^Ϫ2

the most negative reduction potential was observed at V2A

steel and not at glassy carbon. Only at higher current densi-

ties the dependence of the reduction potential on the cath-

ode material corresponds to the results obtained in the cyc-

¹⁾Dimethylformamide, N-methyl-2-pyrrolidone, 1,2-Dimethoxy-

ethane, Acetonitrile.

Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

www.zaac.wiley-vch.de

1977

J. Grobe, J. Hegge

Table 2 Reduction potentials of halofluoromethanes (V vs. SCE)

at different cathodes in Bu₄NBF₄/DMF deduced from current den-

sity measurements j[mA cm^Ϫ2] ϭ f(V)^a)

Cathode

Substrate

CF₃Br

Red. Potential /V

Red. potential^b)/V

at j > 3 mA cm^Ϫ2

at j ϭ 3mA cm^Ϫ2

GC

Pt

V2A

GC

Pt

V2A

GC

Pt

Ϫ2.1

Ϫ1.8

Ϫ2.2

Ϫ0.9

Ϫ0.8

Ϫ1.3

Ϫ1.4

Ϫ1.35

Ϫ2.1

Ϫ2.6

Ϫ2.1

Ϫ2.4

Ϫ2.9

Ϫ1.9

Ϫ2.6

Ϫ2.3

Ϫ2.5

Ϫ3.0

CF₂Br₂

CF₂BrCl

V2A

^a)Conditions of the current density measurements: ^b)j-values for the substra-

tes CF₃Br: 10 mA cm^Ϫ2; CF₂Br₂: 40 mA cm^Ϫ2; CF₂BrCl: 20 mA cm^Ϫ2

.

lic voltammetry studies. Table 2 compares the reduction po-

tentials derived from current/voltage curves at 3 and 10 mA

cm^Ϫ2, respectively. They allow to roughly estimate the po-

tentials necessary for preparative galvanostatic electrolyses.

The current/voltage curves in Figure 2 differ in their ascent

as function of the potential (V). Whereas for the reduction

of CF₃Br at Pt and V2A similar slopes and current densities

of 30 mA cm^Ϫ2were reached at fairly low potentials, the

current density at GC electrodes grows more slowly and

ends at Ϫ2.8 V with a value of only 10 mA cm^Ϫ2. In accord

with the cyclic voltammetry experiments the variation of

solvents and supporting salts is without decisive influence

on the current/voltage curves. The observed effects of the

electrode materials on the reduction of CF₃Br can be attri-

buted to adsorption phenomena.

Figure 4 Cyclic polarogram of CF₂Br₂at a Pt cathode (a) in a 0.1

molar Bu₄NBF₄solution in DMF and (b) after addition of a small

amount of Bu₄NBr

In summary, it can be concluded from the electrochemi-

cal investigation, that in principle all tested electrode mate-

rials are well suited for the reduction of CF₃Br. However,

platinum was found to be the best material, because it leads

to higher current densities already at considerably lower po-

tentials compared to stainless steel or glassy carbon.

In view of the planned use of CF₂Br₂or CF₂BrCl as pre-

cursors for introducing functional groups CF₂X (X ϭ Br,

Cl), we also studied their electrochemical behaviour at the

same electrodes as used for CF₃Br.

The redox behaviour was studied both by cyclic voltam-

metry and with the aid of current/voltage curves as de-

scribed for CF₃Br. The results obtained are presented in

Figures 4a/b and in Tables 1 and 2.

Figure 4a/b shows a comparison of the cyclic polaro-

grams of CF₂Br₂and CF₂Br₂after addition of a small

amount of Bu₄NBr, respectively, Figure 5a/b those of

CF₂BrCl and CF₂BrCl after addition of Bu₄NCl to the

electrolyte Bu₄NBF₄/DMF. The reduction potentials and

the oxidative peak potentials observed by subsequent an-

odic scans are presented in Tables 1 and 2.

Figure 5 Cyclic polarogram of CF₂BrCl at a Pt cathode (a) in a

0.1 molar Bu₄NBF₄solution in DMF and (b) after addition of a

small amount of Bu₄NCl

Referring to the discussion of the prototype investigation

of CF₃Br, the results can be summarized as follows:

In Figure 6 the current density/voltage curves of CF₂Br₂

(a) and CF₂BrCl (b) at different cathodes are shown which

allow to deduce important information for the planned pre-

parative electrosyntheses.

1. For the halomethanes CF₃Br, CF₂Br₂and CF₂BrCl a

very similar behaviour was found both in cyclic voltamme-

try and current/voltage measurements. In general, variation

of solvent and/or supporting electrolyte has only minor ef-

1978

www.zaac.wiley-vch.de

Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

Syntheses of Trifluoromethylating Reagents and Trifluoromethyl Substituted Compounds

a) The use of inert anodes like V2A, Glassy Carbon

(GC) or Platinum (Pt) with oxidative decomposition of the

electrolyte (solvent/supporting salt) as anodic process,

b) Employment of sacrificial anodes like aluminium or

its higher homologues with formation of their cations M^3ϩ

.

´ ´

Since studies of Nedelec et al. [4, 34] of electrochemical

reductions of organic halogen compounds have shown that

magnesium, zinc or aluminium are well-suited sacrificial

anodes, we started our reactions of CF₃Br with aluminium

anodes. Two possible following reactions can be considered:

Either the generation of the active species at the cathode

and the dissolution of the anode or the combination of the

halofluoromethyl species with the metal cations to form

fluoroalkylmetal derivatives. Since trifluoromethyl cad-

mium compounds had successfully been used for trifluoro-

methylation reactions [33], aluminium was tested as an-

odic material.

Because of the high volatility of the halofluoromethanes,

the reactions were carried out in high pressure electrolytic

cells of the type shown in Fig. 3 and functionally described

in the experimental section (see also ref. [1]).

The corpus of the steel autoclave served as an inert cath-

ode, whereas the metallic anode was fixed to the lid of the

cell immersing into the electrolyte solution DMF/Bu₄NBr.

The halofluoromethanes CF₃Br and CF₂BrCl, respectively,

were introduced in gaseous form under pressure. The elec-

trolyses were performed typically with cathodic current

densities of 5-10 mA cm^Ϫ2and anodic current densities of

100 mA cm^Ϫ2. The process was controlled by fluorine

NMR spectra of small samples collected from the autoclave

by opening a valve to a capillary immersed in the solution.

Figure 6 Current density/voltage curves (j/E) of CF₂Br₂(a) and

CF₂BrCl (b) at different cathodes

fects on the reduction potentials, whereas the material of

the cathode leads to significant differences. As a rule, the

substrates are most easily reduced at platinum cathodes,

while their reduction demands higher negative potentials at

glassy carbon and V2A electrodes. This is partly due to the

lower reproducibility of these materials. The tabulated data,

therefore, were collected as mean values of five measure-

ments carried out under identical conditions. In conclusion,

Pt-cathodes are best suited for such studies.

Electrolyses of CF₃Br with zinc as sacrificial anode

Our first experiments served as tests for the possibility and quality

of the experimental set-up, because the principle had been proved

2. The results reflect the expected dependence of the po-

tentials on the strength of the carbon-halogen bonds

decreasing in the series C-F > C-Cl > C-Br together with

the so-called “Perfluoroalkyl Effect” (increase in stability

with increasing number of F substituents), as shown by the

reduction potentials: Ϫ1.6 V (CF₃Br)/ Ϫ0.8 V (CF₂Br₂and

CF₂BrCl) [33]. This is in accord with the result that the

second reduction of CF₂Br₂and CF₂BrCl, which obviously

leads to the formation of difluorocarbene, occurs at Ϫ1.7 V

for the former and at Ϫ2.1 V for latter.

´ ´

before by the group of Nedelec et al. [4, 34]. They were carried out

with an excess of CF₃Br in DMF with a potential of 1 V at room

temperature and occurred without any passivation.

Besides the signal of the non-reacted CF₃Br at δ_FϪ18.2 two

additional resonances are observed at δ_FϪ42.0 and δ_FϪ42.8 with

the intensity ratio of ca. 1 : 3.2.

Similar experiments in the polar solvents N-methyl-2 pyrrolidone

(NMP), 1,2 dimethoxyethane (DME) acetonitrile (AN) gave

analogous results. Additional electrolyses with different amounts

of CF₃Br led to the same products, indicating that the stoichi-

ometry has no effect on the product ratio. This result suggests a

Schlenk-type equilibrium as described for Grignard compounds

(Equ. (1)) [35]:

3. The current density/voltage curves allow to deduce the

reduction potentials necessary for the generation of the ac-

•

Ϫ

tive groups CF₃/CF₃or CF₂X^•/CF₂X^Ϫsuited for pre-

parative investigations.

