Synthesis of asymmetric perfluoro-substituted  -diketones according
to Meerwein method
L.M.Popova, E.V.Ireesova, A.Yu.Trishina,S.V.Vershilov
Usually, to create nitrogen-containing cycle,reactions of intramolecular dehydro-,
dehydra- and deaminocyclization and also reactions of intermolecular condensation of
amino-, imino- and oxygen-containing compounds (aldehydes, ketones, alcohols) are used. In
particular, one of widely used methods to synthesize derivatives of 1,3-diazine is
condensation of 1,3-diketones (or their analogues) with different derivatives of guanidine
and urea; a range of compounds produced in a similar way is extremely wide [1]. Since  -diketones in this method are used
as C-C-C fragments, it is expedient to consider main methods to synthesize such compounds.
It is known, a catalyzed by bases reaction of ethers of carbonic acids with ketones
containing active hydrogen in  -position
(Claisen condensation) is mostly used [2].
R-COOEt + R'-CH2-COR'' ->
R-CO-CHR'-CO-R'' + EtOH
The reaction takes place in the presence of metal sodium (potassium), alcoholates
of alkali metals in suspension or in alcohol solutions. Stronger condensating agents
such as sodium hydride or amide, mesitylmagnesium bromide, triphenylmethyl sodium [2a] are
also used. Numerous symmetric and asymmetric -diketones of linear structure containing different substituents have
been synthesized using Claisen method [2b].
Meerwein condensation also gives -diketones in a good yield by acylation of
ketones with acid halides in the presence of Lewis acids as catalysts [3]:
R-C(=O)X + Me-C(=O)R ' -> RC(=O)CH2 C(=O)R',
where R,R'=alkyl, aryl, hetaryl; X= halogen, RC(O)O-.
But the Meerwein condensation is used rather seldom in applied organic chemistry.
Systematic investigation of fluorinated -diketones containing different numbers of fluorine atoms in the
molecule was begun in the early 60-s.
An impressive series of polyfluorinated -diketones containing one fluorinated
substituent has been produced by the Claisen method: RF = CH2F, CCl2F,
CHF2, CClF2, CF2OCF3, CF3, C2F4H,
CF2(CF2)4O, C2F5, C3F7,
C4F8H, C4F9, C5F11, C7F15,
C8F17, C6H5XF (X = CH3, Cl, OH, CF3),
C6H4CF3, C6F5 (in this case the
second substituent R= alkyl, aryl, or hetaryl). Yields vary from 11 to 80% in dependence
on solvent, condensing agent and the nature of carbonyl and methylene component [4].
Other methods to produce polyfluorinated -diketones are also known. I.L.Knunants et
al. [5] synthesized 1,3-diketones containing trifluoromethyl group by difluoronitromethane
detachment from oxyketones obtained by nitropentafluoroacetone condensation with
methylketones (100oC,20h, 33.5-38%).
A perspective method to synthesize -diketones difficult to access was proposed.
It consists in interaction of fluoroolefins with fluoroanhydrides of acids under pressure
in the presence of antimony pentafluoride at 20oC (ratio of F-acyl/SbF5 = 6/1,
40% yield). The authors of method [6] assume that in the reaction ketone is formed first,
then it reacts with fluoroanhydride according to the scheme of the Claisen condensation
catalyzed by acid.
For the purpose of separation and purification, -diketones are fractionated or
converted to copper chelates, which further are recrystallized from organic solvents and
decomposed with sulfuric acid or hydrogen sulfide [2b]. Chelates of high purity are
produced by means of vacuum sublimation [4].
b-Diketones are known to be a prototropic system existing as
three tautomeric forms (A)- (C), for two of them [(B) and (C)] cis-trans-isomerism
[(B') and (C')] is possible. A number of investigations has shown that acyclic b-diketones exist only in tautomeric forms (A) – (C) [7]:
Introduction of electron-deficient fluorine-containing groups changes considerably
electron density distribution in the -diketone molecule affecting keto-enol
equilibrium. It has been found that polyfluorinated b-diketones
are distinguished by a higher degree of enolization in comparison with nonfluorinated
ones. On the basis of IR, UV, NMR spectroscopy data, existence of -diketones in cis-form
(B)- (C) with intermolecular hydrogen bond has been proved [7,8]. Analysis of NMR spectra
of liquid 1,2-diketones (RF = C3F7, C5F11,C7F15)
demonstrates that an increase in fluorine content leads to a shift of keto-enol
equilibrium to enol side; and in the presence of four and more fluorine atoms in the
molecule enolization becomes complete [8], mainly regarding oxygen bound to more
electron-deficient substituent. It is in good agreement with data on enolization of ethers
of fluorine-containing -keto-acids and connected with high electron-deficiency of
fluoroalkyl groups [7]. More over, it is necessary to note substantial influence of the
solvent nature on keto-enol equilibrium [9,10].
