Synthesis of mono- and difluoroacetyltrialkylsilanes and the corresponding enol silyl ethers

Article abstract of DOI:10.1021/jo049551o

A variety of mono- and difluoroacetylsilanes and the corresponding silyl enol ethers were prepared from trifluoroethanol and chlorotrialkylsilanes in the presence of LDA through retro-Brook rearrangement. Sterically demanding silyl groups, especially those bound to oxygen, resulted in higher yields of difluoroacetylsilanes. The yields of difluoroacetylsilanes were also dramatically affected by the method of the termination of the reaction. Difluorohaloacetylsilanes were prepared from the corresponding difluoroethenyl silyl ethers with electrophilic halogenating reagents in good yields. A γ-fluorinated β-diketone 9a was prepared from monofluoroacetyltriisopropylsilane by nucleophilic acylation with methyl trifluoroacetate.

Full text of DOI:10.1021/jo049551o

Syn th esis of Mon o- a n d Diflu or oa cetyltr ia lk ylsila n es a n d th e
Cor r esp on d in g En ol Silyl Eth er s
Seiichiro Higashiya, Woo J in Chung, Dong Sung Lim, Silvana C. Ngo, William H. Kelly IV,
Paul J . Toscano, and J ohn T. Welch*
Department of Chemistry, University at Albany, State University of New York, 1400 Washington Avenue,
Albany, New York 12222
jwelch@uamail.albany.edu
Received March 18, 2004
A variety of mono- and difluoroacetylsilanes and the corresponding silyl enol ethers were prepared
from trifluoroethanol and chlorotrialkylsilanes in the presence of LDA through retro-Brook
rearrangement. Sterically demanding silyl groups, especially those bound to oxygen, resulted in
higher yields of difluoroacetylsilanes. The yields of difluoroacetylsilanes were also dramatically
affected by the method of the termination of the reaction. Difluorohaloacetylsilanes were prepared
from the corresponding difluoroethenyl silyl ethers with electrophilic halogenating reagents in good
yields. A γ-fluorinated â-diketone 9a was prepared from monofluoroacetyltriisopropylsilane by
nucleophilic acylation with methyl trifluoroacetate.
SCHEME 1. F or m a tion of En ol Silyl Eth er via
Br ook Rea r r a n gem en t a n d Deflu or in a tion
In tr od u ction
Acylsilanes, recognized as synthetic equivalents of
aldehydes due to the facile scission of the carbon-silicon
bonds, have been studied extensively for more than 30
years.1 The Brook rearrangement,2 a unique transforma-
tion of acylsilanes that involves 1,2-silyl group migration
from the carbonyl carbon atom to the adjacent oxygen
atom, has been utilized for the synthesis of versatile
building blocks3 and complex natural products.4 The
incorporation of fluorine atoms into acylsilanes offers a
new route to impart the unexpected and often unique
properties that result from fluorination. For example, the
reactions of fluorinated acylsilanes with organometallic
reagents such as Grignard or lithium reagents resulted
in the formation of fluorinated enol silyl ethers via Brook
rearrangement and subsequent defluorination (Scheme
1).5 The reactions of nonfluorinated acylsilanes with
trifluoromethyltrimethylsilane have been reported to
form enol silyl ethers by a similar mechanism.6
reported, as well as several interesting synthetic applica-
tions.10 The utility of nucleophilic reactions typical of
enolizable carbonyl compounds, such as the Reformatsky
reaction or the aldol condensation, for carbon-carbon
bond formation at the R carbon atom of acylsilanes has
been restricted because of the difficulty in the formation
of enolates of fluoroacylsilanes. In addition, synthetic
methods for the preparation of fluorinated acylsilanes
have been limited.5a,8,11,12
The preparation of simple mono- and difluoroacetyl-
silanes and the corresponding enol silyl ethers, capable
of forming substituted acylsilanes on reaction with either
Electrophilic reactions of fluoroacylsilanes5,7-9 and
intramolecular rearrangements of enol ethers have been
(1) (a) Ricci, A.; Degl’Innocenti. Synthesis 1989, 647-660. (b) Page,
P. C. B.; Klair, S.; Resenthal, S. Chem. Soc. Rev. 1990, 19, 147-195.
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1993, 814-816. (c) J in, F.; Xu, Y.; Huang, W. J . Chem. Soc., Perkin
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Bonini, B. F.; Comes-Franchini, M.; Fochi, M.; Mazzanti, G.; Nanni,
C.; Ricci, A. Tetrahedron Lett. 1998, 39, 6737-6740.