2 CF₃ZnBr v Zn(CF₃)₂ϩ ZnBr₂

(1)

2 Electrochemical reactions of halofluoromethanes

with sacrificial anodes

In order to test, whether the cathodic reaction is the reduction of

the substrate CF₃Br or consists in the deposition of zinc from the

anodically produced Zn^2ϩions, the Zn dissolution was separately

investigated in the electrolyte DMF/Bu₄NBF₄by cyclic voltamme-

try. The deposition of Zn at the platinum cathode was found to

start at a potential of Ϫ1.2 V, the peak current was reached at

Whereas the cathodic reduction of the halofluoromethanes

chosen is determined by their first reduction potential, two

alternatives can be considered for the anodic process:

Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

www.zaac.wiley-vch.de

1979

J. Grobe, J. Hegge

Ϫ1.5 V; the dissolution of the deposited Zn occurs by scanning to

positive voltage values at ϩ0.2 V. These results suggest that the Zn

deposition is the preferred cathodic process and the generation of

the observed CF₃Zn compounds from the activated Zn and CF₃Br

must be considered as following reaction.

Electrolysis of CF₃Br with cadmium as sacrificial

anode

In view of the early chemical preparation of (CF₃)₂Cdи2AN

by Naumann et al. [38] from Cd(CH₃)₂and CF₃I in aceto-

nitrile and the electrochemical synthesis by the French

group [4, 34], the electrolysis of CF₃Br with cadmium as

sacrificial anode with our set-up was of interest for com-

parison with the results described for zinc. Without dis-

cussing our results in detail the following aspects deserve

mentioning:

The current yield amounts to 250 % relative to the production of

Zn^2ϩions and can only be explained by chemical corrosion of the

zinc anode by CF₃Br in addition to the electrochemical dissolution.

This process certainly is furthered by the electrochemical activation

of the anode.

The unexpectedly high current yield suggests that the formation of

the observed CF₃Zn compounds should be possible without elec-

trochemical activation. This assumption could be verified by

pressing an excess of CF₃Br to a suspension of zinc powder in

an autoclave, after a spontaneous start of the reaction at room

temperature leading exothermically to complete consumption of

the zinc with formation of a green solution of the products. After

removal of the solvent by vacuum condensation a light yellow solid

was isolated containing CF₃ZnBr and (CF₃)₂Zn, as expected.

(i) Due to the more positive reduction potential of Cd (Ϫ0.40 V)

as compared to Zn (Ϫ0.76 V), the reductive deposition of metallic

Cd is the preferred process at the cathode and reduction of CF₃Br

does not occur.

(ii) CF₃Cd compounds, therefore, result from the chemical reac-

tion of the activated deposit with CF₃Br to give the derivatives

(CF₃)₂Cd and CF₃CdBr, stabilized by the donating solvent. Highly

reactive Cd(CF₃)₂without donor stabilizationwas also prepared in

the 1990^thby Eujen and Hoge [39].

The Grignard analogous reaction of CF₃Br with Zn was found and

used already in 1988 by Naumann et al. [3, 36] employing a special

pressure glass bulb and iodine or ultrasonic activation. The

isolated product CF₃ZnBrи2DMF was used for the preparation of

CF₃I (> 90 % yield) by reaction with ICl [3, 36].

(iii) In contrast to the observation of the French group [34], we

found current yields > 150 %, indicating additional chemical cor-

rosion of the anode after the oxidative activation of the surface not

only for DMF, but also for NMP and DME as solvents. Obviously,

the increased pressure of CF₃Br in our system is responsible for

the chemical corrosion.

Electrolyses of CF₂BrCl with zinc as sacrificial anode

(iv) Due to the instability of Cd(CF₃)₂in acetonitrile, only

CF₃CdBrи2AN was formed with a current yield of 100 % based on

the anodic dissolution of the metal to Cd^2ϩ

.

In contrast to CF₃Zn compounds ZnCF₂Cl or ZnCF₂Br

derivatives were still unknown when we started our electro-

chemical studies with the corresponding halones. The first

(v) The direct chemical reaction of Cd powder with excess CF₃Br

in DMF and NMP, respectively, could be initiated by warming the

mixture at 40 °C resulting in an exothermic process with complete

consumption of the metal and formation of the same derivatives in

a similar ratio. In DME or AN, however, the chemical reaction at

moderate temperatures does not occur even with acid-activated or

electrochemically produced Cd-powder. Only at temperatures

>100 °C dissolution takes place, but according to NMR spectra,

the resulting brown solution does not contain the expected CF₃Cd-

compounds, which obviously decompose with elimination of di-

fluorocarbene.

reduction potential of Ϫ0.8 V is very close to that of Zn^2ϩ

,

and, therefore, no definite prediction could be made about

the electrochemical processes at the cathode.

The electrolyses were carried out in analogy to those of CF₃Br, but

the potential was chosen just below Ϫ0.8 V. Passivation again was

not observed, but Ϫ based on the expected Zn^2ϩdissolution Ϫ

the current yield amounted to more than 250 %. This indicates a

considerable chemical attack of the halone in addition to the elec-

trochemical process. Surprisingly, the ¹⁹F-NMR spectrum of the

green product solution was found to be identical with that of the

CF₃Br electrolysis, suggesting that ZnCF₂X species cannot be pro-

duced, because the electrochemical activation supports reactions

already observed by Burton et al. [37] in 1985 for activated Zn or

Cd.

(vi) Electrolyses of CF₂ClBr with Cd-anodes in DMF, in accord

with the results for Zn and the studies of Burton et al. [37], led to

the formation of CF₃CdBr·2DMF. In NMP, DME or AN again

electrochemical dissolution of Cd was observed with current yields

>100 % (NMP) or 100 % (DME, AN), but no product could be

detected by ¹⁹F-NMR in the resulting solutions.

A plausible mechanism was suggested including electrochemical

formation of CF₂X^Ϫ, difluorocarbene and, with participation of

DMF, a fluoride ion transfer to generate CF₃^Ϫions, which combine

with Zn^2ϩand X^Ϫto the observed products.

Electrolyses with sacrificial aluminium anodes

The electrochemical investigations of CF₃Br and CF₂ClBr with Zn

and Cd anodes were shown to be suitable for the preparation of

trifluoromethylating reagents, but not for the direct cathodic gener-

With N-methylpyrrolidone (NMP), dimethoxyethane (DME) or

acetonitrile (AN) instead of DMF as solvent similar results were

obtained; only in AN besides the known ¹⁹F resonances of

(CF₃)₂Zn(AN) and CF₃ZnBr(AN) additional signals were de-

tected, probably indicating the existence of small amounts of

ZnCF₂X species.

•

Ϫ

ation of the radical or anionic fluorohalon species CF₃or CF₃

,

because the reduction of the M^2ϩions is the preferred reaction.

Therefore, a more electropositive metal like aluminium has to be

used to favour the fluorohalon reduction.

1980

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Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

Syntheses of Trifluoromethylating Reagents and Trifluoromethyl Substituted Compounds

The standard potential Al^3ϩϩ 3e ϭ Al(s) of Ϫ1.66 V [40] should

allow the preferred reduction of CF₃Br for in situ trifluoromethyl-

ation reactions of substrates like organoelement halides or organic

carbonyl compounds.

[45] δ_FϪ72.8; J_FHϭ 62.7 Hz) and BrCF₂CF₂Cl (2 t, δ_FϪ66.5/

2

Ϫ69.3; ³J_FFϭ 11 Hz). In accord with the general experience in the

spectroscopy of fluorohalones, the triplet at Ϫ66.5 was assigned to

the CF₂Br, that at Ϫ69.3 to the CF₂Cl fragment of tetrafluoro-1-

bromo-2-chloroethane [46]. In view of the preferred formation of

CF₂ClH it is not surprising that only a trace of tetrafluoro-1,2-

dichloro-ethane was detected (s, δ_FϪ71.04; [47] δ_FϪ71).

The anodic dissolution of Al was investigated in DMF, NMP,

DME and AN with the supporting electrolytes Bu₄NPF₆,

Bu₄NBF₄and Bu₄NCO₄, but, due to passivation of the anode sur-

face, was impossible even at potentials >3 V (vs. SCE). However,

already at potentials below 2 V the anodic corrosion was possible

with halide anions, using Bu₄NBr or Bu₄NCl as supporting salts.

Thus, Al-electrodes could be dissolved with 100 % current yield in

0.1 m Bu₄NBr solutions of DMF, NMP or DME. Only in case of

AN as solvent passivity was observed by deposition of a white layer

on the anode.