A scheme of synthesis of 1,3-diketones, given in this investigation, is most convenient
because it is based on commercial perfluoroacylfluorides C6F13COF, C8F17COF,
C3F7[OCF(CF3)CF2]nCOF (n =1,2,3,9).
The process was carried out in one stage according to the Meerwein method [3], this is a
difference from the Claisen condensation and is more attractive from technological point
of view. The reaction was carried out at boiling by treatment of perfluoroacylfluorides
with 3-5- fold excess of acetone in the presence of hydrogen fluoride acceptor (NaF) for
10-15 hours. As a result, asymmetric perfluorosubstituted -diketones were produced (1-6):
RF-C(=O)F + CH3C(=O)CH3 ->
RF-C(=O)CH2C(=O)-CH3
RF = C6F13 (1), C8F17
(2), CF(CF3)OC3F7 (3), CF(CF3)OCF2CF(CF3)OC3F7
(4), CF(CF3)[OCF2CF(CF3)]2OC3F7
(5), CF(CF3)[OCF2CF(CF3)]nOC3F7
(n = 9) (6).
The structure of synthesized -diketones (1-6) was proved by data of IR
spectra, which show characteristic bands of valence vibrations of the methyl group in a
range of 3450-2985cm-1 and valence vibrations of the carbonyl group at 1770 cm-1,
which is in accordance with reference values [11]. The analytical and spectral data are
given in the Table.
Table. Yields, boiling points, refraction coefficients and
data of IR spectra of asymmetric perfluoro-substituted -diketones (1-6)
Compound |
Yield |
b.p.,oC |
nd20 |
IR spectrum, cm-1 |
1 |
21 |
176-177 |
1.3160 |
3500, 2950, 1780, 1660, 1600, 1450,
1350-1000 |
2 |
15 |
139-141/ 25Torr |
1.3095 |
2985,2940,1770,1650,
1590,1455,1350-980 |
3 |
18 |
78-80 |
1.3048 |
3450, 3000,2500, 1770,
1650, 1430,1350-980 |
4 |
22 |
115-117 |
1.2980 |
3410, 2990,2450,1770,
1590,1440,1350-950 |
5 |
23 |
156-160 |
1.2950 |
3480,2960,1770, 1670,
1620,1450,1350-980 |
6 |
23 |
180/
0.1 Torr |
1.3070 |
3300,2910, 2300, 1770,
1720,1430,1350-950 |
Experimental
IR spectra were recorded on a Shimadzu IR-470 ( Japan) instrument (film).
1,1,1,3,3-Pentahydroperfluorodecane-2,4-dione (1). 200g (0.55 mole) of
acylfluoride of tridecafluoroheptanic acid was added dropwise to a mixture of 23.5g (0.56
mole) of NaF and 210 mL of acetone. The reaction mass was kept at 70oC, washed
with water to pH =6-7, extracted with R-113 (1,1,2-trifluorotrichloroethane), filtered and
the solvent was distilled. The yield was 46.7g (21%), yellow oily liquid, b.p.=176-177oC,
nd 20 1.3160.
1,1,1,3,3-Pentahydroperfluorododecane-2,4-dione (2),
1,1,1,3,3-pentahydroperfluoro(5-methyl-6-oxanonane)-2,4-dione (3),
1,1,1,3,3-pentahydroperfluoro (5,8-dimethyl-6,9-dioxadodecane)-2,4-dione (4),
1,1,1,3,3-pentahydroper-fluoro (5,8,11-trimethyl-6,9,12-trioxapentadecane)-2,4-dione (5)
and 1,1,1,3,3-pentahydroperfluoro (5,8,11,14,17,20,23,26,29,32-decamethyl-6,9,12,15,18,
21,24,27,30,33-decaoxahexatriacontane)-2,4-dione (6) were produced similarly to compound
(1) from the appropriate acylfluorides of perfluorocarbonic acids. The physical and
chemical characteristics are given in the Table.
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