Synthesis and structure of O-silylated 2-polyfluoroacyl-cycloalkanones

Article abstract of DOI:10.1007/s007060170054

2-Acyl-cycloalkanones containing polyfluoroalkyl groups react with dichlorodimethylsilane to form the corresponding heterocyclic bis-enol derivatives. The reaction of 2-polyfluoroacyl-cycloalkanones with chlorotrimethylsilane leads to mixtures of the corr

Full text of DOI:10.1007/s007060170054

Monatshefte fuÈr Chemie 132, 911±918 (2001)
Synthesis and Structure of O-silylated
2-Poly¯uoroacyl-cycloalkanones
Ã
È
Dmitri V. Sevenard and Gerd-Volker Roschenthaler
Institute of Inorganic and Physical Chemistry, University of Bremen, D-28334 Bremen, Germany
Summary. 2-Acyl-cycloalkanones containing poly¯uoroalkyl groups react with dichlorodimethylsi-
lane to form the corresponding heterocyclic bis-enol derivatives. The reaction of 2-poly¯uoroacyl-
cycloalkanones with chlorotrimethylsilane leads to mixtures of the corresponding (Z)-con®gured
O-silylethers. The silatropy process in this case is slow on the NMR time scale.
Keywords. Chlorosilanes; Enoles; 2-Poly¯uoroacyl-cycloalkanones; Silyl ethers; Tautomerism.
Introduction
O-Silylated derivatives of 1,3-diketones are an important class of precursors in
heteroatom chemistry [1±5]. One of the possible 1,3-diketone tautomeric forms is
®xed in these compounds which have been obtained reacting 1,3-dicarbonyls with
chlorosilanes. Thus, the reaction of 2-acetyl-cycloalkanones with dichlorosilanes
R2SiCl2 (R ˆ Me, Ph) has afforded heterocycles with a bis-enol structure [6]. Such
silyl ethers are not simply the substitution products of the precursor diketones;
for example, 1,1,1,5,5,5-hexa¯uoroacetylacetone exists as a (Z)-enol stabilized
by hydrogen bonding, whereas the O-trimethylsilyl substituted derivative is
(E)-con®gured [7], resulting in different chemical properties [2±4]. (Z)-Con®gurated
silyl ethers of 1,3-diketones equilibrate fast and are indistinguishable by GLC and
NMR spectroscopy at room temperature [1, 7, 8]. The transformation of (Z)-forms
has been considered either as a 1,5-sigmatropic rearrangement [1] or as an internal
nucleophilic displacement [7, 8].
We report here the synthesis and structure of O-silyl ethers of 2-poly¯uoroacyl-
cycloalkanones as a part of our systematic investigation of this class of compounds.
Results and Discussion
We found that 1,3-diketones 1 and 2 react with dichlorodimethylsilane in the
presence of a twofold excess triethylamine to furnish the cyclic bis-enol derivatives
3 and 4 in 63 and 77% yield (Scheme 1).
In the 13C NMR spectra of 3 and 4 (Table 1), through-space coupling of C3 with
¯uorine (4JC F) was observed. An interesting feature was displayed by the carbon
Ã
Corresponding author. E-mail: sevenard@chemie.uni-bremen.de