By cyclic voltammetry measurements of the electrochemically pro-

duced Al^3ϩsolutions with Pt electrodes, we could show that ca-

thodic Al deposition does not occur up to potentials of reductive

decomposition of the electrolyte.

Reactions of CF₃Br with aluminium in DMF

(i) Electrochemical reaction with sacrificial Al-anodes

Electrolysis of a 0.1 m Bu₄NBr/DMF solution saturated with

CF₃Br occurs with complete dissolution of the anode at an applied

voltage <2 V and 100 % current yield. Deposition of Al at the

cathode was not observed.

Figure 7 ¹⁹F NMR spectrum of the product solution in CDCl₃

after electrolysis of CF₂BrCl in the high pressure cell (Fig. 3) with

a sacrificing Al anode and V4A cathode in the Bu₄NBr/DMF elec-

trolyte.

The resulting reaction mixture was investigated by ¹⁹F NMR. Be-

sides the signal of CF₃Br the spectrum shows a doublet at δ_FϪ78

1

with a coupling of J_HFϭ 79 Hz, typical for CF₃H. The amount

of this compound is too large for a reaction of the CF₃-species

generated at the cathode with traces of water in the thoroughly

dried DMF and can only be explained by a reaction with the sol-

The autoclave reaction of CF₂BrCl with Al powder in DMF, in

contrast to that of CF₃Br, already starts at temperatures <70 °C

yielding a solution which Ϫ besides signals of aluminium fluorides

Ϫ shows two weak doublets for CF₂BrH and CF₂ClH. The re-

duction with Al powder is obviously less selective than the electro-

chemical process at room temperature.

´

vent which already had been observed by Medebielle et al. [41] in

1991. On the other hand, the result of Sibille and co-workers [42]

of the formation of trifluoroacetaldehyde could not be certified in

our study.

(ii) Chemical reaction of CF₃Br with Al powder

Electrochemical reactions of CF₃Br with Al in NMP

Whereas Zn and Cd powder react with CF₃Br in strongly coordin-

ating solvents with formation of isolable trifluoromethyl metal

complexes, similar solvent adducts are not known for aluminium.

Therefore, we tried to react Al powder with CF₃Br in DMF under

pressure in an autoclave without electrochemical activation. At

room temperature and up to 80 °C no reaction was observed. Only

under more drastic conditions (>100 °C, 2h) a reaction occurs giv-

ing a cloudy brown solution with broad signals at δ_FϪ158 and

Ϫ177, indicating the formation of aluminium fluorides [43]. Signals

of oligomers (CF₂)_nformed from the eliminated difluorocarbene

were not detected.

These experiments proceed in analogy to those in DMF without

problems, but with current yields of 110 %, thus indicating some

chemical corrosion in addition of the anodic dissolution of Al. ¹⁹F-

NMR spectra, besides CF₃Br show the expected strong doublet

signal for CF₃H and a singlet at δ_FϪ64.4. Closer inspection of this

product by ¹H- and ¹³C-NMR revealed the formation of 1-methyl-

2-trifluoromethyl-4,5-dihydropyrrole, which was detected by Bürger

et al. already in 1995 in the reaction of CF₃Br with BCl₃and

P(NEt₂)₃in NMP in 30 % yield [48]. Following the mechanism pro-

posed, we assume that in our electrochemical process solvent stabil-

ized AlBr₃иNMP is generated in the CϭO coordinated form and

plays the same role as BCl₃in the study of Bürger et al. [48]. There-

fore, the mechanism can be presented as shown in Figure 8.

Electrolyses of CF₂BrCl with aluminium in DMF

Electrolyses of CF₂BrCl in the Bu₄NBr/DMF electrolyte system

with sacrificial Al-anodes occur with current yields of 100 % for Al

dissolution. The ¹⁹F-NMR spectrum (Fig. 7) of the resulting solu-

tion exhibits four signals in addition to that of the substrate, indi-

Chemical reaction of CF₃Br with Al in NMP

The amount of Al dissolved in the electrochemical reaction dis-

cussed above was larger than that expected for the current used,

indicating an additional chemical corrosion. In order to test

2

cating the formation of CF₂BrH (d, δ_FϪ68.8; J_FHϭ 59 Hz; [44]

2

δ_FϪ63.3; J_FHϭ 60 Hz), CF₂ClH (d, δ_FϪ72.1; J_FHϭ 62 Hz;

Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

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1981

J. Grobe, J. Hegge

and AlF. Better chances are, therefore, expected for the homolo-

gous members of Group 14, Gallium and Indium, respectively.

The use of Ga or In as sacrificial anodes has been reported only

rarely in literature.

Tuck et al. have described the electrochemical reaction of alkyl and

aryl halides in AN at Pt cathodes with indium anodes [53], giving

as a rule the corresponding RInX₂compounds, in the presence of

2,2Ј-bipyridine the adducts RInX₂·bipy and with RЈ₄NX salts of

the type RЈ₄N[RInX₃]. Yields of these products were not reported.

Schick et al. [54] have studied electrochemical Reformatsky reac-

tions and obtained excellent results with sacrificial indium anodes

for the preparation of β-lactones from ketones and α-halogen-car-

bonic acid esters.

While CF₃-boron compounds are known since 1960 [55], trifluoro-

methyl-gallium and -indium derivatives were generated in prepara-

tive amounts for the first time by Naumann et al. [56] in 1991 re-

acting GaCl₃and InCl₃, respectively, with the reactive complexes

Cd(CF₃)₂и2AN and product mixtures of M(CF₃)₃, M(CF₃)₂X and

M(CF₃)X₂(M ϭ Ga, In) could be prepared. All derivatives are

stabilized either by donor molecules of the solvent (AN, DME) or

by addition of DMF. Therefore, the chance for an electrochemical

synthesis of such representatives was promising.

Figure 8 Mechanism of the formation of 1-methyl-2-trifluoro-

methyl-4,5-dihydropyrrol from NMP and CF₃in the presence of

AlBr₃.

Ϫ

whether the electrochemical process is necessary for the observed

trifluoromethylation of NMP or not, the autoclave reaction of

CF₃Br with Al powder (molar ratio 3 : 2) was carried out without

electric activation whereby NMP served both as solvent and sub-

strate. The reaction needed thermal activation and started at tem-

peratures > 70 °C. The ¹⁹F-NMR spectrum proved the complete

reaction of CF₃Br yielding CF₃H and 1-methyl-2-trifluoromethyl-

4,5-dihydropyrrol. After vacuum condensation and distillation the

ene-amine was isolated in 45 % yield, rel. to the amount of CF₃Br.

The preparations of trifluoromethyl ene-amine derivatives by

Bürger’s method or with aluminium electrochemically or chemi-

cally from amino carbonyl compounds Ϫ as described here Ϫ have

some advantages over the literature methods (Wittig reaction of

R¹(H)CϭPPh₃with trifluoro acetylamides [49], addition of diethyl-

amine to CF₃-CϭC-CF₃[50] and nucleophilic substitution of fluor-

Electrolyses of CF₃Br with Ga-anodes at V4A-

cathodes

Electrolyses of CF₃Br have been carried out in steel autoclaves

using the same list of solvents as before. Because of the low melting

point of Ga (30 °C), the reaction must be performed significantly

below this temperature. During the experiments we found that the

addition of a supporting electrolyte was not necessary, if the elec-

trolysis is started with low anodic current densities (1 mA cm^Ϫ2).

Already after applying several coulomb, a sufficient current flow is

reached in DMF, NMP or DME to increase the anodic current

density to 50 mA cm^Ϫ2. In case of AN as solvent, passivity is ob-

served due to a red microcrystalline layer on the anode surface even

on addition of supporting salts.

Ϫ

ine by NR₂in XC₆H₄-CFϭCF-CF₃[51]), which are based on

complicated trifluoromethylated substrates. Our procedure has the

additional advantage of avoiding the carcinogenic compound

P(NEt₂)₃necessary for the generation of CF₃^Ϫ. The in situ forma-

tion of AlBr₃avoids the addition of BCl₃. With the successful reac-

tion of CF₃Br and NMP with aluminium powder we could demon-

strate, that the electrochemical pathway for the halone reduction is

not necessary, because the reductive power of Al is sufficient. The

novel preparation of α-trifluoromethyl ene-amines thus could be

considerably improved.

In the suitable solvents, electrolyses are possible without problems

up to more than 3000 C. The current yields calculated for the dis-

solution of Ga^3ϩamount to 280 % of the theoretical value and lead

to the deposition of Ga crystals on the autoclave wall used as

cathode. The amount of deposited Ga corresponds to the value

expected on the basis of Faraday’s Law, indicating that the only

cathodic reaction consists in the refinement of Ga. This result is

plausible, because the standard potential of Ga/Ga^3ϩ(Ϫ0.53 V) is

less cathodic than the reduction of CF₃Br.