(8) (a) Lee, J .; Tsukazaki, M.; Snieckus, V. Tetrahedron Lett. 1993,
34, 415-418. (b) Crowley, P. J .; Percy, J . M.; Stansfield, K. Tetrahedron
Lett. 1996, 37, 8237-8240.
(9) (a) Ngo, S. C.; Chung, W. J .; Lim, D. S.; Higashiya, S.; Welch, J .
T. J . Fluorine Chem. 2002, 117, 207-211. (b) Chung, W. J .; Higashiya,
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(10) (a) Metcalf, B. W.; J arvi, E. T.; Burkhart, J . P. Tetrahedron
Lett. 1985, 26, 2861-2864. (b) McCarthy, P. A.; Wint, L. T.; Diaz, C.
L. Bioorg. Med. Chem. Lett. 1992, 2, 119-122. (c) Garayt, M. R.; Percy,
J . M. Tetrahedron Lett. 2001, 42, 6377-6380.
(11) (a) Brook, A. G.; Kallury, R. K. M. R.; Poon, Y. C. Organo-
metallics 1982, 1, 987-994. (b) Dubuffet, T.; Sauve´tre, R.; Normant,
J .-F. Bull. Soc. Chim. Fr. 1989, 677-682. (c) Horiuchi, Y.; Taniguchi,
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10.1021/jo049551o CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/17/2004
J . Org. Chem. 2004, 69, 6323-6328
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Higashiya et al.
SCHEME 2. En oliza ble F lu or oa cetylsila n e
Der iva tives Allow in g Electr op h ilic Mod ifica tion
on th e r-Ca r bon Atom
SCHEME 5
SCHEME 6
SCHEME 3
SCHEME 4
TABLE 1. P r ep a r a tion of Diflu or oa cetyltr ia lk ylsila n es 3
yield/
3
SiR3
%
conditions
3a SiEt3 (TES)
3b Si(i-Pr)3 (TIPS)
60 0 °C, 3 h or -20 °C, overnight
63 0 °C, 1.5 d then rt, 1 d,
with HMPA
3c Si(t-Bu)Ph2 (TBDPS)
74 0 °C, 4 h then rt, overnight,
with HMPA
3d Si(t-Bu)Me2 (TBDMS) 13 -20 °C, overnight with HMPA
3e SiMe3 (TMS)
3f SiPh3 (TPS)
3f SiPh3 (TPS)
0
0
-15 °C, overnight
-15 °C, overnight
18 acidic workup/controlled
hydrolysisa
electrophiles or nucleophiles, would facilitate the ex-
ploration of the utility of fluorinated acylsilanes in
synthetic transformations. These compounds will also be
useful tools in the study of the chemistry of fluorinated
organosilicon compounds (Scheme 2).
desired difluoroacetylsilanes 3. Reductive defluorination14
of 3 followed by the hydrolysis of 4 might then form
monofluoroacetylsilanes 5.
In a model reaction, the triethylsilyl ether of trifluoro-
ethanol 1a was prepared and treated with LDA and then
H2O. However, difluoroacetyltriethylsilane 3a was found
instead of the anticipated difluorovinyl silyl ether A. It
was postulated that 3a was formed directly by retro-
Brook rearrangement (Scheme 5).
During the course of exploring the generality of the
formation of 3, it was found that silyl trifluoroethyl ethers
1 prepared in situ could be used without purification
(Scheme 6) in this transformation. In every case, the
intermediate difluorovinyl silyl ether A was not detected,
implying that the rate-determining step is dehydro-
fluorination or, in the case of especially bulky silyl groups,
the formation of the silyl trifluoroethyl ether (Table 1).