È
D. V. Sevenard and G.-V. Roschenthaler
912
Scheme 1
nuclei of the 1,3-diene system: there is not only a triplet or quartet splitting for C5
(2JC F) and C2 (3JC F), but also for C1 (4JC F) and, unexpectedly, for C4 (5JC F).
1
Moreover, in the H NMR spectrum of 4 (Table 2) further ®ne structure was
detected for the ole®nic triplet ± its single components consist of badly resolved
quartets (6JH ˆ ca: 0:6 Hz). Probably, the presence of conjugated C±C double
F
bonds and the favourable W-arrangement of nuclei is responsible for the effective
through-bond spin information transmission between C or H and F.
When 2-poly¯uoroacyl-cycloalkanones 1, 2, and 5 were allowed to react with
TMSCl in the presence of triethylamine, the O-silyl ethers 6±8 were formed as
yellowish moisture sensitive liquids in 63±79% yields (Scheme 2).
Compounds 6±8 exist in CDCl3 solution as an equilibrium mixture of endo- and
exo-enol forms A and B (Scheme 2). The rapid rearrangement between the (Z)-enol
forms of silylated unsymmetrical 1,3-diketones at room temperature resulting in
only one average set of signals in the respective NMR spectra has been the subject
of earlier reports [1, 7]. However, for compounds 6±8, separate signals of forms A
and B could be observed at 25ꢀC in CDCl3. The ratio of tautomers in these mixture
was determined by the integral intensities of the corresponding signals in the 1H and
Scheme 2

O-Silylated 2-Polyfluoroacyl-cycloalkanones
913

È
D. V. Sevenard and G.-V. Roschenthaler
914
19F NMR spectra. The equilibrium depends not only strongly on the size of the
1,3-diketone carbocycle, but also on the chain length of the poly¯uoroalkyl group.
The NMR spectra (Tables 3±5) of 6±8 show two sets of signals. The structure
determination of the coexisting isomeric forms A and B could be achieved by 13C
NMR spectroscopy (Table 5). The ꢀC values of C2 in both sets (111.8±128.5 ppm)
represent unequivocally their ole®nic character, excluding the C-silylated isomer.
The assignments for CCO (C1 and C5) could be performed via their 2JC coupling.
F
The respective ꢀC values of form B differ very strongly (ÁꢀꢁC1-C5† ˆ ca: 64 ppm),
the signal of the the poly¯uoroacyl keto carbon C5 appearing at higher ®eld (137.5±
141.8 ppm). The shielding of the carbonyl carbon nuclei increases substantially
upon enolization [9], referring to an exo-enol structure for B. The magnitude of
1JC in the tri¯uoromethylated compounds 7 and 8 differs markedly for the two
F
1
coexisting forms; for B, JC ˆ 277:6 (7) and 278.0 (8) Hz was observed
F
1
(analogously to the exo-enol compound 4 with JC ˆ 274:2 Hz). The exo-enol
F
structure of form B in 7 is additionally con®rmed by a quartet splitting
(5JC ˆ 0:8 Hz) found for the carbons of Me3Si. The C3 signal exhibits a ®ne
F
4
structure resulting from a JC
con®guration of the exocyclic C±C double bond [10, 11].
coupling, an unequivocal proof for the (Z)-
F
In form A, the signals of CCO are in a similar region (ÁꢀꢁC1-C5† ˆ
1
3.7±13.9 ppm). For the CF3 group, JC values of 289.2 (7) and 289.4 (8) Hz
F
were found. For compounds containing tri¯uoroacetyl groups in conjugation with a
1
®xed endo-enol fragment a coupling constant of JC ˆ 289:2 Hz has been
F
1
Table 3. H NMR data of compounds 6±8 ꢁꢀ=ppm; J=Hz†
endo-enol form (A)
exo-enol form (B)
Me
ꢁCH2†
1.78±1.97 (m, CH2),
2.40±2.67 (m, 2 CH2)
1.49±1.72 (m, 2 CH2)
Me
ꢁCH2†
1.78±1.97, 2.73±2.86 (two m)
n‡2
n‡2
6a
7b
8c
0.26
0.23
2.27 (t, CH2; 3JH ˆ 7:8)
H
0.23
0.17
0.19
0.13
1.65±1.93, 2.39±2.50 (two m)
2.27 (t, 2 CH2; 3JH ˆ 5:5)
H
1.71±1.85 (m, CH2),
2.47 (t, CH2; 3JH ˆ 7:6)
1.71±1.85 (m, CH2)
2.16 (t, CH2; 3JH ˆ 7:7)
H
H
2.59±2.76 (m, CH2)
2.47 (t, CH2; 3JH ˆ 7:6)
H
A:B ˆ 33:67; b A:B ˆ 90:10; c A:B ˆ 60:40
a
Table 4. 19F NMR data of compounds 6±8 ꢁꢀ=ppm; J=Hz†
endo-enol form (A)
exo-enol form (B)
6a
127.37 (s, CF2)
119.16 (q, CF2; 4JF ˆ 8:6)
128.11 (s, CF2)
116.43 (q, CF2; 4JF ˆ 5:2)
F
F
81.60 (m, CF3)
75.58 (s)
82.30 (m, CF3)
66.54 (s)
7b
8c
77.33 (s)
70.48 (s)
a
A:B ˆ 31:69; b A:B ˆ 90:10; c A:B ˆ 60:40