After finishing, the electrolyte contains a fine white precipitate of

the CF₃Ga-compounds. Figure 9 shows the ¹⁹F-NMR spectrum of

a small portion of the DMF/electrolyte solution in CDCl₃. Besides

the signals of the non-reacted CF₃Br and the side product CF₃H,

three broad singlets were observed which on the basis of literature

data [56] have been assigned to Ga(CF₃)₃иDMF (δ_FϪ51.3;

[56] Ϫ51.0), Ga(CF₃)₂BrиDMF (δ_FϪ54.4; [56] Ϫ53.7 for

Ga(CF₃)₂ClиDMF) and Ga(CF₃)Br₂иDMF (δ_FϪ57.4; [56] Ϫ56.1

for Ga(CF₃)Cl₂иDMF).

Electrochemical reactions of CF₃Br with gallium and

indium

Our investigations of the use of Al for the electrochemical and

chemical reduction of CF₃Br have shown that F₃CAl derivatives

are not produced as isolable compounds. This result does not ex-

clude their existence as highly reactive intermediates, but indicates

that Ϫ under the conditions employed Ϫ they cannot be preserved,

but spontaneously decompose with CF₂-elimination and formation

of AlF-bonds. Obviously the combination of the hard acid Al^3ϩ

with the soft base CF₃^Ϫ, according to Pearson’s HSAB-Concept

[52], is not stable with respect to the exothermic formation of CF₂

The extremely high current yield of the CF₃Ga-derivatives is the

result of the chemical reaction of CF₃Br with the electrochemically

activated Ga-anode and, in contrast to Zn- or Cd-anodes, not a

1982

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Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

Syntheses of Trifluoromethylating Reagents and Trifluoromethyl Substituted Compounds

Figure 9 ¹⁹F NMR spectrum of the GaCF₃products in CDCl₃

formed by electrolysis of CF₃Br in the high pressure cell (Fig. 3)

with a sacrificing Ga anode and V4A cathode in the Bu₄NBr/

DMF electrolyte

following reaction of the Ga deposition at the cathode. Therefore,

it was of particular interest, whether trifluoromethyl gallium com-

pounds can also be prepared by reaction of the cathodically pre-

cipitated Ga with CF₃Br without electrochemical activation.

The reaction was tried in DMF-solution in an autoclave under

pressure with gradually increasing the temperature, but without

success. Even at T > 100 °C no reaction was observed.

Autoclave electrolyses of CF₃Br with sacrificial

indium-anodes

This investigation was carried out similarly to that with Ga. The

dissolution of the metal in DMF, NMP or DME was also possible

without problems with current yields of 250Ϫ300 %, again indicat-

ing considerable chemical corrosion of the anode.

Figure 10 Cyclic polarogram of Me₃SiCl in DMF/Bu₄NClO₄at

Pt electrodes (a), of Ph₃SiCl (0.025, 0.050 and 0.075 molar) in

DMF/Bu₄NClO₄at Pt electrodes (b) and Ph₃SiCl (0.025 m) in

DMF/Bu₄NClO₄at Pt electrodes before ······ and after ᎏᎏ addi-

tion of 20 μL H₂O

The amount of metallic indium, expected for the electrochemical

process, was precipitated as powder at the cathode. The additional

amount of In reacts with CF₃Br to give a mixture of In(CF₃)₃иD,

In(CF₃)₂BrиD and In(CF₃)Br₂иD (D ϭ DMF, NMP, DME). The

¹⁹F-NMR spectrum of the resulting solution in DMF, for example,

exhibits singlet signals at δ_FϪ44.8; Ϫ45.6 and Ϫ47.0 for the above

given formulae. The signal intensities prove that the mono-substi-

tuted compound is the main product, whereas the tri-substituted

derivative is formed in lowest yield. Literature values [56]: δ_FϪ44.9;

Ϫ46.15 and Ϫ47.25.

propellants on the basis of halofluoro alkanes (Frigens,

Freons, Halons).

In the experimental studies described, with all metals ex-

cept aluminium stable or donor stabilized trifluoromethyl

metal compounds are formed, very often with current yields

much larger than expected for the dissolution of the metals

either to M^2ϩor M^3ϩions. In these cases a considerable

chemical corrosion of the anode is observed in addition to

the electrochemical dissolution. This result gave rise to the

question, whether or not the trifluoromethyl metal com-

pounds can also be prepared without electrochemical meth-

ods in Grignard-type reactions. For practical use it is par-

ticularly important that the metal anode does not react with

the fluoroalkanes in interfering side reactions. Table 3 sum-

marizes the results of the preceding chapters as basis for

the in situ investigations of group 14 organoelement halides,

mainly Me₃MЈCl (MЈϭ Si, Ge, Sn).

As for Ga also for In no CF₃In-compounds could be obtained by

reacting the freshly deposited In with CF₃Br without electrochemi-

cal activation.

Discussion of the electrochemical and chemical studies

of CF₃Br and CF₂BrCl of group 12 and 13 metals

The special aim of the electrochemical investigations with

bromo-trifluoromethane and partly with bromo-chloro di-

fluoromethane was the evaluation of the optimal sacrificial

anode for electrochemical trifluoromethylations, especially

for the preparation of Me₃SiCF₃, with the simultaneous de-

velopment of a simple method for the disposal of aerosol

In electrolyses with Al-anodes no trifluoromethyl alu-

minium compounds were detected, obviously because reac-

Ϫ

tions of the cathodically generated CF₃nucleophile with

the solvents are preferred. With DMF proton abstraction

Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

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1983

J. Grobe, J. Hegge

V4A steel wall served as cathode, and Bu₄NBr was used as support-

ing salt in a 0.1 m solution. 50 mmol Me₃SiCl and 53 mmol CF₃Br

(small excess) were transferred into a 100 mL autoclave by vacuum

condensation (liquid N₂bath). After warming at room temperature

the electrolysis was started and carried out with a cathodic current

density of 3 mA cm^Ϫ2and an anodic current density of 30 mA

cm^Ϫ2until a complete reaction of Me₃SiCl (2 F mol^Ϫ1, i.e. 9648.5 C

for 50 mmol) could be expected.

Table 3 Trifluoromethylmetal compounds stabilized by donor li-

gands L^a)from reactions of metal anodes or metal powder and

CF₃Br and CF₂BrCl, respectively

Halone Zn Anode

CF₃Br

Zn Powder Cd Anode

Cd Powder M^b)Anode

CF₃ZnBrи2L CF₃ZnBrи2L CF₃CdBrи2L (CF₃)₂Znи2L (CF₃)₃MиL

CF₃MBr₂иL (CF₃)₂Znи2L (CF₃)₂Znи2L (CF₃)₂Cdи2L (CF₃)₂Cdи2L

(CF₃)₂MBrиL

CF₂BrCl CF₃ZnBrи2L CF₃ZnBrи2L CF₃CdBrи2L CF₃CdBrи2L mixture

(CF₃)₂Znи2L (CF₃)₂Znи2L (CF₃)₂Cdи2L (CF₃)₂Cdи2L

The ¹⁹F-NMR spectrum showed besides the signal of non-reacted

CF₃Br and the doublet of traces CF₃H only one singlet at δ_FϪ67.3

for the expected Me₃SiCF₃which is in very good agreement with

the literature value [5, 6]. The GC-MS analysis proved the complete

reaction of Me₃SiCl and in addition the formation of little

(Me₃Si)₂O as hydrolysis product. The current yield for the dissol-

ution of Al^3ϩwas 100 %. Me₃SiCF₃could be isolated from the

resulting electrolyte solution in 80 % yield by fractional distillation.

Carrying out the electrolysis in DMF instead of DME, the amount

of CF₃H formed is ca. 5 % as compared to the yield of Me₃SiCF₃

which was isolated in 75 % yield. In NMP besides the main product

Me₃SiCF₃about 5 % 1-Me-2-CF₃-4,5-dihydropyrrole were formed

together with the by-products CF₃H and (Me₃Si)₂O. The current

yield on the basis of Al^3ϩdissolution amounts to 120-130 %, indi-

cating additional chemical corrosion of Al by CF₃Br.

^a)L ϭ DMF, NMP, AN, DME; ^b)M ϭ Ga, In

yields fluoroform as main product, whereas reactions in

DME give only traces of CF₃H besides DME-solvated Al^3ϩ

together with Br^Ϫ. Of particular interest is the reaction of

NMP with CF₃^Ϫ, forming 1-methyl-2-trifluoromethyl-4,5-

dihydropyrrole, thereby opening a novel preparative route

to α-trifluoromethyl-eneamines from carbonic acid amides.

