36
J. Voss et al. / Journal of Fluorine Chemistry 84 (1997) 35–40
Table 1
Fluorinated diols obtained via mercury-photosensitized dehydrodimerization of commercially available fluorinated alcohols
Starting alcohol/boiling point
(8C)
Reaction temperature
(start/end)
(8C)
Reaction product (diol)/yield
(%)
2H-Hexafluoro-2-propanol (A1)/58
2-Fluoroethanol (A2)/102
2,2-Difluoroethanol (A3)/95
2,2,2-Trifluoroethanol (A4)/74
2,2,3,3-Tetrafluoro-1-propanol (A5)/109
2,2,3,3,4,4,5,5-Octafluoro-1-pentanol (A6)/141
2,2,3,3,4,4,5,5,6,6,7,7-Dodecafluoro-1-heptanol
(A7)/172
100/120
140/160
135/155
115/135
150/175
180/205
210/235
Perfluoropinacol (D1)/50
Decomposed (D2) dark residue
1,1,4,4-Tetrafluoro-2,3-butanediol (D3)/90
1,1,1,4,4,4-Hexafluoro-2,3-butanediol (D4)/90
1,1,2,2,5,5,6,6-Octafluoro-3,4-hexanediol (D5)/90
1,1,2,2,3,3,4,4,7,7,8,8,9,9,10,10-Hexadecafluoro-5,6-decanediol (D6)/90
Decomposed (D7) dark residue
where A denotes alcohol and D denotes diol (A1, D1:
R1'CF3, R2'CF3; A2, D2: R1'H, R2'CFH2; A3, D3:
R1'H, R2'CF2H; A4, D4: R1'H, R2'CF3; A5, D5:
R1'H, R2'CF2CF2H; A6, D6: R1'H, R2'(CF2CF2)2H;
A7, D7: R1'H, R2'(CF2CF2)3H).
Even though perfluoropinacol has been known for a long
time [9], the mercury-photosensitized dehydrodimerization
provides a very convenient laboratory-scale preparation
method, which reduces the risk of contact with this acutely
toxic material [9]! Because of this toxicity, the analysis of
perfluoropinacol has been restricted to the determination of
its boiling point and 13C NMR data.
The fluorinated diols obtained exhibit a similar behaviour
to perfluoropinacol with various solvents, i.e. the formation
of complexes with the solvent [9]. Furthermore, their solu-
bility in conventional solvents is limited. Therefore the sep-
aration of the diastereoisomers via column chromatography
is inappropriate. About 30% of the meso-diols of 1,1,4,4-
tetrafluoro-2,3-butanediol and 1,1,1,4,4,4-hexafluoro-2,3-
butanediol can be recovered via recrystallization from
CH2Cl2. However, complete separation of the dl-diols from
the meso-diols is difficult. Therefore fractionation of the
dl-diols via distillation is necessary, even though some
meso-diol usually remains as an impurity. Both the meso-
and dl-diols of 1,1,2,2,5,5,6,6-octafluoro-3,4-hexanediol
and 1,1,2,2,3,3,4,4,7,7,8,8,9,9,10,10-hexadecafluoro-5,6-
decanediol are solids. Nevertheless, distillation was also cho-
sen for their purification.
3
On excitation with UV light at 254 nm, the P1 excited
state of mercury (HgU) is formed initially, which induces
cleavage of the O–H bond (Eq. (1)). In a subsequent step
(Eq. (2)), the xCR1R2OH radical is formed via H abstraction
from HCR1R2OH by Hx or HCR1R2Ox. Two of the substrate-
derived radicals of the type xCR1R2OH can either recombine
to give the dimer (Eq. (3)) or disproportionate to the starting
alcohol and a ketone (Eq. (4)).
After several days, depending on the amount of alcohol,
the reflux starts to slow down. NMR measurements indicate
that only partial conversion to the diol has been achieved. To
attain high yields of dimerization products, the temperature
must be increased during the reaction time (i.e. continuous
reflux must be maintained) (Table 1). Due to the reflux con-
ditions, subsequent reactions of the product are prevented
because of its much lower vapour pressure relative to that of
the starting alcohol (‘‘vapour pressure selectivity’’) [6].
2,2-Difluoroethanol, 2,2,2-trifluoroethanol, 2,2,3,3-tetra-
fluoro-1-propanol and 2,2,3,3,4,4,5,5-octafluoro-1-pentanol
dimerize in yields of about 90% to the corresponding dl-
and meso-glycols in a 50 : 50 ratio. 2-Fluoroethanol and
2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoro-1-heptanol yield only a
dark residue, which cannot be identified.
1,1,1,4,4,4-Hexafluoro-2-propanol dimerizes in a yield of
about 50% to perfluoropinacol. Generally, the disproportion-
ation reaction (Eq. (4)) competes with the transfer reaction
(Eq. (2)), leading to undesirable products. Secondary alco-
hols exhibit a significantly higher disproportionation rate in
comparison with primary alcohols [6]. For the dimerization
of 1,1,1,4,4,4-hexafluoro-2-propanol, reaction (4) seems to
be the major route, producing hexafluoroacetone, which
reduces the yield of perfluoropinacol. Hexafluoroacetone is
very volatile (b.p., y28 8C) and therefore it is readily
removed from the irradiation set-up. The perfluoropinacol so
obtained is a liquid, which is further purified via distillation
(Table 1).
1
2.1. H and 19F NMR spectroscopy and simulation of the
spectra
The NMR characteristics of the symmetrically substituted
diols depend strongly on their stereochemical nature (meso
and dl) and substituents (Fig. 1) [10,11]. Furthermore, as
shown by Gutowsky [12], the –CF2H end group, e.g. in
1,1,4,4-tetrafluoro-2,3-butanediol and 1,1,2,2,5,5,6,6-octa-
fluoro-3,4-hexanediol, causes an additional inequivalence in
the meso-form due to molecular asymmetry based on differ-
ences in the conformational population [13].
2.2. meso-1,1,4,4-Tetrafluoro-2,3-butanediol
(HCF2CHOH)2
In the 1H NMR spectrum of meso-1,1,4,4-tetrafluoro-2,3-
butanediol, we assign the triplet (5.96 ppm) to the protons