The triethylsilyl derivative 3a was obtained in 60%
yield in the absence of HMPA; however, 3b and 3c, which
Resu lts a n d Discu ssion
Having failed in our initial attempts to prepare mono-
fluoroacetyltrialkylsilanes from brominated acetyltri-
alkylsilanes by a halogen-exchange reaction (Scheme 3),
we turned to the reactions of difluorovinyl anions to
construct the desired fluoroacetylsilanes.
Difluorovinyl anions, generated from protected trifluoro-
ethanol derivatives 1 by treatment with a strong base
such as LDA (Scheme 4), have been described.13 The
reaction of protected difluorovinyl anion with trialkyl-
chlorosilanes, followed by deprotection of the resulting
enol ethers 2, was therefore proposed as a route to the
(13) (a) Patel, S. T.; Percy, J . M.; Wilkes, R. D. Tetrahedron 1995,
51, 9201-9216. (b) Howarth, J . A.; Owton, W. M.; Percy, J . M.; Rock,
M. H. Tetrahedron 1995, 51, 10289-10302. (c) Cox, L. R.; DeBoos, G.
A.; Fullbrook, J . J .; Percy, J . M.; Spencer, N. S.; Tolley, M. Org. Lett.
2003, 5, 337-340.
(14) (a) Uneyama, K.; Amii, H. J . Fluorine Chem. 2002, 114 (2),
127-131. (b) Mae, M.; Amii, H.; Uneyama, K. Tetrahedron Lett. 2000,
41, 7893-7896. (c) Amii, H.; Kobayashi, T.; Hatamoto, T.; Uneyama,
K. J . Chem. Soc., Chem. Commun. 1999, 1323-1324.
6324 J . Org. Chem., Vol. 69, No. 19, 2004

Synthesis of Mono- and Difluoroacetyltrialkylsilanes
SCHEME 7
contain larger silyl groups, were obtained in better yields
at elevated temperatures employing HMPA. On the other
hand, the much less hindered TMS derivative 3e and the
relatively labile triphenylsilyl derivative 3f formed little
to none of the desired product in the crude mixture. The
products were not isolable by the same protocol used for
3a -c.
The low yield of 3d was unexpected since the TBDMS
group is, in general, a more stable and hindered protect-
ing group in comparison with the TES group. The reason
for this low yield is not clear, although it may be a
consequence of the diminished steric demand of the
geminal dimethyl groups of TBDMS relative to the ethyl
groups of TES (Scheme 7). This effect may be especially
significant in the Brook rearrangement, which leads to
the hydrolytically unstable enol silyl ether A (Scheme 6).
This is in contrast to the thermal stability of the corre-
sponding copper â-diketonate complexes where TBDMS
derivatives exhibit greater stability suggesting that
decomposition in that case is dependent upon backside
attack on silicon.15
During the preparation of 3, it was found that the
workup procedure profoundly affected the yields of
isolated product. The best results were obtained when
the reaction mixtures were quenched with water or
weakly basic solutions such as aqueous sodium bicarbon-
ate at room temperature (Figure 1).
F IGURE 1. Effects of workup procedure on 19F NMR spec-
trum: spectra 1 and 2, quenched with H2O at rt; spectra 3
and 4, quenched with 2 M HCl in MeOH-H2O (5:1) at -78
°C; spectra 1 and 3, triphenylsilyl (TPS); spectra 2 and 4,
triisopropylsilyl (TIPS).
a symmetric material such as D formed by the condensa-
tion of enolate C with adventitiously formed 3 as shown
in Scheme 8. Treatment of the TPS derivative of com-
pound D with iodine in a 5:1 mixture of methanol and
concentrated hydrochloric acid yielded a difluoroiodo-
acetylsilane derivative 7f. Furthermore, controlled hy-
drolysis of the TPS derivative of compound D gave the
crystalline difluoroacetylsilane 3f in 18% yield. The
molecular structure of 3f was determined and confirmed
its formulation. The most notable structural feature of
3f is the strikingly long Si-C(carbonyl) bond length
(1.947(3) Å), as compared to the Si-C(phenyl) bond
lengths (1.865(2), 1.858(2), 1.865(2) Å). Such a difference
has been previously observed for the parent compound,
MeC(O)SiPh3;15 the longer former bond probably is a
result of the juxtaposition of the electropositive Si and
carbonyl C atoms.