Zn- and Cd-anodes are only suited for the planned elec-

trochemical syntheses, if the resulting trifluoromethyl metal

complexes are reactive enough for the nucleophilic transfer

Ϫ

of the CF₃substituent. In conclusion, the most suitable

Similar results were obtained by Olah, Prakash et al. [66] with Al-

anodes from CF₃Br and a mixture of Anisol (PhOMe) and HMPA

procedure for the direct trifluoromethylation of group 14

organoelement halides seems to be the electrolysis with alu-

minium anodes in DME solution.

´ ´

as well as by Nedelec et al. in DMF/Bu₄NBr with nickel anodes

and up to 90 % isolated yield of Me₃SiCF₃[67]. Both groups used

one-compartment electrolysis cells at temperatures between Ϫ30

and Ϫ15 °C.

Electrochemical trifluoromethylation of organo-

chlorosilanes

Preparation of Me₃SiCF₃from CF₃Br and aluminium

A decisive essential for this task is the cathodic generation of

“CF₃^Ϫ” at potentials which are less negative than those of the or-

gano-chlorosilanes. There is a number of investigations in the lit-

erature dealing with the electrochemical reduction of such com-

pounds [57Ϫ63]. The results differ considerably, but in several pre-

liminary investigations under experimental conditions suitable for

the planned electrochemical trifluoromethylations in autoclaves, es-

pecially the cyclic voltammetric reduction of Me₃SiCl and Ph₃SiCl

in DMF, NMP and DME, we could verify the results of Corriu et

al. [60, 61] and Shono et al. [62], proving that the reduction of

organo-chlorosilanes only starts at more negative potentials than

Ϫ2 V. The reduction of Ph₃SiCl obviously is preferred over that of

Me₃SiCl. Reduction potentials between 0 and Ϫ1 V, found in the

literature, are obviously due to hydrolysis of the chlorosilanes by

traces of water giving HCl and probably salts like [DMFH]^ϩCl^Ϫ.

Because chloride and bromide ions are introduced into the planned

electrolyses by supporting electrolytes or by electrochemical re-

duction of the fluorohalomethanes, the investigation of the electro-

chemical trifluoromethylation seemed promising, in particular so,

since the literature methods of Ruppert [64] and Pawelke [65] have

disadvantages either in the carcinogenity of P(NEt₂)₃or the expens-

ive educt CF₃I.

powder [68]

The current yields of 120-130 % observed in our investigation in-

itiated attempts to prepare Me₃SiCF₃by reacting CF₃Br directly

with aluminium powder in NMP without electrochemical acti-

vation.

The reaction was carried out in an autoclave to generate the neces-

sary concentration of the volatile halone. To a suspension of Al-

powder in NMP first Me₃SiCl was added and after closing the

autoclave a small excess of CF₃Br was introduced under pressure.

The reaction was started by thermal activation, keeping the auto-

clave several minutes at 50 °C, followed by stirring the mixture at

room temperature until the Me₃SiCl had completely reacted.

On working up, Me₃SiCF₃could be isolated in more than 80 %

yield. The by-products CF₃H, (Me₃Si)₂O and 1-Me-2-CF₃-4,5-

dihydropyrrole are easily separated from the main product by

distillation.

Employing a 500 mL autoclave up to 0.3 mol Me₃SiCl can be trans-

formed to Me₃SiCF₃, but after thermal activation the very exother-

mic process has to be controlled either by adding the halone in

small portions or by thoroughly cooling the mixture. If the reaction

is carried out at 70 °C, the side-reaction with NMP already yields

ca. 30 % of the α-trifluoromethyl ene-amine.

Electrochemical reactions of Me₃SiCl with CF₃Br

The reactions were carried out in the electrolytic autoclaves em-

ployed for the preliminary investigations. The Al-anode is fixed in

an insulating Teflon holder in the middle of the autoclave lid. The

The results of the electrochemical and chemical preparations of

Me₃SiCF₃are summarized in Table 4.

1984

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Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

Syntheses of Trifluoromethylating Reagents and Trifluoromethyl Substituted Compounds

Table 4 Electrochemical^a)and chemical^b)trifluoromethylation of

Table 6 Electrochemical^a)and chemical^b)trifluoro- and difluoro-

trimethylchlorosilane using Al anodes or Al-powder.

halo-methylations of various alkyl-, phenyl- and vinyl-chlorosilanes

with CF₃Br and Al-anodes or Al-powder in NMP,respectively.

Me₃SiCl /mol CF₃Br /mol Al /mol

Solvent Products/Yields (%)

Substrate

Product

Yield /%^a)Yield /%^b)

δ_F(ref. CCl₃F)

0.05

0.055

50 g

Al-anode DME Me₃SiCF₃(80); CF₃H (5);

(Me₃Si)₂O (2)

Al-anode DMF Me₃SiCF₃(75); CF₃H;

(Me₃Si)₂O

Al-anode NMP Me₃SiCF₃(70); ene-amine (5);

CF₃H; (Me₃Si)₂O

Me₃SiCl

Me₂SiCl₂

Me₃SiCF₃

70

Me₂Si(CF₃)₂

80^c)

75

5

80-90^c)

Me₂SiCl(CF₃)

5

80

75

10

20

50

30

Me₂Si(Cl)CH₂Cl Me₂Si(CF₃)CH₂Cl 80

Et₃SiCl

Ph₃SiCl

Ph₂MeSiCl

PhMe₂SiCl

Vi₃SiCl

Ϫ64.7

Et₃SiCF₃

75

10

Ϫ61.2 (Ϫ61.3 [81])

Ϫ57.6 (Ϫ57.4 [82])

Ϫ61.0

Ϫ64.5

Ϫ62.8

Al-powder NMP Me₃SiCF₃(>80); ene-amine (5);

CF₃H; (Me₃Si)₂O

Ph₃SiCF₃

Ph₂MeSiCF₃

PhMe₂SiCF₃

Vi₃SiCF₃

20

NMP Me₃SiCF₃(84) ^c); ene-amine

55^c)

70

33 g

(0.3 mol)

5.4 g

(0.34 mol) (0.2 mol) 300 mL (<5); CF₃H; (Me₃Si)O

Al-powder

Vi₃SiCl

Vi₃SiCF₃

70^d)

70^e)

60^e)

90

Ϫ62.6

Me₃SiCl/CF₂BrCl Me₃SiCF₂Cl

Me₃SiCl/CF₂Br₂Me₃SiCF₂Br

Me₃GeCl/CF₃Br Me₃GeCF₃

Me₃SnCl/CF₃Br Me₃SnCF₃

30^f)

Ϫ63.4 (Ϫ63 [85])

Ϫ57.6 (Ϫ58 [85])

Ϫ61.4 (Ϫ61.0 [76])

25^f)

^a)Conditions: One-compartment high pressure electrolysis cell,

Bu₄NBr as electrolyte, cathodic current density j ϭ 3mA cm^Ϫ2

g)

,

g)

70

anodic current density j ϭ 30 mA cm^Ϫ2, low to moderate

T < 20 °C; ^b)Autoclave (500 mL, V4A steel), ice/water cooling;

CF₃Br is introduced under its vapour pressure in 5 portions to keep

temperature below 20 °C; ^c)isolated yield: 35 g (0.24 mol), 84 %

(ref. to amount of Me₃SiCl).

^a)Electrochemical preparation with Al-anodes; yields, estimated

from GC and NMR data; ^b)Chemical preparation; ^c)isolated yield;

^d)Yield in DME solution with 2 Ϫ 4 % of product; ^e)Electrochemi-

cal preparation in DME; ^f)Chemical preparation in NMP with

considerable side reactions; ^g)Chemical preparation at higher tem-

peratures leads to a complicated product mixture.

Table 5 Chemical trifluoromethylation of Me₂SiCl₂with CF₃Br

and Al-powder^a)under variation of the stoichiometry

The electrochemical trifluoromethylation of Me₃SiCl with Al-an-

odes was found to be possible with excellent yields in DME as

solvent of choice. Similarly good yields were obtained with Al-pow-

der/CF₃Br ϭ 2 : 3 in NMP as solvent at sufficiently low tempera-

tures to avoid the side-reaction with the solvent affording the α-

trifluoromethyl-ene-amine derivative.

