Fluoride ion-initiated a-fluorovinylation of carbonyl compounds with
a-fluorovinyldiphenylmethylsilane
Takeshi Hanamoto,* Shigeo Harada, Keiko Shindo and Michio Kondo
Department of Chemistry and Applied Chemistry, Saga University, Honjyo-machi 1, Saga 840-8502, Japan.
E-mail: hanamoto@cc.saga-u.ac.jp
Received (in Cambridge, UK) 4th October 1999, Accepted 20th October 1999
Table 1 Reaction of a-fluorovinyldiphenylmethylsilane with various
carbonyl compounds
a-Fluorovinyldiphenylmethylsilane was synthesized in one
step from 1,1-difluoroethylene; the TBAF-initiated reaction
of the silane with carbonyl compounds smoothly proceeded
to give the corresponding a-fluoroallylic alcohols in good
yields.
Fluorinated allylic alcohols have recently received much
attention due to their versatility as building blocks for complex
structures.1 These alcohols should be accessible from carbonyl
compounds by fluorovinylation in a straightforward way.
Although the di- or tri-fluorovinylation of carbonyl compounds
to afford the corresponding fluorinated allylic alcohols has been
reported,2 to the best of our knowledge, no monofluorovinyla-
tion as the simplest case has been reported so far. The reason
seems to be the lack of the generation of the mono-
fluorovinylanion species.3 We have considered the title com-
pound as a promising candidate for a monofluorovinylation
reagent which would generate the monofluorovinyl anion
species by action of the fluoride source.4 This report describes
the first preparation and reactions of the a-fluorovinyldiphenyl-
methylsilane 2 as a useful reagent for introducing a monofluoro-
ethylene moiety into carbonyl compounds.5
Entry
R
RA
Yield (%)a
1
2
3
4
5b
6c
7
Ph
H
H
H
H
H
H
Me
Me
61
77
61
65
60
50
18
13
4-MeOC6H4
4-PhC6H4
2-Naphthyl
(E)-PhCHNCH
CH3(CH2)9
PhCH2CH2
Ph
8
a
b
c
Isolated yield. TBAF (20 mol%) was used. TBAF was added to the
reaction mixture at 270 °C.
Our synthetic route for the preparation of a-fluorovinyldi-
phenylmethylsilane is depicted in Scheme 1. Since we have
recently reported the convenient preparation of a-fluorovinyldi-
phenylphosphine from 1,1-difluoroethylene 1,6 this method was
applied to the preparation of the silane. As expected, the
reaction of diphenylmethylsilyllithium7 and 1,1-difluoroethyl-
ene in THF at 278 °C proceeded smoothly to give the desired
product in 56% yield in one step.† We believe that the key to
this reaction is use of chlorodiphenylmethylsilane instead of
chlorophenyldimethylsilane. When the latter was employed
under similar reaction conditions, the yield of the corresponding
a-fluorovinylphenyldimethylsilane was low and the contami-
nant 1,2-diphenyl-1,1,2,2-tetramethyldisilane could not be
separated from the desired a-fluorovinylphenyldimethyl-
silane.
We have examined the use of a-fluorovinyldiphenylme-
thylsilane as a potential reagent for introducing an a-fluorovinyl
group into various carbonyl compounds. The reaction of a slight
excess of a-fluorovinyldiphenylmethylsilane (1.2 equiv.) and
benzaldehyde in the presence of a catalytic amount of TBAF (1
M THF solution, 10 mol%) in THF gave the desired product in
34% yield. Increasing the amount of catalyst (30 mol%) did not
significantly improve the yield (39%). Based on TLC analysis
of the reaction mixture, we observed that the a-fluoro-
vinyldiphenylmethylsilane was completely consumed although
the benzaldehyde still remained. This finding may be explained
as follows: the generated a-fluorovinylanion species decom-
posed to the vinylcarbene or abstracted a proton from the
reaction medium due to its modest reactivity prior to its addition
to benzaldehyde. On the basis of this idea, increasing the
amount of the silane gave better results (1.5 equiv., 42%; 2.0
equiv., 61%). The reaction was applied to the silane and various
carbonyl compounds under optimized conditions. These results
are shown in Table 1. Although ketones afforded the adducts in
low yields, probably due to their steric hindrance, aldehydes
gave the adducts in fairly good yields. It is noteworthy that the
reaction proceeded under mild conditions in contrast to the
reaction of difluorovinyllithium.8 Attempts to improve the
ketone adducts at an elevated reaction temperature (55 °C)
resulted in formation of a complex mixture of products.
A typical experimental procedure (Table 1, entry 2) is as
follows: TBAF (1 M THF solution, 40.3 ml, 0.04 mmol, 10
mol%) was added to a mixture of a-fluorovinyldiphenyl-
methylsilane (97.7 mg, 0.40 mmol) and 4-methoxybenzalde-
hyde (24.5 ml, 0.20 mmol) dissolved in THF (1 ml) at 0 °C. The
resulting mixture was stirred for 1 h at 0 °C and for 16 h at room
temperature. After the usual workup, column chromatography
(silica gel, hexane–EtOAc = 9:1) of the residue afforded 28.5
mg of 2-fluoro-1-(4A-methoxyphenyl)-2-propen-1-ol (77%
yield).‡
In conclusion, we have demonstrated the first preparation of
a-fluorovinyldiphenylmethylsilane from 1,1-difluoroethylene
in one step. The generated a-fluorovinyl anion species derived
from the silane by action of TBAF reacted with various
carbonyl compounds to give the corresponding a-fluoroallylic
alcohols in good yields. Further studies on its synthetic utility
are now in progress in our laboratory.
Notes and references
† Preparation of 2: To a solution of lithium wire (0.14 g, 20 mmol) in 10 ml
of THF was added a catalytic amount of iodine and chlorodiphenylmethylsi-
lane (1.03 ml, 5 mmol) at room temperature under argon. The solution was
Scheme 1
Chem. Commun., 1999, 2397–2398
This journal is © The Royal Society of Chemistry 1999
2397