It was also possible to trap enolate intermediate C with
chlorotrialkylsilanes to form the enol silyl ether 2. In all
cases, the products were obtained in good yields, even
with the TMS, TPS and TBDMS derivatives that previ-
ously had not given satisfactory results in the preparation
of the corresponding ketone derivatives 3 (Scheme 9,
Table 2).
Interestingly, the initial O-silylation leads to formation
of C-silylated 2 after the retro-Brook rearrangement,
while the addition of a second equivalent of silylating
agent results in O-silylation. This process generally
occurs without interchange of the silyl groups to give
predominantly the expected products; however, in the
cases of runs 1-4 and 7, small amounts of disilylated
products were formed, such as 2e, with both silyl groups
TMS. Two regioisomers 2f and 2g were isolated in two
cases (runs 6 and 7), respectively. The molecular struc-
ture of each isomer, as well as that of 2h , was obtained.
The structures contained no unusual bonding parameters
and were in accordance with the proposed formulations.
These results clearly establish that the intermediates
exist in the enolate form C rather than the siloxyvinyl
anion form B.
In the case of TIPS (spectrum 2), the 19F NMR
spectrum of the crude organic phase, obtained after
quenching with water, resulted in predominant formation
of a doublet at -127 ppm (3b), while this resonance was
nearly undetectable in the case of TPS (spectrum 1).
On the other hand, workup with an acidic solution gave
quite different results. It was postulated that acidic
solutions, miscible with the reaction mixtures at low
temperatures, would effectively protonate the enolate and
result in improved yields. However, quenching with a 5:1
mixture of methanol and concentrated hydrochloric acid
at -78 °C or with acetic acid at room temperature
resulted in two sets of doublets in the fluorine NMR
spectra for both the TIPS and TPS cases (spectra 3 and
4). From the similar chemical shifts for the corresponding
silyl ethers (δ -96 and δ -115 ppm for TIPS derivative
2b and δ -89 and δ -108 ppm for TPS derivative 2g,
respectively) and the lack of additional H-F coupling
known to be associated with intermediate A (Scheme 6),
the products generated by the acidic workup possess 2,2-
difluoro-1-trialkylsilylvinyl moieties.
Attempted purification of the unexpected products
failed, as these products underwent facile decomposition
even in pentane at -78 °C. The relatively simple NMR
spectra of the crude isolates suggest the intermediacy of
(15) (a) Higashiya, S.; Banger, K. K.; Ngo, S. C.; Lim, P. N.; Toscano,
P. J .; Welch, J . T. Inorg. Chim. Acta 2003, 351, 291-304. (b) Banger,
K. K.; Ngo, S. C.; Higashiya, S.; Claessen, R. U.; Birringer, C.;
Bousman, K. S.; Lim, P. N.; Toscano P. J .; Welch, J . T. J . Organomet.
Chem. 2003, 678, 15-32.
J . Org. Chem, Vol. 69, No. 19, 2004 6325

Higashiya et al.
SCHEME 8
TABLE 3. P r ep a r a tion of Diflu or oh a loa cetylsila n es 7
SCHEME 9
reaction
time
yield/
%
2
7
SiR3
reagent
T/°C
X
2b 7a TIPS NCS (5 equiv)
rt
3 d
Cl
Br
I
Cl
Cl
Br
I
81
85
76
32
75
70
53
7b
7c
NBS (2 equiv)
I2 (2 equiv)
0 °C
0 °C
rt
30 min
30 min
4 d
2 d
30 min
30 min
2f 7d TPS
NCS (5 equiv)
7d
7e
7f
NCS (10 equiv) rt
NBS (2 equiv)
I2 (2 equiv)
0 °C
0 °C
conditions, the 19F resonances corresponding to D disap-
peared over time.