Me₂SiCl₂

mol

CF₃Br

mol

Al-powder

mol

solvent

Products/Yield

%

0.03

0.15

0.03

0.15

0.03

0.15

0.09

0.06

0.30

0.02

0.10

0.06

0.04

0.20

NMP

Me₂Si(Cl)CF₃(75),

Me₂Si(CF₃)₂(5)

Me₂Si(Cl)CF₃(50),

Me₂Si(CF₃)₂(15)

Me₂Si(Cl)CF₃(10),

Me₂Si(CF₃)₂(45)

Me₂Si(Cl)CF₃(15),

Me₂Si(CF₃)₂(55)

Me₂Si(Cl)CF₃(20),

Me₂Si(CF₃)₂(40)

In case of Me₂SiCl₂the amount of CF₃Br has to be adjusted close

to 1 : 1 for complete reaction of the chlorosilane and yields of

70 Ϫ 75 % of Me₂SiCl(CF₃) besides about 5 % of Me₂Si(CF₃)₂.

Et₃SiCl and Me₂SiCl(CH₂Cl) were trifluoromethylated with

yields of 75 and 80 %, respectively. The phenylchlorosilanes

Ph_3ϪnSiMe_nSiCl, depending on the number of Ph-substituents, give

only low to moderate yields with decreasing number of Ph groups,

obviously due to steric effects. For functionalized chlorosilanes like

Vi₃SiCl the electrochemical method turned out to be the prefer-

able route.

^a)Yields estimated on the basis of NMR and GC data; Side pro-

ducts: Me₂SiF₂, Me₂Si(F)CF₃. Products with very similar boiling

points!

The application of both the electrochemical and chemical method

to the fluorohalomethylation of Me₃SiCl with CF₂BrCl or CF₂Br₂

in combination with Al-anodes or Al-powder in DME solution was

successfully used for the preparation of chlorodifluoro- and bromo-

difluoro-trimethylsilane, respectively, in good (60 Ϫ 70 %) or fairly

low (25 Ϫ 30 %) yields. Examples of our preparations are given in

the Experimental section. Improvement seemed possible by varia-

tion of the experimental conditions. This was clearly demonstrated

by Prakash and Olah [70] for the chemical method on the basis of

our preliminary note [68, 71]. They also described the application

of Me₃SiCF₂Cl in fluoride catalysed chlorodifluoro methylations

of aldehydes and other substrates.

Trifluoro- and difluorohalo-methylation of further

organochlorosilanes

The high yields of Me₃SiCF₃both in the electrochemical reaction

and in the direct process of CF₃Br with Al-powder encouraged fur-

therstudies with various organosilicon substrates. The following or-

ganochlorosilanes were chosen for these screening experiments:

Me₂SiCl₂, MeSiCl₃, Me₂Si(Cl)CH₂Cl, Et₃SiCl, Ph_3ϪnSiMe_nSiCl,

Vi₃SiCl. The results should enable an estimation of the value of

the electrochemical and chemical procedure, respectively, for the

preparation of CF₃- or CF₂X-substituted organosilane (X ϭ Cl,

Br) derivatives.

The general procedures for the alternative routes are described in

the experimental section. The results obtained with various ratios

of Me₂SiCl₂/CF₃Br are summarized in Table 5, a summary of the

observations without too many details, but with some remarks on

the effects determining the reactivity of the substrates and both

type and yields of products is presented in Table 6.

Reactions of Me₃MЈCl (MЈ

؍

Ge, Sn) with CF₃Br

and aluminium

The successful development of two routes to CF₃-substituted or-

ganosilanes encouraged us to test these methods for the prep-

Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

www.zaac.wiley-vch.de

1985

J. Grobe, J. Hegge

aration of the homologous group 14 element compounds

Me₃MЈCF₃(MЈϭ Ge, Sn).

the corresponding derivatives Me₃MЈCF₃is well suited for

their preparation. The highest yield (> 90 %) is obtained for

the Ge-, the lowest for the Sn-compound (ca. 70 %). In spite

of intensive efforts, the reduction potential of Me₃SnCl was

not unequivocally accessible. However, both literature data

and our own measurements indicate a potential probably

less negative or very close to that of CF₃Br, so that both

reductions may occur simultaneously and lead to side reac-

tions. The results of these investigations are presented in

Table 6together with those of the organochlorosilane reac-

tions.

Me₃SnCF₃, the first CF₃-derivative of a group 14 element, was pre-

pared by Clark and Willis [72] already in 1960 in 80 % yield by

reacting hexamethyldistannane with CF₃I at 80 °C in a sealed tube.

This Sn-Sn bond cleavage is also possible at room temperature with

UV irradiation [73]. Lemal et al. [74] applied the trifluoromethyl-

ation method of Burton et al. [75] by reacting Me₃SnCl with CF₂Br₂

in the presence of PPh₃and obtained Me₃SnCF₃in 36 % yield.

Me₃SnCF₃is an excellent difluorocarbene source, giving at 150 °C

in a sealed vessel Me₃SnF and hexafluorocyclopropane [72]. At the

boiling point of DME suitable olefins can thus be transformed to

the geminal difluoro cyclopropanes [73].

Conclusions

In spite of the fact that cyclic voltammetry measurements with

Me₃SnCl give Ϫ as function of solvent and supporting electrolytes

Ϫ broad and irreversible reduction peaks at potentials between

Ϫ1.0 and Ϫ1.8 V vs. SCE, which also suffice to reduce CF₃Br, we

tried the electrochemical reaction in the electrolytic autoclave in

DME/Bu₄NBr with a sacrificial Al-anode. The volatile components

obtained from the electrolyte solution by vacuum condensation

were studied by NMR and GC/MS measurements proving that

70 % of the Me₃SnCl had been transformed to Me₃SnCF₃, the only

tin containing product. Since formation of Me₃SnSnMe₃from the

additional 30 % Me₃SnCl could not be verified by NMR of the

less-volatile residue of the electrolyte, no further studies were

undertaken.

The results presented in this paper have clearly demon-

strated possibilities and measures for the electrochemical

and chemical activation of the halones CF₃Br, CF₂Br₂and

CF₂BrCl to be used in preparative trifluoro- and partly in

difluorohalo-methylation reactions with

a variety of

electrophiles giving useful synthons like Me₃MЈCF₃or

Me₃SiCF₂X ( MЈϭ Si, Ge, Sn) and CF₃M compounds with

M ϭZn, Cd, Ga, In.

The interesting results deserve further investigations,

which Ϫ due to my retirement Ϫ cannot be carried out by

me. Therefore, young chemists are welcome to continue

work along the lines pointed out in this paper. The increas-

ing interest in fluorine substituted bioactive compounds

both in pharmaceutical and agricultural chemistry has led

to considerable efforts during the last decade. For example,

trifluoromethyl- and difluoromethyl-sulfone and -sulfoxide

were found to be excellent precursors to prepare R₃SiCF₃

and R₃SiCF₂H derivatives (R ϭ Me, Et) for the nucleo-

philic attack of numerous electrophiles, as demonstrated in

a recent review of Prakash and Hu [80].

The chemical trifluoromethylation reaction of Me₃SnCl with

CF₃Br and Al-powder in NMP was found to be of no practical

use, because up to 50 °C no reaction was observed and at higher

temperatures a complex product mixture was formed.

As the preparation of Me₃GeCF₃by cleavage of Me₃GeGeMe₃

with CF₃I under UV irradiation was not possible, due to the more

stable Ge-Ge bond [74], the electrochemical method was of special

interest. Trialkyl-trifluoromethylgermanes were prepared before by

Bardin [75] applying Ruppert’s method to Et₃GeCl giving 52 % of

the expected product. Electroanalytical investigations of the re-

duction of organochlorogermanes usually have been performed

parallel to those of organochlorosilanes. Corriu et al. [61] Ϫ on the

basis of a polarographic study of Ph₃GeCl in DME Ϫ found a

reduction potential of E_1/2ϭ Ϫ2 V (vs. SCE).

Experimental Section

Techniques, Chemicals, Gases, Electrodes

For preparative electrolyses power was supplied by a current/volt-

age constanter Straton 3224 with potentials up to 30 V and currents

up to 4 A. These power sources guarantee stabilities of 50 mV and

0.1 mA. The control of electrolyses was carried out with a HEKA-

elektronic PG 285 IEC potentiostat/galvanostat equipped with a

IEEE LabCard 848 in combination with an IBM-AT compatible

computer supplying currents up to 1 A with up to potential differ-

ences of 90 V between working and auxiliary electrode. The largest

voltage sweep was 1000 V/s. All working parameter were generated

and controlled with the computer with the software “Quickbasic

4.5 of Microsoft” in the actual version “Electrochemical Synthe-

sizer ECS 6.2” developed by H. Lademann [81] and R. Beckmann

[82]. This program allows to compile the parameters time, charge,

potential and current during an electrochemical process (synthesis

or cyclic voltammetry measurement).