O-Silylated 2-Polyfluoroacyl-cycloalkanones
915

È
D. V. Sevenard and G.-V. Roschenthaler
916
Fig. 1. Temperature dependence of the molar concentration of form B of compound 6 (from 19F NMR
data); ÁHAꢀ
ˆ
3685 Æ 324 J=mol, ÁSAꢀ
ˆ
6:22 Æ 1:48 eu
B
B
observed [12]. These arguments account for the endo-enol structure of A (cf. Ref.
[13]). In the case of a third possible form with (E)-con®guration of the exo-C-C-
double bond, the shifts of the carbonyl carbons would obviously be similar to those
in form B and should show considerably larger ÁꢀC values, an additional argument
for the endo-structure of the second form.
1
In the H NMR spectra (Table 3) the Me3Si signals of A und B are distinctly
separated. The respective signal of the exo-form B is shifted for all three compounds
to higher ®eld compared to endo-form A.
The 19F NMR spectra of the CF3 substituted compounds 7 and 8 (Table 4) show
the endo-enol A singulet at higher ®eld than in the exo-enol B (ꢀF ˆ 75:58 and
66.54 (7), 77.33 and 70.48 (8) ppm). The CF3 signal of the exo-enol B in 7
was observed in the same region as the corresponding resonance of the related
compound 4 with a ®xed exo-enol fragment (ꢀF ˆ 66:54 vs. 65.87 ppm), which
is in good agreement with the signal assignment.
Low temperature 19F NMR measurements of 6 prove the presence of the
!
equilibrium A B. With increasing concentration of B that of A decreases (Fig. 1),
thus con®rming the structure of the coexisting isomers (the (E)-exo-enol form is
unable to be converted into the other forms [7]).
In conclusion, 2-poly¯uoroacyl cycloalkanones act as O-nucleophiles in
reactions with Si-electrophiles. For monosilylated derivatives of 2-poly¯uoroacyl-
cycloalkanones (unlike for linear CF3-1,3-diketones), the silatropy process in
CDCl3 is slow on the NMR time scale.
Experimental
General
Boiling points are uncorrected. Mass spectra (EI, 70 eV) were measured on a MAT 8200
19
spectrometer. NMR spectra (standards: TMS (1H; 13C), CFCl3ꢁ F†) were recorded as 50% CDCl3