TABLE 2. P r ep a r a tion of 2,2-Diflu or o-1-tr ia lk ylsilylen ol
Tr ia lk ylsilyl Eth er s 2
Next, since 2 was found to react with halogenating
reagents, the corresponding R-difluorohalo derivatives 7
were prepared by treating 2 with electrophilic halogenat-
ing reagents (Scheme 8, Table 3).
run
2
SiR1
SiR2
yield/%
3
3
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
TES
TIPS
TBDPS
TBDMS
TMS
TMS
TPS
TPS
TMS
TMS
TMS
TMS
TMS
TPS
80
69
71
65
62
68
67
43
The reaction of 2b and 2f with electrophilic halogena-
tion reagents N-chlorosuccinimide, N-bromosuccinimide,
and iodine formed the corresponding difluorohaloacetyl-
silanes 7a -f, respectively, in good yields. Chlorination
reactions with N-chlorosuccinimide did require longer
reaction times and higher reaction temperatures com-
pared to bromination and iodination. Chlorination of TPS
derivative 2f with 10 equiv of N-chlorosuccinimide gave
a satisfactory yield in shorter reaction times. However,
when the excess of chlorinating agent was decreased to
5 equiv of N-chlorosuccinimide, there was an increase in
reaction time, as well as an increase in byproduct
formation resulting in lower yields. The molecular struc-
ture of 7f was determined; the Si-C(carbonyl) bond
length (1.950(5) Å) again was found to be significantly
longer than the Si-C(phenyl) bond lengths (1.870(5),
1.863(5), 1.861(5) Å), just as in 3f.
2g
2h
TMS
TPS
No general method for the hydrolytic deprotection of
the O-silyl groups of 2 to form 3 (Scheme 8) has led to
satisfactory results. Under basic conditions, the results
were the same as those found on quenching with water;
for example, the TIPS derivative 2b exclusively formed
the corresponding difluoroketone 3b. The TBDMS and
TMS derivatives 2d and 2e gave mixtures of the corre-
sponding ketone 3 and unknown products with the same
19F resonances previously attributed to D. The TPS
derivative 2g formed a trace of 3f along with significant
amounts of monofluoroacetophenone 6, probably gener-
ated by an anion-promoted 1,3-phenyl rearrangement of
the Brook-rearranged intermediate B. Under acidic
The reductions of the iododifluoro derivatives 7c and
7f were attempted in an effort to prepare the difluoro-
6326 J . Org. Chem., Vol. 69, No. 19, 2004

Synthesis of Mono- and Difluoroacetyltrialkylsilanes
SCHEME 10
SCHEME 12
TABLE 4. P r ep a r a tion of Mon oflu or oa cetylsila n e
Der iva tives 4 a n d 5
3
SiR3
TES
4
yield of 4/%
5
yield of 5/%
3a
3b
3c
3d
3f
4a
4b
74
76
5a
5b
5c
5d
5f
75
82
69
85
68
TIPS
TBDPS
TBDMS
TPS
4d
69
SCHEME 11
revealed that the introduction of a fluorine atom at the
γ-position resulted in little change in the temperature
of maximum rate of weight loss (TMWL; Scheme 12).16
ketones 3; however, only 19F resonances corresponding
to compound D were observed in both cases. For the TPS
derivative 7f, the product of reduction (D) was treated
additionally with cesium fluoride and found to form
monofluoroacetophenone 6 (Scheme 8).
Su m m a r y
A variety of mono- and difluoroacetylsilanes and the
corresponding silyl enol ethers were prepared from
trifluoroethanol and chlorotrialkylsilanes in the presence
of LDA through retro-Brook rearrangement. Sterically
hindering silyl groups, especially those bound to oxygen,
resulted in higher yields of difluoroacetylsilanes. The
yields of difluoroacetylsilanes were also drastically af-
fected by the method of the termination of the reaction
with acidic conditions seemingly solely and unexpectedly
forming dimerized-dehydrated unstable products that
could be successfully hydrolyzed to give a difluoroacetyl-
silane in the case of the triphenylsilyl derivative.