Cyclic voltammetry measurements of Me₃GeCl revealed that the

reduction behaviour is similar to that of Me₃SiCl. Besides weak

peaks of hydrolysis products, resulting from traces of water in the

solvent, down to a cathodic potential of Ϫ2.2 V (vs. SCE) no

further reduction peaks were observed. The electrochemical trifluo-

romethylation of Me₃GeCl with CF₃Br in DME/Bu₄NBr at a V4A

steel cathode (wall of autoclave) using a sacrificial Al-anode in our

attempts gave excellent yields of Me₃GeCF₃(>90 %). ¹⁹F and

¹H NMR spectra showed an almost complete transformation of

the starting compound. Characterization: δ_FϪ61.4 (s); [79]:

δ_FϪ61.0 (s).

As in case of Me₃SnCl the chemical reaction of Me₃GeCl with

CF₃Br in NMP with Al-powder was not suited for the preparation

of Me₃GeCF₃, because below 50 °C no reaction occurred; at higher

temperatures side reactions of CF₃Br and NMP with Al were ob-

served, and Me₃GeCl remained unchanged.

Electrolyses and measurements under normal pressure, i. e. without

applied or produced gases were performed in commercially avail-

able glass apparatus of Metrohm. Whereas electrochemical reac-

tions or investigations under pressure have been carried out in

In summary, it can be concluded that the electrochemical

trifluoromethylation of Me₃MЈCl (MЈϭ Si, Ge, Sn) to yield

1986

www.zaac.wiley-vch.de

Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

Syntheses of Trifluoromethylating Reagents and Trifluoromethyl Substituted Compounds

newly developed high-pressure non-magnetic autoclaves of 100 or

Reference electrodes and SSE-systems

200 mL volume, allowing magnetic stirring of the electrolyte solu-

tion and their use as auxiliary cathodes. A highly developed ex-

ample is given in Figure 3, equipped with facilities for the use of

up to three electrodes and for introducing or taking off gaseous

components. In case of the three-electrode version for cyclic vol-

tammetry measurements a pressurable silver/silver chloride refer-

ence electrode in a small glass container is used.

Reference electrodes used in our electrochemical investigations

were the saturated calomel electrode (SCE) and as pseudo-reference

a silver wire in the respective 0.1 molar SSE-system. The latter was

shown to be sufficiently stable by regular control measurements.

Especially, for reactions under pressure in autoclaves this electrode

system is advantageous, because by introducing gaseous partners

under pressure an exchange of the electrolytes between the reaction

compartment and the reference electrode can not be excluded. The

potentials vs. SCE given in the Tables of this paper have been meas-

ured against the pseudo-reference and recalculated for SCE as ref-

erence on the basis of the half-wave potential ferrocene/ferrocene^ϩ

(FeCp₂/FeCp₂^ϩ), measured both in the pure SSE-system and after

the electroanalytical investigation. If both results differ only slightly

the conversion of the data to the SCE scale seems acceptable. Thus,

a tolerance of plus/minus 0.05 V could be guaranteed.

Chemicals, gases and electrodes

Me₃SiCl, Me₂SiCl₂, MeSiCl₃, Et₃SiCl, Ph₃SiCl, Vi₃SiCl, Me₃SnCl,

CF₂Br₂are commercially available and were purified and dried, if

necessary. Me₂Si(Cl)CH₂Cl, Ph₂MeSiCl, PhMe₂SiCl and Me₃GeCl

were provided from the stored chemicals of the research group.

All solvents used were of analytical grade and were applied after

drying by conventional procedures. The supporting electrolytes also

were of analytical or of electrochemical grade and stored in an

electric drying oven.

Electroanalytical studies

CF₃Br was a donation of Hoechst AG and CF₂ClBr of Hüls AG.

Sacrificial anodes: Aluminium foil (0.3 mm, Fluka), zinc foil m3N

(0.62 mm, Alfa), cadmium foil m3N (0.62 mm, Alfa), self-prepared

indium and gallium foils (0.5 mm, Fluka). Metal powder for chemi-

cal reactions: Aluminium (> 98 %, <160 μm, Fluka), zinc

(99.995 %, 100 mesh, Merck), cadmium (99.5 %, <100 μm, Fluka).

The cyclic voltammetry investigations were carried out with con-

ventional equipment of Metrohm generally with supporting elec-

trolyte concentrations c ϭ 0.1 mol/L in 20 mL of the respective

solvent. Physically dissolved oxygen was removed by nitrogen gas.

Scan rates as a rule were 200 mV/s.

Since the cyclic voltammetry peaks often depend not only on the

scan rate, but also on the concentration of the electroactive com-

pound, 0.01 molar solutions have been investigated in order to

guarantee comparable conditions and results. In case of the gase-

ous fluorohalo-methanes, solutions of the respective concentrations

were prepared as follows: The autoclave (Fig. 3 or simpler versions

in [1]) is filled with the actual solvent and three times pressurized

with analytical grade nitrogen gas up to 50 bar and after 10 minutes

stirring the pressure was released. Then the actual fluorohalome-

thane was introduced under 2.5 bar pressure to produce a saturated

solution, which results after releasing the gaseous pressure to 1 bar.

The concentration of the fluorohalomethanes in the respective sol-

vent was determined from the weight difference (e. g. the solubility

of CF₃Br in DMF is 40 g/kg). From the saturated solutions pre-

pared according to this procedure the amount necessary to gain

the 0.01 molar SSE-system is transferred into the conventional Me-

trohm analytical cells. The software used allowed simple variations

of the potential intervals for the scanning, the number of cycles

and the scan rates.

Most of the cyclic voltammetry investigations were carried out with

conventional platinum electrodes of Metrohm; the working elec-

trodes used included sheet platinum of 0.56 cm², glassy carbon of

2 cm²(Sigradur G) or V2A steel sheet of 1 cm²). Auxiliary elec-

trodes: platinum wire (L ϭ 5 mm, diameter ϭ 1 mm); reference

electrode: Silver wire in a glass container with a diaphragma.

Spectroscopy

NMR-Spectra: AF-200 instrument of Bruker for ¹H, 200.13 MHz,

CDCl₃; ¹³C, 50.32 MHz, CDCl₃; ¹⁹F, 188.13 MHz, CDCl₃, CCl₃F

(0 ppm) as external standard; ³¹P, 81.01 MHz, CDCl₃, 85 % H₃PO₄

(0 ppm) as external standard.

GC-MS: Gas chromatograph 1400 of Varian with an ITD instru-

ment of Finnigan.

General Procedure

The stationary current density/voltage curves [j(mA cm^Ϫ2)/V] ob-

tained by single sweep voltammetry to study the redox behaviour

of the fluorohalomethanes were measured in the autoclave fitted

with the three-electrode arrangement. First the autoclave was

charged with 40 mL of the respective SSE-system, then degassed

with analytical grade nitrogen, and finally the amount of the actual

fluorohalomethane, necessary for a 1 molar concentration (e. g. 6 g

CF₃Br) was introduced under pressure. The single sweep j/V curve

was then recorded at a scan rate of 1 mV/s between 0 and Ϫ2.8 V.

All reactions with gaseous fluorohalomethanes were carried out in

electrolytic autoclaves. The reactants were introduced after filling

with the solvent-supporting electrolyte system (SSE) chosen for the

particular electrolytic reaction directly from the gas cylinder via

suitable pressure tubing under a pressure of about 2.5 bar or by

vacuum condensation. The amount of the gaseous partner was de-

termined by weight difference.

Volatile reaction products and unreacted fluorohalomethanes were

transferred from the autoclave to a standard vac line under vacuum

(10^Ϫ2mbar) using a special steel adapter. Conventional methods

were applied for isolating the synthesized compounds from the con-

densate or from the SSE-system. Thus, escape of considerable

amounts of the fluoro- halomethanes into the laboratory atmos-

phere was avoided.

Preparative Electrolyses

As some preparations have already been described above in re-

porting on the development of the electrochemical procedure, here

only several typical examples will be given.

Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

www.zaac.wiley-vch.de

1987

J. Grobe, J. Hegge

thodic current densities of 2 mA cm^Ϫ2allowed the isolation of ca.

3.01 g (0.021 mol) Me₃SiCF₂Cl (70 %, rel. to CF₂BrCl).