O-Silylated 2-Polyfluoroacyl-cycloalkanones
917
solutions on a Bruker DPX-200 spectrometer operating at 200.1 (1H), 50.3 (13C), and 188.3 (19F)
MHz. 2-Poly¯uoroacyl-cycloalkanones 1 [14], 2 [15, 16] and 5 [15] were prepared by a Claisen-type
condensation of cyclopentanone or cyclohexanone and alkylpoly¯uoroacylates in the presence of
sodium methylate in dry ether [15, 16].
General method for the synthesis of compounds 3 and 4
To a solution of 52 mmol 1,3-diketone and 100 mmol dry triethylamine (10.4 g) in 100 cm3 dry
diethyl ether, a solution of 52 mmol (6.7 g) freshly distilled dichlorodimethylsilane in 20 cm3 dry
diethyl ether was added within 0.5 h. After stirring for 16 h, triethylamine hydrochloride was ®ltered
off and washed with cold diethyl ether (2 Â 10 cm3). The solvent was removed, and the residue was
distilled in vacuo.
7-Per¯uoropropyl-1,2-dihydro-5,5-dimethyl-4,6-dioxa-5-silaindene (3; C11H11F7O2Si)
‡
‡
Yellow oil; yield: 63%; b.p.: 94±96ꢀC (20 torr); MS: m/z (%) ˆ 336 (M , 100), (‰M-C2F5Š , 72), 167
‡
(‰M-C3F7Š , 38), and other fragments; HRMS: found 336.0409, calcd. 336.0417.
4-Tri¯uoromethyl-6,7-dihydro-2,2-dimethyl-5H-1,3-dioxa-2-silanaphthalene (4; C10H13F3O2Si)
Colorless liquid; yield: 77%; b.p.: 48±51ꢀC (0.8 torr, ®rst distillation), 41±44ꢀC (0.9 torr, second
‡
‡
‡
distillation); MS: m/z (%) ˆ 250 (M , 100), 235 (‰M-MeŠ , 62), 181 (‰M-CF3Š , 70), 166
‡
(‰M-Me-CF3Š , 10), and other fragments; HRMS: found: 250.0633, calcd. 250.0637.
General method for the synthesis of compounds 6±8
To a solution of 8 mmol 1,3-diketone and 8 mmol (0.8 g) dry triethylamine in 10 cm3 dry diethyl
ether, a solution of 8 mmol (0.9 g) freshly distilled chlorotrimethylsilane in 5 cm3 dry diethyl ether
was added within 0.5 h. After stirring for 0.5 h, triethylamine hydrochloride was ®ltered off and
washed with cold diethyl ether (2 Â 5 cm3). The solvent was removed, and the residue was distilled
in vacuo. The products are yellowish liquids (compound 8 could not be isolated in pure state, but was
characterized by MS and NMR spectroscopy).
2-Per¯uorobutanoyl-1-trimethylsiloxycyclopentene (6; C12H15F7O2Si)
‡
‡
Yield: 79%; b.p.: 52±55ꢀC (1 torr); MS: m/z (%) ˆ 352 (M , 13), 337 (‰M-MeŠ , 100), 183
‡
‡
‡
(‰M-C3F7Š , 45), 73 (Me3Si , 21), 15 (Me , 10), and other fragments; HRMS: found 352.0720,
calcd. 352.0730.
2-Tri¯uoroacetyl-1-trimethylsiloxycyclohexene (7; C11H17F3O2Si)
‡
‡
Yield: 63%; b.p.: 114±116ꢀC (10 torr); MS: m/z (%) ˆ 266 (M , 18), 251 (‰M-MeŠ , 70), 197
‡
‡
‡
‡
‡
(‰M-CF3Š , 100), 169 (‰M-CF3COŠ , 6), 97 (CF3COŠ , 8), 73 (Me3Si , 76), 69 (CF3 , 10), and other
fragments; HRMS: found 266.0966, calcd. 266.0950.
2-Tri¯uoroacetyl-1-trimethylsiloxycyclopentene (8; C10H15F3O2Si)
‡
‡
Yield: 74%; b.p.: 50±51ꢀC (1 torr); MS: m/z (%) ˆ 252 (M , 12), 237 (‰M-MeŠ , 100), 183
‡
‡
(‰M-CF3Š , 32), 73 (Me3Si , 56), and other fragments.

È
D. V. Sevenard and G.-V. Roschenthaler: O-Silylated 2-Polyfluoroacyl-cycloalkanones
918
Acknowledgements
We gratefully acknowledge generous gifts of chemicals from Dr. K.-H. Hellberg, Solvay Fluor and
Derivate GmbH, Hannover, Germany. I. Erxleben and Dr. P. Schulze are thanked for recording mass
spectra.
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Received February 26, 2001. Accepted (revised) March 27, 2001