Difluorohaloacetylsilanes were prepared from the cor-
responding difluoroethenyl silyl ethers with electrophilic
halogenating reagents in good yields. A γ-fluorinated
â-diketone 9a was prepared from monofluoroacetyltri-
isopropylsilane by nucleophilic acylation with methyl
trifluoroacetate. The homoleptic Cu(II) complex 10a of
the corresponding γ-fluorinated â-diketonate anion was
prepared, and the effect of γ-fluorination on the volatility
of the copper complex was investigated. The introduction
of a γ-fluorine atom had little or no effect on volatility.
Several difluoroacetylsilanes 3 were reduced to the
corresponding monofluoro derivatives 4 and 5 by reduc-
tive defluorination with magnesium in the presence of
TMSCl. The addition of HMPA apparently prevents
passivation of the magnesium surface. The mono-
fluoroenol trimethylsilyl ethers 4 and the corresponding
hydrolysis products, monofluoroacetylsilanes 5, were
prepared in good yields (Scheme 10, Table 4). In the case
of monofluoroenol ethers 4, the hydrolysis was successful
in contrast to the attempted hydrolyses of difluoroenol
ethers 2. The structure of monofluoroacetylsilane 5f was
confirmed by single-crystal X-ray diffraction methods; in
this case, the asymmetric unit contained two crystallo-
graphically independent molecules. Like 3f and 7f, the
Si-C(carbonyl) bond lengths (1.948(4) Å avg) in the two
independent molecules of 5f were significantly longer
than the Si-C(phenyl) bonds (1.870(4) Å avg).
In preliminary findings, attempts to trap the enolate
C of difluoroacetylsilanes 3 with alkyl halides such as
allyl bromide or benzyl bromide, or benzaldehyde, were
unsuccessful. The reaction of the enolate C of the TPS
derivative with benzoyl chloride gave solely the O-
benzoylated product 8a instead of the expected C-
benzoylated product 8b (Scheme 11), presumably due to
the low nucleophilicity of the R-carbon atom and the
relatively high polarity of the solvent system.
Exp er im en ta l Section
Syn th esis of Diflu or oa cetyltr ia lk ylsila n es 3: Gen er a l
P r oced u r e. To a round-bottom flask (flame-dried, three-neck
with a septum cap, 500 mL) under an inert atmosphere,
containing THF (50 mL), trifluoroethanol (5.00 g, 50 mmol),
the appropriate chlorotrialkylsilane (50 mmol), and HMPA (5
mL) as required for hindered chlorotrialkylsilanes (Table 1)
in a dry ice bath, was added dropwise a solution of LDA (175
mmol, 3.5 equiv, in 100 mL of THF). After the addition was
complete, the solution was stirred at an elevated temperature
for the requisite time (Table 1) and then was injected into
slowly stirring H2O (500 mL) in a 1000-mL flask at rt via
syringe. After being stirred for 10 min, 5 M aqueous HF (100
mL) was added, and the solution was vigorously stirred for
30 min at rt to allow the conversion of the silanol to the
The structure of 8a was confirmed by X-ray diffraction
methods; bond lengths and angles were within normal
ranges. On the contrary, the Claisen condensation of
monofluoroacetylsilane 5b and methyl trifluoroacetate
gave the expected γ-fluorinated â-diketone 9a , along with
a small amount of the reduced product 9b (Scheme 12).
Subsequently, copper complex 10a was prepared by
standard methods.16 The volatility of 10a , as assessed
by thermogravimetric analysis (TGA), was compared to
that of 10b, which lacks a γ-fluorine atom. The results
(16) Chieh, P. C.; Trotter, J . J . Chem. Soc. A 1969, 1778-1783.