Electrochemical preparation of Me₃SiCF₃

The electrolysis was carried out in an electrolytic autoclave (Fig. 3)

of 100 mL volume, charged with the 0.1 molar SSE-system DME/

Bu₄NBr after introducing 5.43 g (50 mmol) Me₃SiCl and 7.9 g

(53 mmol) CF₃Br by vacuum condensation employing an alu-

minium anode and using the V4A autoclave wall as cathode. After

warming at room temperature the electrolysis was carried out at a

cathodic current density of 3 mA/cm²and an anodic current

density of 30 mA cm^Ϫ2until 9648.5 C were used (i. e. 2 F/mol

Me₃SiCl). A GC-MS analysis demonstrated the complete reaction

of the silane and in addition the formation of small amounts of the

disiloxane (Me₃Si)₂O as hydrolysis product. From the electrolyte

solution 5.7 g (40 mmol) Me₃SiCF₃were isolated by fractional dis-

tillation.

The analogous electrosynthesis with CF₂Br₂and Me₃SiCl gave

about 60 % Me₃SiCF₂Br besides small amounts of CF₂BrH and

CF₂BrCl.

Characterization by comparison of the NMR-data with those of

Broicher and Geffken [85].

Chemical preparations

Current yields > 100 % in the electrolytic preparations indicated

considerable chemical corrosion in addition to anodic dissolution

and led to a number of chemical reactions without applying electric

current. Thus, it could be demonstrated, that some of the tested

trifluoromethylations can be performed with Al powder as the re-

ducing agent for the fluorohalomethanes. Therefore, three chemical

preparations are described here as examples:

Yield: 80 %, rel. to Me₃SiCl; b. p. 54 °C; ¹⁹F-NMR (CDCl₃): δ_F

Ϫ67.4 s (Lit. Ϫ67.3 [5]); ¹H-NMR (CDCl₃): δ_H0.25 s (Lit. 0.25

[66]); ¹³C-NMR (CDCl₃): δ_CϪ5.2 s; 131.8 q, ¹J_FCϭ 321.4 Hz (Lit.

Ϫ5.2; 131.9/321.9 [66]).

Electrochemical preparation of Vi₃SiCF₃

Preparation of 1-methyl-2-trifluoromethyl-4,5-

dihydropyrrol

Using a similar experimental set-up, the electrolysis was again per-

formed in the SSE-system DME(50 mL)/Bu₄NBr (1.6 g, 0.005 mol)

with 2.0 g (0.014 mol) Vi₃SiCl and 2.4 g (0.016 mol) CF₃Br. The

reaction was carried out with a cathodic current density of 2 mA

cm^Ϫ2, till a charge of 2700 C was applied. The volatile components

of the resulting electrolyte solution were transferred by vacuum

condensation to a trap cooled at Ϫ196 °C. After warming up to

room temperature, the product solution was stirred for 15 minutes

with passing a slow stream of analytical grade nitrogen through it

to remove non-reacted CF₃Br and traces of CF₃H formed. Because

of similar boiling points of DME and Vi₃SiCF₃no further dis-

tillative separation was undertaken, thus avoiding loss of the valu-

able product. The DME solution contains ca. 4 % of Vi₃SiCF₃with

95 % purity. The amount of dissolved anode material was deter-

mined by weight difference to 252 mg Al corresponding to a cur-

A 150 mL autoclave was charged with a suspension of 1 g Al pow-

der (0.038 mol) in 50 mL NMP, closed and degassed three times

with 50 bar analytical grade nitrogen. While stirring the suspen-

sion, 8 g (0.053 mol) CF₃Br were then introduced under pressure.

The mixture was stirred at 80 °C for three hours. After cooling at

room temperature, the volatile components of the reaction mixture

were collected in a trap (Ϫ196 °C) by vacuum condensation. To

remove the CF₃H formed as by-product, the mixture was ventilated

with nitrogen gas for 15 minutes at ambient temperature. The solu-

tion was then distilled over a Vigreux column (10 cm), giving 3.5 g

(0.023 mol) of pure 1-methyl-2-trifluoromethyl-4,5-dihydropyrrol.

Yield: 44 %, rel to CF₃Br; b.p. 102 °C (Lit. 102 °C [48]).

rent yield of 100 % for the generation of Al^3ϩ

.

NMR-spectra in CDCl₃: δ_FϪ64.6 s (CF₃) (Lit. Ϫ64.7 [48]); δ_H

2.56 s (CH₃), 2.44 m (CH₂), 3.15 t, (CH₂), 5.34 m (ϭCH).

Spectroscopic data: NMR-spectra, observed from a solution-mix-

ture CDCl₃/DME with 2 % Vi₃SiCF₃: δ_FϪ62.8 s (CF₃); δ_H5.89

Ϫ 6.45, m (CHϭCH₂); δ_C126.0 s (ϭCH₂); 139.2 s (SiCHϭ); CF₃

not detected.

MS (GC-MS): m/z ϭ 151 (85, M^ϩ), 150 (86, M^ϩϪH), 130 (100,

C₆H₆F₂N^ϩ), 110 (35, [MeCNCCF₃]^ϩ), 89 (20, [C₃HF₂N]^ϩ), 82 (78,

M^ϩϪCF₃), 81 (55, [C₅H₇N]^ϩ), 69 (80, CF₃^ϩ), 42 (75 [C₂H₄N]^ϩ).

Mass spectrum: GC-MS spectrometer: m/z ϭ 109 (100, [Vi₃Si]^ϩ),

83 (68, 109 Ϫ C₂H₂), 81, 38, 109 Ϫ C₂H₄), 69 (25, CF₃^ϩ).

Chemical preparation of Me₃SiCF₃

A 500 mL autoclave was charged with a suspension of 5.4 g

(0.2 mol) Al-powder in 300 mL NMP and 33 g (0.3 mol) Me₃SiCl,

closed and degassed three times with 50 bar N₂. While cooling in

an ice-bath, 50 g (0.34 mol) CF₃Br in five portions (every 20 mins.)

were introduced under pressure. An exothermic reaction was ob-

served already starting with the first portion. The reaction mixture

was stirred at room temperature over night. Then the product mix-

ture was separated by fractional vacuum condensation (trap tem-

peratures: Ϫ20 °C for NMP, Ϫ95 °C for Me₃SiCF₃, Ϫ196 °C for

excess CF₃Br). The product was washed with ice-water to remove

traces of NMP and dried over anhydrous CaCl₂. Fractional distil-

lation over a Vigreux column gave 35 g (0.24 mol) of pure

Me₃SiCF₃(proved by GC).

Electrochemical preparation of Me₃SiCF₂Cl and

Me₃SiCF₂Br

The difluorohalo derivatives Me₃SiCF₂X (X ϭ Cl, Br) have been

prepared for the first time in preparative amounts (20 and 10 %,

respectively) by Broicher and Geffken [85] on the basis ofRuppert’s

method [5].

For our preparation the autoclave-reaction of 2.81 g (0.03 mol)

Me₃SiCl with 4.96 g (0.03 mol) CF₂BrCl in 50 mL DME was

tested, using aluminium as sacrificial anode, [Bu₄N]Br as support-

ing electrolyte and the V4A autoclave wall as cathode. The ¹⁹F-

NMR spectrum of the resulting reaction mixture indicated besides

little CF₂ClH an almost exclusive formation of Me₃SiCF₂Cl. Only

traces of Me₃SiCF₂Br could be detected. Applying only low ca-

Yield: 84 % (rel. to Me₃SiCl); b.p. 54 °C.

1988

www.zaac.wiley-vch.de

Z. Anorg. Allg. Chem. 2008, 1975Ϫ1990

Syntheses of Trifluoromethylating Reagents and Trifluoromethyl Substituted Compounds

NMR (CDCl₃): δ_FϪ67.4 s (CF₃) (Lit. Ϫ67.3 [5]); δ_H0.25 s (Lit.

0.25 [66]; ¹³C-NMR (CDCl₃): δ_CϪ5.2 s (CH₃); 131.8 q (CF₃)

¹J_FCϭ 321.4 Hz (Lit. Ϫ5.2; 131.9/321.9 [66]).

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Chemical preparation of Me₃SiCF₂X (X ϭ Cl, Br)

The autoclave-reactions of Me₃SiCl with Al-powder and CF₂BrCl

or CF₂Br₂in NMP under similar conditions as described for

CF₃Br were found to be less effective due to side reactions and

gave only 30 and 20 %, respectively, of the corresponding products,

detected by ¹⁹F-NMR measurements of the resulting reaction

mixtures.

Acknowledgments. Financial support by Fonds der Chemischen In-

dustrie, Deutsche Forschungsgemeinschaft and Ministerium für

Wissenschaft und Forschung Nordrhein-Westfalen is gratefully

acknoledged. Thanks are due to Hoechst AG for gifts of CF₃Br

and to Huels AG for CF₂BrCl.

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