J . Org. Chem, Vol. 69, No. 19, 2004 6327

Higashiya et al.
corresponding fluorosilane. Pentane, for low-boiling products
3a and 3d , or hexane (50 mL) was added, and the organic
phase was separated, washed with aqueous NaHCO3, and
dried over anhydrous MgSO4. After removal of the drying
agent, the solvent was evaporated under gentle vacuum
(important for low-boiling products 3a and 3d ). The residue
was subjected to distillation or silica gel column chromatog-
raphy to afford the product.
To obtain the corresponding monofluoroacetyltrialkylsilanes
5, 2 M HCl was added to the reaction mixture, and the
resulting solution was stirred for 3 h at rt (a condenser was
attached) except 5b that the solution was refluxed overnight
to the complete hydrolysis with addition of methanol (10 mL).
The organic layer was separated, washed with saturated
NaHCO3 solution, dried over anhydrous MgSO4, filtered, and
concentrated under reduced pressure. The product was puri-
fied by silica gel column chromatography or distillation under
reduced pressure.
P r ep a r a tion of Diflu or oa cetyltr ip h en ylsila n e 3f by
th e Con tr olled Hyd r olysis of th e Dim er ized In ter m ed i-
a te D. To a flame-dried 100-mL Schlenk flask equipped with
a large stirring bar and an Ar inlet was added Ph3SiCl (2.95
g, 10.0 mmol); the atmosphere in the flask was then displaced
with Ar using a vacuum manifold. THF (10 mL), CF3CH2OH
(1.00 g, 10.0 mmol), and diisopropylamine (1.41 mL, 10.0
mmol) were added; the mixture was allowed to react with
vigorous stirring for 10 min and then chilled in a dry ice bath.
A solution of LDA (38 mmol in 20 mL of THF) was then slowly
transferred to the Ph3SiOCH2CF3 solution with vigorous
stirring. The temperature of the reaction mixture was raised
to -10 °C, and stirring was continued overnight. To a 200-
mL Erlenmeyer flask with a stirring bar were added concen-
trated HCl (10 mL) and MeOH (50 mL); this solution was
chilled in a dry ice bath, followed by injection of the reaction
mixture into the solution. Pentane was added, and the organic
layer was separated, quickly washed with ice-cold H2O, and
put into a 200 mL-round-bottom flask on ice. The solvent was
removed at ice temperature using a vacuum pump and a dry
ice-cold trap. To the residual solid were added CHCl3, aq
NaHCO3, and diisopropylamine (0.2 mL), and the mixture was
stirred on ice for 3.5 h. The organic phase was separated,
washed with aqueous citric acid solution and H2O, respectively,
and then dried over anhydrous Na2SO4. After the removal of
the drying agent by filtration, the solvent was evaporated and
the residue was purified by silica gel column chromatography
(0-20% CH2Cl2 in hexane) to give the fluorescent solid 3f in
18% yield. The product was kept cold in the dark.
Syn th esis of 2,2-Diflu or ovin yl-1-tr ia lk ylsilylen ol Tr i-
a lk ylsilyl Eth er s 2: Gen er a l P r oced u r e. Each of the 2,2-
difluorovinylsilylenolate intermediates C (Scheme 9) was
prepared by the same method as for the preparation of the
corresponding 3 (Table 1). To the enolate solution, 1.2-1.5
equiv of an appropriate chlorotrialkylsilane (Table 2) was
added, and then the reaction was allowed to stir for 4 h for
TMS derivatives or overnight for TPS derivatives. Hexane and
H2O were added to the resulting solution, and the organic layer
was separated, washed with two portions of H2O, and dried
over anhydrous Na2SO4. After the removal of the drying agent,
the solvent was evaporated in vacuo and the residue was
subjected to distillation or silica gel column chromatography
to afford the product.
P r ep a r a tion of Diflu or oh a loa cetyltr ia lk ylsila n es 7. To
a solution of an appropriate quantity of halogenating reagent
(Table 3) in THF (5 mL) was added a difluoroenol silyl ether
2 (1 mmol) in 5 mL of THF at 0 °C. After the required period
of time (Table 3), stirring at 0 °C or at r, the reaction was
quenched with satd sodium thiosulfite solution and extracted
with hexane. The organic layer was dried over anhydrous
MgSO4, filtered, and concentrated. The product was purified
by silica gel column chromatography using hexane as an
eluent.
P r ep a r a tion of γ-F lu or in a ted â-Dik eton e 9a a n d th e
Cop p er Com p lex 10a . To a solution of LDA (2.4 mmol, 1.2
equiv, in 15 mL of Et2O) at -78 °C was slowly added 3b (0.437
g, 2.0 mmol), followed by stirring for 5 min. Methyl trifluoro-
acetate (0.24 mL, 2.4 mmol, 1.2 equiv) was added, and then
the solution was brought up to -20 °C and allowed to stir
overnight. The reaction was terminated by addition of aqueous
NH4Cl and hexane, and the organic layer was separated,
washed with H2O, and dried over anhydrous MgSO4. After the
removal of the drying agent, the solvent was evaporated in
vacuo, and the residue was subjected to silica gel column
chromatography. Reduced 9b was eluted by hexane, and then
the product 9a was eluted by 4% CH2Cl2 in hexane.
Copper complex 10a was prepared by mixing 9a with
copper(II) acetate hydrate in Et2O-H2O as described else-
where.15a The resulting crude product was dried at 60 °C in
vacuo and was recrystallized from methanol to provide crystals
suitable for X-ray diffraction studies. The TGA measurements
on 10a were performed as previously reported.15
Ack n ow led gm en t. We gratefully acknowledge the
kind suggestions and advice of Prof. Kenji Uneyama and
Dr. Hideki Amii of Okayama University, Okayama,
J apan. We thank Prof. Evgeny Dikarev for collecting
the experimental data set for the X-ray structural
determination of 5f. This research was supported by the
New York State Science and Technology Foundation,
Center for Advanced Thin Film Technology, and the
Focus Center-New York for Gigascale Interconnects. We
also thank the National Science Foundation for funding
the CCD diffractometer (CHE-0130985) at the Univer-
sity at Albany.
P r ep a r a tion of 2-F lu or ovin yl-1-tr ia lk ylsilylen ol Tr i-
m et h ylsilyl E t h er s
4 a n d Mon oflu or oa cet ylt r ia lk yl-
sila n es 5. To a stirred mixture of Mg (0.486 g, 20 mmol) and
chlorotrimethylsilane (1.63 g, 15 mmol) in THF (40 mL) and
HMPA (25 mL) was added a solution of difluoroacetyltrialkyl-
silane (5 mmol) in THF (10 mL) at 0 °C; after addition, stirring
was continued at room temperature overnight. DMF also can
be used as a solvent instead of THF-HMPA. Usually, the
ratios of the resulting cis-trans isomers were close to 1:1,
while one of the isomers was hydrolyzed faster than the other
during the subsequent purification. Each of the 2-fluorovinyl-
1-trialkylsilylenol trimethylsilyl ethers 4 was obtained by
addition of hexane and aqueous NaHCO3; then the organic
phase was separated, washed with two portions of H2O to
remove HMPA, and dried over anhydrous Na2SO4. After the
removal of the drying agent, the solvent was evaporated in
vacuo and the residue was subjected to distillation or silica
gel column chromatography to afford the product 4.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
1
tails and characterization data (including 300 MHz H NMR
chemical shifts, 75 MHz 13C NMR chemical shifts, 282 MHz
19F NMR chemical shifts, and elemental analyses) for 2a -h ,
3a -c,f, 4a ,b,d , 5a -d ,f, 7a -f, 8a , and 9a . 1H, 13C, and 19F
NMR spectra and chemical shifts for intermediate D. Experi-
mental details for single-crystal X-ray structural determina-
tions, thermal ellipsoid drawings, and X-ray crystallographic
data in CIF format for 2f-h , 3f, 5f, 7f, 8a , and 10a . This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O049551O
6328 J . Org. Chem., Vol. 69, No. 19, 2004